Interference sources and interference types that affect the accuracy of analog sensors and solutions

Foreword

Analog sensors are used in a wide range of applications, from industrial, agricultural, and defense construction to everyday life, education, and scientific research. However, in the design and use of analog sensors, there is a problem of how to maximize the measurement accuracy.

However, many disturbances have always affected the measurement accuracy of the sensor. For example, there are many large energy-consuming devices on site, especially the start and stop of high-power inductive loads often cause the grid to generate spikes of several hundred volts or even thousands of volts. Pressure or overpressure (the supply voltage of the county steel plant fluctuates between 160V and 310V), often reaching about 35% of the rated voltage. This kind of poor power supply sometimes lasts for a few minutes, hours, or even days; various signal lines are tied. When the same multi-core cable is used together, the signal will be disturbed, especially when the signal line and the AC power line are in the same long pipeline. The performance of the multi-way switch or the keeper is not good, and the channel signal is also caused. Disturbance; various electromagnetic, meteorological conditions, lightning, and even changes in the Earth's magnetic field can interfere with the normal operation of the sensor.

In addition, changes in the temperature and humidity of the site may cause changes in circuit parameters, the role of corrosive gases, acid and alkali salts, wind and sand in the wild, rain, and even biting insects may affect the reliability of the sensor. The output of the analog sensor is generally a small signal, which has small signal amplification, processing, shaping and anti-interference problems, that is, the sensor's weak signal is accurately amplified to the required unified standard signal (such as 1VDC ~ 5VDC or 4 mADC ~ 20mADC) and achieve the required technical specifications. This requires the designer to pay attention to some of the problems not shown on the analog sensor circuit diagram, namely the anti-interference problem. Only by understanding the interference source of the analog sensor and the mode of interference, designing the circuit to eliminate the interference or the measures to prevent the interference can achieve the best state of the analog sensor.

Interference source, interference type and interference phenomenon

Sensors and instrumentation are subject to various disturbances in the field operation. Specific analysis of specific situations and different measures for different interferences are the principle of anti-interference. This flexible and flexible strategy and universality are undoubtedly contradictory. The solution is to adopt a modular method. In addition to the basic components, the instrument can be equipped with different options for different operational situations to effectively resist interference and improve. reliability. Before further discussion of circuit component selection, circuit and system applications, it is necessary to analyze the sources of interference and the types of interference that affect the accuracy of the analog sensor.

Main source of interference

(1) Static induction

Electrostatic induction is due to the parasitic capacitance between two branches or components, so that the charge on one branch is transmitted to the other via parasitic capacitance, so it is also called capacitive coupling.

(2) Electromagnetic induction

When there is a mutual inductance between two circuits, the change of current in one circuit is coupled to the other through a magnetic field. This phenomenon is called electromagnetic induction. For example, magnetic flux leakage of transformers and coils, parallel wires for energization, and the like.

(3) Leakage current sensing

Due to poor insulation of component brackets, terminals, printed circuit boards, internal dielectrics or casings of the electronic circuit, especially in the application environment of the sensor, the insulation resistance of the insulator is reduced, resulting in an increase in leakage current, which may cause interference. Especially when leakage current flows into the input stage of the measuring circuit, its effect is particularly severe.

(4) Radio frequency interference

It is mainly the start of large power equipment, interference of operation stop and high harmonic interference. Such as the interference of the thyristor rectifier system.

(5) Other interference

In addition to being susceptible to the above disturbances, the on-site safety production monitoring system is also susceptible to mechanical interference, thermal interference and chemical interference due to poor working environment.

Type of interference

(1) Normal mode interference

Normal mode interference means that the intrusion of interference signals is consistent on two round trips. The source of normal mode interference is generally a strong alternating magnetic field around it, which causes the instrument to be affected by the surrounding alternating magnetic field and generate AC electromotive force to form interference, which is difficult to remove.

(2) Common mode interference

Common mode interference means that the interference signal flows through a part of each of the two lines, and the ground is a common circuit, and the signal current flows only in two lines. Common sources of common mode interference are equipment leakage to ground, ground potential difference, and the line itself has ground disturbance. Due to the unbalanced state of the line, common mode interference is converted to normal mode interference, which is difficult to remove.

(3) Long-term interference

Long-term interference refers to long-term interference. The characteristics of such interference are that the interference voltage exists for a long time and does not change much. It is easy to measure with the instrumentation. For example, the electromagnetic interference of the power line or the adjacent power line is continuous AC 50 Hz. Power frequency interference.

(4) Unexpected transient interference

Accidental transient interference occurs mainly during the operation of electrical equipment, such as closing or opening, and is sometimes accompanied by lightning or the operation of radio equipment.

Interference can be roughly divided into three aspects:

(a) local production (ie unwanted thermocouples);

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

(c) External generation (interference of Bp power frequency).

Interference phenomenon

In the application, the following main interference phenomena are often encountered:

When the command is issued, the motor rotates irregularly;

When the signal is equal to zero, the digital display table value jumps;

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;

When the measured parameter is stable, the difference between the value of the sensor output and the signal value corresponding to the measured parameter is a stable or periodically changing value;

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

There are two main types of channels that interfere with the positioning control system: signal transmission channel interference, interference through the signal input channel and output channel connected to the system; power supply system interference.

The signal transmission channel is the way for the control system or driver to receive the feedback signal and issue the control signal. Because the pulse wave will have delay, distortion, attenuation and channel interference on the transmission line, the long-line interference is the main factor in the transmission process. Any power supply and transmission line have internal resistance. It is these internal resistance that cause noise interference of the power supply. If there is no internal resistance, no matter what kind of noise will be absorbed by the short circuit of the power supply, no interference voltage will be established in the line; The AC servo drive itself is also a strong source of interference, which can interfere with other devices through the power supply.

Anti-interference measures

Anti-interference design of power supply system

The most serious hazard to the normal operation of sensors and instruments is the peak pulse interference of the power grid. The electrical equipment that produces spike interference is: electric welder, large motor, controllable machine, relay contactor, and inflatable lighting with ballast. Even electric irons and so on. Spike interference can be suppressed by a combination of hardware and software.

(1) Use hardware lines to suppress the effects of spike interference

There are three main methods:

1 Intersect the interference controller designed according to the principle of spectrum equalization at the input end of the AC power supply of the instrument, and distribute the energy concentrated by the peak voltage to different frequency bands, thereby reducing its destructiveness;

2 Add a super isolation transformer at the input end of the AC power supply of the instrument to suppress the spikes by the principle of ferromagnetic resonance;

3 Connect the varistor in parallel with the input of the AC power supply of the instrument. When the spike is applied, the resistance value is reduced to reduce the voltage of the instrument from the power supply, thus weakening the influence of interference.

(2) Using software methods to suppress spike interference

For periodic interference, time filtering can be performed by programming, that is, the program can be used to control the thyristor conduction without sampling at the instant, thereby effectively eliminating interference.

(3) Using a hard, software-coupled watchdog technique to suppress the effects of spikes

Software: Before the timer expires, the CPU accesses the timer once, let the timer restart, and the normal program runs. The timer will not generate an overflow pulse, and the watchdog will not work. Once the "flying program" occurs in the spike interference, the CPU will not access the timer before the timing is reached, and the timing signal will appear, causing a system reset interrupt to ensure that the smart instrument returns to the normal program.

(4) Power supply is implemented. For example, separate the drive power of the execution motor from the control power supply to prevent interference between devices.

(5) A noise filter is used. It can effectively suppress the interference of the AC servo drive to other devices. This measure can effectively suppress the above several interference phenomena.

(6) Using an isolation transformer

Considering that the high-frequency noise passes through the transformer mainly by the mutual inductance coupling of the primary and secondary coils, but by the primary and secondary parasitic capacitance coupling, the isolation transformer is isolated by the shielding layer to reduce the distribution between the primary and secondary. Capacitance to improve resistance to common mode interference.

(7) Power supplies with high anti-interference performance, such as high anti-interference power supplies designed with spectrum equalization. This kind of power supply is very effective against random interference. It can convert the high-peak disturbance voltage pulse into a low-voltage peak (voltage peak is less than TTL level), but the energy of the interference pulse is constant, which can improve the sensor and instrumentation. Anti-interference ability.

Anti-jamming design of signal transmission channel

(1) Photoelectric coupling isolation measures

In the long-distance transmission process, the optocoupler can be used to cut off the connection between the control system and the input and output channels and the input and output channels of the servo drive. If opto-isolation is not used in the circuit, external spikes will enter the system or go directly into the servo drive, creating the first type of interference.

The main advantage of optoelectronic coupling is that it can effectively suppress spikes and various noise interferences, and greatly improve the signal-to-noise ratio of the signal transmission process. Although the interference noise has a large voltage amplitude, the energy is very small, and only a weak current can be formed, and the light-emitting diode of the input part of the optocoupler operates in a current state, and the general on-current is 10 mA to 15 mA, so even if there is Very large interference, which can also be suppressed by not providing enough current.

(2) Long-distance transmission of twisted-pair shielded wires

Signals are affected by interference factors such as electric field, magnetic field and ground impedance during transmission. Grounding shields can reduce the interference of electric fields. Compared with the coaxial cable, the twisted pair has a high frequency band, but has high wave impedance and strong anti-common mode noise, which can cancel the electromagnetic induction interference of each small link. In addition, in the long-distance transmission process, differential signal transmission is generally adopted, which can improve anti-interference performance. Long-distance transmission with twisted-pair shielded wires can effectively suppress the generation of (2), (3), and (4) interferences in the interference phenomenon mentioned above.

Elimination of local error

In low-level measurements, strict attention must be paid to the materials used in the signal path (or constituents), and the actual thermoelectric potential may be generated by solders, wires, and posts encountered in simple circuits.

Since they often appear in pairs, it is very effective to keep these pairs of thermocouples at the same temperature. For this reason, heat shields, heat sinks are arranged along the isotherms or high power circuits and low power are used. Circuit separation, etc., the purpose is to minimize the thermal gradient. The junction of two standard manufacturers (such as nickel-chromium-constantan) can produce a temperature drift of 0.2mV/°C, which is equivalent to high-precision and low-drift. The temperature drift of the op amp (OP·27CP) is twice the temperature drift of the chopper amplifier (7650CPA).

Although the use of socket switches, connectors, relays, etc. can make it easier to replace electrical components or components, the disadvantage is that contact resistance, thermoelectric potential, or both may occur, at the expense of increasing the resolution of the low level. Stability, that is to say it is worse than the direct connection system, low precision, increased noise, and reduced reliability. Therefore, it is an important measure to reduce the number of faults and improve the accuracy by using switches and connectors as much as possible in low-level amplification.

In a microvolt signal amplifying circuit, solder may also become a low level fault because a thermoelectric potential is also generated at the solder joint. Therefore, special low-temperature solder should be used in the input circuit of microvolt level, such as kesterl544 solder, and there is even an example: one must be carefully cut in one line, and then soldered to compensate for another The thermoelectric potential generated by a lap or solder joint in a line.

Grounding problem handling

Reasonable "grounding" in low-level amplifier circuits is an important measure to reduce "ground" noise interference and must be given special attention. When using a single power supply to supply multiple sensors and instruments, the interference introduced by the grounding resistance should be minimized. If the voltage drop of the power supply must be minimized, the "high" end of the power supply can be wired in a similar manner. Systems that include multiple power supplies and multiple sensors, instrumentation need to be considered more often, regardless of who supplies the power supply, gather the ground wire to a common point, and then connect to the common end of the system, all power supplies 1 The load is returned to the common end of the power supply 1, all the power supply 2 loads are returned to the common end of the power supply 2, and finally the common ends are connected by a thick wire. In multiple power systems, a judging test may be required to determine the ground connection to achieve the best solution.

To facilitate signal transmission and conversion, the DINIEC 381 standard specifies the permissible current and voltage values. The commonly used voltage signal is 0V ~ 10V, the current signal is 0mA ~ 20mA or 4mA ~ 20mA. These signals are often used for long distance transmission. The voltage signal is limited by conditions such as the transmission distance during the transmission process, and the interference of the current signal during the transmission process has less influence on it, so the current signal should be used as much as possible. If there is ground in the measurement loop, a potential difference will occur between the two ground points. This potential difference has a large effect on the measurement results and should be avoided as much as possible.

However, if it is necessary to ground, then the ground loop must be isolated to avoid measurement errors. Active digital components generate a rapid current change on the power line when turned on and off. This current not only causes a positive voltage drop on the wire inductance, but also causes a negative voltage drop. This change in voltage is transmitted as interference on the main line. In addition, the commutation operating unit (such as the frequency) in the power supply also generates interference, which is coupled into the conductor as a narrowband frequency energy and propagates. The circuitry connected to the rear must filter these high frequency interference voltages through a low pass filter.

Software filtering

Software filtering is unique to intelligent sensors and instrumentation. It can filter various interference signals including interference signals with very low frequency (such as 0.01Hz), and a digital filtering program can be shared by multiple input channels. Commonly used software filtering methods are:

(1) The average value filtering, that is, the self-reported average value of the M times of sampling is used as the output of the filter, and the weight of the freshly sampled value may be increased as needed to form a weighted average filter;

(2) Median filtering, that is, sorting M consecutive samples and taking the median value as the output of the filter. This method has a good effect on the impulse interference filtering of the ramping process;

(3) Limiting filtering, this method is to determine the maximum possible difference Δ of the adjacent two samples according to the sampling period and the normal rate of change of the real signal, and the difference between the current sampling and the last sampling is less than or equal to Δ. It is considered to be a valid signal, and a signal larger than Δ is treated as noise.

(4) Inertial filtering, which is a digital implementation of an analog PC filter, suitable for effective signals with frequent wave stops.

Other anti-jamming technology

(1) Voltage regulation technology

At present, there are two types of regulated power supplies commonly used in the development of smart sensors and instrumentation: one is a series-regulated power supply provided by an integrated voltage regulator chip, and the other is a DC-DC regulated power supply, which interferes with the grid voltage fluctuations. Normal work is very effective.

(2) Suppression of common mode interference technology

Using a differential amplifier to increase the input impedance of the differential amplifier or reduce the internal resistance of the signal source can greatly reduce the effects of common-mode interference.

(3) Software compensation technology

External factors such as changes in temperature and humidity can also cause changes in certain parameters, causing deviations. We can use the software to correct according to changes in external factors and error curves to remove interference.

summary

Anti-interference is a very complicated and practical problem. An interference phenomenon may be caused by several factors. Therefore, in the design of intelligent sensors, instruments and measurement and control systems, we should not only take anti-interference measures in advance, but also timely analyze the phenomena encountered during the debugging process, the circuit principle of the sensors, instrumentation, and specific wiring. Shielding, power supply immunity, digital or analog ground handling, and protection are constantly being improved to improve sensor reliability and stability.