"UAV has rapidly become a popular product at the phenomenon level in 2015. Therefore, through special interviews with many companies in the industry, I hope to understand the hardware structure of UAV and its future technology development trend. Compared with the fixed wing UAV, the flight of multi axis aircraft / UAVs is more stable and can hover in the air.
The hardware structure of the host and the remote control system are mainly shown in the figure:
Figure 1: analytical diagram of four axis aircraft system
Figure 2: analysis diagram of standard remote control system
In addition, some more advanced UAV Systems, such as devices for model aircraft players and aerial photographers, also require more modules such as PTZ, camera, video transmission system and video reception.
Brain of UAV: MCU
Not many chips are needed on the flight control motherboard of four axis aircraft. At present, toy grade aircraft only fly or stay in the air. As long as they can receive the command sent by the remote controller and control four motors to drive the paddles and wings, they can basically realize the function of flying or hovering.
Ren Yuan, senior marketing engineer of St, introduced that the main components of UAV / multi axis aircraft include flight control and remote control. Flight control includes electronic transmission (ESC) / motor control, aircraft posture control and PTZ control. At present, the mainstream ESC control methods are mainly divided into BLDC square wave control and FOC sine wave control. ST's stm32f051 and stm32f301 series are widely used because of their high integration, small package and cost performance.
In terms of aircraft attitude control, different series of stm32f0 / stm32f3 / stm32f4 can be provided according to different external sensors to meet the needs of customers.
In terms of pan tilt control, stm32f301 / stm32f302 / stm32f405 and other series have also been widely used in aerial photography products.
In addition, in terms of remote control, in addition to the application of stm32f0 / stm32f1 series to traditional products without display, stm32f429 is gradually used in remote control with color display due to the built-in TFT color driver.
The person in charge of MCU of Xintang said: the multi axis aircraft is composed of different modules such as remote control, flight control, power system and aerial photography. According to the needs of different levels of products, different CPU cores such as 8051, Cortex-M0, cortex-m4 and ARM9 are adopted. Xintang technology has applied a variety of MCU in multi axis aircraft.
For example, due to its simple function and small volume, the small four-axis flight main controller must integrate remote control reception, flight control and power drive functions at the same time, and adopt Cortex-M0 mini54 series encapsulated by qfn33 or tsop20; The cortex-m4 m451 series with built-in DSP and floating-point operation unit is used for medium and high-order multi axis aircraft, which is responsible for the flight master control function, and the ESC board driving brushless motor is designed with mini5 series. The low-level remote controller uses 4T 8051 n79e814 encapsulated in sop20; Cortex-M0 and m051 series are used for medium and high-order remote controllers. In addition, n329 series SOC with built-in ARM9 and H.264 video codec is applied to 2.4G and 5.8G air capture systems.
Next page: aircraft main control panel
On the flight control motherboard, MCU is still the most used for control and processing. Because the flight control is mainly floating-point operation, the simple arm cortex-m4 core 32-bit MCU can meet the requirements. Some sensor MEMS chips have integrated DSP, and a simpler 8-bit MCU can realize the design.
Qualcomm and Intel demonstrated a multi axis aircraft with more functions at this year's CES, using a CPU or arm cortex-a series processor more powerful than MCU as the flight control master chip.
For example, the snapdragon cargo UAV displayed by Qualcomm is a flight controller developed based on Qualcomm snapdragon chip, which has functions such as wireless communication, sensor integration and spatial positioning.
Brian Krzanich, CEO of Intel, personally demonstrated on CES that the UAV using its realsense technology can build 3D maps and perceive the surrounding environment, fly like a bat and automatically avoid obstacles.
Intel's UAV is developed in cooperation with ascending technologies, a German industrial UAV manufacturer, with up to six Intel's realsense 3D cameras built in, and a PCI Express custom card based on four core Intel Atom processor, which is used to process the real-time information of distance and sensors, and avoid close obstacles. The two companies display powerful UAVs at CES. First, they are optimistic about the UAV market. Second, the United States will soon launch relevant regulations to strictly control the flight of UAVs.
In addition, XMOS, a European processor manufacturer active in the robot market, has also begun to enter the field of UAVs. Paul Neil, vice president of marketing and business development of XMOS, said that XMOS Xcore multi-core MCU series has been adopted by some OEM customers of UAVs / multi axis aircraft. In these systems, XMOS multi-core MCU can be used for flight control and MCU internal communication.
Paul Neil said, "Xcore multi-core MCU has between 8 and 32 32 32-bit RISC cores with frequencies up to 500 MHz. Xcor components also have a hardware response I / O interface that provides excellent hardware instant I / O performance with low latency. " This multi-core solution supports completely independent execution of system control and communication tasks without any real-time operating system (RTOS) overhead. The hardware real-time performance of xcoremcu can help customers realize very accurate control algorithms without jitter in the system.
St said that stm32f7 series adopts the latest generation cortex-m7 architecture and integrates high performance, real-time function, digital signal processing and high integration to provide solutions for aircraft customers with high-precision control needs. STM32 dynamic efficiency MCU series achieves a perfect balance between dynamic power consumption and processing performance, making the aircraft design more perfect.
Multi axis aircraft need four to six brushless motors to drive the rotor of the UAV. The motor drive controller is used to control the speed and direction of UAV. In principle, one motor needs to be controlled by one 8-bit MCU, but there is also a scheme that one MCU controls multiple BLDC motors.
Next page: MEMS / sensors for multi axis UAVs
MEMS / sensor of multi axis UAV
Chen Yimin, general manager of Shenzhen Fuwei Kechuang Electronics Co., Ltd., a UAV solution provider, believes that although most toy grade aircraft in the industry have been upgraded from three-axis to six axis MEMS, they usually use consumer products, such as price sensitive models commonly used on tablets or mobile phones. For professional aerial photography and medium and high-level UAVs specially developed for model aircraft players, sensors with better quality and higher price will be used to ensure more stable and safe flight of UAVs.
Zhao Yanhui, market and application manager of MEMS products in ADI Asia Pacific region, introduced that ADI's industrial gyroscopes adxrs652, adxrs620, adxrs623, adxrs646, adxrs642 and industrial accelerometers ADXL203 and adxl278 have been widely used in professional air shooting equipment. Commercial accelerometers adxl335, adxl326, adxl350, adxl345, etc. also continue to be widely used in various aircraft.
These MEMS sensors are mainly used to realize the stability control and auxiliary navigation of aircraft. The reason why the aircraft can hover and shoot in the air is that the MEMS sensor can detect the changes of pitch angle and roll angle of the aircraft during flight. After detecting the changes of angle, it can control the motor to rotate in the opposite direction, so as to achieve a stable effect. This is a typical closed-loop control system. As for measuring the angle change with MEMS sensor, it is generally necessary to select the combined sensor, which can neither rely solely on the accelerometer nor only use the gyroscope, because each sensor has certain limitations.
For example, the gyroscope outputs angular velocity, and the angle can be obtained through integration. However, even in the zero input state, the gyroscope still has output. Its output is the superposition of white noise and slowly varying random function. Therefore, in the process of integration, cumulative error is bound to be introduced. The longer the integration time, the greater the error.
This requires an accelerometer to correct the gyroscope, because the accelerometer can use the force decomposition principle to judge the inclination through the components of gravity acceleration in different axes. Because there is no integration error, the accelerometer can correct the error of the gyroscope under the condition of relative static. But in motion, the credibility of the accelerometer output will decline, because it measures the combined force of gravity and external forces. The more common algorithm is to use complementary filtering, combined with the output of accelerometer and gyroscope to calculate the angle change.
Zhao Yanhui said that the main advantage of ADI products is to obtain high-precision output under various harsh conditions. Taking the gyroscope as an example, its ideal output is to respond only to the change of angular velocity, but in fact, due to the limitation of design and process, the gyroscope is also very sensitive to acceleration, that is, the common index of deg / sec / g in the gyroscope data book. This index is particularly important for the application of multi axis aircraft, because the motor in the aircraft will generally bring strong vibration. Once the damping control is not good, it will produce great acceleration in the flight process, which will inevitably lead to the change of gyro output, then cause the angle change and lead to the wrong action of the motor, Finally, the intuitive feeling for end users is that the aircraft is not stable.
In addition, in some cases, if the aircraft turns suddenly, it may cause the input speed to exceed the test range of the gyroscope. Ideally, the output of the gyroscope should be saturated output. After the speed returns to the limit range of the gyroscope, the gyroscope will correctly respond to the immediate angular speed change, but some gyroscopes are not. Once the input exceeds the limit range, The gyro will produce an oscillating output, giving a completely wrong angular velocity.
In some cases, the aircraft will be impacted by large acceleration. Ideally, the gyroscope should try to suppress this impact. ADI gyroscope also fully takes this situation into account in its design, uses the mechanical structure of double core and four core, and adopts the principle of differential output to suppress this "common mode" impact and accurately measure the angular velocity change of "differential mode". However, some gyroscopes will produce very large error output or even oscillation output in this case.
Zhao Yanhui said: "for aircraft, the most important thing is safety. Whether it is hardware design or software design, we must first ensure safety, and then the ultimate user experience. ADI's MEMS sensor design first ensures the stability under various extreme conditions, and then pursues the ultimate index. According to the customer's measured feedback, after the aircraft operates incorrectly and falls accidentally, the output of ADI's gyroscope will not be affected, while some other gyroscopes will have a very large zero offset. ADI accelerometers will not have any reliability problems after being impacted, while some other accelerometers will have no output at all with a high probability. The differences measured by these users benefit from the full consideration of various extreme conditions in the design of ADI MEMS sensors. "
"In the future, MEMS products on aircraft will also develop in the direction of integration, such as integrated products with 3-axis acceleration and 3-axis gyroscope, and even SOC, which integrate the processor and directly provide angle output for the back-end processor to call. Since the application scenarios of aircraft are generally outdoor, customers are bound to make temperature compensation within the full temperature range. The development direction of MEMS products in this field will be to make temperature compensation within the full temperature range for MEMS products before leaving the factory, or to design ultra-low temperature drift sensors. "
Chen Yimin believes that with the increasing functions of UAVs, more and more GPS sensors, infrared sensors, air pressure sensors and ultrasonic sensors are applied to UAVs. Solution suppliers are already using infrared and ultrasonic sensors to develop UAVs that can automatically avoid collision, so as to meet the requirements of relevant regulations in the future. The UAV integrated with GPS sensor can realize the one click return function to prevent the flight loss of UAV. The UAV with built-in GPS function can set the warning of approaching the airport or aviation restrictions in the software to avoid entering the control area by mistake.
Next page: wireless control and video transmission
Wireless control and video transmission
As an entertainment grade UAV, it may only need wireless remote control technologies such as 2.4GHz or 5.8GHz to meet the transmission requirements. Although the 433MHz frequency band has strong penetration and long communication distance, and can transmit up to 2km, it is not widely used in remote-controlled UAV or aircraft because of its weak anti-interference ability. Many chip manufacturers can provide 2.4GHz or 5.8GHz wireless RF chips. Most of them have been used by remote control toy manufacturers, and there are many chips to choose from.
Due to its very stable flight, multi axis UAV can shoot video at high altitude after loading PTZ and camera, and transmit it to the ground in real time through wireless communication (5.8G, Wi-Fi or LTE), which is more widely used. Pan Jianzhang, special director of Hetai semiconductor product technology development office / motor product technology department, believes that the four-axis aerial photography aircraft has been used for a few purposes (traffic monitoring, air monitoring, urban planning, border patrol, disaster monitoring, monitoring of agricultural crop growth and pests, precision pesticide spraying, border cruise, urban anti-terrorism, etc.), It began to move towards mass consumers (air shooting) and become a smart hardware commodity.
At present, Broadcom's Wi Fi and Bluetooth chips have been widely used in UAVs, and are involved in control signal transmission and video transmission. The main feature of WLAN chip applied to UAV is to maintain the reliability and stability of video channel and control channel at the same time. These chips have advanced receiving architecture, high receiving sensitivity and various advanced channel management capabilities, including MIMO and dual band technology for high-speed and anti-noise connections“ Broadcom's wiced development kit supports Linux, Android and RTOS based operating systems, "said Brian bedrosia, senior product marketing director of Broadcom's wireless connection portfolio division.
In terms of video transmission of UAV, the general practice is to carry a camera on the PTZ, shoot at high altitude, and then fly back to the ground for inspection. This method can not meet the requirements of air shooting because it can not see the shooting picture immediately. Chen Yimin introduced that at present, many schemes use the 5.8GHz frequency band to transmit analog video to the ground, with a maximum distance of more than 600 meters. However, this method requires high resolution (1080p or 1080p) on the aircraft
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