Principal investigator: Mr Tok Wee Keong
Co-investigator: Ling Shifu (Prof), Du Hejun (Assoc Prof)
Date started: 07/1999
To develop a MEMS Microgripper that is cheap and easy to fabricate and is equipped with a wide gripping range and force sensing capability.
As the race for miniaturization continues, there present a need to design and develop a microgripping device that is capable of gripping small objects (in the microns) accurately and without causing damages to the gripped object. Such a microgripper device also has its potentials in the biomedical field where it can be use, for example in the repair of nerves. Researches on the microgripper reveals that most of the current microgrippers proposed by other research institutions are only capable of achieving small opening of the gripper tips, hence limiting the range of size of objects that can be gripped and handled. Furthermore, most of the proposed microgripper are without force sensing capabilities and requires varying amount of assemblies that are difficult and expensive to do.
Microgripper DesignFigure 1 shows the schematic of the proposed design for the microgripper. It is a symmetrical structure consisting of six major parts: (1)thermal expansion element, (2)displacement transfer element, (3)arms, (4)supporting spring structure, (5)compliant displacement amplifying mechanism and (6)substrate. In operation, electricity current will be applied to the thermal expansion element causing the thermal elements to heat up and expands, causing an angular rotation of the gripper arm and then the gripper tips will open. Normally, the gripper is in its close position. Applying voltage to the thermal expansion element will open the gripper and switching off the voltage will close it again and thus an object can be gripped. The force sensing piezoresistive film located at the bottom of the gripper arm will signal the operator on the gripping force of the microgripper on the gripped object, and subsequent gripping force can be varied to suit the purpose, by varying the voltage supplied to the heating element.
Fig. 1: The Proposed Microgripper
An initial prototype was fabricated using stainless steel by EDM wire cutting machine. The dimension was scaled twenty times larger than the actual design based on the highest precision that can be achieved by the EDM machine. Instead of using a thermal expansion actuator, the prototype was driven by a PZT plate. Thermal expansion actuating method could not be used to drive this prototype because the size had been scaled up, and the temperature distribution would also be different. Another advantage of the PZT actuator was its output displacement could be precisely controlled. To install this PZT plate, part of the thermal expansion element of the prototype was cut off, then a PZT plate was inserted and bonded.
Fig. 2: The Prototype
Fig. 3: Test Result of the Prototype
- Fabrication of MEMS Microgripper using microchips making equipment.
- Modifying of fabrication steps to suit the making of the Microgripper.
- Modification of photo-masks to reduce number of fabrication steps.