The use of direct-reading pressure gauges is mainly restricted to small air- craft with a limited number of gauging requirements. The more complex the aircraft and its systems, the greater the number of pressure measurements required and the less practical it becomes to pipe these to an instrument array in the cockpit. Consequently, the various pressures are measured at source and transmitted electrically to the pilot's instrument displays, which may comprise individual electrically operated indicators or a computerised electronic display.
The conversion of pressure into a proportional electrical signal and its transmission to a calibrated indicating instrument necessarily involves the conversion of mechanical movement into an electrical output at the mea- suring source and, in the case of a mechanical indicator on the flight deck, a reversal of this conversion. There are various types of device for achieving this, including the synchronous transmission, or synchro, system, the inductive transmitter and the potentiometer system.
Synchronous transmission
The principle of synchronous transmission was described in Chapter 2 under the direct reading compass, but is repeated here as it applies to the measurement and transmission of engine oil pressure, and as illustrated in Figure 7.3.
In the example, oil pressure is sensed in a capsule that expands against a spring to create linear movement proportional to the measured pressure.
This movement is transmitted mechanically to a rotor upon which is wound a coil carrying alternating current. The rotor is positioned centrally within a stator having three coils at 1208 spacing. The electro-magnetic field created around the rotor induces a current flow in each of the stator coils, the strength of the current in each coil being dependent upon the orientation of the rotor. These three transmitter currents are fed to, and repeated in, an identical receiver stator system located behind the cockpit instrument panel, where they create a magnetic field identical to that in the transmitter. This field interacts with the a.c. induced field surrounding the receiver rotor coil, causing the receiver rotor to rotate and adopt an orientation corresponding to that of the transmitter rotor. The receiver rotor is mechanically connected to the pointer of a pressure-indicating instrument.
Induction transmitter
In this type of pressure transmission system the pressure to be measured is led to a capsule inside the pressure transmitter, which is located as close as possible to the pressure source. The capsule is mechanically connected to a permanent magnet armature, and linear expansion or contraction of the capsule moves the armature linearly against the opposition of a spring. The armature is surrounded by two sets of coils, supplied with current and
spring
oil pressure pressure gauge
24 v 400 Hz a.c.
transmitter
stator coils
rotor
receiver
connected to a moving coil indicator on the flight deck. As the armature moves, its position relative to each of the coils differs, and the inductance of the two coils will vary in direct proportion to the pressure being measured. This will cause the output current from the coils to vary, positioning the pointer of the moving coil indicator pressure gauge accordingly. The prin- ciple is illustrated in Figure 7.4.
The type of moving coil indicator typically associated with this type of remote reading pressure gauge is the ratiometer, which is illustrated in Figure 7.4 above. Current from the transmitter coils is supplied to two coils wound around armatures on a spindle-mounted iron core. The core is positioned eccentrically between the poles of a permanent magnet, so that the air gap between the core and the magnet poles is greater on one side than on the other. Where the gap is greatest, the strength of the permanent magnetic field will be weakest and vice versa. Current flowing through the core coils creates electro-magnetic fields that will interact with the perma- nent magnetic field. If the two current flows are equal, their magnetic field strengths will be equal and of opposite polarity, thus cancelling each other and the iron core will be held stationary. If, however, the current flow in coil B is greater than that in coil A the stronger magnetic field surrounding coil B will be attracted toward the larger air gap where the field strength is weaker, rotating the iron core on its spindle and moving the attached gauge pointer against a calibrated scale. At the same time, the weaker induced field sur- rounding coil A is moved into a narrowing air gap where the permanent magnetic field is stronger. This will eventually arrest the rotation of the core when the opposition of the permanent field matches the attraction of the induced field and the gauge pointer will indicate the changed pressure that caused the current imbalance. If the current flow in coil A is greater than that in coil B, the effect will be the reverse of that described above.
coil 1 coil 2 coil 2 coil 1 armature oil pressure iron core A B
N
S
pointer and scale springratiometer induction transmitter
Potentiometer transmission
This type of pressure transmission system uses an inductance transmitter similar or identical to that described above, but its output current is amplified and used to drive an a.c. motor, which is connected to the pointer of the pressure gauge and a potentiometer. In simplistic terms, the poten- tiometer is a variable resistance connected to an a.c. supply, the output of which is fed back to the transmitter amplifier. As the motor drives the gauge pointer, the potentiometer resistance varies until its output current and phase balances the transmitter signal and supply to the motor ceases, holding the gauge pointer at its new position.
Piezo-electric transmitters
In most large modern aircraft the transmission of low pressure utilises solid- state transmitters that operate on the piezo-electric principle. These com- prise a thin stack of quartz discs impregnated with metallic deposits. When acted upon by pressure the disc stack flexes and small electrical charges are produced. The polarity of the induced charge depends upon the direction of flexing, due to increased or decreased pressure, and the output is amplified and used to actuate an electronic representation of a pressure indicator.
Pressure gauge indications
Traditionally, aircraft pressure gauges use a central pointer moving against a circular calibrated scale, as illustrated in Figure 7.5. The gauge scale is calibrated in the chosen units of pressure measurement and coloured markings are added in many cases to indicate operating limits
80 60 40 20 0 100 120 PSI oil pressure green arc red red white slip indicator