Sensor to detect slippage in stepper motors

[xydrex]

Hi all, I have recently cnc automated a large mill via retrofit. Main problem is occasionally the motors slip, so the software (mach 3) thinks motion has occured while obviously it has not. Is there any sensor to detect if slippage occurs in these motors? Perhaps by the voltage or current responses of the motors? Eventually we intend to create a closed loop system with feedback via encoders, but until then it would be good if we can get a simple devise that detects slippage.

 

[BobM3]

You could measure the travel of the table of course but that sounds like more than you want to do at this point. I'd try asking this question in the "Electric motors..." forum. There's a bunch of good motor people there that might suggest a way you can do it my monitoring the motor current and/or voltage.

 

[itsmoked]

The standard method is encoders! If you want to detect slippage buy and install the proper encoders. Furthermore mach3 will understand encoders and likely not some other kludge technique.

I'm surprised a "large mill" is using stepper motors. Why not servos?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MikeHalloran]

Forty years ago I worked in a shop that was using CNC mills to cut circular pieces from aluminum plate... except that they got a little joggle in the part and made scrap when there was a hard spot, or the ways got dry, or for some other reason the motors missed a step. That's why commercial CNC mills haven't used open loop steppers in many years.

Since then, I have messed with steppers a bit myself, and noticed that the current waveforms change a lot when the motor misses a step. So, theoretically, you could add a current sensor and a fast microprocessor and some software that I don't yet know how to write, and you could detect that you had indeed lost a step... except that I haven't figured how the detector would know that you've lost _two_ steps, or N steps. So you still wouldn't know what had happened exactly, only that you had already produced some scrap. Lots of effort for useless information.

Buy the encoders.




Mike Halloran
Pembroke Pines, FL, USA

 

[nornrich]

xydrex,

The only way to get a truly accurate system is to close the loop. Implementing some type of encoder, either rotary on the motor itself or linear on the ways or table that you are moving is relatively simple. I am not familiar with the mach 3 software, but any motion control software should be tied to some type of motion controller that should have standard inputs for encoders or resolvers. Close the loop and take advantage of the precision of true motion control.

Regards,


Rich.....

Richard Nornhold, PE