RU2173858C1 - Device for measuring small resistance values - Google Patents

Abstract

FIELD: electric engineering. SUBSTANCE: device has stabilized current source, analog-to-digital converter, current direction switch unit, digital display and two potential clamps for attaching the resistance member under measurement. High precision analog-to-digital converter, controller and by-pass are additionally introduced into the design. EFFECT: high accuracy of measurements. 5 dwg

Description

 The invention relates to electrical engineering and can be used for postoperative quality control of electric contact welding, quality control of collapsible electrical contacts in multi-ampere current conductors and in other cases when measurement of small resistance values is required.

 The well-known "Small resistance meter" (See AS N 1368810, class G 01 R 27/02, 1988), containing two sources of reference voltages of opposite polarity, input analog switches, current stabilizer, DC amplifier, output analog keys, two analog memory cells made on storage capacitors, an operational amplifier that acts as a matching unit, an indicator and a control unit, two current and two potential clamps connected to the output of the current stabilizer and the input of the constant voltage amplifier Iya, respectively.

 The disadvantages of this low resistance meter include the fact that it does not provide strict identity in magnitude of both sources 1 of the reference voltage in time and when changing external conditions. In addition, it also does not provide a strict identity of the two output channels of the current stabilizer, forming sequentially DC pulses of the same amplitude, but of opposite polarity. All this leads to a complication of the device as a whole, and, accordingly, to a decrease in the measurement accuracy.

 Of the similar technical solutions known from the prior art, the "Low resistance meter" according to the RF patent N 2099722, class. G 01 R 27/02, published in 1997

 The known low resistance meter contains a constant voltage amplifier, a digital indicator, current and potential clamps for connecting the measured resistance, a control unit and a series-connected reference voltage source, an analog switch, a current stabilizer, and also contains an analog switch (current direction switch) and analog connected in series -digital converter (ADC), register storage device, digital subtractor.

 The known low resistance meter has the following disadvantages.

 - There is no control of the current flowing through the measured resistance. The value of this current is considered constant, stable in time and equal to a predetermined value. Under the influence of various factors, such as the aging of the elements of the device, the presence of interference, changes in the ambient temperature and others, the current value may change, which will lead to a decrease in the measurement accuracy.

- The use of a constant voltage amplifier at the input of the ADC with a large gain (K _у = 100000 ... 1000000) and, therefore, a narrow dynamic range of the input signal can lead to the fact that during measurement this amplifier due to thermoelectric effects will become saturated, which will lead to a decrease in measurement accuracy.

 - The measurement algorithm adopted in the prototype compensates for errors caused by thermoelectric effects, but does not compensate for random measurement errors caused by various noise and noise, which also reduces the accuracy of measurements.

 Thus, the known device cannot provide high measurement accuracy.

 The objective of the proposed technical solution is to improve the accuracy of measurements of low resistances.

 The problem is solved in that a low resistance meter containing a stabilized current source, an analog-to-digital converter, a current direction switch, a digital indicator, as well as two current and two potential clamps for connecting the measured resistance, and the outputs of the stabilized current source are connected to power inputs a current direction switch, a precision analog-to-digital converter, a controller and a shunt are introduced into it, and the controller outputs are connected respectively to digital indicator and to the lock and control inputs of the current direction switch, one power output of which is connected directly to the current clamp for connecting the measured resistance, and the other power output is connected to the second current clamp through a shunt connected simultaneously to the analog inputs of the analog-to-digital converter, and the analog the inputs of a precision analog-to-digital converter are connected to potential terminals for connecting the measured resistance, while the interface group of inputs the I / O of the analog-to-digital converter and the interface group of inputs / outputs of the precision analog-to-digital converter are connected to the interface group of inputs / outputs of the controller.

 Distinctive features of the proposed technical solution are the precision analog-to-digital converter, controller and shunt, as well as new connections between the elements.

The set of essential features allowed in the proposed device:
- to continuously monitor the current flowing through the measured resistor by measuring the voltage drop across the model resistor R _W ;
- measure the voltage drop across the measured resistor R _{x to} produce a precision ADC with a wide dynamic range;
- after the end of the measurement cycle, perform statistical processing of the results stored in the controller memory.

 Reducing the effect of interference is possible with the statistical processing of multiple measurements. Statistical processing of measurement results is a universal tool that allows you to reduce the influence of interference and noise, regardless of the point of application (See E. I. Gitis, E. A. Piskulov. Analog-to-digital converters. Textbook for universities. M. Energoizdat. 1981. Page 72).

 Thus, the proposed meter circuit provides high accuracy for measuring low resistances, while the meter uses the measurement algorithm adopted in the prototype, according to which the measured value is determined as the half-difference of two sequentially performed samples (measurement results): the 1st sample is performed at one direction of working current; 2nd — in the opposite direction. At the same time, the final measurement result is not affected by errors caused by the presence of noise in the form of a constant voltage (thermopower, ADC zero offset, etc.), and errors caused by the presence of noise in the form of an alternating voltage (random interference and noise as in the input) are also significantly reduced signal, and inside the ADC).

 As a result of this, we can conclude that the proposed meter of low resistances allows to obtain a technical result, which consists in increasing the accuracy of measurements of low resistances, and meets the criterion of "novelty."

 The use of a low resistance meter in electrical engineering to control the quality of electrical welding or to control the quality of collapsible electrical contacts in multi-ampere current conductors ensures it meets the criterion of "industrial applicability".

The proposed low resistance meter is illustrated by drawings, where:
- in FIG. 1 presents a block diagram of the inventive low resistance meter;
- in FIG. 2 shows an example implementation of a current direction switch;
- in FIG. 3 is a flowchart illustrating the operation of a low resistance meter.

The low resistance meter (Fig. 1) contains a stabilized current source 1, a current direction switch 2, an analog-to-digital converter (ADC) 3, a precision analog-to-digital converter 4, a controller 5, a digital indicator 6, a shunt (R _w ) 7, two current 8 and 9 and two potential 10 and 11 terminals for connecting the measured resistance (R _x ) 12.

The outputs of the stabilized current source 1 are connected to the power inputs of the current direction switch 2, the control inputs of which are connected to the controller 5. One power output of the current direction switch 2 is connected directly to the current terminal 8 for connecting the measured resistance (R _x ) 12. Another power output of the switch 2 the current direction is connected to the second current output terminal 9 via the shunt (R _w) 7 which is also connected to the analog inputs of analog-to-digital converter 3. The analogue inputs precision analog-to-digital of the converter 4 are connected to potential terminals 10 and 11. An I / O interface group of the analog-to-digital converter 3 and an input / output interface group of a precision analog-to-digital converter 4 are connected to an input / output interface group of the controller 5, the corresponding output of which is connected to a digital indicator 6.

 As a stabilized current source, it is possible to use a stabilizing power supply unit ЕК100.18.00, manufactured by Power Electronics, 000 in Moscow (See stabilizing power supply unit ЕК100.18.00. Passport. Appendix 1).

 The current direction switch can be made on the basis of two half-bridge modules with optical isolation 5P64P-50-0,6-D22 manufactured by CJSC PROTON-IMPULSE, Oryol (See Half-bridge module with optical isolation 5P64P-50-0,6-D22 Label. CJSC "PROTON". Appendix 2).

 As analog-to-digital converters (ADC and precision ADC), it is possible to use the two-channel version of the precision voltmeter board for direct current signals LA-I24 manufactured by Rudnev-Shilyaev CJSC in Moscow. This board, which contains two channels of a 24-bit ADC and is designed for high-precision measurements, is designed for use as part of digital computers (See Boards for collecting and controlling the input and output of analog and digital information. Product Catalog of the Center for ADC of Rudnev-Shilyaev CJSC 1999 Moscow. Page 38. Appendix 3).

 The controller that controls the operation of all other nodes can consist, for example, of a PCA-6135 processor board and a PCL-724 discrete input / output board, connected using a PCA-6104 cross-board. All boards manufactured by Advantech (Taiwan) (See Everything necessary for PC-based automation. Advantech catalog. Russian edition. Volume 91. 1999 p. 4-19, 10-32, 4-18).

 As a digital indicator, a vacuum-fluorescent sign-synthesizing display model 03602-105-05220 of the Century series of IEE firm (USA) can be used (See Modern Automation Technologies N 1, 1998. STA-PRESS Publishing House. Moscow. Index for catalog " Rospechat "- 72419. Pages 114-120).

 As an exemplary resistor Rш, you can use, for example, a calibrated shunt 75ШСМ3-10-0.5 (See the Handbook of Electrical Measuring Instruments. Edited by KK Ilyunin. L. Energy. 1973. P. 660).

The low resistance meter works as follows. In the initial state, the current direction switch 2 is in the open state when its outputs are blocked. No current flows through the measured resistance R _x 12. The digital indicator 6 displays the value of the previous measurement or the symbol of readiness for work.

The control input of the switch 2 of the current direction from the output of the controller 5 receives a signal that sets the switch 2 in one of the possible positions: forward direction of current or reverse direction of current. Then, from the output of the controller 5, a signal is released that unlocks the outputs of the current direction switch 2. Through the model resistance R _W 7 and the measured resistance R _x 12 begins to flow current from the source 1 of the stabilized current. According to the commands of controller 5, the ADC 3 measures the voltage drop across the model resistor R _w 7, and the precision ADC 4 measures the voltage drop across the measured resistance R _x 12. After being converted to a digital code, the measurement results are recorded in the memory of controller 5. These measurements are made repeatedly for the purpose of subsequent statistical processing of results (See Novitsky P.V., Zograf I.A. Evaluation of errors of measurement results. L. Energoatomizdat. 1985. Chapter Four. Methods of statistical processing of multiple samples. Page 120). After carrying out a given number of measurements when the current flows in one direction, the controller 5 sets the current direction switch 2 to the open state, blocking its power outputs.

Then, the controller 5 supplies a signal to the control input of the current direction switch 2, which sets the switch 2 to a position that allows the current to flow in the opposite direction, and then unlocks its power outputs. Through the model resistance R _W 7 and the measured resistance R _x 12, current flows from the stabilized current source 1 in the opposite direction. According to the commands of the controller 5, measurements are made similar to measurements in the forward direction of the current. After a given number of measurements, the controller 5 puts the switch 2 of the current direction in the open state, blocking its outputs.

 This completes the measurement cycle, and begins the cycle of statistical processing of the measurement results and the calculation of the resistance value.

At the end of the measurement cycle, data consisting of:
- an array of results of measurements of the voltage drop across the measured resistance R _x in the forward direction of the current;
- an array of the results of measurements of the voltage drop across the measured resistance R _x with the reverse current direction;
- an array of results of measurements of the magnitude of the direct current;
- an array of results of measurements of the magnitude of the reverse current.

 Each of these data sets is subjected to statistical processing in order to find a more accurate, averaged result for each data set (see ibid.). This processing makes it possible to reduce the influence of noise and interference, which are the main component of the random error of the analog-to-digital conversion, the source of which can be located both inside and outside the analog-to-digital converters (See E.I. Gitus, E.A. Piskulov. Analog -digital converters. M. Energoizdat. 1981).

The main problem of precision measurements is the thermoelectric effects arising from the contact potential differences of dissimilar metals at different temperatures (See Integrated Circuits. Operational Amplifiers. Volume I. M. Publishing House Physics and Mathematics Literature VO Nauka. 1993 . Page 77). The result of thermoelectric effects is a zero offset of the analog-to-digital converter. Under real conditions of the measurement, a change in the ambient temperature, and hence the temperature of the contact joints in the measuring circuit, is many times slower than the measurement process itself. Therefore, the ADC zero offset associated with thermoelectric effects for a given measurement can be considered constant. To exclude the influence of this bias on the measurement result, measurements are made twice. First, the voltage drop is measured at the measured resistance when current flows in one direction, and then the voltage drop when current flows in the opposite direction. The measurement result is determined as the modulus of semi-difference of these measurements:
U = | (U1-U2) / 2 | (1)
where U ₁ is the statistically processed average result of measuring the voltage drop across the measured resistance with the passage of current in one (forward) direction,
U ₂ - statistically processed average result of measuring the voltage drop across the measured resistance when the current flows in the opposite (reverse) direction.

In turn:
U ₁ = + U _and + U _s , (2)
a
U ₂ = -U _and + U _s , (3)
where U _and is the voltage drop directly at the measured resistance (+ U _and - in the forward direction of the current, -U _and - in the reverse direction of the current),
U _with - ADC zero bias voltage due to thermoelectric effects.

Аналогичным образом вычисляется значение величины тока от источника стабилизированного тока I, пропускаемого через измеряемое сопротивление в процессе измерений:
I = |(I1-I2)/2| (5).
где I₁ - статисчически обработанный усредненный результат измерения тока при прохождении тока в одном (прямом) направлении через измеряемое сопротивление,
I₂ - статистически обработанный усредненный результат измерения тока при прохождении тока в противоположном (обратном) направлении через измеряемое сопротивление.Substituting (2) and (3) in (1), we obtain:

In a similar way, the value of the current value from the stabilized current source I, passed through the measured resistance during the measurement, is calculated:
I = | (I1-I2) / 2 | (5).
where I ₁ - statistically processed average result of measuring the current when the current passes in one (forward) direction through the measured resistance,
I ₂ - statistically processed average result of current measurement when current flows in the opposite (reverse) direction through the measured resistance.

The value of the measured resistance is determined by Ohm's law:
R _x = U / I. (6)
This completes the calculation process, and the result is displayed on a digital indicator.

Tests of the proposed low resistance meter showed that the device provides a measurement of low resistances in the range from 5 mOhm to 0.03 μOhm with a basic error reduced to the measurement point:
- in the sub-range of 5 mOhm - 0.1 μOhm 1%;
- in the sub-range of 0.1 μm - 0.5 μm 3%,
- in the subrange of 0.5 μOhm - 0.05 μOhm 8%;
- in the sub-range of 0.05 μΩ - 0.03 μΩ 21%.

 Thus, the proposed meter provides a measurement of small resistances of the microohm range with higher accuracy.

Claims (1)

 A low resistance meter containing a stabilized current source, an analog-to-digital converter, a current direction switch, a digital indicator, as well as two current and two potential clamps for connecting the measured resistance, and the outputs of the stabilized current source are connected to the power inputs of the current direction switch, characterized in that a precision analog-to-digital converter, controller and shunt are introduced into it, and the controller outputs are connected respectively to a digital indicator to the lock and control inputs of the current direction switch, one power output of which is connected directly to the first current clamp for connecting the measured resistance, and another power output is connected to the second current clamp for connecting the measured resistance through a shunt connected simultaneously to the analog inputs of the analog-to-digital converter , and the analog inputs of a precision analog-to-digital converter are connected to potential terminals for connecting the measured resistance, at this interface group of inputs / outputs of the analog-to-digital converter and the interface group of inputs / outputs of a precision analog-to-digital converter are connected to the interface group of inputs / outputs of the controller.

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