Can drones eliminate external interference through intelligent adjustment systems? 【full text】

The dynamic automatic hovering of the security exhibition network technology means to fix the drone at a preset height and horizontal position, which is actually a set of three-dimensional coordinates. But how does the drone know its location? In fact, it’s very simple to say, altitude is generally measured by ultrasonic sensors (measure the distance from the ground, relatively rare) or barometer (altitude will affect the change of atmospheric pressure), and the coordinates of the horizontal position are measured by GPS Module to determine.

Of course, GPS can also provide altitude information, but for mainstream drones, it is more inclined to use barometers, because the data refresh rate of low-cost GPS is too low, and the data lag will cause the drone when moving at high speed. High drop. In addition to GPS mode for positioning, the drone also has a "attitude mode", which relies on the internal IMU (inertial measurement unit, actually a group of gyroscope + accelerometer sensors) to identify its own flight status and relative displacement .

Intelligent adjustment system eliminates external interference

After knowing its height and horizontal position through various sensors, how does the drone hover over this preset position? This is actually a set of negative feedback automatic control system (meaning that it will automatically adjust back when it deviates from the preset value).

Take GPS mode as an example. When the drone is affected by the outside and the altitude tends to increase or decrease, the control unit adjusts the power of the motor to compensate for movement in the opposite direction; if the drone is blown laterally from the hovering position by the wind The control unit can start the side flight mode to counteract it-these reactions are relatively fast, as long as the external influence is not too big (professional multi-axis drones generally have no problem with resistance to category 4 wind), professional unmanned The machine can handle it, all you see is that it stays there firmly.

The attitude mode comes in handy when the weather is not very good and GPS searching for stars is difficult. Relying on the IMU unit inside the UAV, the system can recognize the current flight attitude, perform automatic balance compensation, and also achieve the lock of height and horizontal position.

How accurate is the drone hovering?

Generally speaking, a player-grade drone with a fever of less than 10,000 yuan can automatically hover within the accuracy range of 0.8 meters vertically and 2.5 meters horizontally. The hovering accuracy of other models is indicated in the product parameter table. Of course, this level of drone also has an "expert mode", you can manually perform high-precision fine-tuning. There are also models with GPS systems for 2,000 yuan drones, which can realize smart hovering. Of course, the stability will be worse than that of enthusiasts. For models with 800 to 1,000 yuan, you can generally rely on IMU units to achieve hover. Stop, the accuracy and stability are worse; as for the more entry-level entertainment models-you can only rely on your dexterous hands to stabilize. DJI's "Wu" series can achieve fixed-point hovering through "visual positioning" indoors without GPS signals.

The drone must know its own coordinates in the three-dimensional space, that is, know where it is, in order to find the position that needs to hover, and this coordinate is realized by GPS, barometer or ultrasonic sensor and camera. GPS is easy to understand. Everyone has used the GPS navigation of mobile phones, and the same is true for drones. You can easily understand your horizontal coordinates through GPS readings.

In addition to the horizontal coordinates, an altitude value is also needed to determine where the drone is hovering. Although GPS can also read altitude parameters, the data refresh rate is not ideal, which may cause the drone to fall at a height. Therefore, professional drones generally use barometers to read altitude parameters (the principle is very simple, the atmospheric pressure varies Altitude changes). The rest is left to the flight control, which will use negative feedback (turn down when it is high, turn it down when it is far away) to stabilize the drone and hover on the preset coordinate point.

In addition, if there is no GPS signal (the weather is bad and the situation of not being able to find satellites is more common), the drone can also rely on its own IMU (inertial unit) to achieve the attitude flight control mode, relying on the manual operation of the flight controller, Let it reach the predetermined position to hover. DJI's "Wu" series of high-end products can use ultrasonic sensors and cameras to perform "visual positioning" indoors without GPS signals, which is more prominent among similar products. In terms of hovering accuracy, products such as DJI Phantom 2 can achieve a horizontal accuracy of 2.5m and a vertical accuracy of 0.8m (the highest-end "Wu" series only increased the vertical accuracy to 0.5m).

Ultrasonic Ranging

The ultrasonic wave is emitted by the ultrasonic transmitter, and 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 ultrasonic waves in a certain direction and starts timing at the same time as the launch time. The ultrasonic waves propagate in the air and return immediately when encountering obstacles on the way. The ultrasonic receiver stops timing immediately after receiving the reflected waves. (The propagation speed of ultrasonic waves in the air is 340m/s. According to the time t recorded by the timer, the distance (s) between the launch point and the obstacle can be calculated, namely: s=340t/2).

Ultrasonic waves have strong directivity and travel far in the medium. Therefore, ultrasonic waves are often used for distance measurement. For example, rangefinders and level measuring instruments can be realized by ultrasonic waves. Ultrasonic detection is often quick, convenient, simple to calculate, easy to achieve real-time control, and can meet industrial practical requirements in terms of measurement accuracy, so it has also been widely used in the development of mobile robots. In order for a mobile robot to automatically avoid obstacles and walk, it must be equipped with a distance measuring system so that it can obtain the distance information (distance and direction) from the obstacle in time. The three-direction (front, left, and right) ultrasonic distance measurement system introduced in this article is to provide a movement distance information for the robot to understand its front, left and right environment.

In order to study and use ultrasonic waves, many ultrasonic generators have been designed and made. Generally speaking, ultrasonic generators can be divided into two categories: one is to generate ultrasonic waves electrically, and the other is to generate ultrasonic waves mechanically. Electrical methods include piezoelectric, magnetostrictive, and electric, etc.; mechanical methods include Galton flute, liquid whistle, and air whistle. The frequency, power and sonic characteristics of the ultrasonic waves they produce are different, so their uses are also different. At present, the piezoelectric ultrasonic generator is more commonly used.

How does GPS locate the horizontal position and vertical height?

GPS positioning is actually to determine the location of the GPS receiver through four satellites with known locations. The GPS receiver is the current device to determine the position. Satellites 1, 2, 3, 4 are the four satellites used for this positioning:

Position1, Position2, Position3, and Position4 are the current positions (spatial coordinates) of the four satellites respectively. It is known that d1, d2, d3, and d4 are the distances from the four satellites to the GPS receiver to be positioned.

â–ºWhere does the location information come from?

In fact, each of the GPS satellites operating in space constantly broadcasts their current position coordinate information to the world through satellite signals. Any GPS receiver can easily receive this information through the antenna, and can read this information (this is actually one of the core functions of every GPS chip). This is the source of these location information.

â–ºWhere does the distance information come from?

We already know that every GPS satellite is working tirelessly to broadcast its location, so when sending location information, it will also attach the time stamp when the data packet is sent. After the GPS receiver receives the data packet, it subtracts the time on the timestamp from the current time (the current time can only be determined by the GPS receiver itself), which is the time taken for the data packet to be transmitted in the air.

Knowing the transmission time of the data packet in the air, then multiplying its transmission speed is the distance of the data packet transmission in the air, that is, the distance between the satellite and 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 di( i=1, 2, 3, 4).

â–ºWhy do you need 4 satellites

Theoretically speaking, with the three-dimensional coordinates (N, E, H) of the ground point as the undetermined parameter, it is indeed only necessary to measure the distances from three satellites to the ground point to determine the three-dimensional coordinates of the point. However, the satellite-to-ground distance is obtained by multiplying the signal propagation time difference Δt by the signal propagation speed v. Among them, the signal propagation speed v 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 Δt. If there is a slight deviation, the measured distance between the guard and the ground will be thousands of miles away. The time difference Δt is obtained by calculating the difference between the signal transmission time tS measured at the satellite and the arrival time tR of the signal measured at the receiver. Among them, the atomic clock placed on the satellite has a high degree of stability. We believe that the time of this clock is consistent with GPS time; the clock at the receiver is a quartz clock, which is stable. We believe that there is a time synchronization error between its clock time and GPS time. , And regard this error as a pending parameter. In this way, there are actually 4 undetermined parameters that need to be solved for each ground point, so at least 4 satellites need to observe the satellite distance data from the ground point.

Barometer measuring height principle: The working principle is to convert the input signal (pressure) into resistance change, that is, the pressure applied on the thin diaphragm is sensed by the piezoresistive pressure sensor of the Wheatstone bridge structure. An important parameter of the pressure sensor is sensitivity. The small high-resolution pressure sensor enables the application of barometer/altimeter to be implemented in mobile terminals, such as on a navigator. The altimeter can accurately determine whether the position is on the bridge or under the bridge. .

The Wheatstone Bridge is a measuring device used to accurately measure the median resistance (10-105W). The simplest and direct method of measuring resistance is voltammetry. When measuring resistance by voltammetry, by measuring the current I flowing through the resistance R and the potential difference V between the two ends of the resistance, the measured resistance value can be obtained according to Ohm's law R=V/I. But this method has a large measurement error. Since the electric meter itself has internal resistance, no matter whether the ammeter is connected internally or externally, the current I flowing through the resistor and the potential difference V between the two ends of the resistor cannot be accurately measured at the same time, so there is inevitably an error caused by the defect of the circuit itself. The error is called the access error of the meter. The connection error of the electric meter is a determinable system error. If we can determine the internal resistance of the ammeter or voltmeter in advance, we can eliminate this error by adding a correction value. However, the accuracy of the ammeter and voltmeter used in the voltammetry measurement can not be very high (the accuracy level of the ammeter is up to 0.1), and the measurement uncertainty caused by the instrument error limit cannot be reduced. For example, if the ammeter and the voltmeter are both 0.5 grade, the measured current and voltage are both close to one-half of the meter range, and the measurement error caused by the limitation of the meter's accuracy level may reach 1.5%. Using the bridge method to measure resistance is essentially comparing the measured resistance with a standard resistance to determine its value. Because the resistance can be manufactured with high accuracy, the bridge method can achieve high accuracy in measuring resistance.

Electric bridges are divided into two categories: DC bridges and AC bridges. DC bridge is divided into single-arm bridge and double-arm bridge. The Wheatstone bridge is a single-arm bridge in the DC bridge; the double-arm bridge is also called the Kelvin Bridge, which is suitable for measuring low resistance (10-6-10W). Because the bridge measurement method is more sensitive, accurate, and easy to use, it has been widely used in electrical technology and non-electricity electrical measurement methods.