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DIGITAL ALTIMETER FOR UCAV Radek BYST!ICKÝ 1 Abstract:

This paper deals with digital altimeter development for UCAV. The altimeter design is based on Atmel AVR microcontroller AT90CAN128 and Freescale pressure sensor MPX4115AP. Altimeter has additional software to calculate vertical air speed. Computed output data are transmitted through CANaerospace interface to an onboard data acquisition unit.

Keywords: UCAV, Altimeter, AVR, FIR, CANaerospace

1. INTRODUCTION Information Technologies (IT) plays a dominant role in development of Network Enabled Capability (NEC) and in professional Czech Armed Forces, in general. NEC is network communication and information system, which combines strategy, recent tactics, ways, procedures and organization methods, which military forces can use to achieve superiority over the enemy. Each commander is able to make right decision and issue orders only on the basis of enough necessary information. Network communication and information system NEC is being developed for this reason. Any comprehensive study on required structure of information from aircraft, their collecting, processing and providing to the NEC has not been published yet. However, well and logically ordered information from aircraft onboard systems and sensors are essential prerequisite for effective NEC realization at aviation area. A basic hierarchical structure of proposed Aircraft onboard Electronic System (AES) is described in full details in [1]. AES is a modular system composed from several subsystems. Communication among particular modules is based on CANaerospace protocol, which is an enhancement of the Controller Area Network (CAN) [2].

Figure 1 Proposal of electrical subsystem Electrical subsystem blocks (Fig. 1) process information concerning inertial measurement of the aircraft space position (Earth's magnetic field, geographic position), airframe and engine operation conditions (operational pressures at systems of oil, fuel, air and oxygen), temperature at important points of the aircraft, surrounding temperature, air pressures, voltage of onboard power supply system and positions of selected actuators.

2. ALTIMETER HARDWARE STRUCTURE One of the subsystems mentioned above is a combined altimeter with climbing rate indicator using commercially widespread available static pressure sensor MPX4115AP. With regard to NEC architecture, we can not adapt it to all altimeters that are used in military and civilian areas. It is 1 - Cpt. Ing., [email protected], Faculty of Military Technology, University of Defence, Kounicova 65, 612 00 Brno, Czech Republic, telephone +420973445234

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necessary to build a measurement system, with at least comparable parameters. One from the low coasts and available sensor is a Freescale MPX family. The sensor is connected regarding the simple recommended schematic [4], and connected with other electronic devices shown in Figure 2. First of all, the impedance match is taking place, done by Rail-Rail operational amplifier. Theoretical voltage range varying between (0.2÷4.8) V corresponds with static pressure between (15÷115) kPa. Because this altimeter is primary dedicated to UAV, especially radio-controlled, it is not necessary to use the complete voltage range. For UAV operating among the area of the Czech Republic, having the vertical relief varying between (115-1602) m, the pressure measurement between (101325÷82000) Pa could be sufficient. Figure 2 shows the altimeter processing chain. The signal is after input impedance match shifted by offset to have 0V output at 0m and 2,56V output at 1800m this means this signal is normalized to the range of AVR internal A/D converter. A period between two measurements is controlled by microcontroller, and has been set for 50Hz.

Figure 2 Altimeter processing chain Using the Atmel microcontroller AT90CAN128 with only 10 bits resolution give us maximum measuring precision corresponding to 1.6m, which is sufficient precision comparable with other onboard mechanic altimeters, at a fraction of their price. Figure 3 shows the measured calibration characteristic. To reduce the barometric error inside the calibration standard to minimum, the measurements were made with and without vibrations. Arithmetic mean of the outcome variables was then interleaved with a polynomial of 2nd order. Based on the calibration measurement, the linear calculation formula given by sensor manufacturer was refused. Primary because the difference between measurement and formula was approximately (20÷80) m. Table 1 and Figure 3, show the calibration measurement used to determine the coefficient for the polynomial curve. Figure 3 also depicts the difference between linear and polynomial curve.

Figure 3 Calibration measurement

3. SOFTWARE DEVELOPMENT 3.1 AVR programming Used microcontroller is offering us to use software filtering instead of analogue input anti-aliasing filter, because filtering is anyway needed. The advantage is that we can easily achieve the same result

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with software, than with pricey electronics components. Even long-window filters could be implemented with only minor time delays. Input parameter for the calculations is a voltage value measured by A/D converter and transformed by polynomial curve to the static pressure pC. Altitude is then calculated by Equation 1, as follows

'T $ ' p $ H=% 0 " ! %%1- C "" & ! # & p0 #

!! R g

(1)

Where T0 is temperature at sea level, p0 is pressure at sea level, alpha is temperature gradient, g is acceleration of gravity, and R is the universal gas constant. Parameters of the International standard atmosphere can be easily modified, even at process time with help of the internal CANaerospace messages. To improve accuracy and stability the FIR filtering was applied. Using a Matlab low pass 50-point Chebyshev window [3] with 60dB of side lobe attenuation was created. This filter was set to substitute the missing anti-aliasing filter. Figure 4 shows the filter curve and a step response. As Figure 5 describes, this filter improved significantly the accuracy, which was initially without filtering around 1.6m. This accuracy allowed as calculating the vertical speed as the derivation of the altitude. Unfortunately using the 10bit converter is causing a serious vertical speed fluctuation. 0.045 0.04 0.035 0.03 0.025 0.02 0.015 0.01 0.005 0

0

20

40

60

80

100

120

140

160

20 0 -20 -40 -60 -80 -100 -120 -140

0

50

100

150

200

250

Figure 4 Chebyshev windows with 60 dB of side lobe attenuation With additional FIR filter applied at climbing rate, the fluctuation can be stabilized better. The second filter is this time set to 150-point Chebyshev window with the same parameters of side lobe attenuation. Used sampling rate and the window-length of the second filter are causing indication time delay about 1.5s. Common mechanic climbing rate devices have approximately the same delay. ISBN 978-80-970202-0-0

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With a help of second filter we was able to improve accuracy and stability to 1ms-1. Figure 4 shows also this filter as well.

3.2 Labview programming During the testing phase LCD display was used as primary output. It served for debugging the calculations and filters and so on. After the debugging phase the output as redirected to a serial line RS-232 to see the measurement history necessary for adjusting the filters coefficients. Easiest way to see the data history is to create simple program in a Labview which is a platform and development environment for a visual programming language. Figure 5 presents Labview code for an interface depicted in Figure 6. Figure 6 shows data history plot and graphical interpretation as the aircrafts onboard indicators, in particular, the altitude measurement on the left side, and climbing rate on the right side.

Figure 5 Labview code

Figure 6 Labview interface during a descent at 5ms-1 As you can see the signal from the altimeter is very stable, in contrast to the vertical speed, which is varying around the correct value. These fluctuations reach about ±1.6m. As seen in Table 1, the precision of the polynomial curve limits the precision of whole device to the units of meters, which correspond to the accuracy of the sensor. Caused by these fluctuations the usage of the climbing rate is limited, and can be used only tentatively. However if we change the 10bit A/D converter for 12bits or even more, the resolution could be only about 0.4m(aprox.1ft.) or better, and the climbing range can be used as needed.

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Table 1 Calibration table Without!vibrations Voltage!# pc [V] [Pa]

Altitude [m]

Voltage!" [V]

pc [Pa]

1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100

2,419863 2,279726 2,139589 1,996950 1,856813 1,719179 1,576539 1,428895 1,287749 1,128602 0,975953 0,823304 0,663148 0,513001 0,366608 0,220215 0,050049

83126,3 84141,7 85157,9 86193,2 87211,2 88211,9 89249,8 90325,2 91354,1 92515,3 93630,1 94746,0 95917,9 97017,5 98090,7 99164,8 100414,5

2,482424 2,342287 2,204653 2,059511 1,919374 1,779237 1,636598 1,483949 1,341310 1,196168 1,043519 0,895875 0,738221 0,590577 0,432923 0,277771 0,120117

82673,3 83688,3 84686,0 85739,0 86756,6 87775,1 88812,7 89924,1 90963,5 92022,1 93136,5 94215,3 95368,4 96449,2 97604,4 98742,4 99899,8

With!vibrations Voltage!#! pc![Pa]! [V] (meas)

hysteresis [m]

Voltage!"! [V]

pc![Pa]! (meas)

45,42 45,02 46,41 44,22 43,84 41,73 41,37 37,59 36,26 45,34 44,94 47,86 49,07 50,29 42,63 36,70 44,29

2,439883 2,299746 2,159609 2,016970 1,876833 1,736696 1,589052 1,443910 1,288758 1,143617 0,995973 0,840821 0,680665 0,535523 0,392884 0,230225 0,072571

82981,3 83996,6 85012,7 86047,8 87065,7 88084,5 89158,7 90215,8 91346,7 92405,7 93483,8 94617,9 95789,6 96852,5 97898,0 99091,3 100249,0

2,462405 2,324770 2,179629 2,041994 1,899355 1,761720 1,616579 1,476442 1,326295 1,173646 1,020997 0,878358 0,720704 0,563050 0,415406 0,247742 0,102600

82818,2 83815,2 84867,5 85866,2 86902,1 87902,5 88958,4 89978,8 91073,0 92186,5 93301,0 94343,4 95496,6 96650,9 97732,8 98962,7 100028,4

Freescale hysteresis! Voltage!mean Freescale!pc Error [m] [V] [Pa] [m] 16,35 18,01 14,28 17,69 15,78 17,39 18,96 22,21 25,40 20,15 16,64 24,76 26,17 17,84 14,48 11,16 18,98

2,451144 2,311007 2,172121 2,028231 1,888094 1,749208 1,606569 1,456422 1,314530 1,162385 1,009736 0,859590 0,700685 0,551789 0,399766 0,248993 0,085083

83308,3 84357,9 85398,1 86475,8 87525,4 88565,6 89633,9 90758,5 91821,2 92960,7 94104,0 95228,6 96418,7 97533,9 98672,5 99801,8 101029,4

$40,32 $44,61 $46,95 $51,90 $53,11 $52,45 $53,50 $58,81 $57,37 $62,03 $66,00 $67,27 $73,40 $71,88 $71,50 $69,38 $74,78

Altimeter!pc [Pa] 82899,8 83915,0 84921,9 85966,1 86983,9 87993,5 89031,2 90124,6 91158,8 92268,7 93383,3 94480,6 95643,1 96733,3 97847,5 98953,5 100157,1

Error [m] 0,56 $0,73 $0,23 $2,39 $1,02 2,07 3,39 0,42 3,95 1,41 $0,58 $0,02 $4,35 $1,27 0,59 4,06 0,01

4. CONCLUSIONS Conception and features of proposed Aircraft Electronic System (AES) have been shown in Figure 1. AES can be used in any transport or combat aircraft or e.g. unmanned aerial vehicle (UAV). AES is a modular and universal system, which is the reason that only few modifications in certain modules are necessary during implementation to particular aircraft. Only selections of measured quantities (needed by other modules in the network) and optimization of their scale are required. The benefit of this system lies in utilization of proven, flexible and relatively high speed CAN enhanced by CANaerospace protocol for data fusion from all available aircraft sensors and their real-time providing to the integrated environment NEC. Even with the widespread sensors we could achieve a resolution corresponding to 1.6m or even better, which is really good for a low coast altimeter. Future work will improve the error with help of 12 bits A/D converter and better filtering method and full CANaerospace implementation. The work presented in this paper has been supported by the Ministry of Defence of the Czech Republic (Project No. OVUOFVT200802 defence research).

REFERENCE 1. JALOVECKY R., BAJER J.: Development of the aircraft electronic system using CAN with CANaerospace protocol. In JALOVECKY, R. and STEFEK, A. (ed.) Proceedings of the International Conference on Military Technologies 2009. Brno: University of Defence, 2009, p. 360-365. ISBN 978-80-7231-649-6. 2. JANU P., BYSTRICKY R. BAJER J.: Proposal of a time-triggered avionic electrical subsystem using CANaerospace. In JALOVECKY, R. and STEFEK, A. (ed.) Proceedings of the International Conference on Military Technologies 2009. Brno: University of Defence, 2009, p. 387-393. ISBN 978-80-7231-649-6. 3. VÍCH, Robert, SMÉKAL, Zden!k. "íslicové filtry. 1. vyd. Praha : Academia, 2000. 218 s. ISBN 80-200-0761-X. 4. Freescale Semiconductor, Inc. MPXx4115: -115 to 0kPa and 15 to 115kPa Integrated Silicon Pressure Sensor, Temperature Compensated and Calibrated [online]. 31.01.2009 [cit. 2009-0630].WWW:http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPXx4115&tab =Documentation_Tab&pspll=1&nodeId=01126990368716&SelectedAsset=Documentation&Pro dMetaId=PID/DC/MPXx4115&fromPSP=true&assetLockedForNavigation=true&componentId= 2&leftNavCode=1&pageSize=25&Documentation=Documentation/00610Ksd1nd%60%60Data% 20Sheets>. Opponent: doc. Ing. Rudolf JALOVECKÝ, CSc., [email protected], Kounicova 65, Brno 662 10, +420973445217 ISBN 978-80-970202-0-0

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