Automatic hovering means to fix the UAV at the preset height and horizontal position, which is actually a set of three-dimensional coordinates. But how do drones know where they are? In fact, it’s very simple. Generally speaking, the height is measured by ultrasonic sensor (measuring the distance from the ground, which is rare) or Barometer (the height will affect the change of atmospheric pressure), while the coordinate of horizontal position is determined by GPS module.
Of course, GPS can also provide altitude information, but for the mainstream UAV, barometer is preferred, because the data refresh rate of low-cost GPS is too low, and the data lag will lead to the height drop of UAV at high speed.
In addition to GPS mode for positioning, UAV also has an “attitude mode”, which relies on the internal IMU (inertial measurement unit, actually a group of gyroscope + accelerometer sensors) to identify its flight status and relative displacement.
Intelligent regulation system to eliminate external interference
How can the UAV hover in the preset position after knowing its height and horizontal position through various sensors? This is actually a set of negative feedback automatic control system.
Take the GPS mode as an example, when the UAV is affected by the outside world and the height tends to increase or decrease, the control unit adjusts the motor power to compensate in the opposite direction; if the UAV tends to be blown away from the hovering position by the wind, the control unit can start the side flying mode to counteract it – these reactions are relatively fast, as long as the outside influence is not too big（ Professional multi axis UAVs are generally resistant to level 4 wind (there is no problem). Professional UAVs can cope with it. What you can see is that it is stable and does not move.
When the weather is not very good and it is difficult for GPS to search satellites, attitude mode will come in handy. Depending on the IMU unit inside the UAV, the system can recognize the current flight attitude, carry out automatic balance compensation, and also realize the locking of altitude and horizontal position.
What is the hovering accuracy of UAV?
Generally speaking, the fever player UAV below 10000 yuan can automatically hover within the accuracy range of 0.8m vertically and 2.5m horizontally. The hovering accuracy of other models is indicated in the product parameter table. Of course, this level of UAV also has “expert mode”, you can manually fine tune with high precision. The 2000 yuan UAV also has a model with GPS system, which can realize intelligent hovering. Of course, the stability will be worse than that of the fever level. The 800-1000 yuan UAV can generally rely on the IMU unit to realize hovering, with worse accuracy and stability. As for the more accessible entertainment model, it can only be stabilized by your dexterous hands. Dajiang’s “Wu” series can hover in the room without GPS signal through “visual positioning”
The UAV needs to know its coordinates in three-dimensional space, that is, where it is, to find the position where it needs to hover, which is realized by GPS, barometer, ultrasonic sensor or camera. GPS is very easy to understand. Everyone has used the GPS navigation of mobile phones, as well as UAVs. They can easily understand their own horizontal coordinates through GPS readings.
In addition to horizontal coordinates, an altitude value is needed to determine the hovering position of UAV. Although GPS can also read altitude parameters, the data refresh rate is not ideal, which may cause the height of UAV to drop. Therefore, nowadays, professional UAVs generally use barometer to read altitude parameters (the principle is very simple, the atmospheric pressure changes with the height).
The rest is left to the flight control, which will make the UAV hover stably at the preset coordinate points through the automatic control mode of negative feedback (high, low, far, close).
In addition, if there is no GPS signal (the weather is bad, and it is quite common that the satellite can not be found), the UAV can also rely on its own IMU (inertial unit) to realize the attitude flight control mode, and let it reach the predetermined position to hover with the manual operation of the flight controller. Dajiang’s “Wu” series of high-end products can carry out “visual positioning” through ultrasonic sensors and cameras in the room without GPS signal, which is more prominent in similar products.
In terms of hovering accuracy, the horizontal accuracy of products like Dajiang spirit 2 can reach 2.5m, and the vertical accuracy can reach 0.8m (the most high-end “Wu” series only improves the vertical accuracy to 0.5m)
The distance can be known according to the time difference when the receiver receives the ultrasonic wave. This is similar to the principle of radar ranging. The ultrasonic transmitter emits the ultrasonic wave in a certain direction, and starts timing at the same time. The ultrasonic wave spreads in the air, and returns immediately when it encounters obstacles on the way. The ultrasonic receiver stops timing immediately when it receives the reflected wave. (the propagation speed of ultrasonic wave in the air is 340m / s. according to the time t recorded by the timer, the distance (s) between the starting point and the obstacle can be calculated, i.e. s = 340t / 2)
Because of its strong directivity and long distance in the medium, ultrasonic wave is often used in distance measurement, such as range finder and level meter. Ultrasonic detection is often more rapid, convenient, simple calculation, easy to achieve real-time control, and the measurement accuracy can meet the requirements of industrial practice, so it has been widely used in the development of mobile robot.
In order to make the mobile robot walk automatically, it must be equipped with a distance measurement system to obtain the distance information (distance and direction) from the obstacle in time. The three direction (front, left and right) ultrasonic ranging system introduced in this paper is to provide a moving distance information for the robot to understand its front, left and right environment.
In order to study and utilize ultrasonic, many ultrasonic generators have been designed and made. Generally speaking, ultrasonic generator can be divided into two categories: one is to produce ultrasonic by electrical way, the other is to produce ultrasonic by mechanical way. Electrical methods include piezoelectric, magnetostrictive and electric; mechanical methods include Galton whistle, liquid whistle and air flow whistle. The frequency, power and acoustic characteristics of the ultrasonic waves produced by them are different, so their uses are also different. At present, piezoelectric ultrasonic generator is commonly used.
How does GPS locate horizontal position and vertical height?
GPS positioning, in fact, is to determine the position of the GPS receiver through four known satellites.
GPS receiver is the equipment to determine the current position, and satellites 1, 2, 3 and 4 are the four satellites to be used in this positioning
Position 1, position 2, position 3 and position 4 are the current positions (spatial coordinates) of the four satellites, and D1, D2, D3 and D4 are the distances from the four satellites to the GPS receiver to be located
1. Where does location information come from?
In fact, every GPS satellite running in the space continuously broadcasts its current position and coordinate information to the world through satellite signals. Any GPS receiver can easily receive these information through the antenna, and can read these information (this is actually one of the core functions of every GPS chip). This is where the location information comes from.
2. Where does distance information come from?
We already know that every GPS satellite is sparing no effort to broadcast its own position, so when sending the position information, it will also attach the time stamp when the data packet is sent. After receiving the data packet, the GPS receiver subtracts the time on the time stamp from the current time (the current time can only be determined by the GPS receiver itself), which is the time that the data packet is transmitted over the air.
If you know the transmission time of the data packet in the air, then multiply by its transmission speed, it is the transmission distance of the data packet in the air, that is, the distance from the satellite to the GPS receiver. Data packets are transmitted by radio waves, so the ideal speed is the speed of light C. if the propagation time is recorded as Ti, the formula is: Di = C * Ti (I = 1, 2, 3, 4); this is the source of di (I = 1, 2, 3, 4).
3. Why four satellites
Theoretically, taking the three-dimensional coordinates (n, e, H) of the ground point as the undetermined parameters, it really only needs to measure the distance between three satellites and the ground point to determine the three-dimensional coordinates of the point. However, the satellite ground distance is obtained by multiplying the signal propagation time difference Δ t by the signal propagation speed v. Among them, the propagation speed V of the signal is close to the speed of light in vacuum, and the magnitude is very large. Therefore, this requires a very accurate measurement of the time difference Δ T. If there is a slight deviation, the measured distance between the earth and the satellite will be thousands of miles away. The time difference Δ t is obtained by subtracting the transmission time Ts measured at the satellite from the time tr measured at the receiver. Among them, the atomic clock installed on the satellite has a high degree of stability, and we think that the time of this kind of clock coincides with the GPS time; the clock at the receiver is a quartz clock with a general degree of stability, and we think that there is a time synchronization error between its clock time and the GPS time, and take this error as an undetermined parameter. In this way, for each ground point, there are actually four parameters to be solved, so it is necessary to observe at least four satellite to ground distance data.
Barometer height measurement principle: the working principle is to convert the input signal (pressure) into resistance change, that is, to sense the pressure exerted on the diaphragm through the piezoresistive pressure sensor of Wheatstone bridge structure. Sensitivity is an important parameter of pressure sensor. The small pressure sensor with high resolution enables the application of Barometer / altimeter in mobile terminal. For example, on the navigator, the altimeter can accurately determine whether the position is on or under the bridge.
Wheatstone bridge is a measuring device used to accurately measure the median resistance (10-105w). The simplest and most direct method of measuring resistance is voltammetry. When the resistance is measured by voltammetry, the measured resistance value can be obtained by measuring the current I flowing through the resistance R and the potential difference V at both ends of the resistance, and according to Ohm’s law r = V / I. But this method has large measurement error. Because the ammeter itself has internal resistance, whether the ammeter is connected internally or externally, the current I flowing through the resistance and the potential difference V at both ends of the resistance can not be measured accurately at the same time, so there is inevitably an error caused by the defect of the circuit itself, which is called the connection error of the ammeter. If we can determine the internal resistance of ammeter or voltmeter in advance, we can eliminate this error by adding correction value. However, the accuracy of ammeter and voltmeter used in voltammetry measurement can not be very high (the highest accuracy level of ammeter is 0.1), and the measurement uncertainty caused by instrument error limitation cannot be reduced. For example, if the ammeter and voltmeter are both of class 0.5, and the measured current and voltage are close to half of the range of the ammeter, the measurement error may reach 1.5% only due to the accuracy level limit of the ammeter.
The essence of measuring resistance by bridge method is to compare the measured resistance with the standard resistance to determine its value. Because the manufacturing of resistance can achieve high accuracy, so the bridge method can achieve high accuracy.
The electric bridge is divided into DC bridge and AC bridge. DC bridge is divided into single arm bridge and double arm bridge. Wheatstone bridge is a single arm bridge in DC bridge; double arm bridge is also called Kelvin bridge, which is suitable for measuring low resistance (10-6-10w). Because the bridge measurement method is sensitive, accurate and easy to use, it has been widely used in electrical technology and non electric measurement.
Editor in charge: PJ