Challenges and interesting observations associated with feedback-controlled nanoindentation

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.