Measuring displacement of a point or surface, rather than measuring the deformation of a flexure, offers many possibilities for compact, low-cost load cells. This page gathers some resources on this topic and presents work towards a simple six degree-of-freedom (DOF) loadcell based on these ideas.
Force measurement is almost always done by measuring the strain of a structure. Most commonly, this is done using a strain gauge, which is bonded to a surface of a structural member. With good bonds, the strain seen by the member is also seen by the strain gauge and a change is resistance is measured. This approach has long been favored because it produces measurable results while allowing the strained member to remain as stiff as possible (and hence, not interfere with the operation of the measured device).
We can also measure strain of a member by measuring the displacement of a point or surface, rather than measuring the material strain directly. This can be advantageous because it eliminates uncertainty from the bond between the member and a strain gauge. These bonds can exhibit hysteresis, can be temperature dependent, and can be a point of failure of the device. If we measure the displacement of a point or surface using a non-contact method, we can circumvent many of these disadvantages.
Non-contact position measurements have historically not had high enough resolution for load cell applications, but have been used commonly for applications that can tolerate larger displacements. For instance, joystick inputs for machines are often sensed using a non-contact magnetic field measurement (c.f. US5831596A). Similarly, many automotive applications (gas pedals, shifter positions, etc.) that sense large displacements are turning to non-contact measurement for the robustness it offers.
