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.
Schematic of Vision-Based Micro-Force Sensor Concept
Prototype Vision-Based Micro-Force Sensor: PDMS compliant mechanism with beryllium copper base (left); Schematic showing part dimensions (right).
Prototype about to manipulate a meso-scale part (top); Picture of force sensor on a penny (bottom).
Movie of force sensor manipulating meso-scale part in the microscope.
Extracted micro-Newton level force data from manipulation test.
Systematic, Decoupled Designs
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.
The design domain for the force sensor showing the rigid probe and the workspace around it.
Steps in the design.(a) Step 1. Design the symmetric half: symmetric half with Center of Elasticity (CoE) along the horizontal. (b) Step 2. Rotate the symmetric halves and combine: (i) symmetric halves rotated by theta, (ii) addition of stiffness coupling vectors, and (iii) addition of stiffness ellipses. (c) Step 3. Include the rigid probe: adjust design to accommodate the rigid probe in between the symmetric halves. (click picture to enlarge)
Scaled macro-scale prototypes for experimental testing.
Deformations for X and Y forces for three different decoupled, equal stiffness designs.
D. Cappelleri, G. Piazza, V. Kumar. “Two-Dimensional, Vision-Based µN Force Sensor for Microrobotics”. Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), Kobe, Japan, May 12-17, 2009.
D. Cappelleri, G. Krishnan, C. Kim, V. Kumar. “Towards the Design of an Optimized, Decoupled, Two-Dimensional, Vision-Based µN Force Sensor for Microrobotics”, Proceedings of the ASME International Design Engineering Technical Conference (IDETC), San Diego, August 29 - September 2, 2009.
D. Cappelleri, G. Krishnan, C. Kim, V. Kumar. “Towards the Design of a Decoupled, Two-Dimensional, Vision-Based µN Force Sensor”, ASME Journal of Mechanisms and Robotics, Vol 2, Issue 2, May 2010.
D. Cappelleri, G. Piazza, V. Kumar, "A Two Dimensional Vision-Based Force Sensor for Microrobotic Applications", Sensors & Actuators: A. Physical, 171 (2011) pp. 340-351.
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