Display of interference analysis and countermeasures in the use of the instrument - News - Global IC Trade Starts Here.

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After the chemical display instrument is matched with various sensors and transmitters, it can be used to display different parameters. However, the conditions of digital pressure gauges used in a wide range of applications in petroleum and chemical industry or in test sites are often complicated, and there are a large number of strong alternating magnetic fields, electric fields, vibrations, thermal noise, strong radiation, temperature effects, and power. Power sources, etc., may affect the correct collection of test data and the automatic control of the production process to become a source of interference. These voltages or currents unrelated to the signal under test are coupled to the detection, control, and display devices in various forms, causing signal acquisition misalignment, recording display distortion, useful signal quality degradation of the measured parameters, automatic control cannot be performed in time, or even operation. Out of control, directly affects normal production, product quality and economic efficiency. Most of these disturbances (disturbances) are difficult to change, but it is necessary to try to effectively suppress them. 1. The principle and composition of digital display instruments. Digital display instruments are generally composed of analog-digital conversion, nonlinear compensation and scale conversion. . It takes an electrical signal as an input and directly measures it with a digital display. The key to digital display is to convert continuously varying analog quantities into intermittent digital quantities through an A/D converter. In production, it is required to display the display value reflected by the meter as a function of the measured parameter, and it is required to automatically compensate other interference factors. Some of these functional relationships are linear, but most of them are nonlinear. In order to display the measured parameters in absolute value, for the display instrument, it is necessary to perform some necessary calculations, processing and nonlinear compensation on the measured parameters, and compensate for the influence of other parameters on the measured parameters. In the A/D conversion, a continuously varying analog quantity is quantized using a certain unit of measure to obtain an approximate intermittent digital quantity. The smaller the unit of measurement, the smaller the error of the quantization, the higher the frequency response of the A/D converter, the stability of the preamplifier, etc., and the closer the digital quantity is to the value of the continuous quantity itself. This process is realized by an A/D converter such as a double integration type, a voltage frequency conversion type, a pulse width modulation type, and a successive comparison voltage feedback coding type. Nonlinear compensation or scale conversion is the necessary calculation of the detected signal so that the digital display instrument can express the direct number of the measured parameter. Analog nonlinear compensation compensates for different ranges of input voltage by changing the amplification factor of the operational amplifier. Digital nonlinear compensation is to convert the analog quantity of the measured parameter into A digital quantity and then enter the nonlinear compensation. The link has the advantages of high precision and strong versatility. With the development of electronic digital computers, the scale conversion and nonlinear compensation tasks are completed by computers. Second, the display of anti-jamming measures in the application of the instrument (a) the generation of interference, the measured parameters are often converted into a weak low-level voltage signal, and transmitted over long distances (sometimes up to hundreds of meters or more) To the display instrument, due to the complexity of the display instrument application environment (a large number of strong alternating magnetic fields, electric fields, vibration, thermal noise, strong radiation, temperature effects, power supply, etc.), electrical interference is also added to the input of the display instrument. In addition, interference sources such as power transformers, relays, switches, and power lines inside the instrument have an impact on the measurement. When there is a large disturbance (the interference of the detection signal mainly has a strong magnetic field and an electric field: when the interference source is a low voltage and a large current, the interference source is mainly a magnetic field; when the interference source is a high voltage and a small current, the interference source is nearby. Mainly electric field), often superimposed on the signal line through some of the following methods (such as series mode interference, common mode interference, etc.) into the meter. 1. Electromagnetic induction (refers to magnetic coupling). In the surrounding space of high-power transformers, AC motors, high-current power grids, etc., there is a strong alternating magnetic field, and the control system (detection, transmission, conversion, adjustment, calculation, execution, auxiliary, display, etc.) lines are formed. In the closed loop, the magnetic field will be induced in this changing magnetic field, so that the connecting wire between the signal source and the instrument and the wiring inside the instrument are magnetically coupled to form interference in the circuit. This electromagnetic induction potential is connected in series with the useful signal. When the signal source is far away from the display instrument, the interference is more prominent. In addition, high-frequency generators, motors with commutators, etc., also generate high-frequency interference. 2. Electrostatic induction (referring to the coupling of electricity). Static induction is the result of the interaction of two electric fields. In the opposite two wires, if the potential of one of them changes, the potential of the other wire also changes due to the change in capacitance between the wires, and the interference source forms a disturbance in the loop by capacitive coupling. 3. Additional thermoelectric potential and chemical potential. DC electrical interference is formed in the circuit loop due to the thermoelectric potential generated by different metals and the chemical potential generated by metal corrosion. 4. Vibration. In a highly vibrating environment, the conductor generates an induced potential due to its motion in the magnetic field. This interference is in series with the signal and enters the instrumentation in the form of series mode interference. 5. Interference introduced by different ground potentials. In the vicinity of high-powered electrical equipment, when the insulation performance of the equipment is poor, the introduction of the potential difference of different ground potentials forms interference, and in the use of the instrument, there are often more than two connection points on the input end. This will introduce the potential difference of different grounding points into the instrumentation in the form of common mode interference, which occurs simultaneously on the two signal lines. 6. The signal source is an unbalanced bridge. When the bridge power supply is grounded, except for the unbalanced voltage (ie, signal voltage) of the diagonal of the bridge, both signal lines have a common common mode interference voltage to ground. Although the common mode interference does not overlap with the signal and does not directly affect the meter, it can form a leakage current to the ground through the measurement system. The coupling of the resistor can directly act on the meter (or amplifier) ​​to generate interference. 7. Some pulse-shaped interference voltage de-energization acts outside the analog circuit, and sometimes it can directly enter the digital circuit to give interference. The sources of these interference voltages are inductive loads such as switches, motors, and relays, and machines that generate discharges. (II) Interference suppression The formation of interference problems is due to the existence of interference sources and influence on instrumentation through certain coupling channels. In order to reduce these effects, the suppression of interference should be considered when designing the instrument, and its ability to resist interference should be maximized. In practical applications, find and combine the methods of twisting, shielding, grounding, balancing, filtering, and isolating to cut off the coupling channel to suppress interference. At the same time, the display instrument is required to have high temperature resistance, low temperature, high pressure, corrosion, high viscosity and other good dynamic characteristics to reduce the measurement error of the measured parameters. 1. The suppression method of series mode interference (serial mode interference is the interference voltage superimposed on the detected signal at the input end of the instrument). The serial mode interference may be generated at the signal source, and it may also be induced or received from the lead. . Since the serial-mode interference is in the same position as the signal under test, once the serial-mode interference is generated, its harmful effects are often not easily eliminated, so it should be prevented from being generated first. (1) Wring of signal line: For electromagnetic induction, it is necessary to keep the wire away from the high-voltage equipment and power network, adjust the direction of the wire and reduce the wire loop area. Only adjust the direction of the wire and the two signal lines. Stranded at a short pitch, the interference voltage can be reduced to the original 1/10~1/100; for electrostatic induction, when the two signal lines are twisted and twisted, the two signal lines are When the distances of the interference sources are approximately equal (the wire is often twisted to a pitch of 20 times the diameter), the area enclosed by the signal loop can be greatly reduced, and the electric field can enter the loop through the inductive coupling on the two signal lines. The mode interference potential difference is greatly reduced. (2) Shielding: In order to further prevent the interference of the electric field, the signal wire may be wrapped with a metal mesh (or metal skin), and then a shielded cable may be directly used on the outer bread or a signal line, and the shielding layer is grounded. Since the non-magnetic shielding layer has no effect on the magnetic field of 50 Hz, the signal wire can be inserted into the iron pipe if necessary, so that the signal wire is magnetically shielded. After the electrostatic shielding, the induced potential can be reduced to the original 1/100~1/1000. (3) Filtering: For DC signals with very slow changing speed, a filtering circuit is added at the input end of the instrument to minimize the interference mixed with the effective signal. It is common to add two to three stages of RC filter circuits before the input stage, and it is better to use a double T type filter with lower internal resistance. (4) Cancellation: Digital instruments such as double integral type and pulse width adjustment type perform A/D conversion on the average value of the input signal instead of the instantaneous value, and can average some serial mode interference. (5) Try to separate the signal line from the power line. Reasonable wiring, under the allowable conditions, the current flow of the wire is reversed to reduce the interference of the magnetic field generated by each other; the signal wire and the power wire are not allowed to be laid together in parallel, and the instrument should not enter the instrument through the same threading hole. Inside. The low-level signal line should be connected to the adjacent position of the signal terminal with the shortest untwisted wire to reduce the area of ​​inductive interference. It is absolutely forbidden to use the same cable for the power line and signal line. Do not use the same wiring plug for the high and low lines. When you have to, leave the high and low lines separately next to the connector, with the ground terminal and the spare terminal separated. 2. Common mode interference suppression (common mode interference is interference between any input of the instrument and the earth) (1) Correct grounding. The meaning of grounding can be understood as obtaining an equipotential point or plane, which is the reference potential of a circuit or system, but not necessarily the ground potential. For safety reasons, the instrumentation and signal source housings are connected to ground and remain at zero potential. However, when the grounding method is not handled well, a ground loop will be formed to introduce interference into the instrumentation. In order to improve the anti-interference ability of the instrument, the amplifier is usually insulated from the instrument case (ground) in the low-level measuring instrument (that is, the amplifier is "floating") to cut off the leakage path of the common mode interference voltage, so that the interference cannot be made. enter. In the low-level test, the signal line should only be grounded at one point and the shield of the signal line must be grounded at the same time. No matter the signal line and instrumentation, etc., it is necessary to shield the grounding and shielding correctly. Solve most of the interference problems. When an ungrounded source is connected to a grounded amplifier, the signal line shield should be connected to the common side of the amplifier. When there is a ground signal source connected to a non-grounded amplifier, even if the signal source is not connected to the ground, the signal line shield should be connected to the common end of the signal source to keep it at zero potential, which can effectively cut off the leakage current of the potential. To improve the anti-interference ability of the measurement signal, which is a commonly used method in the measurement system. (2) The instrument adopts double-layer shielding floating protection technology: in order to improve the anti-common mode interference capability of the instrument, the instrument instrument adopts double-layer shielding floating protection while the input part of the amplifier is floating. In addition to using the case as a shield, an internal shield is used to shield the input section of the amplifier. There is no electrical connection between the two shield layers between the amplifier input section and the inner shield. Do not connect the inner shield to the instrument case. Instead, separate a wire as a protective shield to connect to the shield of the signal cable. This extends the protective shield to the full length of the signal line. The shield of the signal is at the source. Grounded at one point, so that the input protection shield and signal shielding of the instrument are stabilized to the signal source and are in an equipotential state. Therefore, the shield can be used to reduce the common mode voltage coupled to the conductor. (3) Application of balanced circuits: The stability of a system depends on the balance of the signal source, signal leads, load balance, and other spurious distribution parameters. In order to improve the anti-common mode interference capability of the instrument, balance measures are used to equalize the voltages converted on the two lines, thereby reducing the common mode voltage coupled to the load. (4) Suppression of power supply introduction interference: The main interference inside the instrumentation comes from the leakage current generated by the small power transformer. In order to prevent leakage current interference, the primary winding of the transformer can be placed in the shielding layer, and the shielding layer is grounded. At this time, the phase voltage on the primary winding of the transformer passes through the distributed capacitance of the shielding layer, so that the leakage current flows directly into the ground without Interference occurs in the amplifier, measurement circuit, and signal source. In order to prevent the power transformer from introducing interference, a three-layer shielding structure is adopted, that is, the primary shielding layer of the power transformer is directly grounded to the case, the secondary winding of the power supply device is connected to all the shielding layers, and the shielding layer of the secondary winding of the amplifier power supply and the amplifier ground are waiting. Potential state. The pulse-like interference caused by the power supply has a great influence on the digital circuit. A high-frequency filter should be installed on the power supply line. The filter should be installed in an iron shielding box whose input and output leads are filtered by the feedthrough capacitor. . These factors also apply to the #p# page header #e# wireless pressure gauge and wireless thermometer.