Question about Stepper Motors
Ben wrote 10/18/2023 at 16:06 • 0 pointsI'm building a tabletop claw machine game with some middle schoolers. It's arduino-based and using gantry parts and nema 17 steppers from an Ender 3. We're stuck with a question about the weight of the claw, we want to also use a nema 17 motor to wind and unwind the claw to raise and lower but are concerned about the holding weight of the stepper when it is turned off. We're thinking now of a 51:1 planetary gear stepper. Does anyone have experience with these. Is the holding strength of the output shaft a lot greater than a normal nema 17? Any other ideas for a worm gear or something else that would hold position of a stepper when power is cut off completely?
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Could you put a shelf in the park position and rest the claw on it just before shutdown?
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Not a bad idea 👍. Or maybe the spool where it winds has a flat side and a cheap servo flicks out to brake? I ordered one of these to see if I can modify it to fit a stepper: https://amz.run/7DWs
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Something to think about with stepper motors is that their torque falls off as the step rate increases. Modern driver chips help that, but there are still limits. If you use too much reduction gearing, you may run short on torque to lift or carry the claw and prize.
How much the torque will fall off and at what speed is very dependent on the motor and the driver chip, so I can't give you good numbers for your situation. There may be some tuning required.
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Thanks for the info on the TMC2226, those sound efficient, I'll have to check them out! Yes, the goal is to be in a completely powered-off state with the claw positioned at the top of the gantry so that it doesn't unwind if the power is cut. It'll be coin-operated so most likely, the Arduino will be in a run state but we won't kick on power to the steppers until a coin is inserted. The kids are developing the code so I'm trying to help them come up with a design that is as simple/safe as possible. We're using an arcade solenoid-powered 24V claw mechanism for the grabber and it is a bit heavy. I think a standard arcade claw machine uses DC motors and a worm gear on the claw mechanism but I'm hoping the kids will be able to control steppers/limit switches easier than DC motors
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The 51:1 reduction will help with the holding capability when the stepper motor is un-powered. A worm gear drive should also work for this.
Are you thinking of turning off the lift motor while the machine is active or is it just when the whole machine is shutdown? Are you trying to avoid having to home the lift mechanism on powerup?
A some stepper drivers allow reducing the motor current in an idle condition. The Trinamic TMC2226 chips will do this automatically. I expect that there are others that will do this, but that is the only one that I am aware of specifically. Other drivers allow you to set the motor current directly.
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