An Introduction to Motion Sensors: PIR, Tilt, Force, and More

Learn about various movement-type sensors including accelerometers, tilt sensors, vibration sensors, PIR (passive infrared) sensors, and rotation sensors.

Sensors Are Everywhere

We are surrounded by all kinds of sensors in this modern and magical age we live in. Consider the many sensors in a late-model automobile: The engine compartment has crankshaft and camshaft angle sensors, the seats have pressure sensors (to determine if a child or an adult is occupying the seat) and seat belt sensors, and of course life-saving crash sensors are used to inflate airbags in the event of a collision. It is estimated that current model cars have between 60 and 100 sensors per car, and newer (smarter) cars will have close to 200 sensors.

Ever wonder how your smartphone knows when to rotate its screen—from landscape to portrait—depending on its orientation? You can thank accelerometers for that magic trick. These same accelerometers get credit for allowing you to play games on your phone that require movement of the phone itself. While accelerometers are the actual gravity sensors, both magnetometers and gyroscopes play a role by removing linear acceleration from the accelerometer data.

PIR

The PIR, or passive infrared, detector is commonly used in intruder alarm systems. A PIR is similar to a thermal sensor. These infrared sensors/detectors are the secret sauce inside motion sensor security lights that illuminate driveways when someone approaches. Infrared is outside our eyes' light-detecting abilities (see Figure 1 below).

Figure 1. Visible and Infrared spectrum. Image courtesy of Digital Earth Watch from UC, Berkeley

PIRs are pyroelectric devices—i.e., they can generate a voltage when heated or cooled—that detect motion by sensing changes in the radiant heat (aka infrared) emitted by surrounding objects.
PIR sensors typically include a field-of-view and/or a linear distance specification, and most come with a plastic lens. Figure 2 below is an example of a PIR sensor.

Figure 2. PIR sensor. Image taken from Parallax Inc's 555-28027 datasheet.

Accelerometer

As mentioned previously, accelerometers are inside smartphones (and tablets) resulting in accelerometers being low-cost and readily available given the ultra-mass production of smartphones.
Many accelerometer devices use MEMS (microelectromechanical systems) as the critical technology for sensing acceleration. STMicroelectronics accelerometers use a proprietary process that allows suspended silicon structures to freely move in the direction of the sensed acceleration. According to STMicroelectronics, "When an acceleration is applied to the sensor the proof mass displaces from its nominal position, causing an imbalance in the capacitive half-bridge. This imbalance is measured using charge integration in response to a voltage pulse applied to the capacitor" (LIS2DH datasheet, page 18). See Figure 3.

Figure 3. STMicroelectronics MEMS 3-axis accelerometer. Image taken from the LIS2DH datasheet.

Tilt Sensors

Somewhat similar to the accelerometer, but far less complicated in design, is the tilt sensor. The tilt sensor, also referred to as a motion detector or a tilt switch, is exactly as it sounds: it's a sensor that detects tilting, specifically the tilting from the horizon. There are multiple tilt sensing design types, but a common one is called the rolling ball sensor switch (see Figure 4). The rolling ball sensor switch works by means of a rolling ball which, when tilted to a critical point, either makes or breaks contact with two or more conducting pins.

Tilt sensors' specifications normally include the sensor angle, which is given in units of degrees.
Although not commonly used today, the original type of tilt switch—the mercury tilt switch—can still be found, if you look hard enough. If you find a tilt switch that is RoHS compliant, rest assured that it will not contain mercury.

Figure 4. Rolling ball sensor switch. Image taken from C&K's datasheet (p/n: RB-220-07 R).

Vibration Sensors

There are many different methods and sensors used for measuring and detecting vibrations. One such sensor is the aforementioned accelerometer. Many digital accelerometers have user-selectable sensitivity settings which allow the devices to detect accelerations ranging from very large to very small.

The piezoelectric sensor (see Figure 5) is ideal for vibration sensing technologies. A piezoelectric sensor uses the piezoelectric effect to measure changes in acceleration, strain, or force by converting the physical change to an electrical change. In addition to detecting vibrations, these same types of sensors are used for detecting mechanical shock.

Figure 5. A piezoelectric vibration sensor. Image courtesy of SparkFun

Rotational Sensors

Perhaps the most simple rotational sensor is using a potentiometer as a voltage divider where the voltage is proportional to the angle of rotation.
Quadrature encoders, commonly used for measuring rotational position, provide a specific number of equally spaced pulses per revolution. A quadrature encoder is a type of incremental encoder. Incremental encoders provide relative position feedback (and can also provide speed and direction) by generating a stream of binary pulses that are proportional to the rotation of a motor's shaft. Quadrature encoders have two channels that are 90 electrical degrees out of phase with one another. See Figure 6.

Figure 6. Quadrature encoder's binary pulses. Image courtesy of EmbeddedRelated.com

Other rotational sensors include hall-effect-based rotational sensor ICs and rotary position sensors.

Force Sensors

There are many types of force sensors available, two of which are strain gauges and load cells.
A load cell converts the deformation of a material, measured using a strain gauge, into an electrical signal. One type of a load cell uses a bending beam. Let's assume a strain gauge is mounted on the beam. As a force is applied to the beam the beam begins to slightly bend resulting in bending the strain gauge, leading to a change in its electrical output signal. See Figure 7 for an example of a load cell.

Figure 7. Load cell depiction. Image adapted from Tekscan

Pressure Sensors

There are many different types of pressure sensors and technologies used to make pressure sensors.
Types of pressure sensors:

• Absolute pressure sensor: This sensor measures a pressure relative to a perfect vacuum.
• Gauge pressure sensor: This type of pressure sensor measures a pressure relative to the atmospheric pressure.
• Sealed pressure sensor: This sensor measures a pressure relative to some other known and fixed pressure.
• Differential pressure sensor: This pressure sensor measures the difference between two pressures. The sensor "sees" each of the two pressures at the same time. See Figure 8 below.

Figure 8. Differential pressure sensor. Note the two ports. Image courtesy of Modern Device

Technologies used in pressure sensors:

• Piezoresistive strain gauge technology is used to detect a force or a tension as a result of an applied pressure.
• Capacitive pressure sensors use a diaphragm and a pressure cavity to create a variable capacitor.
• Piezoelectric sensors use the piezoelectric effect to measure the resulting strain from applied pressures.