The concept of robots is already very broad. This article discusses the servo motors for robot joints required by the industrial automation industry, and does not involve the composite and integrated joint servo motors of service robots.
Industrial robots are generally classified into linear robots (also called Cartesian robots), multi-degree-of-freedom robots (also called multi-joint robots), parallel robots (also called deltaΔ robots), and horizontal multi-joint robots (also called scara robots). An automation island composed of various types of joint 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 automation workshops.
Between these automated industrial robots and automated units, the servo motor is always in the key role of accurately, timely and stably transmitting the mechanism in place according to the requirements of the control instructions, so it is a core component.
Basic Concepts of Permanent Magnet Servo Motors
Servo means that it can be executed according to the instructions of the control computer system. It is not limited to electric motors and hydraulics, including pneumatics. All components that can complete the task are servo originals.
An electric motor is an electromechanical conversion component that converts electrical energy into mechanical energy. Servo motors are motors that can be used in motion control systems, and their output parameters, such as position, speed, acceleration, or torque, are controllable.
Due to different control indicators, servo motors can have different types. According to the type of power supply, it can be divided into AC servo motor and DC servo motor; according to the operation mode, it can be divided into linear servo motor and rotary servo motor. Linear motors directly generate Newtonian force, and rotary motors output rotational torque. A rotary motor drives a linear load and requires a mechanical mechanism such as a lead screw to convert rotary motion into linear motion.
Rotary AC servo motors are divided into AC asynchronous servo motors and AC synchronous servo motors according to the rotor structure. The rotor of the AC asynchronous servo motor is made of aluminum or copper squirrel cage, and the speed of the squirrel cage always has a certain speed difference with the synchronous rotating magnetic field. This type of motor can obtain the same perfect torque control characteristics as the DC motor under the vector speed regulation technology, but the rotor has the characteristics of large inertia, good constant power characteristics and wide speed regulation range, which is suitable for machine tool cutting and printing machinery rewinding and unwinding. The disadvantage is that the starting torque is small, the electromagnetic response speed is not as good as that of the permanent magnet servo motor, and the electromagnetic time constant value is about 10 times that of the permanent magnet motor made of permanent magnet materials, and due to the low power density, the rotor The size is large, so it is not suitable for high dynamic servo occasions.
The rotor of the rotary AC synchronous servo motor is made of permanent magnet material, which directly generates the excitation magnetic field. There is no process that requires the 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 materials, this type of motor has a high power density, which provides the possibility to further design servo motors with various characteristics. The high dynamic response can be designed with a slender small rotor inertia, or a stubby large rotor inertia. The use of rare earth permanent magnet materials makes permanent magnet motors the first choice for servo motors. Because rare earth permanent magnet material is still the most expensive part of servo motor materials. The differences in the materials used by different manufacturers divide the product quality into different levels. A good permanent magnet material may not demagnetize at a working temperature above 150°C, and a poor permanent magnet material may demagnetize when the motor working temperature is less than 120°C. The permanent magnet material directly determines the different characteristics of the servo motor.
The linear servo motor directly outputs the Newton force, and can obtain high acceleration without the need for mechanical conversion. In recent years, the technology has progressed rapidly, and it is widely used in the feed axis of high-performance machine tools. In industrial robots, it is only partially used for linear robotic arms. , not the focus of this article. This article focuses on the rotary permanent magnet servo motor and its application in industrial robots.
Structure of Rotary Permanent Magnet Motor
Figure 1 Structure diagram of permanent magnet servo motor
Figure 1 shows the structure of a typical permanent magnet servo motor. For a comprehensive description, try to see the whole picture of the permanent magnet servo motor structure through a picture. 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 the installation feet, and the installation of lifting rings is also unnecessary. concise. In this way, the shape of the motor becomes as shown in Figure 2(a). If the motor is small, within 1kw, the aviation plug for the lead wire is unnecessary. b) as shown.
Figure 2 Outline drawing of small power permanent magnet servo motor
It is assumed here that the reader understands the principle of the motor, and only differentiates the structure of the permanent magnet servo motor from the characteristics of the robot motor.
Bearings: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 ensure a service life of at least 30,000 hours. In this way, according to the 8-hour work system, at least the service life of the robot is more than 10 years, and the rotation speed of the bearing must ensure that the 6000rpm can work intermittently.
Stator laminations and windings:Because the robot motor requires high power density, in order to be small in size and low in iron consumption and heat generation, the punching material should be cold-rolled silicon steel sheets below 0.35mm. The winding must withstand the impact of 16K variable frequency carrier pulse for a long time. In order to prevent it from being broken down and withstand intensive dv/dt impact, the withstand voltage should not be lower than 2500V.
Rotor permanent magnet material:Permanent magnet material is the most expensive part of permanent magnet servo motor. Materials with low content of rare earth elements have low Curie point and poor material stability. If NdFeB permanent magnets are used, it is better to be above UH42, and attention should be paid to rare earths such as dysprosium. The content of the element, in order to ensure high temperature resistance to demagnetization, samarium cobalt permanent magnets are often widely used in small and medium servo motors. In short, it is necessary to ensure that the servo motor will never demagnetize in normal use. Otherwise, the long-term stability of the robot cannot be guaranteed.
Shaft seal:In order to prevent oil pollution and sundries from entering the motor, and to ensure the operation, it is a conventional design to add a shaft seal to the shaft end of the motor. Robots often mill a pinion at the motor shaft end of the servo motor. The motor and the reducer are directly connected. High temperature and oil pollution may enter the motor. Therefore, multi-lip high-temperature shaft seals are required. Nitrile rubber shaft seals are reliable, but of course the cost varies greatly.
Brake: The holding brake is the basic option of the robot motor. More than 95% of servo motors need a brake. To ensure the brakes at all times, especially during an emergency stop, the brakes need to have a sufficient safety factor. The static torque is about 1.5 times the rated torque of the motor. Heavy-duty robots The safety factor of the motor holding brake should reach 2.0 or even 2.5 times. One thing to note is that the holding brake of the robot motor is a safety brake, not a brake brake. The control must ensure that the braking circuit of the servo drive works through the braking resistor in the emergency stop state, and the holding brake operates when the motor speed is close to 0. . In order to improve the response speed of the brake, the permanent magnet brake is better than the electromagnetic spring brake.
Encoder: The encoder is installed at the end of the motor and belongs to the 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 given to the control computer for motion trajectory calculation. Robot motor encoders generally have low accuracy, but they need multi-turn absolute position measurement to ensure that after power failure, it will run again, and the position before power failure can be memorized. There are currently three popular ways to solve the problem of the robot motor encoder. The first way is to use Gray code photoelectric or magnetic code disc for single-turn, and mechanical gear for multi-turn. The advantage of this is that the measurement accuracy is high. After the power is turned off, the operating position of the motor will be remembered through the mechanical position of the encoder, and it can be read directly after power-on. too long. The second is that single-turn confidence is stored by photoelectric or magnetic gray code, and multi-turn is stored electronically through battery power supply, so that the encoder can be made very short, which is very suitable for small servo motors smaller than 60mm. The disadvantage is that the battery life is relatively short, as long as 2-3 years, and some need to replace the battery every 1 year. The third method is to use the resolver to measure the single-turn position only when the precision requirement is not high, and the multi-turn information is completed by the circuit board with battery in the control box.
Rotor shaft extension:Due to frequent forward and reverse rotation, the motor bears a certain shear force, and the material of the shaft is preferably modulated with 42CrMo. If the motor is installed with keys, the keys should be fully loaded anyway, so as to effectively reduce the dynamic balance and runout of the motor. Under high-speed operation, the difference between the no-load running beat of the servo motor with key and the optical axis is as much as 9 times, which should not be underestimated.
Main transmission parameters of permanent magnet servo motor
Workspace: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 the continuous working area; outside the continuous working area, the area where the motor is allowed to run for a short time is called the intermittent working area. The working area is represented by two-dimensional plane coordinates of torque and rotational speed.
Rated power PN:In the continuous working area, the maximum power that the motor can output.
Rated torque MN: Torque when the motor outputs rated power in the continuous working area. The definition of rated torque varies widely from manufacturer to manufacturer. Generally, the corresponding cooling conditions must be specified. The common practice abroad is to explain the area and thickness of the aluminum plate flange to be installed, and the flange temperature is guaranteed to be measured at 20°C or a given temperature. Therefore, in actual work, it is often installed in cast iron, and the high temperature in summer exceeds the test standard temperature, so if there is no margin in use, it will cause overheating demagnetization. The national standard stipulates that the standard condition of the ambient temperature of 40 degrees is more reasonable for the Chinese environment. Serious manufacturers will reserve a certain design margin as the published rated torque under the rated value determined according to the standard, which is safer.
Rated current IN:The current corresponding to the rated torque.
Rated speed nN:The maximum speed at which the motor is allowed to work under the rated torque in the continuous working area.
Continuous stall torque MO: In the continuous working area, the maximum torque that the motor can output when the motor is stalled. Generally, the rotation speed lower than 100rpm is regarded as the locked rotor working area.
Continuous stall current I0:Corresponds to the current at continuous stall torque.
Peak torque Mmax:The maximum torque allowed by the motor. Different manufacturers have different nominal conditions, and the difference is very large. Some are marked as the torque corresponding to the demagnetization current. In fact, such a label cannot be used to change the peak torque. The mechanical designer must leave enough margin to prevent the motor demagnetization failure due to excessive working torque. If the maximum torque is marked according to the working system, it has reference value in engineering. The peak torque marked according to S3-10% is the most reference value for engineering. It can be understood as the maximum working torque allowed by the continuous working time of 3s, which is sufficient for the robot. The repetitive overload of a multi-joint robot is generally around 2.0 times.
Peak current Imax:The working current corresponding to the peak torque.
Electrical time constant Te: The characteristic constant of the response speed of the current to the applied voltage, defined as the time taken for the current to become 1-e-1 (about 63.2%) of the final current after a fixed voltage is applied between the motor terminals. The electrical time constant of the servo motor is generally the ratio of the inductance to the resistance of the specified sub-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 the servo motor is defined according to the definition: tm=R*J/Ke*Kt, which is related to the winding resistance, rotor moment of inertia, motor back EMF coefficient, and motor torque coefficient. The mechanical time constant of the drag motor is approximately equivalent to the time it takes to accelerate from zero speed to 63.2% of equilibrium speed with no load. In a servo system, this constant may be quantitatively comparable to the speed loop step response time of the system.
Back EMF constant Ke:The no-load back EMF value induced by the motor at unit speed. Conventional refers to the no-load back EMF corresponding to every 1000rpm, and the unit is V/Krpm.
Torque constant Kt:Motor output torque corresponding to unit current. The relationship between the back EMF coefficient Ke and the torque coefficient Kt of the motor is generally Kt=9.55*Ke*1.732, where the unit of Kt is Nm/A, the unit of Ke is V/rpm, and Ke=Kt. Ke here is the line back EMF.
If the Kt and Ke parameters are not given in the motor data, Kt can be derived according to the rated torque and rated current, and then the line back EMF coefficient Ke can be derived indirectly according to Kt=9.55*Ke*1.732, namely: Ke=0.1047*Kt/1.732, Unit V/rpm; or: Ke=104.7*Kt/1.732, unit V/Krpm, or mV/rpm.
Due to the limitation of the power supply voltage, in order to ensure high response, the back EMF of the motor should be designed relatively low to ensure that there is enough voltage difference at high speed to obtain sufficient current. The large current increases the heating burden of the motor. As a result, the power density of the robot motor is higher, and it can achieve small volume, 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, which is directly related to the stability of the robot’s work. Because robots are often multi-axis linkage. For example, the second axis of a joint robot requires a large motor inertia to adapt to the huge load inertia changes when the arms are opened and retracted.
Cogging torque:When the winding of the motor with permanent magnets is open, the periodic torque is generated due to the slotting of the armature core, which tends to tend to the minimum reluctance position within one revolution of the motor.
Overload capacity:Under specified conditions, the motor can output a certain power or torque within a specified time without exceeding the specified peak current capability. Usually the ratio of the peak current to the rated current is called the current overload multiple, and the ratio of the peak torque to the rated torque is called the torque overload multiple. Usually, the robot motor should ensure a torque overload of about 3 times.
Maximum speed nN: The maximum speed that the motor can reach in the intermittent working area. Different motor factories have very different definitions of the maximum speed, and the robot motor often gives the maximum speed that can be repeated in actual operation. At the highest speed, the corresponding maximum torque can exceed twice the rated torque, thus ensuring the acceleration response in the full speed range.