more on Bridgeport CNC
Posted by
tommym6@x...
on 1999-07-29 18:31:25 UTC
Here are some more details from my contact familiar with BP CNC machines (in no particular order).
The stepper drive used half step for resonance control,
90V unipolar up to 2500 s/s for traverse (2.5"/s). [Seems a little slow to me. I think microstepping drives would fix this resonance too.]
Additional resonance control comes from the timing
belt coupling the motor to lead screw.
Settling time of the motor is dependent upon its construction, laminated or steel rotor, and can approach 1 second if there is no other damping.
The laminated stuff has less internal damping.
You probably want motors with a half ohm or less winding resistance.
Servos were discussed too, however I didn't pay too much attention since I m convinced stepping is the way to go for me (initially).
Stepper systems have the advantage of excellent repeatability.
The servo can have the advantages of speed, potentially higher accuracy and quiet operation. However compensating the drive/motor control loop to achieve stable operation is no easy task.
The PID variables are not intuitive and can't be guessed at. You need lots of info about the machine and motor variables; friction constants, mass, electro-mechanical motor constants...
and a graduate education in control systems theory to do the math.
Geting the system stable on an unloaded machine is no guarantee it won't oscillate with some load on it, say a part or a vice. Tricky stuff.
I am not ready to roll my own on this. It's real work to me.
If anyone has other questions, I'll compile a list for my next call.
Tom
The stepper drive used half step for resonance control,
90V unipolar up to 2500 s/s for traverse (2.5"/s). [Seems a little slow to me. I think microstepping drives would fix this resonance too.]
Additional resonance control comes from the timing
belt coupling the motor to lead screw.
Settling time of the motor is dependent upon its construction, laminated or steel rotor, and can approach 1 second if there is no other damping.
The laminated stuff has less internal damping.
You probably want motors with a half ohm or less winding resistance.
Servos were discussed too, however I didn't pay too much attention since I m convinced stepping is the way to go for me (initially).
Stepper systems have the advantage of excellent repeatability.
The servo can have the advantages of speed, potentially higher accuracy and quiet operation. However compensating the drive/motor control loop to achieve stable operation is no easy task.
The PID variables are not intuitive and can't be guessed at. You need lots of info about the machine and motor variables; friction constants, mass, electro-mechanical motor constants...
and a graduate education in control systems theory to do the math.
Geting the system stable on an unloaded machine is no guarantee it won't oscillate with some load on it, say a part or a vice. Tricky stuff.
I am not ready to roll my own on this. It's real work to me.
If anyone has other questions, I'll compile a list for my next call.
Tom