Milli Newton force measurement and control for micro ultrasonic machining

Ultrasonic machining is performed using closed-loop Z-axis position and force control. Micromachining requires tools smaller than 100 micrometers in diameter, necessitating the use of forces in the low mN range. A precision Z-axis drive has been developed using an aerostatic bearing, contactless position measurement, and contactless force actuation and measurement. The latter is performed with a voice-coil actuator, with current measurement and force-constant calibration which permit the control and measurement of forces in the low mN range. The calibration method is meant to eliminate force estimation error error due to the nonlinear voice-coil current/force relationship and A/D converter gain and offset errors. The simplicity of the tested machine should result in low machine cost, while accommodating a large range of tool sizes and machining forces.