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A servo-assisted altimeter is more accurate than a simple altimeter because the small movements of:
  • A
    the capsules are inhibited.
  • B
    the capsules are not taken into account.
  • C
    the pointers are detected by a very sensitive electro-magnetic pick-off.
  • D
    the capsules are detected by a very sensitive electro-magnetic pick-off.

Refer to figures.
The sensitive altimeter employs a minimum of two aneroid capsules. This provides for a more accurate measurement of pressure and provides more power to drive the mechanical linkage.
The movement of the capsules in response to changes in altitude (pressure) is transmitted via a suitable mechanical linkage to three pointers that display (against a graduated instrument scale) the aeroplane altitude in tens, hundreds, and thousands of feet. The whole assembly is encased in a container, which is fed with static pressure, but is otherwise completely airtight.

Servo-assisted altimeters use the same basic principles as sensitive altimeters, whereby pressure changes are measured using the expansion and contraction of evacuated capsules. The instrument uses an electrical servomotor to transmit the movement of conventional aneroid capsules to the instrument display. A digital counter system and a single pointer indicate the altitude.

The principle of the servo altimeter is that the small movements of the capsules are detected by a very sensitive electromagnetic pick-off. This produces an electric current which is amplified and used to drive a motor which rotates the counters and pointer.

AC is fed to the middle leg of the E bar, setting up alternating magnetic fields in the outer legs ‘A’ and ‘B’.

The coils on these two legs are wound 180° out of phase. The exciter therefore induces a current in each leg, but since these are 180° out of phase and of equal strength, they cancel each other out when the I bar is equidistant from the legs of the E bar (that is when no pressure change acts on the capsules).

With a change of pressure the capsules expand or contract, moving the I bar on its pivot, closing the gap between the I Bar and E Bar at one end and opening it at the other.

This causes an imbalance of magnetic fields and therefore of the currents induced in the ‘A’ and ‘B’ coils. The imbalance causes an error signal which is passed to the amplifier, where it is amplified and rectified, and thence to the servomotor.

The servomotor drives the counter-pointer system of the altimeter and at the same time, via a cam drive, re-aligns the E Bar with the I Bar. Once re-aligned, the error signal ceases and the altimeter indicates the correct height.

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