Belt/Pulley help

[meanshapes]

I currently have been experimenting with At5 belts/pulleys on my router. I currently have a working system using nema 23 servos with a 10:1 gear box on each axis XYZ using an off the shelf bet drive acuator. I'm trying to design a car that will provide Y/Z motion using the AT5 belt and a 1" diameter 22 tooth pulley or so. My question is that I currently use servos with a 10:1 gearbox. If I were to switch to a stepper system is it possible to direct couple the stepper motor shaft to the pulley to drive? What are the issues with this? I assume it would be torque on the motor shaft? I did order the candcnc 620oz/in blade runner stepper kit to try this. Any help is much appreciated...

[derekj308]

Hi meanshapes

In comparison to a servo system, a reduction in achievable resolution will be the likely result of going to a stepper and if you choose open loop stepping, innaccurate positioning. Even though there are stepper drives available which have high microstepping capability of greater than 10 microsteps per step, the likelihood of actually realizing this resolution is dependent on the quality of your motor and your drive. You can achieve smoother motion by using higher microstepping but your achievable resolution is probably not going to be greater than 10 microsteps per step.

At the moment you would be used to having a resolution equal to your encoder ppr * 4 * your gear ratio of 10:1 and you would be hitting your commanded position each and every time. As a comparison lets say you run your current servo with your 10:1 reduction and the 1" pulley. For sake of simplicity assume the pitch diameter of the pulley to be 1" therefore the circumference of the pulley is 1" * PI so the circumference is 3.141". If you have a 500 ppr encoder on your servo run in quadrature (2000 discrete steps per revolution) you could achieve a resolution of

3.141" / ( 2000 * 10) = 0.00015705" , which is pretty good resolution.

If you direct drive using a 200 step stepper, no encoder and say 10 microsteps per step, this reduces to 200 x 10 increments per revolution.

3.141" / 2000 = 0.0015705" , effectively a loss in resolution of 10 times but still OK resolution in hobby CNC terms.

Depending on your application this reduction may not be such an issue.

Without knowing the torque rating of your servo I cant really compare going to a 620oz.in stepper. I could hazard a guess and assume that the servo can provide a torque of at least 62 oz.in at the rpm you require for you to achieve your desired feed rate under load so I'm thinking there is no real torque advantage in going to a stepper compared with a servo through a 10:1 reduction.

Your question is, "... is it possible to direct couple the stepper motor shaft to the pulley to drive?", and I say absolutely. However, if you use stepper setup with no feedback you may not like the potential performance reduction (lost steps, no fault output) in comparison to your servo setup.

Cheers