The concept of robots is already very broad. This article discusses servo motors for robot joints required by the industrial automation industry, and does not involve composite integrated joint servo motors for service robots.
Industrial robots are broadly classified into linear robots (also called Cartesian robots), multi-degree-of-freedom robots (also called articulated robots), parallel robots (also called delta-Δ robots), and horizontal articulated robots (also called SCRA robots). An automation island is formed by combining various types of articulated robotic arms and automated transmission equipment. Automation islands with different functions are linked to form automation lines, and different automation lines are combined to form an automated workshop.
In these automated industrial robots and automated units, servo motors always play a crucial role in accurately, timely, and reliably transmitting the mechanism into position according to the requirements of control commands, and therefore belong to the core components.
Basic concepts of permanent magnet servo motors
Servo means that it can execute the instructions of the control computer system without any deviation. It is not limited to electric motors and hydraulics, but also includes pneumatics. All components that can complete this task are servo components.
An electric motor is an electromechanical conversion component that converts electrical energy into mechanical energy. A servo motor is an electric motor that can be used in motion control systems, and its output parameters, such as position, speed, acceleration, or torque, are controllable.
Servo motors can be classified into different types due to differences in control parameters. Based on power supply type, they can be divided into AC servo motors and DC servo motors; based on operation mode, they can be divided into linear servo motors and rotary servo motors. Linear motors directly generate Newtonian force, while rotary motors output rotational torque. Rotary motors driving linear loads require mechanical mechanisms such as lead screws to convert rotational motion into linear motion.
Rotary AC servo motors are classified into AC asynchronous servo motors and AC synchronous servo motors according to their rotor structure. AC asynchronous servo motors have aluminum or copper squirrel cage rotors, with the squirrel cage speed always differing from the synchronous rotating magnetic field speed. This type of motor, under vector speed control technology, can achieve torque control characteristics as perfect as DC motors. However, the rotor has the characteristics of large inertia, good constant power characteristics, and a wide speed range, making it suitable for large-scale variable inertia loads such as machine tool cutting and printing machinery winding and unwinding. The disadvantages are low starting torque, slower electromagnetic response speed compared to permanent magnet servo motors (the electromagnetic time constant is about 10 times that of permanent magnet motors made of permanent magnet materials), and low power density and large rotor size, making it unsuitable for high-dynamic servo applications.
Rotary AC synchronous servo motors use permanent magnet materials in their rotors, directly generating the excitation magnetic field without the need for an excitation current to establish the motor's magnetic field, resulting in fast electromagnetic response. Furthermore, the high energy density of current rare-earth permanent magnet materials allows for high power density in these motors, enabling the design of servo motors with various characteristics. High dynamic response can be achieved through designs with slender rotors with small inertia or short, stout rotors with large inertia. The use of rare-earth permanent magnet materials has solidified the permanent magnet motor as the preferred choice for servo motors. However, rare-earth permanent magnet materials remain the most expensive component in servo motors. Differences in materials used by different manufacturers result in varying levels of product quality. Good permanent magnet materials can withstand operating temperatures above 150°C without demagnetizing, while inferior materials may demagnetize even below 120°C. The permanent magnet material directly determines the different characteristics of the servo motor.
Linear servo motors directly output Newtonian force without the need for mechanical conversion, achieving very high acceleration. In recent years, their technology has advanced rapidly, leading to their widespread application in high-performance machine tool feed axes. However, their use in industrial robots is limited to linear robotic arms and will not be the focus of this article. This article focuses on rotary permanent magnet servo motors and their applications in industrial robots.
Structure of a rotary permanent magnet motor
Figure 1. Structure diagram of permanent magnet servo motor
Figure 1 shows a typical structural diagram of a permanent magnet servo motor. To provide a comprehensive description, a single diagram is used to illustrate the overall structure of the permanent magnet servo motor. In fact, low-power permanent magnet servo motors under 15kW can dissipate heat naturally, eliminating the need for a cooling fan. Their compact size eliminates the need for mounting feet, making lifting rings unnecessary. Replacing the junction box with an aviation plug for the lead wires is also simpler. This results in the motor's appearance as shown in Figure 2(a). If the motor is very small, under 1kW, the aviation plug for the lead wires is also unnecessary; a short cable can be directly extended, resulting in the appearance shown in Figure 2(b).
This section assumes the reader has an understanding of motor principles; the structure of the permanent magnet servo motor will be explained in a distinctive way based solely on the characteristics of robot motors.
Bearings: The lifespan of a servo motor is closely related to its bearings. Due to the reliability and durability requirements of robots, bearings must ensure a service life of at least 30,000 hours. Based on an 8-hour workday, this translates to a robot lifespan of at least 10 years, and the bearings must be able to operate intermittently at 6,000 rpm.
Stator laminations and windings: Due to the high power density required by the robot motor, and to ensure small size and low iron loss and heat generation, the lamination material must be cold-rolled silicon steel sheets with a thickness of less than 0.35mm. The windings must withstand the impact of 16K frequency carrier pulses for a long time. To prevent breakdown, they must withstand dense dv/dt impacts, and the withstand voltage must be no less than 2500V.
Rotor permanent magnet material: The permanent magnet material is the most expensive part of a permanent magnet servo motor. Materials with low rare earth element content have low Curie points and poor material stability. If neodymium iron boron permanent magnets are used, it is best to use ones with a UH42 or higher. Furthermore, attention should be paid to the content of rare earth elements such as dysprosium. To ensure resistance to demagnetization at high temperatures, samarium cobalt permanent magnets are also widely used in small and medium-sized servo motors. In short, it is essential to ensure that the servo motor truly never demagnetizes under normal operating conditions. Otherwise, the long-term stability of the robot cannot be guaranteed.
Shaft seals: To prevent oil and debris from entering the motor while ensuring operation, adding shaft seals to the motor shaft is a standard design. Robots often have a small gear milled onto the end of the servo motor shaft, and the motor and reducer are directly connected. High temperatures and oil can enter the motor, therefore, multi-lip high-temperature shaft seals are needed. For example, double-lip fluororubber shaft seals are more reliable than single-lip nitrile rubber shaft seals, although the cost difference is significant.
Brake: A brake is a fundamental option for robot motors. Nearly 95% of servo motors require brakes. To ensure constant brake engagement, especially reliable operation during emergency stops, the brake needs a sufficient safety factor. The static torque is approximately 1.5 times the motor's rated torque, while for heavy-duty robot motors, the safety factor needs to reach 2.0 or even 2.5 times. It's important to note that the robot motor brake is a safety brake, not a traditional brake. Control must ensure that in emergency stops, the servo driver's braking circuit activates through a braking resistor, and the brake engages when the motor speed approaches zero. To improve brake response speed, permanent magnet brakes are superior to electromagnetic spring brakes.
Encoder: The encoder is installed at the tail end of the motor and is a sensor for motor speed and rotor position. It measures the rotor position for servo control magnetic field positioning and provides the actual rotor position and speed to the control computer for motion trajectory calculation. Robot motor encoders generally have low accuracy, but require multi-turn absolute position measurement to ensure that the position before power failure can be remembered after restarting. Currently, there are three popular methods to solve the problem of robot motor encoders. The first method uses Gray code photoelectric or magnetic code disks for single turns and mechanical gears for multi-turn turns. The advantage of this is high measurement accuracy; after power failure, the encoder's mechanical position is remembered, and the position can be read directly after power is restored. However, the disadvantage is that the encoder is too thick, which is excessively long in limited installation space. The second method uses photoelectric or magnetic Gray code memory for single turns and battery-powered electronic memory for multi-turn turns. This allows the encoder to be made very short, which is very suitable for small servo motors with a diameter of less than 60mm. The disadvantage is that the battery life is relatively short, ranging from 2-3 years to sometimes requiring battery replacement after 1 year. The third method is used in situations where high precision is not required to measure the position of a single turn using a rotary transformer, while multi-turn information is obtained through a battery-powered circuit board in the control box.
Rotor shaft extension: Due to frequent forward and reverse rotation, the motor is subjected to a certain shear force, so the shaft material is best made of 42CrMo. If the motor is keyed, the key must be fully installed to effectively reduce the motor's dynamic balance and runout. At high speeds, the no-load runout of a servo motor with a key and one with a flat shaft differs by as much as nine times, which should not be underestimated.
Main transmission parameters of permanent magnet servo motor
Operating range: The region in which a motor can operate continuously under the condition that the temperature rise does not exceed the allowable temperature rise is called the continuous operating range; the region outside the continuous operating range in which the motor is allowed to operate for short periods is called the intermittent operating range. The operating range is represented by two-dimensional plane coordinates of torque and speed.
Rated power PN: The maximum power that the motor can output within the continuous operating range.
Rated torque MN: The torque of the motor when outputting its rated power within the continuous operating range. Different manufacturers have significantly different definitions of rated torque. Generally, corresponding heat dissipation conditions must be specified. The common practice internationally is to specify the area and thickness of the aluminum flange on which this indicator is measured, with the flange temperature guaranteed to be 20°C or below a given temperature. Therefore, in actual work, because it is often installed on cast iron parts, and the high temperatures in summer exceed this test standard temperature, overheating and demagnetization can occur if no margin is allowed during use. The national standard specifying an ambient temperature of 40 degrees Celsius is relatively reasonable for the Chinese environment. Reputable manufacturers will leave a certain design margin below the rated value measured according to the standard when publishing the rated torque, which is safer.
Rated current IN: The current corresponding to the rated torque.
Rated speed nN: The highest speed at which the motor is allowed to operate under rated torque within its continuous operating range.
Continuous stall torque MO: The maximum torque that the motor can output when stalled within the continuous operating range. Generally, speeds below 100 rpm are considered the stall operating range.
Continuous stall current I0: The current corresponding to continuous stall torque.
Peak torque Mmax: The maximum allowable torque output by the motor. Different manufacturers specify different conditions, and the differences can be significant. Some label it as the torque corresponding to the demagnetizing current; however, such labeling is actually unusable. Mechanical designers must allow sufficient margin to prevent excessive working torque from causing motor demagnetization and failure. Maximum torque labeled according to duty cycle is of engineering reference value. Peak torque labeled according to S3-10% is the most valuable for engineering reference, and can be understood as the maximum allowable working torque for 3 seconds of continuous operation, which is sufficient for robots. The repetitive overload of multi-joint robots is generally around 2.0 times.
Peak current Imax: The operating current corresponding to the peak torque.
Electrical time constant Te: A characteristic constant of the response speed of current to applied voltage. It is defined as the time taken for the current to become 1-e-1 (approximately 63.2%) of the final current after a fixed voltage is applied between the motor terminals. The electrical time constant of a servo motor is generally the ratio of the inductance to the resistance of the stator winding (te=L/R), which is related to the current step response time of the servo system, but not necessarily equivalent.
Mechanical time constant Tm: The mechanical time constant of a servo motor is defined as tm = R*J/Ke*Kt, which is related to the winding resistance, rotor inertia, motor back EMF coefficient, and motor torque coefficient. The mechanical time constant of a drive motor is approximately equivalent to the time required for it to accelerate from zero speed to 63.2% of its equilibrium speed under no-load conditions. In a servo system, this constant may be quantitatively equivalent to the system's speed loop step response time.
Back EMF constant Ke: The no-load back EMF value induced in the motor at unit speed. It is typically expressed as the no-load back EMF per 1000 rpm, with units of V/Krpm.
Torque constant Kt: The motor output torque per unit current. The relationship between the motor's back EMF coefficient Ke and torque coefficient Kt is generally Kt = 9.55 * Ke * 1.732, where Kt is in Nm/A and Ke is in V/rpm, Ke = Kt. Ke here represents the line back EMF.
If the motor datasheet does not provide Kt and Ke parameters, Kt can be derived from the rated torque and rated current. Then, the line back EMF coefficient Ke can be indirectly derived from Kt = 9.55 * Ke * 1.732, i.e., Ke = 0.1047 * Kt / 1.732, in V/rpm; or Ke = 104.7 * Kt / 1.732, in V/Krpm or mV/rpm.
Due to power supply voltage limitations, the back EMF constant of the motor must be designed to be relatively low to ensure high response and sufficient voltage difference at high speeds to generate adequate current. However, high current increases the motor's heat load. Therefore, robot motors need to have high power density to achieve small size, high torque, and low heat generation.
Rotor moment of inertia J: The moment of inertia of the motor rotor. The moment of inertia of the robot motor is very important, directly affecting the stability of the robot's operation. This is because robots are often multi-axis linked. For example, the second axis of an articulated robot requires a large motor inertia to accommodate the huge load inertia changes when the arm extends and retracts.
Cogging torque: When the winding of a motor with a permanent magnet is open-circuited, the periodic torque generated during one revolution of the motor is due to the slotting of the armature core, which tends to approach the position of minimum magnetic resistance.
Overload capacity: Under specified conditions, the ability of a motor to output a certain power or torque within a specified time without exceeding the specified peak current. The ratio of peak current to rated current is usually called the current overload factor, and the ratio of peak torque to rated torque is called the torque overload factor. Typically, robot motors need to guarantee approximately 3 times the torque overload.
Maximum speed nN: The maximum speed that the motor can achieve within the intermittent operating range. Different motor manufacturers have very different definitions of maximum speed; robot motors often specify the highest speed at which they can repeatedly operate during actual operation. At the maximum speed, the corresponding maximum torque can exceed twice the rated torque, thus ensuring acceleration response across the entire speed range.
Introduction to the performance requirements and selection of servo motors for robots
Linear robotic arm
As the name suggests, Cartesian robots primarily move in straight lines, as shown in Figure 3. The product parameters of Cartesian robots are mainly based on load and travel distance for selection and design. Typical load capacities are 2kg, 5kg, 10kg, 15kg, 25kg, 35kg…200kg. Travel distance is usually customized; the X-axis can be extended indefinitely, the effective travel distance of the Y-axis is within 3m, and the effective travel distance of the Z-axis in double-segment multiples is within 3.5m.
In Cartesian robots, the speed is generally within 5 m/s, the acceleration is within 10 G, and the repeatability is usually within 0.5. In long-stroke Cartesian robots, accuracy and speed are the foundation, while in short-distance strokes, high acceleration and accuracy are the foundation.
Parallel robots
Delta robots are high-speed, light-load parallel robots. They typically capture target objects through teach programming or vision systems, and use three parallel servo axes to determine the spatial position of the gripper center, enabling operations such as handling and positioning of the target object.
Delta robots are primarily used for sorting, processing, and assembling food, pharmaceuticals, and electronic products. Due to their lightweight, small size, high speed, precise positioning, low cost, and high efficiency, Delta robots are widely used in the market. They are named for their resemblance to an inverted Δ symbol.
Delta robots typically carry a load of 2-3 kg, can reach speeds of up to 10 m/s, and accelerations of up to 150 m/s², capable of grasping more than 300 times per minute. This necessitates extremely fast response times from the servo motors. Since the load is relatively fixed, lower motor inertia is preferable to ensure better response under high acceleration and minimize self-damage.
Planar joint robot
SCARA robots, also known as horizontal articulated robots, are a special type of industrial robot with cylindrical coordinates. They typically have four degrees of freedom, including translation along the X, Y, and Z axes and rotation about the Z-axis. SCARA robots are characterized by low payload and high speed, making them primarily used in industries such as 3C (computers, communications, and consumer electronics) and food processing for rapid sorting and precision assembly.
The configuration of the servo motors is roughly as shown in Table 3.
Multi-joint robotic arm
Articulated robots, also known as articulated arm robots or articulated robotic arms, are one of the most common forms of industrial robots in today's industrial fields, suitable for automated mechanical operations in many industries. They have five or six rotating axes, similar to a human arm. Applications include loading and unloading, painting, surface treatment, testing, measurement, arc welding, spot welding, packaging, assembly, machine tools, fixing, special assembly operations, forging, and casting.
As shown in Figure 6, the mechanism driven by the servo motor changes its posture continuously during operation, and the motor itself is part of the weight of the mechanism. Therefore, the motor inertia must have a certain range of disturbance resistance so that it can work easily in the stable region. This requires that motors from the first axis to the sixth axis be medium to large inertia motors. In particular, the inertia of the second axis is very different in the two postures of fully extended and retracted arm. It is best to use a motor with large inertia.
The topic of inertia ratio and stable control operating range is well-established and can be found in other professional articles. Table 4 shows typical motor selection for articulated robots in engineering; motor inertia varies depending on the response and the nature of the operation.
Servo Motors for Industrial Robots and Selection
Based on the demands of industrial robots for servo motors and aiming at the performance of products from advanced companies in Germany and Japan, Dengqi's latest GK9 series industrial robot-specific servo motors are developed with a power range of 0.1~37kW and a rated speed of up to 6000rpm. These motors come in over 20 specifications across 7 frame sizes and boast a high power-to-volume ratio. Through electromagnetic and structural design, the use of new materials, and research into new processes, key technical indicators such as cogging effect, volumetric power density ratio, overload multiple, temperature rise, and efficiency have been optimized. Furthermore, addressing the issues of high stator coil end ratio and large inter-turn gaps affecting heat transfer, a direct thermally conductive winding insulation system integrating stator vacuum insulation impregnation and vacuum high thermal conductivity resin potting technology has been developed. This significantly improves stator thermal conductivity, enhances the reliability of inter-turn insulation and ground insulation, and strengthens resistance to corona and surge impacts, enabling the servo motor to operate reliably in humid, hot, salt spray, and moldy environments. The use of a shell-less stator eliminates the need for heat-shrink casing, reducing energy consumption. The planar non-stop assembly and integral machining process eliminates the turning, drilling, and tapping processes for the casing, front cover, and rear cover, improving motor manufacturing efficiency and reducing product costs. The servo motor flange face, stop, and front and rear bearing housings are machined as a whole in a single clamping without turning around, eliminating the cumulative errors of separate machining and assembly. This effectively improves the rotational accuracy of the servo motor, enhancing its reliability and service life.
Taking the Dengqi GK9 series as an example, the commonly used robot motor servo parameters are shown in Table 5.
(Note: The explanation of servo motor parameters in this article references the "Performance Test Specification for Servo Motors in Electrical Equipment and Systems of CNC Machine Tools" edited by Harbin Institute of Technology. The linear robotic arm was completed with the assistance of Mr. Chen Furu from Ningbo Weili Robotics. The parallel robot was completed with the assistance of Mr. Liu Jiahua from Foshan Jiuzhou Automation Technology Co., Ltd. The articulated robot was completed with the assistance of Mr. Yan Caizhong from Shanghai Xinshida. Sincere thanks are extended to all the aforementioned industry colleagues.)
