News + Publications

When dealing with certain electronics, such as optical breadboards or torque cells, even slight mechanical distortions can result in malfunctions.

We can mitigate distortions from mounting strain by making the mounting arbitrarily soft, but that’s not always possible because of other design constraints.

As a concrete example, consider a reaction torque cell connecting a motor to an apparatus, where both the steel motor and apparatus warm significantly during operation. Reaction torque cells are designed to give minimal spurious signal from off-axis torques and forces but don’t respond well to even tiny distortion of the flanges themselves, and differential thermal expansion and a bolted connection guarantee distortion. We had this problem on an experimental rig where thermal drift contributed more to the measured torque than the actual motor torque.

In this situation, a soft mounting arrangement isn’t viable because the motor requires support.

We solved this problem by fully constraining—but not over-constraining—the connection. The position of an object is fully specified by three spatial dimensions and three angles, for a total of six degrees of freedom. By using six constraints, the connection between the torque cell and each mating surface is fully but not over-constrained. Any distortions of the motor or apparatus flanges will cause a slight translation or rotation of the connection but no bending of the torque cell flange itself.

Conceptually, each single constraint should specify a distance only and be able to provide no moment. The ends would thus require pivots, but in this example it’s not the most economical or the most rigid. Since the angular deflections we’re isolating are small, using redi-rod that is thick enough to be axially stiff and not buckle under the motor mass, but much less stiff under a bending moment than the torque cell flanges themselves, meets the design intent.

After implementation, thermal effects were negligible.