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SPFX Camera Dolly - Motor Acceleration Amps

SPFX Camera Dolly - Motor Acceleration Amps

SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hi All... We are building a special effects film dolly. The motors have been sized and we are trying to estimate the current draw during the very fast accelerations... Doesn't seem to be too problematic for constant speed segments of our move profiles, but figuring out the current draw for accelerations has us a bit stumped.

Is there a relatively easy short-hand way to do this? perhaps using the torque constant (Kt)of the motor? or another formula that we have just been missing.

We have already figured out all our mechanical loads, wheel frictions, air drag forces, moments, gear- ratio, etc... We have this all in a spreadsheet where we plug in the acceleration rate and top speed. For simplicities sake, it spits out us a constant torque value on the motor shaft over that period of time to achieve that desired acceleration...

Now we want to find out how many amp hours that will approximately take.

We tried a couple motor sizing softwares, but they really didn't give us this information.

We did convert the whole shebang (including losses) into Work Done(Joules), then into Watts and then into Amps. When we compared that to the simple equation, Torque/Kt, the Torque/Kt was about 2-1/2 times higher... maybe this is because the voltage was not changing in this equation? Should we cut the voltage in half?

Its a brushless DC Servo motor (actually 4 of them.. Kollmorgen AKM series)
320Volt DC Battery Pack
Elmo Drivers
200 pound dolly with payload
3.91:1 gear reduction on motors.
6" diameter drivewheels.
27.4.mph top speed
14.65 ft/sec/sec accelerations (typical)

Thank you for reading.




RE: SPFX Camera Dolly - Motor Acceleration Amps

At a given speed the back EMF will be close to the applied voltage. The difference between the applied voltage and the Back EMF results in current through the motor and the source. Generally the source and the armature circuit resistances are very low.
An example:
Ignore the source resistance for now.
Assume that the applied voltage is 12 Volts and the back EMF is 11 Volts. The armature circuit resistance is 1 Ohm. One Volt will drive one Amp through one Ohm.
Now double the applied voltage to 24 volts. The difference between the back EMF and the applied voltage is now 24V - 11V or 13 Volts.
You may expect the initial current to be 13 Amps or 1300%.
As the motor accelerates the back EMF will rise and the current will drop.
However, at 1300% Current the source internal resistance may reduce the terminal voltage somewhat.
This word description may help to understand and estimate the voltage-current relationships in an accelerating DC motor.
Do not use my value of the armature circuit for estimates. Actual values will vary widely from motor to motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
That's a great description! and very helpful... really helped me understand. Thank you.

It seems to explain the current as a "snapshot" within the acceleration however. Perhaps to get a total amp-hours for the entire acceleration, I could calculate with this method at, say, 10 intervals of the voltage rise through an acceleration, then calculate each of those currents in terms of the total voltage of the battery pack.... then sum them all together.

I was initially trying to approach a different way I guess, by calculating the work done by the acceleration, then applying a motor/driver "efficiency" to it.

RE: SPFX Camera Dolly - Motor Acceleration Amps

If you have torque (M) value and time (t), to compute energy. If acceleration (a) is constant, motor acc will be eps=a*k/60/d, where k is gear reduction and d is diameter drivewheels; max motor speed is Omega_max=eps*t, so enegy developed is W=M*Omega_max/2; correct with motor and converter efficiency and obtatain energy from source. Depending of source type may compute current (average or maxim) nedeed.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
hmmm... that one I couldn't follow. sorry, but thanks.

RE: SPFX Camera Dolly - Motor Acceleration Amps

Perhaps I'm incorrect, and this would be very conservative, but locked rotor amps (or equivalent, amperage at 100% torque) for the duration of acceleration could be a value to use. Like I said, LRA/100% torque would be overkill, but it I think it would give you a worst-case scenario.

What amperage can the servo controllers handle without tripping on over-current? That may be a limiting factor too.



SceneryDriver

RE: SPFX Camera Dolly - Motor Acceleration Amps

Remember, as the motor is accelerating, the back EMF is increasing and the current is dropping.
If you are using a servo controller it will probably be in current limit for hard acceleration.
You need some information about the controller.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: SPFX Camera Dolly - Motor Acceleration Amps

And here's a proposal from a dumb Mechanical Guy who has done quite a lot of precision motion control over the years and who isn't smart enough to get into the nuances of motor Electrical Engineering (them electrons scare me):

I suggest you should perform your analysis based on the maximum torque required to move your loads. This involves linear accelerations/decelerations of linear masses, rotational accels/decels of rotary masses, accounting for gravity effects, friction/stiction, dirt on the bearing, torques induced by offset loads, all kinds of things. Develop a value for "PeakTorque". Size your motor for PeakTorque*2 or 2.5 and run with it. Excess torque is not necessarily a bad thing, it gives you strength to overcome loads you never anticipated. It's there in reserve if you ever need it. Make sure your mechanical transmission elements won't get damaged if max torque is applied.

Current draw is amp load at max torque (which you should never achieve anyway).

Just a suggestion.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: SPFX Camera Dolly - Motor Acceleration Amps

To move a load at constant acceleration need a constant torque all time (considering load = constant vs speed). For a DC motor that mean constant current (considering constant flux), so as speed increase, to keep constant current (torque) need that converter increase applied voltage; current being constatnt will impose a constant acceleration and power develoved by motor increase liniary up to maxim speed. Sure all mechanical parts and power converter need to be designed accordingly requested acceleration and load values. If source energy have constant volage (battery), current suplied will have same shape like power, will increase from a start-up value up to maxim corresponding with point very before acceleration become zero (max speed).

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Lots of info here... but ughh! Honestly I've tried for 4 days straight: I've looked at literally hundreds of white papers and they are either way over my head technicaly or seemingly have completely different equations.... either way none of my attempts to calculate the amp draw are coming in remotely close to reality, and Kollmorgen sizing software (motioneering) has nothing about the amperage. (and I've been very careful on my units.) The motor is sized correctly for the application in terms of rms torque, and peak torq..


Maybe I will simplify my query, lets say we forget about acceleration, and just ask how to somewhat accurately calculate the current draw for a steady state mode....

Here's my example...

motor speed: 6000 RPM (continuous: no acceleration)
torque on motor shaft: 3.721 Nm (continuous)
WHAT IS THE CURRENT DRAWN FROM THE BATTERY?

In a purely theoretecial calculation of the mechanism (but complete with friction losses), it would take take from the motor 2598 Watts of power, 8.12 Amps.

But of course the motor and driver are not 100% efficient.

Battery voltage: 320VDC
Motor Torque Constant: .43 N-m/Arms
Motor Back EMF Constant: 27.5 V/krpm
Motor Resistance (line to line): .8 ohm
Motor Inductance: 3.1Mh
Motor Continuous Torque Stall at 100 deg: 3.53 Nm
Motor Continuous Current Stall: 8.4Ics
Motor Max Mechanical Speed: 6000rpm
Peak Torque: 11.6 Nm

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Don't know if this matters, but the motor has 5 pole pairs.

RE: SPFX Camera Dolly - Motor Acceleration Amps

Quote:

...Kollmorgen sizing software (motioneering) has nothing about the amperage.

Well maybe the sizing software package doesn't try to tell you this, so instead look through the controller's datasheet. It really is important for all designers to know the maximum current that can be driven by a controller, and the maximum current draw in puts on the line. Wire sizing, fuzing, circuit breakers, insulation, all that stuff. I think the point you're missing, which a number of members have tried to say already, it that the maximum current your system will see is the maximum it can handle before letting out the "magic smoke" or blowing a fuse. This information is so important to electrical design that I'm surprised you haven't just read it off the datasheet and moved on by now.

Very first hit on google, using "stepper motor 320VDC" is a datasheet with torque-current plots and a max current statement all on one page.
If you are NOT seeing this from the devices you have selected, then you have more questions to ask the manufacturer.

I tried looking for a Kollmorgen servo, took a minute longer, found the catalog for one of their product lines, and just poked around. I picked the AKM but I don't know what you are using. About 1/3 of the way through the nicely detailed specifications, I found current at stalled torque under numerous conditions, and a peak current with its corresponding torque... It looks like the number you want. I won't deny that it can be time-consuming to sift through the mountains of information these catalogs can pile on you - this one is 70 pages long.

STF

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Sparweb, I'm not interested in the "maximum the controller can handle" and have never mentioned that. I know we are under that limit. Rather I am interested in calculating amperages of various lesser motor accelerations so that we can size the amp hour capacity of our batteries according to various situations.

And yes, I've seen the curves: our motor is sized correctly: what I wanted was an underlying formula.

I do believe that iop995 answered the question the best, basically informing me that a current curve would basically mimic the power curves no matter what... (he didn't say it would mimic the torque curve, but I think that might be what was implied)

But nobody here has mentioned the following formula:

Amperage = Torque / Kt (torque constant.)

But I think maybe its just that simple.
And If it is not, please feel free to correct me...

So therefore, If I am simulating a very simplified acceleration with a constant torque value, then, as iop995 says, the corresponding current would also be continuous single value. And if this number was divided by the Kt of the motor:

I=Tl/Kt

... and multiplied by the time of the acceleration and then divided by the battery voltage,

Ah=I*s*/V

where:
Ah = the amp hours used during the acceleration
I = the current
s = the time in seconds for the acceleration
V = the battery voltage
I is the amperage
Tl is the torque required to achieve the acceleration
Kt is the torque constant of the motor

I think this is right... 90% sure... If anybody thinks diffently, please do set me straight.

What really threw me originally is that when you calculate the energy, or Work done, of the mechanism undergoing the acceleration, then convert it to watts and then amps, you guy a pretty low number. I thought that the motors, being fairly efficient would only be a little bit above this, but doing what iop995 suggested is considerably higher... 2.4 times higher in my case, and that really through me off.

If these motors are suppposed to be more than 80% efficient, then why is it only like 40% efficient in this acceleration?









RE: SPFX Camera Dolly - Motor Acceleration Amps

You sound frustrated, and now that you've explained some more, I see that you are past the selection and sizing of components - a process I thought you were still undergoing.
I hope to understand why you are going through this analytical exercise.

Going back to review your earlier postings, I think I can glean some performance requirements from the mention of amp-hour capacity and the battery packs....

Would this be a correct statement, on your part: "I am trying to determine the battery capacity required to satisfactorily power this system at a remote location, without chance of recharge or resupply, for the period of time usually required for a film crew to complete their task, under all foreseeable conditions". Two corollaries of this are that too little battery capacity means the shoot is cut short, and too much battery capacity means carrying dead weight to the four corners of the earth. If this is the case, then I understand your goal much better, and we can dispense with the explanations of wire sizes and over-current devices - which are the usual subject of discussion when "maximum current" is called into question.

Have you tested the system to take measurements yet?

STF

RE: SPFX Camera Dolly - Motor Acceleration Amps

eckener: Formula Amperage = Torque / Kt (torque constant.) tell that motor current is constant for a constant torque, but you need to calculate (estimate) Ah battery; as motor speed increase, power developed increase also. At converter input, voltage battery being nearly constant, current supplied by battery will increase also.
For a given torque, at start-up speed (maybe zero) you have motor current I, compute power developed by motor, correct it with motor and converter efficiency and obtain battery requested power (Pbat) and battery current (Ibmin=Pbat/Vbat), then this current increase liniay with speed and have Ibmax at maxim speed. Or, average current will be Ibavg = (Ibmax + Ibmin)/MaxSpeed/2.
To calculate Ah battery, in this case, when current is not constant, I thing best way is to check battery discharge time at constant power and using a 65-70% from maxim calculate power will lead to a good estimation (or may use maxim power to have a reserve room).

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
hahaha... thanks SparWeb.. can you tell? Most of the systems on this dolly, we have a good handle on, but one of the dangerous of the entertainment industry, and especially of VFX and SPFX supervisors is that we are jacks of all trades, masters of hopefully at least few. I've a solid handle on most of the engineering aspects of this project, but calculating motor at this level of detail is not something I've done before.

And yes, that would be an accurate description of at least one of the reasons we are trying to determine amp-hours carefully! Other performance feats we are calculating include for instance, motoring up and down a football field at 27 mph at 14 ft/sec/sec, and making sure we have enough battery life to get to halftime, or calculating amps required to go straight up the side of a building for the next Spiderman movie.. or shooting a car stunt at zero to 60mph in less than three seconds...

As this is a semi-autonomous vehicle, its a delicate balance of weight, bulk and performance and above all, safety. Batteries are probably one of the costliest and bulkiest and most critical components to the system. (We are currently using a 2.4 amphour 320V LiFEPO4 lithium pack)

But another reason we are trying to nail down the amps is to make sure our drivers can do the job, but are not so big that they increase the volume too much.

We are just embarking on building on prototype now, but haven't ordered parts yet. Wanted to try and get the performance envelope and physical envelope nailed down as much as possible before ordering...

After figuring out the amp usage here, the next thing we are tackling is how to best stop the thing. (handle our back-EMF): deciding whether to use quite bulky and very hot dynamic braking resistors, or whether its worth it to regen into the battery.

RE: SPFX Camera Dolly - Motor Acceleration Amps

As battery life is important, and given that this thing will have a fair amount of kinetic energy, then a regenerative system must make sense. In terms of additional mass it won't add much to the driver, and probably less than a braking resistor bank.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Iop995, Thank you so much for your help. Please forgive me for rewriting your sentences a little bit…

The formula I= T / Kt tells us that motor current is constant for a constant torque, but you need to find the amp hours for the battery:

As motor speed increase, the power developed also increases. At the converter input, voltage battery being nearly constant, the current supplied by battery will increase also.


Ok… so if I am getting this right, you are saying that even though the current and torque is remaining the same, it’s the watts of power being generated by the motor over the range of RPM that will affect the Ah calculation…. and I guess this assumes that at different RPMs the motor will be using less than the full voltage of the battery. (but I am not sure about this, because of the way the drivers control both voltage and current.)

However, I think the way I am doing it now, I'm assuming a max of 320 volts over the whole acceleration, (which would be maximum power over the whole acceleration.)

The method you descibe I think would result in less Ah than the way I am calculating it now…


For a given torque, at start-up speed (maybe zero) you have motor current I,
compute power developed by motor,


If this same as calculating the power required by the mechanism over time, I had been doing this by calculating the mechanisms energy in Joules divided by the duration of the acceleration in seconds, however this gives an average power. Or are you saying the power is based on the current inside the motor at a certain torque?

correct it with motor and converter efficiency

I was not finding this efficiency rating specifically listed in any documentation, and was trying to use just multiplying the value of Kt, but this is probably not correct?

and obtain battery requested power (Pbat)

This is where I was really having difficulty. To find watts here, based on torque and rpm, don’t I need the voltage that the motor is using at this rpm…. How to find it? I tried using Kv, but it seemed to be giving me only half the number of volts at full RPM of the motor

and battery current (Ibmin=Pbat/Vbat),

then this current increase linearly with speed and have Ibmax at maxim speed.

Or, average current will be Ibavg = (Ibmax + Ibmin)/MaxSpeed/2.

To calculate the Ah from battery, in this case, when current is not constant, I think the best way is to check battery discharge time at constant power and using a 65-70% from maximally calculated power will lead to a good estimation (or may use maximum power to have a reserve room).


I believe the method I am using now is the maximum power method, as I am assuming that the motor is always sucking up 320. But would like to be doing this the correct way.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
iop995.. you are awesome! I will check out that spread sheet later on today
Thanks!

RE: SPFX Camera Dolly - Motor Acceleration Amps

I haven't had much time to look at this, but I think a (the?) key thing you are missing is how the switching (PWM) amplifier processes the electrical power.

Let's say that at a particular instant, the amplifier is providing 5 amps to the motor at 80 volts. (With your 0.8 ohm [line/line] resistance, that's 4V for the resistive drop and 76V for back EMF.) This does NOT mean you are drawing 5 amps from your 320V battery pack. To first approximation, at least, you are drawing 5A * (80/320) = 1.25A. This ignores amplifier losses, so the draw will be a little (but not much) higher.

This would not be true in a linearly modulated amplifier, where 320 - 80 = 240V would be dropped in the amplifier, and a full 5A would be drawn from the supply.

The analysis does get tricky, because even for a constant acceleration, the back EMF voltage is steadily increasing, so while the motor is receiving constant current, the current draw from the battery back is continuously changing.

Very few servo systems are battery driven, or even care too much about total energy draw from the source, so this is not the focus of most setup software.

Curt Wilson
Delta Tau Data Systems

RE: SPFX Camera Dolly - Motor Acceleration Amps

That's what I needed to zero in upon: a better "mission spec". I was struggling before to figure out why this all mattered. Now it's crystal clear.

Since the folks with much more electrical background helping you on that side, I think I can make a suggestion to help you with your "missions". In the aircraft world, mission specifications or "profiles" are used to simplify the definition of what the end-product aircraft must accomplish (and to make an even playing field if bidders are competing). It seems an appropriate analogy because in aircraft missions the typical flight profile assumes one fuel tank's worth of range to accomplish that mission. In your case I could see a handful of "mission profiles" for your camera system, each having to fit into the battery pack's charge. In each profile you define a typical routine of motions that the equipment must repeat. Each motion requires functions that require energy. Then you can add up the functions that the system experiences during the job to a sum of energy consumed.

This process may help you focus on things in proper order: I see you working through a lot of math and theory but there are aren't any concretely stated goals yet. Of course you could be unwilling to divulge this in the face of competitors, and no need to tell all here, either. I'm just suggesting that the "Superbowl" mission profile could be defined by a specific camera payload, moving X times on a player in "follow mode", Y times pan and zoom over the field, Z times rapidly traversing to track a pass... and so on. The mission profiles for the African safari and the Swedish rally race are all quite different.

STF

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Dear iop995...

That spreadsheet is like a gold nugget for me! If a picture says a thousand words, than that spreadsheet is worth words*10^23!

I wouldn't have got all those calcs right in a million years... but at least I knew I was wrong!

My Ah's were coming in considerably higher, and was actually much closer to when I was simply calculating the work to be done, converting to watts, then amps, and then applying an efficiency to it.

For my own edification, and perhaps those that are following, the 5 (count 'em) big things I was totally incorrect on were:

  1. Motor Phase Current...your calcs use only 2/3 of the current because of the commutation, while I didn't even consider the phases and (incorrectly) used the entire amount!

  2. Average of Min Current and Max Current... your calcs create an average current that considers the full torque at zero RPM as well as the torque at full speed, while i was only using the maximum torque number, and maybe considering cutting it in half.

  3. Calculating Power in the Motor - Now I understand that power in this sense is Torque x Speed. You are finding average power inside the motor, and then using this to find average current inside the motor based on power. I was not even doing this step... and just trying to calculate amp hours directly from the current.

  4. Current Conversion to RMS to DC current considering 120degree comutation. That one was really tricky. I thought all these equations were for DC, since we are on battery, but I guess all these equations are geared towards creating AC current, so you are converting it back to DC by multiplying by sqrt(2)... and the taking it further by dividing by the 2/3 for the commutation. Smart.

  5. Basic Amp Hour Calculation. You were using going back and forth between watt hours in the motor and amps to determine the average number of watt hours drained from the battery over time and then calculated the amp hours based on this. I was just multiplying the amps used by the time of the acceleration, and skipping the power (watts) altogether. I was probably doing this completely wrong.

If there were a part to question for me, it would be this last part above, though I'm not at all saying it's incorrect.

Also I did notice, you had "motor phase resistance" at .4 ohms.. where the spec shheet say resistance " resistance line to line" is .8 ohms. You probably had a reason to do that, but I am double checking. And of course, your calcs didn't have all the mechanism friction losses, air drag, etc... No problem for me to add those.

While I am going to continue to think about this on my own to see if I can get it to make intuitive sense to me, I am just going to take a guess for now, you being much smarter than I in this area, that you've pretty much got this all correct.

Now, if you are correct the batteries are certainly going to last a LOT longer than I thought.

Thanks so much....

P.S... I realize now that its probably NOT JUST ACCELERATIONS that I've miscalculated, but amperage use in general, when it is cruising with no acceleration. I will try to modify your spreadsheet for that as well....

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hey SparWeb...

Thanks for the insight... that is pretty much exactly what I'm doing, though my profile "missions" are only about 5 scenarios right now, and don't include bombs.

We have a very large spreadsheet set up, with every parameter of the system. There is a of course inputs for about 20 common parameters, and then of course about 500 outputs of all kinds of data. (obviously was having trouble in the amps department),

...tings like peak torque, RMS torque, rolling frictions, air drag, sprocket sizes, peak amps, inertia ratio, net torque to total inertia ratios, you name it.

Right now we are using this to facilitate the comparison of various motors.

I initially looked at seemingly hundreds of motors, finally selecting one of the Kollmorgen's that was giving us a decent inertia ratio, But now Mavilor has these new slotless stator servo motors that are really incredible. They basically have about two to three times the performance of the best of what we'd looked at, but of course this is in part because they are extremely fast... upwards of 18,000+ RPM as opposed to the 6000 RPM kollmorgens we were looking at. If we can figure out how to gear them around 10:1, and do this fairly quietly, we'd might go with them.

A great benefit for using the faster motors, is is that we could achieve a very large speed envelope... from very slooooow speeds to very fast... without the use of a transmission. As this is a repeatable motion control system, shifting gears would be undesirable, and good control at creeping speeds is essential.

RE: SPFX Camera Dolly - Motor Acceleration Amps

After figuring out the amp usage here, the next thing we are tackling is how to best stop the thing. (handle our back-EMF): deciding whether to use quite bulky and very hot dynamic braking resistors, or whether its worth it to regen into the battery.

realize you DO HAVE full batt recharge (regen) by default with batt powered drives: o resistors needed. regen power goes back to dc bus which is ur battery pack.

BTW, that Motioneering software does give amps during all the move segments as well as power required (front page top right - system power supply sizing icon). It would give you all these answers - ask your Kollmorgen supplier to go thru it with you and show you the small details maybe?

RE: SPFX Camera Dolly - Motor Acceleration Amps

eckener: calcs are simple formulas and there are a lot of simplifications. Regarding your points:
1. For 3 phase BLDC motor torque is Tm=3/2*kt*Im; in motor datasheet I can't find details about switching method.
2. This average power allow to obtain an average battery current / capacity. Your battery type exibit almost same discharge rate for a wide discharge current, so errors are small.
3. Motor current is constant for a given acceleration / torque (neglecting drag forces and similar forces speed-dependent), but power developed increase as speed increase; driver must be albe to supply a constant current and increase output voltage up to reach maxim speed. From motor data, for this setup, maxim acceleration is 6.64m/s2 coresponding to a 17.99Arms phase current. This is one system limitation (from motor) and another limitation is maxim speed - due EMF, that reach battery voltage - 320V, at 23.4m/s.
4. This conversion is made considering 2pi/3 current commutation.
5. Here, need to know time working for such profile or/and add another profiles with their working time.
Motor line-line is 0.8ohm and I suposed star connection, so a phase have half.

cswilson: I considered converter like a black box; I have an output load (motor) and need to know power at input, so using efficiency obtain it. Converter is important in operation requested (constant acceleration - constant current capability for entire output voltage range).

mikekilroy: good point about regen brake. Battery used allow high charge current (about 1C or maybe 1.5-2C in pulse) so, may be a point to analize cost / advantages for such feature. Anway, regen must be combined with dinamic brage (resistor) and mechanical also.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Iop995...

I did run this through Motioneering real quick.

Your "Motor power at max speed" (Pmax) does indeed exactly match the result for Motioneering's Buss Watts and Continous Power Watts (same thing i guess)...

But, in your spreadsheet, when you divide this (Pmax) by the Maxim Battery Current (Ibmax), the watts shoots up quite a bit, even if you turn the efficiencies up to 100 percent... and then of course its considerably higher than the buss voltage in motioneering.

Just double checking that this is correct?


The formula for Ibmax (Maxim Battery Current) you are using is:

=Pmax/effm/effc/(2/3)^0.5


Is this trying to account for the use of a DC power supply?

Curious what formula would be for Ibmax if it was an AC power supply instead?


P.S as you can see, am also using spread sheet to calculate amps during non-acceleration segments of profiles, by only looking at "max" numbers. Have incorporated formulas into our large mission spreadsheet and modified to calculate amp-hours for duration of individual segments... and of course are plugging in refined torque request rather than basic m*a*d.... Working great. File Attached

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
If this is correct, its weird that Motioneering wouldn't give the watts required by your power supply.

RE: SPFX Camera Dolly - Motor Acceleration Amps

You right, above formula is not correct. Correct minim/maxim/average battery current with:
Minim battery current Ibmin =Pmin/effm/effc/Ub and corresponding. Now for 100% effm and effc will lead to same average power.

RE: SPFX Camera Dolly - Motor Acceleration Amps

whew......

rick armstrong and others (including me) spent over 20 years tweaking Motioneering equations. They take into account commutation. they take into account all efficiencies. Motioneering is the cumulative result of probably 3,000 hours of tweaking, not counting the 2,000 hours we each spent individually on our own spreadsheets that predated it - before even excel existed. I strongly suggest you go back and learn how to use it for this case rather than inventing your own now. How can this many hours from servo engineer experts not be the way to go?

to arbitrarily begin dividing Kt by 2/3 is wrong. If the Copley drive is NOT sinusoidal commutation, then it's trapazoidal commutation may need to be taken into account. Copley has some good sinusoidal commutation drives - are you not using one - to save money??

BLDC motors are designed specifically for trap waveform or sine waveform; your AKD is designed for sine current; if you are not using a sine drive, why?

You bought the akm motor from a Kollmorgen distributor Why not make them give you the proper help? There are a dozen high tech app engineers sitting at their desks at Kollmorgen in Radford, Va, some of whom would be tickled pink to walk you thru Motioneering, even with a not so good AKD drive.

Also, look into the real specs of your LiFePO4 cells: I know of none that cannot handle 3,4,5x (my 100amphr ones handle 10x for upto 20 sec) discharge rate, and also easily handle 1,2,3x or more CHARGE rate: use this information to decide if you can dump ALL regen (decel) energy back inot the batt pack real time of if you need to dump some of it; this should not be a 'guess;' you have the data to determine it properly.

Last, you said in multiple posts you can get to around IIRC 28mph, then in one said you want to chase a vehicle 0-60mph in 3 sec.... how can you do that?

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Thanks Iop995, thats just got us a few more runs up and down the football field! Actually, this has whole excersise has had such positive impact, that maybe we will cut the capacity of our pack down... thus gaining, a lot of valuable space for electronics, whilst still providing for a sleek design.

mikekilroy:
Yes, we will probably go with the regen to battery, if for no other reason than those breaking resistors seem quite large and hot, especially for a piece of film equipment. I haven't calculated if the 1C charge rate of the LiFEPo4's can take the full amount of energy coming from regen, but if not then yes, as iop says, we may need at least some braking resistors in there. These calcs will certainly make those easier to size, as every drive manufacturer seems to size breaking resistors in a different manner.

We do have additional mechanical failsafe brakes on the motor shafts (not the kind that come built into the motors, but ones that can operate dynamically and repeatedly) for e-stops, and for holding the load when the motors are not spinning. They will be able to be temporarily disabled so they do not "click" when shooting a dialogue scene. There are four of them, one on each of the four motors/drivewheels. Beyond these however, we were not planning to have an additional mechanical brake system.



RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hi Mike
Oh man, I did not mean to offend! sorry! No, the kollmorgen guys have been really nice and the motioneering definitely works and really helped me size the motor correctly, and I am VERY glad to see that you worked on that, thank you.

I did use it for motor selection, but now we are running lots of scenarios in a spreadsheet form, where its very easy to cut and paste and compare column to column, motor to motor, etc...

As a "beginner" also, it will say it was a little frustrating as there doesn't seem to be a real good mechanical "model" for a vehicle... conveyor belt was close I think, but I never could be really sure if was actually doing what I expected, especially with all the automatic computation of continuous and intermittent, and so ended up just calculating it all out and putting the numbers into Motioneering as direct data entry. (its good to have two techniques that corroborate) A more seasoned professional could probably use it with a lot more confidence than I.

Also, it ties you to a motor/drive combo, which I guess is no big deal, but we are not using the AKDs. They are far too large for this application, and can't take DC input, (from what I was told).

We were talking to the talking to the Copley guys, and though the Xenus might work for us, its still pretty large, and CANNOT regen into a battery. Copley does have a new Accelnet coming out soon that is for DC input, and is very small (a few inches by an inch tall) which will handle up to 20amps. That would be really good for us. Elmo has a similar model already out, called the Trombone, which has similar capabilities. Additionally Copley has suggested that they we can run their drives with one master drive running on position or velocity, and the other three slaving to it with torque.... torque assist, if you will. This will ease our programming burden, as this dolly is designed to go around curves and up and down hills, and in doing so, the drivewheels will see slightly different radii.

Regarding the 0 to 60 thing, the rig allows you to quickly swap out the gears for for a high-speed version of the dolly. The trade off is that the inertia-ratio suffers quite a bit, and the rpm at very low speed is kinda low... not ideal for precision motion control work for say, a product shoot, or slow moving camera shot.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Also we are not deadset on Kollmorgen AKM42j motors yet. We were all set, but at last minute were shown these Mavilor XtraforsPrime Slotless Stator Motors.

There are some drawbacks, but pound for pound, we get half the inertia ratio, 75% more RPM at very slow speeds (at the cost of running 30% faster...around 8500 rpm), and more head room on continuous and peak torques.

They do run hotter I think, and as if the 6000 rpm kollmorgens weren't fast enough, I'm still trying to figure out how to gear that high speed 6:1 or more and still keep things quiet.

RE: SPFX Camera Dolly - Motor Acceleration Amps

I didn't mean to sound offended, sorry. I wonder how you would use more than 6000rpm on such a camera slide - would have to be a timing belt arrangement I would imagine cuz most planetary gearboxes do not go over 6000rpm even intermittently. For noise be sure to pick helical gears too rather than straight cut if you do go gears.

whoever told u AKD does not accept dc input is wrong; we have lots running with dc input. the regen then to the batts is all automatic also; If one simply brings the dc in thru the ac input terminals on a drive, ok on maybe no regen, but any drive with access to the dc bus terminals like the akd allow full regen to the batts.

not sure how the mavilor motor can offer you anything over the akm; I don't see it. if you were talking about their older pancake design then yes, they can have higher inertia for a given torque. if you list exact part no I would be happy to compare them to the exact akm part no so you get apples to apples comparison.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Mikekilroy...

The Elmo Trombone definitely says "Vector control sinusoidal commutation"


The Copley Accelnet doesn't actually specify... I was just ass-suminng.

I thought the AKD's were also?

The sinusoidal makes a difference as to how we're calc-ing everything?

RE: SPFX Camera Dolly - Motor Acceleration Amps

no link to page 17. I will not sell you the products since you are outside my territory, but I don't mind helping compare and such.
So please give me a link to the FP model - it is not shown on the link you posted earlier and I dont have time to search it out. You also made comments about pos mode to 1 wheel then torque command to the other 3... You should know all your options - that may not be the best one. Lots of drives can do electronic gearing in pos or velocity mode (what you would send to the other motors) - there are some big and some not so big pros and cons to each approach for what you are doing; if you pick drives that only can do 1 then you cannot try the others; I would pick a drive that can run any of these modes just by software changes. If you want to take this offline here that is ok too; maybe some of the small talk off line and good stuff on? Your choice. my email is mike -at- KilroyWasHere -dot com.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Guess it wont link to the .pdf page. Anyway, its just the product manual on that first link to Mavilor.

All the drives I mentioned do all three modes. We wouldn't be stuck with one, and may use more than one for different uses.

RE: SPFX Camera Dolly - Motor Acceleration Amps

hmmmm. either I am missing something drastic or some salesman is blowing some magic dust? Let's go to emails to compare.... The only data sheet I can find at Mavilor for this motor is here:

http://www.mavilor.es/pdf_products/FP_XTRAFORSPRIM...

It is 2 pages and does not show windings or inertia or lots of other details. But what it does show is this motor is way smaller than the other you compared to, so I am not sure why you think it is equivalent.

Here is the basic data I can compare from the data I found on the Mavilor. Be careful if you go this route, remember iop995 showed over 7nm required for your accel; the speed torque curve you showed for the Mavilor motor is good for 2.2nm MAX - AT 31 AMPS! You won't get that in the 1-2" size musical FET drives upto 350vdc...

Again, I think you should have a servo person look over your shoulder to help guide you. I do go 0-60mph in 5 sec with 160kw worth of BLDC motors in my converted Chevy Equinox on my 350V LiFePO4 batt pack - drives are in torque mode as you suggested; there are definite resonant speeds that shake; electonic gearing might be much better method for you - it can still allow going around corners. I am sure my 8:1 planetaries to my 20 some inch dia wheels is a lot worse inertia match than you will have. Email me if you want.

Take care.

RE: SPFX Camera Dolly - Motor Acceleration Amps

As I said, spreadsheet is an estimation tool, made with no complete info, much simplifications and no much time...
Regarding 2/3 factor, it's valid for sinus comutation and it lead to higher current values for trap comutation also (where is 1/2), respectively 32$ Ah oversizing.

RE: SPFX Camera Dolly - Motor Acceleration Amps

decided to just run a quick Motioneering on your movie cart using the data you gave above:

Its a brushless DC Servo motor (actually 4 of them.. Kollmorgen AKM series)
320Volt DC Battery Pack
200 pound dolly with payload (so 50# per motor)
3.91:1 gear reduction on motors (90% e3ff - easy to beat with any old planetary)
6" diameter drivewheels.
27.4.mph top speed (40/2ft/s)
14.65 ft/sec/sec accelerations (typical)

Your first pick AKM42J is a good one. I assumed a cycle of 0-6000rpm (27.4mph) in just over 2.5sec then decel to stop, wait 10 sec and repeat.

Your motors will each use 3.4nm for that 2.5sec accel (& decel unless you give some frictional info - but it will be minimal - I must use my brake to regen EVERY slight incline on the street in my car). This is 7.9 amps up to 320vdc, then 7.9 amps decel (regen) not counting friction. If my assumption is correct for your cycle of 1 move of 108.8 feet in 5 sec then stop for 10 sec then repeat forever, you will require 566 watts per cycle (regening 140.9 watts of that) per motor for a total of 2264 watts. So your battery usage will be 566 watts*15sec*4 or 566 watt-hours.. As your battery is 325vdc, and will likely droop to around 300v due to internal resistance (assuming you use about 6 amp hour cells - better plan on larger than you said above) you will pull about 7 amps avg from them if this cycle is correct as long as you run the cycle. Your peak amps draw from the batt pack when you are at 27.4mph will be 7.9a*4= 32amps. You will need to think about if average amps has any meaning to you: w/o thinking it thru it seems to me you need to consider rms amps not avg maybe. You had asked earlier if sine vs trap waveform effected the current calculations and I forgot to answer it: yes it does - the motors and drives will be rated in one or the other - they are about 1.6x different than each other since one is a sine wave and the other is basically on or off. So you need to know WHICH your motor Kt, Ic, & Ipk is rated in to make proper comparison if you do not just stick with a good sine drive and motor. Your inertia mismatch is about 89:1 which is probably fine for what you are doing. A good 6000rpm motor should be able to run at 0.1rpm smoothly. Hope this helps.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hi mike, I'll keep this real brief as I'm mid-road trip between LA and Seattle!

The link to the mavilor product manual is right below where the link to the two page thing you were looking at is. All the data for the 48 volt windings are there on page 17, and other voltages above.

The mavilor engineers told that these motors are so much faster mainly because of the slotless stators, a manufacturing technique that They explained is difficult and expensive so many of the larger motor vendors don't bother with it. I don't
know if this is true or not, but the numbers are the numbers and they are quite impressive.

Thanks for the calcs, I'm not looking at my spreadsheet right now but those numbers sound just about right. I do have all the numbers for the various wheel rolling friction forces (18 wheels under various loads) , the friction forces from the spring pressure of the drive wheels and the air drag resistance. I will post these later if you want

One item that did not match from you analysis however was the inertia ratio, which I had, and another application engineer at one of my vendors, at 58:1.

I had been wanted to get this inertia ratio down for two reasons, first because the system has a lot of compliance already with polyurethane coated drives heels and synchronous belts. In addition this is a repeatable system and should be able to maintain positional accuracy to within .15mm. That's why I was striving for the ideal of 10:1. ( though the closest I've gotten even with the mavilor is 24:1)

Again thanks for everyone's help on this, you and this forum have been really great

RE: SPFX Camera Dolly - Motor Acceleration Amps

'so much faster,' 'due to slotless stator,' what data on the spec sheet proves the 'so much faster' claim? faster can mean go higher rpm - nothing to do with stator design but mechanical limits of rotor design. faster can mean quicker acceleration so T=Jw/t calc - I don't see this as a case here for you either - consider.... with your much higher inertia load than motor, a fast accel low inertia motor gains YOU and your system nothing. I would be very interested in what their folks say makes it so much faster? perhaps we can look at their claim in a real engineering light?

on inertia mismatch, what do others say? higher? lower?

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
You offered to compare to other motor, which I greatly appreciate and have told where the spec sheet is a couple times as well as uploaded a curve here. You naysay it, but have you even run it through your calculation to see if your criticism is founded?

RE: SPFX Camera Dolly - Motor Acceleration Amps

i offered and did. posted jpg comparison of all relevant specs except inertia in my post 7 Aug 13 14:33. and no need to compare the inertias since they have little meaning to your project since your load inertia is so much higher than the motor anyway.

RE: SPFX Camera Dolly - Motor Acceleration Amps

bottom line is that fp motor picked is way undersized as shown and not capable of even doing the job. its specs are even lower than the equiv I compared it to. Hence my question why they say it is 'faster?' maybe some other spec I am not considering?

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hmm I dont know how you are calculating it. I do know that I have run it through the exact same calcs as the kollmorgens, that got similar results to your earlier post, and the numbers come in better on every level. I was extremely careful entering the motor data correctly. It does need to geared higher of course. I think I had it spinning at 8500rpm at 27mph.

It had more headroom on the peak torque, more headroom on the rms torque, lower inertia ratio by half, and using the fp0207.8 (48v winding) would go up to 18k rpm (or 57 mph) if I could find gears that would survive.

How is this way undersized in your opinion?

RE: SPFX Camera Dolly - Motor Acceleration Amps

There is no calculating to do.

Your curve shows SYSTEM PEAK limit of 2.2nm (red line) this is almost 4x smaller than you require for your accel spec.

Your curve shows CONTINUOUS of only 1.6nm (green line); this is most likely 2x smaller than you require for your cycle.

The motor peak of 12nm (about 160 amps!) is not achievable any longer due to the hi current it would require at 48v winding.

The 20,000rpm is not achievable due to the encoder 8,000rpm mechanical limit.

IF this motor could achieve 20,000rpm & 12nm, that is 33HP! Where would you find a 1 or 2" size drive to achieve this 25,000.00 watts to drive it?

These are the reasons I said this 48v winding motor is way undersized and will not do your application.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hi mikekilroy....

Sorry I was out of commission for a bit...

So, regarding these Mavilor motors, I just don't see these motors being as underpowered as you make them out to be, even considering the current limited torque. To be sure, some are not as strong perhaps as the Kollmorgen (ie. the 48volt windings), but others definitely are, especially when you consider a higher gearing.

For example, take the case of the Mavilor FP0207.23 compared to the Kollmorgen AKM42J, both geared for a straight and level run at 27.4 mph. Attached is a both a pic and a spreadsheet of the results for a KM42J compared to an FP-0207.23 both of which have been run through the exact same calculations. The only difference in the two sets of inputs is the gear ratio, being 3.9:1 for the Kollmorgen and 5.5:1 for the FP0207.23. Geared thusly, the Mavilor just seems to beat the AKM in most every category, including weight, size, inertia ratio, and most of the margins for torque... also, it would appear to be more efficient on the batteries as well, and by a considerable margin.

Perhaps there is still something I am missing, but I am attached the spreadsheet and results for you to see and compare if you want.

P.S. Indeed the encoders cap out at 8000rpm, but the resolvers are good up to 20k rpm: this is confirmed and tested on the drivers we are considering. Resolvers are fine for our application.





RE: SPFX Camera Dolly - Motor Acceleration Amps

Hi Eckener,

Please don't think I disagree with your basic premise that those motors can have very high torque to weight ratios; I don't. I was just referring to the ONE 48v winding you thought could do more than it could. OK on resolver higher speed too (although I still don't see how you could USE it in a gearbox). Anyway, yes, what I saw were some very good motors. If I get a chance and you want another opinion, I will look at your comparison.

RE: SPFX Camera Dolly - Motor Acceleration Amps

Hi Eckener,

I added a column to the spreadsheet to include a more apples to apples comparison; you were comparing two different frame size motors (70mm vs 84mm). So the added column is the same square size motor.

I am not sure what criteria you feel is more important than others though so cannot comment on it being better or worse. Seems all 3 will do the job. The added motor is very similar (almost identical) to the FP model except it has much lower inertia and higher continuous torque. Often the figure of merit used to compare high performance of servo motors is torque to inertia ratio; the larger this ratio the faster the motor can accelerate. Again, in your case with a huge load inertia, this figure of merit is of little importance to you.

So by picking similar diameter motors with almost identical winding constants, the battery draw should almost the same. I would go for more Trms margin to let me run the motor cooler or increase accel of system if needed. I also would not go 8000rpm with mechanical gears. I added two comments to the top portion of your spreadsheet, correcting inertia units & other reminding that the internal regen capacity of the drive is probably of no importance to you either since all regen will simply go right back into the battery pack instead; my experience with similar LiFePO4 cells is they will pump up to about 3.5v @ same 30amp regen, so your bus with say 108 cells will pump up to around 380v max so never turn on the drive regen IGBT.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Thanks, Mike... I very much appreciate your reply. I'm a bit new to this level of engineering, so I welcome your double check.

It does look like all these motors will all do the job. With that said, the criteria I am really trying to juggle and weigh the options on is the gearing. The primary concerns are, as much as possible, making sure it is:
  1. Vibration Free and
  2. Quiet.
On the one hand, the AKM42J has a "relatively low" 3.9:1 ratio, which I can do with a single synchronous belt drive, (Was thinking Gates GT2, 5mm pitch x 25mm wide). This keeps the large gear under 6" in diameter (which conveniently is just slightly smaller than the drive wheel) and simplifies construction. Whether the belt can handle 2500+ fps around a very small sprocket is something I need to look into.

On the other hand, anything above a 3.9:1 (the Mavilor, or the the other AKM you suggested) will either require a considerably larger drive sprocket, which just doesn't seem wise or compact, or a two stage reduction, possibly belt to helical gear.

If I can figure out a good gearing solution, I'd probably go with the Mavilor, as that does have the highest margins.

Regarding Regen... I just haven't quite got too into it yet, and need to do more research. I had those input parameters in there just to help size dynamic braking resistors for kicks. It was quite confusing, because every drive manufacturer seemed to have a completely method for calculating the power rating and resistance value of the resistor. In any case, it seemed that they were going to be quite large and hot, and not something I really wanted on the dolly, so yes, I do want to do the regen. I just don't know quite enough about it at this point to discuss intelligently.







RE: SPFX Camera Dolly - Motor Acceleration Amps

You seem to have your mind made up to use the Mavilor, so why not go larger frame? Next size up gets into the akm42 frame size; that lets you pick one similar in performance to the akd42. One or 2 more sizes larger gets even bigger allowing lower and lower timing belt gearing for quieter. You could even consider going large enough for direct drive for the quietest 'transmission.'

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Good points... and I will definitely look at them, but first was trying to make sure my calculations were mostly correct. Its a bit frustrating however with those Mavilors, because since they don't have any sizing software or publish the torque curves, you have to send emails requesting individual torque curves. Maybe I don't need them, but otherwise I can't really tell what the top speed of the motor under a certain load would be.

I guess my concern about going bigger and slower is just the "resolution" as I call it, at low speeds. Most often, this dolly will work at very low speeds from mere inches per second to 1 foot per second. It is at these speeds that we will need the most amount of "smoothness" (really, it has to be "glass" smooth for camera work... definitely wouldn't want to see any cogging torque or anything like that)

Also at these lower speeds are where we would see the need for very accurate repeatability or "motion-control" style shooting. (this is a visual effects film term, not to be confused with the more general industrial term)

The AKM42J would be around 150 RPM at 1 foot per second... and and inertia ratio of 58:1

Now... maybe my fear is unfounded, and I would really like to find out that it is, because then I could gear much lower, go for slower motors, and even have one gearing that handles all speeds from a crawl up to 60 mph. That would be great.

RE: SPFX Camera Dolly - Motor Acceleration Amps

Echener, I do not want to hog the replies to your posts; I hope others pop in with comments also!

but first was trying to make sure my calculations were mostly correct.

Echener, You wonder if your motor sizing spreadsheet is accurate; I would remind you that you have been given a few different ones and you should compare yours to those. I did go thru one myself with Motioneering(tm) and it came out very close to another one posted. That said, I did not again go back and compare your latest xls data to it. But IIRC you show needing 0.9xnm Tc and the other 2 showed needing over 2.x nm - I will not go back and compare now after the fact, so if I am wrong, so sorry. But you DID ask. IF we show over 2 nm required and you show 0.9nm required, I would suggest your spreadsheet is not as accurate as it should be.

Maybe I don't need them, but otherwise I can't really tell what the top speed of the motor.


Of course you need each individual curve or you cannot logically pick the motor.

I guess my concern about going bigger and slower is just the "resolution"


I have these same size AKM motors on automotive VERY high performance cam and crankshaft racing engine grinders to 0.0001 or better accuracy.... grinding cams at 0.03rpm & 1/2 Tc rating, and yet rapid traversing at 4000rpm. Good servos are DESIGNED for this. You need to ASk your servo provider if they can do this function instead of guessing.

I would really like to find out that it is, because then I could gear much lower, go for slower motors, and even have one gearing that handles all speeds from a crawl up to 60 mph. That would be great.


I suggest you really need to team with an good servo application engineer on this if it really has to work the first time thru. These guys LIVE for helping you in this kind of application in exchange for selling a few of their products. IT IS usually FREE MAN! If you get a couple iterations of buying wrong hardware and spending time trying to make it work, then you can get away without it!

Such things are REGEN need addressing also; you haven't gotten into it enough yet to realize, but you must decide on how many LiFeP4 cells to use; with your potential regen requirement, this is not a no brainer; if you want to continue to say you will have 320v@30a, then you will have too high a voltage for ANY drive to control on regen. In this kind of application you MUST regen into the battery pack; yet you show voltages that are not consistent to do so.

Your Position controller is just as important to the final result; have you picked it? Delta Tau Kollmorgen built into the AKD drive Other?

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Echener, I do not want to hog the replies to your posts; I hope others pop in with comments also!

Hog Away... i will unabashedly gleam information from anyone who offers it. And please, don't feel compelled to reply just for my sake. I appreciate all the help you've already offered.

I suggest you really need to team with an good servo application engineer on this if it really has to work the first time thru.

I can't argue with this point..... (hahaha) BUT.... there will be some time for R&D and so it doesn't absolutely have to work the first time, but there are so many other systems and design elements that will depend on the size and shape of the "powertrain", that it behooves us to figure out as much as possible beforehand. I do wish we could bring on an expert, but alas, this is the case in many areas of this project, and right now, we just have to figure out as much as possible on our own.

Your point to utilize the app engineers at the vendors and manufactures is a good one, and I actually have been, but up to this point none have really just come out and said to try going "big" with the motors to reduce the speeds. Rather they have kinda just gone along with my initial request for "really fast" motors and we've worked to get the inertia ratio down. Now, partly because of your comments, and partly because of the high decibel levels that I see will be produced by synchronous belts at those high speeds, I am really thinking hard about this strategy.

But IIRC you show needing 0.9xnm Tc and the other 2 showed needing over 2.x nm

The first specs I posted with were for going up an incline... the later specs in the motor comparison were for level track.

I have these same size AKM motors on automotive VERY high performance cam and crankshaft racing engine grinders to 0.0001 or better accuracy.... grinding cams at 0.03rpm & 1/2 Tc rating, and yet rapid traversing at 4000rpm.

This is good to hear. I did run the numbers for the Mavilors up to a 1:1 ratio: an FP0711 with a slightly smaller drive wheel pretty much does the trick... AND theoretecially it gets us all the way up to the top speed of 60mph, without having to offer a separate "high-speed geared" version of the rig.

It basically comes down to the choice between small and very fast motors (noisy)... or very big and slow motors (quiet).
If both can perform equally well at the low rpm, then increasingly, I think the choice has got to be the big and slow motors.

if you want to continue to say you will have 320v@30a, then you will have too high a voltage for ANY drive to control on regen....
That spec is for a battery pack that is feeding 4 controllers, with the current based on the RMS torque of each motor.. so that's conservatively 6 to 7.5 amps for each controller.

you haven't gotten into it enough yet to realize, but you must decide on how many LiFeP4 cells to use; with your potential regen requirement, this is not a no brainer; i

I suppose having more batteries for regen is because we theoretically could generate more power in braking than the 400-ish volts that the pack will be? Though I don't fully understand how we could generate more voltage, unless the rig were actually traveling faster than we could accelerate to in the first place. (higher current than we put into it? sure... but higher voltage?)

I do know that we'll need more cells just to provide the voltage at those high current levels. Right now I was looking at the A123 (bankrupt, yes) and K2 cells, . With the A123 we'd need around 116 cells just for the voltage drop due to current.

Your Position controller is just as important to the final result; have you picked it? Delta Tau Kollmorgen built into the AKD drive Other?

Our favorite right now is the Elmo Gold Trombone. We like it because it is extremely small, (we'll have four onboard), is designed specificaly for DC input up to 400vdc or 800vdc, handles pretty high current ( up to 20 amps), has no noisy fans, and the milspec version has a very good heat rating (if it can be afforded). It can also be soldered directly to our custom driver boards. It doesn't handle regen or shunting directly, but neither does it preclude it as some other drives have done.





RE: SPFX Camera Dolly - Motor Acceleration Amps

glad to hear you are continuing to research all this - i bet you'll end up with a good system you are happy with in the end.

It may not be obvious in basic literature, but all decent gearbox mfgrs have db specs for their various boxes so you can compare.

I can see most servo suppliers not suggesting lower ratios; they will generally accept your given criteria and not suggest very far off other ideas as they don't have time for it often, and a lot of engineers are quite hung up on that darn inertia match number and will have blinders on regarding straying far from it.

Since you are considering other ideas, there is also this newer concept called 'direct drive.' It leaves inertia matching as an cave age idea, no longer important at all. Makes for the quietest and probably smoothest velocity also. I apologize for only knowing 1 good source to point you at to see the concept, but I am sure you can google others after you get the gist of it here:
http://www.kollmorgen.com/en-us/products/motors/di...

RE: SPFX Camera Dolly - Motor Acceleration Amps

I forgot to include a suggestion on your 116 cells also; I think this is 8-16 MORE than you want or you will never regen into the batteries to help recharge on decels, and you WILL need those big hot resistors.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
You know... I looked at those direct drive motors very early on. They are very cool. But most max out at about 500rpm (some 800rpm). We'd either need excessively large drive wheels, or have to gear them UP 3:1 or more which I think would defeat the purpose.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Hi Iop995...

Thanks for posting that...
We hadn't seen those, Though we did take a pretty close look at a this mechanical CVT:Link

There were several concerns with it, namely input speed and torque, the fact that we needed a very compact and stout solution, and the notion that we would be adding another variable into a system that needs to precisely repeat. I suppose we could put additional encoders on the outputs, but it was getting to be quite complex, so we are trying to make work with a single gearing.


iop... in your spreadsheet (attached again with the fix), how do the Arms get converted back into Adc from the battery? or do they even need to?

RE: SPFX Camera Dolly - Motor Acceleration Amps

eckener (Electrical)
20 Aug 13 23:29
You know... I looked at those direct drive motors very early on. They are very cool. But most max out at about 500rpm (some 800rpm). We'd either need excessively large drive wheels, or have to gear them UP 3:1 or more which I think would defeat the purpose.


You need to ask an app engineer from their company; they do NOT max out at 500rpm but can go thousands of rpm. I recently showed them a market for 5-6000rpm motors in this arena and they are now available.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Do you mean ask someone from Kollmorgen about the ones in the link you posted?

RE: SPFX Camera Dolly - Motor Acceleration Amps

eckener: current from battery is in Adc, no need for any conversion. Arms phase current is calculated only to find motor Joule loses (power at zero speed, Pmin) and Pmax is true power calculated by torque x speed (max speed at the end of profile); with these 2 values is calculed average motor power, then corrected by motor and converter efficiency and obtain average power supplied by battery and current. Sure, there are a lot of simplification but may correct result by a 10-15% factor to be more confident with battery capacity.
If you know battery discharge curve, may be more exactly by calculating power at each profile point (choose a step as you need, 0.01 - 0.1second) and needed current from battery will entry in look-up table that describe discharge battery curve (as discharging is not liniary with current supplied). Also may consider pulse (peak) current at converter switching frequency instead of average Adc but maybe will not be much result difference.

RE: SPFX Camera Dolly - Motor Acceleration Amps

Do you mean ask someone from Kollmorgen about the ones in the link you posted?

yes.

as you get more into battery systems you should consider how long this camera mover needs to last/work. If only a dozen times, then no need for a BMS system, if years, BMS is absolutely required to control charge/discharge/balancing those 100 cells. bms on my ev works well with my 108 cells as example:

http://lithiumate.elithion.com/

another battery supplier is ones in my ev:

http://www.gbsystem.com/Products_en.asp

both companies have been great working with me, answering my questions, and even updating/correcting their software with my suggestions/help. I have no affiliation with either other than using them and thus can recommend them based on that.

RE: SPFX Camera Dolly - Motor Acceleration Amps

(OP)
Iop995,

I guess my question then regards what current to use for comparing peak voltage of motor to the peak current rating of the controller (which is listed as Arms) ?

Seems like it would be Ibmax(Adc)/sqrt(2) to get peak current of motor in Arms.

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