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My company is involved in a debugging project on a:
Cut to length system producing 10 ft to 20 ft 'sawlogs' from a 'stem'. A stem is what arrives on a logging truck (most people would call them logs) and is up to 80 ft long.
The stem is carried through an arrangement of conveyors and two spiked rolls. The spiked rolls are driven by hydraulic motors. One roll is fitted with a home brew encoder: two inductive proximity switches sensing lobes on two
steel discs. Quadrature signal derives from angle of one disc with respect to the other. The encoder is mounted on the high speed side of a gearbox, between hydraulic motors and gearbox. The encoder provides position feedback. A PLC closes the position loop. The PLC has a VHSC module that counts the encoder signal. And a simple proportional control ramps the velocity down to a stop at the requested sawlog length. The proportional control is implemented via math blocks in ladder logic. The hydraulic drive is throttled by an analog output from the PLC that signals two Rexroth VT cards. The VT card provides
some kind of PWM signal to proportional valve coils controlling oil flow to the motors.
My company did not supply machinery or hydraulics. Nor did we supply the home brew encoder. But we have agreed to do what we can to improve the accuracy and availability of the equipment. I was recently at the plant and witnessed seven hours of down time over two shifts while maintenance staff adjusted the proximity switches to bring the quadrature signal into the specs required by the VHSC module. That could see the signal on an o-scope and it was having phase problems causing the VHSC to fail to count.
I thought I would ask the members of the list:
Is anyone aware of an encoder which will survive high (exactly how high is unknown) G forces that exist as the rolls contact the stem? If no device meeting the requirements exists, then I will rebuild the home brew encoder to add an adjustment to directly affect the phase of the two proximity switch signals. The duty cycle is adjusted via the air gap between the proximity switch faces and the discs. In the present configuration, the phase is adjusted by playing
with the duty cycles.
Is anyone aware of a 'motion controller' or 'position controller' that can accept a quadrature encoder signal and provide output to Rexroth VT cards or Rexroth proportional valves directly? Ideally, it can also accept an 'at position' signal. My customer has installed photo-electrics at the sawlog length positions to cut
the signal to the proportional valves in an attempt to mitigate the accuracy problems. The motion controller would be under the supervisory control of the PLC in that it would receive 'requested positions' from the PLC and return a 'position done' signal to the PLC.
Any response is greatly appreciated.
Bill Code
MPM Engineering Ltd.
4-6240 202nd St., Langley, B.C., Canada, V2Y-1N2
Phone: 604-534-6605 Fax: 604-534-6693
E-Mail: [email protected] WWW: http://WWW.MPM-ENG.COM
Cut to length system producing 10 ft to 20 ft 'sawlogs' from a 'stem'. A stem is what arrives on a logging truck (most people would call them logs) and is up to 80 ft long.
The stem is carried through an arrangement of conveyors and two spiked rolls. The spiked rolls are driven by hydraulic motors. One roll is fitted with a home brew encoder: two inductive proximity switches sensing lobes on two
steel discs. Quadrature signal derives from angle of one disc with respect to the other. The encoder is mounted on the high speed side of a gearbox, between hydraulic motors and gearbox. The encoder provides position feedback. A PLC closes the position loop. The PLC has a VHSC module that counts the encoder signal. And a simple proportional control ramps the velocity down to a stop at the requested sawlog length. The proportional control is implemented via math blocks in ladder logic. The hydraulic drive is throttled by an analog output from the PLC that signals two Rexroth VT cards. The VT card provides
some kind of PWM signal to proportional valve coils controlling oil flow to the motors.
My company did not supply machinery or hydraulics. Nor did we supply the home brew encoder. But we have agreed to do what we can to improve the accuracy and availability of the equipment. I was recently at the plant and witnessed seven hours of down time over two shifts while maintenance staff adjusted the proximity switches to bring the quadrature signal into the specs required by the VHSC module. That could see the signal on an o-scope and it was having phase problems causing the VHSC to fail to count.
I thought I would ask the members of the list:
Is anyone aware of an encoder which will survive high (exactly how high is unknown) G forces that exist as the rolls contact the stem? If no device meeting the requirements exists, then I will rebuild the home brew encoder to add an adjustment to directly affect the phase of the two proximity switch signals. The duty cycle is adjusted via the air gap between the proximity switch faces and the discs. In the present configuration, the phase is adjusted by playing
with the duty cycles.
Is anyone aware of a 'motion controller' or 'position controller' that can accept a quadrature encoder signal and provide output to Rexroth VT cards or Rexroth proportional valves directly? Ideally, it can also accept an 'at position' signal. My customer has installed photo-electrics at the sawlog length positions to cut
the signal to the proportional valves in an attempt to mitigate the accuracy problems. The motion controller would be under the supervisory control of the PLC in that it would receive 'requested positions' from the PLC and return a 'position done' signal to the PLC.
Any response is greatly appreciated.
Bill Code
MPM Engineering Ltd.
4-6240 202nd St., Langley, B.C., Canada, V2Y-1N2
Phone: 604-534-6605 Fax: 604-534-6693
E-Mail: [email protected] WWW: http://WWW.MPM-ENG.COM