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How to select the appropriate servo motor?

# servo motor

1 Application Scenario 
Control motors in the automation field can be divided into servo motors, stepper motors, inverter motors, etc. Some parts that need more precise speed or position control will choose servo motor drive.
Inverter + inverter motor control method is a control method to change the motor speed by changing the power frequency of the input motor, which is generally only used for speed control of the motor.
Servo motor compared with stepper motor.
a) Servo motors use closed-loop control, stepper motors are open-loop control.
b) Servo motors use rotary encoders to measure accuracy, and stepper motors use step angle. The former's accuracy at the ordinary product level can reach a hundred times the latter's order of magnitude.
c) control mode is similar (pulse or direction signal).
2 power supply 
Servo motor from the power supply distinction can be divided into AC servo motor and DC servo motor.
Both are still relatively good choice. General automation equipment, the party will provide a standard 380V industrial power supply or 220V power supply, at this time, choose the corresponding power supply servo motor can be, eliminating the need for power type conversion. But there are some equipment, such as three-dimensional warehouse shuttle board, AGV trolley, etc., due to its own mobile nature, most of the use of self-contained DC power supply, so generally use DC servo motor.

3 Holding brake 
According to the design of the action mechanism, consider whether it will cause a reversal tendency to the motor in a power outage or in a static state. If there is a tendency of reversal, it is necessary to choose a servo motor with a holding brake.
4 Selection calculation 
Before the selection calculation, the first thing to determine is the position and speed requirements of the end of the mechanism, and then determine the transmission mechanism. At this point, you can choose the servo system and the corresponding reducer.
In the selection process, the following parameters are mainly considered.
4.1. Power and speed
According to the structure form and the speed and acceleration requirements of the final load, the required power and speed of the motor are calculated. It is worth noting that the speed reduction ratio of the reducer usually needs to be selected in combination with the speed of the selected motor.
In the actual selection process, for example, the load is horizontal motion, because of the uncertainty of the friction coefficient and wind load coefficient of each transmission mechanism, the formula P=T*N/9549 often cannot be calculated clearly (the magnitude of the torque cannot be calculated precisely). And in practice, it is also found that the maximum power required to use servo motors is often the acceleration and deceleration phase. Therefore, the power size of the required motor and the deceleration ratio of the gearbox can be calculated quantitatively by T=F*R=m*a*R (m: load mass; a: load acceleration; R: load rotation radius).
There are the following points to note.
a) The power enrichment factor of the motor.
b) Consideration of the transmission efficiency of the mechanism.
c) whether the input and output torques of the reducer are up to standard with a certain safety factor.
d) whether there will be the possibility of increasing the speed later on.
It is worth mentioning that in traditional industries, such as cranes, where ordinary induction motors are used to drive, there are no clear requirements for acceleration and the calculation process uses empirical formulas. Note: In the case of vertical operation of the load, note that the acceleration of gravity is taken into account.
4.2. Inertia matching
To achieve high precision control of the load, it is necessary to consider whether the inertia of the motor and the system are matched.
For the question of why inertia matching is needed, there is no unified statement on the Internet. Personal understanding is limited, so I won't explain here. Interested friends can test themselves and let us know. The principle of inertia matching: consider the system inertia folded into the motor shaft, and the motor inertia ratio is not greater than 10; the smaller the ratio, the better the control stability, but the need for a larger motor, lower cost performance. If you do not understand the specific calculation method, please make up for the university "theoretical mechanics".
4.3. Accuracy requirements
Calculate whether the control accuracy of the motor can meet the requirements of the load after the changes of the reducer and transmission mechanism. The speed reducer or some transmission mechanism has a certain return clearance, which needs to be considered.
4.4. Control matching
This aspect is mainly to communicate with the electrical designer to confirm, for example, whether the communication method of the servo controller matches the PLC, the type of encoder and whether the data needs to be led, etc.
5 Brand 
At present, there are many brands of servo motors on the market, and the performance is also very different. Generally speaking, if not bad money, the choice of Europe and the United States, slightly almost money, choose the Japanese, and then Taiwan and mainland China. The author is not a fan of foreigners, is the actual use of lessons learned. According to past experience, the basic performance of the domestic servo motor body is not a problem, the main servo controller control algorithm, integration and stability of a certain gap. I hope that domestic manufacturers continue to work hard to narrow the gap with foreign products.
It is worth mentioning that the design of automation, to learn to borrow external forces. Especially to do non-standard automation, facing too much equipment selection and calculation, often overwhelmed, overtime tired into a dog is the norm. Now servo motor manufacturers will provide technical support, as long as you provide him with the load, speed, acceleration and other parameters required, they have a set of their own software automatically help you calculate and select the right servo motor, very convenient.


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