CERTIFYING DIRECT-LOADING ELECTROTENSOMETRIC COMPARATOR SYSTEMS Yu. A. Kiselev, A. V. Nazarenko, and S. P. Tyumentseva

UDC 006.063:531.768

It is usual to employ joint measurement of the masses of standard loads and the local acceleration due to gravity in order to certify direct-loading apparatuses with maximum weighing limits of 105 N, including those appearing in standard OSM-200-10 force-measuring machines. These are not difficult to certify if the masses of the loads are determined before the equipment is mounted on the standard balance. Repeat certification on the other hand requires the standard loads to be demounted, which involves lengthy and laborious operations; also, the limits to the random error in reproducing the force with the given apparatus are unknown after such certification. The development of standards in Siberia has includedthe creation at the Siberian Metrology Research Institute of a direct-loading apparatus of elevated accuracy having an upper limit of 105 N, a standard deviation in the measurement of So~5.i0 -5, and a residual systematic error of @ o ~ 1 0 - 4 9 It is economically suitable to use the set of special loads forming part of the OSM-200-10 standard force-measuring machine. However, one can recertify the set of loads by the existing method only after dismounting the apparatus, which is economically undesirable. We have therefore considered the most economical way of carrying out the certification. The direct-loading unit in the OSM-200-10 machines has a set of special loads of each of total mass 500 kg, two loads of 500 kg each, and sets of I000, 2500, and 5000 together with a load-bearing arm, an electromechanical drive for loading the packets, clamping device. Only the load-bearing arm and the set of loads of 50 kg each can be mounted without using the mechanisms. This part of the equipment can be certified on standard balances with maximum weighing limits MWL of 50 kg with an error of not more 0.0025%.

50 kg kg, and a dethan

After the certified and mass-adjusted components (with allowance for the local acceleration due to gravity have been installed, the loads of mass 500 kg may be certified by substitution, using the set of 50-kg loads of total mass 500 kg as the standard certified set together with an electrotensometric comparator for mass measurement. For this purpose, the comparator is placed directly on the apparatus and is loaded in turn with the standard set and with the load to be certified of mass 500 kg, and the readings are compared. The limiting error in measuring the mass of a load of 500 kg is defined by ~,~=~ st

+~cq-~m,

t.;O -~' _ ~

(1)

\

2

,Y

t,

rnin

Fig. 1 Translated from Izmeritel'naya Tekhnika, No. i0, pp. 34-35, October, 1984.

926

0543-1972/84/2710- 0926508.50

9 1985 Plenum Publishing Corporation

where 6st is the error in the set, 6 c is the random-error the error in adjusting the package by mass.

limit

of the comparator,

and 6m is

The other packages are certified similarly by substitution, with the previously certified special loads used as standard ones. Comparators of appropriate capacity must be used. The limiting error will increase with the mass of the package even if the metrological characteristics of the comparators are comparable as are the relative errors in adjusting the packages. For example, in certifying a package of 5000 kg the error is ~=~st-~2.5~c~2.5~m,

(2)

i.e., it is substantially larger than ~i). On using (2), one can determine the limiting random error of the comparators corresponding to the required accuracy. If we put 6m = 6 c , we should have the condition ~:,,~,~,=~ st +5~c'-~ 0 .005 % ,

where

6cK0.0005%.

Therefore, to certify a direct-loading system by mass one needs comparators with MWL of 500, i000, 2500, and 5000 kg and a limiting random error of not more than 0.0005%. Up to the present, it has not been possible to attain such an error by the usual methods. We have used a weighing method based on a preloaded elastic element to construct comp a r a t o r s w i t h the required accuracy level, which was developed under a program for building standard comparators for certifying large-mass weights. Prolonged preliminary loading of the elastic element with a constant force in a few days produces a state characterized by a virtually constant drift rate. The preliminary loading level is decisive. Figure 1 shows the changes in characteristics usually called the elastic aftereffect E as a function of the preliminary loading level P for 0, 30, 60, and 95% of the basic load (curves i, 2, 3, and 4 correspondingly) with a measurement cycle length t. With a preloading level of about 95%, the aftereffect becomes unimportant, and the length of the measurement cycle has virtually no effect on the result. At the same time (Fig. 2), the variation S in the results from repeated measurements decreases considerably, down to 10-4% for preloading levels up to 80100%. We have simulated the certification of a load of mass 500 kg directly on the apparatus, with the use of a primary capacitative transducer as the standard facility taken from a mass comparator for 500 kg. The preliminary loading was provided by clamping the equipment. At equal time intervals, the load-bearing arm was fitted in turn with the set of loads of total mass 500 kg and with a load of 500 kg. The two series of measurements were processed by standard methods with allowance for the drift in the comparator and the time between the successive measurements. Figure 3 shows the results (the ordinate is the comparator drift). The observed discrepancy &M between the mass of the single load and the set was about 2 g with a standard deviation of 0.i g. After correcting the mass of the package at time tl, the results virtually coincided, as is evident from Fig. 3.

2,a

\ . . . . . . .

N~

~Y

a,f

1

t

/o

.;o

I_ yo

Fig.

I

: 7~

YO G ~

2

927

D,g

t2

-

2r~

|

~

,

x

#, rain Fig, 3 It is therefore possible to certify a direct-loading system in the OSM-200-10 machine without completely demounting it for certification by using the substitution method and an electrotensometric comparator as primary transducer in a state of preloading. Similar results are provided by virtually any capacitance dynamometer, including one based on Tokar system elastic elements preloaded by clamping to 90-95% of the load to be measured, with this load maintained for two days. It is also possible to use electrodynamometers of other types if their resolving power and short-term stability are adequate. This enables one to certify direct-loading equipment with existing means of measurement with minimum cost where necessary.

CALCULATIONS ON A VIBROCONTACT MEASUREMENT DEVICE UDC 531.781.087

I. T. Smykov

The accuracy in linear or angular measurements is largely determined by the magnitude and stability of the measurement force. A p~rticular problem arises over accuracy in measuring readily deformed components such as wheels for clocks and instrumental mechanisms, corrugated membranes, silicon wafers, etc. One can improve the accuracy considerably by reducing the measurement force and stabilizing it as far as possible by the use of vibrocontact measurement devices VMD [i]. In developing VMD based on automatic force control, it is necessary to estimate the measurement force as well as the static and dynamic characteristics and the maximum error of measurement. We consider the calculation method in application to a quasistatic VMD. Figure 1 shows the structural diagram for such a VMD. The input to the VMD is the parameter to be measured l, which is dependent on time t, i.e., I = f(t). The sensing element SE is a device that responds to the measurement force at the time of contact between the tip and the surface (when electrical contact occurs or in some other

Fig. i Translated from Izmeritel'naya Tekhnika,

928

0543-1972/84/2710-0928508.50

No. i0, pp. 38-39, October,

1984.

9 1985 Plenum Publishing Corporation