First some background information: the motor choppers are current controlled. This means any disturbance in coil current, resulting from the second coil or other motors, will disturb the motor chopper. It can happen through capacitance between cables for different motor coils. The capacitance will then transfer current between the cables during a switching event.
- The best way to avoid this, is to shield cables of different coils against each other. For example by using shielded twisted pair cables, or when using a flat ribbon cable by inserting one GND cable between each two coils.
- Low current stepper motors are affected easily - even by a small capacitance. It can help to parallel the sense resistor with a capacitor of 1µF to 10µF. Choose the capacitor to form an RC-element with a time constant of 1 to 2 microseconds. This way, a low pass is formed that will help skip the current disturbance.
- Another solution (additional to the one above) is activating StealthChop operation. StealthChop is less sensitive to disturbance of the motor current.
- Optionally, use StealthChop without current measurement and pure feed-forward current control (velocity based scaling). This works perfectly, if the motor and supply voltage are well known. As the current is not measured, no disturbance can occur.