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Controlling four Linak actuators.......

Controlling four Linak actuators.......

Controlling four Linak actuators.......

(OP)
The units are Linak medical bed actuators, run on 12 or 28V, usually run by a hand-controller and usually without installed potentiometers. What I would like to do is operate four of these units with some sort of controller/power board and a standard PC control-stick so that they mimic the control-sticks' inputs as accurately as they can operate under normal operating conditions to basically computer-mix the actuators(stick back, all actuators extend, stick forward, all retract, any position in between, all actuators assume their proper position to reflect the stick's position.) If if this can be done, please let me know.

Thanks so much!!!!

RE: Controlling four Linak actuators.......

It is not easy to do what you want to do with standard linear actuators without position feedback.

One way of doing it would be to use "sensor bearings" in the units and use their output signals to keep track of linear motion. The math from actuator position to target position can be complicated, but if the four Linaks are not dependent of each other (each Linak does its own thing) then the math is simple.

You could also, depending on the resolution you need, put little sensors in the actuators (I think that Linak has versions with sensors built-in) and count output from them in the same way as said above.

A few questions that will have an impact on what solution you should chose: Is this a low or high volume application? Does it need to be non-volatile (be able to resume operation without sync operation after a power outage)?. Is it a cost sensitive application? Is the environment tough or hostile - or is it a normal in-door climat?



Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

(OP)
I can order the Linaks with pots built in, but I'm not sure that is what is needed for accurate feedback. The application is relatively high-volume with numerous continuous position changes, but very little structural load. Not sure what you mean by non-volatile load; my only concern is that each time the system is fired up on it's 12 or 28V power, the stick's position is emulated in the actuators' positions(ie: if the stick is spring-centered, all the actuators reflect those postions.) Relatively cost-sensitive, normal envir, not hazardous to electronic components. THANKS!!!!

RE: Controlling four Linak actuators.......

kitarusapien, if you don't have a sensor acting as a feedback device then you have an open loop system and you will get cumulative error with this type of actuator (motor driven, rack and pinion I believe).

One type of linear actuator which isn't particularly accurate though doesn't suffer from cumulative error is a voice coil type permanent magnet actuator that is spring loaded.  The voice coil actuator is basically the same type as used in a loudspeaker (hence the name).  Because it is spring loaded, the position is a function of the current hence approximate positioning can be achieved BUT the actuator must be energized continuously.  For a given holding force requirement it is relatively large and expensive (in terms of both first cost and energy use), though it can be fast.  One application is active valve control in a piston engine.

Better to get a position sensor I would say.

RE: Controlling four Linak actuators.......

(OP)


I'll go back to the Linak site, and see what they recommend for accurate feedback control...mayhaps they have units with such controls already installed. The unit I have for testing does not. I'll report back with the info they give me to bounce it off of you folks.


RE: Controlling four Linak actuators.......

(OP)


Here's their description of an optical encoder that can be rigged for the actuators:

"The optical encoder is used for positioning and servo control of an actuator."



RE: Controlling four Linak actuators.......

(OP)


As opposed to their description of the potentiometer they offer:

"The potentiometer is used where the absolute position of the actuator is needed."

So, which do you folks think would work best? They also offer a controller board which can work two actuators at once......is it possible to link these actuators by two of these controller boards? If it's possible, I can get the specifics, and post them here.

RE: Controlling four Linak actuators.......

A potentiometer should be more than adequate for most applications. (Don't know what yours is.)

As you mentioned the "bed" control pendants, they usually run one axis at a time so the power supply can be capable of only running one axis at a time. Being able to run multi axi will require a hefty supply. (less cheap)

RE: Controlling four Linak actuators.......

(OP)


Can you elaborate on "hefty supply"? Are we talking power, physical size, both?



RE: Controlling four Linak actuators.......

hmmm.  The supply that it takes to run one unit will certainly be smaller than the one to run 4 at once.  If you run an unregulated supply, typical for this type application, it would probably be two or three times larger and maybe 4 times heavier.  If you run switchers they would probably be a third the size?  But switchers cost maybe twice as much as unregulated supplies.  Can't really recommend knowing so little.

RE: Controlling four Linak actuators.......

Regarding the choice of potentiometer or optical encoder, I would think that the former is cheaper, and is probably fine.

Optical encoders may be used where there is high speed motion.  They generally have very high accuracy as well.

RE: Controlling four Linak actuators.......

Yeah UKpete I agree.  Anything run by a joystick can't possibly need extreme accuracy.

RE: Controlling four Linak actuators.......

As I said, a Sensor Bearing seems to be the best choice if you do not need non-volatile storage of position. I gather that few of you heard about such a device. It is an encoder built into a standard DGBB (Deep Grove Ball Bearing). It has a much lower installed cost than a normal encoder and it has a resolution that makes it ideal for low precision applications. Google "sensor bearing" for more information. As it happens, the second item found has some personal interest.

Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

Sorry Gunnar, I didn't recognize the term "sensor bearing".  I have heard of them but I haven't seen them used but I guess they are straight forward enough and reliable (especially if SKF is supplying them).  

I have used Linak actuators with integral potentiometers in a valve actuation application.  The coupling was via a Bowden cable as they couldn't be located next to the valve; no problems to report.  It's probably better to buy the ready assembled item rather than strip down the actuator and fit sensor bearings.

RE: Controlling four Linak actuators.......

(OP)


OK...I'll get the Linak units with the installed pots, but now, this power supply issue.........LOL!




RE: Controlling four Linak actuators.......

(OP)


I'm thinking one power supply for each actuator mounted closely to each unit would be best, but.......what is the overall controller to which the stick inputs to? Do I run a standard PC board with a subroutine that simply maps the stick's patterning? Should I switch over to a computer forum to figure out the controller-to-power supplies-to-actuator rigging?! LOL! Thanks all, especially for being patient with an obvious newb!


RE: Controlling four Linak actuators.......

If your're talking cheap, one supply per actuator ISN'T!

If you are talking easy, clean, and painless, one PS/actuator IS!

Make your choice.

You should look into a regular PC joystick to "output" card.  This will allow a cheap replacable joystick and different input devices like game controllers etc.

RE: Controlling four Linak actuators.......

(OP)


I'll take the more expensive, but easier route of one power supply per. The electronics hooking up to the actual power supply still boggles me, though. Ah well, a searching and a studyin' I go!

RE: Controlling four Linak actuators.......

I think it is about time to discuss power consumption. How many amperes do your actuators consume? Or, if that is easier; what force do they have to produce and at what speed?

It is not automatically so that a single power supply has disadvantages over the one PS/actuator approach. It all depends and it depends on a lot of factors.

Regarding the positioning accuracy, it is sometimes possible to have a +/- 2 or 3 percent accuracy (=repeatability when moving from fully retracted position to setpoint) without position sensor. The trick is to measure motor EMF (voltage induced in armature winding). That can be done rather cheaply by measuring load current and compensate for it (well, there are a few more things to it than that). It will always be cheaper to do that sort of control using a central (common to all actuators) control unit than having the intelligence spread out over four different units.

I would recommend One PSU and One Control Unit. Definitely more economic and also much easier if you need to have the different actuators communicate or interlock.

Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

(OP)


Here's the basic specs on the Linak actuators:

Max. thrust: 750 N
Max. Speed: 50 mm/s
12V or 28VDC

I only need these units to have a maximum 'throw' of two inches fully retracted to fully extended.

Does this info help, or do I need to find out more? Thanks!

RE: Controlling four Linak actuators.......

TWO INCHES?!

What are you using this for?

RE: Controlling four Linak actuators.......

(OP)


controlling panel movement.


RE: Controlling four Linak actuators.......

Okay.

37.5 Watts

Times 3 = 112.5 Watts

Pretty small potatoes. Use three 100W switchers. Or three 50W unregulated supplies.

RE: Controlling four Linak actuators.......

One also needs to take the rather low efficiency of the worm gears into account. That may mean that you need three or four times the mechanical power to supply the motor.

On the other hand, it is not plausible that the actuators will be delivering full power all of the time. It is not even probable that they have to deliver full power at any time.

What forces do you need to move the panels? The 750 N is the spec of the actuator, but what force is excerted on the panel? And what does the duty cycle look like? Do you move tem once in a while or do you move them back and forth continuously?

All these considerations go into the design of a system like this.

Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

Yes skogs, but it will all be parcelled out a little bit at a time.

This will be a complete design by forum taking 3.5 years and 3,000 posts.

It's a     s  l   o     w     work in progress.

Scotty beam me out.

RE: Controlling four Linak actuators.......

(OP)


FLOL!!! Sorry for the sea of questions from me; I really like your "Scotty" remark! Skogs, the system will have the actuators moving back and forth constantly, but after an initial engagement of full throw and relatively static positioning(approx 450 N), the units will move back and forth only incrementally, and have very little load on them(approx 75 N)for the duration of their duty cycle. The maximum time of duty will be less than 3 hours per unit. THANKS AGAIN ALL!!!


RE: Controlling four Linak actuators.......

Is that a business idea Smoked? Doing design in teams on the Eng-Tips. Sounds like some fun - but I think that the poor customer (Kitarusapien in this case) may want a somewhat faster design cycle.

Your 3.5 years and 3000 posts estimate seems to be quite well underpinned. It is not very unlike some design cycles I have lived through even without computers and internet.Postings were then paper letters, meetings and phone calls.

Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

Hi skogs.

Naw! I was just expressing my frustration at having important required details dribbled out a little at a time in some misplaced desire for secrecy.  As if everything hasn't been done or tried.  Everyone thinks they have the next Ipod brewing. (me included)

The OP can't have it both ways.  If he wants tight secrecy he shouldn't be here. OR.... He should at least have his specific requirements very clearly defined.  Otherwise if he doesn't have the specific requirements or know how to derive them we need a lot more information on what he wants to do so we can cogitate the specific requirements needed in a timely reasonable manner.  I mean, is he trying to keep a ball bearing balanced on the platform, or point something, or heat water by sloshing it? Who knows?!? This would completely change possible control strategies.

  At this point I feel we are providing free engineering services for a secret product for the OP to develop and sell.

  I would rather provide tips.

RE: Controlling four Linak actuators.......

Big grin!

Gunnar Englund
www.gke.org

RE: Controlling four Linak actuators.......

You have probably already looked at this....

Hospital bed actuator motors have an extremely low duty cycle, not much better than a starter motor on a car.  I see that the motors are not going to be fully loaded but if they are going to be starting and stopping at full voltage/speed you may have a problem.

It has been a long time since I looked at those actuators but I think the motor would burn in less than a minute of operation.  These motors have to start hard to overcome the static friction of the worm.  

Barry1961

RE: Controlling four Linak actuators.......

(OP)


ITSMOKED, I can appreciate your frustration at my dribbling out info, but it's not because I have a great idea, it's because what I'm trying to do is not S.O.P., so when I tell people what it's really for, I get a very MUTED response, if any response at all! So, PLEASE, forgive me for dribbling, but truth be told, you folks have gotten me farther with your info not knowing what it's for, than 100 posts on other boards, telling folks exactly what I'm doing!!!! BTW, it's not illegal, immoral, illegitimate, somebody else's idea, or a better mouse trap....FLOL!!!! Now, about that duty cycle question, I don't see that as an issue due to the really low loadings after initial actuation at half-travels back and forth for possibly thirty seconds at the most, BUT I'm not the expert here, so please....elaborate. THANKS!


RE: Controlling four Linak actuators.......

You would probably have to test a unit to see if the motor will burn.  These actuators are made for a very definite purpose so you may not get good data from Linak.  When the engineers designed these they did it to the absolute minimum requirements.  That is part of why they are so cheap.

Acme screws often have a very high static coefficient of friction.  It is often very hard to move a short distance with a motor/controller that can not produce full torque at a very low rpm.  Many DC motor/controllers just reduce the voltage to the stator for low speed with also gives the motor very low torque.  If you give the motor enough voltage to “break” the acme screw free it will often over run the target.  

So depending on your setup you may end up with an imposed mechanical resolution of a few revolutions of the motor.  You can try giving the motor short full voltage pulses but that opens up a new set of problems.  It does not sound like you have the budget for even a pseudo servo control.

Barry1961

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