Instrumentation anti-interference measures - Schneider

The interference encountered by sensors and instruments in the field operation is varied, and the specific situation is analyzed on a case-by-case basis, and the principle of anti-interference is to adopt different methods for different disturbances. This flexible strategy is undoubtedly opposed to universality, and the solution is to choose a modular approach, in addition to the basic components, for different operating occasions, the instrument can be installed with different options to usefully anti-interference, improve reliability. Before further discussing the selection of circuit components and the application of circuits and systems, it is necessary to analyze the interference sources and types that affect the accuracy of analog sensors.

Instrumentation anti-interference measures - Schneider

Instrumentation anti-interference measures - Schneider

1. Primary source of interference

(1) Electrostatic induction

Electrostatic induction is also called capacitive coupling because there is parasitic capacitance between two branch circuits or components, so that the charge on one branch is transmitted to the other branch through the parasitic capacitance.

(2) Electromagnetic induction

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

(3) Leakage current induction

Because the internal component brackets, binding posts, printed circuit boards, capacitors internal dielectric or shells of electronic circuits are poorly insulated, especially the application environment humidity of the sensor is large, the insulation resistance of the insulator decreases, resulting in an increase in leakage current, which will cause disturbance. This is especially true when leakage current flows into the input stage of the measurement circuit.

(4) Radio frequency interference

First of all, it is the disturbance of the start, operation and stop of large-scale power equipment and the disturbance of higher harmonics. Such as the disturbance 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 simply disturbed by mechanical, thermal and chemical disturbances because of the poor operating environment of the system.

2. Varieties of interference

(1) Norm interference

Normal mode interference refers to the intrusion of the interfering signal that is common to the two reciprocating lines. The origin of the normal mode disturbance is generally the strong alternating magnetic field around it, so that the instrument is affected by the surrounding alternating magnetic field and communicates the electromotive force to form a disturbance, which is difficult to remove.

(2) Common mode interference

Common-mode interference means that the disturbance signal flows through a part of each of the two lines, with the ground as the common loop, and the signal current only flows through the two reciprocating lines. The origin of common mode disturbance is generally the leakage of equipment to the ground, the potential difference between the ground, and the ground disturbance of the line itself. Because of the imbalance of the line, the common-mode disturbance will be converted to the norm disturbance, which is more difficult to remove.

(3) Long-term interference

Long-term disturbance refers to the long-term disturbance, which is characterized by the long-term existence of the disturbance voltage and the small change, which is very simple to measure with a detection instrument, such as the electromagnetic disturbance of the power line or the adjacent power line is a continuous communication 50 Hz power frequency disturbance.

(4) Unexpected transient interference

Accidental transient disturbances mainly occur during the operation of electrical equipment, such as closing or opening, etc., and sometimes occur at the moment when lightning occurs or radio equipment is operated.

Interference can be roughly divided into 3 areas:

(a) locally occurring (i.e., unwanted thermocouples);

(b) coupling within the subsystem (i.e., the path problem of the ground wire);

(c) Externally occurring (disturbance of Bp power supply frequency).

3. Interference phenomenon

In applications, the following are the main types of disturbances:

(1) When the command is issued, the motor rotates irregularly;

(2) When the signal is equal to zero, the value of the digital display table jumps randomly;

(3) When the sensor is operating, its output value is inconsistent with the signal value corresponding to the practical parameters, 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) The equipment (such as the display, etc.) that shares the same power supply with the communication servo system is not working properly.

There are two main types of channels for interference to enter the positioning and control system: signal transmission channel disturbance, which enters through the signal input channel and output channel connected with the system; Disturbance of the power supply system.

The signal transmission channel is the way for the control system or driver to receive the feedback signal and announce the control signal, because the pulse wave will present delay, distortion, attenuation and channel disturbance on the transmission line, so in the transmission process, the long-term disturbance is the primary element. Any power supply and transmission line has internal resistance, it is these internal resistance that causes the noise disturbance of the power supply, if there is no internal resistance, no matter what kind of noise will be absorbed by the power supply short circuit, and no disturbance voltage will be established in the line; In addition, the communicative servo drive itself is also a strong source of disturbance, which can disturb other equipment through the power supply.

3. Anti-interference methods

1. Anti-interference planning of power supply system

The most serious hazard to the normal operation of sensors and instruments is the peak pulse disturbance of the power grid, and the electrical equipment that occurs the peak disturbance includes: electric welding machine, large motor, controllable machine, relay contactor, inflatable lighting with ballast, and even electric soldering iron. Spike disturbance can be suppressed by a combination of hardware and software.

(1) The impact of using hardware circuits to press the spike to disturb it

There are three common methods:

(1) The disturbance controller planned according to the principle of spectrum equalization is connected to the input end of the instrument communication power supply, and the energy collected by the peak voltage is distributed to different frequency bands, and then its destructiveness is weakened;

(2) Add a super barrier transformer at the input end of the instrument's communication power supply, and use the principle of ferromagnetic resonance to press the peak pulse;

(3) The varistor is connected in parallel at the input end of the instrument communication power supply, and the resistance value is reduced when the spike pulse arrives to reduce the voltage distributed by the instrument from the power supply, and then weaken the influence of disturbance.

(2) Use software methods to press the spike to disturb

Regarding periodic disturbance, programming can be used to carry out time filtering, that is, the thyristor can be programmed to turn on without sampling instantly, and then usefully eliminate the disturbance.

(3) Use the watchdog technology that combines hardware and software to press the impact of spike pulses

Software: Before the timer is punctual, the CPU visits the timer once, so that the timer starts the timer again, and the normal program is running, the timer will not overflow pulses, and the watchdog will not work. Once the spike disturbance presents a "flying program", the CPU will not visit the timer before the punctuality, so the punctuality signal will be presented, and then the system reset will be interrupted, ensuring that the intelligent instrument returns to the normal program.

(4) Implement power supply grouping, for example: separate the driving power supply of the performance motor from the control power supply to prevent disturbance between the equipment.

(5) The selection of noise filters can also be useful to communicate the disturbance of other equipment by the servo drive. This method can be useful for the above kinds of disturbance phenomena.

(6) Select a barrier transformer

Considering that the high-frequency noise passing through the transformer does not rely on the mutual inductance coupling of the primary and secondary coils, but on the coupling of the primary and secondary parasitic capacitance, the primary and secondary stages of the barrier transformer are blocked by a shielding layer to reduce their distributed capacitance and improve the ability to resist common mode disturbance.

(7) Choose a power supply with high anti-interference performance, such as a power supply with high anti-interference planned by the spectrum equalization method. This kind of power supply is very useful against random disturbance, it can convert high spike disturbance voltage pulses into low voltage peaks (voltage peaks less than TTL levels) voltage, but the energy of the disturbance pulses remains the same, and then can improve the anti-disturbance ability of sensors and instruments.

2. Anti-disturbance planning of signal transmission channel

(1) Photoelectric coupling barrier method

In the process of long-distance transmission, the optocoupler is selected to connect the control system with the input channel, the output channel and the input and output channel interception circuit of the servo drive. If the optical resistance isolation is not selected in the circuit, the external spike disturbance signal will enter the system or directly enter the servo drive device, and the first disturbance phenomenon will occur.

The primary advantage of photoelectric coupling is that it can usefully press the spike pulse and various noise disturbances, so that the signal-to-noise ratio of the signal transmission process is greatly improved. Although the disturbance noise has a large voltage fluctuation, the energy is very small, and only a weak current can be formed, and the light-emitting diode in the input part of the photocoupler is operating in the current state, and the general on-current is 10mA-15mA, so even if there is a large fluctuation of disturbance, this disturbance will be pressed out because it cannot supply a satisfactory current.

(2) Twisted-pair shielded wire long-term transmission

In the process of transmission, the signal will be affected by the disturbance factors such as electric field, magnetic field and ground impedance, and the selection of grounding shield wire can reduce the disturbance of the electric field. Compared with coaxial cable, although the frequency band is poor, the twisted pair has high wave impedance and strong anti-common-mode noise ability, which can cancel out the electromagnetic induction disturbance of each small link. In addition, in the process of long-distance transmission, differential signal transmission is generally used, which can improve the anti-interference performance. The use of twisted pair shielded wire for long-term transmission can be useful to prevent the occurrence of (2), (3) and (4) of the disturbance phenomenon mentioned above.

3. Elimination of local errors

In low-level measurements, it is necessary to pay strict attention to the materials used (or composed) in the signal path, and the thermoelectric potential of solder, wires, and binding posts encountered in simple circuits may occur. Because they are often presented in pairs, it is useful to keep these pairs at the same temperature as much as possible, and for this purpose it is generally possible to use thermal shielding, heat sinks arranged along the isotherm and perhaps high power.

Instrumentation anti-interference measures - Schneider

Sections viewed

Contact

working hours