Save me from my Analog Output!!

Okay guys, here's one we've been wrestling with all day.

I've got a pneumatic servo valve hooked up to a 0-10V analog output on a 1769-OF2 slapped on the side of a Micrologix 1500. As soon as the PLC goes into run mode, the valve starts to jiggle like mad. Not enough that it's actually actuating one way or the other, but you can hear it and feel it if you put your hand right on the solenoid.

We tried: different valve current cable, different valve different cable, bus power and external power on output card with current power supply, bus power and external power with different power supply, different output card in both bus and external configurations, different processor, different base unit, it goes on and on.

We have narrowed the problem down to the control voltage, because if I hook the control voltage up to a variable power supply, the valve reacts as normal and does not jiggle like a ******* (this is using the existing cable, so that eliminates noise generated on the cable as a cause).

I'm about to jump out the window here, I could really use an insight into this issue. Thanks y'all!

Here is a link to the manual if you dont have it

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1769-in049_-en-p.pdf

Are you using the internal power supply from the bus or an external power supply.
Is your output card set for current or voltage as its capable of either I think.
What voltage are you getting on the output terminals and is it stable.
What value have you programmed to drive the analog output.

The one thing I dont follow is, you say when you put your hand on the solenoid you feel it jiggle.
Explain this, as I cant think of an analog controlled valve thats driven by solenoids.


Oh and move slowly to the ground floor and dont look out the window

Thanks for the reply.

I've tried it on both bus power and external power. I've even switched power supplies, to no effect. The card is set to voltage, and the valve does still react properly to increasing and decreasing voltage. I'm getting 5V stable on the output (well, I put a better multimeter on it, and I was getting 4.873-4.875V). It's programmed to output 5V at start up.

The valve we're using is a FESTO MPYE-5-1/4-010-B (151694) proportional directional control valve. I say it has a solenoid since it has a large coil and armature to move the spool of the valve, however the valve is a 'smart' valve with some control guts in there.

An Oscilloscope would find your problem fast.

If you don't have that available, then it is just a guess about the cause.

I would guess that a "jiggle" would be about the same frequency as the scan cycle time of the PLC.

Are you sure your not cycling the output from the signal you want, back to zero, and then back some how?

I would write a super simple program to control just that valve and download that into the PLC.

That would eliminate the program as the problem.

I do have access to an oscilloscope, however it's kinda built into the engineers desk and I'd rather not tear it out without his permission. If need be on monday I can get him to yank it for me.

The reason I've been reluctant to blame the program for the jiggle is because there's an identical machine right next to it (finished last week), and it has the exact same program with absolutely no jiggle. The only difference is it has a different screen (PV600 vs PV550).

Is it possible that you are getting noise on the ground wire from an AC source?

Maybe isolate the power supply from the main ground bar in the machine.

Your 5v could be steady, but if your ground potential is fluctuating, then you are going to bounce.

The card has 2 outputs and they are not isolated from each other, they share the common terminal, is the other output being used.
Are the output signals, wires isolated from earth.

We completely unplugged the machine from AC and used a bench power supply to power just the PLC (including removing the chassis ground). Still jiggling.

Do you have another analog output card to try? New Stuff does come in bad sometimes.

I got a bad intermittant pflex 700 communications card last week and it is not the first time. It happens.

Would it be possible to swap output cards with the "working ok" machine?

Sounds like you have it narrowed down to the card, the PLC, or the program.

We've tried swapping the output card with another brand new one from stock, same results.

We've also swapped the base unit and the processor, as well as all the other I/O cards on the rail.

It's looking like the next course of action is to do what The Starr said write a program that just operates the analog output. I just need to yank all the other I/O.

Gil47 said:

The card has 2 outputs and they are not isolated from each other, they share the common terminal, is the other output being used.
Are the output signals, wires isolated from earth.

Click to expand...

The other output is in fact being used, but we removed that right away.

My conclusion is that you may have an overloaded power supply, have you checked to see if this is the case, somewhere on the ML1500 is a Tab, I think in I/O properties or something like that.

Is the Deadband Adjustable at the Valve Actuator?

I've seen this with some of our actuators when the dead-band is set to zero (in theory, causing the valve to react to very small PLC output changes). In practice, the actuator is in CONSTANT non-productive motion. A dead-band of even just .1 solves the problem.

Bill

Not familiar with FESTO valves. However, if you are in a closed loop/feedback system, there can be a jitter or nervous drive due to the valve spool not being nulled. 0-10vdc at null from the analog output should be program integer 2048. Set the electronics for this and mechanically adjust the spool to null, or, for a temp quicky, move your mechanics to what you believe is null and adjust your analog drive interface output to 0vdc on the feedback. It would be interesting if your swapped valves from the other machine.