A detailed explanation of electric servo systems for industrial robots

The typical structure of an industrial robot's electric servo system consists of three closed-loop controls: a current loop, a speed loop, and a position loop. Generally, for AC servo drives, multiple functions such as position control, speed control, and torque control can be achieved by manually setting their internal parameters.

A servo system, also known as a follow-up system, is a feedback control system used to accurately follow or reproduce a process. A servo system is an automatic control system that enables the output controlled variable, such as the position, orientation, and state of an object, to follow any change in the input target (or given value). Servo systems are products developed based on frequency conversion technology and are automatic control systems that use mechanical position or angle as the controlled object. In addition to speed and torque control, servo systems can also perform precise, fast, and stable position control. In a broad sense, a servo system is a control system that accurately tracks or reproduces a given process; it can also be called a follow-up system. In a narrow sense, a servo system, also known as a position follow-up system, has its controlled variable (output) as the linear or angular displacement of the load's mechanical spatial position. When the position given variable (input) changes arbitrarily, the main task of the system is to enable the output variable to quickly and accurately reproduce the change in the given variable.

The structure of a servo system

Mechatronics servo control systems come in a variety of structures and types, but from the perspective of automatic control theory, a servo control system generally includes five parts: controller, controlled object, execution link, detection link, and comparison link.

Servo System Composition Principle Block Diagram

1. Comparison Component: The comparison component compares the input command signal with the system feedback signal to obtain the deviation signal between the output and the input. It is usually implemented by a dedicated circuit or computer.

2. Controller The controller is usually a computer or a PID (proportional, integral and derivative) control circuit. Its main task is to transform and process the deviation signal output by the comparator to control the actuator to act as required.

3. Execution Components: The function of the execution components is to convert various forms of energy into mechanical energy according to the requirements of the control signals, thereby driving the controlled object to work. In mechatronics systems, the execution elements generally refer to various motors or hydraulic/pneumatic servo mechanisms, etc.

4. Controlled object: The controlled object refers to the object being controlled, such as a robotic arm or a mechanical work platform.

5. Detection Stage: The detection stage refers to the device that can measure the output and convert it into the dimensions required by the comparison stage. It generally includes sensors and conversion circuits.

Features and functions of servo systems

Servo systems differ fundamentally from the feed systems of general machine tools. They can precisely control the speed and position of the actuators based on command signals. The servo system is the link between the CNC device and the machine tool, and is an important component of the CNC system, possessing the following characteristics:

It must have a high-precision sensor that can accurately provide the electrical signal of the output quantity.

Both the power amplifier and the control system must be reversible.

It has a sufficiently large speed range and sufficiently strong low-speed load-carrying capacity.

It has a fast response capability and strong anti-interference capability.

Types of servo systems

Based on control principles, there are three types: open-loop, closed-loop, and semi-closed-loop.

Based on the nature of the controlled variable, servo systems can be categorized into those controlling displacement, velocity, force, and torque.

According to the driving method, there are servo drives in the form of electric, hydraulic, and pneumatic drives.

Based on the actuator, there are stepper motor servos, DC motor servos, and AC motor servos.

Servo system actuators: 1. Types and characteristics of actuators

(1) Electrical Actuators Electrical actuators include DC servo motors, AC servo motors, stepper motors, and electromagnets, which are the most commonly used actuators. In addition to requiring smooth operation, servo motors generally require good dynamic performance, suitability for frequent use, and ease of maintenance.

(2) Hydraulic actuators Hydraulic actuators mainly include reciprocating cylinders, rotary cylinders, hydraulic motors, etc., among which cylinders are the most common. Under the same output power, hydraulic components have the characteristics of light weight and good speed.

(3) Pneumatic actuators: Pneumatic actuators are no different from hydraulic actuators except that they use compressed air as the working medium. Although pneumatic drives can achieve greater driving force, stroke and speed, they cannot be used in applications requiring high positioning accuracy because air has poor viscosity and is compressible.

Differences between the three types

2. Commonly used control motors

The control motor is the power component of the electrical servo control system. It is an energy conversion device that converts electrical energy into mechanical energy. The control motor commonly used in mechatronics products refers to the servo motor that can provide correct motion or more complex actions. Control motors include rotary and linear drive motors. Through voltage, current, frequency (including command pulses), etc., they can realize constant speed, variable speed drive, incremental drive with repeated start and stop, and complex drive. The drive accuracy varies depending on the driven object. (1) Servo drive motors generally refer to: stepping motors, DC servo motors, and AC servo motors.

(2) The control methods commonly used for servo motors are mainly three types: open-loop control, semi-closed-loop control, and closed-loop control. The drive system of a closed-loop system has a position (or speed) feedback loop; the open-loop system does not have a position and speed feedback loop. a. The open-loop CNC system does not have a position measuring device, and the signal flow is unidirectional (CNC device → feed system), so the system has good stability.

Without position feedback, the accuracy is relatively low compared to a closed-loop system. Its accuracy mainly depends on the performance and accuracy of the servo drive system and mechanical transmission mechanism. Generally, a power stepper motor is used as the servo drive element. This type of system has advantages such as simple structure, stable operation, convenient debugging, simple maintenance, and low price, and is widely used in applications where accuracy and speed requirements are not high and driving torque is not large. It is generally used in economical CNC machine tools. b. Semi-closed-loop CNC system: The position sampling points of a semi-closed-loop CNC system, as shown in the figure, are derived from the drive device (commonly a servo motor) or lead screw, sampling the rotation angle for detection, rather than directly detecting the actual position of the moving parts.

A semi-closed-loop system contains little or no mechanical transmission, thus achieving stable control performance. While its stability is not as high as an open-loop system, it is better than a closed-loop system. Motion errors caused by lead screw pitch and gear backlash are difficult to eliminate. Therefore, its accuracy is lower than a closed-loop system but better than an open-loop system. However, these errors can be compensated for, thus achieving satisfactory accuracy. Semi-closed-loop CNC systems are simple in structure, easy to debug, and have high accuracy, making them widely used in modern CNC machine tools. c. Fully Closed-Loop CNC System: The position sampling points of a fully closed-loop CNC system are shown by the dotted lines in the figure, directly detecting the actual position of the moving parts.

Theoretically, it can eliminate errors, backlash, and loss of motion in the entire drive and transmission system, resulting in very high position control accuracy. However, because the frictional characteristics, rigidity, and backlash of many mechanical transmission components within the position loop are nonlinear, system instability can easily occur, making the design, installation, and debugging of the closed-loop system quite difficult. This system is mainly used in high-precision boring and milling machines, ultra-precision lathes, ultra-precision grinding machines, and larger CNC machine tools.

robot servo system

Generally, when we talk about robot servo systems, we are referring to precision servo systems used for multi-axis motion control. A multi-axis motion control system consists of a high-order motion controller and a low-order servo driver. The motion controller is responsible for decoding motion control commands, controlling the relative motion between the axes, acceleration and deceleration contour control, etc., and its main function is to reduce the path error of the overall system motion control. The servo driver is responsible for the position control of the servo motor, and its main function is to reduce the following error of the servo axis.

A robot's servo system consists of three main parts: a servo motor, a servo driver, and a command mechanism. The servo motor is the actuator, responsible for movement. The servo driver provides power to the servo motor, and the command mechanism sends pulses or speed signals to coordinate with the servo driver. The requirements for the servo motor are higher than the other two parts. First, the servo motor must have a fast response time. The time from receiving a command signal to completing the required working state should be short. The shorter the response time, the higher the sensitivity and the better the fast response performance of the servo system. The fast response performance of the servo motor is generally described by the magnitude of its electromechanical time constant. Second, the servo motor must have a high starting torque-to-inertia ratio. When driving a load, the robot's servo motor requires a large starting torque and a small moment of inertia. Finally, the servo motor must have continuous and linear control characteristics. The motor speed must change continuously with the control signal, and sometimes the speed needs to be proportional or approximately proportional to the control signal. Of course, to fit the robot's shape, the servo motor must be small in size, lightweight, and have a short axial dimension. It must also withstand harsh operating conditions, be able to perform very frequent forward and reverse rotation and acceleration and deceleration operations, and be able to withstand several times the overload in a short period of time. Servo drives are actuators that can directly or indirectly drive the robot body to obtain various movements of the robot by utilizing the torque and force generated by various motors. They have advantages such as high torque-to-inertia ratio, no brushes and commutation sparks, and are widely used in robots.

Internationally renowned servo motor/system manufacturers

Lenze (Germany) is a world-renowned manufacturer of electromechanical transmission products. Its main products include DC speed controllers, AC inverters, AC servos, various electric motors, small drive units, gearboxes, transmissions, brakes, and clutches. The company's current product, the MCM synchronous servo motor, is primarily targeted at the robotics market. It features a compact structure, excellent load-to-motor-inertia ratio, a smooth casing, and ease of use, making it suitable for packaging, handling, and other applications.

Bosch Rexroth (Germany)

Bosch Rexroth, a wholly owned subsidiary of the German Bosch Group, is a leading global supplier of industrial technology, with an outstanding reputation in the field of drive and control. The company offers a wide range of products, primarily focusing on high-quality electrical, hydraulic, pneumatic, and mechatronic components and systems.

Luster (Germany)

Lust, a subsidiary of the Kolper Group, is a manufacturing and trading company specializing in providing general-purpose, programmable, and multi-axis motion control servo drive products and related solutions to the robotics/machine tool manufacturing industry. Its standard product portfolio is broad, covering general-purpose, programmable, and multi-axis motion control types, and these products are widely used in various industrial machinery and machine tools.

Beckhoff (Germany)

Beckhoff's development philosophy is based on PC-based automation technology. Its industrial PCs, fieldbus modules, drive products, and TwinCAT automation software constitute a complete and compatible control system, providing open automation systems and complete solutions for various industrial control fields.

Siemens (Germany)

Siemens is one of the world's largest electrical and electronics companies, enjoying enduring success for over 150 years. Its business focuses on six main areas: information and communication, automation and control, power, transportation, medical systems, and lighting. Siemens servo motors are diverse in type and compact in structure, achieving excellent dynamic characteristics and operating efficiency, with a full range of performance levels and various installation specifications. European servo motors are characterized by high overload capacity, good dynamic response, strong driver openness, and bus interfaces, including fieldbus, industrial Ethernet, and even wireless network technologies. However, they are expensive and bulky. (While most high-speed servo buses are currently open standards, the technology is controlled by large international companies. Especially those based on industrial Ethernet technologies, such as EtherCAT, Profinet, EtherNet/IP, and SERCOS III, have played a crucial role in improving the performance of AC servo systems for industrial robots. EtherCAT, launched by the German company Beckhoff, has seen rapid growth in recent years, becoming the preferred network standard for most high-performance drive systems. Foreign servo manufacturers such as Yaskawa and Lenze have adopted EtherCAT as the bus standard for their next-generation products.)

Panasonic (Japan)

Panasonic is a large multinational corporation with products spanning numerous fields, including home appliances, digital audio-visual electronics, office products, and aerospace. Panasonic Electric Works, a subsidiary of Panasonic, primarily provides electrical and electromechanical services. Its small AC servo motors and drives are quite popular in the market. The high-inertia series is suitable for CNC machine tools, while the medium-inertia series is suitable for robots (maximum speed 3000 r/min, torque 0.016–0.16 Nm).

Yaskawa (Japan)

Yaskawa Electric is a world-class manufacturer of drive products and the first company in Japan to produce servo motors. Its products are renowned for their stability, speed, and high cost-performance ratio, making it the world's largest-selling servo brand with the widest range of applications. From general industrial machinery to semiconductor and electronic component manufacturing equipment, Yaskawa provides the most suitable servo and controller products for various applications.

Mitsubishi (Japan)

Mitsubishi is not a single company, but a group of numerous independent companies involved in a wide range of industries including mining, shipbuilding, banking, insurance, warehousing, trading, paper, steel, glass, electrical equipment, aircraft, oil, and real estate. It is a major industrial player in Japan. Mitsubishi Electric is committed to improving product performance, and its FA product portfolio covers programmable logic controllers (PLCs), human-machine interfaces (HMIs), AC servo systems, inverters, industrial robots, and low-voltage power distribution products, all of which hold a dominant position in Asia.

Sanyo (Japan)

Sanyo was an electronics company under Panasonic, targeting the mid-range market. The brand ceased operations on April 1, 2012. Sanyo's Denki AC servo drive system was one of the earliest servo systems to enter the Chinese market, renowned for its superior performance, high speed, precision, flexibility, long service life, and high cost-effectiveness. It could switch between different control modes, perform full closed-loop control, and featured output detection, automatic filtering, and real-time self-tuning functions. It was widely used in metallurgical blast furnaces, printing machinery, packaging machinery, textile machinery, plastics machinery, medical equipment, and automated production lines.

Sanyo AC Servo Series

Japanese servo motors are relatively inexpensive, small in size, and lightweight. However, their disadvantages are also obvious: relatively weak dynamic response and poor adaptability.

ELMO (Israel)

Elmo Motion Control is a company specializing in the development of compact, high-power servo motors. For 25 years, they have been dedicated to designing servo drives for harsh industrial environments, while also providing advanced networked motion controllers and complete motion control solutions. The company claims to build the world's best servo motors.

SERVOTRONIX (Midea)

Servotronix focuses on motion control and is a professional developer of high-performance servo drives and motion control solutions. Acquired by Midea, the company will become a major supplier of servo motors for KUKA robots.

AMC (USA)

AMC (Advanced Motion Controls) is a company that provides reliable and high-performance motion control systems. Founded in 1987, it has maintained a small size for decades, with just over 300 employees, 40% of whom are engineers. The company's goal is to become the best servo company, therefore it only produces products in the servo drive field. Relying on its superb manufacturing capabilities and advanced R&D, the company has maintained a customer retention rate of 91% for the past seven years.

COPLEY (USA)

Like AMC, Copley Controls is a company focused on servo drives, primarily providing high-performance motion control solutions for a wide range of industrial applications, with over 25 years of OEM experience. The company's products are used in semiconductors, life sciences, automation equipment, robotics, test/measurement, and COTS (Catalyst, Transmission, and Telecommunications) for military applications. These companies are leaders in servo control technology; although their market share is not large, they represent the technological direction of servo control development.

Domestic well-known servo motor/system manufacturer

With the popularization of CNC technology in recent years, the sales of servo products in China have maintained rapid growth. The independent research and development, manufacturing, and application of servo systems have basically matured, forming a certain product series and independent supporting capabilities. However, in terms of high performance and high reliability, there is still a significant gap compared to foreign brands. For example, the power range of domestic brands is mostly within 3KW, with a focus on small and medium power, and their technical approach is similar to that of Japanese products. With the booming industrial robot market, the significant gap between my country's servo systems and foreign brands has become a bottleneck restricting my country's industrial robot industry. From a technical perspective, domestically produced servo motors mainly have the following shortcomings:

First, they are generally long and have a rough appearance, making them difficult to use in some high-end robots. This is especially true for desktop robots with a light load capacity of around 6kg, where the installation space for the robot arm is very limited, and there are strict requirements on the length of the servo motor.

Secondly, the reliability of signal connectors needs to be improved, and they need to be designed towards miniaturization, high density, and integration with the servo motor body to facilitate installation, debugging, and replacement.

Thirdly, another core technology is high-precision encoders, especially multi-turn absolute encoders used in robots. This reliance on imports is a major bottleneck restricting the development of high-end robots in my country. Encoder miniaturization is also a crucial technology that cannot be bypassed in the miniaturization of servo motors.

Fourth, there is a lack of basic research, including absolute encoder technology, industrial manufacturing technology for high-end motors, breakthroughs in production processes, practical verification of performance indicators, and the formulation of evaluation standards.

Fifth, insufficient collaboration and cooperation among the various components of the servo system industry makes it difficult to achieve good overall performance of the servo motor and drive system.

It's worth noting that, besides the five inherent technological flaws mentioned above, many domestic servo motor manufacturers continue to use outdated technology and hardware to create "domestic" motors. This results in inconsistent motor quality and stability, disrupting the market and discouraging independent R&D, hindering long-term sustainable development. Currently, there are over 20 large-scale servo brands in my country. Some of these manufacturers offer products with outstanding quality and performance, demonstrating a degree of independence and innovation. Delta (Taiwan) is a well-known Chinese automation brand, holding a unique position in the global automation field. In the early 1990s, Delta began its foray into industrial automation through independent R&D. Leveraging its expertise in power electronics, Delta started with inverters, then expanded to PLCs, servo systems, HMIs, CNC systems, and later to a full range of PC-based PAC controllers, sensors, machine vision, and industrial robots, ultimately focusing on the three major areas of drive, motion, and control.

Delta ECMA Series Standard AC Servo Motors, Shihlin (Taiwan)

Founded in 1955, Shihlin Electric primarily develops products for heavy electrical equipment, electrical systems, machinery, and automation. The company has a close partnership with Mitsubishi Electric of Japan, with a joint venture in Xiamen. It is rumored that the user experience of its products is comparable to that of Mitsubishi Electric. Taking Shihlin's SDH AC servo series as an example, this series of motors uses the best encoders in Taiwan, boasting the fastest response speed in the country. Taiwanese brands are easy to use, offer performance close to Japanese brands, but are cheaper, resulting in high cost-effectiveness and rapid growth in the low-to-mid-range market.

Huichuan

Shenzhen Huichuan primarily engages in the R&D of products related to automation and new energy, and is the domestic brand with the highest market share in servo motors. Huichuan has launched a series of industrial automation products in the fields of intelligent equipment and industrial robots, including core components such as frequency converters, servo systems, control systems, industrial vision systems, and sensors, as well as electrical solutions. It has become the largest supplier of medium and low voltage frequency converters and servo systems in China. Several of Huichuan's servo motors support the Ethercat Ethernet communication protocol.

Gugao

Googol Technology is a leading domestic supplier of automatic control products. Its independently developed PC-based open motion controllers, embedded motion controllers, networked motion controllers, and computer-programmable automation controllers and systems have achieved world-class performance, filling several gaps in the domestic industry. Googol primarily provides systems for robot motion control.

Huazhong CNC

Huazhong Numerical Control, backed by the University of Science and Technology of China, boasts outstanding R&D capabilities and is the largest manufacturer of mid-to-high-end CNC series products in China. Its lathe servo motors have reached the international mid-range level. The company's GK6 and GK7 series permanent magnet synchronous AC servo motors and GM7 series AC servo spindles have achieved mass production. It is currently the only enterprise in China with complete sets of core technologies, independent intellectual property rights, and self-sufficiency in supporting components.

Guangzhou CNC

Guangzhou CNC serves the CNC machine tool industry, automation control field, and injection molding industry. It is the largest CNC system R&D manufacturer for machine tools in China, with its key advantages being a complete industrial chain and large scale. The company's servo systems are mainly used in machine tools and packaging machinery.

Invt

In 2011, INVT acquired Yuneng Power and transformed the brand into a leading player in the textile machinery servo system segment. INVT primarily provides comprehensive solutions for textile machinery companies and offers the industry's most complete range of servo systems. Currently, the company's servo products are widely used in textile machinery, packaging machinery, CNC lathes, dispensing machines, LED beam splitting and taping machines, and other machines, offering high cost-performance. Estun

Nanjing Estun has over a decade of experience in manufacturing and selling universal AC servo drives and motors for fully digital applications. Its products are widely used in various CNC machine tools and automation equipment. In 2012, the company successfully completed the 863 Program's major project, "Development of AC Servo System Drives and Motors for Industrial Robots," establishing its own brand of robot-specific AC servo systems. Currently, all AC servo systems used in the company's robots are self-produced, achieving a breakthrough in domestically made core components. The company has also successfully developed a robot-specific "one-to-six" AC servo system, laying the foundation for developing more economical and efficient domestically produced robots.

The performance of robots is greatly influenced by servo systems; therefore, the key performance indicators of precision servo systems are always the primary factor in comparing their advancement. Advanced foreign servo systems can already well meet the needs of the vast majority of applications, and their R&D resources are concentrated on specific high-end applications and overall performance improvement, remaining in a stage of meticulous refinement. In the context of Industry 4.0, domestically produced servo systems have a long way to go and still need to work hard to catch up.