Controlling a fast servo valve in a closed loop

hello there,

this is my first post and hoping to have your bright ideas/reccomendations, anyway,

we use a moog electro hyd. servo valves for a position control case, and the position loop is closed by a potentiometer +/- 10V feedback signal. The position controller itself is an analog PI control board that includes voltage 2 current converter for servo valve as well. Considering a fast 2 stage mechanical feedback servo valve, 120hz is that servo valve open loop bandwidth, is it possible to replace that analog control board with a digital controller, for example a controller running at 4khz + voltage2current converter… if yes do you think is 4kHz is enough to have a stable valve operation. I m planning to do this replacement for the ease of simple control design with a real time software, so that not deal with analog circuit anymore.




Yes, it sounds like replacing the analog controller with a digital controller will work just fine. You mentioned 4 kHz, but typically, hydraulic control only requires 1 or 2 kHz, even with a 120 Hz valve. The RMC75E controller can do a 4 kHz update rate for a single axis of control. For 2 axes, it can do 2 kHz.

The one concern I have is the analog feedback, which is notorious for causing noise and therefore, poor resolution. What is the position range of this transducer? What position resolution do you require? What exactly is the application?

Hi Jacob,

You’r saying that 120Hz_Bandwidth*10(rule of thumb) = 1kHz is okay, I got it. it is a simple piston position control application with a flow control servo valve. Yes the feedback is from a potentiometer whose signal has a 5 to 8 mV rms noise on it. Considering a +/- 10V feedback, which corresponds to +/- 30mm stroke of a piston. Considering at least 12bit ADC conversion of the analog signal, with a proper low pass filtering, Do you still think that the pot. noise might lead a problem on the high speed servo valve like stability? Or do you have other concerns about it?

Thank you,


If the noise corresponds to +/- 30mm, I would expect that it will cause control problems. However, if the control is slow, it may be possible to filter the noise.

Whether the noise will cause problems depends on the total travel distance, speed, acceleration and required position tolerance. Do you have these values? This will help me determine if it will be a problem.

One of the most common sources of control difficulty is noisy analog feedback signals. If the application is price sensitive, and you have time to try to work around the problems caused by noise, then it may worth trying to keep the potentiometer. If there is room in the budget and if you need a robust solution that will result in short machine startup time, then using a different type of transducer, such as magnetostrictive with SSI output, would be of great help.

hi Jacob,

+/- 30mm stroke is a total piston retraction/vice versa. In analog control case, 5-8mV rms of noise value does not lead a piston move at a 0V position cmd ref. signal. Considering the the pot. as the only solution for a position feedback, Do have other concerns about analog to digital conversion of the control system ?

I see! That should be just fine! There should be no problems with the anlog to digital conversion.

What type of motion will you be doing? How fast? Will you be doing cyclical motion?