Industrial robots consist of four main components: the body, servo, reducer, and controller. Among them, the general structure of the electric servo system in industrial robots is based on three closed-loop controls: the current loop, speed loop, and position loop. Generally, for AC servo drivers, various functions such as position control, speed control, and torque control can be achieved by manually setting their internal functional parameters.The servo system (servomechanism), also known as a tracking system, is a feedback control system used to precisely follow or reproduce a certain process. The servo system allows the controlled output of an object’s position, orientation, state, etc., to follow any changes in the input target (or given value).

The servo system is a product developed based on frequency conversion technology, functioning as an automatic control system that uses mechanical position or angle as the control object. In addition to speed and torque control, the servo system can also achieve precise, fast, and stable position control.

In a broad sense, the servo system is a control system that accurately tracks or reproduces a given process and can also be called a tracking system.

In a narrow sense, the servo system is also known as a position tracking system, where the controlled output is the linear or angular displacement of the mechanical space position of the load. When the position input changes arbitrarily, the main task of the system is to quickly and accurately reproduce the changes in the given quantity.

Structure of the Servo System

The structure and types of mechatronic servo control systems are diverse. However, from the perspective of automatic control theory, a servo control system generally consists of five parts: the controller, the controlled object, the actuator, the detection element, and the comparison element.

Block Diagram of Servo System Composition

1. Comparison Element

The comparison element compares the input command signal with the feedback signal from the system to obtain the error signal between output and input, usually implemented by specialized circuits or computers.

2. Controller

The controller is typically a computer or PID (Proportional, Integral, and Derivative) control circuit. Its main task is to process the error signal output from the comparison element to control the actuator to act as required.

3. Actuator

The actuator converts various forms of input energy into mechanical energy according to the control signal requirements, driving the controlled object to work. In mechatronic systems, the actuator generally refers to various motors or hydraulic and pneumatic servo mechanisms.

4. Controlled Object

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

5. Detection Element

The detection element measures the output and converts it into the dimension required by the comparison element, generally including sensors and conversion circuits.

Characteristics and Functions of the Servo System

The servo system fundamentally differs from the feeding systems of general machine tools, as it can precisely control the motion speed and position of the actuating components based on command signals. The servo system serves as the connecting link between CNC devices and machine tools, and is an important part of the CNC system, possessing the following characteristics:

Must have high-precision sensors that can accurately provide electrical signals for the output.
The power amplifier and control system must be reversible.
Must have a sufficiently large speed regulation range and strong low-speed load performance.
Fast response capability and strong anti-interference ability.

Types of Servo Systems

By control principle: There are three forms: open loop, closed loop, and semi-closed loop
By the nature of the controlled quantity: there are displacement, speed, force, and torque servo system forms
By driving method: there are electrical, hydraulic, and pneumatic servo drive forms
By actuating components: there are stepper motor servo, DC motor servo, and AC motor servo forms

Actuating Components of Servo Systems

1. Types and Characteristics of Actuating Components

(1) Electric Actuating Components

Electric actuating components include DC servo motors, AC servo motors, stepper motors, and electromagnetic actuators, which are the most commonly used actuating components. In addition to requiring smooth operation, DC servo motors generally also require good dynamic performance, suitability for frequent use, and ease of maintenance.

(2) Hydraulic Actuating Components

Hydraulic actuating components mainly include reciprocating hydraulic cylinders, rotary hydraulic cylinders, hydraulic motors, etc., with hydraulic cylinders being the most common. For the same output power, hydraulic components are characterized by light weight and good speed.

(3) Pneumatic Actuating Components

Pneumatic actuating components, in addition to using compressed air as the working medium, are not different from hydraulic actuating components. Although pneumatic drives can provide substantial driving force, stroke, and speed, they cannot be used in situations where high positioning accuracy is required due to the poor viscosity and compressibility of air.

Differences Among the Three Types

Type	Characteristics	Advantages	Disadvantages
Electric Pneumatic Actuating	Can use commercial power; signal and power transmission directions are the same; there are AC and DC distinctions; pay attention to voltage and power usage.	Easy to operate; easy programming; can achieve positioning servo control; fast response, easy to connect with computers (CPU); compact size, strong power, and pollution-free.	Instantaneous output power is large; overload difference; once jammed, can cause burning accidents; greatly affected by external noise.
Pneumatic Actuating	Gas pressure source pressure 5~7×Mpa; requires skilled operation personnel.	Convenient gas source, low cost; no leakage and pollution to the environment; fast speed, easy operation.	Small power, large size, difficult to miniaturize; unstable action, difficult to transmit over long distances; noisy; difficult to servo.
Hydraulic Actuating	Liquid pressure source pressure 20~80×Mpa; requires skilled operation personnel.	Large output power, fast speed, stable action, can achieve positioning servo control; easy to connect with computers (CPU).	Equipment is difficult to miniaturize; hydraulic source and hydraulic oil requirements are strict; prone to leakage, polluting the environment.

2. Common Control Motors

Control motors are the power components of electrical servo control systems. They convert electrical energy into mechanical energy. The commonly used control motors in mechatronic products refer to servo motors that can provide correct motion or complex actions.

Control motors have rotary and linear drive motors, which achieve constant speed, variable speed drive, or repeated start-stop incremental drive through control of voltage, current, frequency (including command pulses), etc., with driving precision varying based on the driven object.

(1) Servo drive motors generally refer to:Stepping Motor, DC Servo Motor, AC Servo Motor

(2) The control methods for commonly used servo control motors mainly include: open loop control, semi-closed loop control, and closed loop control.

Closed-loop systems have position (or speed) feedback loops; open-loop systems do not have position and speed feedback loops.

a. Open-loop CNC system

There is no position measurement device, and the signal flow is unidirectional (CNC device → feeding system), so the system stability is good.

No position feedback, the accuracy is relatively lower than that of closed-loop systems, mainly depending on the performance and precision of the servo drive system and mechanical transmission mechanism. Generally, power stepping motors are used as servo driving components.

This type of system has advantages such as simple structure, stable operation, easy debugging, simple maintenance, and low cost, widely used in applications where accuracy and speed requirements are not high, and driving torque is not large. Typically used in economical CNC machine tools.

b. Semi-closed loop CNC system

The position sampling points of the semi-closed loop CNC system are shown in the figure, sampled from the drive device (commonly used servo motors) or screw, detecting the sampling rotation angle, not directly measuring the actual position of the moving parts.

The semi-closed loop does not include or only includes a small number of mechanical transmission links, thus achieving stable control performance. Although the system’s stability is not as good as that of open-loop systems, it is better than closed-loop systems. The motion errors caused by screw pitch errors and gear gaps are difficult to eliminate. Therefore, its accuracy is worse than that of closed-loop systems but better than that of open-loop systems. However, these errors can be compensated, thus still achieving satisfactory accuracy.

Semi-closed loop CNC systems have simple structures, are easy to debug, and have relatively high accuracy, thus widely used in modern CNC machine tools.

c. Fully closed loop CNC system

The position sampling points of the fully closed loop CNC system are indicated by the dashed line in the figure, directly measuring the actual position of the moving parts.

Theoretically, it can eliminate the errors, gaps, and dead motion of the entire drive and transmission links, achieving high positioning control accuracy. However, due to the nonlinear friction characteristics, rigidity, and gaps of many mechanical transmission links within the position loop, it is easy to cause system instability, making the design, installation, and debugging of closed-loop systems quite challenging.

This system is mainly used in high-precision boring mills, ultra-precision lathes, ultra-precision grinding machines, and larger CNC machine tools.

Robot Servo Systems

Typically, when we refer to robot servo systems, we mean precision servo systems used for multi-axis motion control. A multi-axis motion control system consists of a high-level motion controller and a low-level servo driver. The motion controller is responsible for decoding motion control commands, controlling the relative motion between various position control axes, acceleration and deceleration profile control, etc. Its main role is to reduce the overall system’s motion control path error; the servo driver is responsible for the position control of the servo motor, mainly to reduce the tracking error of the servo axis.

The robot servo system consists of three main parts: servo motor, servo driver, and command mechanism. The servo motor is the executing mechanism, responsible for achieving motion, while the servo driver is the power supply for the servo motor, and the command mechanism provides pulses or speed to facilitate the normal operation of the servo driver.

The requirements for the servo motor in a robot are higher than those for the other two parts. First, the servo motor must have a fast response. The time from receiving the command signal to achieving the required working state should be short. The shorter the response time to the command signal, the higher the sensitivity of the electric servo system and the better the fast response performance, usually indicated by the size of the electromechanical time constant of the servo motor. Second, the starting torque inertia ratio of the servo motor should be large. When driving a load, the starting torque of the robot’s servo motor should be large, and the rotational inertia should be small. Finally, the servo motor should have continuity and linearity in control characteristics, allowing the motor speed to change continuously with changes in control signals, sometimes requiring the speed to be proportional or approximately proportional to the control signal.

Of course, to match 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 capable of frequent forward and reverse operations, and endure several times overload in a short time.

The servo driver utilizes the torque and force generated by various motors to directly or indirectly drive the robot body to achieve various movements of the robot’s executing mechanisms. It has advantages such as high torque inertia ratio, no brushes, and no commutation sparks, making it widely used in robots.

Famous Foreign Servo Motor/System Manufacturers

Lenze (Germany)

Lenze is a world-renowned manufacturer of mechatronic drive products, with major products including DC speed control devices, AC inverters, AC servos, various motors, small drive components, reducers, gearboxes, brakes, and clutches. The company’s active product, the MCM synchronous servo motor, targets the robot market, featuring a compact structure, excellent load-to-motor inertia ratio, smooth casing, and ease of use, suitable for applications such as packaging and handling.

Lenze MCM Synchronous Servo Motor

Bosch Rexroth (Germany)

Bosch Rexroth, a wholly-owned subsidiary of the Bosch Group in Germany, is a leading supplier in the global industrial technology market, renowned in the field of drive and control. The company’s products are diverse, primarily focusing on high-quality electrical control, hydraulic, pneumatic, and mechatronic components and systems.

Bosch Rexroth MS2N Synchronous Servo Motor

Lust (Germany)

Lust is part of the Kölber Group, focusing on providing general, programmable, and multi-axis motion control servo drive products and related solutions for the robot and machine tool manufacturing industries. The standard product spectrum is wide, covering general, programmable, and multi-axis motion control types, widely applied in various industrial machinery and machine tools.

Lust LST Series Servo Motor

Beckhoff (Germany)

Beckhoff, based on PC-based automation technology as its development concept, produces industrial PCs, fieldbus modules, drive products, and TwinCAT automation software, forming a complete, compatible control system that provides open automation systems and comprehensive solutions for various industrial control fields.

Beckhoff AM8000 Synchronous Servo Motor

Siemens (Germany)

Siemens is one of the largest electrical and electronic companies in the world, thriving for over 150 years. The company’s business primarily focuses on six major fields: information and communication, automation and control, power, transportation, medical systems, and lighting. Siemens offers a wide variety of servo motors with compact structures that achieve excellent dynamic characteristics and operational efficiency, with complete performance levels and diverse installation specifications.

SIMOTICS 1FT7 Permanent Magnet Synchronous Motor

European servo systems are characterized by high overload capacity, good dynamic response, strong openness of drivers, and bus interfaces, including fieldbus, industrial Ethernet, and even wireless network technologies. However, they are expensive and large in size and weight. (Although most high-speed servo buses are open standards, technologies are primarily controlled by international corporations. Especially technologies based on industrial Ethernet, such as EtherCAT, Profinet, EtherNet/IP, and SERCOS III, have played a significant role in enhancing the high performance of industrial robot-specific AC servo systems. Beckhoff’s EtherCAT has rapidly developed in recent years, becoming the preferred network standard for most high-performance drive systems. Foreign servo manufacturers like Yaskawa and Lenze have adopted EtherCAT as the bus standard for their next-generation products.)

Panasonic (Japan)

Panasonic is a large multinational company with products spanning home appliances, digital audiovisual electronics, office products, aviation, and many other fields. Panasonic’s motors focus on electrical and mechatronic services, with its small AC servo motors and drivers being well-received in the market. The large inertia series is suitable for CNC machine tools, while the medium inertia series is suitable for robots (with a maximum speed of 3000r/min and torque ranging from 0.016 to 0.16N.m).

Panasonic MINAS A5 Series

Yaskawa (Japan)

Yaskawa Electric is a world-class manufacturer of drive products and the first company in Japan to produce servo motors, known for its stable and fast products with high cost-performance ratio, making it the largest servo brand in terms of sales volume and usage across various industries. Yaskawa provides suitable servo and controller products for various applications, from general industrial machinery to semiconductor and electronic component manufacturing equipment.

Yaskawa Electric Σ-Ⅴ Series

Mitsubishi (Japan)

Mitsubishi is not an independent company but a group composed of many independent companies, involved in mining, shipbuilding, banking, insurance, warehousing, trade, paper, steel, glass, electrical equipment, aircraft, oil, and real estate, among many other fields, making it an industrial giant in Japan. Mitsubishi Electric has been committed to improving product performance, with its FA product lineup dominating the Asian market, including programmable controllers (PLC), human-machine interfaces (HMI), AC servo systems, inverters, industrial robots, and low-voltage distribution products.

Mitsubishi Servo Motor

Sanyo (Japan)

Sanyo is an electric company under Panasonic, targeting the mid-range market. The Sanyo brand was discontinued on April 1, 2012. Sanyo’s Denki AC servo drive system was one of the earliest servo systems to enter the domestic market, renowned for its excellent performance, high speed, precision, flexibility, long lifespan, and cost-effectiveness. It can switch between different control methods, achieve full closed-loop control, and has output detection, automatic filtering, and real-time self-tuning functions, widely used in metallurgy, printing machinery, packaging machinery, textile machinery, plastic machinery, medical equipment, and automated production lines.

Sanyo AC Servo Series

Japanese servo motors are relatively low in price, compact in size, and lightweight. However, they also have obvious disadvantages, such as relatively weak dynamic response and poor openness.

ELMO (Israel)

Elmo Motion Control is a company specializing in the development of compact, high-power servo motors, dedicated for 25 years to designing servo drivers for various harsh industrial environments, while also providing advanced network motion controllers and complete motion control solutions. The company claims to manufacture the best servo motors in the world.

ELMO Servo Motor

SERVOTRONIX (Midea)

SERVOTRONIX (Gaochuang) focuses on motion control and is also a company specializing in developing high-performance servo driver series and motion control solutions. The company has been acquired by Midea and will become a major supplier of KUKA robot servo motors in the future.

SERVOTRONIX PRO2 Series Servo Motor

AMC (USA)

AMC (ADVANCED Motion Controls) is a company providing reliable and high-performance motion control systems, founded in 1987. The company has maintained a small size for decades, with only over 300 employees, 40% of whom are engineers. The company’s goal is to become the best servo company, thus only producing products in the servo drive field. With exquisite manufacturing levels and advanced R&D capabilities, the company has achieved a 91% customer retention rate over the past seven years.

AMC Servo Drive Application Cases

COPLEY (USA)

Similar to AMC, Copley Controls is also a company focusing on servo drives, providing high-performance motion control solutions for a wide range of industrial applications, with over 25 years of OEM cooperation experience. The company’s products are mainly used in semiconductors, life sciences, automation equipment, robotics, testing/measuring, and COTS military applications.

Copley Servo Products

The companies mentioned above are leaders in servo control technology. Although their market share is small, they represent the technological development direction of servo control.

Famous Domestic Servo Motor/System Manufacturers

With the popularization of CNC technology in recent years, domestic servo product sales have maintained rapid growth. The independent research and development, manufacturing production, and application of servo systems have basically matured, forming a certain product series and independent supporting capabilities. However, there is still a significant gap in high performance and high reliability compared to foreign brands. For example, domestic brand products generally have a power range mostly within 3KW, with a majority being small and medium power, and the technical route is close to Japanese products.

With the surge in the industrial robot market, the noticeable gap between domestic servo systems and foreign brands has become a “bottleneck” restricting the development of China’s industrial robot industry. From a technical perspective, domestic servo motors have several deficiencies:

First, their shapes are generally longer and appear rough, making it difficult to apply them to high-end robots. Especially in desktop robots with light loads around 6kg, the installation space for the robot’s arm is very limited, requiring strict length specifications for the servo motor.

Second, the reliability of signal connectors needs improvement, and there is a need for miniaturization, high density, and integrated design with the servo motor body for easier installation, debugging, and replacement.

Third, another core technology is high-precision encoders, especially multi-turn absolute encoders used in robots, which heavily rely on imports, posing a significant bottleneck for the development of high-end robots in China. The miniaturization of encoders is also a core technology that cannot be bypassed when miniaturizing servo motors.

Fourth, there is a lack of fundamental research, including absolute encoder technology, industrialization manufacturing technology for high-end motors, breakthroughs in production processes, practical verification of performance indicators, and the establishment of assessment standards.

Fifth, there is insufficient collaboration between various parts of the servo system industry, leading to difficulties in achieving overall performance for servo motors and drive systems.

It is worth mentioning that in addition to the five technical flaws mentioned above, many domestic servo motor manufacturers continue to adopt outdated technologies and hardware to create “domestic” motors, resulting in varying quality and stability of the produced motors, disrupting the market and dampening the enthusiasm for independent research and development within enterprises, which is detrimental to long-term sustainable development.

Currently, there are over 20 large-scale domestic servo brands, and some of their products exhibit outstanding quality and performance, demonstrating a degree of independence and innovation.

Delta (Taiwan)

Delta is a well-known Chinese automation brand, occupying a unique position in the global automation field. In the early 1990s, Delta adopted an independent research and development approach to enter the industrial automation field. Based on its accumulation in the field of power electronics, Delta started with inverter products and later expanded to PLCs, servos, HMIs, CNC control systems, and PC-Based PAC controllers, sensors, machine vision, industrial robots, and a full range of automation products, ultimately focusing on three major areas: drive, motion, and control.

Delta ECMA Series Standard AC Servo Motor

Shihlin (Taiwan)

Shihlin Electric (Shihlin) was founded in 1955, primarily developing products related to heavy electricity, electrical installations, machinery, and automation. The company has a close partnership with Japan’s Mitsubishi, and it is said that the user experience of its products is comparable to that of Mitsubishi Electric. For example, the Shihlin SDH AC servo series employs Taiwan’s best encoders, boasting the fastest response speed in Taiwan.

Shihlin SDH Series AC Servo Motor

Taiwanese brands are easy to use, with performance close to Japanese brands, but at a lower price, showing high cost performance and rapid development in the mid-to-low-end market.

Inovance

Shenzhen Inovance primarily engages in the research and development of automation and new energy-related products, holding the highest market share for servo motors among domestic enterprises. Inovance has launched a series of industrial automation products, including inverters, servo systems, control systems, industrial vision systems, sensors, and other core components and electrical solutions in the intelligent equipment and industrial robot fields, becoming the largest domestic supplier of low and medium voltage inverters and servo systems. Several of Inovance’s servo motors support EtherCAT Ethernet communication protocol.

Inovance IS620N Servo System

Gooch

Gooch Technology is a domestically leading supplier of automatic control products. The company’s independently developed PC-based open motion controllers, embedded motion controllers, networked motion controllers, and computer programmable automation controller products and systems have reached international first-class levels, filling several gaps in the domestic industry. Gooch mainly provides systems for robot motion control.

Gooch GTHD Series High-Performance Servo Drives

Huazhong CNC

Huazhong CNC, backed by the University of Science and Technology of China, has excellent research and development capabilities and is the largest manufacturer of mid-to-high-end CNC series products in the country. The company’s GK6 and GK7 series permanent magnet synchronous AC servo motors and GM7 series AC servo spindles have achieved mass production, making it the only company in the country with complete core technology independent intellectual property rights and self-supporting capabilities.

Huazhong CNC ST Series Bus-Type AC Permanent Magnet Synchronous Servo Motor

Guangzhou CNC

Guangzhou CNC focuses on CNC machine tools, automation control fields, and injection molding products, being the largest CNC system R&D manufacturer in the country, with a complete industrial chain and a large scale. The company’s servo systems are primarily used in machine tools, packaging machinery, and other areas.

Guangzhou CNC SJTR Series Servo Motor

Inovance

In 2011, Inovance acquired YN Power and has since developed the brand into a leading player in the textile machinery servo system segment. Inovance primarily provides comprehensive solutions for textile machinery enterprises and offers the most comprehensive series models of servo systems in the industry. Currently, the company’s servo products are widely used in textile machinery, packaging machinery, CNC lathes, dispensing machines, LED spectrophotometers, and other machines, with high cost performance.

Inovance DA200 High-Performance General AC Servo System

Estun

Nanjing Estun has over a decade of history in producing and selling fully digital AC servo drives and motors, with products widely used in various CNC machine tools and automation equipment. In 2012, the company successfully completed a major special project under the 863 Program, developing an AC servo system drive and motor specifically for industrial robots, achieving breakthroughs in domestically produced core components. The company has also successfully developed an AC servo system for robots that can control six axes, providing a foundation for developing more economical and efficient domestic robots.

The performance of robots is greatly influenced by the servo system, so the key performance indicators of precision servo systems are always the primary factors for comparison. Advanced foreign servo systems can already adapt well to the needs of most applications, with their R&D resources concentrated on specific high-end applications and overall performance enhancement, currently in a meticulous refinement stage. In the context of Industry 4.0, domestic servo systems have a long way to go and need to strive to catch up.

Transcribed from: Industrial Robots
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Understanding Robot Servo Systems

Famous Foreign Servo Motor/System Manufacturers

Bosch Rexroth (Germany)

Famous Domestic Servo Motor/System Manufacturers

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Famous Foreign Servo Motor/System Manufacturers

Bosch Rexroth (Germany)

Famous Domestic Servo Motor/System Manufacturers

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