Practical in-situ calibration method for the non-linear output from a low cost eddy current sensor

Increasing demand on manufacturing industry to produce tighter tolerance parts at a consistent rate means it is necessary to gain a greater understanding of machine tool capabilities, error sources and factors affecting asset availability. The machine tool spindle can be a significant contributor to both machine tool errors and failures resulting in a requirement for spindle error measurement. The use of eddy current non-contact displacement transducers is currently a popular method for measuring spindle error in a manufacturing environment. This is due to their resistance to harsh conditions where dust and coolant may be present. Unfortunately, many eddy current sensors have non-linear outputs that vary with target material and dimension. Typically, adjustments in the signal conditioning are provided to linearise the output, and calibrate the sensors for a specific target material. It is the purpose of this paper to assess current sensor calibration methods and highlight the potential for error in practical situations. To solve the problem, a method of in-situ calibration is presented, which uses the short range positional accuracy of the machine as a reference and least squares best fit of a low order polynomial. Validation is provided through the use of calibration test results and a practical example.