O/T Servo Hydraulic Motor control.

[gbradley]

Just the other day I saw a hydraulic forming press that didn't seem to have a pressure relief valve.
It had a Servo motor connected to a pump that brought the press up to the desired pressure, and once it hit its mark, the Servo motor shut off.
There were other traditional valves and pumps and motors that seemed to move everything around, but it looked like it used the servo motor/pump to close, and bring it up to pressure.
It was pretty cool, but I am really not familiar with this technology.
Any suggestions on where I can learn more about this?
Thanks

 

[Peter Nachtwey]

Beckwood Press has been doing this for years on their smaller C clamp presses. There is nothing tricky about this technology. It works well on smaller presses. The motor usually isn't stopped but rather must maintain torque or maybe turn slowly to maintain pressure. I am surprised there wasn't a relief valve just in case something went wrong.

 

[gbradley]

...I am surprised there wasn't a relief valve just in case something went wrong.

There may have been a relief valve, I didn't crawl around the back side.

 

[osmanmom]

Google link,

http://nsmwww.eng.ohio-state.edu/650.pdf

 

[CharlesM]

Something like this

Siemens Servo Pump Control

 

[Peter Nachtwey]

The problem with the pump systems is that the pressure on the rod side of the cylinder must be zero. However there must be pressure on the rod side to lift the pump.

In small non critical applications this may be OK but when controlling a press you really want to control the applied force, not the pressure. To control the applied force one needs to use a load cell or use 2 pressure sensors. One on the rod side of the cylinder and the other on the cap side of the cylinder. Then one can measure the differential force across the piston by scaling the pressure by the surface area and subtracting the rod side force from the cap side force. The Siemens video doesn't address that.

In small lab like environments I prefer load cells because they are fast and provide true applied force. The down side is that the load cells near the work piece can get damaged. The 2 pressure sensor technique works well on larger applications and those where a load cell may be easily damaged. The down side is that the seal frictions are not taken into account when calculating the applied force. Hopefully the seal frictions are small. It is possible to apply an offset for the seal frictions.

Another issue not covered it that a lot of the forming force actually comes from the kinetic energy of the press itself. The optimal way to set up a press is for the kinetic energy to match the energy required to form the part. The hydraulic force controller then has only small adjustments to make in its control output. If the kinetic energy is too high then the hydraulic force control must try to compensate for pressure spikes but often the valve is not fast enough even if the controller is. If the press closes withe too little energy the pressure or force can still be corrected by the hydraulic force controller but time is wasted adding the extra old to increase pressure. hopefully the hydraulic force controller can do this quickly.