A multi-channel electrometer voltmeter that employs self-nulling lock-in detection electronics in conjunction with a mechanical resonator with non-contact voltage sensing electrodes has been developed for space-based measurement of an Internal Electrostatic Discharge Monitor (IESDM). The IESDM is new sensor technology targeted for integration into a Space Environmental Monitor (SEM) subsystem used for the characterization and monitoring of deep dielectric charging on spacecraft.
Use of an AC-coupled lock-in amplifier with closed-loop sense-signal nulling via generation of an active guard-driving feedback voltage provides the resolution, accuracy, linearity and stability needed for long-term space-based measurement of the IESDM. This implementation relies on adjusting the feedback voltage to drive the sense current received from the resonator’s variablecapacitance- probe voltage transducer to approximately zero, as limited by the signal- to-noise performance of the loop electronics. The magnitude of the sense current is proportional to the difference between the input voltage being measured and the feedback voltage, which matches the input voltage when the sense current is zero. High signal-to-noise-ratio (SNR) is achieved by synchronous detection of the sense signal using the correlated reference signal derived from the oscillator circuit that drives the mechanical resonator. The magnitude of the feedback voltage, while the loop is in a settled state with essentially zero sense current, is an accurate estimate of the input voltage being measured. This technique has many beneficial attributes including immunity to drift, high linearity, high SNR from synchronous detection of a single-frequency carrier selected to avoid potentially noisy 1/f low-frequency spectrum of the signal-chain electronics, and high accuracy provided through the benefits of a driven shield encasing the capacitance- probe transducer and guarded input triaxial lead-in.
Measurements obtained from a 2- channel prototype electrometer have demonstrated good accuracy (|error| < 0.2 V) and high stability. Twenty-fourhour tests have been performed with virtually no drift. Additionally, 5,500 repeated one-second measurements of 100 V input were shown to be approximately normally distributed with a standard deviation of 140 mV.
This work was done by Brent R. Blaes and Rembrandt T. Schaefer of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47339
This Brief includes a Technical Support Package (TSP).
Self-Nulling Lock-in Detection Electronics for Capacitance Probe Electrometer
(reference NPO-47339) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package for the Self-Nulling Lock-In Detection Electronics designed for Capacitance Probe Electrometers, referenced as NPO-47339 in NASA Tech Briefs. It outlines advancements in measurement technology developed at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under NASA sponsorship. The focus is on improving the performance and reliability of electrometers used in aerospace applications, particularly for monitoring internal electrostatic discharge in spacecraft.
Key features of the technology include a stable sense-signal-nulling loop utilizing a simple Proportional Integral (PI) compensator, which enhances measurement accuracy by minimizing signal drift. Additionally, the design incorporates feed-forward cancellation circuitry to mitigate amplifier saturation issues that can arise when the measurement loop is not fully settled. This ensures that the electrometer can maintain high performance even in challenging operational conditions.
The document also highlights the potential for integrating Digital Signal Processing (DSP) techniques to further enhance the performance of the electrometer. By leveraging DSP, the system can achieve improved noise reduction and signal processing capabilities, which are critical for precise measurements in the aerospace environment.
Figures included in the document illustrate various aspects of the technology, such as the multichannel electrometer architecture and laboratory demonstrations of the two-channel electrometer. These visual aids help convey the practical applications and effectiveness of the developed systems.
The Technical Support Package serves not only as a documentation of the research but also as a resource for potential commercial applications of the technology. It emphasizes the broader implications of aerospace-related developments, suggesting that the innovations could be adapted for use in other scientific and technological fields.
For further inquiries or assistance regarding this technology, the document provides contact information for the Innovative Technology Assets Management office at JPL, encouraging collaboration and exploration of the advancements made in this area.
Overall, the document encapsulates significant progress in electrometer technology, showcasing how NASA's research can lead to advancements with wider technological, scientific, and commercial applications.
