Force-balance (Servo) SensorsThe 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 AccelerometerIf 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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