Run a stepper from 2 DM542 drive

[Doddy]

Ignoring the behaviour of the energised coils causing back-emf when disconnected (Jazz's reply) - there's another issue at play here - the stepper drivers would have their own instance of the X/Y coil phasing - swapping between controllers would introduce a discrete change in motor armature position - which I would expect is not desired. That and blowing up the drivers.

As previously explained - use a control system that allows the step/dir to be multiplexed. If using open-collector drives it's possible to wire-or the cathodes on the driver step/dir inputs.

[BeagleBrainz]

If you disable the drive before switching the switching the STEP & DIR it may help with not generating any spurious steps. Just don't forget to re-enable the drive before kicking off again.

[Snowtech]

But, depending on the weight, disabeling the Z-axis may cause it to lose position.

[BeagleBrainz]

Yes a very good point.

[A_Camera]

If losing position after switching is a problem then you need to zero after each switch. Regardless how you do it you can't expect that the position is the same after switching.

However several people mentioned switching only STEP and DIR. That will not work well. You MUST also switch the GND as a minimum, but if you use the EN for any of the drivers then you must include that also.

[Doddy]

If you disable the drive before switching the switching the STEP & DIR it may help with not generating any spurious steps. Just don't forget to re-enable the drive before kicking off again.

Not intended to drag this conversation onwards, but for clarity on the behaviour of a stepper driver:

A stepper driver can be described as a finite-state machine. It will present a series of currents to each of the two (e.g.) coils according to the number of micro-steps selected. The following is an extract from the data sheet for the TB6600 - though the principle applies to any stepper driver.



You should be able to see that for a 1/8 micro-step, there's 32 discrete states for the A/B coil currents. Whether you move CW or CCW, you're just stepping either forwards or backwards in the state-machine ("phase", in my original post).

My point?, if using two stepper drivers, each will maintain it's own internal state as its operated. If you swap the outputs from one driver to another - unless there's the random fluke of each driver having the same state, there WILL be a discrete change in the coil currents appropriate to the different phase in the state machine that WILL result in a discrete change in shaft position. There's no mechanism available to synchronise the drivers. Disabling the drivers before switching the STEP/DIR will have no impact on this.

A_Camera's point of switching grounds is valid - or my lazy assumption that you share a common ground.

[gabi68]

Skip one driver and switch the control signals step, dir, enable and gnd. This is what I am doing on my DIY lathe / 4th axis combo. I would not switch the stepper wires. Beware that if you have some interference then you might get some extra steps, so you must be careful with the wiring.

Can you be more specific? A diagram will be awesome. Thank you

[gabi68]

Woa. The forum system didn't send any emails regarding a new post on my thread. Thank you everybody for your opinions. Regarding the accuracy, a step or two will not harm (this is a fiber laser not milling machine) the focus is forgiving, but a disabled driver will cause the Z axis to fall, not good.
What is the best solution for this situation? Anything already made it out there?

Thank you
Gabi

[dazp1976]

Woa. The forum system didn't send any emails regarding a new post on my thread. Thank you everybody for your opinions. Regarding the accuracy, a step or two will not harm (this is a fiber laser not milling machine) the focus is forgiving, but a disabled driver will cause the Z axis to fall, not good.
What is the best solution for this situation? Anything already made it out there?
Thank you
Gabi

A motor with a brake then?.
Engage / disengage brake as you switch?.
I'm prob clueless and talking crap though.

[BeagleBrainz]

Not intended to drag this conversation onwards, but for clarity on the behaviour of a stepper driver:

A stepper driver can be described as a finite-state machine. It will present a series of currents to each of the two (e.g.) coils according to the number of micro-steps selected. The following is an extract from the data sheet for the TB6600 - though the principle applies to any stepper driver.



You should be able to see that for a 1/8 micro-step, there's 32 discrete states for the A/B coil currents. Whether you move CW or CCW, you're just stepping either forwards or backwards in the state-machine ("phase", in my original post).

My point?, if using two stepper drivers, each will maintain it's own internal state as its operated. If you swap the outputs from one driver to another - unless there's the random fluke of each driver having the same state, there WILL be a discrete change in the coil currents appropriate to the different phase in the state machine that WILL result in a discrete change in shaft position. There's no mechanism available to synchronise the drivers. Disabling the drivers before switching the STEP/DIR will have no impact on this.

A_Camera's point of switching grounds is valid - or my lazy assumption that you share a common ground.

Very good points.

As long as the outputs driving the STEP & DIR are at a logic LOW (assuming common) is ground there should be an issue, but if the outputs are at a logic HI and using a mechanical switch to change over disabling the drive would prevent spurious steps, due to bounce.

All in all a better mechanical design would help.

Actually it would be worthwhile knowing the circuitry that drives the STEP & DIRECTION, before really delving in too deep. Considering that the design requires switching 2 different sources for driving the stepper who knows how the electrical side has been done.