Active vs passive probe proving experiment.

A sewing needle tip interacts with a magnet. In passive mode, owing to the high compliance of the probe, the interaction becomes unstable when the gap becomes smaller than 0.8 mm, whereas, in active mode, the interaction remains tractable down to 90 µm, until the actuator saturates. In the active mode, approach and retraction curves fall on top of each other, denoting absence of hysteresis, and the gap may be arbitrarily small, subject to actuator saturation (±400 µN). In the passive mode, the measurement is erroneous owing to uncontrolled probe dynamics.