Overcoming disorientation: Practical ways to assemble MEMS inertial sensors

August 13, 2014 // By Marc Smith, Maxim Integrated
In this article we describe how MEMS inertial sensors, e.g., gyroscopes and accelerometers, can help someone or something to overcome spatial disorientation. We explain how external forces and movement will impact system operation, and how component placement and mounting conditions—spatial relationships—directly affect the performance of a MEMS inertial sensor.

Children and dogs (illustration above) effortlessly orient themselves and control gymnastic movements. Some might think that this is as easy as “child's play,” until they try to make a machine or robot duplicate the feat.

The human orientation system is marvelously complex and does a great job when we are on the ground. Conversely, while flying an aircraft we are placed in an unfamiliar three-dimensional environment. That, combined with reduced visual orientation clues, can make spatial (dis)orientation difficult or impossible to manage. Between 5% and 10% of all general-aviation (GA, or “light” aviation) accidents can be attributed to spatial disorientation, 90% of which are fatal. (Ref. 1)

Microelectromechanical (MEMS) inertial sensors are sensitive to motion by design. They effectively detect and process linear acceleration, magnetic heading, altitude, and angular rate information. To fully exploit the performance potential of inertial sensors, designers must remain aware of the overall mechanical system, paying close attention to motion sources and resonances in the application.

Given the many different potential system configurations (e.g., board sizes, material, mounting methods), designers need to adapt unique solutions for each application. We show how to do this: to detect and mitigate erroneous inertial signals. We present practical advice for enhancing sensor system operation where, and when, the real-world environment presents undesirable locomotive signals and system resonances.

Understanding balance, the human kind

We begin by discussing balance. Consider the human ear. In Figure 1 the cochlea is the organ for hearing. The ear drum shakes the cochlea via some of the smallest bones on our body. The cochlea contains little hairs, or cilia, and it is filled with fluid. As the cochlea moves, the fluid does not move because of inertia. The cilia sense this difference of motion and transmit nerve impulses to our brain which are interpreted as sound.

Figure 1. Human balance and hearing are part of the complex organs of equilibrium found in the inner ear.

The human ear also contains a motion detection system for equilibrium, also known as balance. Three semicircular canals, functioning similarly to perpendicular gyroscopes, detect and send impulse signals to the brain about one’s state of balance. Unfortunately, there are limits to how we sense motion.

If the motion is less than about 2 degrees per second, we ignore it. If a smooth motion exists longer than 20 to 25 seconds, we stop sensing movement. These human limitations can cause confusion. There are two other sense organs in the inner ear: the utricule senses linear acceleration, and the saccule senses gravity. All five sensors in our ear help us with equilibrium or balance by informing our brain about our body’s position and movement. This, along with our eyes, helps us maintain balance and keep our eyes focused on an object while our head is moving or the body rotating.

next: spatial orientation in flight...