Analysis of the working principle of servo pressure sensor

Servo pressure sensors are generally large and are generally not suitable for dynamic pressure measurement or physical hostile environments, but are well suited for high precision and high resolution pressure measurements in a more benign physical environment.

The servo accelerometer is shown below, hanging on the hinge is a sagging high permeability mass. The "down" or "zero position" is detected by a zero detector and the balancing force is provided by the magnetic coil.

Servo accelerometer

If an acceleration is applied to this component, a force is applied to the mass that will attempt to move from the zero position. When the zero detector detects motion, the coil current is increased by the servo amplifier to maintain the zero position.

The coil current provides the restoring force required to maintain a zero position that will be proportional to the applied acceleration.

High-precision zero detectors can be easily fabricated because the total range of this deflection is very small. In fact, increasing the resolution of the zero detector will result in a correspondingly higher acceleration resolution.

Since the active components of the servo accelerometer do not significantly shift during normal operation, the hysteresis performance of such a sensor is extremely low, and the more reason is the electrical hysteresis in the circuit, rather than the actual mechanical hysteresis. Damping of seismic devices is done electrically and mechanically using silicone oil.

Compared to strain gauge accelerometers, servo accelerometers have microgravity resolution with high zero Hertz stability and low thermal error. Large-size inertial masses generate large forces during high-impact events, and even in high-impact environments, such sensors may contain excessive long-range shock stops, but such sensors are not suitable for high-impact environments.

Early force balance sensors provide piezoelectric or magnetic "jitter" mechanisms that reduce the viscous effect by constantly oscillating the bearings to keep the bearing friction coefficient within a low dynamic range. The recent design, using a high-resolution zero-detection system, eliminates the need for the bearing to completely replace its simple quartz bend. The excellent mechanical properties of crystalline quartz, as a pivot, provide substantial zero hysteresis performance due to the absence of significant deflection of the mass.

Typical practical flat (±5%) frequency response bandwidths for servo accelerometers are typically less than 100 Hz. Based on the closed-loop control network, the recovery time of the servo accelerometer from the overrange input may be long relative to the design of the strain gauge open-loop accelerometer. In fact, the recovery time of the sensor after the process input is a direct function of the total power available to the restoring force mechanism.

Typical servo sensors are typically limited to 50 or 100 mA of input drive current, thereby "energy limiting" the resulting resiliency mechanism.

The typical overrun recovery time is 100 milliseconds. The large thermal mass of this type of sensor makes the device quite insensitive to thermal transients.

Servo pressure sensor

The figure shows how the servo concept based on the above concept is applied to the manufacture of extremely high precision pressure sensors.

Servo pressure sensors are generally large and are generally not suitable for dynamic pressure measurement or physical hostile environments, but are well suited for high precision and high resolution pressure measurements in a more benign physical environment.