I am looking for a stepper motor that has a range of 0.01 RPM to 100 RPM. The max Torque would ramp up to about 128 oz-in but only when maxed out and running on the low end of the rpm's say about 0.2 rpm. The 100 rpm is to return to home in a unloaded state as fast as possible. I need to keep my speed within 5% error. Do I need a gear reducer to get this to a higher low end rpm? Would a gear motor be better suited? I would like a digital control.
Stepper motors don't have floating speed problems because they are not analogously operated. When purchasing a stepper motor you need to know the maximum index degree movement you want. they come in very fine moment to very coarse. as far as slowness, any one of them can crawl. Because in order to move them, you need to alternate the poles. once you have done this, keeping the phase angle that you currently have will keep the motor locked into that position. To move it to the next index, you need to alternate the poles again and so on.. THat is just a basic description of the operation.
Stepper motors have spec.'s that tell you what the highest pulse rate you can operate them at and the number of degree's each index will give you. so if you have a motor that has an IPR (indexes Per Revolution) of lets say 100. This means it will take 100 Pulses into the controller to make a full revolution. so for 1 RPM you would need 100/60 = 1.666 PPS (PPS = Pulses Per second). 10 RPM 16.66, 100 RPM 160.66 and so on. you must check the specs on the fastest pulse rate and the number of degree's for each index it has.. Smaller degree steps give you better accuracy for positioning but will not spin as fast. Your Speed will only be as accurate as your clock rate.
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You may have to "manually" accelerate the motor to get 100 RPM. Suddenly applying a high pulse rate drive to a stopped motor may just cause the motor to jitter, so you have to start with a slow pulse rate to start the motor, then slowly increase the step rate until you reach the desired speed. For best positioning accuracy, you should also decelerate the motor as you approach the target position.
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No gear reducer. Digital drive is far better than using 2 external sines. The torque should be no problem for some of the better quality size 23 motors and easy for many of the size 34 types. Indeed, that 128 oz-in should be available through to hundreds of steps per second. By comparison "0.2rpm" is near standstill and the 'pullout torque' is at it's best. Most good motors offer 200 steps per rev' and 100 RPM will be 333 full steps per second, so there's still lots of torque available even on a home run. Better if you can step the motor in "half step" mode as the mechanical response is much much smoother but would need 666 sps on a home run.("microstep" is even better but more complex electronics and expense) As mentioned by Jamie, motor speed is -exactly- the rate you send steps to the motor. The whole digital stepper motor idea, is absolutely dependant on and hinges on, -not- ever losing any steps (speed). Doom awaits those who overload their motors. There's lots of misinformation out there. Best data source is the motor manufacturers PDFs. john
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