MoveDone Torque Foldback
This feature was designed to allow the SSt servo drive to excel in three application areas:
- To allow axes to position accurately to a point and then become compliant ("allow itself to be pushed around" within user-defined limits), typically when a pin is engaged for mechanical registration.
- When used with axes that have compliant drive mechanisms and high static friction, this feature eliminates motor overheating and shutdowns that normally occur with other servo systems under these circumstances.
- MoveDone Torque Foldback provides a convenient, automatic method to clamp (apply a fixed force) at the end of a move.
The MoveDone Torque Foldback feature reduces the torque available to the servo automatically once it detects that the move is complete.
You can see how this feature can be useful if you consider the need to allow the endpoint of an axis to be compliant or when it is necessary to clamp something with the axis under control at the end of the move—all without complicated software or encoder processing. It may not be immediately obvious why it would help with axes that would otherwise have motor overheating or shutdown problems, so let’s expand on this:
It is not uncommon for inexpensive mechanics or long-travel axes to have significant compliance in the drive train and significant stiction (break-away friction) at the load. This will happen with heavy cantilevered loads run by belts or when a worm gear box is driven with a compliant coupling, etc. In these situations, the load will periodically stop very near (close enough for the application), but not at the commanded position and then "stick" there (because, for example, it has plain linear bearings under a large load.) A typical servo system responds by adding torque until the motor is at the exact position commanded. However, it is often that this torque is not enough to free the stuck load. So, instead of moving the load, it just winds up the compliant mechanics (stretches the belt, etc.). The motor continuously uses torque to do this and heats up, reducing its continuous output capability and often intermittently shuts down because its ratings are exceeded. Using MoveDone Torque Foldback in these situations will drastically increase the system reliability because this bad behavior is actively suppressed by limiting the torque at the end of the move. In addition, the motor will run cooler or be able to be reduced in size and wear on the axis due to chronic wind-up (tension) is reduced.
With other digital servo drives, the only way around this problem is an expensive mechanical retrofit that may not be required for the application. Rather, it is required to keep the servo system "happy". Alternately, complicated software workarounds may need to be employed with traditional servo control boards. The SSt servo drive’s MoveDone Torque Foldback feature eliminates the need for either of these expensive solutions, helping keep material costs low while simplifying control software development and enhancing design flexibility.
In existing open-loop stepper motor applications where the mechanics exhibit this behavior, this effect is likely to go unnoticed until you try to implement a servo system. MoveDone Torque Foldback eliminates the risk of this problem.
The MoveDone Torque Foldback feature (together with the RAS, Hard Stop Homing, Limit Switch Homing, Torque/Force Foldback Input feature and flexible input resolution) enables the use of simple low cost controls that lack encoder feedback, even in machines that have less than perfect mechanics or where automatic endpoint clamping is required.