Analog sensors have a wide range of applications, appearing in industries, agriculture, national defense, daily life, education, and scientific research. However, a key challenge in the design and use of analog sensors is maximizing their measurement accuracy.
Numerous interferences have consistently affected the measurement accuracy of sensors. For example, the presence of numerous high-energy-consuming devices on-site, especially the starting and stopping of high-power inductive loads, often generates spike interference of hundreds or even thousands of volts in the power grid; industrial power grids experiencing undervoltage or overvoltage, often reaching around 35% of the rated voltage, can sometimes last for minutes, hours, or even days; various signal lines bundled together or running on the same multi-core cable can interfere with signals, especially when signal lines and AC power lines run in the same long conduit; poor performance of multiplexers or holding circuits can also cause crosstalk in channel signals; changes in various electromagnetic and meteorological conditions, lightning, and even the Earth's magnetic field can also interfere with the normal operation of sensors; in addition, changes in on-site temperature and humidity may cause changes in circuit parameters; the effects of corrosive gases, acids, alkalis, and salts; wind, sand, rain, and even rodent bites and insect damage can all affect the reliability of sensors.
Analog sensors typically output small signals, which present challenges in amplification, processing, shaping, and interference suppression. This involves precisely amplifying the sensor's weak signal to a standardized signal (e.g., 1VDC–5VDC or 4mA DC–20mA DC) to meet required specifications. This necessitates that designers and manufacturers pay attention to issues not explicitly shown in analog sensor circuit diagrams, namely, interference suppression. Only by understanding the sources and mechanisms of interference in analog sensors and designing interference-eliminating circuits or preventative measures can the optimal application of analog sensors be achieved.
II. Interference Sources, Types of Interference, and Interference Phenomena
Sensors and instruments encounter a wide variety of interferences during field operation. The principle of interference mitigation is to analyze each situation specifically and adopt different measures for different types of interference. This flexible and adaptable strategy undoubtedly contradicts universality. The solution is to adopt a modular approach. In addition to basic components, instruments can be equipped with different optional components for different operating environments to effectively mitigate interference and improve reliability. Before further discussing the selection of circuit components, circuit and system applications, it is necessary to analyze the interference sources and types that affect the accuracy of analog sensors.
1. Main sources of interference
(1) Electrostatic induction
Electrostatic induction occurs because there is a parasitic capacitance between two branches or components, which allows the charge on one branch to be transferred to the other branch. Therefore, it is also called capacitive coupling.
(2) Electromagnetic induction
When mutual inductance exists between two circuits, a change in current in one circuit will couple to the other circuit through a magnetic field; this phenomenon is called electromagnetic induction. Examples include leakage flux in transformers and coils, and level-carrying parallel conductors.
(3) Leakage current induction
Interference can occur due to poor insulation in components such as the support structure, terminals, printed circuit boards, capacitor dielectric, or casing within electronic circuits, especially in high-humidity environments where sensors are used. This leads to a decrease in the insulation resistance of the insulators, resulting in increased leakage current. The impact is particularly severe when leakage current flows into the input stage of the measurement circuit.
(4) Radio frequency interference
The main sources of interference are the starting, operation, and stopping of large power equipment, as well as high-order harmonic interference, such as interference from thyristor rectifier systems.
(5) Other interference
In addition to being susceptible to the above-mentioned interferences, on-site safety production monitoring systems are also prone to mechanical, thermal, and chemical interferences due to the poor working environment.
2. Types of Interference
(1) Normal mode interference
Normative interference refers to interference signals that occur consistently on both the outgoing and incoming lines. The source of normative interference is generally a strong surrounding alternating magnetic field, which causes the instrument to generate an alternating electromotive force, thus creating interference. This type of interference is relatively difficult to eliminate.
(2) Common-mode interference
Common-mode interference refers to interference signals flowing partially on both lines, with ground as the common loop, while the signal current only flows between the two lines. Common-mode interference typically originates from equipment leakage to ground, ground potential differences, or inherent ground interference in the lines themselves. Due to line imbalances, common-mode interference can be converted into constant-mode interference, making it more difficult to eliminate.
(3) Long-term interference
Long-term interference refers to interference that exists for a long time. The characteristics of this type of interference are that the interference voltage exists for a long time and does not change much. It is easy to detect with testing instruments. For example, electromagnetic interference from power lines or nearby power lines is continuous AC 50Hz power frequency interference.
(4) Unexpected momentary interference
Unexpected transient interference mainly occurs during the operation of electrical equipment, such as closing or opening a circuit breaker, and sometimes it also occurs during lightning strikes or the moment when radio equipment is in operation.
Interference can be roughly divided into three aspects:
(a) Local generation (i.e., unwanted thermocouples);
(b) Coupling within the subsystem (i.e., the ground path problem);
(c) External interference (Bp power frequency).
3. Interference phenomenon
In applications, the following main interference phenomena are often encountered:
(1) When the command is given, the motor rotates irregularly;
(2) When the signal is zero, the digital display value jumps randomly.
(3) When the sensor is working, its output value does not match the signal value corresponding to the actual parameter, and the error value is random and irregular;
(4) When the measured parameter is stable, the difference between the value output by the sensor and the signal value corresponding to the measured parameter is a stable or periodically changing value.
(5) Devices that share the same power supply with the AC servo system (such as monitors) are not working properly.
There are two main channels through which interference enters the positioning control system: signal transmission channel interference, which enters through the signal input and output channels connected to the system; and power supply system interference. The signal transmission channel is the path through which the control system or driver receives feedback signals and sends control signals. Because pulse waves experience delay, distortion, attenuation, and channel interference on transmission lines, long-line interference is a major factor during transmission. All power supplies and transmission lines have internal resistance, and it is this internal resistance that causes power supply noise interference. Without internal resistance, any noise would be absorbed by a short circuit, and no interference voltage would be established in the line. Furthermore, the AC servo system driver itself is a strong source of interference, which can interfere with other devices through the power supply.
III. Anti-interference measures
1. Anti-interference design of the power supply system
The most serious threat to the normal operation of sensors and instruments is power grid spike interference. Equipment that generates spike interference includes: welding machines, large motors, controllable machines, relay contactors, gas-filled lighting lamps with ballasts, and even soldering irons. Spike interference can be suppressed using a combination of hardware and software methods.
(1) Suppressing the effects of spike interference using hardware circuitry
There are three main methods:
① An interference controller designed according to the principle of spectrum equalization is connected in series at the AC power input terminal of the instrument to distribute the energy concentrated by the peak voltage to different frequency bands, thereby reducing its destructiveness;
② Add a super isolation transformer to the AC power input terminal of the instrument to suppress spike pulses using the principle of ferromagnetic resonance;
③ Connect a varistor in parallel at the input of the instrument's AC power supply. When a spike pulse arrives, the resistance value decreases, thereby reducing the voltage that the instrument receives from the power supply and weakening the impact of interference.
(2) Using software methods to suppress spike interference
For periodic interference, time filtering can be performed by programming, that is, the thyristor is controlled by the program to not sample during the moment it is turned on, thereby effectively eliminating the interference.
(3) Employ a hardware and software combined watchdog technology to suppress the effects of spike pulses.
Software: Before the timer expires, the CPU accesses the timer once to restart it. During normal program execution, the timer will not generate an overflow pulse, and the watchdog will not function. However, if a spike interference causes a "program skip" (i.e., the program to "skip"), the CPU will not access the timer before its expiration, resulting in a timing signal that triggers a system reset interrupt, ensuring the intelligent instrument returns to its normal program.
(4) Implement power supply in groups, for example, separate the drive power supply of the actuator motor from the control power supply to prevent interference between devices.
(5) Using a noise filter can also effectively suppress the interference of the AC servo drive to other devices. This measure can effectively suppress the above-mentioned interference phenomena.
(6) Use isolation transformer
Considering that high-frequency noise passes through the transformer mainly through the parasitic capacitance of the primary and secondary windings rather than through the mutual inductance of the primary and secondary windings, the primary and secondary windings of the isolation transformer are isolated by a shielding layer to reduce their distributed capacitance and improve their ability to resist common-mode interference.
(7) Use a power supply with high anti-interference performance, such as a high anti-interference power supply designed using the spectrum equalization method. This type of power supply is very effective in resisting random interference. It can convert high-peak disturbance voltage pulses into low voltage peaks (voltage peaks are less than TTL levels) without changing the energy of the interference pulses, thereby improving the anti-interference capability of sensors and instruments.
2. Anti-interference design of signal transmission channel
(1) Optoelectronic coupling isolation measures
During long-distance transmission, optocouplers can be used to disconnect the control system from the input and output channels, as well as the input and output channels of the servo driver. Without opto-isolation in the circuit, external spike interference signals can enter the system or directly into the servo drive, causing the first type of interference.
The main advantage of optocouplers is their ability to effectively suppress spikes and various noise interferences, significantly improving the signal-to-noise ratio during signal transmission. Although interference noise has a large voltage amplitude, its energy is very small, only forming a weak current. The LEDs at the input of the optocoupler operate under current, typically with a conduction current of 10mA to 15mA. Therefore, even with significant interference, it can be suppressed because it cannot provide sufficient current.
(2) Long-distance transmission using twisted-pair shielded cable
Signals are affected by interference factors such as electric fields, magnetic fields, and ground impedance during transmission. Using grounded shielded wire can reduce electric field interference. Compared with coaxial cable, twisted pair cable has a lower bandwidth but higher impedance and stronger common-mode noise immunity, which can cancel out electromagnetic induction interference in each small component. In addition, differential signal transmission is generally used in long-distance transmission to improve anti-interference performance. Using twisted pair shielded wire for long-distance transmission can effectively suppress the generation of interference phenomena (2), (3), and (4) mentioned above.
3. Elimination of localized errors
In low-level measurements, strict attention must be paid to the materials used (or constructed) in the signal path. Solder, wires, and terminals encountered in simple circuits can all generate actual thermoelectric potentials. Since they often appear in pairs, it is an effective measure to keep these paired thermocouples at the same temperature as much as possible. For this purpose, thermal shielding and heat sinks are generally arranged along isotherms.
