Canadian researchers develop novel ultra-low force microgripper

Citation

(2008), "Canadian researchers develop novel ultra-low force microgripper", Assembly Automation, Vol. 28 No. 4. https://doi.org/10.1108/aa.2008.03328daf.001

Canadian researchers develop novel ultra-low force microgripper

Article Type: Mini features From: Assembly Automation, Volume 28, Issue 4

A widely held view within the nanotechnology industry is that, to mass produce certain types of nanodevices, an assembly line process featuring highly miniaturised robots which will add parts to a product in a sequential manner will be required. Accordingly, researchers are beginning to develop the first rudimentary nano- and micro-manipulation devices. As mechanical end-effectors, microgrippers allow the pick-transport-place of micron-sized objects and are viewed as key components in future micro- and nano-assembly systems. However, to conduct these tasks, some form of force sensing and control is required if components are not to be damaged during manipulation. Now, a team of Canadian scientists from the Advanced Micro and Nanosystems Laboratory at the University of Toronto have developed a microgripper featuring closed-loop force-controlled grasping at the nN (nano-Newton) level for the first time.

The microgripper is a complex microstructure (Figure 1) based on MEMS technology and produced on an silicon on insulator (SOI) wafer. Fabrication is by a multi-stage process involving a range of advanced techniques including plasma-enhanced chemical vapour deposition, reactive ion etching (RIE), deep RIE (DRIE), HF-based wet etching and electron-beam evaporation, used to produce aluminium contacts. This process, and the use of an SOI wafer, permits the fabrication of electrically insulated but mechanically connected microstructures, as well as providing effective thermal insulation.

Figure 1 The microgripper showing the grasping force sensing comb structure (left) and the thermal actuator mechanism (right) (Source: Yu Sun, University of Toronto)

The gripper employs electrothermal actuation of a V-beam which is used to control the opening of the gripping arm, whereby an applied voltage heats the V-beams, causing them to expand and produce motion. Applying 10 V leads to a displacement of 67 μm. Integrated capacitive force sensors are implemented with transverse differential comb drives and enable the measurement of the gripping forces as well as the contact forces applied at the end of gripping arms along the normal direction, with resolutions of 38.5 and 19.9 nN, respectively. The device has a gripping force measurement range of ±30 μN along the x direction and a contact force range of ±58 μN in the y direction. The gripping force sensor allows an object to be securely grasped without applying excessive forces and normal force sensing prevents device breakage when the gripping arms approach a substrate. Four tethering beams are directly connected to the two gripping arms for transmitting forces. A gripping force (along the x direction) or contact force (along the y direction) deflects four unidirectional sensor springs and further changes the comb finger gaps. Eight sensor springs are orthogonally configured to decouple force sensing along the x and y directions. The measured forces are used as the feedback signals to a proportional-integral-derivative controller which regulates the gripping forces for force-controlled micrograsping.

Experiments were conducted with biological cells (porcine aortic valve interstitial cells) in a liquid medium. These were chosen due to their high delicacy, high deformability, variations in sizes and mechanical properties. The micro-manipulation system used in the experiments comprised a three degrees of freedom microrobot for positioning the microgripper; a motorized X-Y stage for positioning the samples; an inverted microscope with a CMOS camera; the microgripper, wire bonded to a circuit board and a control board mounted on a host computer. After the tips of the gripping arms are immersed in the medium, the microrobot controls the microgripper at a constant speed of 20 μm s⁻¹ to approach the object while force data along the y direction of the microgripper are sampled. The contact detection process is completed within 5 s. The microgripper is then positioned a few microns above the detected contact position. The pre-set threshold force used in the experiments was 96 nN which was effective for determining the initial contact between the gripping arm tips and the object. After contact detection, the microgripper grasps a cell using a force of 65 nN, transfers it to a new position and releases it. The research group concludes: “Besides force-controlled micromanipulation of biomaterials in liquid, the monolithic micro-grippers with two-axis nanonewton force feedback can also find important applications in biomaterial mechanical characterization and in electronic component handling as well as the assembly of micro objects”. The group is now improving further the force sensing resolution of the microgrippers for sub-nano-Newton measurements.

The work was supported by the Natural Sciences and Engineering Research Council of Canada and the Ontario Ministry of Research and Innovation. Full details can be found in Kim et al. (2008).