Source: WeChat Official Account “Sensor Technology”
With the advancement of drone technology, drones have gradually expanded from military to civilian applications, becoming a rapidly developing emerging industry in recent years. Today, drones are widely used in meteorological monitoring, land resource enforcement, environmental protection, remote sensing aerial photography, disaster relief, and delivery services. As the Internet of Things (IoT) develops, the application of IoT technology in drones continues to increase. To better control drone flight, the use of various sensors plays a crucial role. Therefore, some refer to drones as “flying sensors.” So, what sensors are needed for a drone to achieve stable flight and perform different maneuvers in the sky?
Drones’ Characteristics
The movements of a drone must be very precise; in addition to stability, it must be able to fly to the expected altitude and communicate effectively. Therefore, a basic drone must have the following characteristics:
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Stability
A drone should be stable, without sudden vibrations, shakes, or tilts, or it will lose balance and crash.
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Precision
The movements of a drone must be very precise. These movements may refer to distance, speed, acceleration, direction, and altitude.
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Resistance to Various Environmental Conditions
A drone must be able to withstand rain, dust, high temperatures, and other environmental conditions. Moreover, not only the external materials but also the electronic components used inside the drone must meet these requirements.
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Low Power Consumption
Drones are becoming lighter, so ensuring ultra-low power consumption to minimize battery size is crucial. The rise of low power consumption technology has made drone technology more widespread.
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Environmental Perception
Environmental sensing technology has gradually emerged as one of the most critical areas of drone development. Today’s drones are equipped with several sensors to monitor the environment. The data collected can be used in various applications, such as meteorological monitoring and agriculture.
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Networking Capability
Networking capability is an important factor for the rise and market acceptance of drones. Drones can be controlled through simple smartphones, remote controllers, or directly via the cloud. Suitable networking solutions should be provided based on different use cases. Some drones may adopt multiple networking solutions to meet the needs of various applications.
Flight Controller
The flight controller (FC) is akin to the brain of the drone, comparable to the operating system on a computer or smartphone. The flight controller obtains data from various sensors mounted on the drone, processes this data, and controls the flight of the drone. Additionally, the flight controller is responsible for information transmission.
The flight controller mainly consists of two parts—IMU (Inertial Measurement Unit) and CPS module. The flight performance of a drone largely depends on this flight controller. The internal sensors (IMU) within the flight controller are essential for stable flight. IMU refers to the Inertial Measurement Unit, commonly used in devices requiring motion control, such as cars and robots, and in scenarios requiring precise displacement calculations based on orientation. Generally, IMUs include accelerometers and gyroscopes.
Sensors on Drones
The IMU is located at the core of the drone, ensuring that the device functions and navigates correctly. These sensors include accelerometers, gyroscopes, magnetometers, and barometric sensors.
Accelerometers
Accelerometers are used to provide the acceleration forces experienced by the drone in the XYZ three-axis directions. They can also determine the tilt angle of the drone when it is stationary. When the drone is in a level stationary state, the X-axis and Y-axis output 0g, while the Z-axis outputs 1g. All objects on Earth experience a gravitational force of 1g. If the drone rotates 90 degrees on the X-axis, then 0g will be applied to the X-axis and Z-axis, while 1g will be applied to the Y-axis. When tilted, the XYZ axes output between 0g and 1g. The related values can be applied to trigonometric formulas to allow the drone to achieve a specific tilt angle.
Accelerometers also provide linear acceleration in both horizontal and vertical directions. The relevant data can be used to calculate rate, direction, and even the rate of change of the drone’s altitude. Accelerometers can also monitor the vibrations experienced by the drone.
For any drone, the accelerometer is a crucial sensor, as it provides key input even when the drone is stationary.
Gyroscopes
Gyroscope sensors can monitor the angular velocity across three axes, allowing the detection of changes in pitch, roll, and yaw angles. Gyroscopes are also essential sensors for conventional aircraft. Changes in angle information can be used to maintain drone stability and prevent shaking. The information provided by the gyroscope will be fed into the motor control driver, dynamically controlling motor speed and providing motor stability. Gyroscopes also ensure that the drone rotates according to the angles set by the user’s control device.
Magnetometers
As the name suggests, magnetometers provide the drone with a sense of direction. They can provide data about the magnetic field experienced by the device along the XYZ axes. This data is then processed by the microcontroller’s algorithm to provide the heading angle relative to magnetic north, which can be used to detect geographical orientation.
To calculate the correct direction, the magnetic data also requires tilt angle data from the accelerometer to enhance the information. With tilt data combined with magnetic data, the correct orientation can be calculated.
Magnetometers are very sensitive to hard iron, soft iron, or operational angles. Hard iron refers to hard, permanent ferromagnetic materials near the sensor, which can cause permanent offsets in compass readings. Soft iron refers to weak ferromagnetic materials nearby, such as circuit traces, which can cause variable shifts in sensor readings. Therefore, it requires magnetic sensor calibration algorithms to filter out these anomalies. The most important thing is to ensure that users do not have to exert effort; the algorithm can quickly perform the calibration.
Besides sensing direction, magnetic sensors can also detect surrounding magnetism and ferrous metals, such as electrodes, wires, vehicles, and other drones, to avoid accidents.
Barometers
The barometer operates on the principle of using atmospheric pressure to calculate altitude. The pressure sensor can detect the atmospheric pressure of the Earth. The data provided by the barometer can assist in drone navigation, allowing it to rise to the required altitude. Accurately estimating the ascent and descent rate is crucial for drone flight control. STMicroelectronics has launched the LPS22HD pressure sensor, which can meet the demand for predicting altitude with a data rate of up to 200Hz.
Ultrasonic Sensors
Drones use ultrasonic sensors to take advantage of the property that ultrasonic waves bounce off other substances for altitude control. As mentioned earlier, when close to the ground, the barometric sensor cannot respond effectively. However, using an ultrasonic sensor near the ground can achieve altitude control. Thus, when the barometric sensor is combined with the ultrasonic sensor, the drone can achieve stable flight at both high and low altitudes.
GPS
Just like cars have navigation systems, drones also have navigation systems. Through GPS, it is possible to know the positional information of the drone. GPS is one of the global navigation systems and is the satellite navigation system of the United States. However, recent drones are not only using GPS; some models combine GPS with other satellite navigation systems to receive multiple signals and detect the position of the drone. Whether setting longitude and latitude for automatic flight or maintaining position for hovering, GPS is an extremely important function.
However, due to the constant movement of satellites and the influence of buildings and magnetic fields, there are situations where GPS signals cannot be received. This is worth noting.
Of course, in addition to the sensors mentioned above, drones may also use sensors to detect voltage and current status, as well as infrared sensors for obstacle detection. It is these sensors, which resemble human senses, that enable drones to fly stably in the air.
Application-Specific Sensors
This type of sensor does not affect the core functionality of the drone but is increasingly used on drones to provide various applications, such as climate monitoring and agricultural purposes.
Humidity Sensors: Humidity sensors can monitor humidity parameters, and the relevant data can be applied in weather stations, condensation height monitoring, air density monitoring, and adjustment of gas sensor measurement results.
MEMS Microphones: MEMS microphones are audio sensors that convert sound frequencies into electronic signals. MEMS microphones are gradually replacing traditional microphones because they provide higher signal-to-noise ratios (SNR), smaller form factors, better radio frequency interference resistance, and are more robust against vibrations. These sensors can be used in drone video recording, monitoring, and surveillance applications.
Processing and Transmission of Sensor Data
To convert raw sensor data into meaningful use cases, software databases play a significant role. Algorithms can expand sensor functionality beyond the originally known range. Algorithms can also combine inputs from different sensors to produce outputs with contextual awareness.
Accelerometers, gyroscopes, and magnetometers each have different advantages and disadvantages. The limitations of sensors include insufficient calibration, and they can drift due to time, temperature, and random noise. Magnetometers and accelerometers are prone to distortion, while gyroscopes inherently exhibit drift. We can use sensor fusion databases to mutually calibrate these sensors to create conditions that yield correct results in all situations. It can provide not only calibrated sensor outputs but also information on angles and heading angles, as well as quaternion angles.
Users can also access various advanced databases through a simple computer licensing agreement. Once tested on the platform, designers can develop their dedicated printed circuit boards and load the firmware they developed on the platform. Users only need to sign the production license for the database when they want to test the dedicated circuit boards.
SensorTile: SensorTile is a square miniaturized design platform that contains everything needed for remote sensing and measuring motion, environmental, and acoustic parameters. Developers can immediately focus on the drone’s aerodynamics, motor control, and physical design without worrying about networking functions and sensor integration.
Drone Networking
Drones have various networking technology options to consider. Low Power Bluetooth (BLE) and Wi-Fi are often used for smartphone connectivity, while Sub-1GHz is used in remote controllers, providing longer-range networking capabilities.
The following image lists the differences between different technologies in terms of effective range and energy consumption. Next, we will further discuss BLE, RF sub-1GHz, and low-power technologies such as Sigfox.
Bluetooth Smart Low Energy Technology (BLE)
Bluetooth Smart, also known as Low Energy Bluetooth (Bluetooth Low Energy, BLE), provides low-power networking capabilities for drones. This technology is suitable for low-end models, particularly toy drones. It allows drones to communicate bidirectionally with smartphones, tablets, laptops, or dedicated remote controllers. Low-power Bluetooth enables drones to achieve excellent battery life, which is not possible with traditional wireless technologies like Wi-Fi or classical Bluetooth.
Low-power Bluetooth operates in the 2.4GHz free ISM frequency band. The relevant standards are managed by the Bluetooth Special Interest Group (Bluetooth SIG) and are supported by major smartphone brands.
Low-power Bluetooth devices have two main approaches:
a. Network Processor
A network processor is a low-power Bluetooth device that executes low-power Bluetooth communication protocols, including controllers, main components, and stack. However, it requires a separate microcontroller to work smoothly with the main microcontroller executing low-power Bluetooth profiles and applications. It is also an independent platform that provides greater flexibility for users to choose the most suitable microcontroller or operating system. BlueNRG-MS is a network processor launched by STMicroelectronics that supports the BLE 4.1 specification. This IC can act as both master and slave, allowing the remote controller to serve as a slave device to the smartphone while also being the master device for the drone.
b. System-on-Chip (SoC)
A system-on-chip is an independent chip set that includes controllers, main components, stack profiles, and applications. STMicroelectronics’ BlueNRG-1 is a system-on-chip certified by BLE 4.2, which includes 15 GPIO, I2C, SPI, UART, PWM, PDM, and 160kb of RAM. Because it supports the BLE 4.2 specification, this IC also provides advanced security and privacy features.
RF sub-1GHz
As the name suggests, RF sub-1GHz transmits signals using frequencies below 1GHz. Each country defines different frequencies, which are freely provided for industrial or scientific research purposes.
The following are the free bands provided by various countries:
• North America: 315, 433, 915MHz
• Europe: 433, 868MHz
• India: 433, 865-867MHz
The advantages of the sub-1GHz frequency are that these bands are relatively quiet, have longer ranges, and consume very low current. The downside is that they cannot directly provide smartphone connectivity, and they are not usable everywhere.
Drones are one of the most important innovative technologies in recent years. With the advent of low-power sensors and networking technologies, today’s drones can be widely used in various consumer and industrial applications. Drones provide new opportunities for developers and innovative companies to solve complex problems that were previously considered impractical or too expensive.
Editor: Lemon
