How far before the end of travel should I trigger the limit switch

Not sure if this question is in the correct forum, sorry if I have it in the wrong place.

I've read that stepper motors coast a bit before stopping.
My question is how far before the end of travel should I trigger the limit switch to allow room for any motor coasting.

I'm figure if I set the soft limits in Mach3 then the motors will be running slow enough before the limit switch trigger resulting in minimal coast.

How much room is everybody allowing? I was figuring that .100 should be enough. any inmput would be appreciated.

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Originally Posted by bkp08817

I've read that stepper motors coast a bit before stopping.
My question is how far before the end of travel should I trigger the limit switch to allow room for any motor coasting.

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The stopping or overrun for steppers should be far less than for servo's, steppers posses residual torque, torque that exists when the power is removed.
Any overrun will depends on the inertia of the machine at the rapid rate of travel, for that particular axis.
Al.

The limits switches can be fairly close to the mechanical limits as long as you ALWAYS have soft limits turned on and reference the machine before use.
The deceleration time for the steppers is in you motor tuning.
Steppers don't coast, but need to decelerated in a controlled manner, which Mach3 does once set up correctly.
During normal operation, after referencing the machine, you never will hit the limits, if soft limits are enable.
Hitting limits instantly stops the stepper motor and if running above a critical speed will overshoot steps during the abrupt stop that occurs.
As for the .100, the value used depends on how well the machine is setup, and the performance of the steppers.
I think .100 is very small, and many stepper/controller combinations, at speed can require much more than .100
A rubber donut on the leadscrew can make a compact bumper, at the expense of losing a little travel.
If using high powered motors this is good insurance against damage.
I have seen older machines with a coil spring that gives a controlled energy absorption during a limit stop crash.
The spring idea I have seen on Pratt and Whitney stepper driven mills - circa 1970's
My powerful machine will use donuts. 6000mm/min with 25mm screws can do damage.

If you have Softlimits on, then you should never hit the switches.
The limit switches should be considered a safety device in case the Softlimits don't work, and the switches are hit at full speed. The amount of space you need would depend on how fast the machine is and how heavy it is.

The hardest part of using soft limits, I have found, is the discipline of ALWAYS turning them back on if they are turned of manually for some temporary reason, like sneaking a little extra travel at the end of the axis, and using the safety margin,
needing to jog off a homing switch that is not at the end of travel is also something that can occur.

It also depends on whether the O.T. limits are soft limits (S/W detected) or hardwired into drive enable or e-stop? which is usually the case when servo's are used, steppers not so important.
Strictly speaking the control should no longer be responsible for control once the limits are hit.
Al.

Just thinking out loud, if you have adjustability in the limit stops you could set them up a far distance off. Then set up a long travle dial indicator and zero it right where the switch activates. Then rapid the machine down (with soft limits turned off) and use the indicator to see how far past the trip location the machine goes. Add a little bit for a safety factor and I would think it would be good.

Quote:

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Originally Posted by bkp08817

Not sure if this question is in the correct forum, sorry if I have it in the wrong place.

I've read that stepper motors coast a bit before stopping.
My question is how far before the end of travel should I trigger the limit switch to allow room for any motor coasting.

I'm figure if I set the soft limits in Mach3 then the motors will be running slow enough before the limit switch trigger resulting in minimal coast.

How much room is everybody allowing? I was figuring that .100 should be enough. any inmput would be appreciated.

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That figure seem right,but I ve installed my hard wire estop right after the limits on all axis. That way if i jog accidentally in my limits switch and reset and accidentally again jog the wrong way pass the limits it will estop before hitting the end of travel.It happened more than one might think.
So in my case .1 inch after my estop (second microswitch)