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May 27 2019

What is a Electronic Potentiometer? Application and Selection

Catalog

Article Core

Electronic Potentiometer

Introduction

Basic Structure

Thermocouple

Measuring Bridge

Amplifier

Reversible Motor

Indicating Recording Mechanism

Adjusting Mechanism

Power Supply

Working Principle

Type

Interference Source

Selection

Requirements for Measuring Circuits

Fault Analysis

Ⅰ Common faults and Calibration

In Measuring and Controlling Temperature

Inspection

(1) Outer inspection

(2) Power Preheating

(3) Insulation Resistor Verification

(4) Inspect Line Connections

(5) Verification Point Selection

(6) Basic Error Instrument Indication and Record Verification

(7) Verification of Return Error

(8) Verification of the Instrument Step Response Time

(9) Verification of Potentiometer Set Points Error

(10) Calibration of Potentiometer Switching Difference


Introduction

This video will show how to control a Digital Potentiometer using a microcontroller. This technique can be used to build a digital volume control, opamp gain adjust, voltage regulator control, comparator threshold setting or any application where a variable resistance is controlled by a microcontroller.

The electronic potentiometer is not only an automatic balance display instrument that based on the compensation principle to measure, but also an automatic instrument that continuously displays and records the changes of the measured devices parameters. The electronic potentiometer has a high temperature measurement accuracy, and is generally 0.5. Since the electronic potentiometer is equipped with different transmitters, it can be used to measure different parameters of different devices, so it has a wide range of applications in the industry.


Basic Structure

Schematic Diagram of Electronic Potentiometer

Schematic Diagram of Electronic Potentiometer

The electronic potentiometer is mainly composed of thermocouple, measuring bridge, amplifier, reversible motor, indicating recording mechanism and adjusting mechanism, power supply, etc. They have different functions:

(1) Thermocouple: Measuring the temperature in the circuit, generating a thermoelectric potential

(2) Measuring bridge: The measuring bridge in the electronic potentiometer is used to generate a DC voltage that is balanced with the thermoelectric potential generated by the thermocouple, therefore, it plays a major role in the meter. It consists resistors and regulated power supply of the bridge arm.

(3) Amplifier: The amplifier in the electronic potentiometer is actually equivalent to a zero-point instrument, which is used for amplifying the difference signal between the thermoelectric potential generated by the thermocouple and the potential of the bridge output, and driving the actuator (reversible motor) at a certain ratio.

(4) Reversible motor: It acts as an actuator in the electronic potentiometer, drives the sliding contact to achieve automatic balance of the measuring bridge, and can drive the pointer and the stylus.

(5) Indicating recording mechanism: The indicating recording mechanism in the electronic potentiometer can automatically record the temperature measured by the meter. 

(6) Adjusting mechanism: The adjustment mechanism in the electronic potentiometer can automatically adjust the temperature to a given value and perform automatic temperature control. And the old electronic potentiometer also has a working current adjustment mechanism for standardizing the bridge operating current.

(7) Power supply: Energizing the circuit.


Working Principle

The basic working principle of the electronic potentiometer is as follows:

The electronic potentiometer matched with the thermocouple, according to the measurement principle of the thermocouple, the magnitude of the thermoelectric potential depends on the temperature difference between the thermoelectric end and the reference end. Therefore, when the temperature of the thermoelectric end is a constant value, the measured potential is due to the temperature at the reference end varies with the ambient temperature of the meter, so the cold end temperature must be automatically compensated. The inside of the cold end temperature compensator is an unbalanced bridge, and the three bridge arms of the bridge are wound by a manganese-copper wire with a small resistance temperature coefficient, to keep the  constantresistance wouldn’t change with temperature. In addition, the three bridge arm resistance values are all 1Ω, and the other bridge arm is wound by a copper wire with a large resistance temperature coefficient, and its Rx is 1Ω at 0°C. At this time, the bridge is balanced and there is no voltage output. If the temperature of the thermo-end is constant and the temperature of the reference terminal rises, the value of the thermoelectric potential decreases, and the resistance of the resistor also increases due to the temperature rise, and the voltage on the resistor will also increase, therefore, the reduced thermoelectric potential is compensated to achieve automatic compensation of the cold end temperature.

1. The DC potential input by the thermocouple is compared with the potential in the measuring bridge. The compared difference voltage is amplified by the electronic amplifier and output, and the value is large enough to drive the reversible motor, so that the reversible motor drives the sliding contact with the sliding wire resistor, which changes the resistance of the sliding wire resistor, balancing the potential of the measuring bridge with the thermoelectric potential generated by the thermocouple.  

2. When the measured temperature changes cause the thermocouple to generate a new thermoelectric potential, the bridge has a new unbalanced voltage output, and being amplified by the amplifier, the reversible motor is driven to rotate, and the position of the sliding arm is changed again until a new balance is reached.

3. While the sliding arm is moving, the pointer and the stylus connected thereto move along the ruler and the recording paper having the temperature index. Each balance position of the sliding arm corresponds to a certain temperature ruler index and a certain coordinate value on the recording paper, so that the corresponding temperature can be automatically indicated and recorded. When the temperature reaches a given value, automatic control of the temperature can also be achieved by an additional adjustment mechanism.


Type

Electronic potentiometers have different types according to different basis:

1. According to the time of production: it can be divided into ew series, xw series and er series, eh series and intelligent pr series.

2. According to the different attachment devices: it can be divided into display electronic potentiometer, adjustment electronic potentiometer, alarm electronic potentiometer.

3. According to the size: it can be divided into: large electronic potentiometer, medium-sized electronic potentiometer, small electronic potentiometer.

4. According to the pointer and recording method: it can be divided into single-needle single-pen type large electronic potentiometer, double-needle double-pen type large-scale electronic potentiometer, multi-point indicating type large electronic potentiometer, multi-point recording type large-scale electronic potentiometer, etc.

Interference Source

The interference of the electronic potentiometer comes from the inside and outside of the meter. 

1. The internal interference is mainly caused by vibration converter, input transformer, power transformer and other components in the electronic amplifier.  

2. The external interference is mainly the use of electrical furnaces such as electric resistance furnaces and induction furnaces in industrial production, and the thermocouple used as instrument transmitters are in close proximity to these devices which can generate strong electromagnetic field.

3.In addition, large transformers, AC contactors, and motors are sometimes installed near the instrument, and the alternating magnetic fields generated by them are transmitted to the electronic amplifier through various means to affect the electric potentiometer. And when install the instrument, having strict requirements for the wiring of the AC power supply and the wiring of the DC signal and the installation position and installation method of the thermocouple, because inappropriate installing ways will cause interference the same.

Selection

Pay attention to the following principle

1. Consider some important parameters such as the measurement range, accuracy, power supply and quality of the electronic potentiometer.

2. For different working needs, it is necessary to choose an electronic potentiometer with different standard.

3. When selecting, it must take care of the performance electronic potentiometer based on different working conditions.

Requirements for Measuring Circuits

1. High degree accuracy.

2. Constant current flows through the electric bridge.

3. The influence of the cold end temperature change on the potential generated by the coupling must be automatically removed.

4. Detailed calculation of requiring component parameters of the measurement system is needed.


Fault Analysis

 Common faults and Calibration

Indicating failure

1. Indicating value: over-error

Turn on the power, reverse the signal line, and input the temperature signal value to the electronic potentiometer, at this time, the pointer indicates the zero position. If not, open the front cover and adjust the “zero” knob until the pointer indicates zero position. The signal line is connected, input the full-scale power value, the pointer indicates the full-scale scale, If not, adjust the “full-scale” knob until the pointer indicates the full scale. The adjustment should be repeated until the zero position and the full position are get correctly. If there is still a phenomenon of over-error value, there are the following performances:

(1) Slightly nonlinear error.

reasons:

 If the resistor of the sliding line in the instrument is corroded and the resistance value is changed nonlinear, it shall be replaced.

 If the amplifier circuit fails, the amplifier can be replaced.

(2) The large error is nonlinear, and the difference is tens of degrees to several hundred degrees. Because the amplifier circuit has failed and replace it.

(3) The error varies linearly. Component failures in the amplifier circuit, such as resistors, amplifier blocks, etc.

(4) The damping is too large or too small. If the damping is too large, the pointer indicating reaction is slow, and it is not easy to reach the corresponding indication value; if it is too small, the pointer indicating reaction speed is too fast, and it is easy to cause the pointer to rotate beyond the indicated value.

2. The indicating needle is unstable and swinging left and right.

reasons:

(1) Poor contact of zero position or full-degree potentiometer.

(2) Poor contact of the amplifier socket and use of anhydrous alcohol for cleaning.

(3) Virtual welding of components on circuit board.

(4) Damping is too small.

3. After the potentiometer is powered on, the power indicator lights up and the running sound is normal, but the pointer does not indicate the temperature.

(1) This is generally caused by damage to the filtering capacitor on the power supply board.

(2) The instrument has more dust, lack of oil or excessive damping, which causes the pointer to be unable to rotate. The instrument should be cleaned and oiled.

(3) After the above is excluded, if it still does not rotate, the servo motor may be generally damaged.

b. Recording malfunction

1. Indicated value of instrument record is out of tolerance. Adjust the corresponding position and adjusting nut of mechanical actuator above the recording pen.

2. Record the non-paper. Generally, the asynchronous motor is damaged.

c. Controlling faults

1. Instrument control does not work. The failure of the fretting switch in the instrument results in the failure in the instrument.

2. The recording table shows that the ripples are large. Because the instrument control has over-error, slightly adjusting the gap between the fretting switch and the contact.

 Fault Analysis of Electronic Potentiometer in Measuring and Controlling Temperature

a. Possible malfunction of the thermocouple line

The thermocouple is used as a transmitter of the electronic potentiometer. The thermal end position is measured at the temperature, and the temperature of the measured point is transmitted to the two ends of the meter measuring bridge in the form of thermoelectric potential. And the magnitude of the potential generated by the thermocouple depends on the temperature difference between the cold and hot ends.

The fault caused by the thermocouple has a lot to do with its installation conditions and is also related to its construction. In general, a thermocouple with a camber is more prone to short-circuit than a thermocouple without it. Thermocouples used in salt bathing furnaces are more susceptible to short circuits than thermocouples used in air furnaces. The most prone to failure is the thermocouple used to measure metal liquids (such as copper, aluminum, and steel, etc.).

1. Short circuit

between two pole of thermocouple

in thermocouple compensation line

when electrode passes through protective sleeve

2. Poor contact

There exists poor contact of compensation wire in the thermocouple connector or the end of the meter, and the pointer points to the maximum or swings up and down.

3. Open circuit

Most situations of open circuit are the electrode itself, because the electrode is damaged by mechanical impact or long-term heating, and the temperature pointed by the pointer will change greatly with time, and this change is made by leaps.

b. Measuring system failure of electric potentiometer

The measuring system of the meter is a current-standardized bridge. Normally, it is applied to the amplifier that can be balanced potential or be a zero potential.

The basic reason of the pointer pointing to the maximum or minimum is that the input stage of the power amplifier has an unbalanced potential, that is, there is a current flowing in the amplifier, and the unbalanced potential is large enough that the balance may be caused by the failure of the measuring system.

c. Fault of electronic balanced bridge measuring system

1. Pointer points the terminal

The cause of the fault may be that R1 is in open-circuit, the RL of the upper branch is poorly connected, and the external line or Rt is grounding.

2. Pointer points the beginning

The cause of the fault may be a short circuit on R1 or other lines. In special cases, it is also possible that the line additional resistor RL of the lower branch and the resistors Rz and R2 are open.

3. The pointer moves slowly

The measuring system power supply is not connected or faulty, the amplifier input terminal is in poor contact, and the sliding line resistor RH is poorly contacted. If the balanced bridge does not work or requires system check, refer to the relevant model manual of the electronic potentiometer for preliminary inspection and repair.


Inspection

(1) Outer inspection

a. The instrument display glass should not have defects that affect the reading.

b. The inside of the instrument should be clean and tidy, the parts should be complete, and the installation should be correct and firm.

c. The indicating scale or brand name of the instrument shall have the license marking and number of the manufacturing measuring instrument, and the accuracy level, unit of measurement symbol and graduation number of the instrument shall be indicated.

d. The instrument shall mark the manufacturer's name or trademark, model number, specification, factory number, and date of manufacture.

e. The text, numbers and symbols of the instrument shall be clear, clear, and shall not be contaminated or defective.

(2) Power preheating: The preheating time of potentiometer with automatic reference temperature compensation is 30 min, and the ambient temperature change within 30min during the verification period is required to be no more than 0.5°C.

(3) Verification of the insulation resistor of the instrument: When the ambient temperature is 15 °C ~ 35 °C, the relative humidity is 45 % ~ 75 %, in the case of cutting off the power, the power switch is set to turn on, using the insulation resistor meter to measure the insulation resistance of the instrument input and output between the power supply and ground (shell) terminals should be no less than 20 , in addition, the measuring time should be kept 5 s before reading.

(4) Inspect line connections

Input Signal Reverse Connection (0 point verification connection diagram)

Input Signal Reverse Connection (0 point verification connection diagram)

 

Verification Connection Diagram


Verification Connection Diagram

(5) The selection of the verification point: the verification point should include not less than 5 points including the upper and lower limits, and should be marked on the scale with the digital mark, that is, the main scale.

(6) The verification of the basic error instrument indication and record: when detecting, the indication value starts from the lower limit value, and the pointer is slowly stopped at the check mark of each check point until the upper limit value is read by the method of increasing the input signal, respectively. The standard device indicates the value; then, the input signal is reduced to perform the next verification in sequence until the lower limit value (the lower limit value is not detected during the upper stroke, and the upper limit value is not detected during the lower stroke). And this performs a loop check.

Error (accepted value) = (upper limit - lower limit) × accuracy level 

Error = measured value - rated value

(7) Verification of return error: The instrument has included the check of the return difference value in the verification process of the basic error, and takes the maximum value when it is repeatedly measured.

Return difference (accepted value ) = | allowable error | × 80

Return difference = | upper stroke - lower stroke |

(8) Verification of the instrument step response time (stroke time): Input the signal so that the meter pointer is at the initial position of the scale 10% (upper stroke) and the scale 90% (down stroke). Then, the step response time is increased (upper stroke) and the step response time is reduced (downstroke) with a step signal of 80% of the input range, and the stopwatch is started. When the meter pointer reaches a stable value (which is allowed to be 1% of the range), the stopwatch is stopped, and the interval time is full stroke time. The average of three measurements taken in each of the upper and lower strokes is taken as the step response time (stroke time) in each direction.

(9) Verification of potentiometer set points error: The test should be performed at set points around 10%, 50%, and 90% of the measurement range. The set points should be adjusted on the digital mark of the scale; and the verification is performed in the recording state; the check is to input the set standard power signal under the premise of less than the set point power signal, and the value indicated by the reader is upper switching value, while input the set point standard power signal under the premise of the set point power signal, that is, lower switching value.

(10) Calibration of potentiometer switching difference: it is performed simultaneously with the verification of the set points error.

Switching value = | upper switching value - lower switching value |

The processing of the verification results: the qualified instruments are verified according to the requirements of the regulations, the verification certificate is issued, while the unqualified instruments are verified, and the verification result notice is issued.

Verification cycle: The calibration period of the instrument can be determined according to the environmental conditions, applying frequency and importance, generally no more than 1 year.


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