Visual measurement of MEMS microassembly forces using template matching

This paper describes a new visual force sensing method for measuring microforces acting upon the jaws of passive microgrippers used in the assembly of MEMS devices. The importance of force measurement during microassembly is to confirm the microgripper-micropart make a successful grasp and to also protect the microparts and microgripper from excessive forces which may lead to damage during the assembly process. In the proposed approach, the force measurement problem is reduced to a problem of determining the microgripper jaw displacement. A 3-D finite element model is developed to study the relation between the forces and the displacements. The resulting nonlinear force-displacement relationship is fitted into a second degree equation. Computer vision is used to measure the relative displacements of the right and left microgripper jaws with respect to the microgripper base during assembly. Patterns that were introduced to the microgripper during the design phase are used to measure those relative displacements through pattern identification. Two-dimensional pattern identification is performed using normalized cross correlation template matching, to estimate the degree to which the image and pattern are correlated