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Author Topic: how to control z-axis brake  (Read 12472 times)

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Re: how to control z-axis brake
« Reply #10 on: June 03, 2010, 06:51:08 PM »
A stepper is a different animal, I was referring to a servo, if a servo that has a transconductance amplifier i.e. torque or current mode of operation, and the motor is clamped and left active, the current will climb in order to attempt to position the motor within the specified following error if any occurs.
You cannot guarantee that positioning a servo to a certain position and then braking it that there will not be any servo error occur at any point.
I personally would not want to chance it and have never seen any servo system that operates under these constraints.


Re: how to control z-axis brake
« Reply #11 on: June 03, 2010, 09:39:07 PM »
I ever use the brake on servo motor system when the servo is still clamp, but i decrease the gain of PID control first....

OK..my point is using the brake to help my stepper holding the position....any good idea?
and why GetDRO(16002) always return "0"??

Offline simpson36

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Re: how to control z-axis brake
« Reply #12 on: June 04, 2010, 07:32:49 AM »
. . . and the motor is clamped and left active, the current will climb in order to attempt to position the motor within the specified following error if any occurs.
This is a function of the servo drive, and as I pointed out, some drives have the ability to automatically engage limits during the condition you describe. Some even allow you to set the range in encoder counts within which this limit is applied.

You cannot guarantee that positioning a servo to a certain position and then braking it that there will not be any servo error occur at any point.
Actually, you can. Again it depends on the servo drive you are using.

I understand the thought process that ends in "do not restrain an active servo motor", but while this advice was valid, I am finding that it is no longer universally applicable. An analogy would be the advice to 'pump your brakes when you are driving on ice' which was of course valid and taught in any driving class . . . until anti-lock brakes came along. I discover that there are some clever people out there designing servo drives that have actually though about this topic and built some interesting features into the drives. Not all drives mind you, but some. There are still a lot of cars on the road without anti lock brakes also, so 'pump the brakes' is still good advice, but not in all cases . . since pumping anti lock brakes is a decidedly bad idea.



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Offline simpson36

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Re: how to control z-axis brake
« Reply #13 on: June 04, 2010, 07:45:06 AM »
I ever use the brake on servo motor system when the servo is still clamp, but i decrease the gain of PID control first....

OK..my point is using the brake to help my stepper holding the position....any good idea?
and why GetDRO(16002) always return "0"??

From the department of redundancy department:
If you are going to go into MACH to get variables, then you have already coupled your process specifically to MACH, so why not just write a macro to engage the brake and add that to the G-code between moves (with delays if needed). That is how I implemented the 4th axis spindle lock initially and it worked fine.

My quest for an autonomous function stems only from the goal to make the operation transparent to MACH, or more broadly, to any CNC controller.

Note that I am simply commenting on your question, not endorsing any particular idea.

Re: how to control z-axis brake
« Reply #14 on: June 04, 2010, 10:49:48 AM »

From the department of redundancy department:
If you are going to go into MACH to get variables, then you have already coupled your process specifically to MACH, so why not just write a macro to engage the brake and add that to the G-code between moves (with delays if needed). That is how I implemented the 4th axis spindle lock initially and it worked fine.

My quest for an autonomous function stems only from the goal to make the operation transparent to MACH, or more broadly, to any CNC controller.

Note that I am simply commenting on your question, not endorsing any particular idea.


the part that I ever use brake on servo motor system is reply the position error for braking and servo motor active, my solution is reduce the PID so that system can hold by motor and brake together...
of course I can put M code for release brake in my process, but my stepper will lose pulse, that is why I want to know if there is a way to auto release the brake.

Offline simpson36

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Re: how to control z-axis brake
« Reply #15 on: June 04, 2010, 11:11:13 AM »
the part that I ever use brake on servo motor system is reply the position error for braking and servo motor active, my solution is reduce the PID so that system can hold by motor and brake together...
of course I can put M code for release brake in my process, but my stepper will lose pulse, that is why I want to know if there is a way to auto release the brake.

I'm afraid I don't understand what you are saying here. Also it seems you are using the terms 'servo motor' and 'stepper' interchangeably, which is not helpful.
Re: how to control z-axis brake
« Reply #16 on: June 04, 2010, 11:30:42 AM »
I'm afraid I don't understand what you are saying here. Also it seems you are using the terms 'servo motor' and 'stepper' interchangeably, which is not helpful.
forget the servo....
my problem is using step and hope system can auto release before z moving and lock when z stop....
z-axis using stepper..

Offline simpson36

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Re: how to control z-axis brake
« Reply #17 on: June 04, 2010, 11:50:39 AM »
forget the servo....
my problem is using step and hope system can auto release before z moving and lock when z stop....
z-axis using stepper..

In this case, have nothing new to add to my previous comments.  I was not aware that steppers had electronic brakes . . . .  live and learn.
Re: how to control z-axis brake
« Reply #18 on: June 04, 2010, 11:54:18 AM »
, my solution is reduce the PID so that system can hold by motor and brake together...

PID loop with a stepper??

Offline BR549

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Re: how to control z-axis brake
« Reply #19 on: June 05, 2010, 02:52:21 PM »
O K here we go again(;-)

YOU "could"  monitor the Z axis velocity and

 IF  Zvel =zero then apply output signal to apply brake

AND
IF Zvel <> zero then turn off output to release brake


BUT you run into the latency problem with using a buffered system. The Z moves are already stacked into the buffer ready to go AND when the signal comes to run there is NOTHING in machs design that will allow the buffer to hold UNTIL the brake is releases fully before the Z can safely move without a loss of steps do to the drag of the brake untill it has fully released.

EVEN IF you had an intelligent drive that could do the function you could run into an accuracy problem as MACH is assuming that ALL drive can react very closely the SAME.  Testing has shown that 1 slow reacting drive out of 3 can cause a mispositional error . Mach has NO way of knowing the timeing points of the drives it just assumes ALL the moves have been made as a coordinated move. It would depend on the resolution and amount of drive dwell as to HOW MUCH error would be involved

With "any" form of monitoring the Gcode pulses to active or deactivate BY the time it "sees" the movement it is TOO LATE to release the brake.

In the normal world brakes are used to HOLD the axis when it is NOT active/enabled. It is UP to the motor to HOLD the axis in position when it is active/enabled. Mostly to HOLD the position when the machine is OFF and to Catch the axis in the event of a power failure and loss of holding force.

IF the axis cannot hold itself you have 2 options mentioned above. Increase the motor to the proper size OR counterweight it to require LESS holding force.

Hope that helps