Vibrations of three electron microscopes

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Vibrations of three electron microscopes

·.

NATIONAL RESEARCH COUNCIL OF CANADA

No.

PREPARED BY Gerald Pernica CHECKED BY _{T.D.N. APPROVED BY C.B.C.}

.2AI!. May 1974

PREPARED FOR

. Record Purposes

SUBJECT VIBRATIONS OF THREE ELECTRON MICROSCOPES

Vibration measurements were taken on 12 February 1973 in

Room B5 of Building M12, Division of Chemistry. Building M12 is five

storeys in height of which two are below ground level. The building is

five bays wide (east-west direction) and ten bays long (north-south

direction). Room B5 is located near the southwest corner of the building

on the basement level which is the first storey below ground level (F ig s . 1 and 2).

Located in the rear of Room B5 (Fig. 1) was an EM6

Metropolitan Vickers electron microscope. For a magnification of

50,000 times or greater important details of the image were severely

blurred. The vibration of the image plane was e specially noticeable in

one direction due to the presence of sizable beats. Four large

com-pressors in the basement of the building were blamed for these poor

image receptions. These compressors were located in a room on the

same side of the building as the electron microscope, but at the centre

of the building (Fig. 2). Tw'o compressors were aligned in the east-west

direction and two in the north-south direction. All compressors had the

same rated horsepower but the speeds of rotation differed slightly.

Three compressors were always on. The fourth came on when three

were insufficient to handle the needs of the building. So as not to have

the same three compressors going all the time, different combinations

of the four were used. It was thought that certain combinations of the

..

2

-In order to ascertain if the vibration levels were affected by the

compressor combination. vibration measurements were taken in Room B5

for all four combinations of three compressors on. The output from the

"Larson" accelerometer was recorded on a Century galvanometer recorder. Measurements in the vertical and two horizontal directions were recorded at three locations (Figs. 3 and 4).'

a) on the floor beside the microscope;

b) on the microscope support table. and c) at the top of the microscope tube.

Preliminary measurements revealed that in the horizontal and

vertical directions the lowest dominant frequency at each location was in the

neighbourhood of 30 Hz. ｃｯｮｳ･ｱｵ･ｮｴｬｹｾ for the recorded measurements the

accelerometer signal was filtered for a band pass of 20-50 Hz on a "Kron-Hite" filter.

Table I is a summary of the test results for all four combinations

of one compressor off. Only the range of maximum peak-to-peak

amplitudes of acceleration of each location component from the four tests

is given. Beats were present in a majority of records. ｈｯｷ･ｶ･ｲｾ a

few location components did not exhibit a beat phenomenon for any of the compl'es sor combinations.

From the results of Table I it can be seen that the amplitude of vibration of the three components at each location appears to be only

slightly dependent on the compressor combination. In order to determine

whether the compressors ｷ･ｲ･ｾ therefore. the major cause of the

vibrations. vibration measurements were taken with all compressors off.

Table II is a summary of these results. Beats again existed in a majority

of the records. When the results of Table II are compared to the results

of Table ｉｾ it is seen that there is very little difference. It was therefore

concluded that the compressors were not the major cause of the vibrations. A close examination of the rooms adjacent to room B5. in the

sub-basement. basement and ground floors disclosed that a large air conditioning unit was directly beneath the electron microscope on the

sub-basement level. Although the unit was not used in its cooling capacity

during the winter months. its fan was still used to circulate air. The

motor which powered the fan rotated at 1725 rpm.

Table III is a summary of the measurements taken with the fan off.

but the compressors on. The presence of beats in a majority of the records

was not as evident as before. A comparison of the amplitudes of Table III

with those of Table II indicates an over -all reduction by a factor of two for floor vibrations and an over -all reduction by a factor of four for the

microscope support table and microscope tube. The fan motor was thus

3

-The microscope support table was closely examined in order to determine how best to reduce its transmissibility to floor vibrations.

The microscope had originally been equipped with four isolation mounts,

one at each corner. On examination it was found that two of the four

mounts needed replacement as one side of the table was now resting

directly on the floor. Until new mounts could be obtained it was suggested

that a material such as rubber be placed under each of the existing mounts. On 21 February 1973 vibration measurements were again taken. A single piece of a rubber stopper. approximately 3/4 in. thick. had

been placed under each of the mounts. A summary of the measurements

taken with the compressors and fan motor on is given in Table IV. All

amplitudes of vibration of the microscope were even less than those

obtained with the fan motor off. Although a further reduction of these

amplitudes might be possible. these levels of vibration no longer interfered with the satisfactory operation of the microscope.

Vibration Survey of Existing Microscopes

Vibration measurements were also taken in Room 103 (Fig. 2) which contained two electron microscopes; a Cambridge Scanning

Micro-scope and a Phillips 100 transmission electron microMicro-scope. No

vibra-tion problem had been reported for either instrument. Measurements

were taken so that satisfactory operating levels for these particular

microscopes would be known. The Phillips 100 was isolated from the

floor by a single rubber mat while the Cambridge Scanning Microscope was isolated by an air mount system which formed an integral part of the microscope unit.

The results of the vibration measurements are given in Table V. For each measurement the output from the accelerometer was filtered

so as to pass frequencies within the 0-50 Hz band. The results indicated

that suitable isolation was provided for each microscope only in the

vertical direction. As there had not been any complaints from the

operator. it appeared that for both instruments the recorded vibration levels were still within the satisfactory operating range.

TABLE I

VIBRA TION MEASU REMENTS ROOM B5 ALL COMBINATIONS OF ONE COMPRESSOR OFF

po-

-

'In Floor Be side In Microscope At Top of

pirection Microscope Support Table Microscope

Lowest Lowest Lowest

Dominant Dominant Dominant

Frequency Amplitude':< Frequency Amplitude':' Frequency Amplitude::

Hz 10-3_g Hz 10-3_{g Hz 1 0Ｍ［ｾｧ} Vertical 33 2.0 -2.4 37 3.8-6.6 37 4.6- 6.0 lHorizontal North -South 36 0.17-0.19 31 7.0-9.6 35 2.2- 5. 4 !Horizontal East- West 31 0.38-0.42 31 4.6-7.0 34 15.0-23.0 TABLE II

VIBRA TION MEASUREMENTS ROOM B5 ALL COMPRESSORS OFF

In Floor Beside On Microscope At Top of

Direction Microscope Support Table Microscope

Lowest Lowest Lowest

Dominant Dominant Dominant

Frequency Amplitude':' Frequency Amplitude':' Frequency Amplitude::

Hz 10-3_{g Hz 10-}3_{g Hz 10-}3_g Vertical 34 2.2 37 5. 4 37 7.0 H:orizontal North -South 38 O. 18 34 6. 2 30 6. 4 :-:.Iorizontal lEast -West 31 O. 44 33 6.8 22 16.0

TABLE III

VIBRATION MEASUREMENTS ROOM B5

FAN OFF ALL COMPRESSORS ON

On Floor Beside On Microscope At Top

Microscope Support Table of Microscope

Direction Lowest Amplitude';': Lowest ａｭｰｬｩｴｵ､･Ｇｾ Lowest Amplitude ':

Dominant Dominant Dominant

Frequency 10 -3 g Frequency 10-3g Frequency 10- 3 g Hz Hz Hz Vertical 35 1. 1 42 1.6 42 2.0 Horizontal 41 30 2.2 34 North-South O. 12 1.5 Horizonal East- West 35 0.22 34 1.9 40 3.4 TABLE IV

VIBRATION MEASUREMENTS ROOM B5 MICROSCOPE ON RUBBER MOUNTS

FAN AND COMPRESSORS ON

.-On Floor Beside On Microscope At Top

Microscope Support Table of Microscope

Direction Lowest Amplitudeli Lowest Amplitude: Lowest Amplitude ':

Dominant Dominant Dominant

Frequency 10 - 3 g Frequency 10-3 g Frequency 10- 3 g Hz Hz Hz Vertical 31 1.9 44 1.6 35 1.6 Horizontal 33 0.20 42 1.2 35 1.2 North-South Horizontal 30 0.26 33 1.6 31 1.4 East- West

TABLE V

VIBRATION MEASUREMENTS ROOM 103

Direction On Floor On CSM On TEM

Lowest Lowest Lowest

Dominant Dominant Dominant

Frequency Amplitude':' Frequency Amplitude':' Frequency Amplitude ;,

Hz 10-3g Hz 10-3g Hz 10-3g Vertical 35 2.6 7 0.40 31 1.7 Horizontal North-South 26 0.98 8 1.5 51 2.8 Horizontal East- West 29 1.1

6

CSM Cambridge Scanning Microscope

TEM Transmission Electron Microscope

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FIGURE PLAN VIEW OF BASEMENT FLOOR OF BUILDING M12

2ND F LOO R 1 S T FLOO R 103 ....J ....J u.J GROUND FLOOR _ｾ ｾ - _ｾ " I

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FIGURE 3 PLAN VIEW OF ROOM B5

LOCATIONS OF ACCELEROMETER

•

SUPPORT TABLE

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