Although MEMS (microelectromechanical system) technology has been on the job for about two decades in airbag deployment and automotive pressure sensors, it has taken the motion-sensing user interfaces of the Nintendo Wii and the Apple iPhone to spur broad awareness of what inertial sensors can do.
To some extent the idea persists that inertial sensors are useful mainly when the end product has a need to detect acceleration and deceleration. This is true enough from a purely scientific view, yet it ignores many of the expanding uses of MEMS accelerometers (Figure 1) and gyroscopes (Figure 2) in areas that include medical devices, industrial equipment, consumer electronics, and automotive electronics.
Figure 1. Analog Devices' ADXL345 three-axis digital iMEMS accelerometer
Figure 2. Analog Devices' ADXRS610 gyroscope
If we examine what's possible in each of the five modes of motion sensing—acceleration (including translational movement, such as position and orientation), vibration, shock, tilt, and rotation—we expand the options beyond today's high-volume MEMS applications.
For instance, an accelerometer with activity detection can enable power management techniques by telling a device to go into its lowest power consumption mode when that device is determined to be inactive by the absence of movement or vibration. Complicated controls and physical buttons are being replaced by gesture-recognition interfaces controlled by the tap of a finger. In other use cases, the operation of end-products becomes more precise, such as when a compass has compensation for the tilt angle at which it is held in your hand.
This article offers a sample of the ways that advanced, commercially available MEMS accelerometers and gyroscopes are set to transform a diverse range of end products though the five types of motion sensing they enable.
Introduction to Motion Sensing and MEMS
Acceleration, vibration, shock, tilt, and rotation—all except rotation are actually different manifestations of acceleration over different periods of time (Figure 3). However, as humans we don't intuitively relate to these motion senses as variations on acceleration/deceleration. Considering each mode separately helps to envision more possibilities.
Figure 3. The five motion senses
Acceleration (including translational movement) measures the change in velocity in a unit of time. Velocity, expressed in meters per second (m/s), includes both the rate of displacement and direction of movement. It follows that acceleration is measured in meters per second squared (m/s2). Acceleration with a negative value—a car slowing down when the driver applies the brakes, for example—is known as deceleration.
If we consider acceleration over various periods of time then vibration can be thought of as acceleration and deceleration that happens quickly and in a periodic manner. Similarly, shock is acceleration that occurs instantaneously but, unlike vibration, a shock is a nonperiodic function that typically happens once.
Let's stretch out the length of time again. When an object is moved to alter its tilt, or inclination, some change in position with respect to gravity is involved. This movement tends to happen rather slowly compared to vibration and shock.
Because these first four modes of motion sensing each involve a certain aspect of acceleration, they are measured by g force, the unit of force that gravity exerts on an object on the Earth (1 g = 9.8 m/s2). A MEMS accelerometer detects tilt by measuring the effect of the force of gravity on the different axes of the accelerometer. In the instance of a 3-axis accelerometer, three separate outputs measure acceleration along the X, Y, and Z axes of motion.
The accelerometers with the largest share of the market today use differential capacitors to measure g force, which is then converted into volts or bits (in the case of a digital-output accelerometer) and then passed to a microprocessor to perform an action. Recent advances in technology have made it possible to manufacture tiny MEMS accelerometers in low-g and high-g sensing ranges with much wider bandwidth than previously, increasing the field of potential applications. A low-g sensing range is <20 g and deals with motion a human can generate. High-g is useful for sensing motion related to machines or vehicles—in essence, motion that humans cannot create.
So far we have discussed only linear motion, the type of motion that includes acceleration, vibration, shock, and tilt. Rotation is a measure of angular rate motion. This mode differs from the others because rotation may take place without any change in acceleration. To understand how that works, picture a 3-axis inertial sensor with the sensor's X and Y axes parallel to the Earth's surface and the Z axis pointing toward the center of the Earth. In this position, the Z axis measures 1 g; the X and Y axes register 0 g. Now rotate the sensor to move only about the Z axis. The X and Y planes simply rotate, continuing to measure 0 g while the Z axis still measures 1 g. MEMS gyroscopes are used to sense this rotational motion. Because certain end products must measure rotation in addition to other forms of motion, gyroscopes may be integrated in an inertial measurement unit (IMU) that embeds a multiaxis gyroscope and multiaxis accelerometer.
Acceleration in Usability, Power Management
Earlier we observed that acceleration comes into play for detecting movement and position. It's possible to use a MEMS accelerometer to notice when a device is picked up and put down; when the motion is detected, the accelerometer can generate an interrupt to turn device functions on and off automatically. Various combinations of functions can be kept active or put into the lowest power state possible. Movement-driven on/off features are human-friendly because they eliminate repetitive actions on the user's part. What's more, they enable power management that lets the device go longer between battery recharging or replacement. An intelligent remote control with a backlit LCD is among the potential scenarios.
Another potential use of an accelerometer to sense movement and to generate an interrupt is in a radio for military or public safety personnel. To keep communication secure, when the radio stops being worn or carried it could require re-authentication before permitting user access. Note that to be practical for a portable or small form-factor design, these two preceding use cases require accelerometers that draw little current; several microamps at most.
Medical equipment presents another application arena for sensing movement. Automatic external defibrillators (AEDs) have been designed to deliver an electrical shock that gets the patient's heart pumping again. When that fails, manual cardiopulmonary resuscitation (CPR) must be performed. A less experienced rescuer might not compress the patient's chest enough for effective CPR. Accelerometers embedded in the AED's chest pads can be used to give the rescuer feedback on the proper amount of compression, by measuring the distance the pad is moved.
Vibration for Monitoring and Energy Saving
Slight changes in vibration serve as a leading indicator of worn bearings, misaligned mechanical components, and other issues in machinery. Very small MEMS accelerometers with very wide bandwidth can be used to monitor vibration in motors, fans, and compressors. The ability to perform predictive maintenance lets manufacturing companies avoid damage to expensive equipment as well as to prevent costly breakdowns. Measuring changes in the equipment's vibration signature could also be used to detect whether machinery is tuned to operate in an energy-efficient manner.
Shock, Gesture Recognition, and More
The disk drive protection found in many notebook PCs is among the most widely implemented applications of shock sensing to date. An accelerometer detects the small shocks generated by accidentally yanking on the power cord or knocking the monitor, which are usually precursors to a shock event (i.e., the notebook hitting the floor). Within milliseconds, the system orders the hard disk drive to park the drive head, stopping it from contacting the disk platter during impact and preventing damage to the drive. Gesture-recognition interfaces are a promising new use for this type of inertial sensing. Defined gestures, such as taps, double-taps, or shakes, allow users to activate different features or to adjust the mode of operation. Gesture recognition increases device usability for situations where physical buttons and switches would be difficult to manipulate. Button-free designs can also reduce overall system cost in addition to improving the durability of end-products such as underwater cameras, where the opening surrounding a button would let water seep into the camera body.
Small-form-factor consumer electronics are only one application area in which accelerometer-driven gesture recognition is finding a place. Tap interfaces can also be a good fit for wearable and implantable medical devices such as medication delivery pumps and hearing aids.
Tilt Sensing for Precision Operation
Tilt sensing, too, has potential for use in gesture-recognition interfaces. For instance, one-handed operation may be preferable in applications such as construction or industrial inspection equipment. The hand not operating the device remains free to control the bucket or platform where the operator stands, or perhaps to hold a tether for safety's sake. The operator could simply rotate the probe or device to adjust its settings.
A 3-axis accelerometer would sense the "rotation" as tilt in this case: measuring low-speed changes in inclination in the presence of gravity, detecting the change in the gravity vector, and determining whether the direction is clockwise or counterclockwise. Tilt detection could also be combined with tap (shock) recognition to let the operator control more functions of the device one-handed.
Compensating for the position of a device is another area where tilt measurement is useful. Take the electronic compass in a global positioning system (GPS) or mobile handset. A well-known problem here is the heading error that results when the compass is not positioned exactly parallel to the surface of the Earth. In industrial weigh scales, the tilt of a loaded bucket relative to the Earth must be calculated to read the weight accurately. Pressure sensors, such as those used in automobiles and industrial machinery, are likewise subject to gravity's effects because these sensors contain diaphragms whose deflection changes depending upon the position in which the sensor is mounted. In all these situations, MEMS accelerometers perform the necessary tilt sensing to compensate for the error.
Rotation: Gyroscopes and IMUs in Action
Real-world applications of MEMS technology benefit when rotation is combined with other forms of inertial sensing. In practice this requires the use of an accelerometer and a gyroscope.
Some IMUs include a multiaxis accelerometer, a multiaxis gyroscope and, to further increase heading accuracy, a multiaxis magnetometer. The IMU may also provide full 6 degrees of freedom (6DoF) measurement, bringing ultrafine resolution to applications such as medical imaging equipment, surgical instrumentation, advanced prosthetics, and automated guidance for industrial vehicles. Another advantage of selecting an IMU is that its multiple functions can be pretested and precalibrated by the sensor's manufacturer.
IMUs are also proving useful in less obvious use cases. For example, they can be incorporated into an intelligent golf club that tracks and records every movement of a swing so that the golfer's technique can be refined. Accelerometers inside the club measure the acceleration and swing plane while gyros measure pronation, or the twist of the golfer's hands, during the swing. The golf club records the data collected during play or practice for later analysis on a PC.
New Wave of Signal Processing
Whether the need is for user-friendly features, minimizing power consumption, eliminating physical buttons and controls, compensating for gravity and position, or more intelligent operation, MEMS-based inertial sensing offers an abundance of options to explore across all five motion senses.
As an innovator, Analog Devices, with its iMEMS Motion Signal Processing portfolio, leads in creating the accelerometers and gyroscopes needed to deliver for this next wave of signal processing. An expanding scope of motion-sensing applications will benefit from the small size, high resolution, low power consumption, high reliability, signal conditioning circuitry, and integrated functionality that these IC solutions provide.
ABOUT THE AUTHORS
Rob O'Reilly is currently addressing the long-term MEMS test strategy as well as specific business opportunities within the MEMS market place. A former flight engineer in the U.S. Navy, he attended Northeastern University with a special focus in shock, vibration, and solid-state physics. He can be reached at Analog Devices Inc., Norwood, MA; [email protected], www.analog.com.
Harvey Weinberg, BSEE, is the leader of the Applications Engineering group for inertial products at Analog Devices' Micromachined Products division, where he has worked for 12 years. Prior to that he worked for 10 years as a circuit and systems designer specializing in process control instrumentation. He can be reached at Analog Devices Inc., Norwood, MA; [email protected], www.analog.com.