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
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
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
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
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
RE: Current sensing using proportional valve
Mark
RE: Current sensing using proportional valve
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
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
Thank you Keith and waross for your input. Most helpful.
Mark
RE: Current sensing using proportional valve
Benta.
RE: Current sensing using proportional valve
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
they have a wide range of transducers at good prices.
respectfully
RE: Current sensing using proportional valve
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!
RE: Current sensing using proportional valve
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
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
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.
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Current sensing using proportional valve
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
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 position
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Current sensing using proportional valve
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
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
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
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
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Current sensing using proportional valve
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
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
Mark
RE: Current sensing using proportional valve
Curt Wilson
Delta Tau Data Systems
RE: Current sensing using proportional valve
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
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
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
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current sensing using proportional valve
Gunnar Englund
www.gke.org
RE: Current sensing using proportional valve
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.
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
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
Love this
Gunnar Englund
www.gke.org
RE: Current sensing using proportional valve
We should go ask some math forum... Maybe we can get a big fight going..
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Current sensing using proportional valve
Gunnar Englund
www.gke.org