Capacitive Accelerometer Closed-Loop Control for Dielectric Charging
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Solution Overview
Problem
Capacitive accelerometers face performance degradation due to dielectric charging, especially in closed loop configurations, which affects accuracy and dynamic range, and existing solutions either compromise on acceleration range or require significant modifications to control electronics.
Innovation Solution
A method for closed loop operation of capacitive accelerometers that applies a DC voltage based on a threshold acceleration value, adjusting pulse width modulation drive signals to maintain the proof mass at a null position, reducing mean voltage offsets and minimizing charge migration-induced errors while maintaining high acceleration range capability without substantial modifications to existing control electronics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If closed loop operation with high DC voltage is used to achieve high acceleration range, then acceleration dynamic range is improved, but dielectric charging effects worsen causing performance degradation
Solution Approach 1:
The patent applies periodic AC drive signals with pulse width modulation instead of continuous DC voltage to the capacitive accelerometer. The drive signal alternates between positive and negative phases, creating time-varying electrostatic forces that maintain the proof mass at the null position while minimizing net DC voltage across the dielectric layers. This periodic action reduces dielectric charging effects while preserving the ability to measure high acceleration levels through the modulation depth of the periodic signal.
Solution Approach 2:
The patent changes the operational parameters by using AC voltage signals with variable amplitude and duty cycle rather than fixed DC voltage. By modulating the amplitude and pulse width of the drive signals, the system can achieve high acceleration measurement capability without maintaining high net DC voltage across the dielectric layers, thus reducing charge migration while preserving dynamic range.
2Measurement precision
If DC offset voltage is applied to achieve force balancing in closed loop mode, then proof mass positioning is improved, but charge migration in dielectric layers worsens
Solution Approach 1:
The patent replaces static DC offset voltage with periodic AC drive signals that oscillate between positive and negative phases. The proof mass positioning is maintained through the time-averaged effect of these periodic signals rather than a constant DC voltage. This eliminates the sustained high field gradient that drives charge migration, while still achieving precise force balancing through the modulation of the periodic signal amplitude and duty cycle.
Solution Approach 2:
The patent inverts the approach by using alternating polarity AC signals instead of unidirectional DC voltage. Rather than applying a constant positive DC offset that creates continuous charge migration, the system applies alternating positive and negative voltage phases that cancel out net charge accumulation in the dielectric layers while maintaining the force balancing function.
3Productivity
If PWM drive signals with high mark:space ratio variation are used to cover wide acceleration range, then acceleration range is improved, but voltage gradients across dielectric layers worsen
Solution Approach 1:
The patent uses symmetric periodic AC waveforms with controlled duty cycles instead of asymmetric PWM signals. By maintaining balanced positive and negative phases with equal peak amplitudes, the system achieves wide acceleration range through duty cycle modulation while keeping the peak voltage gradients symmetric and minimizing net charge migration. The periodic nature ensures that voltage gradients are applied in both directions, preventing cumulative charging effects.
Solution Approach 2:
The patent changes the waveform parameters by using symmetric AC signals with fixed peak amplitude but variable duty cycle, rather than asymmetric PWM with varying peak voltages. This approach maintains constant peak voltage gradients that prevent excessive charge migration, while achieving wide acceleration measurement range through the modulation of the duty cycle parameter that controls the time-averaged force balance.
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
The solution significantly reduces the effects of dielectric charging, allowing for high dynamic range operation while being compatible with existing control electronics, thereby improving the accuracy and reliability of capacitive accelerometers.
Implementation Method 1
applying a first drive signal V1 to the first fixed capacitive electrode, a second drive signal V2 to the second fixed capacitive electrode... so as to provide a net electrostatic restoring force on the proof mass for balancing the inertial force of the applied acceleration
Implementation Method 2
first and second fixed capacitive electrodes arranged symmetrically either side of the proof mass along the sensing axis with a gap defined between each of the first and second fixed capacitive electrodes and the proof mass
Data Source
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AI summary
A capacitive accelerometer comprises a proof mass (103), first and second fixed capacitive electrodes (101, 102), and a DC biasing element (704) arranged to apply a DC voltage of magnitude VB to the proof mass (103) based on a threshold acceleration value. A first closed loop circuit (713) is arranged to detect a signal resulting from displacement of the proof mass (103) and control the pulse width modulation signal generator (700) to apply the first and second drive signals V1, V2 with a variable mark:space ratio. A second closed loop circuit (715) is arranged to keep the mark:space ratio constant and to change the magnitude, VB, of the DC voltage applied to the proof mass (103) by the DC biasing element (704) so as to provide a net electrostatic restoring force on the proof mass for balancing the inertial force of the applied acceleration and maintaining the proof mass at a null position, when the applied acceleration is greater than a threshold acceleration value.