High-Efficiency Multiphase Capacitive Power Transfer in Sliding Carriages With Closed-Loop Burst-Mode Current Control

Abstract

Sliding carriages or gantries that use journal bearings for support are common in industrial automation, e.g., assembly lines, spindle heads, and xy tables, and their moving parts are powered through a flexible wireway or cable track. This paper eliminates the cable connection by using the parasitic capacitance of the stock sliding bearings to capacitively transfer power between stationary and moving parts. At high frequency, ac current can safely pass through the journal-rail capacitance where it is ultimately rectified on a carriage/gantry for final use. A 3.4-MHz resonant voltage sourced inverter utilizing gallium nitride (GaN) high-electron mobility transistors in a three-phase configuration enables this approach. The salient features of this paper are: 1) characterization of commercial-off-the-shelf bearings; 2) development of a three-phase matching network between the tank circuit and GaN inverter for high efficiency; and 3) control of the dc output current on the secondary (moving carriage/gantry) side using only primary (stationary) side sensed quantities and burst-mode control. A controller developed from a small signal operating point model uses average bearing displacement current, measured by ac sense coils embedded in the tank inductors, to accurately regulate dc output current. The experimental tests at a tank resonance of 3.4 MHz demonstrate >100-W power transfer through an off-the-shelf bearing to a sliding carriage load with 95% efficiency. This system, when paired with a burst-mode frequency of 10 kHz, demonstrates current regulation on the carriage load with a bandwidth of 200 Hz.