there are number of options here.
standard in industrial automation is 24VDC (probably about 90%) but of course there are also AC versions with 120VAC or 230VAC being more common than 24VAC. Usually AC is used only when higher power is needed or where voltage drop is an issue (valve is hundreds of meters away from controller) or where simplicity is essential (if you only need one valve, no need to spend money on 24VDC power supply).
Solenoid valves can be individual or in banks (manifolds) and usually are controlled by 24VDC. direct action valves take more current (larger solenoids) but for factory automation much more popular are piloted valves where solenoid draws very little current (makes initial small movement of the valve, which then allows compressed air to complete the movement). The difference is some 2-4A for direct drive solenoid versus some 10-20mA for piloted valve.
But you seem to look for analog control. In industrial world (you mention PLC), analog signals are standardised. Most common are 0..10V, 4..20mA, but there are also 0..20mA, 1..5V, 0..5V and -10..10V. Of course both PLC and controlled device must support same interface (say 4..20mA) otherwise you will need to spend another $150-300 for signal converter (weidmuller, phoenix contact, red lion, etc.)
You would use analog signal to control proportinal valve (earlier we talked about discrete valves). Exact wiring would depend on model (what is the part number and make of your valve). in the PLC you would then write program that sends value to analog output (which is wired to your valve).
One common way to use analog controls is to create PID loops (all PLCs except really really small ones have PID functions built in).
What PLC and valve you use? How long is cable from PLC to regulator? (current loops are favoured in long cabling).
Most common products are Festo and SMC but there are many others. Here is what they usually look like:
http://www.smcetech.com/CC_host/pag...cfm?cc_tvl=&cc_nvl=((CC,smc,INS_US,Node_22816))