Acceleration, vibration, shock, tilt, and rotation—all except rotation are actually different manifestations of acceleration over different periods of time. However, as humans we don’t intuitively relate to these motion senses as variations on acceleration/deceleration. Considering each mode separately helps in envisioning more possibilities.
Acceleration (including translational movement) measures the change in velocity in a unit of time. Velocity is expressed in meters per second (m/s) and 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—imagine a car slowing down when the driver applies the brakes—is known as deceleration.
More Aspects of Acceleration: Vibration, Shock and Tilt
Now consider acceleration over various periods of time. 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 non-periodic 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. That movement tends to happen rather slowly compared with vibration and shock.
Because these modes of motion sensing 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. (One g equals 9.8 m/s2.) A MEMS accelerometer detects tilt by measuring the effect the force of gravity exerts on the 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.
Accelerometers use differential capacitors to measure g-force, which is then converted into volts or bits (in the case of digital output accelerometers) 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, greatly increasing the field of potential applications. A low-g sensing range is less than 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.
New Uses of MEMS Acceleration Sensors
Earlier we observed that acceleration comes into play for detecting movement and position. This creates the possibility of using a MEMS accelerometer to notice when a device is picked up and put down,which when detected can generate an interrupt that powers 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 recharging or replacing the battery. An intelligent remote control with a backlit LCD is among the potential scenarios.
Another way to use an accelerometer to sense movement and generate an interrupt would be in a radio for military or public safety personnel. To keep communication secure, when the radio stops being worn or carried, it could require reauthentication before permitting user access. Note that to be practical for a portable or small form factor design, these two preceding use cases would depend on accelerometers that draw little current: several microamps (μA) at most.
Another application for movement sensing is in medical equipment such as automated external defibrillators. Typically, AEDs have been designed to deliver a shock that gets the patient’s heart pumping again. When that fails, manual cardiopulmonary resuscitation 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.
The disk drive protection found in many notebook PCs is among the most widely implemented applications of shock sensing to date. An accelerometer detects tiny g-forces that indicate the notebook is falling or dropping, which is a precursor to a shock event: hitting the floor. Within milliseconds, the system orders the hard disk drive head to be parked. Parking the head stops contact with the disk platter during impact, preventing damage to the drive and the resulting data loss.
Industrial Uses of MEMS Acceleromters
Slight changes in vibration serve as a leading indicator of worn bearings, misaligned mechanical components, and other issues in machinery, including industrial equipment. Very small MEMS accelerometers with very wide bandwidth are ideal for monitoring vibration in motors, fans, and compressors. Being able to perform predictive maintenance lets manufacturing companies avoid damage to expensive equipment and prevent breakdowns that cause costly productivity loss. 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. Unless corrected, this inefficient operation could hurt a company’s green manufacturing effort and drive up its electricity bills or eventually lead to damaged equipment as well.
Industrial weigh scales are another example. In this application, 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. 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.
Accelerometer Driven Gesture Recognition
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 adjust the mode of operation. Gesture recognition makes devices more usable 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 products are only one application area in which accelerometer driven gesture recognition is finding a place. Thanks to extremely small, low power MEMS accelerometers, tap interfaces can be a good fit for wearable and implantable medical devices such as medication delivery pumps and hearing aids.
Tilt sensing has tremendous potential in gesture recognition interfaces as well. 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.
To learn more about expanding uses of MEMS accelerometers, please download the attached document by Analog Devices called "The Five Motion Senses: Using MEMS Inertial Sensing to Transform Applications."
 | The_Five_Motion_Senses.pdf |
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