Design and Experimental Investigation of a Force Sensor for a 3D Electrothermal Microgripper

This paper presents a force sensor designed for a three-dimensional (3D) electrothermal microgripper. The novel force sensor structure incorporates a strain gauge attached to a compliant beam segment to accurately measure the forces exerted at the microgripper tip. Firstly, a comprehensive theoretical model for the force-strain-voltage relationship of the sensor is established. The model consists of two parts: force-strain with the static analysis of a flexible hinge resembling a composite beam, and strain-voltage with the physics of the sensing interface. In the force-strain part of the model, two equivalent transformations (the equivalent transformation of composite beam cross-Section to T-shaped beam cross-Section and the equivalent transformation of unconnected beam cross-Section to connected beam cross-Section) are used to establish the theoretical formulation between the gripping force and the sensor strain of the microgripper. The model is verified through the finite-element simulation and further validated through experimental testing of an in-house developed single finger with a comparison. According to the experimental results, the force sensor can detect the gripping force in the millinewton range. Finally, three force sensors are integrated into an in-house developed 3D electrothermal microgripper and the force detection with the proposed sensor design is further demonstrated with a series of micromanipulation experiments. The results reveal the dynamic variations in the micro-gripping force during 3D micro-manipulation due to the vibration of the mechanism, the relative sliding motion, and the gravity of the microspheres.