So cutting wood, I noticed that the Y axis stepper motor in particular felt underpowered. It seemed to stall out without a whole lot of resistance and when I had the steppers locked in place it sure didn't feel like 400 oz-in of torque!
At first I blamed sticktion, then I blamed the Oldham coupler locking up at certain rotations. But I could get the handwheel to rotate smoothly, and there wasn't an excessive breaking force to get the axis spinning.
So I did what any rational person would do, I grabbed the food scale out of the kitchen (sorry, wifey) and pushed on the handles of the Y axis handwheel while the stepper was locked and sure enough, I got something just under 200 oz-in of torque! What gives? I figured even if overadvertized, the steppers would at least be north of 300 oz-in, and I knew the motor drivers were sufficiently powerful to give me most, if not all of the advertised torque.
So was it the motor or the drivers? Turns out it was the latter. The DRV8825 stepper driver board comes with a potentiometer to limit current. If you visit the pololu page for the DRV8825 stepper board, and scroll down to the video, you can watch a nice lady explain to you how to adjust the potentiometer. Not mentioned in the video: as shipped they limit to 1A current. The drivers are rated for 2.2A peak, 1.8A continuous. I set mine to 2.0A by setting VREF to 1.0 volt.
A little more information: The 2 amps is per coil. The steppers are rated to a peak of 2.83A for the motor - not for the coils! If you are treating the motor as bipolar (by wiring the four coils as two sets in series) the maximum current would be the square root of 2 amps, squared, for each coil, which works out to 2.83 amps. Or 2 amps per coil. So set the motor drivers to 2 amps knowing you aren't exceeding the capability of the motor. You do need to ensure you aren't microstepping - on my board you need to explicitly set a sequence of jumpers to enable microstepping.
One other thing to note: while bipolar series has more holding torque than bipolar parallel, bipolar parallel has more torque at speed than bipolar serial, however it draws more amps - need a bigger motor driver. See the "series v. parallel" figure towards the end of this article.
At first I blamed sticktion, then I blamed the Oldham coupler locking up at certain rotations. But I could get the handwheel to rotate smoothly, and there wasn't an excessive breaking force to get the axis spinning.
So I did what any rational person would do, I grabbed the food scale out of the kitchen (sorry, wifey) and pushed on the handles of the Y axis handwheel while the stepper was locked and sure enough, I got something just under 200 oz-in of torque! What gives? I figured even if overadvertized, the steppers would at least be north of 300 oz-in, and I knew the motor drivers were sufficiently powerful to give me most, if not all of the advertised torque.
So was it the motor or the drivers? Turns out it was the latter. The DRV8825 stepper driver board comes with a potentiometer to limit current. If you visit the pololu page for the DRV8825 stepper board, and scroll down to the video, you can watch a nice lady explain to you how to adjust the potentiometer. Not mentioned in the video: as shipped they limit to 1A current. The drivers are rated for 2.2A peak, 1.8A continuous. I set mine to 2.0A by setting VREF to 1.0 volt.
A little more information: The 2 amps is per coil. The steppers are rated to a peak of 2.83A for the motor - not for the coils! If you are treating the motor as bipolar (by wiring the four coils as two sets in series) the maximum current would be the square root of 2 amps, squared, for each coil, which works out to 2.83 amps. Or 2 amps per coil. So set the motor drivers to 2 amps knowing you aren't exceeding the capability of the motor. You do need to ensure you aren't microstepping - on my board you need to explicitly set a sequence of jumpers to enable microstepping.
One other thing to note: while bipolar series has more holding torque than bipolar parallel, bipolar parallel has more torque at speed than bipolar serial, however it draws more amps - need a bigger motor driver. See the "series v. parallel" figure towards the end of this article.
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