Last month’s column looked at the rise of motion sensing and its role in expanding the functionality of mobile devices. Against this backdrop, we reviewed the basics of accelerometer technology. This month, let’s look at another leading motion sensor: The gyroscope, a technology that offers a way of measuring angular rotation across three axes, providing data on roll, pitch and yaw. In addition to measuring these parameters, design engineers can use a gyroscope to improve the accuracy of an accelerometer operating in the same inertial measurement unit.
While these features made traditional gyroscopes a logical candidate for mobile interfaces designed to serve context-aware applications, a few obstacles still remained. These included size and sensitivity. Before gyroscopes could achieve their full potential in mobile applications, sensor makers had to address these shortcomings.
Enter MEMS (micro-electro-mechanical systems) technology, a microfabrication technique that delivers greater sensitivity in a smaller form factor. By making the shift to MEMS-based devices, design engineers can take on a new class of applications, changing the mobile landscape in the process.
Although MEMS gyroscopes include piezoelectric- and laser-based designs, many rely on a tuning-fork configuration that uses the Coriolis effect to measure the angular rate. With this approach, the two tines of the tuning fork oscillate, moving in opposite directions. When angular velocity is applied, the Coriolis force on each tine acts in opposite directions, resulting in a change in capacitance proportional to the applied angular rate. The sensors convert this change in capacitance into output voltage for analog gyroscopes or LSBs for digital gyroscopes.
An important strength of this type of sensor is its ability to measure complex motion accurately in multiple dimensions. As a result, it can track the position and rotation of a moving object, unlike accelerometers that only determine that an object is moving in a particular direction. In addition, errors related gravitational and magnetic fields do not affect the performance of this type of gyroscope. Armed with these features, these sensors go a long way toward enabling advanced motion applications in consumer devices, such as gesture control.
The challenge for design engineers has been to identify the parameters that help them choose the most appropriate gyroscope for an application. While much has been written about nonlinearity, noise density and bias repeatability, experts generally consider bias instability and acceleration sensitivity to be the key parameters engineers should focus on during the selection process.
Bias instability describes the resolution floor of the gyroscope, specifying the device’s detection limitations. The bias of a vibratory gyroscope represents the device’s average output when it is at rest, a state referred to as the zero rate output. Bias instability measurements describe how the bias of a gyroscope changes over a specific period of time under constant temperature and pressure. These measurements are usually expressed in degrees per hour or degrees per second.
Designers must also consider sensitivity to acceleration and vibration because these forces affect the output of the gyroscope. Sensitivity results from asymmetries in the gyroscope’s mechanical design or micromachining inaccuracies. Gyroscopes manifest acceleration sensitivity in several ways, and the degree of their reactions varies from one design to another. Sensitivity to linear acceleration–or g sensitivity–produces the most significant errors, especially in mobile devices.
Knowing the importance of these two parameters is half the equation. We also have to minimize the impact of error sources. Environmental factors like temperature adversely affect bias instability. Most MEMS gyroscope data sheets, however, specify the impact of temperature, so a designer can use this information to calibrate the application to compensate for this type of error.
Correcting for g sensitivity is another matter, and it often proves more difficult. The most common approach is to add a mechanical anti-vibration mount, where the gyroscope assembly is isolated by rubber. The trouble with the technique is that the mount can be difficult to engineer because of its flat response over a broad frequency range and the fact that vibration-reduction characteristics change over temperature and the operating life of the device. Ultimately, the most important step is to select a gyroscope based on its vibration-rejection capabilities.