Automation, the current hot field, plays an important role, commonly used for precise control of speed or position of parts in projects.
Automated equipment designers often face a variety of motor selection problems, and the motors offered by suppliers are diverse, with a multitude of parameters, often overwhelming for beginners.
This article shares some practical work experiences, hoping to provide some assistance to those in need.
I. What is a servomotor?
A servo motor is a motor that controls the movement of mechanical components in a servo system; It is essentially an auxiliary engine with an indirect variable speed mechanism.
Servo motors are categorized by their power source: direct current (DC) servo motors and alternating current (AC) servo motors.
The functional difference between the two is that AC servos perform better due to sine wave control, which results in less torque ripple. DC servos, on the other hand, employ trapezoidal waves.
However, DC servos are simpler and more economical. Servo motors can achieve precise control; they will rotate exactly as instructed and provide feedback to ensure accuracy through what is known as a closed loop. This is achieved by using an encoder to check rotation, which increases control accuracy.
The accuracy of stepper motors is measured by the step angle. Common pitch angles on the market include 0.36°/0.72° (for five-phase motors), 0.9°/1.8° (for two-phase and four-phase motors), and 1.5°/3° (for three-phase motors). ). BERGER LAHR, a German company, produces three-phase hybrid stepper motors with step angles selectable via DIP switches: 1.8°, 0.9°, 0.72°, 0.36°, 0.18 °, 0.09°, 0.072° and 0.036°.
Let's consider a stepper motor with a step angle of 0.036°.
0.036 = 360/10000
Assuming an encoder is connected to the rear end of this stepper motor, the formula implies that the motor outputs 10,000 pulses per rotation, indicating an encoder resolution of 10,000.
The accuracy of a servo motor is measured by the resolution of the encoder connected to its rear end. Currently, servo encoder resolutions can reach 2 23 demonstrating that servo motor accuracy far exceeds that of stepper motors.
A standard motor turns on and starts rotating and stops when the power is cut off. In addition to rotation, if we were to give it any additional functionality, it would be its ability to reverse direction.
II. How to choose the right servo motor?
1. Application Scenario
Control motors in the field of automation can be divided into servo motors, stepper motors and variable frequency motors. For components that require precise speed or position control, servo motors are chosen.
The inverter plus variable frequency motor control method changes the speed of the motor by changing the frequency of the power source input to the motor. This is generally only used for engine speed control.
Comparing servomotors and stepper motors:
a) Servo motors use closed-loop control while stepper motors use open-loop control.
b) Servo motors use rotary encoders to measure accuracy, while stepper motors use step angles. At the common product level, the accuracy of the former can reach a hundred times that of the latter.
c) The control methods are similar (pulse or directional signal).
2. Power supply
Servo motors can be classified into AC servo motors and DC servo motors based on the power source.
Both are relatively easy to choose. For general automation equipment, customers generally provide a standard industrial power supply of 380V or 220V, in which case simply select a servo motor for the corresponding power supply, eliminating the need to convert power types.
However, some equipment, such as conveyor plates in three-dimensional warehouses and AGVs, due to their mobile nature, primarily utilize integrated DC power supplies and therefore often utilize DC servo motors.
3. Brakes
Based on the design of the movement mechanism, consider whether there will be a tendency to reverse to
the engine in off or stationary state. If there is a tendency to reverse, a servo motor with brake must be selected.
4. Selection Calculation
Before making the selection calculation, you first need to determine the position and speed requirements of the mechanism end, and then identify the transmission mechanism.
At this point, you can select the servo system and corresponding reducer.
During the selection process, consider the following parameters:
4.1. Power and speed
Calculate the required engine power and speed based on the structural form and the speed and acceleration requirements of the end load.
Notably, in general, it is necessary to choose the reducer reduction ratio in conjunction with the selected motor speed.
In actual selection, for example, if the load is horizontal movement, due to the uncertainty of the friction coefficient and wind load factor of various transmission mechanisms, the formula P = TN/9549 often cannot be calculated clearly ( it is not possible to accurately calculate the size of the torque).
In practice, it has also been found that the place where maximum power is needed when using a servo motor is often the acceleration and deceleration stage.
Therefore, through T=F*R=m* a *R you can quantitatively calculate the required power and the reduction rate of the motor and reducer (m: mass of the load; a: acceleration of the load; R: radius of rotation of the charge).
The following points need attention:
a) The excess power factor of the engine;
b) Consider the transmission efficiency of the mechanism;
c) Whether the reducer input and output torques meet the standard and have a certain safety factor;
d) Whether it will be possible to increase the speed later.
It is worth mentioning that in traditional industries, such as cranes, common induction motors are used for drive, there are no clear acceleration requirements, and empirical formulas are used in the calculation process.
Note: In the case of operation with vertical load, remember to include gravitational acceleration in the calculation.
4.2. Inertia matching
To achieve high-precision control of the load, it is necessary to consider whether the inertia of the motor and system match.
As for why inertia matching is necessary, there is no unified explanation on the Internet.
The principle of inertia matching is: considering the system inertia converted to the motor shaft, the ratio with the motor inertia should not be greater than 10 (Siemens); the smaller the ratio, the better the control stability, but it requires a larger motor and the cost performance is lower.
Consult the university's “Theoretical Mechanics” if you have any questions about specific calculation methods.
4.3. Accuracy Requirements
After going through the reducer and transmission mechanism, calculate whether the motor control accuracy can meet the load requirements. The reducer or some transmission mechanisms have a certain backlash, and all this needs to be considered.
4.4. Control matching
This mainly involves communication and confirmation with electrical designers, such as whether the servo controller communication method matches the PLC, the encoder type, and whether data output is required.
4.5. Steps to follow
The selection of a servo motor is influenced not only by the weight of the mechanism, but also by the operating conditions of the equipment, which can change the choice of servo motor. Greater inertia requires greater torque for acceleration and deceleration, and shorter times for acceleration and deceleration, thus requiring a servo motor with greater output torque.
When selecting a servo motor specification, follow these steps:
- Initially choose a servo motor whose maximum output torque is greater than the sum of the acceleration torque and the load torque. If this is not the case, select and check other models until load requirements are met.
- Calculate the load torque based on the weight of the load, its structure, coefficient of friction and operation efficiency.
- Select the appropriate load inertia correction formula based on operating conditions and calculate the load inertia of the mechanism.
- Choose a suitable tentative servo motor specification based on the load inertia and servo motor inertia.
- Calculate the continuous instantaneous torque by considering the load torque, acceleration torque, deceleration torque and holding torque.
- Define the movement conditions of the load mechanism, which include acceleration and deceleration speeds, operating speed, mechanism weight, and movement.
- Calculate the acceleration and deceleration torque by combining the inertia of the main servo motor with the inertia of the load.
- Finalize the selection.
5. Brand
Currently, there are several brands of servomotors on the market, with varying performances. Generally speaking, if budget is not a concern, choose European or American brands. If you are more budget conscious, choose Japanese brands, followed by those from Taiwan and mainland China.
This is not the author biased towards foreign brands; It's a lesson learned from actual use.
Based on past experience, there are generally no problems with the basic performance of domestic servo motors, but the control algorithm, integration and stability of the servo controller may lag behind.
Some commonly used servo motor brands:
European and American: Siemens, ABB, Lenze, etc.;
Japanese: Panasonic, Mitsubishi, Yaskawa, etc.
It is worth mentioning that in the automation project you must learn to take advantage of external forces. Especially in non-standard automation, faced with the selection and calculation of many devices, it is often tiring and working overtime is the norm.
Now all servo motor manufacturers provide technical support. As long as you provide requirements for load, speed, acceleration and other parameters, they will have their own software to automatically help you calculate and choose the right servo motor, which is very convenient.
