The three motion sensors – accelerometer, gyroscope and magnetic compass – have their own advantages and disadvantages. Sensor limitations include imperfect calibration, drift over time or temperature and random noise. Magnetometer and Accelerometer are subject to distortions and gyroscope has inherent drift. Sensor fusion library is used to calibrate there sensors with each other to create a condition which gives accurate result in all scenarios. It not only give calibrated sensor outputs but also angle and heading angle information and Quaternion angles.
Applications Specific Sensors
These sensors do not impact the core functioning of drone but are increasingly used in drones for variety of applications such as weather monitoring, farming etc.
Humidity sensor can detect humidity parameter which can be used at weather station, condensation level monitoring, air density monitoring and gas sensors measurement correction.
ST has introduced a humidity sensor HTS221 which includes a sensing element and an analog front end to provide the measurement information by digital serial interface. The sensing element consists of a polymer dielectric planar capacitor structure able to detect relative humidity variations.
MEMS Microphone is audio sensor that converts sound signals into electrical signals. MEMS Microphones are increasingly preferred over conventional microphones as they offer higher SNR, small form factor, digital interface, better RF immunity and high robustness against vibration. The sensors are in in drones for applications such as videography, surveillance, spying etc.
Connectivity in Drones
There are various connectivity options considered for drones. BLE and Wi-Fi are used for smart phone connectivity, sub-1GHz is used for connectivity with remote controller and give longer distance connectivity. Cellular and Long range technologies such as LoRa and Sigfox are used to collect information from drone directly to telecom infrastructures.
The map of various technologies on scale of distance and power consumption is listed below. We will discuss lower power technologies such as BLE, RF sub-1GHz and Sigfox in a greater detail.
Bluetooth low Energy – Bluetooth Smart [BLE]
Bluetooth Smart or Bluetooth Low Energy [BLE] enables low power connectivity to drones. It is suitable for low range drones particularly the toy drones. It enable two way communications between drones and control devices such as smartphone, tablet, laptop and dedicated remote controller. BLE allows drones to have incredible battery life which is not possible using traditional wireless technologies such as Wi-Fi and Classical Bluetooth.
Bluetooth Low Energy works in license free 2.4GHz ISM band. The standard is governed by Bluetooth Special Interest Group (SIG) and is support all the major smartphone brands.
There are two major partitioning available in BLE devices.
a. Network processor
A network processor is a BLE device which is running BLE protocol including controller, Host and stack. But it need a separate microcontroller for its functioning with main microcontroller running BLE profiles and application. This platform is also platform independent so offers wide flexibility to user to select best fit microcontroller or operating system. BlueNRG-MS is a network processor offered by ST and compatible with BLE 4.1. This IC can act as Master and Slave at the same time. So it is possible to have a remote controller acting as slave to Smartphone but master of drone.
b. System on Chip
System on Chip (SOC) is a devices which is an independent chipset and includes controller, Host, stack profiles and application. BlueNRG-1 from ST is BLE 4.2 certified SOC and includes 15 GPIOs, I2C, SPI, UART, PWM, PDM and 160kb of RAM. This IC also offers advance security and privacy features offered by BLE 4.2.
RF sub-1GHz as the name suggests carries signal over sub-1GHz frequencies. Different countries have defined different frequencies which can be freely used for Industrial and scientific usage.
Below are some of free bands available in various countries
• North America : 315, 433, 915Mhz
• Europe : 433, 868Mhz
• India : 433, 865-867Mhz
The advantage of sub-1GHz frequencies is that they are relatively quieter bands, have a longer range and consume low current. Down side is that they do not offer direct smartphone connectivity and not universally present.
Sigfox is subscription based LPWAN service based on sub-1GHz frequencies. Sigfox allows connectivity directly to telecom infrastructure and in-turn to cloud. It can give connectivity over several Km. In drones Sigfox can be used for tracking and to send sensor data to the cloud. It is low data rate technology directly to telecom infrastructure so not expected to replace direct control connectivity options such as BLE and RF sub-1GHz direct peer to peer connection.
STMicroelectronics has recently launched RF sub-1GHz and Sigfox transceiver S2-LP. This product can communicate at 430-470 MHz and 860-940 MHz. It has output power up to +16dbm and sensitivity down to -130dbm.This extremely low current IC can be used as peer to peer connection to establish connection between drone and remote controller. It is also a certified Sigfox product so it can be configured to directly connect drone to Sigfox network.
The wave of emerging applications such as drones is mainly driven by startups and young Engineers. Moreover the market is evolving at such a fast pace that chancing of success depends heavily on time and cost involved. It is very important to have a development platform which is open source, affordable, flexible, production ready and scalable.
STMicroelectronics has introduced a STM32 open development environment platform which allows designers to develop products using ST microcontroller, sensors, RF and Analog products in a very flexible and affordable way. The hardware platform is complemented by software drivers, middleware libraries and applications. It also include associated Android and iOS code.
The user can also access many advance libraries by a simple computer based license agreement. Once the platform is tested designer can develop his custom PCB and load the firmware developed on this platform. The user will need to sign the production license for libraries only when he want to test on his custom board.
SensorTile is a miniaturized tile-shaped design platform. It includes all that is needed to remotely sense and measure motion, environmental and acoustical parameters. A drone developer can straightaway focus on aerodynamics, motor control and physical design of the drone without worrying about connectivity and sensor integration.
Drones are one of the most important innovation of recent past. With the advent of lower power sensor and connectivity technology drones are now used is wide range of consumer and industrial applications. Drones offers new opportunities for developers and startups as they are solving many complex problem which were unpractical and expensive earlier. STMicroelectronics recognizes drone an important market to focus and is focusing on products suitable for this segment.