Acceleration Vs. Load - BRUEL & KJAER System V Instruction Manual

_ _
-aro.! &-
KJaw
&rUe1
& J(Pc-r
8,.;;.1 &- Kje»r
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Sp..d,~~.c
Ffg.
7.12.
Frequency response of the Type 4815
7,3 . 2 , Frequency Respo nse of t he Standa rd Acceleromete r Typ e 8 3 05
If the Standard A ccelerometer were removed from its case and inverted, as shown in
Fig. 7 .13 it would showa typical accelerometer response, with a 35 dB peak at 35kHz ,
which is the natural frequency of the spring-mass system of the accelerometer . In the
calibration procedure of an unknown accelerometer using 4815 and 8305, the accelera­
tion level of the Standard Accelerometer is held constant . In this case, the Exciter char­
acteristics and the resonance of the steel case have no influence . However the screw
which connects the two accelerometers is effectively a spring whose influence
IS
particu ­
larly felt at high frequencies. This spring , with the mass of an accelerometer above i1.
has a resonance at about 20 kHz, opposite in direction to that of the mass-spring system
of the Type 8305 . The two effects just about cancel out , and the result is an almost flat
frequency response (see ca libration chart for Type 8305, Fig . 7.14) . The larger the accel ­
erometer being calibrated, the more the resonance curve flattens out.
272099
Fig. 7. 13. Accelerometer 8305 removed from its case and inverted
7 .3 .3 . Acc eleratio n vs. Load
.'
Fig. 7.15 shows the relationship of the possible acceleration level to the mass of the ac­
celerometer being calirated . Accelerometers weighing up to 60 g present no problem.
However, it should be noted that addition of mass on top of the reference accelerometer
i ncreases the cross-motion and testing of large transducers (200 + grams) could cause
base bending of the accelerometer and subsequent error in its output.
~ i f*: ~
Bruel.& Kjcer
Pot.... t~.rl\~ , ~c18
~~
Cop.oho~
..
..,
100
g
Meo~rino
Obi.:
Frequency
.:rs ~seea~f
- Je
I.~
3 ' ,6
g
with d iff eren t'
accelerometers
mo u nt ed. ­
1:::::-.
Constant 20 10
lOg
'vohage
. throughoul
. measu rements.
;:;==" t­
, t::.
3,16 g
o.u....
~7 2
~~.i; 7A
O
o' "
1 00
,
200
LtroLe-..l ' _ ' g
L_U.....
~. , . _ 19 -H1 'Wr. ~
_ _
~teC_ ~
500 1000"2000
8305
8305 + 4339
(16 gm)
305 + 4338
(60g.m)
5000 1 0000
161'2 /'2l!2 A B C lin.
211'03
'20000 40000 D
A.
8 C li n.
WR
6 60
6 ' 5
.JIJ
2 "
o o
50
- '
­