Design of true airspeed measurement system for UAV based on single chip microcomputer

True airspeed refers to the real speed of an aircraft when it moves relative to the air. It is an important parameter for safe flight of an aircraft. Accurate true airspeed data information plays a significant role in improving the safety, accuracy and economy of flight. When an aircraft moves relative to the air, it can be regarded as motionless according to the relativity of motion, while the air flows through the aircraft at equal and opposite speeds. True airspeed cannot be measured directly and can only be calculated indirectly based on atmospheric static pressure and dynamic pressure. This paper introduces the design of a true airspeed measurement system for unmanned aerial vehicles based on C8051F352 microcontroller. The relative error of the measurement results of this system is less than 4%, which can meet the accuracy requirements of the flight control system of unmanned aerial vehicles.

1 Measurement principle
The true airspeed of unmanned aerial vehicles is usually related to atmospheric static pressure and dynamic pressure. For the case where the flight altitude does not exceed 11,000 m and the flight Mach number is not greater than 1, the true airspeed can be calculated by formula (1):

where PH is the atmospheric pressure (static atmospheric pressure) at the corresponding altitude, and PD is the atmospheric dynamic pressure, which is the difference between the total atmospheric pressure and the static pressure. The constants in the formula are: Pob = 101 325 Pa, which is the atmospheric static pressure at the standard sea level; Tob = 288 K, which is the atmospheric temperature at the standard sea level; τb = 0.0065 K/m, which is the temperature lapse rate; R = 29.2746 m/K, which is the atmospheric constant; k = 1.4, which is the air adiabatic index; g = 9.806 65, which is the gravitational acceleration.
It can be seen from formula (1) that as long as the atmospheric static pressure and dynamic pressure are measured, the true vacuum speed can be calculated according to the calculation formula.

2 Overall structure of the system
According to the composition and working principle of the digital atmospheric data measurement system, the author designed a true vacuum speed measurement system with a single-chip microcomputer as the core, and its overall structure is shown in Figure 1. The absolute pressure sensor and the dynamic pressure sensor sense the static pressure and dynamic pressure, and convert them into corresponding voltage signals respectively. Since the output signal of the sensor is relatively weak, it must be processed by the amplification circuit, the zero adjustment circuit and the filter circuit to generate a signal that meets the requirements, and then transmitted to the AD conversion input terminal. The A/D converter built into the single-chip microcomputer converts the analog voltage signals of static pressure and dynamic pressure into digital signals and sends them to the single-chip microcomputer CPU for processing. The CPU calculates the corresponding true airspeed value according to the true airspeed calculation formula. The calculation result is sent to the onboard flight control computer through the serial port and displayed on the display interface.

3 System hardware circuit design
3.1 Pressure data acquisition module The
true airspeed measurement system designed by the author is required to work within the altitude range of 0 to 5 000 m. According to the pressure height formula (2), the static pressure range can be calculated to be between 54 005 and 101 325 Pa. The range of the differential pressure sensor depends on the true airspeed range of the aircraft. The true airspeed range measured in this paper is 50 to 400 km, and according to formula (1), the dynamic pressure range can be calculated to be between 71 and 7 769 Pa.

According to the static pressure and dynamic pressure ranges, the absolute pressure sensor and differential pressure sensor of this paper are respectively selected from the ASDX015A24R sensor and 140PC02D sensor of HONEYWELL. The range of ASDX015A 24R is 0～15 psi(103．419 kPa), and the accuracy is within 2%; the range of 140PC02D is 0～2 psi(13．7892 kPa), and the accuracy is 0．25%. The pressure measuring device uses the GCY-1M high-precision comprehensive tester. According to formula (3), the error caused by the sensor can be calculated to be 3．23%.

The single-chip microcomputer C8051F352 selected in this system has two 16-bit AD conversions (ADC0 and ADC1). After amplification and zeroing, the output signals of the static pressure and dynamic pressure sensors can be sent to the single-chip AD conversion input terminal to be converted into digital signals. [page]

3.2 Single-chip microcomputer processing circuit
The C8051F352 single-chip microcomputer used in this design adopts a 32-pin LQFP package. It is a single-chip microcomputer launched by Cygnal Company of the United States with the characteristics of high speed, high performance, high integration, etc., and is compatible with 8051. In addition to the digital peripheral components of the standard 8052, the chip also integrates the analog components and other digital peripherals and functional components commonly used in data acquisition and control systems.
The two analog channels AIN0 and AIN1 in the C8051F352 microcontroller are used in this system design, which are used for analog/digital conversion of static pressure and dynamic pressure respectively. The AD of the single-chip microcomputer converts the input analog signal into a digital signal, and then sends it to the CPU for data processing.
3.3 Serial communication
The solved true vacuum speed is sent to the host computer through the serial port and displayed on the display interface. The true vacuum speed is sent to the host computer through the RS-232 interface from the serial port of the single-chip microcomputer, and the MAX232 chip is used to realize the single-chip microcomputer level conversion so that it can communicate normally with the host computer. The single-chip CPU uses asynchronous serial output UART0, works in mode 1, interrupt mode, and timer 1 is set to auto-reload mode to generate a baud rate of 115 200.
3.4 Display interface
The host computer communication program is mainly programmed using Visual C++. The display interface is used to display the atmospheric static pressure, dynamic pressure and true air speed values. The program interface is shown in Figure 2.

When writing the host computer program, you need to use the MSComm control to complete the serial port parameter setting and data reception by setting the various properties of the control. The main properties of the MSComm control used in the true vacuum display interface are as follows: CommPort property (used to set the serial port number), PortOpen property (used to open or close the serial port), SetSettings property (used to set or return the serial port baud rate, parity, data bit, stop bit parameters), InBufferCount property (set or return the number of characters waiting for the computer to receive in the receive buffer), Input property (read data from the receive buffer and clear the receive buffer).

4 Design of true vacuum solution method
4.1 Principle of linear interpolation
Segmented low-order interpolation is a function interpolation method. The interpolation interval [a, b] of a relatively complex function f(x) is divided into a series of subintervals. In each subinterval [xi, xi+1], a simple function is used to approximate the original function. When the approximate function used in each subinterval is a linear function, it becomes a commonly used linear interpolation method.
The linear interpolation formula of the sub-interval is:

h is the interpolation step. When calculating the function value, the system collects the x value, and then finds the interpolation interval [xi, xi+h] where the value is located and the corresponding function value [yi, yi+h], and then the y value can be calculated according to the interpolation formula.
4.2 Interpolation node calculation
It can be seen from formula (1) that the true airspeed calculation formula is relatively complex. If it is directly calculated using a single-chip microcomputer, it will occupy a large amount of memory resources, thereby reducing the calculation speed. Therefore, within the allowable error range of the system, the original function is interpolated and the complex function is approximated by a simple function. Since the true airspeed calculation formula has two variables, it cannot be directly solved by piecewise linear interpolation. Considering the convenience of actual calculation, formula (6) is used to calculate the true airspeed value.

Then the true airspeed in formula (6) is divided into two parts: one part is the indicated airspeed Vi containing only the dynamic pressure PD variable, and the other part is the function formula H' of the pressure altitude H containing only the static pressure PH variable. By calculation, it can be obtained that 1≤H'≤1.289 9.
The calculation formulas for indicated airspeed Vi and pressure altitude are shown in formula (7) and formula (8) respectively.

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According to formula (4), the two parts of the true airspeed formula are processed separately.
First, piecewise linear interpolation is performed on Vi. The number of interpolation nodes can be determined by the linear interpolation remainder:

In this design, the relative error of the interpolation calculation is set to 0.5%, then δ=400x0.000 5=0.2 km, , according to formula (9), the interpolation step size is. Let h=29, interpolation node. Because it is an integer, let n=266.
The second part of the true airspeed formula is the function H' of the pressure altitude H. Here, H is not interpolated, but H' is directly interpolated. In this design, the relative error of the interpolation calculation is set to 0.01%, then δ=1.2899×0.01%=0.012 899≈0.0013. M=max|(H')"(Ph)|=.513×10-10, according to formula (9), the interpolation step is . Then the interpolation node n≥ is an integer, so let n=8.
As long as the corresponding values ​​of dynamic pressure and static pressure are calculated by the above interpolation algorithm, and then multiplied, the actual true airspeed value is obtained.

5 Test results
Table 1 is the actual true airspeed value calculated in real time at altitudes of 0 m, 1 000 m, 3 000 m and 5 000 m. It can be seen from the table that the maximum error of the actual true airspeed does not exceed 4%, which can meet the design requirements.

6 Conclusion
The UAV true airspeed test system based on single-chip microcomputer designed in this paper has simple circuit, small size, light weight, stable performance, and can measure the true airspeed value in real time and accurately. It is suitable for small unmanned flight systems.