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Input / Output Devices
Servo Control Accelerometers
Another common accelerometer is the electromechanical force-balance type. These are commonly called servo control accelerometers because they use a closed-loop servo control system to measure acceleration.
Mechanically, the servo control devices consist of a permanent magnet and a sensing mass attached to a moving coil. Servo control accelerometers also require some internal circuitry generally consisting of a position sensor, error amplifier, and a voltage-to-current (V/I) converter.
The sensor output represents the location of the sensing mass with respect to a null or center position. An error amplifier and V/I converter change the position signal into a current which drives the moving coil.
The moving coil and permanent magnet exert a force on the mass. The force is proportional to the current in the coil. When the mass begins to move away from the null position because of an external acceleration, the position error generates a coil current, returning the mass to the null position. A voltage signal representing coil current is used as the servo control accelerometer output signal.
One variation in the construction of the servo control accelerometer uses a bobbin-shaped coil mounted on a disc. The disc supports the coil with two beams, allowing the coil to move axially. The coil is surrounded by a permanent magnet. Current forces the coil into a null position in response to external accelerations. The advantage of this structure is high reliability because the coil needs no pivot.
Servo control accelerometers typically require supply voltage of ±15 Vdc. Output signals of ±5 V are typical. Because the servo control structure is bulkier than suspended-mass accelerometers, frequency response is lower. Most servo control units do not respond above a few hundred cycles/sec. These accelerometers usually come with mechanical stops or fluid dampers to prevent damage from accelerations beyond their range.
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Servo Control Accelerometers: Servo Control Information
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