Enabled by advances in both integrated circuit technology and micro electromechanical systems (MEMS), the continuing miniaturization and integration of electronics, sensors, and actuators and mechanisms will make it feasible to create insect-sized autonomous microrobots. We propose to create a class of autonomous crawling microrobots the size on the order f 1cm^3 and equipped with a power source, low-power CMOS controller, sensors, wireless communications devices and motorized articulated legs. The work presented here demonstrates how articulated insect legs could be created from rigid links, mechanical couplings and low-power electrostatic micromotors.
Articulated legs require rigid links to support the weight of the microrobot and also have joints that allow out-of-place motion. Surface micromachined polysilicon hinges are utilized to satisfy both requirements. Rigid links are created by folding three hinged polysilicon plates into a hollow triangular beam which snap into place using snaplocks. These links can be fabricated in series with hinges as revolute joints. Two-link legs with up to three degrees-of-freedom (DOF) have been demonstrated.
Each link has a mechanical coupling that couples it to its own motor on-chip. Using hinged lever arms, hinged tendons and sliders, mulit-DOF mechanical couplings can be created to convert linear motion at the motor to angular displacement at the joint. A 2-link, 2 DOF leg has been demonstrated with two mechanical couplings. The first mechanical coupling (with 1-DOF) is created from a four-bar linkage (sliding crank) and coupled to the first link. The second mechanical coupling (with 2-DOF) is created from a four-bar linkage insures with a five-bar linkage and coupled to the second link. Higher DOF mechanics couplings could be achieved with higher order n-bar linkages but at a cost of higher complexity.
The main considerations for actuation are output power density, efficiency, force density, and integration with the rest of the microrobot. Of the MEMS actuation technologies that have emerged over the years, electrostatic gap-closing actuators (GCA) in an inchworm motor topology is currently best suited for microrobots. Motors fabricated on silicon-on-insulator (SOI) wafers have been demonstrated with 80um of travel, stepping rates of 1000 full steps/second corresponding to 4mm/s shuttle velocity, and ~260uN of force (~130 times its own weight). In all cases, displacement was limited by contact with a physical constraint (spring travel limits, nearby structures, etc.) rather than an intrinsic limit.
In addition to inchworm motors, mechanical digital-to-analog converters (DAC) have been demonstrated. These DAC's convert a n-bit digital electrical input to an analog mechanical output (displacement) with 2^n position. Based on cascaded lever arms with high output resistance and gap-stop-limited actuator arrays at the input, the DAC are less sensitive to loading effects and input noise. These properties are ideal for possible open loop actuation of microrobots where joint angle feedback is difficult to implement. Four-bit DAC's with electrostatic actuators have been demonstrated in SOI technology with a least significant bit (LSB) of 0.6um, an integrated non-linearity (INL) of +/-0.38 LSB and a differential non-linearity of +/-0.35 LSB. Surface micro machined 6-bit DAC's with hinged micromirrors have also been demonstrated with an LSB of 90nm, INL of +/-3.2 LSB and a DNL of +/-0.7 LSB.