From the robot arm structure above, we can see that the motor and reducer are installed at the joints of the robot to control the joint movement. Then the problem arises: In a robot system, why can't we directly control the servo motor rotor speed to control the joint movement, why do we need a reducer? To answer this question, we must first clarify the working conditions of the joints of industrial robots: 1. The joints of industrial robots need to hold the torque generated by the back-end mechanism due to gravity. 2. Industrial robot joint speed is not high. The angular velocity of the robot joint is very low, and the motor can not rotate smoothly at a very low speed. The control is not easy, and a mechanism is needed to allow the motor to move at a reasonable speed to ensure smooth movement. There are two reasons for using a reducer. The first is to increase the torque, and the second is to provide control resolution and closed-loop accuracy. For example, a 50:1 harmonic reducer can easily increase the rated torque of a 100mNm rated motor to 5Nm, at the cost of: 1. 49 times faster rotor speed than direct drive Originally, the rotational speed of the joints of industrial robots is not high, generally one or two revolutions per second, rated 100mNm motor easily run 6k rev / min, white does not turn so fast. If the transfer is not fast enough, the solution is to raise the voltage, but this should consider whether the bearing and the rotor can hold it. 2. The weight is increased by 3 times For example, maxon EC45's thickest motor has a rated torque of 83mNm and weighs 110g; maxon EC90 has a rated torque of 560mNm and weighs 600g. Starting from these two data, you can make up for the number of times that the rated 5Nm motor weight. 3. When the same torque is maintained, the heating power is 1/2500 without speed reducer Actually, it does not mean that a motor rated at 100mNm is less than 5Nm, but it can be done. As long as the current is good enough for the dead, it will cause the motor to get hot quickly, and it will take a few seconds to smoke. Water-cooled electricity charges will also be more. If you want to achieve the same torque and do not want to be too hot, you have to change the torque / high heat efficiency and thermal resistance, heat capacity of the large motor, but this will go back to the 2 said the problem. In addition, the benefits of using reducers include: 1. Achieve higher resolution with lower price machines An ordinary 5k-ray photoelectric encoder can achieve 1.44mdeg angular resolution (of course, if too much money on the sine cod encoder hard subdivision can also be achieved); or a 5-phase 1000-step stepper motor can Do a resolution of 7.2mdeg (here is Dongfang Motor's 33 step + 50:1 harmonics). The advantage of high resolution is that the speed control can be more accurate, as the high-frequency components generated by the step caused by the quantization become very small and the control is smoother. 2. Improve closed-loop accuracy and better control loop Since there is a large reduction ratio of 50:1, the output shaft of the reducer is transmitted to the motor end by the disturbance and is reduced by 37dB compared with the direct drive, making the closed-loop accuracy higher in the output shaft of the reducer. At the same time, the equivalent inertia of the rotor is increased to 2500 times, which makes the hysteresis loop of the control loop dominated by the rotor inertia, and the rotor is directly driven by the electromagnetic force so that there is no torque lag caused by the stiffness, which is better than the direct drive. In addition to the above technical aspects of the answer, there is a case here, to answer this question indirectly from another perspective. A customer is a model 6150 lathe with 10 inch (about 250mm diameter) hydraulic chuck. The low speed high torque (torque) characteristics of the spindle motor (AC induction motor) are used to achieve a unilateral 7mm cutting capacity. It is proposed to select a 7.5 kw motor with a rated speed of 1000 rpm while using a 1:2 gear ratio (deceleration) gear. It is known that the gear ratio is proportional to the power and torque, that is, using a gear ratio of 1:2, the motor of 7.5 kw achieves a characteristic of 15 kw and 2 times of torque at the same time (note here, not referring to 2 times rated torque). If you choose a 15kw motor, the price will be a lot more expensive, and the installation size will also have to be larger. However, a pair of gears with a 1:2 transmission ratio does not require a lot of money. Do not understand can see the following asynchronous motor power, torque curve. (Note: Unlike DC motors, asynchronous motors may operate beyond the transverse torque area.) In addition to obtaining low speed, high torque characteristics, the DC motors used in robots may also have this reason, and the gear ratios used may be larger (2 stages or higher). Of course, using a speed reducer is not perfect, but there are still some drawbacks, but it is more appropriate to use a speed reducer in comparison. The disadvantages of using a reducer: 1. If a speed reducer is equipped and the encoder is mounted on the motor side, the manufacturing accuracy of the speed reducer will affect the actual accuracy; 2. The multi-stage amplification will still cause the decrease of the repeat accuracy. 3. After all, there are gear reducers or gear flexion, there is a life limit; 4. The non-linear coupling of the multi-linkage mechanism's backlash makes the absolute accuracy of the robot worse, so the industrial robot only talks about the repeatability of positioning irrespective of the absolute accuracy, making it difficult for the robot to perform off-line programming purely, which increases the difficulty of deployment. And costs. The above is why it is not possible to directly control the rotation speed of the servo motor to control the joint motion, but rather the reason for using the speed reducer. Although there are now direct-drive motor-driven robots, due to the above problems, maturity is still worse. In short, the use of speed reducers on industrial robots is to use high speeds that are easily achieved by motors in exchange for high torque and low mass that are not easily achieved by motors. 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