An embedded electrical impedance analyzer based on the AD5933 for the determination of voice coil motor mechanical properties

Abstract

The voice coil motor (VCM) is a simple electromechanical linear motor used in space constrained applications such as pumps, precision positioning, and mobile camera lens actuation. The motion of a VCM is determined by the applied current as well as the mechanical parameters of mass, spring constant, and damping coefficient. VCM motion and mechanical parameters can be determined by a position sensor, but such a sensor may be too bulky for a miniaturized solution. To overcome this limitation, measurements of the VCM electrical impedance versus frequency can be combined with an electromechanical model to identify mechanical parameters. Here we detail an analytical model that relates the electrical impedance to mechanical parameters and demonstrate a miniaturized electrical impedance analyzer for VCMs designed around the AD5933 integrated circuit. The electrical instrument measures impedances of a typical VCM coil of ∼10 Ω with a signal-to-noise ratio of 84.7 dB. We display the effectiveness of the analytical model and impedance analyzer by identifying mechanical parameters of mass, spring constant, and damping coefficient using electrical impedance measurements alone. We experimentally modified the system mass and detected the changes using electrical impedance with a mean error of 5.6% and a Pearson's correlation coefficient of ρ = 0.95. Repeated measurements of a single VCM configuration demonstrated that the natural frequency, knowledge of which is critical for optimal efficiency, was detected with a variation of 0.2%. A departure from harmonic motion was observed at low velocities. We explain this departure by adding static friction to the model of VCM motion. The AD5933-based miniaturized VCM driver and impedance analyzer coupled with a model that relates mechanical motion to electrical impedance is a viable instrument for in-situ diagnostics and tuning of VCMs.