PWM to Proportional Valve

[zmanvortex]

Hi all, I am sending a PWM signal from an AutomationDirect Counter Module to a hydraulic Proportional Valve. The valve coil pulls 1.2 amps at 24 Volts. My plc module can run 1 amp max. I put a 1 amp fuse in series with the signal to protect the module.
It all worked fine when I first fired it up. I never had to go above 50% on the duty cycle and set the run frequency to 200hz, which the valve manufacturer recommended.
After running the process a few times it seemed like the card was current limiting, apparently after the valve coil warmed up enough to pull more amperage. If I paused the process for a couple of minutes it would work fine for a while until the coil warmed up again.
Our mechanical engineer ordered the valve and it is too big for the flow that we need, but I am supposed to make this valve work.
I am thinking of getting a low cost PWM amplifier. Could anyone recommend one or a different solution without changing the plc module or valve.

 

[Greg Gauper]

Your first step should be to try and contact the valve manufacturer as they will more than likely have a PWM card designed to work with their valve.

Otherwise, just about any big name hydraulic companies offer stand alone amplifiers.

The bad news is that you will probably need an analog output module to provide a command source for one of these valves.

I have never heard of someone trying to drive a valve directly with a PLC output before. Most of the PLC's that offer sometype of PWM output are not designed for that but rather are intended to work with some form of high current driver i.e. a solid state relay. For example, we used a PWM output to drive a solid state 480V, 10A relay for heating elements for a temperature control loop. So it is more a matter of picking the right solid state relay for the application.

I am a little bit skeptical that the PWM output of your PLC has enough resolution to provide adequate control of your process, but I am not completely familiar with that brand of PLC, nor your hydraulic application.

As an experiment, you might try and use a solid state relay from Opto-22, Gordos, Grayhill, etc. These companies all offer 'industry standard' (interchangeable) solid state relays that can drive an inductive load and are rated for 3 amps. Don't forget to install a supression diode across the your inductive load (coil)!

 

[rsdoran]

I was looking for something else when I saw this, let me know if it could work.
http://www.fluidshop.com/pr/hydraforce_valve_cont.shtml

 

[zmanvortex]

Thanks

Thanks rsdoran, I think I will switch to an analog module in my plc and use the amplifier to supply the PWM signal to my valve.

 

[shooter]

use a 2n3055 transistor and one resistor of about 100 ohms that will solve your problem.
a better solution could be to use a opto22 coupler