Resonator-based touch-sensitive probe

This paper presents a design of a simple low-cost resonator-based touch-sensitive probe. The design exploits the fact that when a stiff element (probe) oscillating near the resonance frequency comes into physical contact with the surface of another body (workpiece), the frequency of vibrational resonance of the probe changes depending on the latter's stiffness. This change can be detected by monitoring the variations in the phase shift between the probe driving force and the resulting strain, which consequently prompts the trigger signal. Standard phase-lock-loop oscillator circuitry is used to adjust the drive frequency to maintain a pre-set phase difference between the input and output signals, thus ensuring that the oscillator remains locked at the probe resonance frequency as contact takes place. A prototype probe has been designed and tested. The probe is excited to resonate at its fundamental frequency by employing piezoelectric drive and pick-up arrangements. The operating frequency of the probe is set at 25.179 kHz (second-mode frequency), which seems to be an adequate choice for excellent static and dynamic characteristics. The shift in the natural frequencys the contact force progresses follows a non-linear trend before it saturates. The maximum frequency shift measured is about 2 kHz, which corresponds to 140 mN contact force in the axial direction, whilst the minimum detectable trigger force is 3.8 mN.