Design and validation of a thermo-mechanical Self-Calibrating displacement sensor for embedded applications

Structural deformation due to thermal expansion or finite stiffness effects is prevalent in most manufacturing machinery. Structural monitoring can be achieved using direct measurement of displacement or strain. Such sensors require periodic calibration to maintain traceability, and this may be impractical for an embedded solution inside the structure of a machine. [1-2] discuss a novel low-cost design of sensor capable of long-range strain measurement to monitor these effects. The current design of these stain sensors has shown to have sub-micron resolution and repeatability once calibrated but follow up calibration of these sensor has the potential to be difficult and problematic.
This paper presents a new design of the displacement sensor that incorporates an in-situ self-calibration function negating the potential future need to disassemble and remove the sensor assembly from the machine it is monitoring. This paper reviews the design and preliminary FEA analysis of the thermal actuator mechanism that can provide self-calibration, whilst maintaining the sensors primary functionality. Initial bench tests results are also presented.
The system utilises a system of slave/master displaceable caddies, which allows the sensor head to be displaced when subjected to an external force in normal operation but can also be independently manipulated thought its measurement range by the thermal expansion of a calibrated aluminium slug. This initial validation has shown that this system can allow for the recalibration of the sensor in-situ, currently with an uncertainty in the order of 1.3µm. By calibrating the thermal expansion of the slug and accurately monitoring the inputted temperature, by way of PT100, this uncertainty can be decreased.