Came into a problem the other day that has me puzzled..maybe you guys can help.
I've got a customer that has several molding machines made by a now-defunct OEM. Two of the pieces run AB PLC 5's and run reliably and without problems.
The third is one of those "optimized" racehorses that runs a Modicon Quantum.
These machines are basically hydraulically powered and have six operational steps:
1) Prepared sand is blown into a chamber.2) A Ram squeezes the sand to a operator-set pressure (typical 850 psi).
3) Plate (pattern) opposing the ram raises out of the way.4) Ram pushes finished mold onto a string of molds.
5) Ram Retracts.6) Plate opposing ram moves back into position for another mold.
How this is accomplished in the controls scheme is through a Rexroth pump (5,000 psi max) and controller that is straight PID. The Quantum tracks where the machine is in the operational sequence and ramps the Rexroth controller via analog signals (-10 to +10 VDC). A -10 signal "tilts" the pump 100 % one direction, where a +10 signal "tilts" the pump the other direction. Tilting is accomplished through a auxiliary servo with LVDT feedback to the Rexroth.
The main pump does not operate anywhere near its rated capacity except in operation 2 above. Apparently, in that operation, mechanical limits are reached because a "dump" valve opens up to remove the ram pressure for 2 seconds after the 850 psi is reached (this time frame is also selected by the operator) and a flow control is implemented to keep total system pressure from bleeding prior to the next operation.
Two problems here:
1) The pressure in operation 2 is always overshot and2) The mechanicals move way too fast in operations 3 & 6.
Changing the pump commands from the Quantum seem to help somewhat, but it seems pressure is building up in the system. I feel like we're not dumping enough pressure in operation 2, and maybe the flow control needs opened a little. What is a good practice for tuning the Rexroth controller when it apparently worked fine while the mechanicals were new, but now seems to falter? Do I accel/decel faster? Do I set the LVDT gain down/up? Do I offset the output?
I hope I've given enough information here... any suggestions would be appreciated.
Jake