The pid-loop output to control my recip-compressor's loading solenoid valves, for example:
solenoid valve 1 will be energized if pid output > 10 % 
solenoid valve 2 will be energized if pid output > 30 % 
The solennoid on most recip compressors is in fact a pilot device and causes cylinders to unload by either applying or bleeding away oil pressure, which in turn usually causes the suction valves to either seat/unseat automatically in response to compression space pressure or to remain unseated, thus unloading the machine.
If this is a large machine using such an arrangement, you could do a lot of damage to unloader mechanism, valve discs and plates applying pulse width modulation at anything like normal pulse rates and intervals.
If the motor and the system will tolerate it and you need to use P & ID (though I don't know  why you would) consider a VFD.  We have applied them through 600 Hp on recips...

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