Sensing Nanonewton Level Forces by Visually Tracking Structural Deformations

When assembling MEMS devices or manipulating biological cells it is often beneficial to have information about the force that is being applied to these objects. This force information is difficult to measure at these scales and up to now has been implemented using laser-based optical force measurement techniques or piezoresistive devices. In this paper we demonstrate a method to reliably measure nanonewton scale forces applied to a micro scale cantilever beam using a computer vision approach. A template matching algorithm is used to estimate the beam deflection to sub-pixel resolution in order to determine the force applied to the beam. The template, in addition to containing information about the geometry of the beam, contains information about the elastic properties of the beam. Minimizing the error between this elastic template and the actual image by means of numerical optimization techniques, we are able to measure forces to within +/3 nN. In addition, we also discuss how this method can be generalized to measure forces in elastic configurations other than a simple cantilever beam using a micro-tweezer as an example. This provides the opportunity for this method to be used with specially designed micromanipulators to provide force as well as vision feedback for micromanipulation tasks.