Electromagnets calibration utilizing the pull-in instability

Precise determination of electromagnetic characteristics is crucial for many applications employing magnetic actuators such as magnetic bearings and precision machine controls. Traditionally, this task is achieved by the constant air gap type of calibration. Although this method can achieve high accuracy, the requirements for dedicated instrumentation and precise machining impose limitations to most users. In this article, an alternative approach utilizing pull-in instability to calibrate electromagnets is presented. The electromagnet to be calibrated exerts a force on the tip of a cantilever beam and causes deflection. The deflection-current curve is recorded to obtain the exact pull-in current. Finally, the electromagnetic force constant can be derived from the pull-in current, the initial air gap, and the equivalent stiffness of the cantilever beam. Experimental results showed that the calibrated force coefficients agreed with those obtained from the standard constant air gap method within 3–10% accuracy. In comparison with the constant air gap method, this approach is potentially cheaper and can achieve an accuracy level sufficient for subsequent robust control purpose.