Interference problems and solutions in data acquisition and processing system

2021-03-02 08:19:54

With the rapid development and popularization of computer technology, electromagnetic compatibility technology, sensor technology and information technology, data acquisition and processing systems have also been widely used. For example: in the production process, the application of this system can collect, monitor and record the process parameters at the production site, providing information and means for improving product quality and reducing production costs; in scientific research, the application of this system can obtain a large number of The dynamic signal is a powerful tool for studying instantaneous physical processes and one of the important means to obtain scientific mysteries. In short, no matter in which application field, the more timely the data collection and processing, the higher the work efficiency, performance and price ratio, and the better the economic benefits achieved. The working site of the data acquisition and processing system is generally harsh, and it is filled with various interferences (from inside and outside the system). These interferences are usually called noise. When the signal under test is very weak, it will be "submerged" by the noise, resulting in large data collection and processing errors, reduced reliability, and may also cause system failure and even lead to major accidents. Therefore, noise is one of the main obstacles for data collection and processing. In order to accurately collect and process data, the impact of various interferences on the system must be considered, and the anti-interference problem is a crucial content of system design. , Take corresponding measures from three aspects of software and hardware design and EDA simulation technology to enhance the anti-interference ability of the system.

Figure 1 Block diagram of data acquisition and processing system

1. The main source of interference

There are many interference sources in the working environment of the data acquisition and processing system, each with its own characteristics. The following is a classification from different perspectives:

1.1 Classification from sources of interference

1.1.1 Internal interference

Refers to various interferences in the internal electronic circuit of the system, such as parameter changes caused by aging of components, thermal noise of resistors, internal distribution noise and flicker noise of devices such as transistors and field effect tubes, and self-oscillation caused by positive feedback of the amplifier circuit Wait.

1.1.2 External interference

Refers to all kinds of interference that the outside world has entered into the system. Such as electric sparks caused by motor brushes, electromagnetic signals generated by pulse switch contacts of other equipment, lightning in nature, electromagnetic waves radiated by the universe, etc.

1.2 According to the law of interference

1.2.1 Fixed interference

Refers to the interference caused by the operation of fixed electrical equipment near the system. Interference caused by the start and stop of nearby "strong electricity" equipment at a fixed moment.

1.2.2 Semi-fixed interference

Refers to the interference caused by some accidentally used electrical equipment (such as driving, electric drill, etc.), with predictability.

1.2.3 Random interference

Refers to sporadic disturbances, such as lightning, power supply system relay protection and other disturbances, it is difficult to predict the moment of occurrence.

1.3 Classification from the way interference is generated and propagated

1.3.1 Electrostatic interference

Refers to the interference of the electric field through capacitive coupling, which is caused by the parasitic capacitance between the components and the wires. In addition, it also includes the friction between chemical fibers and fibers to make the human body electrified, so that the human body can interfere with electronic equipment.

1.3.2 Magnetic field coupling interference

Magnetic field coupling interference is an inductive interference. It is the AC interference voltage induced on the transmission line or the closed wire due to the alternating magnetic field generated by the power line, transformer, motor, relay, electric fan, etc. passing through the loop formed by the transmission line or the closed wire.

1.3.3 Electromagnetic radiation interference

Interference caused by the radiation of high-frequency electromagnetic waves generated by various high-power high-frequency, intermediate-frequency generating devices and electric sparks into the surrounding space.

1.3.4 Coupling interference in the conductance path

Refers to the interference caused by the conductance path on the common impedance between each unit circuit due to the difference in ground potential. Because it is a multiple ground point, a circulating current will be formed on the ground loop. This circulating current couples the transient noise interference to the next stage circuit through the ground loop impedance.

1.3.5 Leakage coupling interference

It is due to the poor insulation of the circuit inside the instrument, and the interference caused by the resistance coupling caused by the leakage current; it may also be a system composed of high input impedance devices, because its impedance is comparable to the insulation resistance of the circuit board. Interference caused by leakage current.

1.4 Division from the relationship of interfering input signals

1.4.1 Series mode (differential mode) interference

Series-mode interference means that the interfering signal and the signal under test are connected in series. It becomes a part of the signal under test and is sent to the amplifier for amplification, which has a great influence. Cause: The alternating electromagnetic field of the external high-voltage power supply line is coupled into the sensor end through the parasitic capacitance; the alternating electromagnetic field of the power supply is coupled to the leakage current of the sensor end.

1.4.2 Common mode interference

Common mode interference refers to the interference between the signal ground and the instrument ground (earth). Causes: (1) There are high-power electrical equipment near the data collection system, and the electromagnetic field is coupled to the sensor and the transmission wire in the form of inductance or capacitance; (2) The leakage or three-phase power grid load caused by poor power supply insulation is not When the balance causes a large current in the neutral line, there is a large ground current and ground potential difference. If the system has more than two grounding points, the ground potential difference will cause common mode interference; (3) When the insulation performance of electrical equipment is poor, the power supply will be coupled to the signal loop of the data acquisition system through the leakage resistance, forming interference; (4) In AC-powered instruments, the AC current will form a loop to the ground through the parasitic capacitance between the primary and secondary windings, the rectifier filter circuit, the signal circuit and the ground, and form interference.

1.5 Sources of software interference

It is mainly manifested in the following aspects: (1) Incorrect algorithms produce erroneous results. The main reason is that the exponential operation of the program in the computer processor is an approximate calculation, and the resulting results sometimes have large errors and are easy to produce Misoperation; (2) Because the accuracy of the computer is not high, and the order must be aligned when adding and subtracting, the large number "eats" the decimal, resulting in error accumulation, which leads to the occurrence of underflow and is also one of the sources of noise; (3) Since the computer processor is a high-speed digital device, its arithmetic unit, controller and control register are susceptible to electromagnetic interference. The computer caused by the interference of the above hardware, such as: program counter PC value change, data acquisition error increase, control state failure, RAM data interference change and system "deadlock" and other phenomena.

2. Hardware measures against system interference

The general policy is: shielding, grounding, filtering, isolation and absorption. The anti-interference measures of each part of the system are introduced below:

2.1 Anti-interference of power supply system

The power supply part is a very important part. Most of the equipment in the system uses 220V, 50HZ city power. Due to the large frequency and voltage fluctuations of the power grid, it will directly interfere with the data acquisition system. Some measures can be taken:

2.1.1 Using isolation transformer

Since the data acquisition system and the power grid have their own ground wires, if the two are directly connected, there is a potential difference between their ground wires, which will form a circulating current and cause common mode interference. An isolation transformer is used to isolate the two. And in order to eliminate high-frequency noise, the primary and secondary of the transformer are separated by shielding layer. This reduces parasitic capacitance and improves resistance to common-mode interference.

2.1.2 Using power supply low-pass filter

Since most of the interference of the power grid is high-order harmonics, a low-pass filter is used to filter out high-order harmonics greater than 50 Hz to improve the performance of the power supply. In order to prevent the filter from entering the magnetic saturation, a distributed parameter noise attenuator (which consists of nearly 50 meters of twisted pair) should be added in front of the filter. When using a low-pass filter, the following points should be noted: (1) The low-pass filter itself should be shielded, and the shielding box must maintain good contact with the system cabinet; (2) To reduce coupling, the wires used should be close to the ground Line; (3) The input and output of the filter should be isolated; (4) The position of the filter should be as close as possible to the place where the filter is needed, and the connection between it should also be shielded. (5) Power filter is constructed with advanced magnetic powder core material.

2.1.3 Using AC voltage stabilizer

To prevent overvoltage and undervoltage of the power supply, a voltage regulator with sufficient output power must be used.

2.1.4 Separate power supply for the system

In order to prevent the interference from the power supply system, the method of using an AC stabilized power supply in series with an isolation transformer, distributed parameter noise attenuator and low-pass filter; when inductive equipment is used in the system, the data acquisition system and inductive equipment should be The power supply system is separated to avoid mutual interference between the power supply lines; also pay attention to the shielding of the transformer and the low-pass filter to suppress electrostatic interference.

2.1.5 Use power module to supply power separately

Modern chip manufacturing technology is developing rapidly, and various power supply modules are emerging, such as DC-DC, three-terminal voltage regulator and other modules. The use of separate power supply has the following advantages: (1) Each power module separately protects the corresponding board from voltage overload, and will not paralyze the system due to the failure of a certain voltage regulator; (2) It is helpful to reduce the mutual impedance Coupling and mutual coupling of public power supplies greatly improve the reliability of the power supply system and also contribute to the heat dissipation of the power supply; (3) Changes in the voltage on the bus will not affect the voltage on the board, which helps to improve the reliability of the board. Sex.

2.1.6 Reasonable wiring of power supply system

The layout of the power inlet line, output line and common line of the data acquisition system should follow the following points: (1) The feed line from the power inlet through the switching device to the low-pass filter should use thick wires as much as possible; (2) Behind the power supply The section of the application of twisted pair should be short and must be separated wiring; (3) Try to avoid public lines.

2.1.7 Other technologies

Gas discharge tube, varistor, TVS and other components are used for protection. It is also possible to apply shielding layer or metal fiber on the shell of the device.

2.2 Anti-interference in the analog part between data acquisition card, computer, multi-way switch, A / D converter and sensor

Some anti-interference methods in this part have common features with the anti-interference methods of the power supply, so only different places will be introduced.

2.2.1 Using isolation technology

Including: (1) photoelectric isolation (use photoelectric coupling device to achieve isolation on the circuit); (2) electromagnetic isolation (add an isolation amplifier between the sensor and the acquisition circuit).

2.2.2 Use filters to filter out interference

Add a filter on the signal transmission line. However, it should be noted that weak signals are not used because they are greatly attenuated.

2.2.3 Using floating measures to suppress interference

That is, the analog signal of the data acquisition circuit is not connected to the chassis or the ground, blocking the path of the interference current.

2.3 Anti-interference of printed circuit board

Pay attention to the following points: (1) Reasonably arrange the components on the board; (2) Reasonably distribute the pins on the board; (3) Reasonable wiring, the power cord should be wide; (4) The shielding method should also be adopted for the printed circuit board.

2.4 The interference of digital parts such as the peripheral circuits, interface circuits, digital acquisition circuits of the computer

The following measures are adopted: (1) use integrated circuit to suppress interference; (2) use pulse isolation gate to suppress interference; (3) use clipper to suppress interference.

2.5 Grounding technology

The principles to be followed are: (1) One-point grounding principle: ground one point at the input to avoid common mode interference. The digital ground and the analog ground in the circuit are only connected at one point; (2) the principle of multi-point ground: the shorter the ground wire, the better, and each circuit should be grounded nearby; (3) the wiring principle of different nature ground wires: Weak signal analog, digital circuit and high-power drive ground are separated, analog ground is separated from digital ground, high-level digital ground is separated from low-level signal, and each subsystem ground is only connected to the ground at the power supply point; (4) The ground wire should be as thick as possible.

2.6 Other hardware anti-interference methods

A watchdog (WDT) circuit is added to the computer (single-chip computer), which can effectively prevent the program from "running away" and the system from "freezing". Use digital multi-way switches instead of mechanical switches to avoid spark interference. Replacing the traditional analog filter with a digital filter (which uses a software filtering method) can improve the anti-interference strength.

3. Anti-interference software measures of the system

3.1 The method of intercepting out-of-control procedures

Mainly include: (1) Single-byte instructions should be used in program design, and some no-operation instructions should be inserted at key positions, or valid single-byte instructions should be repeated several times, so that subsequent instructions can be protected from being dismantled. Make the program run on the right track; (2) Add a software trap: when the PC value is out of control and the program is out of control, the CPU enters the non-program area. At this time, a boot instruction can be used to force the program to enter the initial entry state and enter the program area. A trap can be set every other period. (3) Software reset: When the program is "flying", run the monitoring system to automatically reset and re-initialize the system.

3.2 Establishing logo judgment

Define a unit as a flag, set the value of the unit as a characteristic value in the main program of the module, and then determine whether the value of the unit remains unchanged at the end of the main program. If the description is incorrect, the program will transfer to error handling Subroutine.

3.3 Increase data security backup

Important data is stored in more than two storage areas, and large-capacity external RAM can also be used to back up the data. The permanent data is made into a table and solidified in EPROM, so that it can not only prevent the data and the table from being destroyed, but also ensure that the program logic is not confused when the data is used as an instruction to run. Write a special data protection subroutine. For the data whose nature is changed by the instruction, save it as much as possible after each change, so that the correct value can be restored when needed.

3.4 Using active initialization and data redundancy technology

Set the various functional ports or methods and states of the single-chip microcomputer and the off-chip expander to ensure that the software can correctly initialize all levels after power-on or reset, and re-correlate the corresponding function before the program uses each function The control register of the device sets the operation mode; add redundant bits to the important data to extend the hamming distance between the data and the code to enhance the ability to detect and correct errors.

3.5 Adopt on-chip soft "watchdog" (WDT) and repeated execution program technology

The idle timer / counter inside the microprocessor is used to form a WDT with an appropriate program. When the system is abnormal, it can effectively and effectively force a "soft reset" to restore the system to normal work; design important instructions into a regular scan module to make it Repeated execution throughout the entire program cycle, so that even if the interference signal rewrites the instruction content, it can automatically return to normal within the reaction time of the controlled data acquisition system. Take the method of reading the input pin data multiple times, and take the average value of the multiple readings as the correct data for the pin.

3.6 Setting method for unused memory area

In order to prevent the program counter from jumping into the unused memory area and malfunctioning, an instruction like "SWI" is added to the unused memory area to fill it. In this way, after the PC value is destroyed and jumps into these areas, it will read the SWI vector, and set the vector to jump to the beginning of the program in advance, so the program runs normally.

4. Use EDA software to simulate the EMC performance of the system during PCB design

In the PCB design, the main considerations from the EMC perspective are: (1) To ensure the signal integrity of the wiring network itself, the electromagnetic radiation, electromagnetic interference that may be generated in the PCB, and the ability of the PCB itself to resist external electromagnetic interference; (2 ) Put forward the principles of layout and wiring according to the requirements of the designer. The methods adopted are: (1) Use the support of databases such as IBIS and SPICE to analyze the electrical characteristics of IBIS (freely provided) data of many components on the circuit board. For the purpose of electromagnetic compatibility, use EDA tools for simulation . (2) Organically combine the designer's experience with the EDA tool simulation technology, and consider the EMC performance factors of the system during the layout and wiring stages. For example, the software package EMC-WORKBENCH of the German INCASES company can be used, which can meet the EMC requirements of circuit designers, improve the PCB design process, and simplify the complicated hardware debugging work in the later stage.

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