Selection steps and precautions of servo motor

Each type of motor has its own specifications. When selecting the type, it is necessary to follow the steps. Select according to its running speed, weight and other data, so as to select the product suitable for you. When selecting the servo motor specification, follow the following steps.

(1) Specify the requirements for the movement conditions of the load mechanism, namely the speed of acceleration/deceleration, the movement speed, the weight of the mechanism, the movement mode of the mechanism, etc.
(2) The load inertia of the mechanism is calculated by selecting the appropriate load inertia calculation formula according to the operating conditions.
(3) According to the load inertia and servo motor inertia, select the appropriate pseudo-selected servo motor specification.
(4) The acceleration torque and deceleration torque are calculated based on the selected inertia of servo motor and load.
(5) The load torque is calculated according to the load weight, configuration mode, friction coefficient and operation efficiency.
(6) The maximum output torque of the primary servo motor must be greater than the acceleration torque+load torque; If the conditions are not met, other models must be selected for calculation and verification until they meet the requirements.
(7) The continuous instantaneous torque is calculated according to the load torque, acceleration torque, deceleration torque and holding torque.
(8) The rated torque of the primary selected servo motor must be greater than the continuous instantaneous torque. For example, if the conditions are not met, other models must be selected for calculation and verification until the requirements are met.
(9) Complete the selection.

Precautions for selection of servo motor
Servo motor is a widely used motor in the field of automation, which is usually used to drive more accurate speed or position control components in the project. The designers of automation equipment often need to face the problem of motor selection with various requirements, and the motors provided by the suppliers are also diverse, with numerous parameters, which often makes beginners confused. This article only makes some sharing based on the actual work experience of the author, hoping to provide some help to those in need.

1. Application scenario
Control motors in the field of automation can be divided into servo motors, stepper motors, variable frequency motors, etc. For components requiring more accurate speed or position control, servo motor drive will be selected. 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, it is only used for speed control of motor.

Compared with stepping motor, servo motor:
a) The servo motor uses closed-loop control, and the stepper motor uses open-loop control;
b) The servo motor uses rotary encoder to measure the accuracy, and the stepper motor uses the step angle. The accuracy of the first one in the ordinary product level can reach 100 times of that of the latter;
c) The control mode is similar (pulse or direction signal).

2. Power supply
Servo motor can be divided into AC servo motor and DC servo motor from power supply.
The two are better choices. For general automation equipment, Party A will provide standard 380V industrial power supply or 220V power supply. At this time, select the servo motor corresponding to the power supply to avoid the conversion of power supply type. However, some equipment, such as shuttle boards and AGV trolleys in the warehouse, mostly use their own DC power supply due to their mobile nature, so they generally use DC servo motors.

3. Holding brake
According to the design of the actuating mechanism, consider whether it will cause the reverse trend of the motor under the power failure or static state. If there is a reverse trend, it is necessary to select the servo motor with holding brake.

4. Servo motor selection calculation
Before type selection calculation, the position and speed requirements of the end of the mechanism should be determined first, and then the transmission mechanism should be determined. At this time, the servo system and corresponding reducer can be selected.

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

4.1 Power and speed
According to the structural 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 in general, it is necessary to select the reduction ratio of the reducer 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 friction coefficient and wind load coefficient of each transmission mechanism, the formula P=T * N/9549 often cannot be calculated clearly (the torque cannot be accurately calculated).

In the process of practice, it is also found that the maximum power required for using servo motor is often at the acceleration and deceleration stage. Therefore, T=F * R=m * a * R can be used to quantitatively calculate the power of the required motor and the reduction ratio of the reducer (m: load mass; a: load acceleration; R: load rotation radius).

The following points should be noted when selecting servo motor:
a) Power surplus coefficient of motor;
b) Consider the transmission efficiency of the mechanism;
c) Whether the input and output torque of the reducer meet the standard and have a certain safety factor;
d) Whether it is possible to increase the speed in the later stage.
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 runs vertically, pay attention to the calculation of gravity acceleration.

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 match.
For the question of why inertia matching is needed, there is no unified statement on the Internet. My personal understanding is limited, so I won't explain it here. If you are interested, you can check and inform yourself.

The principle of inertia matching is: consider that the system inertia is converted to the motor shaft, and the inertia ratio with the motor is not more than 10 (Siemens); The smaller the ratio, the better the control stability, but the larger the motor is required, and the lower the cost performance. If you don't understand the specific calculation method, please study 'theoretical mechanics' by yourself.

4.3 Accuracy requirements
Calculate whether the control accuracy of the motor can meet the load requirements after the change of the reducer and transmission mechanism. There is a certain return clearance for the reducer or some transmission mechanisms, which needs to be considered.

4.4 Control matching
This aspect is mainly to communicate and confirm with the electrical designer, such as whether the communication mode of the servo controller matches the PLC, the type of encoder and whether it is necessary to export data.

Source: Yibu Motor