Back Driving Ball Screws and Dropping Loads

Revisiting last time

In our last posting we discussed the pros and cons of an acme screw. The self-locking characteristic was an important feature that prevented the load from being dropped when the servo amplifier tripped out from an over current condition. The good news was that the system didn’t need to run the test’s high duty cycle, so the application design was fine as it was. But what if it needed to operate at the high duty cycle? The application may have required a ball screw instead of an acme screw.

Vertical ball screw application

The ball screw story isn’t quite as involved as the acme screw story. For simplicity sake let’s call It a high duty cycle, single axis, vertical application with a stepper motor. The power goes out in the building and the stepper motor’s holding torque drops to close to zero. (A stepper motor has a small amount of torque called “detent torque” even if it’s not energized. That is why it doesn’t go all the way to zero.) The load however, is heavy enough to causes the ball screw to back drive and the load drops. If the load or the work piece under the vertical axis is an expensive item, (think of dropping a diamond grinding wheel on to the lens for the Hubble telescope) then dropping it is not be a good thing. If the load or the work piece is inexpensive and you could afford to lose one or both if it dropped, then OK, let it drop.

Brake the shaft on power failure

To prevent the drop a “power off” or a “failsafe” brake could be attached to the ball screw. The brake releases the screw’s shaft as long as there is power and grabs or brakes the shaft as soon as power turns off.

What other failure modes can you think of that you might have to protect against?

We covered a power outage scenario, but what would happen if the motor’s coupling breaks?

I’ve seen that happen, so it’s a real scenario.

Brake the shaft with an encoder

The motor is no longer connected to the ball screw. It’s free to spin and if the load is heavy enough it will fall by back driving the ball screw. The solution to prevent this from happing is more involved. We have to add a rotary encoder to the ball screw’s shaft or a linear encoder that’s attached to the slide’s table. These components are in addition to the brake. And we also need a control that monitors the encoder and can actuate the brake if it ever sees encoder counts clicking away when the stepper motor is not moving.

This application takes advantage of the high duty cycle and efficiency of a ball screw, but adds cost and complexity by requiring additional components such as the power off brake, encoder and the electronic intelligence to control the brake.

Read more in the next post, where we calculate the reflected inertia for linear systems.