Research on electromagnetic compatibility issues in infrared imaging tracking system

1 Introduction

Electromagnetic compatibility (EMC) is a marginal subject based on electromagnetic field theory, including information, electrical engineering, electronics, communications, materials, structures and other disciplines. It is also an applied science and technology that studies the compatibility of various electrical and electronic equipment or systems in the same electromagnetic environment without causing performance degradation under limited space, time and frequency resources [1]. With the rapid development of large-scale integrated circuits and power electronics technology, motors and their control systems have become a comprehensive automation system that integrates motors, power electronics and microelectronics, which has also doubled the complexity of the system's electromagnetic environment. In this complex electromagnetic environment, how to minimize the electromagnetic impact between various devices and ensure the normal operation of the system and various devices is a topic that needs in-depth research. This paper takes a certain infrared imaging and tracking system as the research object, analyzes its electromagnetic environment, and studies and improves the electromagnetic compatibility problems in its design and operation process, solving the instability problems of image transmission and target tracking.

2 Introduction to infrared imaging tracking system

The infrared imaging and tracking system is a platform system that integrates optics, mechanics, and electronics. Its main function is to perform stable imaging and tracking of infrared targets. The system structure block diagram is shown in Figure 1.

The platform adopts a three-degree-of-freedom frame structure, which can realize the movement on the roll, pitch and azimuth axes. The infrared imaging component is installed on the platform to sense the infrared red radiation of the target and background and convert it into electrical signals to send to the signal processing circuit. The signal processing circuit distinguishes the target and the background in the real-time image, intercepts the target, tracks the target image, sends the tracking error signal to the platform control circuit, and sends the image signal to the display for real-time display. The platform control circuit forms a control signal according to the target angle error signal, drives the platform to rotate after power amplification, ensures stable tracking of the target and makes the target image located in the center of the image. The rate gyro and potentiometer sense the movement of the platform, and feed back the speed and position information of the platform movement to the platform control circuit to realize closed-loop control. In addition to the platform and the motor, rate gyro, potentiometer and infrared imaging component installed on the platform, the entire system is installed in a cabin with a length, width and height of 40cm×20cm×20cm respectively. All power and signal routing are located in two wiring grooves on the upper and lower surfaces of the cabin. The longitudinal and cross-sectional schematic diagrams of the cabin are shown in Figure 2. In this system, there are both high-frequency signal circuits and low-frequency signal circuits, both strong current circuits and weak current circuits, both circuits with frequent switching actions and weak signal circuits that are extremely sensitive to disturbances. The electromagnetic environment is relatively complex. If electromagnetic compatibility is not considered during the design, the performance of the system will inevitably be greatly affected by electromagnetic interference and will even fail to work normally.

3 System Electromagnetic Environment Analysis

Theoretical and practical research has shown that, regardless of whether it is a complex system or a simple device, the occurrence of any electromagnetic interference must meet three basic conditions: first, there should be an interference source, second, there should be an interference propagation path, and finally, there must be a response from the interfered object (sensitive equipment) [1]. Therefore, the interference source, propagation path, and sensitive equipment are collectively referred to as the three elements of electromagnetic interference. In the process of analyzing the electromagnetic environment of the system in this article, we focus more on the system level and analyze the electromagnetic interference and electromagnetic compatibility issues between various functional circuits (such as between power amplifier circuits and signal processing circuits).

In this system, the power amplifier and torque motor are strong sources of electromagnetic interference.

This system is a high-precision servo control system with high requirements for positioning accuracy and response speed. Therefore, the power amplifier circuit part adopts a pulse width modulation power amplifier based on power electronic devices, referred to as a pwm power amplifier, which can achieve a wide range of speed and position control and has excellent performance. The pwm power amplifier uses the switching characteristics of fully controlled power electronic devices to modulate the fixed voltage DC power supply. The power electronic devices are connected and disconnected at a fixed frequency, and the length of "connection" and "disconnection" in a cycle is changed as needed. The average voltage is changed by changing the "duty cycle" of the voltage on the armature of the DC servo motor, thereby controlling the speed of the motor. Therefore, this pwm power amplifier is also called a "switch drive device." Since the PWM power amplifier controls the motor speed by periodically operating the power electronic devices in the on-off state, the high-voltage switching of the main circuit power conversion circuit is inevitable. The voltage at both ends of the armature is pulsed, and the amplitude of the voltage change rate du/dt is generally very high. The frequency of the high-voltage switch pulse is high and the edges before and after the pulse are steep, which produces abundant high-order harmonics. The armature current of the motor is triangular, and the current change rate di/dt is also very large during the switch switching. The magnetic or electromagnetic interference caused by the large pulse current flows through the loop formed by the large amplitude and fast-changing current loop and the ground, which will produce magnetic field coupling and form serious interference. The influence of parasitic capacitance and wiring inductance on the circuit that were not described in the original circuit cannot be ignored [3]. The harmonics and interference generated by the power amplifier circuit are coupled to the control circuit logic elements of the servo system and the input end of other systems through the distributed inductance and distributed capacitance of the wires and circuits, affecting the stability and normal operation of other circuits.

In order to meet the system's requirements for tracking speed, this system uses a brushed DC torque motor to drive the platform. Its normal operating current is about 2 amperes, and its stall current can reach more than 3 amperes. During the operation of the DC motor, the current in the coil is constantly changing direction, which will cause electromagnetic wave radiation. When the motor speed is very high, the frequency of the radiated electromagnetic wave will also be higher [8]. In addition, the brushed DC motor has a commutator and brushes. The rotor coil is connected to the brushes and the external power supply through the commutator. During high-speed rotation, the brushes are constantly connected and disconnected. If the current does not have time to reverse, a large amount of charge will accumulate between the commutator and the brushes in an instant, which is very likely to generate brush arcs and form spark disturbances. In essence, it is a current mutation with a small time interval and irregularity. The motor is an inductive load. When the power supply of the load is switched, a high reverse voltage several to dozens of times higher than the power supply voltage will be generated at both ends of the inductive coil. Spark disturbance is a broadband pulse disturbance. The high reverse voltage is a conduction voltage pulse with a very steep front edge. It has the characteristics of wide spectrum and rich harmonics. It has strong radiation disturbance in the frequency range of tens of megahertz to thousands of megahertz [9]. These stray electromagnetic waves are radiated and propagated along the motor housing, power lines, stray inductance and stray capacitance, causing pollution to the electromagnetic environment and affecting the normal operation of other equipment.

Analysis of interfered objects: In this system, the small signal circuit mainly includes the analog video signal output by the infrared imaging component and the sensor signal of the sensitive platform movement. The output voltage level of the infrared imaging component used in this system is about hundreds of millivolts. When the platform position deviation angle and the platform movement angular velocity are not large, the output signals of the potentiometer and gyroscope are also small signals. These small signals are very susceptible to interference, affecting the stability of tracking and even causing the system to malfunction. The quantitative description of the circuit's susceptibility to interference can be expressed by sensitivity. The higher the sensitivity, the lower the circuit's sensitivity level and the worse the anti-interference ability. The definition of the maximum sensitivity index of analog circuit systems and digital circuit systems can be found in reference [6].

Coupling is the channel between the interference source and the sensitive equipment. There are three forms of coupling: direct coupling, electromagnetic induction coupling and radiation coupling. In this system, there is a direct interconnection of signal lines between the signal processing circuit, platform control circuit, power amplifier circuit, motor and sensor. There is also a common impedance between some circuits. At the same time, the f/d noise current of the power electronic device in the power amplifier circuit during the high and low level conversion will cause the ground line to jump, interfering with the level of the digital circuit.

The level fluctuation caused by the spark discharge of the motor brush propagates outward through the power line and the ground line. These factors form a direct coupling channel between the interference source and the sensitive equipment. In the system, high-frequency signals and low-frequency signals coexist, strong electric signals and weak electric signals coexist, and switch signals and sensitive signals coexist. The routing of all these signals is summarized in the wiring trough and then connected to the potentiometer, gyroscope, motor and infrared imaging components on the platform. Due to the high degree of miniaturization and integration of the system, the space available for routing is limited, so it is inevitable that these signal lines are parallel and close to each other, forming electromagnetic induction coupling between each other, causing interference of large signals on small signals and interference of high-frequency signals on low-frequency signals. The cabin space not only limits the size of each circuit board, making it impossible for the PCB routing spacing to be large, but also makes each circuit board close to each other, increasing the distributed parameters between routings, between routings and components, and between circuits. These distributed parameters enhance the radiation coupling within the system and form a radiation coupling channel.

During the initial joint debugging of the system, the following problems were found:

(1) The PWM waveform quality is poor

Intermittent interference noise appears in the low-level part of the power amplifier output PWM waveform, which is manifested as irregular burrs with a relatively fixed frequency. Along with the appearance of noise, you can often hear the "buzzing" sound of the torque motor brush ignition, and the output torque of the motor decreases. When the burr amplitude is large and appears frequently, the output torque loss of the motor is very large.

(2) Image flashes white

During the process of circular tracking of simulated targets, the display image shows white flickering phenomenon, which is manifested as flickering bright spots and multiple irregular horizontal stripes in the image, and the image clarity is reduced. As the tracking speed increases, the white flickering phenomenon becomes more serious and occurs more frequently. It not only brings difficulties to image processing, but also affects the stability of tracking. Appropriate anti-electromagnetic interference measures must be taken.

(3) Unstable tracking

When the tracking speed increases, the platform will intermittently shake on the pitch and azimuth axes, causing the image to shake. The system is a closed-loop tracking system, and the image shaking exacerbates the shaking of the platform frame. The faster the tracking speed, the more frequent the shaking and the more severe the shaking amplitude. When the tracking speed reaches a certain value, it can no longer guarantee that the image is always in the center of the field of view. The shaking of the platform frame sometimes causes the image to suddenly escape the field of view. The escape speed exceeds the maximum tracking speed of the system, causing the target to be lost during the tracking process.

4 Electromagnetic Interference Suppression Technology

Electromagnetic interference suppression technology is centered around the three elements of electromagnetic interference. According to the specific situation, targeted measures are taken. In summary, there are three points: first, suppress the source of electromagnetic interference; second, cut off the electromagnetic interference coupling path; third, reduce the sensitivity of electromagnetic sensitive equipment. According to the electromagnetic compatibility theory and the analysis of the electromagnetic environment of this system, we have taken the following measures for the interference source, interference coupling path and sensitive equipment:

(1) Suppress interference sources

According to the synchronization of the PWM waveform interference noise output by the power amplifier and the sparking of the motor brush, it is judged that the PWM waveform interference is caused by the sparking of the motor brush. For this reason, we add a ceramic capacitor between the motor brushes to play a filtering role, as shown in Figure 3. The capacitance of the ceramic capacitor can generally be 10nf or 100nf.

In order to suppress the di/dt, du/dt and voltage and current surges generated by the switching of power electronic devices in the power conversion circuit of the PWM power amplifier and reduce the impact of circuit spike noise on other circuits, we designed an RC snubber network for the PWM power amplifier, as shown in Figure 4. The snubber network changes the switching trajectory of the power electronic devices, reduces di/dt, du/dt and voltage and current surges, and plays a role in suppressing interference [3]. The resistors in the snubber network should be power resistors. Generally, resistors with a power of more than 1W and a resistance of one hundred to several hundred ohms can meet the requirements. According to the voltage and power requirements of the motor drive of this system, a power resistor of 1W/100ω is selected here. There are also certain rules for selecting the capacitance value of the capacitor. If the capacitance value is too small, the absorption effect on interference is not obvious. If the capacitance value is too large, the energy consumed by the resistor in the snubber network will increase. If the rated power of the selected resistor is not large enough, the resistor may be burned out. It has been verified through experiments that when the power resistor is 1W/100ω, the capacitance should be less than 100nF. A 1nF multilayer ceramic capacitor is selected here, and a capacitor with a larger capacitance can also be selected according to the actual filtering effect.

The design of the power amplifier circuit PCB in the system is a very worthy issue to discuss, because in the power amplifier circuit, not only are there many high-power signals, but also the signal voltage and current are relatively large (the high and low voltages output by the PWM power amplifier in this system are 27V and 0V respectively, and the output current reaches more than 2 amps). In addition, the power amplifier output signal also has very high di/dt, du/dt, surge voltage, and surge current. If the PCB layout and routing are not carefully designed, it may cause greater electromagnetic interference to other circuits, and even make its own normal operation impossible. Based on the author's design experience, the PCB was designed for electromagnetic compatibility to improve the circuit performance. These designs include:

Try to use multi-layer circuit boards. The power amplifier circuit PCB here uses a four-layer board structure, with separate power and ground layers designed, and single-point grounding. This has many advantages: it helps to reduce the signal loop area; it helps to enhance the electromagnetic shielding effect of the PCB; it helps to increase the spacing of large signal traces. If the device density on the board is low enough, you can even use the upper and lower surfaces of the PCB as ground planes, and the middle two layers as signal layers and power layers. The power supply on the signal layer is routed with wide lines, which can make the path impedance of the power supply current lower, and the impedance of the signal path is also lower, enhancing the suppression effect of electromagnetic interference [1].

Different functional circuits should be divided into different areas, analog circuits and digital circuits should be divided into different areas to minimize signal cross-connection. Large signal circuits and small signal circuits should be divided into different areas to minimize coupling between them.

When routing, attention should be paid to minimizing the signal loop area and increasing the spacing between signal lines, especially between large and small signals, and between switch signals and analog signals. At the same time, large signals should be routed with wide lines to reduce the length of the lines and avoid long parallel sections between two signal lines to reduce mutual inductance. The input and output signals of the power amplifier should not be routed in parallel as much as possible.

(2) Cut off the interference coupling channel

The motor and PWM power amplifier circuit in this system are the biggest sources of electromagnetic interference. First, the position and spacing of each circuit board in the cabin are reasonably arranged, and the power amplifier circuit board is as far away from other circuits as possible. Secondly, a separate power supply and ground system is designed for the power amplifier and motor circuit using a DC/DC module. Avoid the interference generated by them from being directly coupled to sensitive equipment through the power line and ground line. In addition, the signal transmission between the signal processing circuit and the platform control circuit, and the platform control circuit and the power amplifier circuit is carried out through optical coupling to block the electrical connection between different circuits and isolate noise and interference. The wiring of the power amplifier circuit and other circuit signals is arranged in different wiring grooves on the cabin body, especially the power amplifier output signal and the analog video signal, and the platform motion sensor output signal must be wired separately to reduce the electromagnetic coupling between them. Even so, at the connection part between the cabin body and the platform body, the wiring must still be gathered together, so it is necessary to perform electromagnetic shielding on each signal line. In this system, we added a metal wire mesh shielding sleeve to the outside of each signal line, and single-ended grounding of the shielding layer. Finally, a common-mode choke is connected in series to the input line of the motor, and a twisted pair is used to reduce the electromagnetic interference of the motor.

On the circuit board, different types of signals should use different connectors as much as possible. In particular, weak signal lines should not share the same connector with strong signal lines and power lines. When different types of signals share the same connector, there should be enough ground pins to isolate different types of signals. To ensure the continuity of the shield, the shielding layers of different types of signal lines should be connected to different pins [2].

(3) Reduce the sensitivity of sensitive equipment

The sensitive devices of this system mainly include image processing circuits and sensor signal processing circuits. The following measures are taken to reduce the sensitivity of these circuits to electromagnetic interference: select low-noise amplifier devices and low-noise resistors and capacitors; minimize the system bandwidth while meeting system requirements; select low-noise power supplies and adopt power supply filtering, ripple suppression and other measures; reasonably design grounding and circuit wiring; adopt isolation and decoupling technologies such as isolated power supplies and optical couplers [2]; and use twisted pair differential transmission or coaxial cable single-ended transmission for small signals.

After taking the above measures, the electromagnetic compatibility of the system has been greatly improved, the quality of the PWM waveform output by the power amplifier has been significantly improved, and the burr noise has disappeared; the image flash problem and tracking instability problem have also been well solved. Even at the maximum tracking speed, the system can still stably track and output high-quality target images.

5 Conclusion

In high-precision servo control systems, pulse width modulation power amplifiers based on power electronic devices are mostly used. The electromagnetic compatibility issue is a very concerning issue here. Only by comprehensively applying various technical means such as grounding, filtering and shielding in electromagnetic compatibility theory can we ensure that the increasingly integrated electronic systems can operate normally and have good performance in a complex electromagnetic environment.