The miniature high-sensitivity accelerometers have been developed using
a high-resolution displacement transducer based on electron tunneling. The
electron tunneling was originally developed for use in Scanning Tunneling
Microscope (STM). In the STM, a sharp metallic tip is positioned about 10
Angstroms above a metallic surface. When a DC voltage bias is applied between
tip and surface, a tunneling current of about 1 nA can be measured. Small
variations (1 Angstrom) in the tip-surface separation appear as large
variations (10-50%) in tunneling current. We incorporated electron tunneling
into micromachined sensors to fabricate miniature high-sensitivity
The above picture shows the dual-element micromachined tunneling accelerometer for NAVY sonobuoy application. Because NAVY is interested in small but high-resolution sensors to identify submarines in water, this device is designed to meet NAVY's performance specifications including resolution, bandwidth, linearity, cross-axis rejection and size. This prototype tunneling accelerometer was originally developed by JPL. The focus of our research is to optimize the sensor performance. We utilize finite element analysis to improve the structure and dynamic response. Modeling (linear and nonlinear) and feedback control are used to improve the accelerometer performance.
This tunneling accelerometer comprises a proof mass, a wide-bandwidth cantilever and feedback control circuit. This design separates the proof mass and the electrical transducer so that their mechanical properties can be independently tailored. Electron tunneling occurs between electrodes on the wide-bandwidth cantilever and on the proof mass. The wide-bandwidth cantilever is controlled electrostatically by high-frequency feedback circuitry to closely follow the motion of the proof mass. Since the proof mass may have a low resonant frequency, its dynamics may be tuned to enhance acceleration sensitivity.
Operation of this accelerometer uses electronic feedback circuit to maintain a constant tip-to cantilever spacing, 10 Angstroms, by controlling the electrostatic deflection voltages on the proof mass and the wide-bandwidth cantilever. When the device is accelerated, the proof mass experiences an inertial force which causes its motion to lag that of the device. The feedback circuit generates the electrostatic re-balance forces on proof mass and wide-bandwidth cantilever to maintain constant tunneling current, 1.5 nA, to go through the constant tunneling junction, 10 Angstroms.
Since this system balances the inertial force on the proof mass to keep the current ( and therefore tunneling junction separation ) constant, we can measure the external acceleration by measuring the electrostatic force the feedback circuit applies to balance the external acceleration.