What are Capacitive Accelerometers?

In the electronic industry, there are various applications requiring accelerometers. For instance, the automotive industry uses accelerometers to activate airbag systems. Cameras use accelerometers to actively stabilize pictures. Computer hard disk drives rely on accelerometers to detect imminent external shocks that may damage the device—the accelerometer protects the device when an external shock is imminent. But, these are only a few applications for accelerometers.

In reality, there are endless possibilities for accelerometers uses. Microfabrication technologies have advanced steadily to enable the low-cost, tiny micro-machined accelerometers that the industry uses today. In fact, the small form and low cost are the two main factors allowing the application of these devices to cover such a broad spectrum.

The most common method of measuring acceleration is using a mass-spring-damper structure, converting the acceleration to a displacement quantity. Applying the capacitive sensing technique makes it easy to convert this displacement to an electrical signal proportional to the applied acceleration.

For the mass-spring-damper structure, a known quantity of mass, also known as test mass or proof mass, connects to the sensor frame through a spring. When the sensor frame senses acceleration because of an external force, the proof mass tends to hang back due to its inertia. This allows the relative position of the proof mass to change with respect to the sensor frame.

An external observer sees the proof mass being displaced to one side of its resting position. At the same time, the displacement of the proof mass compresses or elongates the spring. This exerts a force proportional to the displacement on the proof mass. The force from the compressed or elongated spring pushes or pulls the proof mass to the other side and makes it accelerate in the direction of the external force.

If the designer has chosen appropriate values for the various parameters in the system, the displacement of the proof mass will be proportionate to the value of the frame acceleration, once the transient response of the system subsides.

In summary, a mass-spring-damper structure converts the sensor frame acceleration to a displacement of the proof mass. Now the question is, how to measure this displacement? Although there are several methods of measuring this displacement, one of the most common arrangements is the capacitive sensing technique.

Fixing two electrodes to the sensor frame and a movable electrode to the proof mass creates two capacitors. As the proof mass moves, the capacitance between the moving electrode and that of one fixed electrode decreases, while the capacitance between the others increases. By measuring the change in the sense capacitors, it is possible to detect the displacement of the proof mass. This is then proportional to the input acceleration.

To measure changes in the sense capacitors accurately, it is necessary to apply the technique of synchronous demodulation. It is easy to do this while employing the signal conditioning offered by the ADXL family of accelerometers from Analog Devices. These devices use a 1 MHz square wave as the AC excitation for the sense capacitors.

As the movable electrode moves close to one of the fixed electrodes, the amplifier input bridge receives a larger proportion of the excitation voltage from the moving electrode. If the movable electrode is at rest, the voltage at the amplifier input is zero.