Measuring voice coil impedance on a (power and space) budget

Voice coil motors (VCMs), alternatively known as non-commutated DC linear actuators, are a type of direct drive linear motor. This simple electromechanical linear motor is 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. Technically, the 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. Knowledge of the mechanical parameters in VCM systems enables health monitoring and accurate control that can enhance overall performance of devices containing VCMs.

At present, a miniaturized electronics system that monitors the electrical impedance and identifies mechanical parameters of a VCM has not yet been presented in literature. To address this, Dr. Lucas Koerner and Dr. Thomas Secord at the University of St. Thomas proposed a solution using the AD5933, i.e. a miniaturized integrated circuit impedance analyzer that consumes only ∼30 mW. To be precise, the two scholars detailed an analytical model that could relate the electrical impedance to mechanical parameters and demonstrated a miniaturized electrical impedance analyzer for VCMs designed around the AD5933 integrated circuit. Their work is currently published in the research journal, Sensors and Actuators A.

In their approach, they surrounded the AD5933 with support electronics to create an instrument that measures impedances of 5 ohms at frequencies approximately10 Hz and used this instrument to measure VCM impedance versus frequency. They also used an analytical model that related mechanical parameters to the electrical impedance so as to identify VCM mechanical parameters from a non-linear least-squares fit to electrical impedance measurements. Altogether, they demonstrated the instrument and the mechanical model through two experiments.

The authors reported that their modified electrical system could measure impendences of a typical VCM coil of 10 ohms with a signal-to-noise ratio of 84.7 dB. In addition, 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%. Although a departure from harmonic motion was observed at low velocities, they showed an enhanced model that still accurately describes these conditions

In summary, a repeatable and miniaturized instrumentation system for measuring the impedance of VCMs operating in frequency ranges typical of actuation applications was developed. The study by Dr. Lucas Koerner and Dr. Thomas Secord demonstrated reliable calibration based on known resistance values and prescribed high pass filter characteristics. Given the simplicity of their model identification algorithm, straightforward implementation of their system is anticipated as an online diagnostic tool for VCM health monitoring and control system adaptation.