Your PLC analog output will be in the 10V range or 20mA range. The hydraulic servo valve is going to require anywhere from 200mA to maybe as much as 1600mA. Additionally, the valve coil driver current usually has a dither frequency applied to it to overcome static friction. So at the very least you will requier an amplifier.
That said, I usually use servo valves (Bosch-Rexroth are my pefered vavles) that have on board amplifiers and I send the command signal from the PLC directly to the on-valve amplifier. I have used intermediate controllers in the past, but I find that those are usually "black boxes" and the manufacturers are not too willing to divulge their inner workings - if you need something special, you pay dearly for custom software, and its more trouble than its worth. I prefer to have all the control in the PLC. It does require more software development time and you do need to have an understanding of what you are doing.
I use AB's Control Logix PLC and the Enhanced PID instruction - but I have added additional logic to it, for example there is extra logic so that if pressure is leading setpoint (we cannot tolerate an over pressure situation on the process) by more than a certain ammount, and the setpoint ramp is not going to overtake it within a certian time (I use a line intercept equation to determine this), then I zero the integral of the PID equation, causing the valve to center. I couldn't do that when I used to use an intermediate controller, I had to wait for the integral to unwind.
