I actually use PLC analog outputs for driving a PWM signal regularly in one of my OEM products. So I might be able to steer you in the right direction.
As the others said, you need to be aware of your interface capabilities. Also, the "high rate" must be in the slew rate capability of the output card. (My application required a period of 0.7 seconds, which works fine with a 1769-OF2) You'll need to do your homework to see if your period and the capabilities of the card (and MicroLogix backplane) are a feasible match to the interface circuitry needs.
All interface problems aside, this is what I did to create a fairly high speed PWM: I used an equation (in my case a PID) to define a percent output to the process I was trying to control. That's fairly easy. Then a I made a free running timer with the preset defined as the period of the PWM. Scale the percent output of the PID to the period of PWM to get a duty cycle in units of time.
A greater than (or less than depending on what you need) then compares the duty cycle time to the accumulator of the timer. The result should be a digital response that switches the analog output from the minimum output value to the maximum output value you require. 0 to 10 volts, whatever you need.
This way the duty cycle changes how much your sprayers apply in a relationship to the speed of the conveyor, amount of pressure, whatever you need. You'll need to define this relationship as part of your solution, it may be a PID solution, or it may be a calculation which is linear or nonlinear depending on your inputs of conveyor speed and the pressure available to the sprayers.
