motion of an external ferromagnetic
on landing. If used on a track, since the accelerometer
object. A steel arm extending from the
is moderately sensitive to transverse acceleration, the
carriage -and passing close to the pickups
track should be as smooth as possible.
during carriage travel is used to create
f. Travel Measurement.
field disturbance in the measuring system.
(1) Travel-time curves may be differentiated
(4) In place of a single steel arm, a steel
to determine velocity, acceleration, and
comb may be fastened to the moving
rate of change of acceleration. However,
carriage to measure travel. This comb
the values obtained are relatively
(toothed rod) consists of 23 teeth on 1-
inaccurate. Several means are used to
inch centers.
A series of magnetic
plot travel versus time: high-speed motion
pickups (similar to those described above)
pictures, magnetic pickups, and variable
is mounted along the track at 24-inch
resistors.
intervals. The pickups are mounted so
(2) The use of high-speed movies to
that the teeth of the comb pass close to
determine travel time and all of its
the pickups during carriage travel and
derivatives is obvious; in addition, movies
generate impulses used to mark the 1-
also serve another purpose: they may be
inch intervals of travel. A pulse-shaping
used to look for gas leaks or tube
circuit (fig. 91) may be used to couple the
bending.
impulse generators to the recording
(3) Travel-time curves may be obtained by
(Many pulse-shaping
the use of a vertical tower, with a series of
networks are commercially available.)
travel markers located along a track. The
markers, are placed to mark 1-inch
when the impulse generators are used as
intervals along the region traversed during
travel markers.
the power stroke of the device, and at 3-
(5) Magnetic pickups may also be used to
foot intervals beyond the power stroke. A
determine the velocity of a device at
magnetic pickup impulse generator is
separation by triggering a counter
used as a marker. The magnetic pickup
chronograph with two pickups spaced a
is 3/4 inch in diameter and 2 inches long.
known distance apart. In the past, the
The stainless steel body of the pickup
velocity of propellant actuated devices at
contains an Alnico magnet energizing a
separation was determined by inserting
pole piece surrounded by a coil of wire. A
voltage pulse is generated when its
external magnetic field is distorted by the
Figure 91. Shaping circuit used with magnetic pickup impulse generators.
102
