Servo Accelerometer
A servo accelerometer is illustrated below. A high-permeability mass hangs on a hinge. The "down" or null position is detected by a null detector, and the balancing force is provided by a magnetic coil.
If acceleration is applied to the assembly, a force acts on the mass and tends to displace it from the null position. When the null detector senses motion, a servo amplifier increases the coil current to hold the mass at the null position.
The coil current provides the restoring force needed to maintain the null position. This current is proportional to the applied acceleration.
High-precision null detectors are relatively easy to implement because the total deflection is very small. Increasing the resolution of the null detector directly improves acceleration resolution.
Because the active element in a servo accelerometer experiences negligible displacement during normal operation, the sensor exhibits very low hysteresis. Any remaining hysteresis is largely due to circuit effects rather than mechanical friction. Damping of the seismic element is achieved using silicone oil in the electrical and mechanical domains.
Compared with strain-gauge accelerometers, servo accelerometers provide excellent DC stability and low thermal error for microgravity-level resolution. However, the large inertial mass produces very large forces during high-shock events. Even if long-travel shock stops are incorporated, these sensors are generally not suitable for severe shock environments.
Early force-balance sensors used piezoelectric or magnetic dither mechanisms to reduce stiction by continuously applying a small oscillation to bearings, keeping friction in a low dynamic range. Recent designs, using high-resolution null detection, have eliminated bearings entirely and instead use simple quartz flexures. Crystalline quartz offers excellent mechanical properties as a pivot. Because the mass shows negligible deflection, the design achieves essentially zero hysteresis.
Typical usable flatness (±5%) bandwidth for a servo accelerometer is generally less than 100 Hz. As a closed-loop design, recovery time from overrange inputs can be much longer than that of open-loop strain-gauge accelerometers. In practice, the recovery time after an overrange event is a direct function of the total available power for the restoring mechanism.
Typical servo sensors limit input drive current to 50 or 100 mA, which effectively energy-limits the restoring mechanism. Typical overrange recovery time is on the order of 100 ms. The large thermal mass of these devices makes them relatively insensitive to thermal transients.
Servo Pressure Sensor
The figure shows how the servo concept is applied to produce very high-precision pressure sensors.
Servo pressure sensors are generally large and are typically unsuitable for dynamic pressure measurement or physically hostile environments. They are well suited for high-precision, high-resolution pressure measurement in benign physical environments.
