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Aug 04, 2023
How to choose the appropriate servo motor?




The field of automation refers to the current hot topic, and servo motors play an important role in it, usually used for driving more precise speed or position control components in projects. Designers of automation equipment often face various motor selection issues with different needs, and suppliers provide a wide range of motors with numerous parameters, often confusing beginners. This article only shares the author's actual work experience and hopes to provide some help to those in need.
 
 
    1. Servo Motor Basics:
 
      Servo motors are part of a closed-loop system and are comprised of several parts namely a control circuit, servo motor, shaft, potentiometer, drive gears, amplifier, and either an encoder or resolver.
     A servo motor is a self-contained electrical device, that rotates parts of a machine with high efficiency and with great precision.
     The output shaft of this motor can be moved to a particular angle, position, and velocity that a regular motor does not have.
    The Servo Motor utilizes a regular motor and couples it with a sensor for positional feedback.
    The controller is the most important part of the Servo Motor designed and used specifically for this purpose.
 
    

    The servo motor is a closed-loop mechanism that incorporates positional feedback in order to control the rotational or linear speed and position.
   
   
   
    The motor is controlled with an electric signal, either analog or digital, which determines the amount of movement which represents the final command position for the shaft.
 
     

    A type of encoder serves as a sensor providing speed and position feedback. This circuitry is built right inside the motor housing which usually is fitted with gear system.
 
   
 
     2. Types of Servo Motors:

     Types of Servo Motors are classified into different types based on their application. There are three main considerations to evaluate servos motors:

  1. First based on their current type – AC or DC,
  2. secondly on the type of Commutation used, whether the motor uses brushes,
  3. the third type of consideration is the motor rotating field, the rotor, whether the rotation is synchronous or asynchronous.
   3. Servo Motor Applications:

Servo motors are widely used in various industrial and automation applications where precise control of motion is required. Their ability to provide accurate position, speed, and torque control makes them suitable for a wide range of tasks. Here are some common applications of servo motors:

  1. Robotics: Servo motors are extensively used in robotic systems for precise and smooth movement of robot joints, grippers, and end-effectors. They enable robots to perform complex tasks with high accuracy and repeatability.

  2. CNC Machines: In Computer Numerical Control (CNC) machines, servo motors drive the axes (e.g., X, Y, Z) to precisely control the tool's movement, allowing for precise machining and manufacturing operations.

  3. Packaging and Labeling Machines: Servo motors are used in packaging and labeling machines to control the movement of conveyor belts, cutting blades, and printing heads with precision.

  4. Textile Industry: Servo motors are employed in textile machines like looms and spinning machines to control thread tension and guide the fabric accurately.

  5. Printing and Paper Handling: In printing presses, servo motors control paper feeding, registration, and print head movement, ensuring precise alignment and high-quality output.

  6. Automotive Industry: Servo motors are utilized in various automotive applications, including electric power steering systems, throttle control, and precision robotic assembly lines.

  7. Aerospace and Aviation: Servo motors are used in aircraft control systems for precise and reliable movement of flight control surfaces like ailerons, elevators, and rudders.

  8. Medical Equipment: In medical devices and equipment, servo motors are employed for precise positioning, such as in robotic surgery systems and medical imaging devices.

  9. Camera Stabilization: In photography and cinematography, servo motors are used in gimbals and camera stabilization systems to achieve smooth and steady shots.

  10. Conveyor Systems: Servo motors control the movement of conveyor belts in material handling and logistics systems, ensuring accurate product positioning and sorting.

  11. Industrial Automation: Servo motors play a crucial role in various automation applications, including pick-and-place operations, material handling, and assembly processes.

  12. Food and Beverage Industry: Servo motors are used in food processing and packaging machines for precise filling, sealing, and labeling of food products.

  13. Renewable Energy: Servo motors are used in solar tracking systems and wind turbines to orient solar panels and wind blades for optimal energy capture.

These are just a few examples of how servo motors are employed in diverse industries and applications. Their ability to provide precise and controlled motion makes them a vital component in modern automation and motion control systems.

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).
 
4. 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 a standard 380V industrial power supply or 220V power supply. At this time, the corresponding servo motor can be selected to avoid power type conversion. But some equipment, such as shuttle boards and AGV cars in three-dimensional warehouses, mostly use built-in DC power supply due to their inherent mobility, so DC servo motors are generally used.
 
5. Band brake
 
Based on the design of the action mechanism, consider whether it will cause a reverse trend to the motor in a power outage or stationary state. If there is a reversal trend, it is necessary to choose a servo motor with a band brake.
 
6. Selection calculation
 
Before selecting and calculating, 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, the servo system and corresponding reducer can be selected.
 
During the selection process, the following parameters are mainly considered:
 
6.1. 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, it is necessary to select the reduction ratio of the gearbox based on the speed of the selected motor.
 
In the actual selection process, for example, when the load is in horizontal motion, the formula P=T * N/9549 often cannot be clearly calculated (the torque cannot be accurately calculated) due to the uncertainty of the friction coefficient and wind load coefficient of each transmission mechanism. In practice, it has also been found that the maximum power required for using servo motors is often in the acceleration and deceleration stages. So, by using T=F * R=m * a * R, the power of the required motor and the reduction ratio of the reducer can be quantitatively calculated (m: load mass; a: load acceleration; R: load rotation radius).
 
The following points need to be noted:
 
a) The power surplus coefficient of the motor;
 
b) Consider the transmission efficiency of the mechanism;
 
c) Whether the input and output torque of the reducer are up to the standard and have a certain Factor of safety;
 
d) Is there a possibility of increasing 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 are no clear requirements for acceleration. The calculation process uses empirical formulas. Note: When the load runs vertically, pay attention to the calculation of Gravitational acceleration.
 
6.2. 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 statement online on why inertia matching is necessary. My personal understanding is limited, so I won't explain it here. Interested friends can conduct their own research and let them know. The principle of inertia matching is to consider converting the system inertia to the motor axis, and the inertia ratio to the motor is not greater than 10; 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 method, please study Theoretical Mechanics by yourself.
 
6.3. Accuracy requirements
Calculate whether the control accuracy of the motor can meet the load requirements after changes in the reducer and transmission mechanism. There is a certain return clearance for reducers or certain transmission mechanisms that need to be considered.
 
6.4. 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.
 
7. Brand
 
At present, there are many servo motor brands in the market, and their performance varies greatly. Generally speaking, if you don't need money, you should choose European and American ones and Japanese ones, then Taiwan, China, and Chinese Mainland ones. It is not the author's admiration for foreign things, but a lesson learned from practical use. Based on past experience, the basic performance of domestically produced servo motors is not a problem, but there is a certain gap in the control algorithm, integration, and stability of the main servo controllers. I hope domestic manufacturers will continue to work hard to narrow the gap with foreign products.
 
It is worth mentioning that in designing automation, one should learn to borrow external resources. Especially in non-standard automation, facing too many equipment selections and calculations, it is often unbearable, and overtime is the norm. Nowadays, servo motor manufacturers provide technical support. As long as you provide them with parameter requirements such as load, speed, and acceleration, they have their own software to automatically help you calculate and select the appropriate servo motor, which is very convenient.
 
OstBridge is based on the market in China and Europe to provide our clients with original HEIDENHAIN, PRECIZIKA, SIEMENS, B & R, BOSCH REXROTH, EATON VICKERS, PARKER, Large stock, accept MOQ, fast delivery If you are looking for the fields of precise motion control, electronic control, automation, and hydraulic system, the OstBridge team must be your first choice to bring you the right solution.
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