Velocity control with class D amplifiers

Abstract

A SONAR array’s radiation pattern is affected by the acoustic interactions, which may limit the effectiveness of beamforming algorithms when transmitting. A negative feedback system with a velocity sense signal fed back to the power amplifier can mitigate the array interactions proportional to the loop gain, and be effective across a broad frequency range without requiring a priori knowledge of the input signals. The tradeoffs between motional current velocity control and accelerometer based velocity control are summarized. Class D switching amplifiers can achieve greater than 90% efficiency and are increasingly being used to drive SONAR arrays. When a velocity control system is used with a Class D amplifier, feedback stability becomes a significant concern due to obtaining the feedback signal after the amplifier’s LC output filter. The array equations are still obtained by converting the amplifier into a Thevenin equivalent force and output impedance, which now includes the amplifier’s output filter and the synthesized impedance due to the velocity control loop. Sample beampatterns both with and without velocity control are shown, concluding that velocity control is well suited for use with complex dynamic transmit beamforming.