Solenoid Driven By The PWM Output Of Solenoid Driver Card

A1) no response.

A2) Use the line pressure to the pump as the indication.

A3) There is a programming problem, which cannot be seen from here.

Sir,

actually the card is completely shielded and there is no interface provided in itto connect it to PC. Its just a black box for us. It is Fluid Power Systems make Model MC3202-0111 card. Also, this model is obsolete now and hence I cannot go for direct replacement of the card.

It seems to be a microcontroller type card and I guess programming error cannot self generate in middle of the operation. It was working fine earlier.

Actually I was more interested in the answer to the first question so that I can do some modification in it.

Thanx

A3 rev1) Then something is faulty. Look for a bad connection at the 0V rail of the potentiometer(s) in the first instance.

PWS covered Q2 and Q3, try this link for Q1.

1 You can control same solenoid with variable voltage from 0 to 24 Volts, by using a voltage regulator like LM 317. But then there is going to be certain amount of voltage drop, Means loss of power (drop x current). on other hand With PWM you are controlling the amount of energy fed to solenoid coil, this is done by controlling on and off time of voltage applied, in a way its similar to those energy regulators used to control the heater, or used as dimmer. Beside all this, its much easy to to control PWM digitally in comparison of voltage.

2 You can measure it with an oscilloscope, I doubt any multimeter will work.

3 Cant say anything without seeing circuit layout or more details.

sir,

I have one more doubt.

Since the output is PWM current, therefore the voltage input to the solenoid must also be a PWM wave. Then is it ok to vary (somehow) any of these because I am not confirmed about the current values and voltage values at different positions of the potentiometer.

All the potentiometer is doing is converting an analog signal (the position of the wiper in the potentiometer) to a PWM signal (the mark-space ratio of the output). Look for a faulty connection at the potentiometer in the first instance.

sir,

I've checked the potentiometer. It's working normally.

As we know PWM controls the energy dissipated in the coil and prevents the burn out of the coil. Suppose I feed direct 4-20ma current to the solenoid or a 24VDC 4-20mA power cable (the one used for pressure transmitters / i/p converters etc) and vary the current, then will it be a problem, will it be dangerous for the coil?

No, though it might not work. In the original posting, PWM of 0-500mA was stated. 20mA is 4% of the current that the valve needs, in that case. Why should it open (rhetorical question)? 4mA is 0.8% of the current that the valve needs. Why should it close (rhetorical question)?

Pulse width modulation is actually an ideal method of controlling an adjustable solenoid valve. First, I want to go back to some basics.

A solenoid is by definition a cylindrical inductor. A solenoid operated valve typically has a spring that pulls a ferrous slug out of the solenoid. When the full current rating gets applied to the solenoid, the magnetic field generated by the solenoid imparts a force greater than spring force and the slug fully retracts. The force generated by the spring is proportional to the displacement from rest position and is opposite to the change in displacement. The force generated on a slug by a solenoid is proportional to the current in the solenoid. The number of turns and the geometry of the solenoid along with the permeability of the slug will determine the scaling factor of the current to force relationship.

Electrically a solenoid is an inductor. In an inductor the current lags voltage. So by changing the pulse width of full voltage applied to a solenoid, one is modulating the current in the solenoid. This then allows for fine control of the force generated by the solenoid. The opposing force to the solenoid's force is the spring force. The spring force is proportional to net movement. So fine movements of the valve that then permits fluid to flow is now possible.

Obviously this is a simplification of the whole concept. I did not mention the magnetic hysteresis that happens with the slug, nor the force from the hydraulic on the valve, nor how much of the slug residing inside the solenoid changes the value of the inductance. This is the principle though of using pulse width modulation on a solenoid.

Sir,

will it be problem if i use the normal signal cable and vary current 4-20mA from DCS.

currently the current (as shown in DC milli ammeter connected) is 0-500mA

If you read RedFred (above), you'll see how a PWM signal to a solenoid works to provide proportional control.

How do you propose to get sufficient energy from 4-20mA when the current PWM signal is 500mA?

If it takes 500mA at some duty cycle to produce the solenoid movement to a certain position against spring loading, how much solenoid movement do you think will occur with a mere 20mA DC?

Yes, it certainly will be a problem. The whole range of 4-20mA is not even 5% of the current range to control the solenoid. 4-20 mA is likely a sensor output that is not designed to directly control a solenoid. I have no idea at all what Dick Clark Studios has to do with this.

Here's a 1/4" to 1/2" (25mm) proportional PWM solenoid valve with a 4-20mA control signal:
http://www.ascovalve.com/Common/PDFFiles/Product/PosiflowR3.pdf

excerpt:
Electrical Characteristics
Nominal supply voltage:
24 VDC ± 10%,maximum ripple 10%

Input control signal (selectable):
0-10 VDC or 0-20 mA or 4-20 mA

Maximum full-load current:
1100 mA (factory set at 500 mA)

You appear to have a proportional solenoid valve. This does not vary pressure, it varies flow. The pressure is set on the pressure regulating valve. If you equate pressure with motor torque and flow with motor speed you will understand the system better.

Proportional control valves are very prone to sticking if there are contaminants in the oil. You say that the system was working previously. Did it suddenly fail or did it deteriorate over a period of time? Has anything else altered. e.g. the oil level being topped up, with the wrong oil grade or possibly introducing contaminants? When were the filters last changed?

There are two potentiometer inputs. Do they each have a separate input to the card? Have you tried

a) varying the speed from each potentiometer?

b) disconnecting each pot in turn and then varying the speed with the remaining potentiometer?

Answering these questions will help to identify which part of the system has failed. Do not automatically assume that it is the PWM card.

If it turns out to be the card, there are many manufacturers of replacement cards. Tell a supplier the voltage (24VDC), amperage (0-500mA) and what the card is being used for (Driving a proportional solenoid valve), and the duty cycle (maybe 24hrs/day,7days/week, 50weeks/year). Most cards will only have a single input and finding one with two inputs will be more difficult. It may be easier to find a separate card where you can input two pots and output one variable voltage, then feed the output of the first card to the input of a new PWM card.

Sir,

Yes its a proportional solenoid valve.

There are two potentiomenter giving input to the same card. Actually 1 input is to control the speed locally and other from DCS. both the POTs are working fine.

And yes it suddenly deteriorated and as was reported nothing in the system was changed.

Yesterday I checked that the DC milliammeter reading is 370mA constant and is not varying when we are varying the potentiometers.

Also I put a pressure gauge in output line of the solenoid, it was constant and not varying.

I will upload the pics of the system so I can have more help from u people.

Replacement driver to to match your specification

Axiomatic Remote Mount Solenoid Driver 24V, 0-600mA (internally adjustable to 500mA). Available as card or DIN rail mounted. It only has a single input but if you connect the ends of both pots to the card 'reference' and 'ground' terminals and the wipers of both pots to the 'voltage in' terminal via a local-remote switch or relay, this will meet your requirements. See www.axiomatic.com

I have previously used Axiomatic equipment and found it to be reliable. I have not previously used this driver so cannot comment on it's performance. No recommendation is given or implied

These are the drawings of the card and the solenoid...

http://imageshack.us/photo/my-images/41/solenoidbeingdriven.jpg/

http://imageshack.us/photo/my-images/52/solenoiddrivercarddetai.jpg/

Honestly, I'm a bit lost! The terminology used in this string to me is misleading. With the servo valve, as I presume that is what you mean by proprtional solenoid, the position feedback to the servo amplifier card or the "black box" you referred to, comes from one of two places; the shuttle movement itself or an independant position feedback like an LVDT (linear voltage differential transformer). At the risk of sounding stupid. Have you checked the physical operation of the servo valve?? If the shuttle is not moving there will be no feedback and hence no variation in the output, regardless of what you do with the pots. I actually have encountered this problem before on hydraulic recycle valves. Often the card is not at fault.

A proportional solenoid valve is not a conventional servo valve in that there is no feedback loop. The normal set up is a cone shaped spool in a cone shaped housing. As the spool slides back, the distance between the respective circumferences of the spool and housing progressively increases, allowing an oil flow rate proportional to the movement of the solenoid. By varying the mark-space ratio of the PWM card, the force acting against a fixed spring holding the spool closed can be varied, and the spool moved to a desired position. A high frequency "dither" is superimposed onto the PWM output signal to vibrate the spool slightly and prevent it from sticking. The valves work well if you have clean oil, but contamination causes them to jam in one position and then release suddenly, so their operation becomes unpredictable. There are a few variations created to circumvent the original patents, which I think were filled by Moog (not certain of that), but this is the most common arrangement.

Have you managed to check the PWM with an oscilloscope yet so you know whether you have a non responsive "card" or a shagged solenoid?

If you don't have an oscilloscope then find a multimeter that has a duty cycle function. This may give an indication of whether the "card" is communicating with the solenoid. The % duty cycle will change if the card is actually responding to the pot movements.

The PWM modulation frequency for your solenoid is 33Hz according to the sheet you referenced. That may even have an audio signature (VF intermod products) that changes with the PWM signal. Using a small high impedance speaker (from an analogue fixed line phone handset is perfect) , try to hear if the "card" output changes the audio signature or just smokes the speaker.

There's two more ways to maybe check the PWM signal. Scope is still the best.

Hello prvn619, how is your solenoid adventure progressing?

Did you, somehow, manage to stimulate the solenoid into motion ?