Accelerometers: Motion and Position Sensing is Becoming Very Personal

Because they can accurately determine the speed, direction and position of a body in motion, accelerometers play an indispensable role in providing digital electronic circuits with real-world information for a multitude of applications.

They are most often used to measure:

- Velocity and position

- Inclination, tilt and orientation

- Vibration and impact

The information provided has a multitude of applications. Accelerometers have a long history in inertial navigation systems for aircraft and missiles as well as for detecting and monitoring vibration in rotating machinery. In the past decade, consumer products such as smartphones, tablets, laptops, watches, gaming consoles and athletic shoes began integrating micromachined accelerometers. These devices implement functions such as fitness monitoring, human interface control and device protection.

The basic operating principle of an accelerometer is simple: It consists of a damped mass on a spring. (The damping element is usually the air surrounding the mass.) When a force displaces the mass, the spring exerts a force in the opposite direction. The mass moves; air acts as the damper.

Together, the spring, mass and air comprise a second-order lumped physical system, which can be characterized by a set of formulas. Assuming that we know the physical characteristics of the mass, the spring and the air, acceleration can be calculated by measuring how far the mass is displaced by the force acting on it. Acceleration is measured in units of gravity force (g). Knowing the acceleration makes it possible to calculate velocity, direction, position and other characteristics of a body in motion such as pitch and yaw.

One way to measure displacement is to add a capacitor to the system (Figure 1). When the mass moves, the distance between the capacitor electrodes also moves, changing the capacitance. Measuring the change in capacitance accurately predicts the amount of movement. There are several other ways to measure displacement distance. Prominent technologies are: Piezoresistance, lasers, potentiometers and Hall-effect sensors.

 

Figure 1: Capacitive accelerometer structure. Image credit to Wiki.

Accelerometer Specifications

The most important specification of an accelerometer is its type of output. Analog accelerometers output a constant variable voltage depending on the amount of acceleration applied. The output of digital accelerometers is a variable frequency square wave generated by pulse-width modulation (PWM). A digital accelerometer takes readings at a fixed rate, typically 1 kHz. Acceleration is proportional to the pulse width of the PWM signal. Both analog and digital types are widely used.

Although the list of accelerometer specifications is fairly extensive (output range, sensitivity, dynamic range and bandwidth), another one that must be mentioned is the number of axes. The basic operating principle described above is for a one-axis device. It is useful for applications that need to measure acceleration in only one dimension. It is a three-dimensional world, however, and two- and three-axis devices are also available.

An accelerometer with two orthogonal sensors can determine pitch and roll and is useful for capturing head movements. Adding a third orthogonal sensor provides orientation in three-dimensional space and is used, for example, to detect the orientation of the Apple pencil or human falls.

Going Tiny

Microelectromechanical sensor (MEMS) technology is responsible for the transition from accelerometers the size of a jewelry box into ultraminiature devices realized in silicon. MEMS technology, by the way, is used for many kinds of microminiature sensors and not just accelerometers.

MEMS accelerometers are created with micromachining technology that etches into the chip substrate a tiny bar of silicon supported at each end by equally tiny posts. When the chip moves, the bar deforms. This deformation can be converted into an electrical pulse by measuring the change in capacitance just as in the description above.

Initially, MEMS accelerometers used piezoresistors, but capacitive detection is more sensitive and is now the most popular distance-sensing technology.

One of the earliest applications enabled by MEMS accelerometers were the air-bag safety systems in automobiles. More recently, smartphones and other personal devices began using accelerometers for user-interface control such as automatically presenting a landscape or portrait view of the screen depending on how the device is being held. Since they were first introduced, fitness trackers have used accelerometers to count steps and generally monitor the wearer’s movements.

The latestand likely the tiniestwellness devices to integrate accelerometers are smart rings. In addition to a three-axis accelerometer, the inner rim of the ring has an infrared optical pulse sensor, a gyroscope and a body temperature sensor. Different manufacturers target different aspects of wellbeing. At least one focuses on sleep and uses the sensor to suggest how well the wearer slept overnight and what level of activity is appropriate to the day ahead.

Many commercial and industrial applications use accelerometers to detect if a system has been dropped or is falling. Laptop PCs, for example, park the head of a hard disk to prevent a head crash and resulting data loss upon impact. Sensing vibrations in machinery and recognizing the patterns that will lead to damage is a widely used diagnostic tool in the industry that saves both equipment and labor costs caused by downtime.

Accelerometers are used in medical applications to monitor the movement of patients, including falls. A three-axis accelerometer continuously monitors the person’s velocity and direction. When these variables are graphed, a fall exhibits a very different graph than, for example, the person just sitting down. If the graphs of the two variables match the patterns known to be created by a fall, the incident is classified in the danger category (the length of time that the wearer is on the floor is also measured) and the system automatically sends a fall alert and calls for help.

Conclusion

The explosive growth of accelerometer technology has resulted in these tiny sensors being integrated into any device that might provide engineers with information about variables such as position, velocity inclination and tilt. It is not too surprising that the range of applications is so great. Forty years ago, computers were used primarily to crunch numbers and output reports. Industrial processes were an exception because chemical and mechanical processes had to monitor real-world variables such as temperature, pressure and pH to control the processes. The relatively recent revolution has been very personal: Monitoring human beings or, at least, processes that have human beings at the center of the data gathering. The trend is sure to continue with accelerometers assuming a leading role.

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