Pressing Operation Using AXIS.TorqueReference

Greetings folks!


New member here, but been doing automation work for just North of 25 years.

Thought I'd pop in and ask the brain-trust a question.



I have an application which is a hydraulic press retrofit using a servo-drive.


General:
PLC - 1769-L30ERM
Drive - Kinetix 350
Motor - MPL series low inertia motor
Mechanical - Tolomatic linear actuator coupled to a ram



What I'm wanting to do is emulate the operation of a hydraulic press by manipulating the output torque of the drive by temporarily writing a reduced torque value to the drive (axis.TorqueLimitPositive), performing an absolute position MAM move past the position where the the part would be properly inserted , and monitoring the axis.TorqueReference parameter feedback until a timer is done. Basically, the motor runs at that torque value, pressing on the part until it's commanded to stop and reverse, either because it reached the end of the move or times out.



My question is, will this work? Is it a conventional method for accomplishing this? Is there a better way of doing it?


Thanks in advance!

It will work. However, unless the press is small and the pressing action is very quick, it probably wasn't a good idea change hot the hydraulics for the servo motor.

If you run the servo in torque mode, you can simply output a open loop signal to the drive that will be roughly proportional to the torque. This should be easy but it may not be accurate due to friction and other things. You should have a load cell to measure the applied force. The problem is that load cells need to be protected.

Another problem is that the motor requires a lot of armature current to maintain a force or torque. Hydraulic systems require only a little oil to compensate for leakage. Otherwise hydraulic systems will maintain force with little energy.

On more problem is the time it takes to write the reduced torque value to the drive. It may take too long. The force on presses can increase rapidly depending on the compliance of what is being formed or pressed. Communication delays of a few milliseconds will be a killer.

Thanks for your reply, Peter.


It's a pretty small press. It's actually an individual parts press, not a stamping operation. This is a 2kw drive and the press is ~3.5 tons, with a cycle time in the range of 2-3 seconds for a ~3" stroke. To clarify, I'm running this in a position loop but changing the torque limit prior to the beginning of the move so timing shouldn't be an issue.


The dwell time at that torque value is in the range of 0.1 - 1.5 seconds depending on the part.

I think you will be OK as long as you can switch modes as soon as you see the torque increasing. Even a few milliseconds is a disaster. The update rate over the Ethernet or bus might be problem. Ignorance of what is happening from millisecond to millisecond is not bliss.

Cool, thanks.

I'm switching it well before the torque can start to ramp up. This is an isolated machine, no network connections,a PV7+, drive, and PLC, so Ethernet/IP communications loading "should" be reasonably unencumbered.



I'm curious if anyone has actually done an application in this way. My main concern is getting following errors(velocity or position) during the terminal part of the pressing operation when it's not really moving but mostly pressing at the programmed TorqueLimitPositive value, I'm sure of the multitude of settings, there may be something I can adjust to mitigate that... anyone?

stoic-one said:

I'm curious if anyone has actually done an application in this way. My main concern is getting following errors(velocity or position) during the terminal part of the pressing operation when it's not really moving but mostly pressing at the programmed TorqueLimitPositive value, I'm sure of the multitude of settings, there may be something I can adjust to mitigate that... anyone?

Click to expand...

We have done lots of position force/torque applications. You will get following errors if you keep the position control in closed loop. There is nothing you can do about that. You can control position or force/torque but you can't control both at the same time.


Below is a link to old ( 20 years now ) press project. At about 13 seconds the press closed on a the object in the press. You can see the actual position ( red ) diverges from the target position ( cyan ) until the controller switches over to force control at 1500 psi. When the controller switches to pressure control, the closed loop position control stops so the target position and velocity is set to the actual position and velocity.

https://deltamotion.com/peter/Pictures/RMC100/lawton.png


The torque will not increase as fast with compliant material so the torque will be easy to control. If the material is hard, it may be impossible to keep the torque from over shooting.