Author Topic: Stepper vs Regular DC motor in CNC  (Read 3072 times)

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Offline newInRoboticsTopic starter

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Stepper vs Regular DC motor in CNC
« on: January 18, 2012, 09:27:54 AM »
Hi everyone

What are the benefits of having stepper motors in CNC instead of regular DC motors with shaft encoders?
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Offline kutalinelucas

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Re: Stepper vs Regular DC motor in CNC
« Reply #1 on: January 19, 2012, 06:22:12 PM »
Well I must be honest, I haven't built a CNC (yet) but I have done a considerable amount of work interfacing steppers and DC motors over the past 5 months, so here's what I think...

Stepper motors have some pretty cool characteristics which seem to make them ideal of CNCs. The first is the resolution. Generally you can turn the rotor of a stepper motor in less than 2 degree increments, and coupled with a geared system (Which you'll have to do because the output torque is SHITE)  the open-loop precision that can be achieved is quite impressive. Saying that a similar resolution can be achieved with a DC motor.

If you have say a quadrature encoder which emits 16 pulses/rev, if fitted with a spur gear head the encoder feedback can shoot upto 1200+ pulses/rev at the motor output. The only problem here is that DC motors are more suseptable to back-forces acting on the motors output, where stepper mtors generally have considerable holding torque, which resists rotative influence from system its supposed to be driving.

You could, to a certain extent control the position, and resist outside influence through software. If you used a parallax propeller multi-core processor, you could program the DC motor to operate as a servo, where disparity between the assumed motors position and any renegade encoder pulses can be used to corrent the motors position. But its still not going to be as precise as a stepper...which is the reason we use CNC's in the first place.

Hope this helps and sorry if i've gone on...its my first post and got excited...

All the best

Martyn

Offline rodstar

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Re: Stepper vs Regular DC motor in CNC
« Reply #2 on: July 29, 2012, 09:20:12 PM »
The short answer is: Servo motors are better: Faster as they keep high torque at high speeds so won't loose steps, as they are based on a sensored feedback loop.--

and just in the case you didn't know...
First, DC brush motors with quadrature encoder and an appropiate controller becomes to be a Servo. Servos have the same operation as a Stepper motor, that is 2 signals: step and direction.

The angular resolution of a servo motor depends on the encoder. If encoder is 400 ppr (pulses per revolution) and use 4x mode [edge detect(2) x channels(2) = 4] you will get a 1600 pulses per revolution (0.225 deg per pulse). So, servo resolution depends on encoder resolution.

The angular resolution of a stepper motor is tipically 200 ppr. That is each step will rotate 1.8 degrees per raw step. Depending on your motor driver abilities- you can use microstep. There are half, quarter microsteps, and so on. That is a microstep of 1/8 of step will rotate 1.8/8=0.225 deg per pulse. That is 200 * 8 = 1600 ppr. There are also motors with raw step of 7.2, 3.6, 0.9, 0.72( typical 5-phase Vexta stepper motors)

there are way lots of factors, but i got lazy now sunday night
« Last Edit: July 29, 2012, 10:19:13 PM by rodstar »

 


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