Towards the Design of a Decoupled, Two-dimensional, Vision-based Μn Force Sensor for Microrobotics

In this paper, we present three designs for a decoupled, two-dimensional, vision-based μN force sensor for microrobotic applications. There are currently no reliable, off-the-shelf, commercially-available force sensors to measure forces at this scale, that can be easily integrated into standard microrobotic test-beds. In our previous work, we presented a design consisting of a planar, elastic mechanism with known force-deflection characteristics. It was inspired by the designs of pre-existing MEMS suspension mechanisms. A CCD camera is used to track the deformation of the mechanism as it is used to manipulate objects in a micro/meso-scale robotic manipulation test-bed. By observing the displacements of select points in the mechanism, the manipulation forces can be estimated. Here, a building block approach for conceptual synthesis of compliant mechanisms methodology is used to design for decoupled displacements for the tracking points when the tip is subjected to forces in the XY-plane. 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 with macro-scale prototypes and recommendations for scaling the designs down for microrobotic applications are offered along with a sensitivity analysis of the final designs yielding insights for microfabricating such designs. ∗Address all correspondence to this author. INTRODUCTION The real advantages of a high precision microrobot or manipulation system can only be utilized if the automated system also has high resolution sensors along with good control strategies [10]. Force sensors capable of resolving micro-Newton (μN) level forces, the typical forces encountered when manipulating biological cells and microand meso-scale parts, are generally made using microfabrication techniques. The traditional methods of indirect force measurements, like mounting strain gauges at specific locations [13, 28, 30] is hard to demonstrate at the MEMS scale because it substantially complicates the micro-fabrication process. Methods like capacitance-based indirect force measurement usually require special electronic circuitry to measure the low capacitance of femtoand atto-farads and thus a complicated microfabrication process [2,9,26,27] that can drive up production costs. Indeed there are no commerciallyavailable, inexpensive, multi-axis force sensors at this scale that can be easily integrated into a typical micromanipulation system. Our goal is to come up with a design that can be easily integrated into a microrobotic test-bed like the one shown in Figure 1 [4,6,7], which does not require any alteration of the object being manipulated or require exotic fabrication techniques and can sense μN level forces in two dimensions. With such a design, real-time controlled manipulation of micro-objects is possible. In this paper, we develop an optimal, decoupled, two-dimensional, computer vision-based, force sensing device which consists of an elastic mechanism with known force-deflection characteristics. From observing the deformation of a calibrated structure as it interacts with an object that it is manipulating, the actual 1 Copyright c © 2009 by ASME Figure 1. MICROROBOTIC TEST-BED manipulation force can be extracted. The design for the compliant mechanism consisting of the device is synthesized utilizing a combination of serial and parallel concatenation of compliant building blocks. RELATED WORK The development of micro-force sensors is an active research area. An overview of force sensing for microassembly applications using capacitance, optical, piezoelectric, piezoresistive, SFM (scanning force microscope), AFM (atomic force microscope), and laser Raman spectrophotometer (LRS) techniques is given in [21]. A review of MEMS devices used for cellular force measurements can be found in [25], while a more general summary of micro-force sensor research is presented in [4]. The commercially available force sensor AE 801 Micro Force Sensor from SensorOne (http://sensorone.com) can measure forces from 12 grams down to 12 mg (118 mN to 118 μN) in one dimension. An AFM is commonly used to measure smaller forces, in the pN to nN range. A very detailed description of the various techniques, interpretation and applications of force measurements with an AFM is given in [3]. Sitti et al. used an AFM for manipulation along with integrated force sensing at the mN level in [24]. Koch, et al. [17] designed, fabricated, and tested a compliant surface-micromachined spring in order to calibrate the lateral force field of an electromagnet on a single magnetic microparticle. Due to the high resolution microscope objective used, pN force sensitivity is achieved. The Zyvex Force Characterization Package (FCP) (http://www.zyvex.com) is capable of measuring one dimensional forces ranging from nN to mN but requires extensive (proprietary) hardware, software, as well as a high resolution scanning electron microscope to function. In regards to vision and optical force sensing, the design for a micrograting-based force sensor integrated with a surface micromachined silicon-nitride probe for penetration and injection into drosophila embryos is presented in [31,33]. Penetration forces in the μN range have been recorded and the device has also been used to characterize positioning forces on the drosphilia embryos that are self-assembled in 2-D arrays [32]. Optics are also used in [34] and [14] to sense contact forces. A minimally intrusive, vision-based, computational force sensor for elastically deformable objects is presented in [29]. Force estimation is calculated from the visually measured displacements and known material properties of the deformable object. This approach is only viable if the displacements of the deforming object can be captured accurately. Greminger, et al. demonstrates a method to visually measure the force distribution applied to a linearly elastic object using the contour data in an image in [11]. A sensor resolution of less than ± 3 nN for a microcantilever and ± 3 mN for a microgripper were achieved. Sasoglu, et al. have used highaspect ratio polydimethylsiloxane (PDMS) microbeams to sense one dimensional micro-scale forces specifically targeted for single cell studies [23]. Similar PDMS beam structures have been used by Liu et al. [20] to hold a cell in place during microrobotic mouse embryo injection while tracking the beam displacements, thus extracting force information. A sub-pixel visual tracking algorithm allows for 3.7 nN force resolution. In our previous work [4, 5], we have presented a design that allows for two-dimensional sensing of micro-forces and that can be easily integrated into standard robotic manipulation test-beds without cluttering the workspace or requiring any complicated drive electronics (Figure 2). It can also be used as a manipulation tool to execute a variety of tasks and has many different applications. The design topology was inspired by traditional MEMS suspension mechanisms found in accelerometers and resonators with the final geometries selected based on a MonteCarlo method optimization routine. They were microfabricated out of polydimethylsiloxane (PDMS) material. Other materials such as silicon and soft metals (copper, beryllium copper) were initially considered for the force sensor. However, in order to produce device stiffnesses sensitive enough to resolve μN forces with these materials, designs with very large length-width aspect ratio beams were required which caused the device to buckle under it’s own weight. The designs were also too large to have a sufficient portion of them reside in the field of view of the microscope. Choosing PDMS for the device allows for a much smaller device footprint along with a larger minimum feature size and thus easier manufacturing. The manufacturing process consists of an initial photolithography process with a thick photoresist to create a mold of the desired part geometry in the photoresist, PDMS pouring and curing inside the mold, followed by a wet etching step to plane the PDMS. The photoresist mold is then dissolved and the parts extracted from the substrate. These devices exhibited coupled displacements (in the plane) of the tracking points when subjected to transverse and off-axis loading, which can lead to difficulties in accurately resolving the two force components at the tip. In this paper, we present next generation designs for our 2-D vision-based force sensors, designed for decoupled motions. A building block approach [15, 18] for conceptual synthesis of compliant mechanisms methodology is used to design for the decoupled displacements for the tracking points when the tip is subjected to forces in the XY-plane. Macro-scale prototype testing is also presented, validating this methodology, and recommendations for scaling the designs down for microrobotic applications offered along with some conclusions. FORCE SENSOR SPECIFICATIONS The specific performance specifications for the force sensor are dictated by the constraints imposed by the vision system in the microrobotic test-bed and particular application. A probing section of the force sensor mechanism with tracking points along with a fixed, stationary point need to be in the microscope field of view (FOV) at all times so they can be observed by the CCD camera and displacement information extracted and converted to manipulation forces using the device’s stiffness calibration data. The constraints imposed by the vision system are dependant on the choice of microscope objective. For example, when using the 4X objective in our system the size of the FOV is 3.368 mm × 2.626 mm, with an image resolution of 5.26 μm/pixel. However, 2 Copyright c © 2009 by ASME Figure 2. 2-D VISION-BASED FORCE SENSORS with a 40X objective, the FOV reduces to 344 μm× 258 μm, with a corresponding image resolution of 0.537 μm/pixel. (Note: the accuracy of the vision-based force sensor is not necessarily equal to the image resolution. Rather, it corresponds to the robustness and accuracy of the feature tracking algorithm utilized.) Two initial application areas are considered when designing these vision-based force sensors: meso/micro-scale assembly and biological cell manipulation. The design specifications for each of these are listed in Table1. In the case of meso/microscale assembly, large deflections (≥ 1 pixel = 5.26 μm) are required to be able to optically track them with the camera. The larger objective and corresponding FOV that is used with cell manipulations will require much smaller deflections to be observed. The maximum deflection values listed in the table are the maximum deflection allowed for the device to keep all necessary tracking points in the FOV during a particular type of manipulation. Thus, the range for the designed sensor will be it’s XY stiffness × this maximum deflection value. The desired resolution ranges for each of the applications are 0.25 0.75 μN/pixel and 0.01 0.05 μN/pixel, respectively. This, along with the image resolution for the particular objective being used, determines the acceptable XY stiffness range for the force sensor. As stated previously, capacitive and piezoresistive strain gage MEMS sensors are capable of achieving similar resolutions but suffer from complicated manufacturing processes and require special drive electronics for operation. The advantages to our vision-based force sensor designs are that they can be easily integrated into standard microrobotic testbeds without the addition of this extra hardware, do not have a complicated fabrication process, and can sense forces in two-dimensions while also being used as a manipulation tool. The main objective for the problem at hand is to design a compliant force sensor which has equal stiffness in X− and Y− direction. An equal stiffness design allows for easier force extraction from the observed displacements of the device and uniform performance in each planar direction. In applications that may require unequal stiffnesses, the same methodology shown here may also be applied. Previous attempts to design a compliant force sensor with equal stiffness were inspired by MEMS based suspension designs. These designs were optimized to obTable 1. DESIGN SPECIFICATIONS PER APPLICATION AREA Meso/micro-scale Assembly Cell Manipulation Expected Force Range 0 to 50 μN 0 to 10 μN Characteristic Length 1200 μm × 500 μm 50 μm to 200 μm diameters