I am not 100% sure what would be the behavior of my system in a specific scenario:
I am imagining my bot loses power, and some external physical force pushes the arm that my stepper is controlling and the gearing generates a non-zero several volts of power by spinning the stepper motor.
Ideally, I would like that voltage to go into the stepper and work in a resistive fashion so that there’s some passive braking dependent on how fact the stepper is spun.
Functionally I’m not sure what would happen. So the DRV8825 is a pair of Hbridges…I guess because it’s an H-bridge it may be that the voltage is in fact blocked and the stepper will be “free running” and not with shorted windings.
IS there any way to make my windings short under power-off condition? I considered having a relay that I have to power before running the motor, and this relay is normally closed for shorting the windings. I am just a little concerned that well (1) I am adding a relay more complexity yada yada, and (2) I am not sure what the behavior will do in terms of voltage spikes and blowing up the drivers.
When the DRV8825 stepper motor driver is not powered, the motor outputs of the driver should be floating, and you should be able to freely turn the motor. You will not be able to short the motor leads together without powering the DRV8825 driver; however, your plan for using a relay might work. If you post diagram of your proposed design, I would be happy to take a look at it and give pointers.
I will definitively return to this, and when I do I’ll show up with a better diagram. One concern is naturally that I don’t know exactly how to control that the driver turns off before the relay (because we’re talking about uncontrolled power outages), but I guess that could be discussed after I’ve drawn something proper up. It might be as simple as a capacitor that minutely delays the relay from turning off.
I decided to focus on my pulley actuation system first for the part of the bot that doesn’t need to have any resistance during a power-off fault [there’s a low-risk horizontal moving part.]
Timing the relay with the stepper motor driver outputs might be difficult when the device loses power. As an alternative, you might consider adding a worm gear to the output shaft of the stepper motor to prevent it from back driving. I do not have any specific recommendations for a worm gear, but I have seen gearboxes that use worm gears specifically made for stepper motors.
I looked into wormgears and the enclosed boxes were just a bit pricier than I wanted, and the exposed solutions were giving a headache of how difficult it might be to align and protect the individual gear components (which weren’t too expensive.)
The stepper motor brakes as I believe I mentioned before, were a little too weak and expensive (they were going to be at the limit of their operation and some were spec’ed to as few as 10 emergency power-offs before their pads wore out.
So finally I decided to go to a high-angle threaded rod. I’m not super happy with it, but it seems my best option.
As for power-off shorting the windings:
Normally-closed solid state relays seemed extremely complicated and hard to do. No standard parts could really do it, given that I didn’t want to use relays [perhaps I should have accepted them]. I found I could develop a small board with a supercapacitor that could identify system power-loss and short the windings for 30+ seconds to allow for failure shutdown (to allow for the arm to have some resistance on its fall down.) but the shorting method seemed pretty complicated.
