BEMF and stepper motor reversing as a braking method?

[Oldmanandhistoy]

I have recently bought a break out boards from the US. While looking through the pdf file and reading about E stop setup I came across this:

Quote from the set up document:

“The following will take place upon an emergency-stop button push with this hook-up method: The stepper motors will immediately and instantly reverse direction from the direction they were moving, to aid in initial braking, and simultaneously the step signals to the stepper motor driver board will stop, and the stepper motor driver board will remain powered-on to provide the motor with power-on electrical braking.”

My question is; do I need to worry about increased BEMF using the method described?

John

[ger21]

First, a stepper motor can NOT immediately and instantly reverse direction. Something to do with physics I think.

Back EMF can damage drives if it creates a higher voltage than the drives are rated at. This can be a problem if you are running your drives at close to their voltage limit.

[Oldmanandhistoy]

Originally Posted by ger21 First, a stepper motor can NOT immediately and instantly reverse direction. Something to do with physics I think. Back EMF can damage drives if it creates a higher voltage than the drives are rated at. This can be a problem if you are running your drives at close to their voltage limit.

I was quoting their document.

So back to the question; does their badly described method increase BEMF let say if the stepper is almost stopped immediately and is almost instantly reverse direction?

John

[ger21]

I don't really understand what they are saying. I believe, though, that EMF is generated any time the motor decelerates.

[Oldmanandhistoy]

Now reading your reply gets me thinking stepper probably do what they are describing all the time while cutting parts? So if that is correct then it would not increase the BEMF at all?

John

[CuttersCov]

Originally Posted by Oldmanandhistoy I have recently bought a break out boards from the US. While looking through the pdf file and reading about E stop setup I came across this: Quote from the set up document: “The following will take place upon an emergency-stop button push with this hook-up method: The stepper motors will immediately and instantly reverse direction from the direction they were moving, to aid in initial braking, and simultaneously the step signals to the stepper motor driver board will stop, and the stepper motor driver board will remain powered-on to provide the motor with power-on electrical braking.” My question is; do I need to worry about increased BEMF using the method described? John

The part I'm confused about is why for any reason would you WANT it to stay powered on if you hit the estop? It sort of defeats the estop.

What if backing up does more damage? For example, if the router bit got hooked on something.

[daedalus]

back emf is proportional to rotor velocity, and opposes motion. When braking with an inertial load on the stepper in the way you mention back emf will add to your drive voltage, creating a momentary spike in bus voltage. Some stepper drives have connection for an external dump resistor which is used in this situation, does yours have this? If not you can get dump resistors that connect accross the psu supply, and switch a resistor across the terminals to dump power when the voltage exceeds safe limits.

Personally i would want e-stop to power off the drives, as stop should stop not only the mechanical section, but the electronics too! imagine you have accidentally severed a motor cable, you want e-stop to immediately cut power to it, not sit there.

I would only use the method they describe if your machine is taking too long to stop after e-stop is hit when wired normally. I doubt anything under bridgeport size will need to worry too much about using this method.

[Oldmanandhistoy]

Hi all and thanks do far,

As always is the way for me, 10mins after I switch my laptop off my brain thinks more clearly.

I do not have the board yet as a family member who is visiting the US for 3 weeks will bringing it back with them.

My point about, it is probably doing what they describe during normal cutting was incorrect as the stepper motors are controlled by software where acceleration and deceleration are implemented in a controlled way. The description quoted would be implemented via the BOB so I would think it is just done as fast as possible.

As to why you would want it powered on, I can only presume that it is the method used to stop axis creep and or spindle run away while the spindle is deceleration to a stop (especially if back cutting/climb cutting).

Backing up the drive or reversing I’m guessing would only be momentary.

At present I have my E stop wired to cut power to drives and spindle; I have another switch wired to stop my machine in a controlled manner using software (stops software only).

I will put my question to the board supplier and see what comes back. I should have done this in the first place but hoped it would be a simple yes or no answer.


John

[ger21]

What board is it if you don't mind me asking?

[Oldmanandhistoy]

Originally Posted by ger21 What board is it if you don't mind me asking?

I don’t mind you asking at all the reason I did not post the BOB suppliers name was I did not want to look like I was pushing their product. Far too much of that goes on IMHO.

See this attached pdf

John

[ger21]

How about the manufacturer, or where to get it? I've never seen it before.

[Oldmanandhistoy]

Originally Posted by ger21 How about the manufacturer, or where to get it? I've never seen it before.

See very bottom of pdf.

John