I'm working on a project with a stepper motor and am running into an odd interference issue related to metal contact with the motor.
I'm using a StepperOnline Nema23 2.8A (23HS22-2804S) motor with a StepperOnline DM542T digital driver. Power is supplied to the driver with a DC variable power supply set to 40V. The driver is controlled with a Raspberry Pi (GPIO pin I/O is converted to 5V to the driver via SparkFun logic level converter).
I attached a metal flange coupling connector to the motor shaft.
My Pi is able to control the motor with no load attached without issue. I'm operating the motor at about 1,200 steps per second (1.8 degree steps, no microstepping).
I want my Pi to be rotated by the motor in the end, so I screwed the bottom of my plastic Pi case to the coupler with four M2 metal screws/nuts. With a 150g non-metallic 4" bar ziptied to the case bottom, the motor handles the torque without issue and works properly. The motor is setup to just rotate the object above it, it doesn't lift or translate the object.
When I remove the weight, put electrical tape over the 4 screw heads, and place the Pi on the case bottom, the motor starts making a humming noise. The humming is present whether the Pi is connected to power or not. If I try to control the motor with the Pi in the case attached to the motor shaft, I get a lot of missed steps, and have to severely slow the motor down to prevent them. A similar thing happens if I attach a simple metal bar directly to the shaft coupler instead of the Pi+case. I also hear a humming noise when I bolt the motor frame to a metal bar (as seen in first picture).
Video of the humming noise when putting the Pi in the case here: Humming noise with Pi on top
I later realized that even just having the 4 metal screws/nuts tightened on the shaft coupler causes humming, but doesn't appear to impair motor functionality. Video of the humming going away when I loosen the screws here (Again, volume up): Humming from screws going away
Can anyone explain what is causing humming and semi-related severe degradation of motor functionality when I attach metal or unpowered PCBs to the stepper motor shaft? What can I do to prevent this?
