Originally Posted by mmjpotter During a machining cycle the torque requirements can vary considerably due to changes in feed rates, depths of cut, material hardness, cutter speed etc. Surely the purpose of a servo system is to control these non-linear torque requirements. Running out a bit of backlash seems like a rather trivial task, and I agree that these torque requirements have nothing to do with encoders, linear or rotary.
Does anybody agree with the following:-
A: That linear encoders return actual slide movements regardless of backlash. |
Assuming they are mounted on the slide -- YES.
Originally Posted by mmjpotter B: A DC servo can maintain a constant speed/torque under varying loads. |
NO. -- They can maintain a more constant speed OR torque under varying loads. That's more constant than a system without feedback. The systems we are referring to do not measure the torque, so we're not really talking about maintaining a constant torque.
Originally Posted by mmjpotter C: If the linear encoder is replaced by 2 micro-switches or limit switches, the motor will drive the slide to the limits regardless of backlash. |
One could design a system to do that.
Originally Posted by mmjpotter D: When a slide with backlash is driven by a stepper motor, 2000 pulses in one direction may not equate to the same distance for 2000 pulses in the opposite direction. If a linear encoder is fitted to the slide and the motor is driven by a CONTINUOUS train of pulses, then counting 2000 encoder pulses in any direction will equate to the same distance thus eliminating the backlash. |
In response to the first sentence, it depends. If you mean that applying 2000 pulses in one direction, followed by 2000 pulses in the opposite direction, then, in general, the slide will not wind up in the same position. If one counts encoder pulses, the slide will wind up in the same position.
But you are using steppers in one case and servos in the other. If you move slowly enough that you have zero overshoot, you are correct. But you might not wind up with a useful system. It depends on your application.
Ken