Today, servo systems , like CNC, PLC, and frequency converters, are widely used in various fields of industrial production. Servo drives, as power conversion devices integrating high-voltage and low-voltage, digital and analog signals, inevitably suffer from electromagnetic interference. The mandatory implementation of the YY0505-2012 standard in 2015 has further increased the medical device industry's focus on electromagnetic compatibility issues.
This article addresses the electromagnetic compatibility (EMC) issues of servo systems by focusing on several aspects, including the types of EMC interference, risk assessment, and mitigation methods of a medical robot based on a servo system designed by a certain company.
Types and risks of interference from medical robots
Radiation interference from space
The distribution of radiative interference from space is extremely complex, and it usually propagates through space in the form of electromagnetic induction. This type of interference is ubiquitous, just like air; for example, when using a mobile phone, television images will produce static. The prohibition of mobile phone use during flight is also based on considerations of avoiding radiative interference.
Therefore, the interference in medical standard YY0505 mainly focuses on radiation immunity. The risk lies in the possibility that radiation interference may cause malfunctions in the PC, such as PC damage, abnormal LCD display, or uncontrolled robotic arm movements. As the primary function of a medical robot, the robotic arm must not exhibit any abnormalities, and uncontrolled movements are absolutely unacceptable.
Radiated interference from internal wiring harnesses
Such interference often originates from noise generated by frequency converters and servo drives in the automatic control system. This noise radiates to other cables in the system via power cables, signal cables, or faulty grounding cables, thereby affecting the stable operation of mechanical equipment.
Interference from radiation between low-voltage DC wiring and AC220V wiring, as well as interference between motherboard interface lines, absolute position encoder lines, CAN communication lines, DC power lines, motor lines, encoder lines, etc., can all interfere with the normal operation of the PC and the robotic arm.
Conducted interference from outside the system
The power grid in use may experience surge interference due to transients in the main power system switching and lightning transients; the surge capacity is relatively large and is very likely to damage the power supply part of the unprotected robot.
Interference from power frequency magnetic fields, generated by power frequency currents in conductors or by leakage flux from nearby devices (such as transformers), generally affects the accuracy of data acquisition and transmission by the robot's sensors, thus impacting the robot's normal operation.
Caused by sudden, large changes in the power grid (mainly short circuits) or load. In some cases, two or more consecutive dips or interruptions may occur, causing some internal circuits of the robot to fail to reset successfully or data to be saved incorrectly. Electromagnetic fields from radio frequency transmitters may act on the entire cable connecting the installation equipment. Conducted through the cable, this may affect the normal operation of the robot.
When static electricity carried by a human body approaches or comes into contact with a robot, the resulting discharge can affect the robot's normal operation, damage components, and so on.
System launch into space
Inside the robot, the rectifier sections of the switching power supply and servo driver often use nonlinear rectifier diodes, which inevitably generate harmonic currents that distort the grid-side voltage. The differential-mode current caused by these harmonics is transmitted in the electrical circuit via direct wire coupling, while the common-mode voltage forms a common-mode interference loop through capacitive coupling with parasitic capacitance. Simultaneously, the high-frequency electromagnetic noise generated by the power-mode switch interferes with external circuitry through energy radiation.
Interference suppression technology for medical robots
layout
The electrical cabinet design must use metal materials and be rationally laid out according to EMC zoning principles. Different devices should be planned in different areas, and servo drive units should be installed as close as possible to the bottom of the cabinet. Use grounded metal isolation plates to isolate the areas, or install them independently in a metal electrical cabinet.
Ensure good ventilation and heat dissipation in the electrical cabinet, and do not obstruct the normal flow of the fan.
Wiring in the electrical cabinet should separate strong and weak current lines, and signal lines and power lines should be run separately.
AC contactors and DC relays should be installed away from I/O components and signal cables, and proper RC suppression components and diodes should be used to reduce noise pollution when the coil engages.
Grounding
Proper and reliable grounding is the most effective way to solve conducted interference and also eliminates the effects of common-mode interference. Incorrect grounding not only fails to reduce interference but also becomes an accomplice to it.
Grounding can be classified into signal ground, shielding ground, and protective ground according to its purpose.
All electrical components in the electrical cabinet (inverter, PLC) must be reliably connected to the common grounding point or the grounding busbar PE using short and thick grounding wires.
Do not use paint to isolate the joints of the robot's frame. After achieving an effective connection, connect it to the system ground PE for effective grounding, which can reduce the impact of pulse groups and radiated interference.
Filtering
The filtering of the main power supply and each branch power input (AC380/220V) mainly uses EMI filters and ferrite cores.
DC power supply output filtering, EMI filter, and ferrite core.
Input filtering for the driver, EMI filter, and ferrite core.
PCB filtering mainly involves low-frequency and high-frequency filtering, using common-mode inductors, RC circuits, ferrite beads, etc.
Motor noise filtering
During the rotation of a brushed motor, the carbon brushes continuously generate electric arcs, producing high-frequency noise. This high-frequency noise radiates outwards through the motor leads and gaps in the casing. The motor's metal casing can reflect and absorb the internal radiated noise, suppressing its outward radiation. However, a significant amount of radiated noise still escapes through the motor leads. Therefore, our efforts to mitigate motor radiation primarily focus on addressing the motor leads. This section will highlight the application of BDL filters in motor design.
shield
The cable from the shielded servo drive amplifier unit to the motor power cable should use a metal shield. The cable should be as short as possible, and the shield should be fixed to the grounded mounting plate with metal clips to avoid power loss and reduce interference. For cabinet ventilation, a dense metal mesh should be used, with the smallest possible openings. Narrow openings may radiate high-frequency signals within the cabinet. The cabinet door and cable inlets must be reliably grounded to prevent interference magnetic fields from leaking out through the shielded cable. The cabinet door should use a conductive sealing gasket that fits tightly against the cabinet body.
Effective shielding of servo drives, communication ports, PCs, and LCD displays can significantly reduce the impact of EMI.
Conclusion
Interference in control systems is a very complex problem. Only when design engineers take precautions and nip problems in the bud during the design process, and adhere to the principle that "prevention is the most effective and economical method," and comprehensively consider all aspects of factors, can they effectively avoid the occurrence of system interference and thus ensure the stable and reliable operation of the equipment.
