Controlling force and displacement: Instrumentation design and application in thermal actuation and nanoindentation

The motivation of this research is to build systems that precisely control displacement in the presence of external load (both linear and rotation force). To achieve this, two experimental platforms have been created. Those instruments incorporate actuation stages, rotary apparatus, and rotary encoder and displacement sensors. During experiments the temperature of the stage and the environment are recorded. Characterization of these processes necessarily requires generation and monitoring of forces and measurements of displacement, rotation, and environmental temperature. The actuator methods include thermal expansion and piezoelectric (PZT) actuation, and the displacement sensing includes optical knife-edge and capacitive gage sensors, and the rotation sensing includes digital camera and rotary encoder. An automated control strategy comprising PID closed loop control (for heating) and On/Off switching between air and mist control (for cooling) is described for the thermal expansion actuator. The translation stage of this study produces a displacement range of 100 µm and 200 µm (using 240 W and 480 W power sources) in the presence of preloads up to 1 kN. Depending on the power source used, the root mean square (rms) controller error at steady-state is within 15 nm (240 W) and 35 nm (480 W). The PZT actuation stage has a displacement range of 16 μm with a resolution of sub-nanometer, and this is used to generate a penetration force between a sample and a diamond indenter tip and this force passes through, and is measured by the load-cell stage. Numeric experiments have been performed with penetration depths from 200 nm to 2000 nm and with penetration forces between 20 mN to 200 mN.