Gyros and accelerometers: what they do and how to choose one

If you use a smartphone, handheld electronics gaming accessory or an electronic fitness band, it’s very likely you are using one or more accelerometers, gyroscopes (gyros) or an inertial measurement unit (IMU), which combines these devices, sometimes with other electronic parts such as signal processors. As the IoT revolution progresses, and more devices are connected to the Internet, many of them will use gyros and accelerometers, together with a host of other sensors, to detect and measure physical attributes that tell us what is going on in the world around us.

 

Gyroscopes were discovered thousands of years ago, apparently by a number of ancient civilisations. First used as spinning toys, it was only in the 18th and 19th centuries that they started to be used to make measurements. Gyros measure how fast something spins around a given axis. They come in 1-, 2- and 3-axis versions. What we normally call x, y and z axes are sometimes referred to as roll, pitch and yaw when we talk about gyros, the terminology originating from the rotation motions of ships, which gyros are used to measure.

 

Figure 1: 3-axis gyros, like the L3G4200D from STMicroelectronics, are now available for just a few Euros (pounds or dollars) each

 

Today, most gyros used in electronic products are based on MEMS technology. 2- and 3-axis gyros used to be expensive but their cost has fallen significantly in recent years. In addition to being found in the consumer products mentioned earlier, they’re used for image stabilisation in cameras and in a host of other applications from industrial robotics to automotive and crane stabilisation systems, and radio controlled planes and helicopters. Units of measurement are usually revolutions per minute, or RPM. The RS Components range of gyros can be found here.

Accelerometers measure acceleration. The units of measurement are usually metres per second per second (m/s2) or G-force (g). They can be also be used to detect orientation (as in the iPhone) because they detect gravitational acceleration when used in static mode. In dynamic mode, they sense acceleration during movement. The RS Components range of accelerometers can be found here.

Figure 2: Analog Devices offers the ADXL335BCPZ 3-axis accelerometer in a tiny 16-pin, LFCSP EP package and there’s this option of an evaluation board

 

When choosing an accelerometer or gyroscope you need to consider:

 

  • The number of axes you want to measure
  • The operating range – maximum and minimum values, and whether these are selectable or fixed
  • How sensitive it is
  • Its resolution – how accurately you can make measurements
  • How it interfaces to your system: including whether analogue or digital outputs are provided (nearly all are now digital)
  • How much power they consume, and if there are sleep modes to help you save energy, which is particularly important in battery-powered devices
  • Form factor – how well they will fit in the space you have available. Some devices are available in BGA or LGA packages, others in conventional surface mount packages

 

If you want the best of both worlds, MikroElektronika’s InvenSense MPU-6000 is a 6-axis motion-tracking device that combines a 3-axis accelerometer, a 3-axis gyro and a digital motion processor. It’s an IMU on a small PCB and it communicates with your target board processor through mikroBUS™ SPI, I2C, RST and INT lines.

 

Figure 3: The MPU-6000 IMU provides 6-axis motion tracking

 

There’s also the Murata SCC1300-D04, which combines a gyro with a 3-axis accelerometer. This is a high-performance, wide range, robust device that provides ±300 °/s angular rate measurement and ±6 g 3-axis acceleration measurement.

 

Figure 4: Murata’s SCC1300-D04 is designed for applications in demanding environments where wide range, high accuracy and mechanical ruggedness are paramount considerations