Solutions for Suppressing Interference Problems in Digital Display Meters in the Chemical Industry

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Solutions for mitigating interference issues with digital display instruments in the chemical industry. In the chemical industry, display instruments, when used in conjunction with various sensors and transmitters, can be used to show different parameters. However, the conditions under which display instruments are used on a wide scale in the petroleum and chemical industries or at testing sites are often very complex. The presence of strong alternating magnetic fields, electric fields, vibrations, thermal noise, intense radiation, temperature effects, and power supplies can all affect the accurate collection of measurement data and the automatic control of production processes, thus acting as sources of interference. These voltages or currents, which are unrelated to the signal under test, are coupled into the detection, control, and display devices in various forms, causing inaccurate signal acquisition, distorted recording and display, a decline in the quality of the useful signals related to the parameters being measured, delays in automatic control, and even loss of control over operations. This directly affects normal production processes, product quality, and economic efficiency. Most of these disturbances are difficult to change, but it is essential to find effective ways to suppress them. I. Principle and General Composition of Digital Display Meters Digital display meters generally consist of three main components: analog-to-digital conversion, nonlinear compensation, and scale transformation. It takes electrical signals as input and displays the measured value directly in digital form. The key to achieving digital display is to convert continuously varying analog signals into discrete digital values using an A/D conversion device. In production, it is required that the values displayed by the instruments be functions of the parameters being measured, and that they be able to automatically compensate for other interfering factors. Some of these functional relationships are linear, but most are nonlinear. In order to display the parameter under test in absolute value, the display instrument needs to perform certain necessary calculations, processing, and nonlinear compensation on the parameter under test, while also compensating for the influence of other parameters on it. In A/D conversion, a certain measurement unit is used to quantize the continuously varying analog value, resulting in an approximate discrete digital value. The smaller the unit of measurement, the smaller the error in integer quantization; moreover, the frequency response of the A/D conversion device and the stability of the pre-amplifier improve, resulting in digital values that are closer to the actual value of the continuous quantity. This process is achieved through A/D converters such as those of the dual-integration type, voltage-frequency conversion type, pulse width modulation type, and successive comparison voltage feedback coding type. Nonlinear compensation or scaling transformation are necessary operations performed on the detected signal, enabling digital display instruments to present the measured parameter in its direct digital form. Analog nonlinear compensation compensates for input voltages within different ranges by adjusting the gain of operational amplifiers, whereas digital nonlinear compensation first converts the analog value of the parameter being measured into a digital form through an A/D converter before applying nonlinear compensation. It offers advantages such as high precision and strong versatility. With the development of electronic digital computers, tasks related to scaling transformation and nonlinear compensation are now carried out by computers. II. Anti-interference measures in display instrument applications (1) Generation of interference In production, the parameters to be measured are often converted into weak low-level voltage signals, which are then transmitted over long distances (sometimes hundreds of meters or even more) to the display instruments. Due to the complex operating environment of these instruments – characterized by the presence of strong alternating magnetic and electric fields, vibrations, thermal noise, intense radiation, temperature effects, and power supplies – electrical interference also reaches the input terminals of the display instruments. Additionally, internal components such as power transformers, relays, switches, and power cables contribute to interference, thereby affecting the accuracy of measurements. When large disturbances occur (the main interferences to the detection signal are strong magnetic and electric fields; when the interference source consists of low voltage and high current, then the interference source is primarily a magnetic field) ； When the interference source consists of high voltage and low current, the field present in the vicinity of the interference source is primarily an electric field); it often reaches the instruments via various mechanisms such as common-mode interference and differential-mode interference. 1. Electromagnetic induction (referring to magnetic coupling). Strong alternating magnetic fields exist in the surrounding areas of high-power transformers, AC motors, high-current power grids, etc. The closed circuits formed by the wires of control systems (such as detection, transmission, conversion, regulation, calculation, actuation, auxiliary functions, display units, etc.) are exposed to these changing magnetic fields, which induces voltages. This results in interference in the circuits through magnetic coupling, affecting the connection wires between the signal sources and instruments as well as the internal wiring of the instruments. This electromagnetic induction potential is connected in series with the useful signal, and the interference becomes more prominent when the signal source is far away from the display instrument. In addition, devices such as high-frequency generators and motors with commutators can also generate high-frequency interference. 2. Electrostatic induction (referring to electrical coupling). Electrostatic induction is the result of the interaction between two electric fields. In two opposite 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; thus, the interference source creates disturbances in the circuit through capacitive coupling. 3. Additional thermoelectric potential and chemical potential. Due to the thermoelectromotive forces generated by different metals, as well as the chemical electromotive forces resulting from metal corrosion, direct current electrical interference is created in the circuit. 4. Vibration. In an environment with strong vibrations, the wire generates an induced potential due to its movement in a magnetic field; this interference combines with the signal and enters the instruments in the form of series mode interference. 5. Interference caused by different earth potentials. Near high-power electrical equipment, when the insulation quality of such equipment is poor, the difference in potential between different ground levels creates interference. In the use of instruments, it is common for there to be two or more connection points at the input side, either intentionally or unintentionally; this leads to the introduction of the potential difference between different ground points as common-mode interference into the instruments. This type of interference appears on both signal lines simultaneously. 6. The signal source is an unbalanced bridge. When the bridge supply is grounded, in addition to the unbalanced voltage across the bridge diagonals (i.e., the signal voltage), there is a common common-mode interference voltage with respect to ground for both signal lines. Although common-mode interference does not superimpose on the signal and does not affect the instrument directly, it can create a leakage current to ground through the measurement system; through resistance coupling, this current can act directly on the instrument (or amplifier), thereby causing interference. 7. Some pulsed interference voltages can not only affect analog circuits but sometimes also enter digital circuits directly to cause interference; the sources of such interference voltages are inductive loads such as switches, motors, and relays, as well as devices that generate discharges. (II) Suppression of interference issues? Interference issues arise due to the presence of interference sources, which exert an influence on instruments through certain coupling pathways. To reduce these effects, the issue of interference suppression should be considered during the design of instruments, with an effort to enhance their resistance to interference. In practical applications, it is necessary to identify and combine methods such as twisting, shielding, grounding, balancing, filtering, and isolation in order to cut off the coupling paths and suppress interference. At the same time, it is required that the display instruments possess properties such as resistance to high and low temperatures, high pressure, corrosion, and high viscosity, as well as good dynamic characteristics, in order to reduce measurement errors of the parameters being measured. 1. Methods for suppressing common-mode interference (common-mode interference is an interference voltage that is superimposed on the signal to be measured at the input of instruments). Common-mode interference can originate from the signal source, or it may be induced or received through the leads. Since crosstalk interference shares the same location as the signal being measured, once it occurs, its harmful effects are often not easy to eliminate; therefore, it should be prevented from arising in the first place. (1) Twisting of signal wires: To mitigate electromagnetic induction, it is necessary to keep the wires away from strong electrical devices and power networks. Adjusting the routing direction and reducing the area of the wire loops are also important measures; by simply adjusting the routing direction and twisting the two signal wires together at short intervals, the interference voltage can be reduced to 1/10 to 1/100 of its original value ； In the case of electrostatic induction, by twisting two signal wires together in a twisted pair configuration and ensuring that the distance between the two wires and the source of interference is roughly equal (the wires are usually twisted at a pitch corresponding to 20 times their diameter), the area enclosed by the signal loop can be significantly reduced. This, in turn, greatly reduces the differential voltage of the common-mode interference that enters the loop through inductive coupling across the two signal wires. (2) Shielding: To further prevent interference from electric fields, the signal wires can be wrapped in a metal mesh (or metal foil), with an insulating layer added on the outside, or the signal wires can simply be shielded cables whose shielding layer is grounded. Since the non-magnetic shielding layer has no effect on a 50 Hz magnetic field, it is possible to insert the signal wires into iron tubes when necessary, thereby providing magnetic shielding for them. After electrostatic shielding, the induced electric potential can be reduced to 1/100 to 1/1000 of its original value. (3) Filtering: For DC signals with a very slow rate of change, a filtering circuit is added at the input of the instrument to minimize the interference mixed in with the useful signal. Two to three stages of R-C filtering circuits are often added before the input stage, and a dual-T filter with lower internal resistance yields better results. (4) Cancellation: Digital instruments of the dual-integration type and pulse-width modulation type perform A/D conversion on the average value rather than the instantaneous value of the input signal, which allows certain series-mode interferences to be averaged out. (5) Try to run the signal wires separately from the power wires. Arrange the wiring properly; where feasible, reverse the direction of current flow in the wires in order to reduce the interference caused by the magnetic fields generated between them ； It is not allowed to run signal wires parallel to power wires, nor should they enter instruments through the same conduit. Low-level signal wires should be connected to adjacent positions on the signal terminals using as short, untwisted wires as possible, in order to reduce the area affected by induced interference. It is strictly prohibited to use the same cable for power lines and signal wires. High-level and low-level lines should also not use the same wiring connector. As a last resort, separate the high-level and low-level wires beside the connector, with ground terminals and spare terminals in between. 2. Suppression of common-mode interference (common-mode interference is the interference applied between either input terminal of the instrument and ground). (1) Proper grounding. The meaning of grounding can be understood as obtaining an equipotential point or surface, which serves as the reference potential for a circuit or system, but it is not necessarily the potential of the earth. For safety reasons, the enclosures of instruments and signal sources are connected to ground to maintain a zero potential. However, if grounding is not handled properly, a ground loop will be formed, introducing interference into the instruments. To improve the resistance of instruments to interference, amplifiers in low-level measuring instruments are usually insulated from the instrument’s casing (ground), that is, the amplifier is placed in a floating ground condition, in order to cut off the pathways through which common-mode interference voltages can penetrate and cause interference. In low-level testing, the signal line should be grounded at one point, and its shielding layer as well should be grounded at one point. Both the signal line and instruments need to be shielded, and by using grounding and shielding together in a proper manner, most interference problems can be resolved. When an ungrounded signal source is connected to a grounded amplifier, the shield of the signal cable should be connected to the common terminal of the amplifier. When a grounded signal source is connected to an ungrounded amplifier, even if the signal source is not connected to ground, the shielding layer of the signal cable should be connected to the common terminal of the signal source to keep it at zero potential. This helps to prevent leakage currents caused by potential differences, thereby improving the resistance of the measurement signal to interference. This is a commonly used method in measurement systems. (2) The instrument adopts a double-layer shielding floating ground protection technique: To enhance the instrument’s resistance to common-mode interference, it utilizes double-layer shielding with a floating ground at the amplifier input stage. In addition to using the case as a layer of shielding, an additional internal shield is used inside the instrument to shield the input section of the amplifier. There is no electrical connection between the two shielding layers, nor between the amplifier input section and the inner shielding layer. The inner shielding layer should not be connected to the instrument’s casing; instead, a separate wire should be used to connect it to the shielding layer of the signal line. This allows the protective shielding to extend along the entire length of the signal line. The shielding of the signal line is grounded at the signal source, thereby ensuring that both the input protective shielding and the signal shielding of the instrument are at the same potential level, thus stabilizing the signal source. Therefore, shielding can be used to reduce the common-mode voltage coupled to the wires. (3) Application of balanced circuits: The stability of a system depends on the balance of the signal source, signal leads, load, as well as the balance of other stray distributed parameters. To improve the resistance of instruments and meters to common-mode interference, balancing measures are employed to equalize the voltages converted on the two lines, thereby reducing the amount of common-mode voltage that is coupled to the load. (4) Suppression of power supply-induced interference: The main source of interference inside instruments is the leakage current generated by low-power transformers. To prevent interference from leakage current, the transformer’s primary winding can be placed inside a shielding layer, which is then grounded. In this case, the phase voltage applied to the primary winding causes the leakage current to flow directly into the ground through the distributed capacitance of the shielding layer, rather than flowing into the amplifier, measurement circuits, and signal sources and causing interference there. To prevent interference introduced by the power transformer, a three-layer shielding structure is employed: the primary shielding layer of the power transformer is directly grounded to the enclosure, the secondary winding of the power supply unit is connected to all the shielding layers, and the shielding layer of the amplifier’s power supply secondary winding is at the same potential as the amplifier’s ground. Pulsating interference caused by the power supply has a significant impact on digital circuits; high-frequency filters should be installed on the power supply lines. These filters ought to be placed within an iron shielded box in which both the input and output wires pass through bypass capacitors for filtering