MEMS accelerometers and gyroscopes

Accelerometers and gyroscopes are common in many mobile devices today, and are used in positioning and movement tracking, such as with pedometers and fitness trackers. These devices will use a MEMS piezoelectric to produce a voltage in response to movement. Gyroscopes detect rotational motion, and accelerometers respond to changes in linear motion. The following diagram illustrates the basic principle of an accelerometer. Typically, a central mass that is fixed to a calibrated location via a spring will respond to changes in acceleration that are measured by varying capacitance in a MEMS circuit. The central mass will appear to be stationary in response to acceleration in one direction.

An accelerometer will be synthesized to respond to multiple dimensions (X, Y, Z) rather than one dimension, as shown in this illustration:

 Accelerometer: The principle of acceleration measurement using a central mass suspended by a spring. Typically, these will be used in multiple dimensions.

Gyroscopes operate slightly differently. Rather than relying on the motion response to a central mass, gyros rely on the Coriolis effect of a rotating reference frame. The following figure demonstrates the concept. Without increasing velocity, the object would move in an arc, and not reach the northbound target. Moving towards the outer edge of the disk requires additional acceleration to maintain a northbound course.

This is the Coriolis acceleration. In a MEMS device, there is no spinning disk; rather, there is a resonant frequency applied to a series of MEMS-fabricated rings on a silicon substrate:  

Accelerometer: Effect of a rotating disk on a path moving northward. 

The rings are concentric, and cut into small arcs. Concentric rings allow for more area to gauge the accuracy of rotational movement. Single rings would require rigid support beams, and are not as reliable. By bifurcating the rings into arcs, the structure loses rigidity, and is more sensitive to rotational forces. The DC source creates an electrostatic force that resonates within the ring, while the electrodes attached to the rings detect the changes in the capacitor. If the resonating rings are perturbed, the Coriolis acceleration is detected. The Coriolis acceleration is defined by the following equation:  

This equation states that the acceleration is a product of the rotation of the system and the velocity of the rotating disk, as shown in the preceding diagram, or the resonant frequency of the MEMS device, as shown in the following diagram. Given a DC power source, a force changes the gap size and the overall capacitance of the circuit. The outer electrodes detect deflection in the ring, while inner electrodes provide capacitance measurements:

Right: Concentric cut rings representing the gyro sensor sitting upon a silicon substrate. Left: the disk gap connected to a corresponding.

Both gyroscopes and accelerometers will require power supplies and an op-amp for signal conditioning. After conditioning, the output is ready to be sampled by a digital signal processor.  

These devices can be synthesized in very small packages, such as the Invensense MPU-6050, which includes a 6-axis gyro and accelerometer in a small 4 mm x 4 mm x 1 mm package. The device draws on 3.9 mA of current, and is good for low power sensing.