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Current sensing using proportional valve
3

Current sensing using proportional valve

Current sensing using proportional valve

(OP)
Hi everyone. First post here, and not even sure if I'm in the correct forum.

I am currrently working on a new product, whereas I am controlling a proportional valve using a PWM output. The PWM is controlled by a dsPIC. The actual PWM driver is an ST VND600SP. I am using the current sensing ability of the VND to determine output current. The problem I am having is how do I calculate the actual output current to the coil based on the PWM signal? Basically, without knowing an exact coil resistance, how do I figure out my output current in order to compare it to the sensed current to determine if any compensation is required?

Best regards,

Mark

RE: Current sensing using proportional valve

I think you should consider not trying to read the current.  Just run the valve open loop on a timed basis.

Why?
1) Because measuring the current in that heavily inductive RLC circuit is going to be a monumentally big headache.
2) Aren't you actually trying to control flow of something?  Well the flow is going to change at a different rate than the openness/closedness of the valve so trying to find the actual valve position is not going to help you control the flow much. (Certainly not worth the hassle you are embarking on!)

I would suggest that you profile the valve in your system by setting a bunch of fixed PWMs into the valve and actually measuring the steady-state flows that occur.  Then graph this and finally tabularize it.  Then you can quickly (very quickly with that dsPIC) interpolate from your look-up table what PWM you need to control your fluid.

Take lots of measurements near the closed position.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing using proportional valve

(OP)
Thanks for the response Keith. One thing I didn't mention is that the user will have a device with an analog joystick which he will use in order to vary the speed of the system, or 'openness' of the valve. Also, there is not one valve that will be used, as customers may use different valves. The coils will likely have very similar characteristics, but since they can be from different manufacturers, there will be a difference.

I feel the only way I can control this valve is to read back the current and compensate. At higher currents (over 1 amp), the resistivity of the coil changes fairly quickly, thus the current through the coil drops fairly quickly. The monitoring needs to be fairly precise, since these valves tend to be fairly sensitive to current changes (i.e. large flow changes).

RE: Current sensing using proportional valve

Hi markus.
I still think you are not going to have the control you are hoping for.  Different valves just makes the 'current' method more complicated yet!

Is there any way you could put an orifice and a differential pressure transducer in the line?  This would give very accurate flow and not care about the valve style or drive vulgarities at all.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing using proportional valve

(OP)
Hmm...interesting. Are you talking of reading the pressure back to the micro and compensating that way? i.e. if the differential was a certain size, compensate by increasing the PWM duty cycle. So, where abouts would the 2 sense lines from the transducer be connected? (forgive my ignorance regarding these transducers)

RE: Current sensing using proportional valve

(OP)
I probably should have read your post better. You would put the orifice after the valve, then make one connection before the valve and one connection after the orifice?

Mark

RE: Current sensing using proportional valve

Hi markus506 and itsmoked.
How about a low value series resistor and measure the voltage drop? Feed the output through  a bridge rectifier circuit to a resistor-capacitor circuit with a time constant calculated to smooth the pwm frequency.
If you need isolation wind a small C/T.
respectfully

RE: Current sensing using proportional valve

Hi again markus.

  Yes! That is a very common "cheap" method used for flow measurement.  Any orifice will cause a pressure drop exactly like a resistor in an electrical circuit.  And just like a resistor/orifice, the greater the current/flow the greater the voltage/pressure drop.  Mind you it doesn't have to be a large pressure drop to get you some nice feedback.  Sort of like waross' suggestion except measure the actual thing you want to control not something that only marginally affects what you are controlling.

  Measuring the actual flow can let you report totals and other things that are not available by looking at the coil current.  Things like, "is there any flow at all?  Or is a tank empty questions?"

  You can either use two different flow sensors one on each side of the orifice or you can use a differential sensor (which has two ports one on each side of its diaphragm).  The individual sensors can tell you the actual system pressure whereas the differential sensor cannot.

  In bigger pipes they call the orifice a restrictor plate.  They use this method in a whole lot of places.

  If your system can utilize this technique you will be far happier with your control results. (dare I say Happy as compared to Not Happy)

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

(OP)
Hmm..seems like a neat idea...however, from some of the prices of differential transducers, that option is likely out! I will look into individual flow sensors, but it looks like I may have to balance out system accuracy versus price at this point.

Thank you Keith and waross for your input. Most helpful.

Mark

RE: Current sensing using proportional valve

If you want to measure the current, why not measure the DC current supplied to the whole circuit? I can't imagine that the current consumed by the PIC and the internal VDN600 current can skew this a lot.

Benta.

RE: Current sensing using proportional valve

(OP)
No, it probably wouldn't Benta. The problem I have with measuring current is the ever changing resistance of the coil. For instance, the user immediately uses full output and has 1.5 A flowing through the coil. If this is held on for 1 minute, the current would conceivably reduce to 1.25 A. If the user then left the system alone for 1 minute, then used the system full on, the coil would not be fully cooled and the output current might be 1.35 A. Thus, I am always going to be compensating against new current outputs versus 'true' current output (through a cold coil). Thus, without being able to record the cold coil current output for each PWM output, I don't see how I can reliably compensate the system using current feedback so the user can expect to achieve the same solenoid position with a certain joystick position time and again.

Ugh. This is becoming more problematic the more I think of it. I also don't know if I can use the transducer approcach because of cost/lack of inputs to the system.

Mark

RE: Current sensing using proportional valve

Check out omega.com
they have a wide range of transducers at good prices.
respectfully

RE: Current sensing using proportional valve

(OP)
Thanks for the tip waross.

I'd like to say that I am extremely impressed and grateful for the help I have received here. Being a fairly new engineer (almost 2 years field experience) I am always looking for tips/opinions from more experienced people. Once again, thank you to all the contributed to my first thread! 2thumbsup

RE: Current sensing using proportional valve

Waross I beg to differ with you.  Omega's prices are truly horrendous.  BUT!  They do have a nice selection.  They only re-brand then stack on large mark up.  You can always find the same product with the original maker's markings for less. (Well 'always' may be the wrong word as sometimes you just can't figure out who the maker is.)

markus;  I re-read my last post.  I said, "You can either use two different flow sensors one on each side of the orifice.....".    Flow should have been pressure!   Was thinking about flow so I slipped up there.

Questions:
What is the fluid you are expecting to be controlled?
What is the pressure it will be at?

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

Hi itsmoked;
Horrendous is relative. I am usually comparing Omega prices to industrial instrumentation. I am usually comparing with outfits like Foxboro, Rosemount, Honewell, Partlow and similar companies.
I look at their prices and then look at the Omega price and usually stop there.
Thanks for your comments. The next time I need instrumentation I will consider searching a little more.
And I bet you a cup of coffee that you have the Omega library on your shelf. If not, get it, it's free.
Respectfully

RE: Current sensing using proportional valve

Yeah my back is healing from the last time I hoisted part of it.lol

And if your basis is the likes of Foxboro, Rosemount, Honewell, Partlow...  I guess I see your point!!

Omega isn't a bad place to go if you just need "one".  It does save time hunting around...time is money.. etc...

I'm still ticked at them for a shipping blunder.  I needed something for a trade show in the middle of the UPS strike.  They made it very clear that things shipped overnite were not subject to delays.  So I made it very clear I wanted it shipped  overnite.  I would pay umpteen extra bucks.. etc..  Then they promptly thru it in the ground bin.  Took weeks to get it.  Then the sales guy was utterly unapologetic.curse

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

(OP)
I will be controlling hydraulic fluid. Probably in the range of 2000 - 2500 psi.

I noticed that slip of the keyboard regarding flow and assumed you meant pressure. Thanks for clarifying though.

Mark

RE: Current sensing using proportional valve

Ouch 2000-2500psi... :(
That does run the price up.  ~ $70 times two.  Probably too much money..?

Low pressure sensors for air and water can be really cheap $5 kinda things.

If $70 won't break the bank and the system is pretty static you  could probably make assumptions and then use just one pressure transducer and no restriction.

Here's a suggestion that is wacky but may actually work pretty well depending on your system.

You could put a strain gauge or gauges on or around a hose and measure the hose diameter which will change with pressure.  I keep grasping for something because valve position is just so poor a relation to flow and a novel approach to something, can win you a lot.

Still brainstorming... I watch hydraulic hoses squirm and change position according to what's going on all the time.  What about a tilt sensor on a hose that relates to pressure..

Or back to the valve positionlookaround.  Any way you can observe the valve position? Use an armature follower or something?  Then you could just control to the actual position.


Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

I like the valve positioning suggestion. If there is any way that the operating position of the valve can be accessed you may be able to fit a throttle position sensor to measure valve position.

Still brainstorming... I watch hydraulic hoses squirm and change position according to what's going on all the time.  What about a tilt sensor on a hose that relates to pressure.. It worked for Bourdon, it should work for us.

Another thought.
Impedance versus resistance. At even moderate frequencies the inductive reactance will probably be high in proportion to the resistance. The higher the ratio the less effect the coil resistance will have on the current.
A transformer will serve the double duty of providing AC and increasing the voltage to the higher level that will be needed by the higher impedance.

By using a higher voltage and frequency AC current you can reduce the effect of the change in resistance of the coil with heat.
Whether you use PWM, duty cycle or a combination, a given output should give good repeatability on valve position.
Comments Keith?
respectfully

RE: Current sensing using proportional valve

Hmmmm Not sure waross, how much that would help..  I agree if you are PWMing fast then the inductive component should dominate.. But the R part will remain..

I think looking at that part markus.. Its Current Sense outputs are horrendous.. +/- 10%!!  Varying with time of day and position of moon.

You might have more luck using a  +/- 0.5%:
http://pdfserv.maxim-ic.com/en/ds/MAX4172.pdf

Then either average the current reading in hardware or software.   

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

Hi Keith;
Hope I get this right.
The inductive reactance and the resistance act at 90 degrees to each other. If the impedance is 5 times the cold resistance the impedance will be 5.01 times cold resistance.
If the hot resistance is 125% of the cold resistance then the hot impedance will be 5.154 times the cold resistance of the coil. Our original 25% increase in resistance is now a 2.9% increase in impedance.
This may be acceptable.
respectfully

RE: Current sensing using proportional valve

(OP)
Yes, Keith, that VND device does vary greatly in its 'K' factor. However, there are other aspects of the chip that we like. Since the product is still in development, alot can change, hopefully this current sensing method will be one of them.

Note that most applications will use a PWM of aproximately 250 Hz...fairly low frequency.

And I'll make sure to check out that Maxim chip on Monday morning first thing.

Again guys, thanks for all the suggestions. Keep 'em coming!!

Mark

RE: Current sensing using proportional valve

Hi waross; I see...  I like it.  Works for me!

Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>

RE: Current sensing using proportional valve

Back from vacation, so late to this thread, but puzzled by the direction it has taken. I want to bring up a very basic issue.

Mark: a device like this with PWM outputs provides a time-averaged voltage to the load. For example, with a 36V supply and a 50% duty cycle, it will apply a time-averaged 18V to the load, regardless of the impedance of the load. Now, the current that this voltage produces does depend on the impedance of the load, but the current-sense circuit (I presume some sort of "current mirror" in the chip) will detect the current reasonably accurately (I'm sure some devices better than others).

In control-loop terms, your control algorithm has a voltage output and current feedback, and you will need some sort of feedback law for compensation. Usually, PI (proportional plus integral) is sufficient. This is every-day stuff -- done all the time for both motors and hydraulic valves. If the resistance of the coil increases, as it will with temperature, the output voltage require to get a given current level will increase, but your feedback law should handle this automatically.

Because the resulting current waveform is a sawtooth, you do have to be careful about how and when you sample the current. Some here have suggested oversampling and averaging. You may also just be able to sample at the midpoint of your on-time and/or the midpoint of your off-time, which does a pretty good job of catching the average current level.

A valve that has built-in position feedback of the spool is a servo valve. These tend to be much more expensive and finicky (easily clogged) than proportional valves, although they tend to do better, particularly in the zero-crossing for bi-directional applications.

Curt Wilson
Delta Tau Data Systems

RE: Current sensing using proportional valve

(OP)
Well, Curt, better late than never!

I will admit (sheepishly) that I had not even considered using a PI controller in my firmware. I guess this is what I get for falling asleep during my controls classes. I will look in to this right away.

(start of ramble)
As for the feedback, the current sense circuit uses a 5.9K resistor and the voltage is measured across that and fed into an ADC. The resulting waveform is pretty flat, almost as flat as the PWM pulse. I am interrupting on the falling edge of the sense voltage so my sample voltage is approximately at the highest point of the waveform. But since I am using formulae to calculate the output current based on a number of factors, I am sensing current. (End of ramble).

Thanks again for joining in on the discussion, your post was most valuable. I'll let you all know how I progress.

Mark

RE: Current sensing using proportional valve

(OP)
Hmm...well, I tried to do some modelling of the coil I have here. However, the step response of the coil shows that it is behaving almost like a pure inductor (minimal resistance), thus a first order system. Is this something that I am able to compensate using a PI or PID contoller? I don't believe that this type of system can be over/under damped, etc. Excuse my ignorance, but this is the first time I have run into step responses since school, and, from memory, we only ever really dealt with 2nd order systems. Can someone shed some light on this for me?

Mark

RE: Current sensing using proportional valve

If you are PWMing at a high enough frequency for the system to be responsive, the inductance will dominate the immediate response. So in the time frame of a PWM cycle, you can usually ignore the resistance and just get the slope of the current response from the inductance. Over a longer time frame -- multiple PWM cycles, where you can use the average value for each cycle, you will need to take into account the resistance of the coil.

Curt Wilson
Delta Tau Data Systems

RE: Current sensing using proportional valve

Markus,

You might like to explore the servo valves produced by Moog or Bosch-Rexroth. Moog are hideously expensive, but are among the best in the business. Their website has a fair bit of technical information on how their valves and controllers work. It might trigger an idea.

http://www.moog.com/Industrial/Products/

----------------------------------
  I don't suffer from insanity. I enjoy it...

RE: Current sensing using proportional valve

Also sorry to be late in this thread.

In my world, proportional valves always have a feed-back signal called "actual position". It is usually a +/- 10 (or sometimes 5) V DC signal that accurately tells where the spindle is. 0 V meand centered and +10 V means 100 % open in the plus direction.

Have you looked for such a signal? It is really difficult to do any serious work without having position feedback from the valve. The hydraulics guys seem to rely heavily on actual valve position for many control tasks.

Gunnar Englund
www.gke.org

RE: Current sensing using proportional valve

(OP)
Well Gunnar, it seems that the application we have, the guys are using just a cheap, PWM controlled flow control valve. This valve is supplying flow to the directional valve, which is on/off.

I agree that it would be much more convenient if the valve had a feedback signal for position, but since we are planning to interface to existing valves, it does not look like it is going to be an option.

Mark

RE: Current sensing using proportional valve

skogs; I gather this is an application where a human is part of  the immediate control loop.  So accuracy is not going to matter 'all' that much.  Mark maybe needs 3 or 3-1/2 bits of accuracy.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing using proportional valve

+/- 12.5 percent or just 12.5 percent? Can't figure out what 3.5 bits would be. 8.8 percent?

Gunnar Englund
www.gke.org

RE: Current sensing using proportional valve

Well Mr. Skogs,

3 bits ->  8
4 bits -> 16
delta = 16-8 = 8
1/2 delta = 4

3-1/2bits = 3 bits + 1/2 delta = 12

3bits     ->  100%/8 = 12.5%

3-1/2bits ->  100%/12 = 8.3%

That's my theory and I'm sticking with it.infinity

Although actually now that I've revisited it, Mark really may only need 2-1/2bits to 3.  Muwhahahaha.


Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing using proportional valve

I think repeatability is important. With good repeatability, the absolute precision is not particularly important. Enough linearity to avoid excessively low P or high P at the low and high speeds will be nice. (P; proportional band, or ratio of output change to input change.)

Quote (markus506):

Basically, without knowing an exact coil resistance, how do I figure out my output current in order to compare it to the sensed current to determine if any compensation is required?
This may be the answer all along. If you can control on current, the amp turns will remain the same. When the coil heats up the output will rise to maintain the set current and thus automatically compensate for the change in resistance of the coil. The same current means the same amp turns and the same valve position.
I agree with skogsgurra on the position feedback. I do understand that it may not be possible to attach a feedback potentiometer.
respectfully

RE: Current sensing using proportional valve

So, we have a discrepancy there. I reasoned like this: 2^3 = 8, 2^4 = 16. 2^3.5 = 11.3 which gives 100/11.3 = 8.8 percent.

Love this  winky smile

Gunnar Englund
www.gke.org

RE: Current sensing using proportional valve

I like yours better...

We should go ask some math forum... Maybe we can get a big fight going..
flamecrymachinegunbugeyedcursehairpullbanghead


Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Current sensing using proportional valve

That's BIG, smoked. No, no fight. It's getting late over here.

Gunnar Englund
www.gke.org

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