Permanent magnet servo motor for key parts of industrial robot
The concept of robots is already very extensive. What is discussed in this paper is the robot joint permanent magnet servo motor needed by the industrial automation industry, while the composite integrated joint servo motor of service-oriented robots is not involved for the time being.
Industrial robots are generally classified as linear robots (also known as rectangular coordinate robots), multi-degree-of-freedom robots (also known as multi-joint robots), parallel robots (also known as delta △ robots) and horizontal multi-joint robot (also called scara robot). An automation island composed of various types of joint mechanical arms and automatic transmission equipment. Automation lines are formed by linking automation islands with different functions, and these automation lines combine into the automation workshop.
Between these automatic industrial robots and automation units, servo motor is always in the key role of transmitting the mechanism accurately, timely and steadily according to the requirements of control instructions, so it belongs to the core components.
Basic concept of permanent magnet servo motor
Servo is the meaning that it can be executed according to the instructions of the control computer system. It is not only limited to motor and hydraulic pressure, including pneumatic pressure, but all components that can complete this task are servo originals.
A motor is a Electromechanical conversion component that converts electrical energy into mechanical energy. Servo motor is a motor that can be applied to motion control system. Its output parameters, such as position, speed, acceleration or torque, are controllable.
Due to different control indicators, servo motors can have different types. According to different power types, it can be divided into AC servo motor and DC servo motor; According to the mode of operation, it can be divided into linear servo motor and rotary servo motor. Linear motor directly generates Newton Force, and rotary motor outputs rotating torque. The linear load driven by the rotary motor needs to convert the rotary motion into linear motion through mechanical mechanisms such as screw screws.
According to the rotor structure, rotary AC servo motor can be divided into AC asynchronous servo motor and AC synchronous servo motor. AC asynchronous servo motor rotor is aluminum or copper mouse cage mouse cage has a certain speed difference with synchronous rotating magnetic field. Under the vector speed regulation technology, this type of motor can obtain the perfect torque control characteristics as DC motor, but the rotor has the characteristics of high inertia, good constant power characteristic and wide speed regulation range, it is suitable for large-scale variable inertia load such as machine tool cutting and winding and unwinding of printing machinery. The disadvantage is that the starting torque is small, and the electromagnetic response speed is not as good as permanent magnet servo motor, the constant value of electromagnetic time is about 10 times that of permanent magnet motors made of permanent magnet materials. Due to low power density and large rotor size, it is not suitable for high dynamic servo occasions.
The rotor of rotary AC synchronous servo motor uses permanent magnet materials to directly generate the excitation magnetic field, so there is no process of needing excitation current to establish the motor magnetic field, so the electromagnetic response is fast. Moreover, due to the high energy density of the current rare earth permanent magnet material, the power density of this kind of motor is high, which provides the possibility for further designing the servo motor with various characteristics. High Dynamic response can be designed into slender small rotor inertia, and also can be designed into coarse and short large rotor inertia. The use of rare earth permanent magnet materials has laid the first choice for permanent magnet motors as servo machines. As rare earth permanent magnet materials are still the most expensive parts of servo motor materials. The difference of materials used by different manufacturers divides the product quality into different levels. Good permanent magnet materials can be not demagnetized above the working temperature of 150℃, poor permanent magnet materials may be demagnetized when the working temperature of the motor is less than 120℃. Permanent Magnet Materials directly determine the different characteristics of servo machines.
Linear servo motor outputs newton force directly, and high acceleration can be obtained without mechanism conversion. In recent years, technology has made rapid progress, and it is widely used to feed shafts of high-performance machine tools. In industrial robots, it is only partially used for linear mechanical arms, not as the focus of this article. This paper focuses on rotary permanent magnet servo motor and its application in industrial robots.
Structure of rotary permanent magnet motor
Fig. 1 structure chart of permanent magnet servo motor
Figure 1 shows the structure chart of a typical permanent magnet servo motor. In order to describe comprehensively, the whole structure of permanent magnet servo motor can be seen through one figure as far as possible. In fact, the low-power permanent magnet servo motor is within 15kw, which can naturally dissipate heat without designing a cooling fan. The motor is small and does not need to be fixed by installing a Foundation pin. It is not necessary to install a ring, it is more concise to change the lead line from the junction box to the Aviation plug. In this way, the shape of the motor will become as shown in Figure 2(a). If the motor is small and within 1kw, The Aviation plug used for the lead-out is also unnecessary, directly leading to a section of cable, it becomes as shown in Figure 2 (B).
Fig. 2 outline diagram of low-power permanent magnet servo motor
It is assumed here that the readers understand the principle of the motor and only explain the structure of the permanent magnet servo from the characteristics of the robot motor.
Bearing: the service life of the servo motor is closely related to the bearing. Due to the requirements of the robot for reliability and durability, the bearing must at least ensure the service life of 30000 hours. In this way, according to the 8-hour working system, at least the service life of the robot is more than 10 years, and the rotation speed of the bearing must ensure that 6, 000rpm can work intermittently.
Stator punching and winding: because the robot motor needs high power density, in order to be small in volume and low in heat consumption, the punching material should be cold-rolled steel sheets below 0.35mm. The winding has to endure 16k variable frequency carrier pulse impact for a long time. In order to prevent breakdown, it bears dense dv dt impact, and the withstand voltage is either lower than 2500v.
Rotor permanent magnet material: permanent magnet material is the most expensive part of permanent magnet servo motor. The material with low content of rare earth elements has low center point and poor material stability. If neodymium iron boron permanent magnet is used, it is better to be above UH42, and, attention should be paid to the content of dysprosium and other rare earth elements. In order to ensure high temperature anti-demagnetization, samarium-cobalt permanent magnets are also widely used in small and medium servo motors. In short, it is necessary to ensure that the servo motor is really never demagnetized in normal use occasions. Otherwise, the long-term stable performance of the robot cannot be guaranteed.
Shaft seal: in order to prevent oil and sundries from entering the motor, and to ensure operation, the shaft seal on the motor shaft end is a conventional design. Robots often mill a small gear on the motor shaft of the servo motor. The motor is directly connected to the reducer. High temperature and oil may enter the motor. Therefore, multi-lip high temperature shaft seal is needed, for example, the double lip fluorine rubber shaft seal is more reliable than the single lip nitrile rubber shaft seal, of course, the cost difference is also very large.
Lock: the lock is the basic option of the robot motor. Nearly 95% of the servo motor needs to be locked, so it is necessary to ensure the timing of the brake, especially for reliable operation during emergency shutdown, and the brake needs to have sufficient safety factor, the static torque is about 1.5 times of the motor’s forehead torque, and the safety factor of the motor brake of the heavy-duty robot is 2.0 or even 2.5 times. There is one thing to be noticed, the brake of robot motor is a safety brake, not a brake. In the brake control, it should be ensured to let the brake circuit of servo driver work through brake resistance in the state of emergency stop, when the motor speed is close to 0, the brake acts. In order to improve the response speed of the brake, the permanent magnet brake is better than the electromagnetic spring brake.
Encoder encoder is installed at the end of the motor, which belongs to motor speed and rotor position sensor. The position of the rotor can be measured for servo control magnetic field positioning and the actual position and speed of the rotor can be used by the control computer for motion trajectory calculation. The robot motor encoder generally has a low precision, but it requires multiple laps of absolute position to be measured to ensure that after the power is off, it runs again, and the position in front of the power off can be memorized. At present, there are three popular ways to solve the problem of robot motor encoder. The first way is to use gray code photoelectric or magnetic code disc for single circle and mechanical gear. The advantage of this is that the measurement accuracy is high. After the power is off, the date remembers the running position of the motor through the mechanical position of the encoder. After the power is on, it can be read directly, but the disadvantage is that the encoder is too thick, it is too long under the limited installation space. The second is that the single-circle confidence is powered by battery and electronic memory through photoelectric or magnetic gray code memory, so that the encoder can be made very short, which is very suitable for small servo motors less than 60mm. The disadvantage is that the service life of the battery is relatively short, which is 2-3 years long, and some batteries need to be replaced in 1 year. The third way is to use the rotary transformer to measure the single circle position and multi-circle information in the case of low precision requirements, which is completed by the circuit board with battery in the control box.
Rotor shaft extension: Due to frequent forward and reverse rotation, the motor is subject to a certain shear force, and the shaft material is preferably modulated by 42crmo. If the motor is installed with a key, the key must be fully installed in any case, so as to effectively reduce the dynamic balance and jitter of the motor. Under high-speed operation, the no-load running runout of the servo motor with key and optical shaft is as much as 9 times, which cannot be underestimated.
Main transmission parameters of permanent magnet servo motor
Working area: under the condition that the temperature rise of the motor does not exceed the allowable temperature rise, the area where the motor can work for a long time is called continuous working area; Outside the continuous working area, the area that allows the motor to operate for a short time is called the intermittent working area. The work area is represented by two-dimensional planar coordinates of torque and speed.
Forehead fixed power PN: The maximum power that the motor can output in a continuous working area.
Rated torque MN: the torque when the motor outputs certain power in the continuous working area. Different manufacturers have very different definitions of rated torque. Generally, the corresponding cooling conditions should be specified. The common practice abroad is to show that the change index is to install on the aluminum flange with the area and thickness, and the flange temperature is guaranteed to be measured at 20℃ or a given temperature. Therefore, in actual work, it is often installed in iron castings, and the high temperature in summer exceeds the standard temperature of the test. If there is no allowance during use, it will cause overheating and demagnetization. The standard condition of 40 degrees ambient temperature stipulated by the national standard is more reasonable for china’s environment. Serious manufacturers will keep a certain design allowance as the published rated torque under the rated value measured by the standard, which is safer.
The forehead current IN: the current corresponding to the forehead torque.
Forehead fixed speed nN: The motor is in the continuous working area, and the highest rotation speed is allowed to work under the rated torque.
Continuous locked-rotor torque MO: the maximum torque that motor can output when it is locked in the continuous working area. If the general rotating speed is lower than 100rpm, it is regarded as a blocked working section.
Continuous locked-rotor current I0: The current corresponding to the torque of continuous locked-rotor.
Peak torque Mmax: maximum torque allowed for motor output. The nominal conditions of different manufacturers are different and vary greatly. Some are marked as torque corresponding to demagnetization current, which is actually not allowed to be changed to peak torque, and mechanical designers should keep enough allowance to prevent the motor from demagnetization failure due to excessive working torque. If the maximum torque is marked according to the working system, it is of reference value in engineering. The peak torque marked according to S3-is the most valuable for engineering reference, which can be understood as the maximum working torque allowed by 3s of continuous working time, which is sufficient for the robot. The repeated overload of multi-joint robots is generally about 2.0 times.
Peak current Imax: The operating current corresponding to the peak torque.
Electrical time constant Te: the characteristic constant of the current’s response speed to the added voltage is defined as the 1-e-1 of the final current (about 63.2%) the time taken. The electrical time constant of servo motor generally refers to the ratio of inductance and resistance of stator winding (te = L R), which is related to the current step response time of servo system, but not necessarily equivalent.
Mechanical time constant the mechanical time constant of Tm servo motor tm R J /Ke* Kt according to the definition, that is, related to winding resistance, rotor moment of inertia, motor back potential coefficient and motor torque coefficient. The mechanical time constant of the drag motor is approximately equivalent to the time required by the no-load to accelerate from zero speed to the balance speed of 63.2%. In a servo system, this constant may be equivalent in number to the speed Loop step response time of the system.
Back-electromotive force constant Ke: the no-load back-potential value induced by the motor at unit speed. Conventional refers to the corresponding no-load back potential per 1000rpm, in V /Krpm.
The torque constant Kt unit current corresponds to the motor output torque. The relationship between the back potential coefficient Ke of the motor and the torque coefficient Kt is generally Kt = 9.55 * Ke * 1.732, of which the unit of Kt is Nm/A and the unit of Ke is V /rpm,Ke= Kt. Ke here is the line back potential.
If no Kt and Ke parameters are given in the motor data, Kt can be derived according to the rated torque and temporal current, and then the line back potential coefficient Ke can be indirectly derived according to Kt = 9.55 * Ke * 1.732, I .e: ke = 0.1047 * Kt/1.732, unit V/rpm; Or: Ke = 104.7 * Kt/1.732, unit V/Krpm, or m V/rpm.
Due to the limitation of power supply voltage, in order to ensure a high response, the back potential of the motor should be designed relatively low to ensure sufficient pressure difference at high speed to obtain sufficient current. However, large current increases the heating burden of the motor. Therefore, the power density of the robot motor is relatively high, which can realize small volume, large torque and low heat.
Rotor moment of inertia J moment of inertia of rotor of motor. Moment of inertia of robot motor is very important, which is directly related to the stability of robot work. Because robots are often multi-axis linked. For example, the second axis of the joint robot needs a large amount of motor inertia to adapt to the huge load inertia change when the arm is expanded and contracted.
Cogging torque: when the winding of the motor with the permanent magnet is open, and within one week of the motor’s rotation, the periodic torque is generated with the trend of the position of the minimum reluctance because the armature core is slotted.
Overload capacity: under the specified conditions, the motor can output a certain amount of power or torque within the specified time without exceeding the specified peak current. The ratio of peak current to forehead current is usually called current overload multiple, and the ratio of peak torque to rated torque is called torque overload multiple. Generally, the robot motor must ensure about 3 times torque overload.
Highest speed nN: the maximum speed of the motor in the intermittent working area. The definition of the highest speed varies greatly from motor factories. Robot motors often give the highest speed at which work can be repeated during actual operation. At the highest speed, the corresponding maximum torque can exceed twice the rated torque, which ensures the acceleration response in the full speed range.
Performance requirements and selection of servo motor for robots
Linear mechanical arm robot
Figure 3 linear mechanical arm robot
As the name implies, the rectangular coordinate robot mainly moves in a straight line, as shown in the figure. The product parameters of rectangular coordinate robot are mainly based on load and stroke, which are 2k, 5kg, 10kg, 15kg, 25kg, 35kg…… 200kg, the stroke is usually mainly customized, the x axis can extend indefinitely, the effective stroke of y axis is within 3m, and the effective stroke of z axis double segment multiple is within 3.5.
In the rectangular coordinate robot, the speed is generally within 5 m/s, the acceleration is within 10g, and the repeated positioning accuracy is usually within 0.5, based on the accuracy and speed of the long-stroke rectangular coordinate robot, based on the high acceleration and accuracy of the short distance stroke.
Parallel robot
Delta robots belong to parallel robots of high speed and light load, and they usually capture target objects through teaching programming or visual system, three parallel servo axes are used to determine the spatial position of the gripper center to carry and locate the target object.
Delta robots are mainly used in the sorting, processing and assembly of food, medicine and electronic products. Delta robots are widely used in the market due to their light weight, small volume, fast moving speed, accurate positioning, low cost and high efficiency. And it derives from the fact that its shape looks like inverted Δsymbol.
The weight of the Delta robot is generally 2kg-3kg, the speed can reach 10 m/s, the acceleration can reach 150m/s2, and it can grab more than 300 times per minute, which requires the response of the servo motor to be very fast. Because the load is relatively fixed, the smaller the inertia of the motor, the better, so as to better response and smaller self-loss under high acceleration.
Fig. 4 parallel robot
Planar joint robot
SCARA robot is also called horizontal multi-joint robot. It is a special type of industrial robot with cylindrical coordinates. There are generally 4 degrees of freedom, including translation along the X 、Y、 Z direction and rotation around the Z axis. SCARA robot is characterized by small load and fast speed, so it is mainly used in 3C industry, food industry and other fields such as rapid sorting and precision assembly.
Fig. 5 plane joint robot
The configuration of servo motor is shown in Table 3.
Multi-joint manipulator
Joint robots, also known as joint arm robots or joint mechanical arms, are one of the most common forms of industrial robots and are suitable for mechanical automation in many industrial fields. Has five or six rotating axes, similar to human arms. Application fields include loading, unloading, painting, surface treatment, testing, measurement, arc welding, spot welding, packaging, assembly, chip cutting machine tool, fixation, special assembly operation, forging, casting, etc.
Figure 6 multi-joint manipulator
As shown in figure 6, the attitude of the mechanism driven by servo motor changes constantly during the working process, and the motor itself is a part of the weight of the mechanism, so the inertia of motor has a certain anti-disturbance range, so that it is easy to work in the stable area, which requires that the motor requires a large inertia motor from the first shaft to the sixth shaft, in particular, the inertia of the second shaft is very different between the two postures of the arm opening and retraction, and the motor is best to use a large inertia motor.
Special servo motor for industrial robot and its selection
Based on the demand of industrial robots for servo motors and aiming at the product performance of advanced enterprises such as germany and japan, aiming at the current ac servo motors is mainly toward high power density, high response, high precision, high speed and high efficiency, in the direction of low temperature rise, the power range of the special servo motor for GK9 series industrial robots newly developed by dengqi is between 0.1 ~ 37kW, and the rated speed can reach more than 7 frame numbers of 6000rpm, high power volume ratio, using new materials through electromagnetic and structural scheme design, researching new technology, optimizing key technical indicators such as motor cogging effect, volume power density ratio, overload multiple, temperature rise and efficiency. At the same time, aiming at the problems of high proportion of the end of the fixed coil of servo motor, large interturn gap and affecting heat transfer, the direct thermal winding insulation system combines vacuum insulating paint dipping of stators with vacuum highly heat-conductive resin potting technology, which greatly improves the thermal conductivity of stators, and improves the reliability of inter-turn insulation and insulation to the ground, the ability to resist corona and surge impact is enhanced, so that the servo motor has the ability to run reliably under the environment of damp-heat, salt mist and mold. The adoption of the coreless stator saves the heat set of the shell and reduces energy consumption; The plane no-stop assembly and the overall processing technology save the turning, drilling, the tapping processing procedure improves the manufacturing efficiency of the motor and reduces the cost of the product; Complete the integral processing of the end face, the cutting edge and the front and rear bearing rooms of the servo motor without turning around at one time, without cumulative error of split processing and assembly, the rotation accuracy of the servo motor is effectively improved, and the reliability level and service life of the servo motor are improved.
Author: fu jiangzhi
(Note: for the explanation of servo motor’s parameter terms, refer to standard for performance test of cnc machine electrical equipment and system servo motor edited by harbin institute of technology. The linear mechanical arm robot part was completed with the help of mr. chen fumu, a ningbo weili robot. The part of the parallel robot is completed with the assistance of mr. liu jiahua of foshan zhuozhou automatic control technology co., ltd. The joint robot part was completed with the assistance of mr. yan caizhong, xinshida, shanghai. I would like to extend my heartfelt thanks to the above-mentioned industry colleagues.)
Source: China Permanent Magnet Manufacturer – www.rizinia.com
