Servo Accelerometer
A servo accelerometer is shown below. A pendulous, high-permeability mass is suspended on a hinge. The "down" or null position is detected by a null detector, and the balancing force is supplied by a magnetic coil.
If acceleration is applied to the assembly, a force acts on the mass and it will tend to move away from the null position. When the null detector senses motion, the servo amplifier increases coil current to hold the null position.
The coil current provides the restoring force required to maintain the null position, and this current is proportional to the applied acceleration.
A high-precision null detector is relatively easy to build because the total deflection range is very small. Increasing the null detector's resolution will directly improve acceleration resolution.
Because the active element of a servo accelerometer undergoes negligible displacement during normal operation, hysteresis is extremely low. Any hysteresis is more likely due to electrical hysteresis in the circuitry rather than mechanical hysteresis. Damping of the seismic element is provided by silicone oil in both electrical and mechanical domains.
Compared with strain-gauge accelerometers, servo accelerometers offer high DC stability and low thermal error with microgravity resolution. The large inertial mass can produce very large forces during high-shock events; although the sensor may include remote shock stops to limit travel, servo accelerometers are generally not suitable for high-shock environments.
Early force-balance sensors used piezoelectric or magnetic "dither" mechanisms that applied small continuous oscillations to the bearing to reduce stiction and keep the bearing friction coefficient in a low dynamic range. Recent designs using high-resolution null detection have eliminated bearings entirely, replacing them with simple quartz flexures. The excellent mechanical properties of crystalline quartz used as a pivot provide essentially zero hysteresis since the mass undergoes negligible deflection.
A typical useful flat (±5%) frequency-response bandwidth for a servo accelerometer is generally less than 100 Hz. Because the design is based on a closed-loop control network, recovery time from an over-range input can be relatively long compared with an open-loop strain-gauge accelerometer. In practice, the sensor's recovery time after an over-range event is a direct function of the total power available to the restoring-force mechanism.
Typical servo sensors are current-limited in the input drive to 50 or 100 mA, thereby energy-limiting the available restoring force. Typical over-range recovery times are on the order of 100 milliseconds. The relatively large thermal mass of this sensor type makes the device fairly insensitive to thermal transients.
Servo Pressure Sensor
The figure shows how the servo concept described above is applied to produce very high-precision pressure sensors.
Servo pressure sensors are generally large and are not normally suitable for dynamic pressure measurements or physically hostile environments, but they are well suited for high-precision, high-resolution pressure measurements in more benign environments.
