How to choose the appropriate servo motor

1. Application scenarios
Control type motors in the field of automation can be divided into servo motors, stepper motors, variable frequency motors, etc. In components that require more precise speed or position control, servo motor drive will be chosen.

The control method of frequency converter+variable frequency motor is to change the motor speed by changing the power frequency of the input motor. Generally only used for speed control of motors.

Compared to stepper motors, servo motors:
a) The servo motor adopts closed-loop control, while the stepper motor adopts open-loop control;
b) The servo motor uses a rotary encoder to measure accuracy, while the stepper motor uses a step angle. The accuracy of the former at the ordinary product level can reach a hundred times that of the latter;
c) The control method is similar (pulse or directional signal).

2. Power supply
Servo motors can be divided into AC servo motors and DC servo motors based on their power supply.
The two are relatively easy to choose from. For general automation equipment, Party A will provide standard 380V industrial power supply or 220V power supply. At this time, the corresponding servo motor of the power supply can be selected, eliminating the need for power type conversion. But some devices, such as shuttle boards and AGV cars in three-dimensional warehouses, mostly use built-in DC power due to their mobility, so DC servo motors are generally used.

3. Holding brake
According to the design of the action mechanism, consider whether it will cause a reversal trend of the motor in a power outage or a stationary state. If there is a reversal trend, it is necessary to choose a servo motor with a holding brake.

4. Selection calculation
Before selecting and calculating, the first thing to determine is the position and speed requirements at the end of the mechanism, and then determine the transmission mechanism. At this point, the servo system and corresponding reducer can be selected.

During the selection process, the following parameters are mainly considered:

4.1. Power and speed
Calculate the required power and speed of the motor based on the structural form and the speed and acceleration requirements of the final load. It is worth noting that in general, the reduction ratio of the gearbox needs to be selected based on the speed of the selected motor.

In the actual selection process, such as when the load is in horizontal motion, due to the uncertainty of the friction coefficient and wind load coefficient of each transmission mechanism, the formula P=T * N/9549 often cannot be accurately calculated (the magnitude of torque cannot be accurately calculated). In practice, it has also been found that the maximum power required for using servo motors is often during the acceleration and deceleration stages. So, by using T=F * R=m * a * R, the required power of the motor and the reduction ratio of the gearbox can be quantitatively calculated (m: load mass; a: load acceleration; R: load rotation radius).

There are several points to note:
a) The power surplus coefficient of the motor;
b) Consider the transmission efficiency of the mechanism;
c) Is the input and output torque of the reducer up to standard and has a certain safety factor;
d) Is there a possibility of increasing speed in the later stage. Official account Mechanical Knowledge Network: share knowledge and spread value.

It is worth mentioning that in traditional industries such as cranes, ordinary induction motors are used for driving, and there is no clear requirement for acceleration. The calculation process uses empirical formulas.
Note: When the load is running vertically, pay attention to calculating the gravitational acceleration.

4.2. Inertia matching
To achieve high-precision control of the load, it is necessary to consider whether the inertia of the motor matches that of the system.
There is no unified explanation online for why inertia matching is needed. My personal understanding is limited, so I won't explain it here. Interested friends can take the exam and let me know on their own. The principle of inertia matching is to consider converting the system inertia onto the motor shaft, with a ratio of no more than 10 to the motor inertia (Siemens); The smaller the ratio, the better the control stability, but a larger motor is required, resulting in lower cost-effectiveness. If you have any doubts about the specific calculation methods, please study "Theoretical Mechanics" in university by yourself.

4.3. Accuracy requirements
Calculate whether the control accuracy of the motor can meet the requirements of the load after changes in the reducer and transmission mechanism. Decelerators or certain transmission mechanisms have certain return clearances that need to be considered.

4.4. Control Matching
This aspect mainly involves communicating and confirming with electrical designers, such as whether the communication method of the servo controller matches the PLC, the type of encoder, and whether data needs to be extracted.

Source: Mechanical experts