AB Linear Thruster braking

phuz · Mar 30, 2016

I'm working with two AB linear thrusters that have the 24vdc brake option. These axes are parallel and are normally coupled by an intermediate piece so they need to operate synchronously. For homing, they will detach the intermediate piece and home each axis independently. After homing, they will open the guards, and attach the intermediate piece which weighs ~70lbs. There is also torquing of bolts required, and this will cause the axis to move about 10mm sometimes, which takes them out of synchronization. We can initiate a slow jog to bring the axis back to its "0" position, but isn't there a better way to brake the axis during the E-stop condition to prevent any movement at all? We found out that, by attaching the airlines, we send the axis to its bottom position, which is not helpful.

RET · Mar 30, 2016

Sounds like you need a position encoder.

phuz · Mar 30, 2016

Encoder won't help with braking.

RET · Mar 30, 2016

phuz said:
Encoder won't help with braking.

Not braking, per se, but an out of position indication by the encoder would ideally cause the servo motor to act with torque to try to put it back in position, essentially supplementing the braking.

Or, just design a simple pin/clevis stop, with an actuated cylinder.

phuz · Mar 30, 2016

RET said:
Not braking, per se, but an out of position indication by the encoder would ideally cause the servo motor to act with torque to try to put it back in position, essentially supplementing the braking.

Or, just design a simple pin/clevis stop, with an actuated cylinder.

Sorry, this is occurring an e-stop condition when the drives have zero power, so the brake should be in place. I think there may have been an issue with a 6-pin cable being needed and only having a 4.

OkiePC · Mar 30, 2016

phuz said:
Sorry, this is occurring an e-stop condition when the drives have zero power, so the brake should be in place. I think there may have been an issue with a 6-pin cable being needed and only having a 4.

I haven't used those motors, but in most cases, you apply power to release the brakes, so they should be holding (or attempting to hold) when there is no power.

I may have it backwards in your particular application. I have seen servo motors with the brake circuit set up the opposite way although much more rarely...like only once...

phuz · Mar 30, 2016

Yes, exactly. And that's where we are confused. They hold, but can be moved with about 25 lbs of force, and a rep was telling us that they should NOT move whatsoever when its de-powered.

OkiePC · Mar 30, 2016

Sounds like you need an auxiliary method to hold them in place as suggested earlier. They are probably rated with enough braking power for most applications, and yours is one with exceptional needs.

phuz · Mar 30, 2016

OkiePC said:
Sounds like you need an auxiliary method to hold them in place as suggested earlier. They are probably rated with enough braking power for most applications, and yours is one with exceptional needs.

And that would be OK if we could get a solid answer from AB. The rep was insisting that they shouldn't be able to be moved by human force.

Pete.S. · Mar 30, 2016

Maybe the brake has been worn out or it needs adjustment.

For E-stop it often be better to do a category 1 emergency stop. That means a controlled stop by the servo motor followed by removal of power (or activation of STO) after a predetermined time.

This often results in a quicker stop and more importantly, the brakes are just used to hold the load in place, not slow it down. So a lot less wear.

phuz · Mar 30, 2016

That's exactly what we are doing.

Pete.S. · Mar 30, 2016

phuz said:
That's exactly what we are doing.

But how is the drive set up to stop?

They should not get misaligned if the servos are syncronized and the speed drops to zero.

PS. But if the drives just stop the servo motors without syncronization...then they might get misaligned.

phuz · Mar 30, 2016

Pete.S. said:
But how is the drive set up to stop?

They should not get misaligned if the servos are syncronized and the speed drops to zero.

We bring it to a stop using an MAS. The problem is when they open the doors after it is stopped and begin connecting the intermediate piece that couples the two axes together. Tightening it down causes the axis to move off the position it was on. Ergo, no longer synchronized.

kamenges · Mar 31, 2016

I would agree with Pete.S. that the brake is likely out of adjustment or the friction surfaces are contaminated with something that is significantly reducing their coefficient of friction. If I were you I would look to mechanical brake issues.

Keith

AB Linear Thruster braking

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