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', , 4 STUDY 0F MEANS FOR ADAPTTNG THE OSCILLOGRAPH

F0 THE RECORDING OF MOTION

,jTHEsm FOR DEGREE OF. B s;

iOCJCmNQEcKER';Qg"‘L

1923‘

 

 

 

 

LIBRARY
Michigan State
University

 

 

 

 

 

 

 

PLACE IN RETURN BOX to remove this checkout from your record.
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6/01 cJCIRC/DateDuo.p65-p.15

 

A STUD! OF EEAI‘IS FOR ADAPTING THE OSCILLOGRAPH
TO THE RECORDING
OF MOTION

A
THESIS SUBMITTED TO THE FACULTY
OF MICHIGAN STATE COLLEGE OF
AGRICULTURE AND APPLIED SCIENCE

BY

CECIL CARTER NOECKER
CANDIDATE FOR THE DEGREE
OF BACHELOR OF SCIENCE

JUNE, 1926.

A STUDI OF LEAH“ FUR ADAPTING THE OSCILLUGHAEH

TO TEE RECORDING C-F LOTION

A considerable amount of work has been done in
connection with the measurement of mechanical motion
by use of mechanical methods alone? Some of these
methods seem to be sufficiently accurate and satis—
factory. The mechanical methods have the advantage
of requiring no complicated electrical circuits and,
of course, no oscillograph which is a veny expensive
instrument. but owing to the fact that the oscillo-
graph is an exceedingly sensitive recording device
it was considered wise to develops the project of
designing a suitable method of motion measurement
employing it. A

There are a great number of impOrtant uses to
which such a device could be applied . Vibrations of
machines, ships,structures and air could be studied.

It is possible to take oscillograms of the human

voice and Sounds from.musical instruments and study

*? The Use of Vibration Instruments an Electrical
Machinel, by J. Ormondroyd. Journal of A.I.E.E.

Apr-‘1926. pase 330-

their preperties. By observing oscillOgrams taken
from a genuine Stradivarus violen one might be able
to determine what number of overtones were present,
and what their relative intensities were—-for it is
upon these two that the quality of music depends.

Ry analyzing vibrations of the human voice re-
corded on oscillograms it is possible to determine
the harmonic characteristics of a given voice.

Swaying of buildings and tower: could be stud-
ied under diffennt conditions of disturbance. The
effect of acceleration and deceleration of heavy
traffic on such structures is sOmtimes very pro-
nounced and destructive?-

Iataly,due to the collapse of bridges, roofs
and other structures, engineers have begun to more

fully realize the necessity for studying_the vibrat—

ion of mechanical structures‘fFrOm an investigation

* Apparatus for Studying the Vibration pro-
duced in Buildings by Traffic. Bul. de la
Socie'té l'Encouragement pour l'Industrie Rationale.
Vol. l34,no. 3, Larch 1922. pp 177-186

*4‘ Field for Research Work. By Prof. Foltz

1.1.8.0. Record, iay‘3, 1926, page 452.

of vibration it might be possible to obtain records
of the deterioration in structures and prevent dis-
astrous failure. Analysis of periodic shocks and
their resultant vibrations will reveal conditions.
jeopardizing the safety of structures before mater-
ial damage results.

In a good many electrical machines due to an
unbalanced condition of the rotor there arises cer—

tain speeds,termed critical speeds, where the period

of the rotor vibration is equal to the natural period

of the stator and foundation or acme multiple there-
of} IV taking an oscilloaram of the natural period
of the later it should bespossible to predetermine

the critical speeds cf the rotor.

" In any vibration problem there are five things

which should be know :*
l. Frequency of vibration.
2. Amplitude of vibration .
3. Type of vibration —— simple-harmonic or complex.
4. She elastic properties and mass distribution

of the vibrating body .

*- The use of Vibration Instruments on
Electrical Lachines. By J. brmondrcyd. Journal

5. A general knowledge of the possible mech—
anical and electrical forces acting on the body.

The first three must be gotten by quantitative
neasurement. Observations of these without measure-.
ment is usually valueless. The fourth is gotten
fr0m a knowledge of the material and dimensions of
the body. The fifth involves a clear understanding
of the mechanical and electrical functioning of the
body. Io instrument can supply the fourth and fifth.
Trained human intelligence is necessary here".

In the problem of measuring the magnitude,
period and direction of vibration or motion by means
of an osciIIOgraph one is immediately confronted
with the determination of a suitable of a suitable
beans for transforming small ard delicate mechanical
motions into electrical impulses of sufficient mag-
nitude. The magnitude of the electrical impulse must
be great enough to give a good deflection on the
cscillograph. The mass of the moving part of the
device that will obsorb energy directly from the
moving body should be as small as possible in order
that the inertia of it will be relatively small. If
the inertia of it is too large there is apt to be a
'change in the phase positions of the reaction to the

true vibrations. iherefore, in this problem it is of

prime importance to evolve a device that will have
but a small value of inertia compared to that of
the movin: body.

another characteristic which this device must
possess is that for a small motion there will be
a comparatively large change of current in the
oscilloaraph.

Bearing these points in mind numerous exper-
iments were performed which are listed and explained
a little later. Some of the experiments seemed to
be theoritically sound yet they failed to w rh out

in pI‘QCtiCeo

I

I

vibrating
V body

+ .—
'-—il|IlII.L

 

30 V.

 

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Figure 1.

300 v.

milli

 

mmet er

One of the first experiments Was that of hav-
ing s Loving body change the especitJ of the Variahle
condenser in Fig. l. Thenﬁitically when the especitJ
of the condenser is changed there should accompany it
a change of potentiel of the grid. The grid end fil-
ement form a condenser b; virtue of the feet that
they are separated bJ a leJer of air or gas. The
equivalent circuit can be represented as shown in

Pi . 2.

is

oh; I
7H 7

 

 

 

._J__

Figure 2.

In Fig. 2 we know that $1 the charge in cond-
enser Cl is equal to “Q the charge in condenser 02.
This is true because the condensers are connected

in series.

Let the capacity of Cl 1 cl and 02 = 02.
Also let the voltage drop across 02 = v2 at first,
and after C1 has been changed let the voltage drop
across 02 =‘vé .

V total impressed voltage,”

s charge in the circuit at first, and

g'a charge in circuit after Cl has been changed

In a series circuit

(1) ‘1':- (ﬁ) V = c2v2

170w change the capacity of Cl to cl+Ac. Now

 

Iv: (01+4c) c2 V ' cgv'
c1“ 0 +6}; 2

(2)
Solving for v2 in (1) yields
(3) V2 3 cl

01+ C2

Solving for vé in (2) yields

 

(4) v. : Cl+Ac

 

cl+A C + 02
how dividing (3) by (4) to obtain the ratio of the

voltage drops across C2 we have

 

cl
(5) 32 = “I“: -
vg 61+A°

cr+Ac+c2

. i . .
Equation (5) shows that v; is larger than v2. ln

other words the potential drop across the grid and

filehent nust have been increased. lf the capacity

0f Cl had been decreased then from similar reasoring

.e would find that the voltage drop across 0. would

C.
decrease. Practically, however, there seehed to be
inc change of current in the plate circuit, Fig.1, so

probably there was not 2 sufficiert charge in grid

potential as the vsrieble condenser was moved.

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r----- ----_-- ___---

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In Fig. 3 the pick-up coil recieves enerog

from the high-frequenq; oscillator and has a voltage

E0 3 E: sin t

iii.
induced in it, Fig.4. The currert Io which tends
to flow in the Circuit is leading the voltage EQ, and
maJ be represented by Fig. 5. Since E0 is changed
into a pulsating direct current by means of the rect-
ifier the resultant current has a shape similar to
Fig. 6, and is termed 11' The average value of 11

is equal to one half the value of 10 or

- .6
I1 " "32" L10

With the current I1 flowing throuah the resistance

R1 there is a voltage drop of IlRl across the resis-

tgnce, Fig. 7, which effects the potential of the

grid. The absolute potential of the grid for a

given position of the Variable condenser is egual to
Eg 3 EC - IlRl

and is represented by Fig. 8. Thus as the conden-

3
ser capacity is varied b4 the vibrating bod; various
values of 11 result and by virtue of the fact that

this will cause a vanying voltage drop across the

resistance there will be a changing grid potential.

The changing potential of the grid, Eg, will change
the value of current in the plate circuit. A mﬂli-
amneter was connected in series with the plate cir-
cuit and an extremely small Variation of plate cur-
rent was noticed as the condenser capacity was
varied. There was not enough variation to Justify
connecting the oscilloaraph in place of the mini-
mmeter. Perhaps the variation in current could be
increased b4 two or three stages of resistance
coupled amplification to a proper value. Due to the
fact that for low frequencies the impedance of the
condensers used in the ampliﬁying would be relative-
ly high it would be necessary to use verJ larbe
condensers to eliminate distortion of the low period
vibrations.

This device will not give a straight line
function because the relation of a given movement of
the condenser to another value of movement is not
directlg proportional to the values of their respect-

ive plate currents.

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The inertia recorder, Fig. 9, gave very poor
results. lheoriticallj when this device is attached
to a machine which is vibrating in the horizontal
plane the inertia of the brass slug will cause a
Varying pressure on the carbon pile thus verging
the resistance. rracticallg, with the simple series
circuit of recorder, ndﬂdanmeter and battery, the
sensitivity was-extremely low. This sereitivitj might
be increased with two or three stages of resistance
coupled amplification with the added danger of dis-
tortion of the lower frequencies. This device does
not give a straight line relation between pressure

and resistance but a curve similar to a hyperbole.

Friction tape covering
the windings

wt,
. l3.

‘

 

 

 

 

 

 

' l
a E |
a Secondary
l " /w1m1ng
"orctin~
badyq % Iron armature
Primary
winding 1
Iron
core

FI GURE 10

This device, Fig. 10, operates directly by
the change in reluctance of the magnetic circuit by

movement of the armature in the air-gap. Since

- ll?
¢

it is evident that a change in the reluctance

causes a change of flux when the LLF is constant.
- d . ' . .
Because e - k Eg', there is generated in the

secondary winding a voltage e.

By quickly inserting the armature the voltage
induced in the secondarJ coil was only about two
or three mﬂlivolts. lhis could be amplified. The
relation here between movement of the armature and
the induced e.m.f. will not be a straight line.

She curve taken from a device of this tape

would be a first derivative of the true vibration.

Permarent magnet

   

[ Vibrating
body

0011

Figure 11

ihe meter-movement, Fig. ll, also gave rather.
poor results. The ends of the coil were connected
to a millivoltmeter. When the coil was moved through
the field of the permanent magnet there was a small
e.m.f. generatEd in it and this caused a small de-
flection on the millivoltmeter. This device will not
produce a straight line for the relation of coil-
movement and induced voltage. The curve in this case

will also represent the first derivative of the true

vibration.

Bridge

 

 

 

 

 

 

 

 

 

 

 

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' . contact
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Terminal
WW7?
Vibrating
body

FIGURE} 12

The apparatus in Fig. 12 will produce a straight
line relationship between the movement of the vib-
rating bod; and the resistance between terminals.
This gave a relativels large per cent variation in
current for a given movement of the arm.

Practically the arm had too much inertia and
there was too much lost motion at the pivot "1".
There is apt to be trouble in attempting to keep
a uniform Contact resistance at "m" for different

positions of the arm.

T ermi na 18

Carbon disks

 

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BRADLRYSEAT

FI GURE l3

Vibratinz
body

Stiff wire

Fig. 13 represents a Bradleystat which has the
screw replaced Hith a stiff wire. In this device
the relation between the pressure and resistance is
not a straight line but has a shape similar to that
of a hpperbole as illustrated in the accompanying
graph. I

The Bradlejstat was mounted on the end of a
section of rail that was clamped to a table. The
table was then adjusted until the stiff wire made
firm contact with the_machine Glin the A.C. lab-
oratory. Fig. 14 shows the set—up. as the machine
vibrated the resistance of the carbon pile was var-
ied which in turn caused a varying current to be-
sent through a simple series circuit of a 1.5 volt
dry-cell. The oscilloeraph was connected in series
with the Bradlejstat and then some oscillograns-were
taken.‘ Figures A, B, C, D, and E are oscillograms
taken from this set-up. In figures A and B curves
no. 1 represent the 60 cicle timing wave of voltage
from the power plant and curves no. 2 show the record
of the vibration of G1 without a field. The irreg-
ularities in the vibration curves are probably due
to microLhonic action of the carbon pile.

Curve no. 2, Fig. C shows the vibration of G1

 

 

 

 

 

F mass 14?

With a field. In figures D and E curves no. 1
represent the 60 cycle A.C. voltaae supply from the
powerhouse, curves no. 2 represent the voltage wave
of G1 and curves no. 3 show the record of the vib—
ration of the same machine. In obtaining figures D
and E the pressure between the stiff wire and G1 was
changed from the value used in taking figures a, b
and C.

Pram an observation of curves no. 2 and 3 in
figures D and E it can be noted that one chle of
vibration takes place in the time required to mahe
three CJCleS in the voltage wave. Since G1 is a six
pole machine it is equivalent to easing that a chle
of vibration is made during every revolution of the

rotor.

The following data and results were obtained from a

test on a Lradlegstat.

Pressure Volts amperes Resistance
in lbs. in ohms.
l 4.4 _ .35 12.7
2 3.8 .5 ' 7.6
3 3.4 .61 5.58
4 4.85 .73 3.9
s 2.6 .78 3.33
6 2.35 .815 2.88
7 .95 .9 2.28
8 1.9 .915 2.08
9 1.8 .9.4 1-915
10 1.6 .99 1.615
11 -1.5 1.01 1.455
12 1.4 1.02 1.37
13 1.3 1.035 . 1.255

14 1.2 1.06 1.13

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reassess 01: 1am: snare 1: 121a:

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Hollow fibre cy‘li nd er

   
 
   
 

Metal diaphram
‘ ‘? ‘ Vibrating
. ‘ body

. Carbon grains

hlchorhoNE BUTTUN

FIGURE 15

 

 

 

 

 

FIGURE 16

 

 

 

 

 

 

 

 

 

 

List of References Pertaining to the

Subject of Vibration

heasuring Vibrations in Euildings.
C.R. ioung
Engineering and contracting (Buildings)

Vol. 58, no.1, Oct} 1922, pp 9-12, 1 fig.

High—Speed Photography of Vibration
( Sound, Lechanical, Electrical, etc.)
Augustus Trowbridge
Franklin Inst, Jl., vol. 194, no. 6,
Dec. 1922, pp 713-729, 11 figs.
Rechanical Application of the Oscillograph
A.I.E.E. Journal, tJan.192S, page 45,.
Discussion of above paper
A.I.E.F. Journal, June 1925,page 640.
The Vibration Problem in Engineering
C.R. Soderburg

The Electric Journal, Dec. 1924, page 579.

Apparatus for imudying the Vibration Produced in
Buildings by Traffic.

Eul. de la Sociéte 1‘ Encouragement pour
l‘Industrie Rationale. Vol. 134,Larch 1922,
pp 177-186, 7 figures.

Field for Research Work
ITof. Foltz
K.S.C. Record, Kay 3, 1926, page 452.

The use of Vibration Instruments on Electrical
Eachines.
J. Crmondroyd

T5

Journal of A.I.E.s. Apr. 1926, page 330.

1293 02290 9802 '