Dynamic Structural and Contact Modeling for a Silicon Hexapod Microrobot

This paper examines the dynamics of a type of silicon-based millimeter-scale hexapod, focusing on interaction between structural dynamics and ground contact forces. These microrobots, having a 5 mm × 2 mm footprint, are formed from silicon with integrated thin-film lead–zirconate–titanate (PZT) and high-aspect-ratio parylene-C polymer microactuation elements. The in-chip dynamics of the microrobots are measured when actuated with tethered electrical signal to characterize the resonant behavior of different parts of the robot and its piezoelectric actuation. Out-of-chip robot motion is then stimulated by external vibration after the robot has been detached from its silicon tethers, which removes access to external power but permits sustained translation over a surface. A dynamic model for robot and ground interaction is presented to explain robot locomotion in the vibrating field using the in-chip measurements of actuator dynamics and additional dynamic properties obtained from finite element analysis (FEA) and other design information. The model accounts for the microscale interaction between the robot and ground, for multiple resonances of the robot leg, and for rigid robot body motion of the robot chassis in five degrees-of-freedom. For each mode, the motions in vertical and lateral direction are coupled. Simulation of this dynamic model with the first three resonant modes (one predominantly lateral and two predominantly vertical) of each leg shows a good match with experimental results for the motion of the robot on a vibrating surface, and allows exploration of influence of small-scale forces such as adhesion on robot locomotion. Further predictions for future autonomous microrobot performance based on the dynamic phenomena observed are discussed.

Design of a MEMS Force Sensor for Quantitative Measurement in the Nano- to Pico-Newton Range

We describe the design and fabrication of a MEMS nano- to pico-Newton force sensor with SI traceability. There has been much recent interest in developing instrumentation for the quantitative measurement of forces in the nano- to pico-Newton range. Forces in this range are frequently encountered when investigating mechanical properties of nanomaterials, in nanobiotechnology, and in single-molecule biophysics. Various methods of measuring forces at these levels include using AFM cantilevers, scanning probe microscopy, and nanoindentation. However, such measurements are relative, and in order to obtain precise quantitative measurements, it is necessary to be able to calibrate such sensors in a manner that is traceable to fundamental SI units. One such method of calibration is using an Electrostatic Force Balance (EFB) that has been established at NIST. We thus describe the design and fabrication of a MEMS-based force sensor that may be directly calibrated with the EFB and thus has the potential to measure nano- to pico-Newtons of force with SI traceability. The sensor consists of a silicon rigid arm supported on silicon tethers and which are attached to capacitive electrodes. The bar, tethers and electrodes are made from the device layer of a double side SOI wafer. A glass wafer with patterned metal electrodes is anodically bonded on both the top and bottom of the wafer to form symmetrical capacitive electrodes. An external force moves the silicon arm and the resulting capacitive force gradient of the electrodes is measured with the EFB. The mechanical structure and electrodes are designed for force sensitivity in the nano- to pico-Newton ranges and for operation in UHV to reduce thermomechanical noise. We discuss the design, initial fabrication and testing of this force sensor as a step toward the ultimate goals of quantitative nanomechanical testing of materials, NEMS, and engineered surfaces at the nanoscale.

Piezoelectrically Actuated Microvalves for Micropropulsion Applications

This paper presents a piezoelectric microvalve technology with a high pressure handling capability for micropropulsion applications. The device is a normally closed valve fabricated mostly by the micromachining of silicon. The valve consists of a custom designed piezoelectric stack actuator bonded onto silicon valve components in a stainless steel housing. Major elements of the silicon valve design include narrow edge seating rings and tensile-stressed silicon tethers that contribute to the desired normally closed leak-tight operation. No leak has been detected from a soap solution test at differential pressures of 0∼500 psi for a normally closed valve structure, indicating a leak rate of 0.001sccm or lower has been achieved. Piezoelectric operation has been successfully demonstrated at a differential pressure of 500 psi. A flow rate of 20 sccm at 100 psi has been obtained at 50 V.