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ALTIMETER Multiply alt by 10 . 1'o r heig ht in feet Digital altimeter for gliders & ultralights This compact digital altimeter can display altitude up to 19,990 feet with 10ft resolution. It has a barometric pressure offset adjustment for heights up to 5000ft & is ideal for use in ultralights, gliders, hang-gliders & balloons. By JOHN CLARKE An altimeter is one of the most useful instruments on board any aircraft but, unfortunately, they are expensive. A brand new altimeter for general aviation will typically cost about $800, while secondhand units in working order usually start at about $300. For this reason, many recreational flyers of gliders and ultralights do without an altimeter, or use a secondhand instrument of doubtful accuracy. 28 SILICON CHIP Many old altimeters can be as much as 30% out and often have sticking dials and a slow response time as well. Most also suffer from significant hysteresis- ie, they give different readings at the same heights, depending on whether the aircraft is climbing or descending. Even new altimeters are only guaranteed to an accuracy of about 10% and can suffer significant hysteresis problems. Another problem with some mechanical altimeters is that they only provide barometric pressure offset for altitudes up to about 2000 feet. This offset adjustment is necessary to compensate the unit for the altitude of the landing strip and to compensate for daily variations in air pressure. Unfortunately, an offset adjustment of 2000 feet is inadequate for some landing strips if you want to zero the altimeter at ground level. By contrast, the SILICON CHIP Digital Altimeter does not suffer from any of these mechanical problems. It is accurate to better than 3.5% and responds quickly to changes in height with insignificant hysteresis. And because it has a fast response time, it can easily indicate rising thermals which should make it particularly attractive to glider pilots. But perhaps the best news is the cost of this unit. We reckon that a complete kit of parts should set you back no more than about $250. Digital display Unlike a conventional altimeter, this unit features a 3½-digit liquid crystal display (LCD) that's very easy to read. All the pilot has to do is multiply the reading on the display by 10 to get the height (or altitude) in feet. There are just three user controls on the front panel: two toggle switches to the left of the display and a rotary control knob (BAR ADJ) to the right. The top toggle switch is used to select the display mode for the instrument. When ALT is selected, the unit functions as an altimeter with a resolution of 10 feet. When BAR is selected, the unit displays the barometric setting with a resolution of lhPa (one hectaPascal). The barometric reading is set using the BAR ADJ control, which provides a calibrated adjustment from 1051hPa to 842hPa. This corresponds to a height adjustment from -1000 feet to +5000 feet when compared to the standard sea-level pressure (1013hPa). This means that the altimeter can easily be set to show altitude (ie, height above sea level), height above a local aerodrome, or flight level (ie, height above the standard mean sea-level pressure of 1013hPa). In most cases, where takeoff and landing are from the same strip, the BAR ADJ control is simply adjusted (in ALT mode) so that the altimeter displays zero feet when the aircraft is on the ground. After that, the altimeter will indicate the height of the aircraft above the landing strip, provided of course that the BAR ADJ control is h=;ft untouched. Alternatively, the pilot can set the altimeter before takeoff so that it displays the altitude of the aerodrome. Thus, the BAR ADJ control functions in exactly the same manner as the barometric (or sub-scale) adjustment control found on a conventional altimeter. It can be used either to set the height or the barometric pressure. Incidentally, the hectaPascal (hPa) is the standard metric unit for atmospheric pressure. It is directly equivalent to the old millibar unit which was in common usage until a few years back; ie lhPa = lmb. The second toggle switch is for powering up the Digital Altimeter and ,.,..-· .. ,-·,.,- An SCX15ANC solid state pressure sensor (lower, right) forms the heart of the Digital Altimeter. This device is designed for measuring air pressure from about 1033hPa down to a vacuum & provides an output voltage which is proportional to air pressure. for checking the battery condition. This toggle switch has three positions: down for ON; centre for OFF; and up for battery check (BAT). A light emitting diode (LED) positioned immediately above this switch indicates the battery condition. It shines brightly if the battery is in good condition but goes dim if the battery is flat. The expected life of a 9V alkaline battery is approximately 50 hours of continuous usage. Maximum ceiling To keep the design as simple as possible, the Digital Altimeter is limited to a maximum reading of 20,000 feet (19,990 feet to be precise). We don't really regard this as a limita- tion, since most recreational aircraft don't get above 10,000 feet and even gliders seldom exceed this altitude. In any case, oxygen is necessary above 10,000 feet and the temperature goes well below freezing above this altitude. These environmental limitations should keep even the most enthusiastic recreational fliers to altitudes well below 10,000 feet. How an altimeter works An altimeter is really a barometer. that's been calibrated to display air pressure changes directly in feet. It relies on the fact that the air pressure drops by about lhPa for every 30ft rise in height. However, this figure is only approxi- Specifications Range .................................................................. -1000ft to+ 19,990ft Barometric offset range ....................................... 843-1051hPa calibrated (-1000ft to +5000ft) Readout ....................................... ........................ 3½-digtt LCD Resolution ............................................................ 1O~in ALT. mode, 1hPa in BAR. mode Accuracy .............................................................. 3.5% (typically better than 2%) Operating temperature range .............................. -5°C to +50°C Current consumption ........................................... 1OmA Power requirements ............................................ internal 9V DC battery; or 12V DC external supply (with optional regulator) Battery life ............................................................ 50 hours Dimensions .......................................................... 140 x 110 x 46mm Weight .................................................................. 0.4kg SEPTEMBER 1991 29 mate because, in practice, the change in pressure with altitude is non-linear. For example, from sea level to 1000ft, the pressure drops by lhPa for every 27.7ft rise, while at about 5000ft the pressure drops by lhPa for every 31ft increase in height. This non-linearity is mainly due to the compressibility of air and the drop in air temperature with altitude. To ensure accuracy, the non-linearity of the pressure vs altitude curve must be taken into consideration. The SILICON CHIP Digital Altimeter has the necessary correction factors built in to the circuit. We'll talk more about this later on. In practice, the air pressure at a given height in a standard atmosphere can be calculated from the following formula: TABLE 1 2.5 2.4 2.3 Altitude (Feet) Pressure (hPa) Temp. (OC) -1000 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 11 ,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 1051 1013 977 942 907 875 843 812 782 752 724 697 670 644 619 595 572 549 527 506 485 466 17.0 15.0 13.0 11.0 9.1 7.1 5.1 3.1 1.1 -0.8 -2.8 -4.8 -6.8 -8.8 -10.8 -12.7 -14.7 -16.7 -18.7 -20.7 -22.6 -24.6 P = 1013.25(1 - 73.999 x 10·6 x Height) 5 -2563 Table 1 shows the air pressure values for various altitudes from -1000 to 20,000ft, together with the expected temperatures. This table assumes a standard atmosphere (1013hPa and 15°C at sea level), whereas the actual pressures and temperatures will depend on the weather· and ground temperature conditions on the day. As can be seen, the air temperature drops at a rate of approximately 2°c/1000ft. Note that although we usually think of an altimeter as a device that measures height above ground, it is really a device that 2.2 2.1 20000 1.9 19000 AMPLIFIED PRESSURE SENSOR OUTPUT (WITH RESPECT TO 2V) 1.8 18000 "' 1.7 1.6 • A-0 OUTPUT CHANGE POINTS 17000 16000 1.5 15000 1.4 14000 "' ~ 1.3 13000 > 12 12000 1.1 11000 E 10000 ...g 0 0.9 9000 0.8 8000 0.7 7000 0.6 6000 0.5 5000 0.4 4000 0.3 3000 0.2 2000 0.1 1000 0 1013 963 913 863 813 763 713 663 613 563 513 5 "" 0 463 PRESSURE (hPa) Fig.1: unlike the amplified pressure sensor output, the altitude vs. pressure curve is non-linear, as this diagram clearly shows. For this reason, the sensor output is fed through a curve shaper (actually, a weighted AID converter) to obtain a corrected response. This corrected response is represented here by the stepped graph & has a maximum error of 1.1 %. 30 SIL/CON CHIP All the parts for the Digital Altimeter (except for the LCD & switches) are mounted on two PC boards which sit one above the other to give a compact assembly. Power is derived from a 9V battery or you can add the optional 9V regulator board & power the instrument from an external 12V battery. -measures height above a set pressure level. For example, let's say that the air pressure at ground level is 1025hPa. If the instrument is set to this value using the BAR ADJ control, it then displays the height above this pressure level. So what happens if the air pressure at ground level changes (eg, due to changing weather patterns)? Our altimeter will no longer display the height above ground level; instead it will continue to display the height above the 1025hPa pressure level. The way around this problem of course is to set the altimeter to the new ground pressure level using the information transmitted by the aerodrome's local weather beacon. Pressure sensor Unlike conventional altimeters, the SILICON CHIP Digital Altimeter derives its accuracy from a solid-state pressure sensor. This device is designated the SCX15ANC and is made by SenSym in the USA. It is designed for measuring air pressure from about 1033hPa down to a vacuum - just the shot for altimeter air pressure measurements. In operation, the SCX15ANC produces an output voltage which is proportional to air pressure. It is supplied calibrated to within ±5% and is also temperature compensated, which means that its output voltage remains relatively constant with changes in temperature. This is important for an altimeter pressure sensor, since the temperature difference between sea level and 20,000ft is about 40°C. Because the sensor output voltage increases linearly with increasing pressure, it must be corrected so that we get a true altitude reading. This is done by feeding the amplified pressure sensor output through a curve shaper. Fig.1 shows the details. The bottom curve in Fig.1 is a plot of altitude vs pressure for altitudes up to 20,000ft. From this, it can be seen that any altimeter which did not correct for this non-linear curve would be extremely inaccurate. In fact, we could expect errors of 25% or more, depending on how the output of the sensor was amplified. The top graph in Fig.1 plots the amplified pressure sensor output, while the stepped graph shows the corrected response after it has been fed through the curve shaper. This clearly shows the accuracy of the in~ built correction circuitry. Note that the corrected response from the curve shaper tracks the required altitude vs. pressure curve in a stepwise linear fashion. The tracking error is less than or equal to 1.1 % but this is not the overall accuracy of the altimeter since we must also take the non-linearity of the sensor into consideration (±1 %). Verification So how did we verify the design in practice? We did this in two ways: (1) by flying the unit in a glider; and (2) by comparing it against a conventional altimeter of known accuracy in a vacuum chamber. For the flight test, the Digital Altimeter was compared with a conventional altimeter up to a test ceiling of about 6000ft. Because of its fast reSEPTEMBER 1991 31 sponse, the electronic unit quickly became the pilot's preferred reference. In particular, the pilot reported that this fast response enabled him to track thermals without recourse to a variometer. The Digital Altimeter also gave the . same reading on the ground at takeoff and landing. By contrast, the conventional unit was nearly 50ft out. The vacuum chamber was used for design verification and for calibra- tion. One of the accompanying photographs shows the test rig. It used a small vacuum pump to evacuate air from a glass bowl placed upside down on a rubber seal attached to an old turntable platter. As shown in the photograph, the mechanical altimeter was placed inside the vacuum chamber while the Digital Altimeter's sensor was connected to the vacuum chamber via a plastic hose. By using this test setup, we were able to calibrate the Digital Altimeter so that it tracked the commercial unit to within 1 %. Finally, we used two methods to check the Digital Altimeter for temperature variations. First, we used spray freezer to test the temperature sensitivity of various parts of the circuit. When these tests were complete, the unit was placed in a freezer for several hours and then tested on the vacuum chamber setup. i-:+.::..:3V~-------------------tREF REFERENCE l-'-+-'-'1.2=5V'--------, VOLTAGE +ZV LIQUID CRYSTAL DISPLAY BAROMETER ADJUST PRESSURE SENSOR CURVE SHAPER BAROMETER/ ALTIMETER SELECT INH1 3·1/2 DIGIT DISPLAY DRIVER +2V Fig.2: block diagram of the Digital Altimeter. The pressure sensor produces a voltage which is a function of air pressure. This voltage is then amplified & fed to the curve shaper to correct for the nonlinearity of the altitude vs. pressure curve. The curve shaper output is then applied to a 3½-digit LCD display driver via a mode selection circuit & this in turn drives the liquid crystal display. 32 SILICON CHIP The prototype was tested by comparing it with a conventional mechanical altimeter placed in a vacuum chamber. This vacuum chamber used a small pump to evacuate air from a glass bowl which was placed upside down on a rubber seal attached to an old turntable platter. You can now afford a satellite TV system For many years you have probably looked at satellite TV systems and thought "one day". You can now purchase the following K band system for only These tests showed that the main temperature variations came from the pressure sensor itself, while any variations produced by the remaining circuitry were negligible. For the prototype, the reading varied by only 100ft over a 30°C temperature range. Block diagram Refer now to Fig.2 which shows the general arrangement of the altimeter. In addition to the pressure sensor, it includes an amplifier, a curve shaper, a voltage reference, a display driver IC and the 3½-digit LCD. There is also the barometer adjustment control and a switching circuit to select either the altimeter mode or the barometric adjustment mode. The output from the pressure sensor is first amplified to increase the signal to a usable level and then applied to the curve shaper. Although shown as a single stage in Fig. 2, this amplifier actually consists of three separate op amps. Its job is to amplify the differential output from the bridge circuit in the sensor and provide a noise-free output with stable gain over a wide temperature range. To ensure temperature stability, the reference voltage block provides a stable +6V supply for the pressure sensor. This reference voltage block also provides +3V, +2V and +1.25V reference voltages for the curve shaper circuit and 3½-digit display driver IC. If you now refer back to Fig, 1, you will see that the amplified output from the pressure sensor is a straight line. Its output (with respect to 2V) is 0V at 1013hPa barometric pressure and +2.5V at 463hPa (20,000ft). As explained previously, this straight line response has to be shaped (by the curve shaper) so that it follows the altitude vs. pressure curve. This curve shaping function is performed using a 32-step staircase approximation. Although not shown here, this circuit uses a resistive divider network which reduces the amplified output from the sensor so that it is within 1.1 % of the required altitude vs pressure curve. It works by switching in a different divider ratio for every 0.078V (2.5/32) increase in the amplified pressure sensor output as the aircraft gains height. The 1.25V reference voltage ensures that the correct divider ratios are switched in at the correct positions on the altitude vs pressure curve. In the altimeter mode, the output from the curve shaper is switched through to the INm input of the 3½digit display driver (IC13) . This device is an Intersil ICL7106CPL and it contains all the circuitry necessary for A/D conversion and for driving the LCD. Note that there are four external inputs applied to the display driver: REFm, INm, INw and COM. The REFm input is set to +3V (which sets the input voltage range), while COM is set to +2V. All input voltages applied to INm and INw are with re- $995. 00 This is about 1/3 the price of comparable systems Here's what you get: .. A 1.8 metre pressed steel prime focus dish antenna, complete with all the mounting hardware - as well as a self supporting ground stand. .. One super low noise LNB (low noise block converter) l.4d:B or better. .. One KU band feedhorn and all the mounting hardware as well as a magnetic signal polariser. .. 30 metres oflow loss coaxial cable with a single pair control line. .. A 99 channel infrared control satellite receiver with adjustable IF and audio bandwidth, polarity, and dual digital readout. The IR control unit has a range of approx. 10 metres. 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SEPTEMB ER 1991 33 PARTS LIST 1 plastic case (SY-110), 140 x 110 x 46mm (Arista UB14) 1 PC board, code SC04108911 , 114 x 97mm 1 PC board, code SC04108912, 97 x 102mm 1 PC board, code SC04108913, 87 x 40mm 1 front panel label , 98 x 36mm 1 SPOT centre-off toggle switch, C&K 7103 (S1) 1 SPOT toggle switch, C&K 7101 (S2) 2 C&K toggle switch dress nuts 5 9.5 mm ID x 19mm OD fibre washers 1 Sensym SCX15ANC pressure sensor (Farnell , NSD) 1 3½-digit LCD (Farnell Cat. H1331CC) 1 9V battery holder 12 PC stakes 1 clear Perspex panel, 45 x 18 x 2mm 1 knob, 14.5mm outside diameter, 0.25-inch shaft 1 34mm-length of plastic right angle strip, 19 x 19mm 2 25mm 4BA nylon screws 2 12mm 4BA brass screws 4 48A nuts 3 2mm x 5mm-long screws 3 2mm nuts 1 6mm ID grommet 1 piece of polyurethane packaging for heat insulation, 200 x 130 x 2mm thick 1 200mm-length of light duty hookup wire 1 1-metre length of 0.8mm tinned copper wire Semiconductors 1 LM10CN op amp and reference (IC1) 6 OP77GP precision op amps (IC2-IC6,IC11) 1 ADC0804LCN 8-bit AID converter (IC?) 1 4093 quad Schmitt NANO gate (IC8) 2 4051 8-1 analog multiplexers (IC9,IC10) 1 4053 triple 2-1 analog multiplexer (IC12) 1 ICL7106CPL 31/2-digltA/D LCD driver (IC13) 1 BC548 NPN transistor (01) 1 BUZ71 N-channel Mosfet (02) 1 LP2950CZ-5 5V regulator (REG1) 1 3mm red LED (LED 1) 1 4. 7V 400mW zener diode (ZD1) 11N4148, 1N914diode (D1) Potentiometers 2 100kQ 25-turn top adjust trimpots, Bourns 3296W (VR2,VR4) 1 1OkQ 25-turn top adjust . trimpot, Bourns 3296W (VR3) 2 5kQ 25-turn top adjust trimpots, Bourns 3296W (VR1 ,VR5) 1 10kQ 10-turn pot, Bourns 3590S-1 Ok (VR6) Resistor networks 2 100kQ 4-resistor 8-pin SIL thick-film resistor networks, Bourns 4608X-102 (R2,R4), (Farnell 107-064) 3 22kQ 4-resistor 8-pin SIL thick-film resistor networks, Bourns 4608-102 (R1 ,R3,R5) , (Farnell 107-062) Resistors (all Philips MRS25 0.6W 1% metal film) 1 1MQ 1 3.3kQ 1 470kQ 1 1.2kQ 1 360kQ 2 1kQ 1 200kQ 2 330Q 1 160kQ 1 300Q 4 100kQ 2 270Q 1 75kQ 2 240Q 1 51kQ 1 220Q 2 47kQ 1 200Q 1 27kQ 4160Q 2 20kQ 2150Q 1 13kQ 1 100Q 2 10kQ 1 24Q Wire & cable 1 80mm-length of 6-way 0.1-inch spacing rainbow cable (Farnell 150-432) 1 200mm-length of 8-way 0.1 inch spacing rainbow cable (Farnell 150-433) Capacitors 1 10µF 16VW tantalum 2 10µF 16VW PC electrolytic 1 4.7µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 1 0.47µF polyester (Wima MKS2) 1 0.22µF polyester (Wima MKS2) 8 0.1 µF monolithic ceramic 6 0.1 µF polyester (Wima MKS2) 1 .047µF polyester (Wima MKS2) 2 .01 µF polyester (Wima FKC2) 1 220pF ceramic 1 100pF ceramic Where to buy the kit A complete kit of parts for the Digital Altimeter will be available from Altronics Pty Ltd, PO Box 8350, Perth Stirling St, WA 6000. Phone (09) 328 4459. sp ect to this +ZV common voltage. Thus, any voltage above ZV applied to the IN8 1 input will give a positive reading, while voltages less than ZV will give n egative readings. The IN10 input works a little differently. When the input voltage is greater than ZV, it subtracts from the voltage on the INH1 input. Conversely, when the voltage on IN10 is less than 2V, it adds to the voltage on IHHJ. Thus, if both INHI and IN10 are at +ZV, the display will read zero (ie, 000). In this circuit, the INLo input is used for the barometer setting and to provide the required offset adjustment in the altimeter mode. We've already covered the function of the barometer adjust (BARADJ) control. It allows the barometric reading to be set anywhere in the range from 843-1051hPa (calibrated); or it can be used for height adjustment. Finally, the LCD sp ec ifi ed has 12.7]Jlm-high digits which are easily read , even in bright sunlight. Unlike a conventional instrument, it can b e read at a glance and the digital display avoids any possible confusion. That's all we have space for this month. Next month, we will publish the circuit details and tell you how it works. SC 34 SILICON CH IP