Sensor interference and anti-jamming measures

I. INTRODUCTION The use of analog sensors is very extensive. No matter in the areas of industry, agriculture, national defense construction, or in the fields of daily life, education, and scientific research, analog sensors can be seen everywhere. However, in the design and use of analog sensors, there is a problem of how to achieve the highest measurement accuracy. The numerous disturbances have affected the measurement accuracy of the sensor, such as: many large-scale energy-consuming devices in the field, especially the start-stop of high-power inductive loads will often cause the grid to generate several hundred volts or even several thousand volts of spike interference; Pressure or overvoltage, often reaching about 35% of the rated voltage, this poor power supply can sometimes reach minutes, hours, or even days; the signal cables will be affected if the signal cables are bundled together or the same multi-core cable is taken. Interference, especially in signal lines and AC power lines, is especially disturbing in the case of long pipelines. The poor performance of multi-channel switches or cages can also cause nuisances in the channel signals; various electromagnetic, meteorological conditions, lightning, and even space Changes in the magnetic field will also interfere with the normal operation of the sensor; in addition, changes in the field temperature and humidity may cause changes in the circuit parameters, the role of corrosive gases, acid and alkali salts, wild sand, rain, and even rat biting insects. Affects the reliability of the sensor. Analog sensor output is generally a small signal, there are small signal amplification, processing, shaping and anti-jamming problems, that is, the sensor's weak signal accurately amplified to the required uniform standard signal (such as 1VDC ~ 5VDC or 4 mADC ~ 20mADC) and achieve the required technical specifications. This requires that the designer must pay attention to some problems that are not shown on the analog sensor circuit diagram, namely the anti-jamming problem. Only by clarifying the interference sources of the analog sensors and the modes of interference and designing circuits for eliminating interference or measures for preventing interference, can the best state of application of analog sensors be achieved.

Sensor interference and anti-jamming measures II. Sources of interference, types of interference and interference phenomena Sensors and instrumentation in the field are subject to a wide range of interference, specific analysis of specific conditions, different measures for different interference is the principle of anti-interference . This flexible and flexible strategy is undoubtedly in conflict with universality. The solution is to adopt a modular approach. In addition to the basic components, the instrument can be equipped with different options for effective anti-jamming and improvement for different operating situations. reliability. Before discussing further the selection of circuit components, circuits and system applications, it is necessary to analyze the interference sources and interference types that affect the accuracy of analog sensors.

1, the main source of interference (1) electrostatic induction electrostatic induction is due to the existence of parasitic capacitance between the two branches or components, so that the charge of a branch through the parasitic capacitance to another branch, and therefore also known as capacitive coupling.

(2) Electromagnetic induction When a mutual inductance exists between two circuits, the change of current in one circuit is coupled to another circuit through a magnetic field. This phenomenon is called electromagnetic induction. For example, leakage of transformers and coils, energization of parallel conductors, etc.

(3) Leakage Current Sensing Due to the poor insulation of the component supports, terminals, printed circuit boards, internal dielectrics, or housings of the electronic circuit, especially the humidity of the sensor's application environment, the insulation resistance of the insulator decreases, resulting in increased leakage current. It will cause interference. Especially when the leakage current flows into the input stage of the measuring circuit, its influence is particularly severe.

(4) RF interference is mainly the start of large-scale power equipment, the interference of operation stop, and the interference of high-order harmonics. Such as silicon controlled rectifier system interference.

(5) Other interferences The on-site safety production monitoring system is not only susceptible to the above disturbances, but it is also vulnerable to mechanical interference, thermal interference and chemical interference due to the poor working environment of the system.

2. Types of interference (1) Normal mode interference Normal mode interference means that the interference signal is intruded on the two lines. Normal mode interference source is generally a strong alternating magnetic field around the device, so that the instrument is affected by the surrounding alternating magnetic field to generate AC electromotive force interference, this interference is more difficult to remove.

(2) Common-mode interference Common-mode interference refers to that the interference signal flows through a portion of each of the two lines, and the ground is a common circuit. The signal current only flows through two lines. The source of common mode interference is generally equipment ground leakage, ground potential difference, and the line itself has interference to ground. Due to the unbalanced state of the line, the common mode interference will be converted into normal mode interference, which is more difficult to remove.

(3) Long-term interference Long-term interference refers to long-term interference. The characteristics of such interference are long-term interference voltages and small changes. It is easy to measure with a detection instrument. For example, the electromagnetic interference of a power line or a nearby power line is It is continuous AC 50 Hz power frequency interference.

(4) Unexpected transient disturbances Unexpected transient disturbances mainly occur during the operation of electrical equipment, such as closing or opening, etc., and are sometimes accompanied by lightning or radio equipment.

Interference can be roughly divided into 3 areas:

(a) Local production (ie, unwanted thermocouples);

(b) Coupling within the subsystem (ie, the path of the ground);

(c) Externally generated (Bp power frequency interference).

3. Interference In applications, the following types of interference are often encountered:

(1) When the instruction is issued, the motor rotates randomly;

(2) When the signal is equal to zero, the digital display table jumps.

(3) When the sensor is working, the output value of the sensor does not agree with 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 sensor output value and the signal value corresponding to the measured parameter is a stable or periodically changing value;

(5) Devices that share the same power supply as the AC servo system (such as monitors) do not work properly.

There are mainly two types of interference channels into the positioning control system: signal transmission channel interference, interference through signal input channels and output channels connected to the system, and power supply system interference. The signal transmission channel is a way for the control system or driver to receive the feedback signal and send out the control signal. Because the pulse wave will have delay, distortion, attenuation and channel interference on the transmission line, long-term interference is the main factor in the transmission process. Any internal power supply and transmission line has internal resistance. It is these internal resistances that cause noise interference in the power supply. If there is no internal resistance, no matter what kind of noise will be absorbed by the power supply, no interference voltage will be established in the line. The AC servo drive itself is also a strong source of interference. It can interfere with other devices through the power supply.

Sensor interference and anti-jamming measures III. Anti-interference measures 1. Anti-jamming design of power supply system The most serious damage to sensors and instrumentation is grid spike interference. The electrical equipment that produces spike interference includes: welding machine, Large motors, controllable machines, relay contactors, inflatable lights with ballasts, and even soldering irons. Spike interference can be suppressed by a combination of hardware and software.

(1) Using hardware circuits to suppress the influence of spike interference There are three main methods that are commonly used:

1 Insert an interference Controller designed according to the principle of spectrum equalization at the input of the AC power source of the instrument to distribute the energy of the peak voltage concentration to different frequency bands, thereby weakening its destructiveness;

2 Add a super-isolation transformer to the input of the instrument's AC power supply, and use the principle of ferromagnetic resonance to suppress the spike;

3 The varistor is connected in parallel with the input of the instrument's AC power supply. With the arrival of the spike, the resistance value is reduced to reduce the voltage of the instrument from the power supply, thereby weakening the influence of interference.

(2) Use of software methods to suppress spike interference For periodic interference, programmatic time filtering can be used, that is, the thyristor conduction is not sampled at the instant of program control, thus effectively eliminating interference.

(3) Hard and software watchdog technology is used to suppress the influence of spikes. Software: Before the timer expires, the CPU accesses the timer once, the timer restarts, and the normal program runs. No overflow pulse will occur, and watchdog will not work. Once spike interference occurs with a "flying program," the CPU will not access the timer before the timer expires, so the timing signal will appear, causing a system reset interrupt and ensuring that the smart instrument returns to the normal program.

(4) Power supply is provided for group power supply. For example, separate the drive power of the executing motor from the control power to prevent interference between devices.

(5) The noise filter can also effectively suppress the AC servo driver's interference to other devices. This measure can effectively suppress the above several interference phenomena.

(6) The use of an isolation transformer allows high-frequency noise to pass through the transformer, which is not primarily based on the mutual inductance coupling of the primary and secondary coils, but is coupled by primary and secondary parasitic capacitances. Therefore, the primary and secondary isolation transformers are shielded. Layer isolation, reducing its distributed capacitance to improve resistance to common mode interference.

(7) Power supplies with high anti-interference performance, such as high anti-interference power supplies designed using spectrum equalization method. This kind of power supply is very effective against random interference. It can convert high-spike perturbation voltage pulses into voltages with low voltage peaks (voltage peaks less than TTL level), but the energy of the interference pulses does not change, which can increase the resistance of the sensor instrumentation. Interference ability.

2. Anti-interference design of signal transmission channel (1) Photoelectric coupling isolation measures In the process of long-distance transmission, optocouplers are used to cut off the control system from the input and output channels of input channels, output channels, and servo drivers. Contact. If optical isolation is not used in the circuit, the external spike interference signal will enter the system or directly enter the servo drive device, resulting in the first interference phenomenon.

The main advantage of optocoupler is that it can effectively suppress spikes and various noise interferences, which greatly improves the signal-to-noise ratio of the signal transmission process. Although the interference noise has a large voltage amplitude, but the energy is very small, only a weak current can be formed. The light-emitting diode of the input part of the optocoupler operates in the current state. Generally, the conduction current is 10mA to 15mA, so even if A great deal of interference, this interference will also be suppressed because it can not provide enough current.

(2) Twisted-pair shielded long-term transmission signals will be affected by interference factors such as electric field, magnetic field, and ground impedance in the transmission process. The use of a grounded shield line can reduce electric field interference. Twisted pair and coaxial cable, although the frequency band is poor, but the wave impedance is high, strong resistance to common mode noise, can make the electromagnetic induction interference of each small part of each other to cancel each other. In addition, in the long-distance transmission process, differential signal transmission is generally used to improve anti-interference performance. The use of twisted-pair shielded long-line transmission can effectively suppress the generation of (2), (3), and (4) types of interference in the aforementioned interference phenomena.

3. Elimination of Locally Generated Errors In low-level measurements, rigorous attention must be paid to the materials used (or formed) in the signal path. Solder, conductors, terminals, etc. encountered in simple circuits may be Actual thermoelectric power is generated. Since they often occur in pairs, it is an effective measure to keep these paired thermocouples at the same temperature as much as possible. For this reason, heat shields, radiators along the isotherm or high power are generally used.