Manipulation and puncture forces on biological cells with diameters of a few hundred microns are on the order of micro-Newtons (µN's). Manipulation forces as a result of pushing manipulations on micro- and meso-scale parts are also on the order of micro-Newtons. There are no low-cost, reliable, off-the-shelf, commercially available force sensors to measure forces at this scale. Therefore, we are developing force sensors to resolve forces at this scale to use for these types of microrobotic manipulation tasks. It is desired to have as few as possible extra parts cluttering the workspace and interfering with the manipulation tool. In order to take advantage of the pre-existing components of a typical manipulation system, a compliant mechanism, computer vision based, force sensing device has been developed. From observing the deformation of a calibrated structure as it interacts with an object that it is manipulating, the actual manipulation force can be extracted. The force sensor is directly mounted to the micromanipulator at one end, while the other end is used to manipulate the parts. The device is designed with fiducial markers that can be tracked in two dimensions in the images from the CCD camera, providing two dimensional (in the XY-plane) µN level force sensing. Thus, only the tip of the device is required to be present in the field of view of the microscope. Due to the image size and microscope objective, the desired resolution for the force sensor is = 0.25 µN/pixel. This corresponds to a maximum stiffness in each direction of 0.0475 N/m. The design topology is inspired by traditional MEMS suspension mechanisms found in accelerometers and resonators made from silicon wafers. However, silicon wafers are much too stiff to produce a device at the desired stiffness level in the workspace constraints of the system as well as with sufficient out-of-plane stiffness. Therefore, the force sensors are made out of a much more compliant polydimethylsiloxane (PDMS) material. The manufacturing process consists of photolithography with a thick, negative photoresist to create a photoresist mold on a silicon wafer substrate. The PDMS is then poured in the mold, allowed to cure, and then released producing the finished device.
More systematic design methods for decoupling the two dimensional micro-force sensor readings has also been performed. By designing mechanisms with circular compliance and stiffness ellipses along with zero magnitude compliance and stiffness vectors, we are able to achieve our design requirements. Validation of this approach has been illustrated through the testing of macroscale prototypes as well as scaled designs for microrobotic applications. The sensitivity analysis conducted also yield insights for microfabricating such designs, which is currently underway.
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