Chapter 6. Sensors

6.1. Air Data System

Air Data System evaluates flight parameters of aircraft by means of measurement of environmental parameters (air). The main parameters of environment are:

-static and dynamic pressure;
-rate of change in pressure;
-direction of air movement;

The following basic parameters are assessed:

-absolute altitude of flight;
-indicated velocity;
-vertical speed;
-Mach number;
-air temperature;
-true airspeed;
-angle of attack and grazing angle.

In the past the values of absolute flight altitude and indicated airspeed were enough for flight performance. However, modern information systems on the board of aircraft need more quantity of parameters, such as static and dynamic air temperature, maximum allowable parameter values and others necessary control automatic systems for more precise work.

Air preasure and Air Data System

One of the main environmental parameters is an air pressure outside the aircraft. There are dynamic, static and total air pressure.

Dynamic pressure – it is a pressure that is caused by movement of aircraft in the air. Dynamic pressure is used for measurement of indicated aircraft velocity.

Static pressure – it is a pressure that surround aircraft on the specified altitude. Static pressure is used for measurement of absolute altitude and velocity of flight.

Total pressure consists of static pressure and dynamic pressure.

The measurement of pressure parameters is important because the altitude of flight and velocity are calculated with the help of it (fig. 40). According to this, the special hole system on the fuselage of aircraft is used for air sucking and group of sensors that are coupled by system of air conductors.

Fig. 40 Using of pressure for different parameters measurement

Total pressure is created by means of Pitot tubes (fig. 41) that are arranged outside the aircraft and distanced from fuselage for decrease in aerodynamic errors influence on the measurement of navigation parameters. It connected with air pressure. Since the air receivers are very sensitive to ice presence, there is heating element in the construction of air receivers.

Fig. 41 Air preasure probes

In the past only combined pressure receivers are used but now additional static pressure receivers as small holes on the fuselage of aircraft are used for achieving more precise measurement. Pressure receivers are arranged on the both sides of aircraft in the different places for more precise (fig. 42).

After air sucking it moves using special ducts to devices for speed parameters, absolute altitude and corresponding sensors measurements.

Fig. 42 Arrangement of air pressure sensors on the board of B-737

Pressure sensors

There are different methods of construction of sensors for measurement of gas pressure:

- Piezoelectric;
- Piezoresistive;
- Tenzometric;
- Capacitive;
- Inductive;
- Resonance;
- Ionization.

Manometric pressure sensors structure

One of the checked methods for pressure measurement is manometric that is based on measurement of change in size of aneroid box. It proportional to pressure change (fig. 43).

Fig. 43 Manometric method of altitude measurement

Such principle of construction is inherent for most of the mechanical barometric altimeters that are used as a reserve devices (ВД-10, ВД-17, ВД-20). The measurement devices of such class have great errors of altitude measurement that make up from ±3 to ±1% of measured value.

Piezoelectric method of air pressure sensing

Today piezoelectric method has obtained popularity. The principle of operation is based on the use of semiconductor piezoresistors that convert distortions material to change in resistance [20; 95]. Principles of sensors absolute and differential pressure shown in Fig. 44.

Absolute pressure sensors consist of case 1, airtight vacuum chamber 2 and diaphragm 3. in the chamber 4 action of air pressure P1 causes deformation of the diaphragm 3 as the sensing element. In accordance with the provisions of the diaphragm 3 depending on the pressure in the air chamber 4.

Differential pressure sensor is built along similar scheme except that incoming air pressure P2 goes to the airtight chamber 5. It is necessary to measure ratio air pressure P2.

Usually for construction of diaphragm silicone diaphragm structure is used because their physical properties it bends perfectly perceiving attached to her pressure. In addition, the silicon crystals is very flexible to stretch that ensures long term of exploitation.

Fig. 44 Principles of sensors constraction of absolute and differential pressure

Usually the bending of diaphragm is measured using four identical piezoresistors that are attached to the surface of a thin circular silicon diaphragm firmly (Fig. 45) [95]. Effects of applied pressure or force causes bending of the thin diaphragm and piezoresistors. Resistor will vary according to the change of the material from which they are made, and therefore the air pressure applied to the diaphragm. Thus, the change in pressure is converted to the change in resistance.

Resistors can be connected by bridge or semi bridge scheme. Bridge circuit connection of resistors shown in Fig. 46. Each of the four piezoresistors has the same nominal value. On the Fig. 46 symbol R is a resistor that is a resistance of the resistor that corresponds to not curved diaphragm without air pressure action (P1 = 0); ΔR - change resistance of the resistor that is the result of applied pressure.

Resistors are placed on the silicone diaphragm so that the change in resistance of each was similar approximately. In addition, two resistors increase in the nominal during the bending but other two reduce it depending on their orientation on the diaphragm.

Fig. 45 Arrangement of piezoresistors on the diaphragm

Fig. 46 Bridge scheme of piezoresistors connection

As a result, output voltage of bridge scheme connection of the resistors (Uout) will be proportional to the applied voltage supply and applied air pressure that is caused resistance change ΔR:

The accuracy of air pressure for the piezoresistor method is accounted for 1 – 0,25%. Piezoresistor and piezoelectric methods of measurements of pressure are the basic methods that are used for construction of MEMS sensors.

Absolute altitude of flight

Barometric altimeter measures absolute and relative altitudes of flight based on information about static pressure of air. Principle of its action are based on functional depending of air pressure from altitude. (“barometric depending”) [24]:

Where – air pressure on the elevation h ; g0 – average value of gravitational acceleration (approximately 9.8 m/s2); р0 and ρ0 – air pressure and its density at sea level.

The negative sign in the exponent reflects the fact that atmospheric pressure decreases with increasing altitude as a result of reducing the height of the column of air. The height of the flight can be determined by the formulas:

for h ≤ 11000 м; for 11000 m ≤ h ≤ 20000 m,

Where Т0 =288, 15 К; τв = 0,0065 К/m – temperature gradient of altitude that includes average change of temperature with climbing to the altitude until 11000 m for the standard atmosphere; р0 = 101300 Pa – air pressure at sea level; Rпит = 287,05287 J / (kg • K)- specific gas constant for air; g0 – average value of gravitation acceleration; р11=22632 Pa – altitude pressure; h11=11000 m – altitude; Т11=216,65 K – altitude temperature.

AVIONICS training course materials represented only in demonstrative mode. If it useful, all grammar mistakes will be corrected and more documents will be added. Let me know if you interest in avionics.
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Avionics training in details by
Ostroumov IvanOSTROUMOV Ivan, PhD
Associate Professor, www.sciary.com/ostroumov