Two main conditions in practice violate the ideal assumption of exact symmetry among the measured strains. First, strain gauges exhibit variations in their gauge factor, which is the ratio of the fractional change in resistance to strain. Second, the strain gauges are placed on areas with high strain gradients, which makes the gauge outputs sensitive to placement errors. Therefore, exact cancellation of the nontorsional components may not be achieved with the theoretical gain vector alone. This design requirement is consistent with a decoupling property of the sensor.
Torsional deflection degrades the position accuracy of the joint angle controller.
Therefore, one of the critical design challenges is to maximize the stiffness while maintaining high sensitivity. This torque sensor has two elastic elements linked to two rigid discs that carry the external loads. This dimensionless index is a decisive factor in the sensor design and should be maximized. It is independent of material properties, and captures the ratio of the local and global strains. The index is maximized in elastic structures that produce high strain concentration in torsion.