Acceleration - Capacitive Type Sensors

Introduction

Capacitive acceleration measurement technology is widely used in commercially available accelerometer sensors. These sensors find application in mobile devices, gaming systems, disk drive protection, image stabilisation, sports & health devices, industrial instrumentation and so on.

How It Works

The capacitive acceleration sensor is based on the properties of one or more capacitors.

This is a very common design for this type of sensor.

A known quantity of mass, referred to as the proof mass, is connected to the sensor frame through a spring.

There are two plates fixed to the sensor frame along with a movable plate connected to the proof mass. This creates two capacitors.

As a force is applied to the assembly, the mass experiences movement in the opposite direction. As the proof mass moves, the distance between the plates of one capacitor increases while the distance between the plates of the other capacitor decreases.

This results in a capacitance difference between the two capacitors. This difference in capacitance is directly proportional to the experienced acceleration and (beyond the scope of this article) this is converted to an electrical signal.

An alternative design to that shown above is a micro-machined structure built on a silicon wafer, suspended by polysilicon springs that allow movement along the X, Y, and Z axes when subjected to acceleration.

The proof mass is the bar with the attached polysilicon springs attached at each end. There are plates attached along the length of the proof mass. Another set of plates, fixed to the sensor's structure, exist in parallel to the proof mass movable plates.

Acceleration moves the proof mass plates, changing the distance between them and the fixed plates. This changes the capacitance and on-chip circuitry detects this changed capacitance, converting it into an electrical signal.

This design is used in Analog Digital's accelerometer offerings such as the ADXL335 (analog interface) and ADXL345 (serial interface).

Advantages

It has many advantages including:

• High sensitivity and resolution.
• Low noise and high signal-to-noise ratio
• Excellent stability and low drift.
• Able to measure static (DC) acceleration, such as constant gravitational forces or tilt, as well as low-frequency vibrations.
• Robustness to environmental factors
• Low Power Consumption i.e. useful for battery operated portable and wearable devices.
• Highly compatible with standard semiconductor manufacturing processes (e.g., MEMS fabrication).

Disadvantages

Even good things usually have some disadvantages:

• Response degrades in the presence of high-frequency components especially at high amplitudes.
• Sensitive to temperature variations and operate within a limited temperature range.
• Suffer from hysteresis error and reduced longevity over time.