Passive Z axis brake - ** idea / theory only **

1

I know that a few people have had an issue with their Z-20 slider creeping down (a.k.a. ‘back-driving’ the screw) from the weight of a hefty spindle with motor power off. I do not have that issue, as my spindle is a relatively small BLDC spindle. But I have given it thought just the same.

Anyone who has played much with stepper motors is familiar with how the motor turns without much resistance with no power hooked up. However, if you short out either of the 2 motor coils, A or B, makes no difference, the motor suddenly becomes very resistant to turning. Probably enough to prevent the gravity back-driving, I’m guessing.

So how do you accomplish this? With the e-stop button. There are 3 contacts on a typical e-stop button, and normally only one set is used. With the button pulled out, it creates a NC condition between the contacts. The other contact is now at a NO condition. With the button pushed in, it toggles these NC/NO conditions.

I can’t speak for Buildbotics, but this is the normal ‘run’ condition on my machine with an Acorn controller and Gecko G540 driver. It’s looking for that ‘closed’ condition that applies holding torque to the motors.

If one were to splice into the Z wire and create a NC condition (button pushed in) across a pair of coil wires, it would effectively lock the Z axis motor when with the button pushed in. ‘Resetting’ the button (out position) would remove this connection at the same time holding torque is being applied to the motor.

Disclaimer: this is only a conceptual idea. My controller has no voltage applied to the e-stop button, so this would be safe to do in my case. Again, I can’t speak for the Buildbotics or Masso controllers. This is just an idea that would remove the need for a special motor with an active brake, and should work on virtually any stepper motor.

2

I can’t find the link but there is a circuit available that does what you are describing - forget who sells it.

Using the current E stop connected to a relay would work. The Masso relay is relatively expensive but a useful addition for Elite owners who can mount it into the Touch. It would be easy to add this functionality.

Here is a patent doc that explains it - https://patents.google.com/patent/CN107959448A/en

3

I think it would work without the relay. That just adds unnecessary complexity to the circuit, unless it’s necessary to keep that circuit isolated.

4

Hey Bill,

this is a known way to prevent motion on a powerless stepper. Someone already proposed that in one of the numerous “spinlde drop issue” threads. However it is not really comparable to a brake that acts immediately, as it needs to have the motor first move a bit to create the current (acting as generator) that then will stop it again.

2 Likes
5

Hey Aiph5u,

I see, I had not actually considered that, but that is fact.

6

I have a working circuit board that monitors the stepper currents for both an over current and under current condition. I haven been able to truly test it as I’m waiting for my stiffy to show up. I’m also upgrading to drag chains. Just moving it around manually and creating a fault condition it triggers the brake. I designed it for the Buildbotics controller.

brake board
brake board1920×2162 284 KB

1 Like