Requirements for anti-interference of industrial electronic scales and handling of complex working conditions of mixing systems

Preface
As a core component of industrial automation, weighing instruments are different from commercial scales and often face more complex working conditions. In the case of a relatively harsh electromagnetic environment in a mixing plant, some large-scale integrated circuits are often interfered with, resulting in malfunction or operation in an erroneous state, and the consequences are often very serious. Therefore, understanding the anti-interference performance is the key to selecting weighing instruments. In the process of repeatedly comparing UNI800 and TR600 produced by Zhuhai Changlu Industrial Automatic Control System Co., Ltd. and other similar manufacturers' products, we have gained analytical experience in the anti-interference performance that a good single-chip microcomputer system (weighing instrument) should have. I would like to share this with my peers in the hope of promoting the improvement of the industry's technical level.

Electromagnetic compatibility (EMC) of instruments is an important indicator, which includes two aspects of the system's emission and sensitivity. If a single-chip microcomputer system meets the following three conditions, the system is electromagnetically compatible:

1. No interference to other systems;

2. Insensitive to the emission of other systems;

3. No interference to the system itself;

if the interference cannot be completely eliminated, it should be minimized. Interference is generated either directly (through conductors, common impedance coupling, etc.) or indirectly (through crosstalk or radiation coupling). Electromagnetic interference is generated through conductors and through radiation. Many electromagnetic emission sources, such as light, relays, DC motors and fluorescent lamps, can cause interference; AC power lines, interconnect cables, metal cables and internal circuits of subsystems can also radiate or receive unwanted signals. In high-speed microcontroller systems, clock circuits are usually the largest source of broadband noise. These circuits can generate harmonic distortion up to 300MHz and should be removed from the system. In addition, the reset line, interrupt line and control line are the most susceptible in the microcontroller system.

1. Interference coupling mode

(1) Conducted EMI

One of the most obvious and often overlooked paths that can cause noise in a circuit is through conductors. A wire passing through a noisy environment can pick up noise and send it to other circuits to cause interference. Designers must avoid wires picking up noise and use decoupling methods to remove noise before it causes interference. The most common example is noise entering the circuit through the power supply. If the power supply itself or other circuits connected to the power supply are interference sources, the power line must be decoupled before entering the circuit.

(2) Common impedance coupling

[page] Common impedance coupling occurs when currents from two different circuits flow through a common impedance. The voltage drop across the impedance is determined by the two circuits, and the ground currents from the two circuits flow through the common ground impedance. The ground potential of circuit a is modulated by current b, and the noise signal or DC compensation is coupled from circuit b to circuit a through the common ground impedance.

(3) Radiated coupling

Coupling through radiation is generally called crosstalk. Crosstalk occurs when current flows through a conductor and generates an electromagnetic field, which induces transient currents in adjacent conductors.

(4) Radiated

emission There are two basic types of radiated emissions; differential mode (DM) and common mode (CM). Common mode radiation or monopole antenna radiation is caused by an unintentional voltage drop, which raises all ground connections in the circuit above the system potential. In terms of electric field size, CM radiation is a more serious problem than DM radiation. To minimize CM radiation, practical design must be used to reduce common mode current to zero.

2. Factors affecting EMC

(1) Voltage.

The higher the power supply voltage, the greater the voltage amplitude, the more emissions, and low power supply voltage affects sensitivity.

(2) Frequency. High frequencies generate more emissions, and periodic signals generate more emissions. In high-frequency microcontroller systems, current spikes are generated when the device switches; in analog systems, current spikes are generated when the load current changes.

(3) Grounding. Among all EMC problems, the main problem is caused by improper grounding. There are three signal grounding methods: single-point, multi-point, and mixed. When the frequency is below 1MHz, the single-point grounding method can be used, but it is not suitable for high frequencies; in high-frequency applications, it is best to use multi-point grounding. Mixed grounding is a method of using single-point grounding for low frequencies and multi-point grounding for high frequencies. Ground layout is critical, and the grounding circuits of high-frequency digital circuits and low-level analog circuits must not be mixed.

(4) PCB design. Proper printed circuit board (PCB) wiring is crucial to preventing EMI.

[page] (5) Power supply decoupling. When the device switches, transient currents will be generated on the power line, and these transient currents must be attenuated and filtered out. Transient currents from high di/dt sources cause ground and traces to "emit" voltages. High di/dt generates a wide range of high-frequency currents, which excite components and cables to radiate. Current changes and inductance flowing through the wires will cause voltage drops. Reducing inductance or current changes over time can minimize this voltage drop.

3. Hardware requirements for weighing instruments to resist interference and handle complex working conditions

In terms of hardware, we require instrument manufacturers to have the following measures:

(1) PCB and circuit anti-interference measures

The anti-interference design of printed circuit boards is closely related to the specific circuits. Here we will only explain several common measures for PCB anti-interference design.

① Power line design

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance; at the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which will help enhance the anti-noise ability.

② Ground line design

In the design of single-chip microcomputer systems, grounding is an important method to control interference. If grounding and shielding can be used correctly, most interference problems can be solved. The ground structure in the single-chip computer system generally includes system ground, chassis ground (shielded ground), digital ground (logic ground) and analog ground.

The following points should be noted in the ground design:

a. Correctly select single-point grounding and multi-point grounding. In low-frequency circuits, the operating frequency of the signal is less than 1MHz, and the inductance between its wiring and devices has little effect, while the loop formed by the grounding circuit has a greater impact on interference, so a single-point grounding method is used. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible, and multi-point grounding should be used nearby. When the operating frequency is between 1 and 10MHz, if a single-point grounding is used, the ground length should not exceed 1/20 of the wavelength, otherwise a multi-point grounding method should be used.

b. Digital ground and analog ground are separated. There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed. They should be connected to the ground wire of the power supply end respectively. The ground of the low-frequency circuit should be connected to the ground at a single point as much as possible. When the actual wiring is difficult, it can be connected in series and then connected to the ground in parallel; a large area of ​​grid-shaped ground foil should be used around the high-frequency components as much as possible, and the grounding area of ​​the linear circuit should be

increased as much as possible. C. The ground wire should be as thick as possible. If the ground wire uses a very thin line, the ground potential will change with the change of current, causing the timing signal level of the electronic product to be unstable and the anti-noise performance to be reduced.

Therefore, the ground wire should be as thick as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

d. The ground wire forms a closed loop. When designing the ground wire system of a printed circuit board composed only of digital circuits, making the ground wire into a closed circuit can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power. Due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground wire, causing the anti-noise ability to decrease; if the ground wire is formed into a loop, the potential difference will be reduced, and the anti-noise ability of the electronic equipment will be improved.

[page] ③ Decoupling capacitor configuration

One of the conventional practices in PCB design is to configure appropriate decoupling capacitors at various key parts of the printed board. The general configuration principle of decoupling capacitors is:

a. Connect a 10~100μF electrolytic capacitor across the power input terminal. If possible, it is better to connect more than 100μF.

b. In principle, each integrated circuit chip should be arranged with a 0.01pF ceramic capacitor. If the printed board space is not enough, a 1~10pF tantalum capacitor can be arranged for every 4~8 chips.

c. For devices with weak anti-noise ability and large power supply changes when turned off, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the power line and the ground line of the chip.

d. The capacitor lead cannot be too long, especially the high-frequency bypass capacitor cannot have leads.

In addition, the following two points should be noted:

a. When there are contactors, relays, buttons and other components in the printed circuit board, they will generate large spark discharges when they are operated, and an RC circuit must be used to absorb the discharge current. Generally, R is 1~2kΩ and C is 2.2~47μF.

b. The input impedance of CMOS is very high and is easily inductive. Therefore, when using it, the unused end should be grounded or connected to the positive power supply.

(2) Input/output electromagnetic compatibility design

In the single-chip microcomputer system, the input/output is also the transmission line of the interference source and the pickup source of the receiving radio frequency interference signal. When designing the weighing instrument, effective measures should generally be taken:

①. Use necessary common mode/differential mode suppression circuits, and also take certain filtering and anti-electromagnetic shielding measures to reduce the entry of interference.

②. Take various isolation measures (such as photoelectric isolation or magnetoelectric isolation) as much as possible when conditions permit, so as to block the propagation of interference.

(3) Design of MCU reset circuit

In the MCU system, the watchdog system plays a particularly important role in the operation of the entire MCU, because all interference sources cannot be completely isolated or removed. Once the CPU interferes with the normal operation of the program, the reset system combined with software processing measures becomes an effective error correction defense barrier. There are two commonly used reset systems:

①. External reset system. The external "watchdog" circuit can be designed by yourself or with a dedicated <