Page 20 - PDF_Flip_Book

Rod Machado’s Private/Commercial Pilot Handbook
5-42

which alters the position of (precesses) that gyro (no, there’s no gyro in the
HSIs found in most airplanes). As a result, the remotely located directional
gyro is kept aligned to the airplane’s current magnetic heading. The informa-
tion from the remotely located directional gyro and flux valve is then sent to the
HSI. Small motors in the HSI unit turn its vertical compass card to provide the
airplane’s correct magnetic heading. This process is called slaving, and it’s
what’s being referred to when someone speaks of a slaved gyro (and no, you
don’t need to try and free all the world’s slaved
gyros, either).
The other component is a slaving meter-com-
pensator unit (Figure 85). The slaving meter tells
you when there’s a difference between the air-
plane’s actual magnetic heading and the heading
displayed on the heading indicator. In the event an
error between these two readings exist, the pilot
could use the slaving meter to temporarily correct it
Fig. 84 before having the unit checked or repaired. Fig. 85

Postflight Briefing #5-4

Three Ring Laser Gyro

To provide pitch and bank information, an attitude and heading
reference system (AHRS) typically uses three laser gyros, one for
each airplane axis (Figure 86). Computer assessment of all these
three gyros (along with other components of the AHRS) provides
the basic heading and attitude reference along with present posi-
tion, groundspeed, drift angle and attitude rate information. The
onboard computer begins assessing this information once it has
been initialized by determining the initial vertical position and
heading.
The ring laser gyro uses laser light to measure angular rota-
tion. Each gyro (one for each airplane axis) is a triangular-
shaped, helium-neon laser that produces two light beams, one
traveling in the clockwise direction and one in the counterclock-
Inside the 3-Ring Laser Gyro Fig. 86
Courtesy of JAXA
Readout Detector Corner Prism
wise direction (Figure 87). Production of the light beams, or
Fringe Pattern A small amount of light lasing, occurs in the gas discharge region by ionizing a low
passes through this pressure mixture of helium-neon gas with high voltage to
Clockwise mirror produce a glow discharge. Light produced from the lasing
is reflected around the triangle by mirrors at each corner of
Light Beam
Counter Clockwise the triangle to produce the clockwise and counterclockwise
Light Beam light beams.
Anode
When the laser gyro is at rest, the frequencies of the two
opposite traveling laser beams are equal. When the laser
gyro is rotated about an axis perpendicular to the gyro unit,
Anode
a frequency difference between the two laser beams
Gas Discharge results. The frequency difference is created because the
Region speed of light is constant. One laser beam will thus have a
greater apparent distance to travel than the other laser
As a small amount of laser light from the two lasers
passes through the mirror at the top of the diagram. Both
light beams are now combined. If movement of the gyro
has changed the frequency of the laser light, then the
combined beams will produce a fringe or interference pat-
tern. This is a pattern of alternate dark and light stripes.
Mirror (1 of 3) The onboard computer’s analysis of this fringe pattern
Cathode provides pitch and bank information to the airplane’s
Fig. 87 instrument systems.