Servo motor definition: A servo motor is an engine that controls the operation of mechanical components in a servo system. It is a type of auxiliary motor with indirect speed change.
Depending on the power supply used, servo motors can be divided into DC servo motors and AC servo motors.
The functional difference between the two is that AC servos are better because they use sinusoidal wave control, resulting in less torque ripple. DC servos use trapezoidal waves. However, DC servos are simpler and cheaper.
At this point, you might think servo motors aren't particularly special. Simply put, servo motors offer precise control; you tell them to rotate a certain amount, and they rotate exactly that amount. Furthermore, they provide feedback, achieving a closed-loop control system where an encoder checks if the rotation amount was indeed correct, resulting in even higher control precision.
We know that the accuracy of a stepper motor is measured in step angle. Stepper motors on the market typically have step angles of 0.36°/0.72° (five-phase motors), 0.9°/1.8° (two- and four-phase motors), and 1.5°/3° (three-phase motors). The three-phase hybrid stepper motors produced by the German company BERGER LAHR have step angles that can be set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036° via DIP switches. We will take a stepper motor with a step angle of 0.036° as an example.
0.036 = 360/10000
If we add an encoder to the back end of this stepper motor, then the formula is equivalent to the encoder emitting 10,000 pulses for every revolution of the motor, and the encoder resolution is 10,000.
The precision of a servo motor is measured by the resolution of the encoder at the rear of the motor. Currently, the resolution of servo encoders has reached 2 to the power of 23, which shows that the precision of a servo motor is much higher than that of a stepper motor.
A regular motor turns on when powered and stops when the power is off. If we have to say what other function it has besides turning, it's that it can rotate in both directions. :)
Provide servo motor selection process
1. Loading mechanism (determine the mechanism type and its detailed data, such as ball screw length, ball screw diameter, stroke, and pulley diameter, etc.)
2. Motion Mode (Determines the motion mode of the controlled object, the relationship between time and speed; converts the motion mode of the controlled object into the motion form on the motor shaft; determines the operating mode, including parameters such as acceleration time (ta), constant speed time (tu), deceleration time (td), stopping time (ts), cycle time (tc), and travel distance (L)).
3. Load inertia, torque, and speed (We select the power based on the torque during the model selection process!)
4. Positioning accuracy (confirm whether the encoder pulse count meets the system's required resolution specifications)
5. Operating environment (such as ambient temperature, humidity, atmospheric conditions, vibration, and shock).
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, 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 variable frequency motors. Servo motors are chosen for components requiring precise speed or position control. The variable frequency drive (VFD) + variable frequency motor control method changes the motor speed by altering the input power frequency. It is generally only used for motor speed control. Compared to stepper 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) Their control methods are similar (pulse or direction signals).
2. Power supply
Servo motors can be categorized into AC servo motors and DC servo motors based on their power supply. Choosing 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, a servo motor with the corresponding power supply can be selected, eliminating the need for power type conversion. However, some equipment, such as shuttles and AGVs in automated warehouses, due to their inherent mobility, mostly use built-in DC power supplies, and therefore generally employ DC servo motors.
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. The following parameters are mainly considered during the selection process:
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 the reduction ratio of the gearbox usually needs to be selected in conjunction with the speed of the chosen motor. In actual selection, for example, when the load is horizontally moving, the formula P=T*N/9549 often cannot be precisely calculated due to the uncertainty of the friction coefficient and wind load coefficient of each transmission mechanism (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 the acceleration and deceleration phases. Therefore, the required motor power and the reduction ratio of the gearbox can be quantitatively calculated using T=F*R=m*a*R (m: load mass; a: load acceleration; R: load rotation radius).
Each type of servo motor has specifications including rated torque, maximum torque, and servo motor inertia. Each parameter must be correlated with the load torque and load inertia. The servo motor's output torque must meet the motion requirements of the mechanism's acceleration and weight. The mechanism's motion conditions (horizontal and vertical rotation) are not directly related to the servo motor's output, but generally, a higher servo motor output corresponds to a higher relative output torque.
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. Greater inertia results in greater acceleration/deceleration torque, shorter acceleration/deceleration time, and ultimately, greater output torque from the servo motor. When selecting servo motor specifications, please follow these steps.
(1) When the initial selection of the servo motor's maximum output power torque must be greater than the acceleration torque + load torque, other models must be selected for verification until the sign meets the requirements.
(2) The load torque is calculated based on the load weight, structure, friction coefficient and operating efficiency.
(3) Select an appropriate load inertia correction formula according to the requirements of the operating conditions, and calculate the load inertia of the mechanism.
(4) Select the appropriate hypothetical servo motor specification based on the load inertia and the servo motor inertia.
(5) Calculate the continuous instantaneous torque based on the load torque, acceleration torque, deceleration torque and holding torque.
(6) Defines the motion conditions of the load mechanism, namely acceleration and deceleration speed, motion speed, mechanism weight, mechanism motion, etc.
(7) The acceleration torque and deceleration torque were calculated by combining the inertia of the main servo motor and the load inertia.
(8) Complete the selection.
