servomotor to control a hydraulic pump


Thread Starter

Rodger Furey

I am considering using a servomotor to control a hydraulic pump. It is a simple single acting press application where I would drive the pump (slowly) in one direction until the servomotor stalled at its torque setting. The servomotor would hold, near stationary, at that torque setting for 8 hours or more. Then I would reverse the motor and pump the fluid from the press back into the reservoir. I have found a fixed displacement pump that can handle very low RPMs (to zero) and is reversible. I currently have a conventional system with an array of proportional valves and a bank of control cards which I must connect to individually to change variables. I like the idea of putting all the control in the motor, partially because I can easily communicate with a PLC and change the variables on the fly. I like the idea of a simple system with no feedback. Speed and linear displacement of the press would be determined by the pumps fix displacement and the motor RPMs, max pressure would be determined by the motor torque setting. I like the idea of a servomotor quietly applying torque (pressure) for 8 hours instead of a variable displacement pump roaring away, like my current system. I also like the idea of no valves and simple plumbing, only a safety pressure relief valve. What I don’t like is the fact that I can’t find anyone else who has done anything like this. Has anyone seen an application like this?

Jason Christopher

I am very surprised to hear of an application like this, as it is most unusual. I do have a couple of questions, though:

1. Are you sure the servo won't overheat by holding it at its stall torque for 8 hours?
2. Are you planning on running the servo in torque mode or position mode?

What you may want to try instead is a pneumatic driven power unit (Hey-pak, for example), depending on your flow requirements. A directional control valve or two should allow you to pump the fluid in reverse. This will afford you easy control and quiet operation. You may want to investigate using poppett style valves for your control, instead of traditional directional control valves. These will have less leakage, so the Hey-pak will cycle less.
I'm not familiar with hydraulics, but the motor control seems feasible.

The only thing I can think of that would mess you up is the servo tripping out when it stalls (i.e. following error tripping your controller). If you are using a smart system like Delta Tau you can monitor the torque output or position in a fast program loop and detect a stall, and then command the motor to stop, or hold a constant (open loop) torque at this point.

This has been done before and there are a few applications of this that have been patented in the tunnel boring machine industry.

You should have no problem with this application if running the drive in torque (or current) mode operation.

Both Servo Motors and ACVector motors of TENV design will have no difficulty at rated torque 24x7.

If you use the Delta Tau controller - it is fairly easy to disable fatal following error and set the notch filters up such that the PID is driven from Velocity error. Turn off all integrators and then following error is of no consequence and you can use the DAC limit to set the torque to achieve the desired pressure.

With an ACVector motor, you would need to consider inertia effects as they would undoubtedly produce a pressure spike when the motor stalled.

In short, it has been done, it works well and I would not hesitate to try it. :)