Automation is a hot field these days, and servo motors play a crucial role, typically used to drive components requiring precise speed or position control in projects. Designers of automation equipment often face the challenge of selecting motors to meet a wide variety of needs. This article shares some insights based on the author's practical experience, hoping to provide some assistance to those who require it.
Servo motors are widely used in the field of automation, typically for driving components requiring precise speed or position control in projects. Designers of automation equipment often face the challenge of selecting motors to meet a wide variety of needs, and suppliers offer a bewildering array of motors with numerous parameters, often leaving beginners confused. This article shares some insights based on the author's practical experience, hoping to provide some assistance to those in need.
1. Application Scenarios
In the field of automation, control motors can be categorized into servo motors, stepper motors, and frequency converter motors. Servo motors are chosen for components requiring precise speed or position control.
The inverter + variable frequency motor control method changes the motor speed by altering the frequency of the power supply input to the motor. It is generally used only for motor speed control.
Compared to stepper motors: Servo motors
a) Servo motors use closed-loop control, while stepper motors use open-loop control;
b) Servo motors use rotary encoders for accuracy measurement, while stepper motors use step angles. In standard products, the former's accuracy can be hundreds of times that of the latter.
c) Similar control methods (pulse or direction signal).
2. Power supply
Servo motors can be classified into AC servo motors and DC servo motors based on their power supply.
The choice between the two is relatively straightforward. For most automated equipment, the client will provide a standard 380V industrial power supply or 220V power supply. In this case, you can simply choose a servo motor with the corresponding power supply, eliminating the need for power type conversion. However, some equipment, such as shuttles and AGVs in automated warehouses, mostly use their own DC power supply due to their mobile nature, so DC servo motors are generally used.
3. Brake
Based on the design of the actuation mechanism, consider whether the motor will reverse in a power outage or stationary state. If there is a reverse reversal tendency, a servo motor with a brake should be selected.
4. Selection Calculation
Before performing selection calculations, the first step is to determine the position and speed requirements of the mechanism's end effector, followed by 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 design and the speed and acceleration requirements of the final load. It is worth noting that the reduction ratio of the gearbox usually needs to be selected in conjunction with the speed of the chosen motor.
In practical selection, for example, when the load is horizontally moving, the formula P=T*N/9549 often cannot be precisely calculated due to the uncertainties in the friction coefficient and wind load coefficient of various transmission mechanisms (the magnitude of torque cannot be accurately calculated). Furthermore, in practice, it has been found that the maximum power required by the servo motor is often during acceleration and deceleration. Therefore, the required motor power and the reduction ratio of the reducer can be quantitatively calculated using T=F*R=m*a*R (m: load mass; a: load acceleration; R: load rotation radius).
The following points should be noted:
a) Power margin of the motor;
b) Consider the transmission efficiency of the mechanism;
c) Whether the input and output torques of the reducer meet the standards and have a certain safety factor;
d) Is there a possibility of increasing the speed later?
It is worth mentioning that in traditional industries, such as cranes, ordinary induction motors are used for driving, and there are no specific requirements for acceleration. The calculation process uses empirical formulas.
Note: When the load is running vertically, be sure to take gravitational acceleration 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.
There's no universally accepted explanation online regarding the necessity of inertia matching. My understanding is limited, so I won't elaborate here. Those interested can research it themselves and let me know. The principle of inertia matching is: considering the system inertia converted to the motor shaft, the ratio of inertia to the motor's inertia should not exceed 10 (Siemens). The smaller the ratio, the better the control stability, but a larger motor is required, resulting in lower cost-effectiveness. If you don't understand the specific calculation methods, please study university-level "Theoretical Mechanics."
4.3 . Accuracy Requirements
Calculate whether the motor's control accuracy can meet the load requirements after the changes are made through the reducer and transmission mechanism. The reducer or some transmission mechanisms have a certain backlash, which must 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 is compatible with the PLC, the encoder type, and whether data needs to be exported.
5. Brand
There are numerous servo motor brands on the market, with vastly different performance levels. Generally speaking, if budget isn't a concern, choose European or American brands; if a slightly lower budget, choose Japanese brands; followed by Taiwanese and domestic brands. This isn't about blindly favoring foreign brands, but rather lessons learned from practical experience. Based on past experience, domestically produced servo motors themselves generally have no issues in terms of performance; the main gap lies in the control algorithms, integration, and stability of the servo controller. It is hoped that domestic manufacturers will continue their efforts to narrow the gap with foreign products.
If readers have any recommendations for domestic brands that have proven effective, please share them with everyone. After all, the rise of domestic brands requires our support.
Some commonly used servo motor brands:
European and American brands: Siemens, ABB, Lenze, etc.;
Japanese: Panasonic, Mitsubishi, Yaskawa, etc.
It's worth mentioning that when designing automation systems, it's essential to leverage external resources. This is especially true for non-standard automation, where the sheer volume of equipment selection and calculations can be overwhelming, often resulting in overwork and exhaustion. Servo motor manufacturers now offer technical support; simply provide them with parameters such as load, speed, and acceleration, and they have their own software to automatically calculate and select the appropriate servo motor – a very convenient solution.
