LINEAR AND ANGULAR MEASUREMENTS

E F F E C T OF V I B R A T I O N S ON THE P R E C I S I O N

OF L I N E A R M E A S U R E M E N T S

Yu. S. M i r o n o v UDC531.71.088.2

A characteristic feature in the application of many modern instruments used in testing linear dimensions con- sists of bringing them closer to the work bench, thus making it possible to test more efficiently the dimensions of the manufactured articles, to prevent rejection in good time, and in m a n y instances to control technological pro- cesses. However, this transition from laboratories (with a relatively constant temperature, normal humidity, small vibrations, etc.) to production entails, as a rule, a pronounced change in the instruments' operating conditions. In the first instance the conditions are more favorable for obtaining the smallest possible error, whereas in the second instance the effect of various interfering quantities can produce additional errors which reduce the measurement precision.

Let us examine the operation of instruments under vibration conditions. One of the sources of considerable vibrations under production conditions consists of metal-working machines. Thus, forced oscillations arise in grind- ing machines under the effect of various disturbing forces (for instance, due to the unbalance of the grinding wheel or the motor). The cutting and friction forces produce self-oscillations. The intensity of oscillations depends on the machining conditions. According to the data of the ~.NIMS (Experimental Scientific Research Institute of Metal- Cutting Lathes),the basic vibration frequency range of grinding machine stocks extends from 20 to 500-1000 Hz for a swing attaining 1-40 g. The swing of oscillations has a tendency to decrease with a rising frequency.

How are the instruments affected by these vibrations ? Without dealing here with the instruments' vibration stability (their capaci ty to perform their functions after subjection to vibrations), we shall examine the vibration strength of instruments (their capacity to perform their functions under the effect of vibrations).

The theoretical problem of the effect of vibrations on various measuring mechanisms has been investigated in a general form by Yu, I. Iorish, S.S. Tikhmenev, A. E. Kobrinskii, S. A. Nozdrovskii, et al. They demonstrated in their work the possibility of the "blurring" of an instrument's pointer, its displacement by various dynamic dis- turbances in the casing, and a discrepancy between the static and dynamic balances.

Oscillations of the reading device pointers (especially on mechanical instruments) which impede considerably the taking of readings are often observed in practice during testing in the course of machining. In correct operation, chatter of contacts and accelerated wear are often observed in electr ical contact transducers. Vibrations also af- fect the reading precision of pneumatic instruments [1]. Testing devices which contact the measured component in one point only are particularly sensitive to vibrations. In order to prevent a process-control instrument's measuring tip breaking away from the tested component 's surface under the effect of vibrations, it is sometimes necessary to raise the measuring effort up to several tens of newtons [2]. Vibrations of measured components produce, according to experimental data [3], considerable addition errors in three-contact snap gauges.

Vibrations affect not only instruments mounted on machines. Although vibrations decrease with an increasing distance from their sources (for instance, from compressors, generators, ventilators, motors, machines, as well as traffic) they can still be felt at a distance of tens and even hundreds of meters, since they are transmitted through baseplates, foundations, etc. For instance, according to the data of [4], horizontal and vertical ground vibrations with basic frequencies of about 25 and 15-20 Hz and amplitudes of 15 g and up to i g respectively, were recorded at a distance of 20 m from a Moscow thoroughfare. These oscillations in turn often produce in building forced vi- brations which sometimes are resonant, since the ground vibrations have a fairly wide frequency spectrum. Thus, a stand whichhad a mass of 1000 tons, carried metrological equipment and was at a distance of 100 m from the ef- fect of any normal industrial or transport disturbances oscillated at an amplitude of 6 ~ and a frequency of 5 Hz [5]. Moreover, vibration amplitudes in the upper stories of buildings are normally higher than in the lower ones.

Translated from Izmeri te l 'naya Tekhnika, No. 1, pp. 16-19, January, 1968. Original article submitted Sep-

tember 25, 196q.

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i- F

Fig. 1

G f

Precision instruments are fairly sensitive to vibrations even with the low accelera- tions. According to our observations a low-power vibrator placed on rubber dampers produced at a distance of 10 m strong resonant vibrations (with swings up to 20-a0 divi- sions) in the pointer of an opticator fixed in an operating condition. The switching-in of a low-power motor produced trembling and sometimes a complete blurring of the in- terference pattern in instruments located in adjacent premises (including adjacent floors). This is most dangerous for instruments with an automatic measurement cycle, since it can produce larger undiscovered errors.

It is true that the majority of the working instruments is insensitive in the first ap- proximation to normal industrial vibrations (provided these instruments are not close to or on a vibration source). This is due to the particular features of their design, includ- ing solid links with forced locking, large measuring errors, and high damping (for in-

stance, by means of dry friction). However, even among these instruments there exist models which are highlysen- sitive to inertial disturbances (for instance magnetic thickness gauges MT-2 and MT-DAZ).

In precision measurements of less solid or soft objects (paper, rubber, films, paint and varnish coatings, etc.) it is also necessary to take in~.o account the possibility of additional errors due to variations of the measuring effort, since vibrations produce additional dynamic efforts on the measuring stem.

At present the necessity for raising the precision of instruments has become glaringly obvious. For this purpose designers strive to produce instruments with a higher sensitivity for the measured quantity, with a small measuring effort, and a reduced friction. The latter two factors do not increase the instrument response to the effects of t em- perature, moisture, pressure, etc., but they raise sharply its sensitivity to the effect of vibrations. At present the largest errors are normally produced by temperature effects; however, with an increased precision of instruments the errors due to vibrations may become commensurate with those due to temperature. In this connection the problem under consideration appears to be even more serious.

All the above statements indicate the necessity to normalize the level of vibrations and shaking permissible in linear measurements of various types of instruments, and to test the instruments for their resistance to vibrations and shocks. In existing normalizing documents for linear measurements this problem is obviously insufficiently clarified. The majority of precision linear measurement instruments has no requirements for their vibration strength. In certain existing norms and recommendations only the qualitative aspect of the question is dealt with:

"Ultraoptimeters are highly precise laboratory instruments and, therefore, work with them should be carried ou t . . , in premises free of vibrations (from the appendix to instruction 109-55 of the Committee of Standards on the Testing of Ultraoptimeters); the basic error of a level indicator is guaranteed for the conditions that "there should be no shaking or vibrations" (from GOST [All-Union State Standard] 11846-66 on level indicators),

Since shaking and vibrations are never completely absent in practice, it is not clear how it is possible to refer an error evaluated under actual conditions to the basic error specified in the requirement of the standard.

Instruction 147-58 on the testing of double microscope reads: "The table which carries the instrument must be carefully protected from vibration." It is not at all clear how it should be protected and how fulfillment of this requirement should be checked (normally all the operations and means of testing are specified by instructions).

In the supplement of instruction t14-56 on testing optical height gauges it is stated: "Optical height gauges are located in a morn free from shaking on a table whose legs are mounted on rubber pads." In this case the follow- ing questions may arise: if the room is "free from shaking," what is the purpose of the rubber, and what kind of rub- ber should be used (the standard should specify such characteristics of rubber pads as their type, thickness, and de- gree of hardness) ?

The requirements and recommendations for optimeters, horizontal comparators, end gauges, universal gear- measuring instruments, and lever evolventometers have been compiled in a similar manner to the one mentioned above.

Let us assume that having spent appropriate efforts and means we had attempted to meet the recommendations as far as possible by isolating from vibrations the premises or the location of an instrument. Would this be necessary in all cases? For many instruments such recommendations do not exist at all. Perhaps these instruments should provide the required measurement precision even with a certain amount of vibration. It may be advisable in deter-

mining the basic error of instruments in general to specify the least possible disturbances for their operation.

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Fig. 2

I " ' - - II z ,% ae --'-'B, C"--~-- ~2

1 g ~ z/ Acceleration, g

Fig. 3

nevertheless some instruments will have under certain vibra- tion conditions even a higher precision then in a static state. All these questions can be answered on!y after developing meth- ods and means for testing instruments under conditions of vi- brations and shaking, selecting optimum criteria for evaluat- ing their quality, and carrying out careful and comprehensive testing of the instruments.

In this field relatively few investigations have been car- ried out. Among them one should mention those described in [6]. The authox~ of this work have developed for testing in- struments a special mechanical vibration rack to which the tested instruments are fixed. By raising continuously the rota- tion speed of an eccentric the instant is fixed at which the in- strument pointer begins to oscillate. This so-called critical rotation speed is determined for different positions of an in- strument and taken as a characteristic of its vibration strength. It is possible, in the authors' opinion, to determine whether a measuring instrument is suitable for its intended purpose from the knowledge of the natural vibration frequency of the mech- anism with which the instrument is intended to be used. A similar criterion of vibration strength was adopted in [7]: "Quantitatively vibration strength can be evaluated by the fre- quency range to which the measuring head mechanism reacts."

However, it is not clear how to understand the "reaction of a mechanism" to frequency in an instrument intended for measuring length. Theory and practice both indicate that changes in a mechanism's reaction (as compared with the static state) can occur even for insignificant vibration intensities (which is evident, for instance, in changes of the measuring effort).

P. I. Rnsin et al. [8] have arrived at opposite conclusions (as compared with [6, 7]). They state: "Variation in the frequency of vibrations does not affect the precision of readings. The vibration amplitude is directly related to the readings precision."

In analyzing these contradictory opinions expressed in [6, 7] and in [8], it is necessary to note that if the vi- bration strength of an instrument is evaluated only by its frequency range [7] or by its critical frequency [6], it be- comes possible to obtain for the same instrument, depending on the vibration amplitude (or acceleration), either a complete insensitivity to vibrations at all these frequencies, or a lack of vibration strength over the greater part of the range. The same can be stated if only the amplitude of vibrations is taken into account.

The effect of vibrations on measuring devices has also been investigated in [3]. However, since the author of this work dealt with an essentially different probiem of selecting optimum design elements for process control instru- ments which operate under vibration conditions, he confined his research to testing a superposable snap gauge of a special design at 780 Hz and an amplitude of 0.4-3 g.

Thus, the above-mentioned research has not solved the basic and most important problems of selecting sub- stantiated criteria for evaluating the metrological characteristics of linear measurement instruments under vibration conditions, and developing methods and means for testing in accordance with the above criteria.

For carrying out such investigations we developed a special device (Fig. 1) similar to the one described in [6], but with a design modified according to the considerations expressed above.

Frame 5 carries support 1 and bracket 3 for fixing to it tested instrument 4. Adjustable anvil 2 serves to set the instrument precisely to the required reading. The device is fixed to the vibration rack table 6 which provides harmonic oscillations in the range of 20-1000 Hz with accelerations up to 10 g. Accelerations are measured by means of a piezoelectric transducer (not shown in the figure). The applied inertial effects which simulated the actual vibration disturbances produced a change in the operating conditions of the instruments.

In qualitative investigations of scale instruments (microindicators, c lock-type indicators, lever and gearheads), a definite vibration frequency was set and the behavior of instruments (their pointers) with a rising accelerationwas observed. When pointers were not set along the direction of vibrations, their position with respect to the scale for

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large amplitudes was determined by means of the stroboscopic effect. The following were observed in these tests: 1. a reduction in reading variations within the range of the instrument's static readings dispersion field; 2. rattling of the instrument's components at various intensities (as a result of impacts between links); it normally preceeded the displacement of the pointer; 3. unstable oscillations which a) start only on unlocking the measuring tip, b) or on lowering it abruptly after unlocking, c) or else stop spontaneously; 4. stable (gradual or sudden) reading varia- tions which are due to changes in the aeceieration or frequency of oscillations and comprise a) a displacement rising with acceleration, b) a change in a displacement's direction (for a further rise in acceleration) with the pointer possibly returning to its initial position; 5. stable pointer oscillations which have a rising amplitude with accelera- tion (at a constant frequency) and occur a) both at the frequency of external vibrations and at comiderably lower frequencies, often with a combination of high and low (beating) frequencies; b) symmetrically or asymmetrically with respect to the initial position of the pointer; in the latter case the blurring of the pointer can be considered as a combination of its displacement and symmetrical vibration; c) with different starting and stopping conditions of oscillations (for instance, the hysteresis with respect to acceleration at a constant frequency amounted to 26%).

Some of these phenomena were absent for certain instruments and oscillation frequencies. The experimental- ly observed aequenee of these phenomena with a rising amplitude (acceleration) and a constant frequency is shown in Fig. 2 (its figures correspond to the numbering of the qualitative test result clauses given above).

Items in clause 1 were investigated quantitatively. The effect of oscillations intensity on reading variations was investigated (for a given number of Iock releases). For this purpose variations in a static state were first deter- mined, and then definite accelerations were provided (in a rising series) at a given frequency, with its proper read- ing variation determined for each operating condition. Characteristic measured relationships of the instrument's reading variations to the vibrations acceleration at a constant frequency of 200 Hz are shown in Fig. 3 (curves 1 and 2 represent reading variations respectively for 20 and 10 lock releases of a dock-type indicator and a microindica- tot). Over segment OAI(OA2) a reduction in the instrument's reading variations was observed (as compared with the static state), over segment A1BI(AzBz) variations attained their minimum, remaining approximately constant, where- as over segment BIC 1 a sharp rise in variations was observed. Such a characteristic is produced in the above instru- ments by considerable friction in the links of the measuring mechanism. Segment A1BI(AzBz) is preferable from the point of view of reduced reading variations. Instruments are most frequently used over segment OAI(OA2). It is not advisable to utilize segment B1C 1.

LITERATURE CITED

I. O.B. Balakshin, Izmeritel '. Tekh., No. 2 (1966).

2. V.V. Kondashevskii, Automatic Instruments' Adjustment for Testing the Dimensions of Components [in Rus- sian], Mashgiz, Moscow (1960).

3. V.S. Pogorelyi, in Coll.: Transactions of the Institute of Engineering, Seminar on the Precision in Engineer- ing and Instrument Making [in Russian], No. 19, Nauka, Moscow (1965).

4. Yu. I. Nemchinov, in Coil.: Seismic Strength of Large-Panel and Stone Buildings [in Russian], Stroiizdat, Moscow (1967).

5. V.S. Shkalikov, Transactions of the State Committee's Institutes, No. 76 (136) Izd. standartov, Moscow (1965). 6. A.B. Zill and C. D. Ehrhardt, in ColI.: Acta Imeco, Budapest (1961). 7. Ya. M. Tseitlin, Reliability of Spring Mechanisms in Measuring Heads and Transducers [in Russian], Izd.

LDNTP, Leningrad (1964). 8. P.I . Rusin et al., in Coll.: Automation of Technological Process Testing in Agricultural Engineering [in Rus-

sian], Rostovskoe knizhnoe izd. (1965).

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