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Accurate feedback control of the nanoindentation process is particularly challenging from the perspectives of many orders of magnitude change in contact stiffness and the use of the ramp/hold/ramp protocol. The conventional proportional-integral-derivative feedback control algorithm is not well suited for such an application. Here we provide a description and present performance data for a newly developed digital control algorithm that augments the familiar proportional-integral-derivative routine with an adaptive feedforward control having inputs related to open-loop device physics and encountered contact mechanics. This novel approach results in reproducing even a steep demand ramp with minimal feedback error. Additionally, we discuss interesting observations made with respect to the varied displacement-controlled nanoindentation responses of materials. Metals, in particular, are found to expose especially rich nanomechanical phenomena when the force-displacement curves are measured under displacement control. The findings of this study nicely illustrate the decided scientific advantages of displacement control over load control.

Challenges and interesting observations associated with feedback-controlled nanoindentation