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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Remote measurement of electrical resistance. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The author proposes a method for measuring the resistance of a variable resistor, thermistor or a sensor of any physical quantity, the output parameter of which is electrical resistance. The distance between the measurement object and the device can reach several hundred meters, and only two wires are enough to connect them. Sometimes it becomes necessary to measure the electrical resistance of an object located at a considerable distance. For example, if you put a pulley on the axis of a variable resistor and throw a cable through it with a float fixed at one end and a load at the other, you can determine the water level in a tank or in a reservoir. Similarly, you can control the degree of opening of windows, air dampers, doors. Numerous commercial instruments exist for remote resistance measurement. But in some cases, their use turns out to be too expensive, and, most importantly, they do not have anti-vandal protection, and controlled objects are often located in places rarely visited by service personnel. I would like to connect a small and cheap sensor to a pair of wires going to a measuring device located one or two kilometers away. Connection schemes that require a larger number of wires are not considered, because free wires are always in short supply in existing communication and control cables. And the common four-wire resistance measurement circuit on such extended communication lines, for a number of reasons, does not provide the required accuracy. I propose a method for remote measurement of resistance, requiring only a two-wire communication line, and the resistance of the wires does not introduce errors into the measurement result. The measurement principle is illustrated in fig. 1, where Rx - measured resistance; Rn - resistance of the wires of the communication line; GI1 - current source. When the switch SA1 is in the upper position according to the circuit, the source current flows through the communication line, diode VD1 and the measured resistance. Voltmeter PV1 shows voltage U1=UVD1+I (Rn+Rx), where UVD1 - direct voltage drop across the diode VD1. After switching the switch SA1 to the lower position, the current will flow through the communication line and the diode VD2, and the voltmeter PV1 will show the voltage U2=UVD2+I Rn, where UVD2 - direct voltage drop across the diode VD2. If the diodes VD1 and VD2 are identical, then UVD1=UVD2 и Rx=(U1-U2)/I.
On fig. 2 shows a diagram of the implementation of this measurement method. A current stabilizer is assembled on the transistor VT1. On the DD1 chip - a multivibrator that controls the operation of the switch on electronic keys DD2 and DD3. During the presence of a high logic level voltage at pin 10 DD1, the current from the stabilizer will pass through the closed key DD2.1, the first wire of the connecting line, the diode VD1, the measured resistance Rx, the second wire of the connecting line and a closed key DD2.4 to a common wire. The voltage drop on this circuit will be applied through the closed key DD3.1 to the capacitor C6 and charge it to voltage A.
In the next half-cycle of oscillations of the multivibrator, the current will pass through the closed key DD2.3, the second wire of the connecting line, the diode VD2, the first wire of the connecting line and the closed key DD2.2 to a common wire. The voltage drop on this circuit through the closed key DD3.2 will charge the capacitor C7 to voltage U2. The circuits R4C5VD3 and R5C4VD4 delay the closing moments of the keys DD3.1 and DD3.2 for the time required for the attenuation of transients in the communication line. The high-resistance voltmeter PV1 measures the proportional Rx voltage difference across the capacitors. If you set the output current of the stabilizer to 1 mA, then the readings of the voltmeter in volts will be numerically equal to the measured resistance in kiloohms. In real conditions, a communication line can pass through telephone and signal cables with different electrical parameters. The amplitude of transient processes in them can reach 3 V (actually measured value). These processes are especially noticeable if the measured resistance has a significant inductive component. For example, if it is a relay coil used as a temperature sensor. In some cases, transient processes are quite long. To eliminate their influence, it is necessary to increase the oscillation period of the multivibrator and the time constants of the delay circuits. As a communication line, it is recommended to choose a twisted pair of wires with minimal current leakage. It should not be not only between the wires of the pair, but also between them and other wires of the cable used. If we take into account that at the moment of making a call to the subscriber, the voltage in the telephone line exceeds 120 V, then it is clear that even a small leak can create severe interference and even damage the resistance measuring device. Setting up the meter basically comes down to adjusting the current stabilizer. To do this, break the wire connecting the current stabilizer with electronic keys in the place marked on the diagram with a cross, and turn on the milliammeter between points A and B. Set the required current (for example, 1 mA) by selecting the resistor R3. If this is not done, then you can accidentally exceed the current allowed for the keys of the K561KT3 chip. The microcircuit after an overload may even continue to work, but the measurement results will become strange. Then, having restored the connection of the current stabilizer with the keys, connect a resistor of precisely known resistance to the device as Rx and finally select the resistor R3 according to the readings of the voltmeter PV1. Now about the components of the error of the method under consideration. The first is a different voltage drop across the diodes VD1 and VD2. This component of the error is clearly noticeable when measuring a resistance of 200 ohms and increases with its decrease. To lower it, you need to select diodes with the same voltage drop at a given measurement current and try to provide them with the same temperature conditions. The second component of the error is associated with the low quality of current stabilization. It manifests itself at large values of the measured resistance. To reduce it, you should choose as VT1 a field-effect transistor with the lowest possible threshold voltage and the highest possible steepness of the characteristic. If increased measurement accuracy is required, then a current stabilizer on an operational amplifier should be used. The third component of the error is related to the variation in the resistance of the closed keys of the K561KT3 microcircuit, which can reach ± 5 ohms. If you need to remove this error, close the terminals of the diode VD2 to each other and pay attention to the readings of the voltmeter PV1. If it shows a positive voltage, then turn on the equalizing resistor in series with the DD2.2 or DD2.3 key and select it so that the readings become zero. If the voltmeter shows a negative value, then the equalizing resistor must be connected in series with the key DD2.1 or DD2.4. On fig. Figure 3 shows a diagram of the implementation of the considered method for remote measurement of resistance using a microcontroller, which can be any one with a built-in ADC. Unlike the diagram in Fig. 2, to simplify switching, two current stabilizers are used here, which should be identical. AN0 is the ADC input of a microcontroller not shown in the diagram (it can be, for example, PIC16F8T3A), RA1 and RA2 are its general-purpose discrete I/O lines. The microcontroller is powered by 5 V.
In the first measurement cycle, the microcontroller program configures the RA2 line as an output, and the RA1 line as an input with a large input resistance. At the output of RA2, it sets a low logic level. As a result, the stabilizer current on the transistor VT1 flows through the communication line through the diode VD1 and the measured resistance Rx, and then flows into the common wire through the low-resistance output RA2. After a pause necessary to complete the transients, the ADC of the microcontroller measures the voltage U1. In the second cycle, the functions of the lines RA1 and RA2 mutually change. As a result, the stabilizer current on the transistor VT2 flows through the communication line through the diode VD2 and goes into the common wire through the low-resistance output RA1. ADC measures voltage U2. Then the program finds the difference U1-U2, computes Rx, after which the process is repeated. The current of one of the stabilizers (for example, on the transistor VT1) is set by selecting the resistor R1 according to the previously described method. Then, a 1 kΩ variable resistor is included in series with a break in any wire of the communication line, and as Rx connect a resistor of known resistance. By selecting the resistor R2, the minimum influence of the variable resistor (in the entire range of its resistance change) on the measurement result is achieved. Zener diodes VD3, VD4 protect the inputs of the microcontroller in the event of an open in the measuring circuit. Diodes VD5, VD6 decouple voltage measurement circuits U1 and U2. The lower limit of the measured resistance in both considered cases is practically zero. The upper limit for a device assembled according to the scheme shown in fig. 2, at a current of 1 mA - about 7 kOhm. With a further increase in the measured resistance as a result of a violation of current stabilization, the error sharply increases. For the circuit shown in fig. 3, the maximum voltage drop across Rx is equal to the allowable input voltage of the ADC (5V). Therefore, at a current of 1 mA, resistance of no more than 5 kΩ can be measured. It should be noted that the considered method allows you to measure the difference between two resistances, one of which is connected in series with the VD1 diode, and the second with the VD2 diode. This is convenient, for example, when using a thermistor as a temperature sensor, the resistance of which at a temperature of 0 оC is not equal to zero. If you turn on the thermistor as Rx (in series with the VD1 diode), and turn on a compensating resistor in series with the VD2 diode, the resistance of which is equal to the resistance of the thermistor at zero temperature, then the instrument readings will be positive at a temperature above zero and negative if it is below zero. In a practically implemented device, the measured resistance and diodes VD1, VD2 were located at a distance of about 700 m from the meter. For their connection, a free twisted pair of telephone cable wires was used. The readings of the instrument were unstable until a measurement delay was introduced for the duration of the transients. Practice has shown that if there is no urgent need for a high measurement speed, then it is better to make the switching frequency of the measuring current lower. Author: L. Elizarov
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