Back EMF Problem Stepper Motor / Line Follower Robot

Mister G. (mister_garfield) OP 2023-10-17 22:23 #7517889

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Hello all,

I have built a line follower robot with stepper motors to drive the wheels. It is controlled with TMC stepper motor drivers from the 3D printing area and a Teensy / Arduino microcontroller. It is powered by an 18 volt drill battery.

As far as everything works wonderfully, however, I have now encountered the following problem: If I move or drive the car by hand when it is switched off, the stepper motors behave like a generator, i.e. various LEDs of the circuit start to light up.

I am therefore concerned that this can damage the controller, drivers or battery due to this reverse current if you move the trolley too fast.

So now the question:

What is the best way to get the coils of the stepper motor disconnected from the rest of the circuit when it is off?

I have now read quite a bit about relays, optocouplers, solid state relays, mosfets etc.

So for me as a hobbyist / electronics layman it is hard to assess what would be the best / easiest solution to the problem.

I would be very grateful for helpful tips.

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Lothar M. (lkmiller) Moderator (Company: Titel) 2023-10-18 07:32 #7518006

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Mister G. wrote:

I am therefore concerned that this can damage the controller

It depends on the controller, but usually it should be able to handle the voltage and the current if its not higher as the usual working range. So at first I would do some simple tests and check wether the supply voltage overrates the battery voltage and the following step down when moving it powerd off.

What is the best way to get the coils of the stepper motor disconnected from the rest of the circuit when it is off?

If you want to get it really disconnected with no concern about voltages and currents you must use a mechanical relay. Becaues an open mechnical contact is the only mean to get absolutely no current back into your circuit.

                 decoupling 

  + -------o------>|---o----.

       stepdown       ===   |

battery    |           |    |

          µC ----------+-> driver ===> motor

  - -------o-----------o----'

what would be the best / easiest solution to the problem.

The "easiest way" would be: do nothing. Most of the devices you can buy are like that.

The "easy way" would be to seperate the supplies of the motor driver and the logic with a diode, so that the inducted current cannot find its way to logic. And if the inducted voltage then overrates the motor driver you can add a resistor with additional circuit (like a crowbar) to absorb the energy.

The "hard but absolutely safe way" would be the mechanical relays.

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