MEMS sensors: the heart of drones

The size and scope of the drone market continues to grow, with new applications emerging all the time. Whether it’s delivering mail or packages, providing entertainment for children and seniors, security monitoring, agricultural or industrial management, or opening up new horizons in aerial photography, drones are becoming more and more common.

Initially, most drones were relatively simple toys. However, recently, their flight capabilities have improved significantly, making them safer, more stable, and easier to control, allowing them to be used in a wider range of real-life applications.

One of the key factors for this improvement is the use of high-performance micro-electromechanical systems (MEMS) sensors . And the drone sensor market is growing rapidly:

According to IHS Markit (Consumer and Mobile Device Motion Sensors – 2017), the market for MEMS motion sensors (i.e., accelerometers, gyroscopes, IMUs, and pressure sensors) in drones and toy helicopters is expected to reach approximately 70 million units by 2021, with a compound annual growth rate of 17% from 2018 to 2021.

The impact of MEMS sensors on UAV flight performance

Thanks to the use of inertial MEMS sensors, drones can ensure their orientation is stable and can be precisely controlled by the user or even fly autonomously. However, some challenges make drone system design very complicated, such as motors are not perfectly calibrated, system dynamics may change depending on the payload, operating conditions may change suddenly, or sensors have errors. These challenges will cause deviations in positioning processing and ultimately lead to position deviations during navigation, or even cause drone failure.

To make drones more than toys, high-quality MEMS sensors and advanced software are essential. The accuracy of a drone’s inertial measurement unit (IMU), barometric pressure sensor, geomagnetic sensor, application-specific sensor node (ASSN), and sensor data fusion has a direct and substantial impact on its flight performance.

Size constraints as well as harsh environmental and operating conditions such as temperature changes and vibrations raise the demands on sensors to a new level. MEMS sensors must be as immune to these influences as possible and provide precise, reliable measurements.

There are multiple approaches to achieving excellent flight performance: software algorithms, such as sensor calibration and data fusion; mechanical system design, such as vibration reduction; and MEMS sensor selection based on the drone manufacturer’s own requirements and needs. Let’s take a closer look at MEMS sensors and reference some examples.

The heart of a drone is its attitude and heading reference system (AHRS), which includes inertial sensors, magnetometers, and a processing unit. The AHRS estimates the device orientation, such as roll, pitch, and yaw angles. Sensor errors, such as offset, sensitivity errors, or thermal drift, can cause positioning errors. Figure 1 shows the positioning error (roll, pitch angles) as a function of the accelerometer offset, which is often the core source of continuous sensor errors. For example, an accelerometer offset of just 20 mg can result in a 1-degree error in the device orientation.

Figure 1: Tilt error caused by accelerometer offset

Inertial Measurement Unit (IMU)

An IMU consists of an accelerometer and a gyroscope, along with corresponding embedded processing. This enables it to identify motion in terms of both linear movement and rotation.

The BMI088 from Bosch Sensortec is a six-axis IMU with a low-noise 16-bit accelerometer and a low-drift 16-bit gyroscope. This high-precision device is derived from high-end automotive sensors and therefore offers excellent bias and temperature stability over long periods of time, as well as high vibration stability, making it ideal for drone applications.

Figure 2 shows the typical offset drift of the BMI088 at different temperatures.

Figure 2: Typical zero gravity and zero rate offset drift of the BMI088 at different temperatures

The accelerometer offset drift is shown to be in the range of only 10 mg, while the gyroscope sensor offset drift is less than 0.5 dps. In addition, the BMI088 exhibits a linear trend over temperature with very low hysteresis. This makes the BMI088 ideal for drone and robotic applications.

Air pressure sensor

A high-performance air pressure sensor built into the drone measures altitude precisely and is used in conjunction with the altitude control readings from the IMU. The air pressure sensor must be as immune to external influences and inaccuracies as possible.

Nowadays, distance sensors can be used to improve the reliability of the system and reduce position errors in combination with additional sensors such as GPS and optical flow.

The new BMP388 air pressure sensor from Bosch Sensortec provides altitude information to improve flight stability, altitude control, take-off and landing performance. This makes drone control a breeze, making it more appealing to a wider range of users.

The requirements for pressure sensors in drones are often very demanding. Due to the impact of non-ideal weather and temperature conditions, the altitude accuracy must be maintained within a tight tolerance range, and the sensor must have low latency and very low drift over long periods of time. The BMP388 meets these demanding requirements with a relative accuracy of +/-0.08 hPa (+/- 0.66m), an absolute accuracy of 300 to 1100 hPa +/- 0.5 hPa, and a low TCO of typically less than 0.75 Pa/K. It has an attractive price/performance ratio, low power consumption and an extremely small package size of only 2.0 x 2.0 x 0.75mm².

In addition to TCO improvements, multiple factors contribute to overall precision: relative accuracy, noise, stability, and absolute precision. From clumsy toys to high-precision aircraft; the potential for innovative industrial and commercial drone applications is now limitless, as long as engineers can imagine it.

Magnetometer

A magnetometer is like a compass that can help determine the direction of a drone based on the Earth’s magnetic field. An example is the BMM150 from Bosch Sensortec, a three-axis digital geomagnetic sensor.

The BMM150 is used in conjunction with the BMI088 IMU to provide a nine-degree-of-freedom (DoF) solution for heading estimation and navigation. Stable performance over a wide temperature range, 16-bit resolution, and immunity to strong magnetic fields (no magnetism for stable sensor offset) make the BMM150 ideal for UAV applications and minimize the effort required for sensor offset calibration.

Application Specific Sensor Nodes

The Application Specific Sensor Node (ASSN) provides a highly integrated smart sensor hub that combines multiple sensors in a single package with a programmable microcontroller. It provides a flexible, low-power solution for motion sensing applications.

For example, the BMF055 from Bosch Sensortec is an ASSN with an integrated accelerometer, gyroscope, magnetometer, and a 32-bit Cortex M0+ microcontroller for software management including sensor output. The BMF055 can be used as an AHRS in conjunction with positioning processing software. The device is available in a small 5.2 x 3.8 x 1.1 mm3 package, saving valuable space and weight. The sensor provides an all-in-one package for drone applications. Figure 3 shows the use of the BMF055 in a drone application as a positioning processing unit with an integrated sensor fusion algorithm.

Figure 3: The BMF055 (ASSN) is used as an AHRS in a UAV application.

Signal processing and software

In addition to individual sensors, we can also look at the overall signal processing structure of the drone at a system level and determine the software required to integrate sensor readings and control.

Figure 4 shows the different signal processing functions in a typical consumer drone. The left column shows the individual sensors, and the right column represents the derived software processing functions, such as localization processing and flight control algorithms. The dark blue sensor modules represent the most advanced sensors, which are mainly used to achieve stability for indoor and toy drones, and the gray sensor modules represent the extended optional functions required for outdoor flight and automatic waypoint navigation.

With integrated sensors, certain software functions, such as positioning processing, can be performed directly on the chip by mainly fusing various sensor readings. In addition to MEMS sensors, Bosch Sensortec also provides sensor data fusion software for positioning processing, which includes functions such as sensor calibration, sensor data preprocessing, and positioning processing. For drone manufacturers, this can significantly reduce engineering and software complexity, avoid unnecessary risks, and shorten time to market.

However, manufacturers still need to provide their own software, especially code unique to the mechanical design and dynamics of the drone, such as control loops and use-case specific functions.

Figure 4: Overview of signal processing for consumer drones

Typical drone functions

Let’s take a look at how innovative MEMS sensor technology is combined with software to enable modern drone capabilities.

Even in low-cost toy drones, complex functions are common today. First, the stabilizer uses the IMU output to keep the drone in a horizontal position. By integrating data from the air pressure sensor to maintain the drone at its altitude and position, for example, in toy applications, the drone can be flipped without changing altitude. As a result, pilots do not need many hours of practice to master basic maneuvers, and the risk of accidental collisions is significantly reduced.

Data fusion with the GPS module provides the drone with some interesting outdoor flight features, such as autonomous flight between multiple waypoints, and a "return to home" function, where the drone automatically returns to its starting position and lands safely.

Other newer features include “orbit mode” or “follow me mode,” where the drone orbits around a specific point or has the ability to autonomously follow a person. By incorporating cameras, pilots can now view themselves from a bird’s eye view while “taking the drone for a walk,” and even interact with the drone through hand gestures.