Force-balance (Servo) Sensors

The force-balance accelerometer is shown below where a pendulous, high-magnetic permeability mass is hung from a hinge as shown. The "down" or "null position" is detected by the null detector and the counterbalancing force is provided by a magnetic coil.

The Servo Accelerometer

If acceleration is applied to this assembly, a force is exerted on the mass and it will attempt to move from the null position. When the null detector detects motion, the coil current is increased by means of a servo amplifier to maintain the null position.

The coil current provides the restoring force required to maintain the null position and this current will be in direct proportion to the applied acceleration.

Highly-precise null detectors can be easily fabricated as the total range of this deflection is extremely small, in fact, increasing null detector resolutions will result in proportionately improved acceleration resolution. Since the active member of the force-balance accelerometer does not substantially displace in normal operation, the hysteresis performance of this type of sensor is extremely low and is due more to electrical hysteresis in the circuitry than to actual mechanical hysteresis. Damping of the seismic assembly is accomplished both electrically and mechanically with silicone oil.

The servo accelerometer is physically large relative to strain gauge accelerometers but provides microgravity resolution with high zero-hertz stability and low thermal errors. The large size of the inertial mass results in large forces during high shock events, and even though overrange-ilmiting stops may be incorporated, this type of sensor is not suited to high shock environments.

Early force balance sensors were provided with piezoelectric or magnetic "dithering" mechanisms to reduce stiction effects by constantly oscillating the bearing slightly to keep the coefficient of bearing friction in the lower dynamic range. Recent designs, utilizing high-resolution null detect systems, eliminate the bearing altogether replacing it with a simple quartz flexure. The superb mechanical characteristics of crystalline quartz, used as a pivot, provides essentially-zero hysteresis performance due to the fact that the mass does not deflect significantly.

The typical useful flat (±5%) frequency response bandwidth of the servo accelerometer is generally less than 100 Hertz. Based upon a closed-loop control network, recovery time of a servo accelerometer from an overrange input can be lengthy relative to the strain-gaged open-loop accelerometer designs. In fact, the recovery time of the sensor, after an over-ranged input, is a direct function of the total power available to the restoring force mechanism. Typical force balance sensors are usually current-limited to 50 or 100 ma of input drive current thus "energy limiting" the restoring force mechanism resulting
in typical overrange recovery times of 100 milliseconds.

The large thermal mass of this sensor type renders the device reasonably insensitive to thermal transients.

The Servo or Force-balanced Pressure Sensor

The diagram shows how the servo force-balance concept can be applied to fabricate extremely-high-precision pressure sensors based upon the above concepts. The force-balance pressure sensor is generally large, and typically not suited to dynamic pressure measurements or physically-hostile environments, but is admirably suited to the high-precision and high-resolution measurement of pressure in more benign physical environments.

This article is taken from the Handbook
'The Art of Practical and Precise Strain Based Measurement' by James Pierson


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