From the robot arm structure diagram above, we can see that motors and reducers are installed at the robot joints to control joint movement. So, the question is:
In a robot system, why can't the joint movement be controlled directly by controlling the rotor speed of the servo motor? Why is a speed reducer still needed?
To answer this question, we must first understand the working conditions of the joints of industrial robots:
1. The joints of an industrial robot need to support the torque generated by gravity in the rear mechanism.
2. Industrial robots have low joint rotation speeds. The angular velocity of robot joints is very low, and motors rotating at extremely low speeds are unstable and difficult to control. A mechanism is needed to allow the motors to move at a more reasonable speed, ensuring smooth movement.
There are two reasons for using a speed reducer: first, to increase torque, and second, to improve control resolution and closed-loop accuracy.
For example, a 50:1 harmonic reducer can easily increase the rated torque of a motor rated at 100 mNm to 5 Nm, at the cost of:
1. Rotor speed is 49 times higher than direct drive.
Industrial robots typically have low joint rotation speeds, usually only one or two revolutions per second. A motor rated at 100mNm can easily run at 6,000 revolutions per minute, so why not make it that fast? If you think it's not fast enough, the solution is to increase the voltage, but this requires considering whether the bearings and rotor can withstand it.
2. The weight has increased to 3 times the original amount.
For example, the thickest Maxon EC45 motor has a rated torque of 83 mNm and weighs 110g; the Maxon EC90 has a rated torque of 560 mNm and weighs 600g. Based on these two figures, you can imagine how much heavier a motor with a rated torque of 5 Nm would be.
3. When maintaining the same torque, the heat dissipation power is 1/2500 of that without a speed reducer.
It's not that a motor rated at 100mNm can't reach 5Nm; it can, by simply increasing the current to its maximum. However, this will cause the motor to overheat rapidly and start smoking within seconds. Even with water cooling, the electricity bill will be significantly higher. To achieve the same torque without overheating, you need to switch to a motor with high torque/heating efficiency, low thermal resistance, and large heat capacity. But this brings us back to the problem mentioned in point 2.
In addition, the advantages of using a speed reducer include:
1. Achieve higher resolution with lower-cost machinery.
A standard 5k-line photoelectric encoder can achieve an angular resolution of 1.44mdeg (of course, with sufficient funds, a sine/cosine encoder with finer subdivisions can also achieve this); or a 5-phase 1000-step stepper motor can achieve a resolution of 7.2mdeg (this refers to Dongfang Electric's 33-step + 50:1 harmonic motor). The advantage of higher resolution is more precise speed control, as the high-frequency components caused by the step transition due to quantization become very small, resulting in smoother control.
2. Improve closed-loop accuracy and better control the control loop.
Due to the large 50:1 reduction ratio, the transmission of disturbances from the reducer output shaft to the motor is reduced by 37dB compared to direct drive, resulting in higher closed-loop accuracy at the reducer output shaft. Simultaneously, the rotor's equivalent moment of inertia is increased to 2500 times, making the lag in the control loop dominated by rotor inertia. Furthermore, since the rotor is directly driven by electromagnetic force, there is no torque lag due to stiffness, making it easier to control than direct drive.
In addition to the technical answers above, here is another case study that can indirectly answer this question from another perspective.
A customer has a 6150 lathe with a 10-inch (approximately 250mm in diameter) hydraulic chuck. To achieve a single-sided cutting capability of 7mm, they plan to utilize the low-speed, high-torque characteristics of the spindle motor (AC asynchronous motor) and select a 7.5kW motor with a rated speed of 1000rpm, while using a 1:2 gear ratio.
It is known that the transmission ratio is directly proportional to power and torque. That is, using a 1:2 transmission ratio, a 7.5kW motor achieves the characteristics of a 15kW motor while having twice the torque (note that this does not refer to twice the rated torque). If a 15kW motor is chosen, the price will be significantly higher, and the installation size will also need to be larger; while machining a pair of gears with a 1:2 transmission ratio does not cost much.
For those who don't understand, please refer to the power and torque characteristic curves of the asynchronous motor below. (Note: Unlike DC motors, asynchronous motors may operate beyond the transverse torque region.)
In addition to achieving low-speed, high-torque characteristics, the DC motors used in robots may also have this effect, and the gearboxes used may have a larger transmission ratio (2-stage transmission or higher).
Of course, using a speed reducer is not perfect and has some drawbacks, but compared to other options, using a speed reducer is more suitable.
Disadvantages of using a speed reducer:
1. If a speed reducer is configured and the encoder is mounted on the motor end, the manufacturing precision of the speed reducer will affect the actual accuracy;
2. Small errors such as changes in the backlash oil film thickness in a multi-stage reducer will still cause a decrease in repeatability after being amplified through multiple stages;
3. Furthermore, speed reducers, due to gear meshing or flexural deformation, have a limited lifespan;
4. The nonlinear coupling of backlash in multi-link mechanisms results in poor absolute accuracy of robots. Therefore, industrial robots only focus on repeatability and not absolute accuracy, making it difficult to program robots purely offline, which increases the difficulty and cost of deployment.
This explains why joint movement cannot be controlled directly by controlling the rotor speed of a servo motor; instead, a speed reducer is required. Although robots driven by direct-drive motors exist, their maturity is still somewhat lacking due to the aforementioned issues.
In short, the use of speed reducers in industrial robots is to exchange the high speed that motors can easily achieve for the high torque and low mass that motors cannot easily achieve.
