A servo motor is a motor that controls the operation of mechanical components in a servo system; it is an auxiliary motor with indirect speed control. Servo motors enable highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and can respond quickly. In automatic control systems, it is used as an actuator and possesses characteristics such as a small electromechanical time constant, high linearity, and low starting voltage. It can convert received electrical signals into angular displacement or angular velocity output on the motor shaft. Servo motors are divided into two main categories: DC and AC. Their main characteristic is that they do not rotate when the signal voltage is zero, and their speed decreases uniformly as the torque increases.
Features and characteristics of servo motors
Features of DC brushless servo motors
It features low rotational inertia, low starting voltage, and low no-load current; it abandons the contact commutation system, greatly increasing the motor speed, with a maximum speed of up to 100,000 rpm; when performing servo control, the brushless servo motor can achieve speed, position, torque, and other controls without an encoder; there is no brush wear, and in addition to high speed, it also has the characteristics of long life, low noise, and no electromagnetic interference.
Features of DC brushed servo motors
1. Small size, fast action and response, large overload capacity, and wide speed range.
2. High torque at low speeds, minimal fluctuations, and smooth operation.
3. Low noise, high efficiency
4. Advantages of back-end encoder feedback (optional) forming a DC servo motor, etc.
5. Wide transformer range and adjustable frequency
Advantages of servo motors
First, let's look at the advantages of servo motors compared to other motors (such as stepper motors):
1. Precision: It achieves closed-loop control of position, speed, and torque; overcoming the problem of stepper motor step loss;
2. Speed: Good high-speed performance, generally rated speed can reach 2000-3000 rpm;
3. Adaptability: It has strong overload resistance and can withstand loads three times the rated torque, making it particularly suitable for occasions with instantaneous load fluctuations and requirements for rapid start-up.
4. Stability: Smooth operation at low speeds, without exhibiting the stepping motion characteristic of a stepper motor. Suitable for applications requiring high-speed response;
5. Timeliness: The dynamic response time of motor acceleration and deceleration is short, generally within tens of milliseconds;
6. Comfort: Heat generation and noise are significantly reduced.
Simply put: a regular motor, even after power is cut off, will continue to rotate for a while due to its inertia before stopping. Servo motors and stepper motors, on the other hand, stop and start instantly, with extremely fast response times. However, stepper motors are prone to step loss.
The applications of servo motors are numerous. Generally, any equipment requiring a power source and demanding precision will involve servo motors. These include machine tools, printing equipment, packaging equipment, textile equipment, laser processing equipment, robots, and automated production lines—equipment with relatively high requirements for process precision, processing efficiency, and operational reliability.
Types and classifications of servo motors
Servo motors are divided into two main categories: DC and AC. DC motors are further divided into brushed and brushless types; AC motors can be divided into asynchronous and synchronous types.
Brushed motors are low-cost, simple in structure, have high starting torque, wide speed range, and are easy to control. However, they require maintenance, which is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
Servo motors are motors that control the operation of mechanical components in servo systems; they are a type of auxiliary motor with indirect speed change. Servo motors are continuously rotating electro-mechanical converters. Servo motors used in hydraulic valve controllers are micro-motors with very low power; permanent magnet DC servo motors and shunt-wound DC servo motors are the most common.
The function of a servo motor: A servo motor enables very accurate control of speed and position.
Servo motors can be classified into DC servo motors and AC servo motors.
The output speed of a DC servo motor is proportional to the input voltage and can achieve forward and reverse speed control. It has advantages such as high starting torque, wide speed range, good linearity of mechanical and adjustment characteristics, and convenient control. However, wear of the commutating brushes and the tendency to generate sparks can affect its service life. In recent years, brushless DC servo motors have emerged, avoiding brush friction and commutation interference. Therefore, they have high sensitivity, small dead zone, low noise, long lifespan, and minimal interference to surrounding electronic equipment.
The transfer function of the output speed/input voltage of a DC servo motor can be approximated as a first-order lag element, with its electromechanical time constant typically ranging from tens of milliseconds to hundreds of milliseconds. In contrast, the time constant of some low-inertia DC servo motors (such as coreless rotor, printed winding, and slotless types) is only a few milliseconds to twenty milliseconds.
Low-power DC servo motors have rated speeds above 3000 r/min, and even greater than 10000 r/min. Therefore, controllers for hydraulic valves require high-ratio reducers. DC torque servo motors (i.e., low-speed DC servo motors), on the other hand, can operate at low speeds of tens of rpm, even under prolonged stall conditions, and can therefore directly drive the controlled object without the need for reduction gears.
DC servo motors are divided into brushed motors and brushless motors.
Brushed motors are low in cost, simple in structure, have high starting torque, wide speed range, and are easy to control. They require maintenance, but maintenance is convenient (replacing carbon brushes). They generate electromagnetic interference and have environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications.
Brushless motors are small in size, lightweight, powerful, fast-responding, high-speed, low-inertia, smooth-rotating, and stable in torque. While their control is complex, they are easily made intelligent. Their electronic commutation is flexible, allowing for either square wave or sine wave commutation. The motors are maintenance-free, highly efficient, operate at low temperatures, have minimal electromagnetic radiation, and a long lifespan, making them suitable for various environments.
AC servo motors are also brushless motors, and they are divided into synchronous and asynchronous motors. Currently, synchronous motors are generally used in motion control because they have a wide power range and can achieve very high power. They have high inertia, low maximum rotational speed, and their speed decreases rapidly as power increases. Therefore, they are suitable for applications requiring low-speed, stable operation.
Working principle of AC servo motor
The rotor inside a servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver creates an electromagnetic field, causing the rotor to rotate under the influence of this magnetic field. Simultaneously, the motor's built-in encoder feeds back signals to the driver. The driver compares the feedback value with the target value and adjusts the rotor's rotation angle accordingly. The accuracy of a servo motor depends on the accuracy (line count) of the encoder.
What are the functional differences between AC servo motors and brushless DC servo motors?
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.
Permanent magnet AC servo motor
Since the 1980s, with the development of integrated circuits, power electronics technology, and AC variable speed drive technology, permanent magnet AC servo drive technology has made remarkable progress. Leading electrical manufacturers worldwide have successively launched their own series of AC servo motors and servo drives, continuously improving and updating them. AC servo systems have become the main development direction of contemporary high-performance servo systems, putting the traditional DC servo system at risk of obsolescence. Since the 1990s, commercially available AC servo systems worldwide have adopted fully digitally controlled sinusoidal wave motor servo drives. The development of AC servo drive devices in the transmission field is progressing rapidly. Compared with DC servo motors, permanent magnet AC servo motors have the following main advantages:
(1) It has no brushes and commutator, so it is reliable and requires little maintenance.
(2) The stator winding is relatively easy to dissipate heat.
(3) Small inertia makes it easy to improve the speed of the system.
(4) It is suitable for high-speed and high-torque working conditions.
(5) It has a smaller volume and weight for the same power.
Applications of servo motors
Applications of servo amplifiers and automobiles
You will gain a better understanding of servo amplifier operation if you see some typical applications. Figure 11-90 shows an example of a motor-controlled press feed. In this sheet application, the press is fed in where it is cut to length by a blade or simply. The sheet may have a sign or other markings and must be queued at the demarcation point with a registration number. In this application, the speed and slope position of the sheet must be synchronized to achieve the correct cut point. Feedback sensors can be coded or parsed, and photoelectric sensors are added to determine the position of the registration mark. The operator provides a panel that allows the operator to run the system to maintain the blade or when loading a newly introduced material. The operator panel can also be used to call parameters, drive, and throttle for each type of material being used. The system can also be integrated with a programmable logic controller (PLC) or other type of controller, and the operator panel can be used to select the correct demarcation point for each type of material or product.
Servo-controlled bottle filling applications
The second application is shown in the diagram. In this application, multiple filling heads are connected to bottles because they follow a continuous line. Each filling head must match a bottle, with the bottle track facing the direction it is aligned with. The product is moved as the nozzle moves with the bottle. In this application, 10 nozzles are mounted on a carriage driven by a ball screw mechanism. The ball screw mechanism is also called a screw. When the motor rotates the ball screw shaft, the carriage moves along the transverse length of the ball screw shaft. This movement is smooth, allowing each nozzle to dispense the product onto the bottle without oil contamination.
The servo drive system uses software from the positioning driver controller to track the position and speed of the moving bottles on the conveyor line. A main encoder tracks the bottles as they move along the conveyor line. A spiral feeding system is also used before the bottles enter the filling station. The spiral creates specific spaces between each bottle upon entering the filling station. The bottles can be packed tightly into the spiral, but they pass through the spiral with their spaces being exactly the same, so that the neck of the bottle will conform to the filling nozzle interval. A detector is also incorporated into the dispensing system to ensure that no product is exempted from the nozzle if a bottle is lost or if large gaps appear between bottles.
Graphical beverage and gas station control servo motors
The servo drive system compares the position of the bottle with the feedback signal from the main encoder, indicating the position, and fills the conveyor with the ball screw. The servo drive amplifier increases or decreases the speed of the ball screw mechanism, causing the nozzle to match the speed of the bottle to the bottom.
Servo-controlled precision spiral filling system
The third application provides a diagram of the servo system. In this large application, filling tanks are used to fill their containers along a conveyor line. The material is dispensed into the container; it can be a single material replenishment or several containers, with each container being added to a mixer for mixing. Due to the amount of material being dispensed, the containers must be accurately measured and quantified in blocks, controlled by a servo system. Feedback sensors allow this system to function as a weighing system. The command signal can come from a programmable controller, or the operator can manually select a recipe from the operator's terminal. The amount of material can be from different recipes.
The application uses a precise spiral filling station controlled by a servo motor. The speed of the spiral can be adjusted so that when it runs at high speed, the container will be filled first, and the speed can be slowed down slightly at the last minute to accurately measure the amount of material and fill the container at the appropriate point. Due to rising material prices, precision filling equipment can provide savings as well as the quantity of product formula used.
The application server used by the tag
The fourth application uses a servo-controlled speed tag feed mechanism. Pre-labeled tags are pulled and applied to the encapsulated moving continuous conveyor system, similar to previous tagging mechanisms. Feedback signals are provided to the encoder, displaying the position of the conveyor belt; a loop generator displays the conveyor speed; and a sensor displays the registration number on each tag. The servo positioning system is powered by a microprocessor, providing error signals, a servo amplifier, and power signals to the servo motor. This application is illustrated in Figure 11-93.
The random timing material control system consists of a servo
Fifth, the application diagram shows a series of packaging equipment, which can be used as three independent machines. Each station of the packaging system operates independently of the others. The packaging system includes a material conveyor, a conveyor positioning station, and a packaging station. The material conveyor and packaging machine stations are connected so that they operate at the same speed. The positioning of the packages at the packaging station must be strictly controlled so that the packages do not become too close to each other. A metal plate, called a flight guide, is connected to the specific point of the packaging conveyor station to ensure that each package remains in its position. A sensor is installed at the beginning of the positioning to determine when the leading edge of the package begins to move onto the positioning conveyor. A second sensor is located at the bottom of the conveyor belt to detect the package flight. These two signals from the sensors are transmitted to the motor, providing information so that the servo can adjust the speed of the positioning conveyor, making each package agree with one of the flight actions as it is packaged on the conveyor belt. This application demonstrates that the servo positioning controller is capable of handling a variety of different signals from multiple sensors because the controller employs a microprocessor.
