Capacitive Gyroscope Pickoff Using High-Frequency Electrode Bias
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Solution Overview
Problem
Capacitive angular rate sensors suffer from charge trapping due to DC offset voltages, leading to slow shifts in bias and scale factor, which recur upon device power-up and limit performance.
Innovation Solution
Applying a high-frequency voltage to the pick-off electrodes in the vibrating structure angular rate sensor, eliminating the need for a constant high-magnitude voltage across the electrodes, thereby reducing charge trapping effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a constant high-magnitude voltage is applied across the pick-off electrodes to generate a detectable current, then the measurement precision is improved, but charge trapping effects occur causing bias and scale factor shifts
Solution Approach 1:
The patent applies a periodic AC voltage at a frequency above the resonant frequency of the ring structure to the pick-off electrodes, replacing the conventional constant DC voltage. This periodic action allows the circuit to detect motion through capacitance changes while avoiding the charge trapping effects caused by sustained high-magnitude DC voltage, thus resolving the contradiction between measurement precision and bias stability
Solution Approach 2:
The patent changes the voltage parameter from a constant high-magnitude DC voltage to a time-varying AC voltage with frequency above the resonant frequency. This parameter change enables the pick-off circuit to maintain detection capability through capacitance modulation while eliminating the charge accumulation problem that occurs with DC voltage, thereby improving both measurement precision and reliability
2Ease of operation
If a DC offset voltage is applied to the capacitive electrodes, then the pick-off transducer can detect motion, but charge trapping occurs due to migration of charged species in the presence of electrical field gradients
Solution Approach 1:
The patent replaces the static DC offset voltage with a periodic AC voltage signal whose frequency exceeds the resonant frequency of the ring structure. This periodic voltage creates time-varying electric field gradients that prevent charged species from accumulating on the dielectric layers, thereby eliminating charge trapping while maintaining the transducer's ability to detect motion through capacitance changes
Solution Approach 2:
The patent transitions from a static DC voltage regime to a dynamic AC voltage regime. The time-varying nature of the AC voltage creates continuously changing electric fields that prevent charge accumulation, making the system dynamically stable and free from charge trapping effects while preserving operational capability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Reduces charge trapping, allowing for improved sensor performance by minimizing bias and scale factor errors, even with reduced DC bias voltage application.
Implementation Method 1
the first pick-off circuitry is arranged to apply a voltage having a frequency above the resonant frequency of the ring structure to the first electrode and the second electrode; and wherein the first pick-off circuitry is further arranged to detect changes in the voltage on the first electrode and the second electrode resulting from motion of the ring structure with respect to the first electrode and the second electrode
Implementation Method 2
pairs of drive transducers can excite the planar ring into a primary mode of resonance, in which a constant amplitude of motion is maintained at all times
Implementation Method 3
When a rotation is applied around an axis perpendicular to the plane of the vibrating ring, Coriolis forces couple energy into a secondary mode of vibration, with the amplitude of the vibration being proportional to the applied angular rate
Data Source
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AI summary
A vibrating structure angular rate sensor comprising a MEMS structure comprising a vibrating structure (101), a primary drive transducer arranged to cause the structure (101) to oscillate in a primary mode at a resonant frequency, a first pick-off transducer arranged to sense oscillation of the structure (101), the first pick-off transducer comprising a first electrode (115a) and a second electrode (115b), and first pick-off circuitry. The first pick-off circuitry is arranged to apply a voltage having a frequency above the resonant frequency of the structure (101) to the first electrode (115a) and the second electrode (115b). The first pick-off circuitry is further arranged to detect changes in the voltage on the first electrode (115a) and the second electrode (115b) resulting from motion of the structure (101) with respect to the first electrode (115a) and the second electrode (115b).