Analog Dialogue 50-05, May 2016

MEMS gyroscopes offer a simple way to measure the angular rate of rotation in packages that easily attach to printed circuit boards, so they are a popular choice to serve as the feedback sensing element in many different types of motion control systems. In this type of function, noise in the angular rate signals (MEMS gyroscope output) can have a direct influence over critical system behaviors, such as platform stability, and is often the defining factor in the level of precision that a control system can support. Therefore, low noise is a natural, guiding value for system architects and developers as they define and develop new motion control systems. Taking that value (low noise) a step further, translating critical system-level criteria, such as pointing accuracy, into noise metrics that are commonly available in MEMS gyroscope data sheets, is a very important part of early conceptual and architectural work. Understanding the system’s dependence on gyroscope noise behaviors has a number of rewards, such as being able to establish relevant requirements for the feedback sensing element or, conversely, analyzing the system-level response to noise in a particular gyroscope. Once system designers have a good understanding of this relationship, they can focus on mastering the two key areas of influence that they have over the noise behaviors in their angular rate feedback loops: 1. developing the most appropriate criteria for MEMS gyroscope selection and 2. preserving the available noise performance throughout the sensor’s integration process. Motion Control Basics Developing a useful relationship between the noise behaviors in a MEMS gyroscope and how it impacts key system behaviors often starts with a basic understanding of how the system works. Figure 1 offers an example architecture for a motion control system, which breaks the key system elements down into functional blocks. The functional objective for this type of system is to create a stable platform for personnel or equipment that can be sensitive to inertial motion. One example application is for a microwave antenna on an autonomous vehicle platform that is maneuvering through rough conditions at a speed that causes abrupt changes in vehicle orientation. Without some real-time control of the pointing angle, these highly directional antennas may not be able to support continuous communication while experiencing this type of inertial motion. The system in Figure 1 uses a servo motor, which will ideally rotate in a manner that is equal and opposite of the rotation Figure 1. Example of a motion control system architecture. Servo Motor Digital Filtering ADXRS290 MEMS Gyroscope Integrator –