BRUEL & KJAER System V - Mo de Stud Yhea D

For 981 m / s
2
(100 g) acceleration level and 0,18 kg moving element mass (Type 4811) .
380
m
= 981 - 0,180 =0 , 207kg
Fig.7.4 shows a diagram of bare table acceleration limits with respect to frequency for
the Type 4811 High g Head mounted on a 4801 Body and a 4805 Body with and with-
out cooling. Power is supplied in all cases by a Type 2707 Power Amplifier . Above
400 Hl it can be seen that the limits are based sol ely on the amount of force available,
as in the preceding example. At lower frequencies, however, other factors must be con-
sidered . The displacement, for example, must be held below 13 mm (0 ,5 in) in order to
avoid hitting the overtravei stops . At 10 Hl, therefore the acceleration is limited to 2,6 g.
At 32 Hl the limit has risen to 27 g. At this point a velocity limit of 1,27 m/ s (50 in / s)
begins to controi. This is due to a limitation on the voltage which can be obtained from
the Power Amplifier . A further limitation appears from 150 Hl to 400 Hl caused by the
power limit of the amplifier . If it were necessary to exceed these limits for high g, low
frequency operation, a Type 2708 Powe r Amplifier could be used, because of its higher
voltage capability . However, caut i on should be exercised when operating outside the
specification limits .
Fig.7 .5 shows a plot of maximum acceleration vs. load for the 4811 Head mounted on
the different bodies . To find the maximum acceleration possible for a particular combina-
tion, find the TOTAL mass , including test object , fixture and moving element, on the
abscissa, then move vertically upward to the intersection with the applicable curve . The
acceleration can be found on the ordinate opposite.
Fig.7.6 shows bare table acceleration limits with respect to frequency for the 4812
Head . Above 120 Hl, the limits are based on force available. Between 29 Hl and 120 Hl ,
the velocity is limited to 1,14 m / s (45 in / s) . Below 29 Hl , the displacement limit of
13 mm (0,5 in) controis .
Fig.7 .7 shows a pl ot of maximum acceleration vs. load for the 4812 Head mounted on
the different bodies .
Fig.7 .8 shows bare table acceleration limits with respect to frequency for the 4813
Head . The displacement limit controls to 26 Hl and the velocity limit to 90 Hl.
Fig. 7 .9 shows a plot of maximum acceleration vs. load for the 4813 Head mounted on
the di fferent bodies .
7.2. MODE STUDY HEAD
Fig . 7.10 shows the frequency response of the Mode Study Head Type 4814. The reso -
nance peak is at 6,4 kHl.
The loading limits diagram is shown in Fig.7.11. In this case force is used as the ordi-
nate instead of acceleration , as the Mode Study Head is usually force - controlled . Dis-
place ment and velocity (proportional to voltage) limits still apply , in this case 25,4 mm
(1 in) and 1,01 m/ s (40 in / s).
45