I found this interesting piece on driving stepper motors at speed:
Using a cheap $5 92oz Nema 17 stepper, I was able to achieve around 600 rpm using the misfit tech board.? Wanting to improve performance, I changed out the driver using a TI DRV8823 and 24v at 1.5amps per phase max with PSOC 5LP as the driving MCU updating the PID loop at 1Mhz (1us loop time) and the encoder in ABI mode gives me 2500rpm in 90 degree advance and 9,900 rpm (165 rotations per second) at 180 degrees out of phase.? More analysis needs to be done as there are some special edge cases where the motor ends up going backwards when driven at 180 degrees but it's clearly possible.?
Ant?
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On Tue, Jan 7, 2020 at 02:10 PM, John Dammeyer wrote:
Micro-steppers tend to run out of torque at around 700 RPM in general.? It's the physics of the design.? Two things have to happen to make them turn.? You have to change the direction of the current through the windings.? The back emf from the RPM which is what limits standard DC motors is the second problem.
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Think of it this way.? Turn a DC motor at a specific speed and as a generator it produces a voltage.? If the applied voltage is say 12V then that generator voltage rises to 12V and that's what holds the motor speed to a given RPM. ?Increase the applied voltage to increase the speed until the back emf matches again and the speed stays steady.
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Stepper motors have very low voltage windings and use current feedback to determine when to shut off the voltage.? That's when the drive starts chopping the power to hold the current at a given value and that torque (amp x turns) is what you get to rotate the armature.? But now you have to reverse the current through the windings and if the motor is turning it's generating that back EMF.?
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Plus the current through the winding has to decay (inductance slows that) before it can reverse. The more pressure? (voltage) the faster the change in current.? That's why stepper motors with 1.5V windings have 24V power supplies.? But at a certain RPM the back EMF matches that power supply voltage and it takes too long to reverse the current in the windings before the next step.? End result no torque. Motor locks up.
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So even with an encoder, you still have speed limitations.? And, BTW, the motors with encoders come in larger than size 17.? I used one with a gearbox on the end of it as size 23 and 400oz-in with an encoder built in.? Used CANopen messaging for control. Real sweat motor.? Still not more than about 700 RPM before there were issues.
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OTOH, the Bergerda AC Servos down side is they require 220VAC so you can't run them on 110VAC or a 24VDC battery system.? But with the 2500 line encoder they run 3000 RPM.? I swapped on in place of a 90VDC DC Brushed Servo. ?Worked well for the mill X axis at 150 ipm.? (3:1 to 5 TPI lead screw).? And not nearly as mechanically noisy as steppers.
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John
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I'm surprised, what speed is that?
Smooth and quite is what the the new driver modes do well using clever microstep control. Spread, stealth, chop.
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On Tue, Dec 17, 2019 at 03:07 PM, John Dammeyer wrote:
But then there is the problem with steppers is they just can't turn as fast as you need
