Accelerometer Types: A Comprehensive Comparison

This article compares different types of accelerometers, namely Piezoelectric, Piezoresistive, Capacitive, and Servo accelerometers. It details their working principles, advantages, disadvantages, and key specifications.

Introduction:

Accelerometers are transducers that output a signal proportional to acceleration, vibration, or shock. They are used in diverse applications ranging from research and analysis to everyday devices such as airbag sensors and security alarms.

Piezoelectric Accelerometer

This type of sensor measures acceleration or vibration based on the piezoelectric effect.

• Principle: Piezoelectric materials generate an electrical charge in response to applied mechanical stress.
• Construction: A piezoelectric crystal (e.g., quartz, ceramic, or piezoelectric polymer) is sandwiched between two electrodes.
• Working: Acceleration or vibration causes mechanical deformation of the piezoelectric material, leading to a change in electrical charge proportional to the applied force. This charge is measured and converted into a corresponding voltage or digital signal.

Piezoelectric accelerometers are categorized into:

• IEPE (Internal Electronic Piezoelectric) Accelerometer: This type incorporates an internal charge amplifier, converting the charge into a low impedance voltage signal.
• Charge Mode Accelerometer: The crystal directly generates an electrical charge proportional to the applied acceleration. They’re used in high-frequency or high-temperature measurements.

Advantages of IEPE Accelerometer:

• Wide dynamic and frequency range.
• Durable.
• Can be miniaturized.

Disadvantages of IEPE Accelerometer:

• Limited temperature range (typically up to 175°C).

Advantages of Charge Piezoelectric Accelerometer:

• Operates at high temperatures (up to 700°C).
• Wide dynamic and frequency range.
• Durable.

Disadvantages of Charge Piezoelectric Accelerometer:

• Requires careful installation and maintenance.
• High impedance circuitry must be kept clean and dry.
• Requires additional cables and charge amplifiers.

Piezoresistive Accelerometer

Similar to piezoelectric accelerometers, piezoresistive accelerometers measure acceleration or vibration, but based on a different principle.

• Principle: The piezoresistive effect describes the change in electrical resistance of certain materials when subjected to mechanical stress or strain.
• Construction: A sensing element made of piezoresistive materials like silicon or polysilicon, often in a micro-machined structure with built-in piezo-resistors.
• Working: Applied acceleration deforms the silicon structure, causing strain on the piezoresistors and changing their resistance. This resistance change is measured and converted into voltage or current.

These accelerometers are mainly designed for measuring dynamic acceleration and are less suitable for static or low-frequency measurements. Temperature variations can affect performance, requiring regular calibration.

Advantages of Piezoresistive Accelerometer:

• DC response.
• Small size.

Disadvantages of Piezoresistive Accelerometer:

• Low shock protection.
• Small dynamic range.

Capacitive Accelerometer

This type of accelerometer measures acceleration by detecting changes in capacitance.

• Principle: The capacitance between two parallel plates changes as the distance between them varies.
• Construction: Two parallel plates separated by a small air gap or dielectric material. One plate is fixed, while the other is attached to a proof mass that moves in response to acceleration.
• Working: Acceleration causes the proof mass to move, changing the capacitance between the plates. Electronic circuits detect and measure this change, converting it into a voltage or current proportional to the applied acceleration.

Advantages of Capacitive Accelerometer:

• High linearity.
• Wide frequency range.
• Low power consumption.

Disadvantages of Capacitive Accelerometer:

• Sensitive to environmental factors like temperature, humidity, and shock.

Servo Accelerometer

Servo accelerometers, also known as force balance accelerometers, utilize a closed-loop control system for precise acceleration measurements.

• Principle: Force balance between a known reference acceleration and the acceleration being measured.
• Construction: Feedback loop that continuously adjusts force to keep a proof mass at a fixed position. Force is generated by an electrostatic or electromagnetic actuator.
• Working: Applied acceleration displaces the proof mass. A position sensor detects this displacement and provides feedback to control the actuator. The control system maintains the proof mass in a balanced position by adjusting the force as per the applied acceleration. The amount of force needed to maintain balance provides a precise measure of the applied acceleration.

Advantages of Servo Accelerometer:

• High accuracy.
• Wide dynamic range.
• Low noise.
• Stability over temperature and time.

Disadvantages of Servo Accelerometer:

• Larger size.
• Complex construction.
• Expensive.

Comparison Table: Accelerometer Types

The following table compares the specifications of Piezoelectric, Piezoresistive, Capacitive, and Servo accelerometers. Note that values can vary based on design and construction.

Reference: The values mentioned in the comparison table have been derived from “Sensor Technology Handbook” by Jon Wilson.