The emergence of drones has provided many conveniences to people's lives and has been widely used in all walks of life in society. But its limited flight time has always been a headache for researchers. At present, drones mainly rely on six kinds of power sources to complete complex tasks.
1. Lithium battery: Most drones are equipped with lithium batteries, but the effect can only last for about 20 minutes, and the batteries need to be removed and replaced frequently, which is time-consuming and laborious. To deal with this situation, researchers have explored two new sources of power, greatly improving the efficiency of UAVs.
2. Hydrogen fuel cell: instead of a lithium battery, the hydrogen fuel cell can support the drone to operate continuously for two hours, it can be charged very quickly but in a little bit complicated way.
3. Laser transmitter: The laser transmitter powers the drone. The laser beam emitted from the ground is converted into power by the receiver on the fuselage, which can almost support the drone to work all the time.
4. Solar power: drones that use solar power are usually equipped with both lithium batteries and solar batteries. When there is sunlight, solar energy can be used to provide flight power, and lithium batteries are used as backup batteries.
5. Internal combustion engine power generation: The use of internal combustion engine power generation can support the drone to fly at a speed of 100 kilometers per hour for 1 hour, but it is noisy and poses a safety hazard because there is flammable gas in the drone.
6. Wired cable power supply: The use of a wired cable power supply can almost make the drone operate permanently, and can also speed up the transmission of data from the drone to the computer. However, due to the limitation of the wired connection, it cannot complete the long-distance flight.
Hydrogen fuel cells and laser transmitters, two power sources, are favored by the research and development community because of their remarkable effects and high safety factor. They can also provide inspiration for the exploration of power sources for other aircraft in the future.
Besides, as the brain connects all devices, drone flight controllers have more and more types of interfaces. What are the common interfaces and what are they used for on drones?
PWM:
All model airplanes and drones are inseparable from an interface. Single-wire signal, sending positive pulse periodically, changing pulse width as a way to transmit information, one pin transmits one channel, often with ground wire and power wire to control a steering gear or an ESC. It is the first interface to learn to get started with drones or model airplanes. The advantage is a simple and stable transmission of a variable signal. The disadvantage is a low speed. The current common standard is 50 times or 300 to 400 times per second.
PPM:
It is an upgraded version of PWM, that is, each signal cycle becomes a set of multiple pulse width combinations to transmit the change information of multiple channels at the same time. In the early days, it was also used for remote control radio signals and aircraft model simulator signals. Now it is mostly used to connect receivers and flight controllers. There are many receivers with PPM signal output, which are necessary for the introduction of drones. The advantage is the stable transmission of multiple channels, but the disadvantage is that the speed is slower.
S.BUS:
It is a protocol designed by Japanese remote control manufacturer FUTABA to use single-channel digital signals to transmit multi-channel information. It has only one signal pin and one ground reference wire. It supports HUB expansion and multiple servos and ESCs are connected to one signal source, so S. BUS is actually a kind of bus, and its principle is actually a changed serial port protocol. Its advantage is a pure digital signal, very reliable, with bus function. The defect is that limited compatible devices due to the manufacturer's technology restriction. It is currently the best choice for the flight controller to connect to the receiver.
Relay:
In fact, it is a digital IO signal, which only has two states of 0 and 1. It exists in certain pins of the flight controller to automatically control the camera shutter and pesticide spray head. The advantage is reliability, but the disadvantage is that the amount of information is extremely small, resources are wasted, and there is no verification and other functions.
Serial port:
The serial port is currently the most common device interface in the control field. The hardware forms are TTL, 232, 422, and 485. TTL is a basic signal. It is commonly used with three pins. One pin is called RX for signal input, one pin is called TX for signal output, and the other pin is the signal reference ground. Generally, 0 volts and 3-5 volts represent 0 and 1. The flight controller comes with this kind of serial port, and there will be a lot of them for connecting multiple devices, and there are 5 PIXHAWK flight controllers. The voltage of the TTL signal is low, and it is not suitable for long-distance transmission over 1 meter in experience, so the 232 interfaces are developed, using positive and negative levels to indicate 0 and 1. Others are the same as TTL, which greatly extends the transmission distance, but the speed is still insufficient.
So the 422 interfaces were developed. Each pin of RX and TX changes to a pair of signal wires with positive and negative voltages. The interference signal is cleverly offset, and the transmission distance and speed are double-leaped, but 5 wires are needed for debugging. Caused a lot of trouble, currently, only military products use this kind of serial port. The 485 serial port combines the advantages of 232 and 422. It uses a pair of positive and negative voltage signal lines to send and receive, but it needs an additional signal to control the transceiver conversion. The 485 bus has the characteristics of a bus and can be connected between two wires and the ground wire. There are multiple devices, but you need to write programs to coordinate tasks such as sending and receiving, chip selection, arbitration, and verification. The workload is huge, and few people use it now.
SPI:
This is a high-speed interface for board communication. It uses a master-slave design and a dedicated clock line. Each SPI has 4 pins, master-in-slave-out, master-out-slave-in, clock, and ground. The master device is responsible for information management, clock synchronization, and communication with all devices. One SPI can connect multiple devices, but each must have a chip select. Because of the clock line, it is easy to achieve synchronous communication of all devices. Due to its stability and high-speed characteristics, it is used to connect all sensors and the main single-chip microcomputer on the flight control board.
I2C:
In fact, it is I square C, which is a bus used to connect high-speed devices on the board. It has three signal wires, signal, clock, and ground wires. It also adopts a master-slave design with a clock synchronization design, but the signal line needs to bear the input and output of multiple devices, which is managed by the master device. All devices on I2C have their own address or label, and the master device uses this address to identify the device. It is used in flight control to connect many devices that are not of high importance, such as indicator lights, magnetic compass, airspeed, ultrasonic, laser ranging, and so on. Actually, some manufacturers use this bus to connect multiple ESCs in order to save trouble. This is very dangerous because the 3-5v level is easy to be interfered with in long-distance transmission.
CAN:
The earliest bus interface designed for automobiles is said to be the current transmission, so it has super anti-interference performance and is specially used for multiple long-distance transmissions between devices in a large interference environment. There are only two signal lines H and L, all devices are connected to it, and the bus chip is responsible for arbitration. This is actually the best choice for drones, especially multi-rotor ESCs, but due to its complexity and cost of interface chips, relatively few people use it. Pixhawk flight controllers and ESC32 ESCs have had this interface many years ago. No one cares so far.
AD:
The analog-to-digital conversion interface, which is easy to be disturbed, still exists, and its use is to measure voltage. At present, flight controllers are used to measuring voltage and current. This method has a low cost and can also be used on small drones. Other distance detection devices are still in use, but they are basically on the verge of obsolescence, and the effect is far inferior to digital devices, but the cost is lower, such as airspeed and ultrasound.
SDIO:
Used to connect SD card or TF card for flight data recording. Due to the widespread use of TF cards, low cost, and satisfactory capacity and speed, it has gradually become essential equipment for advanced flight control, used for flight records, accident analysis, fault diagnosis, and so on.
USB:
The civilian bus interface can connect multiple devices through the HUB, which can be said to be a perfect interface. However, the protocol is too complicated, the programming workload is huge, and the interface connection form is likely to cause problems, and a special wire must be used, and it cannot exceed 2 meters. The flight control is mainly used for ground debugging, reading and writing parameters, and other ground operations.
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