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

This article presents a force sensor designed for a 3-D 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. First, 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 3-D 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 microgripping force during 3-D micromanipulation due to the vibration of the mechanism, the relative sliding motion, and the gravity of the microspheres.