Ok :0)
What you are lookig for is some sort of frequency to voltage convertor.
Does your PLC only provide a variable frequency output , or can it also do a PWM output.Frequency is fixed , but the duty cycle changes.
If so , things are easy.
If you only have a variable frequency it's a little harder.Do a search for frequency to voltage convertor.
If you have the ability to program a micro the solution is simple. Measure the input frequency and output a proportional PWM.If you can't use a micro , go analog :0)
Frequency to voltage convertor -> PWM.
I'll attach a LTSpice cct to this post. It will have a simple frequency to voltage convertor , using a 555 , followed by a PWM cct to drive the solenoid.
There are much better chips than the 555 designed exactly for this job(lm2907 comes to mind) , but I'll use what you can probably get hold of easely. The accuracy you need is also a factor.If you need very precise control , with little delay(Voltage to frequency convertors using this scheme have some delay before the voltage settles) , then some other solution is needed. In this case the delay is about 100ms untill the voltage is stable and hence stable pwm is reached. At the low frequency end there will also be a bit of jitter on the pwm , because the voltage out of the F->V convertor has ripple on it.If your input frequencies are higher , say 10khz to 100khz , then the ripple is significantly reduced. This circuit uses frequencies from about 6Khz to 18Khz to give a voltage 0 to 100% pwm.Below 6Khz the FET is off and above about 18Khz it's hard on.PWM'ed in between :0) Just scale the R's and C's for the frequency range you want to use. (within reason :0) )
It runs off 12V with a 5V input. It's not the best circuit you can get for this sort of job , but it will get the job done and uses easy to get components and is fairly simple.If you don'y have LTSpice , you can get it from Linear.com. Its simple to use , unlimited and best of all , free. Ive made the circuit with components that are supplied with the program except for the opamp , I've used a lm358. The subcct is in the schematic , so you don't have to download it.I've go no specs on the solenoid , so I guessed at 1mH inductance. The fet u will use will obvoiusly have to be sized accordingly.
This was supposed to be a quick post , but has become a little long winded. If you have any qusetion , just shout. Hope this will at least get you started though :0)