A drone, formally called as unmanned aerial vehicle, is an unmanned flying object. It is primarily controlled by remote control or inbuilt intelligence for navigation. It is a prime example of how embedded systems work together to bring a complete system which can navigate, self-balance, attain height and transfer information with the remote user or machine.
Traditionally drones have been used for many defense and scientific applications such as reaching remote areas, surveillance, weather monitoring and delivering goods in tough conditions. In the recent times with the advent of low-cost, small-size and low-power embedded technologies drones are increasingly used in consumer applications. Drones are now used as toy, photography assistance, cargo delivery vehicle, pesticide spraying vehicle etc.
Drones are attracting significant level of interests from startups, students, system integrators, solution providers, e-commerce and technology providers. Drones are expected to make our lives better, safer and more efficient.
This article elaborate major advancements in drone sensor and lower power connectivity technologies which are key to its exponential growth in recent times. .
Drones are expected to be very precise in their movement, be stable, attain expected height and communicate effectively. So a typical drone should have following attributes.
A drone is expected to be stable and do not shake, wobble or tilt unexpectedly. Else it will lose its balance and fell down.
A drone should be very precise its movement. The movement may be in terms of distance, velocity, acceleration, direction and altitude.
c) Resistant to environment
A drone should be resistant to rain, dust, heat and other environment conditions. This applies not only to the outer material but also to the electronics which is used inside a drone.
d) Low Power
Drone are expected to be lighter so it is important to ensure that it consumes ultra-low power to minimize the size of battery. The emergence of low power technologies has enabled democratization of drones’ technology.
e) Environment aware
Environment sensing is emerging as a key growth area for drones. Today drones are powered by several sensors to monitor the environment. The data is used for variety of applications such as weather monitoring, agriculture etc.
Connectivity is the most important factor for emergence and acceptance of drones. Drones can be controlled by simple smart phone, a remote controller and directly the cloud. Depending upon the use case a right connectivity solution should be used. Sometime drones use multiple connectivity solution for multipurpose use cases.
Role of Sensors in Drones
These sensors form the core part of drone and ensure its proper functioning and navigation. These sensors are accelerometers, gyroscope, magnetic compass and barometer sensor.
Accelerometer is used to provide the acceleration force which the drone is subjected to in all three axis X, Y and Z. It also determines the tilt angle of drone in stationary position. If the drone is stationary in horizontal position then its X and Y axis will give 0g output whereas z-axis will give 1g output. 1g is the gravity which is experienced by every object on earth. If the drone rotate by 90deg on X axis then X and Z will give 0g and Y axis will start will start giving 1g. During the tilt X, Y and Z will give output which lies between 0 and 1g. The values can then be applied to trigonometry formulas to arrive at tilt angle of drone.
Accelerometer are also used to give linear acceleration in horizontal and vertical direction. This data can be used to calculate velocity, direction and even rate of change of altitude of the drone. Accelerometer is also used to detect the vibration which the drone is experiencing.
Accelerometer is a very important sensor in any drone as it provide critical inputs even when drone is in stationary position.
Gyroscope sensor detects angular velocity in three axis. So it can detect rate of change of angle in pitch, roll and yaw. Gyroscope is a critical sensor even in regular craft. The change in angle information is used to provide stability to drone and to prevent it from wobbling. The information from gyroscope is fed to motor control drivers to control the speed of motors dynamically to provide the stability to motor. Gyroscope also ensure that drones rotate at exact angle which is expected by user controls.
STMicroelectronics latest innovation, LSM6DSM, is an IMU with combination of accelerometer and gyroscope. It also offers two independent gyroscope outputs – a feature that can be used for flight control and, at the same time, for the camera stabilization (OIS/EIS function), if available in the drone.
Magnetic compass as the name suggests gives the sense of direction to the drone. It gives data of magnetic field in three X, Y and Z which the device is subjected to. This data is then fed into algorithm in the microcontroller to give heading angle w.r.t magnetic north. This information is then used to detect geographical directions.
To get accurate direction the magnetic data should be complemented by tilt angle data from the accelerometer. The tilt data along with magnetic data will then be used to calculate accurate direction.
A magnetic compass is very sensitive to hard iron, soft iron or angle of operation. Hard iron is the presence hard permanent ferro-magnetic material in the vicinity of sensor. It create a permanent shift in compass reading. Soft iron is the presence of weak ferro-magnetic material, circuit traces etc. It create a variable shift in the sensor reading. So a magnetic sensor calibration algorithm is needed to filter out these anomalies. It is important for the algorithm to do fast calibration with minimum effort by the user.
Apart from direction sensing, magnetic sensor can also be used to detect surrounding magnetism and ferro-metals such as electricity poles, wires, vehicle, other drones etc. to help avoid accident.
A barometer working principle is to convert atmospheric pressure into altitude. Pressure sensor can detect earth’s atmospheric pressure. The data from Barometer helps in drone navigation and achieve desired altitude. Very good estimation of ascend and descend speeds is very vital for drones flight control. STMicroelectronics has introduced pressure sensors, LPS22HD, with 200Hz of data rate to address this requirement of altitude estimation.
Important characteristics for sensors in drones
Drones are subjective to extreme conditions including vibration, noise and environment. A sensor used in drone should have high shock survival capability, should capture less noise and fast enough to capture all vibration. Its performance should not vary with change in environment parameters such as temperature and humidity. And finally it should consume ultra-low power to have enhanced battery life.
Role of Algorithms
Software libraries play an important role to convert raw sensor data into meaningful use case. The algorithm augment the function of sensor beyond their stated features. The algorithm also combine the input from various sensors and create a context aware output.