Servo systems can be controlled in either an open-loop or closed-loop manner.
I. Overview of Servo Systems
A servomechanism is a system that controls the position, speed, and acceleration of a mechanical system through closed-loop control. A servo system consists of a controlled object, actuators, and a controller (load, servo motor and power amplifier, controller and feedback device).
1. The function of an actuator is to provide power to the controlled object. Its main components include a servo motor and a power amplifier. The servo motor includes feedback devices such as photoelectric encoders, rotary encoders or gratings (position sensors).
2. The function of the controller is to provide closed-loop control of the entire servo system, such as torque control, speed control, and position control. Servo drivers typically include a controller and a power amplifier.
3. In addition to position sensors, feedback devices may also require voltage, current, and speed sensors.
The diagram below shows the components of a typical industrial servo system, where red represents the servo driver and yellow represents the servo motor.
The word "servo" originates from the Greek word for "slave." People envisioned a "servo mechanism" as a docile tool, obeying the demands of control signals: before a signal arrives, the rotor remains stationary; after the signal arrives, the rotor immediately begins to rotate; and when the signal disappears, the rotor automatically stops. It is named a servo system because of its "servo" performance.
II. Commonly Used Parameters
1. Servo motor nameplate parameters
1. Flange dimensions
2. Number of motor pole pairs
3. Rated output power of the motor
4. Power supply voltage specifications: Single-phase/Three-phase
5. Motor inertia: Divided into large, medium, and small inertia, this refers to the rotor's own inertia. From a response perspective, a smaller rotor inertia is better; from a load perspective, a larger rotor inertia is better.
6. Motor output shaft types: keyway, flat shaft, smooth shaft, gearbox adapter…
7. Motor power line definition: U:REDV:BLACKW:WHITE
8. Rated speed
9. Encoder lines: 2500/1250/1000/17B/20B
A flange is a part used to connect shafts to each other, and is used for connecting pipe ends.
2. Servo driver nameplate parameters
1. Rated output power
2. Power supply voltage specifications
3. Encoder line count
3. Performance indicators of the servo system
1. Detection error: This includes errors from the given position sensor and the feedback position sensor. These errors are inherent in the sensor itself and cannot be overcome.
2. Systematic error: The type of system determines the systematic error.
As long as p+q>0, the system has sufficient tracking capability for step input signals; for velocity input signals, the tracking capability of the Type I system is greatly weakened, and the tracking error is inversely proportional to the proportional coefficient of the open-loop transfer function, while the Type II system still has excellent tracking capability; for acceleration input signals, only the Type II system can barely follow.
III. Servo Motor Related
1. Servo Motor Selection
1. System accuracy: Factors such as rotor inertia, motor type, and torque fluctuation must be comprehensively considered.
2. Motor power: Calculate the output torque based on the load type and size.
3. Electric speed
4. Optional brake: The brake is used to lock the position when the motor stops, preventing the motor from moving due to external forces; it is not used to brake during operation.
5. Overload capacity
2. Servo motor feedback device / Number of pulses required for one revolution of the motor
The most common detection element used in servo systems is the photoelectric encoder.
An encoder is a device that encodes signals (such as bit streams) or data and converts them into a signal form that can be used for communication, transmission, and storage.
Based on their detection principles, encoders can be classified into optical, magnetic, inductive, and capacitive types. Based on their calibration methods and signal output formats, they can be classified into incremental, absolute, and hybrid types.
Incremental : A pulse signal is emitted for every unit angle rotated.
Absolute type : This means that for each reference angle in a revolution, a unique binary value corresponding to that angle is emitted. Multiple positions can be recorded and measured using an external recording device.
The encoder and current loop are not connected; its sampling comes from the rotation of the motor.
Encoder line count : This refers to the encoder resolution, which is the number of pulses emitted per revolution. For example, 2500 lines means that 2500 pulses are needed to make one revolution. This indicates that the number of pulses required for a servo motor to make one revolution is fixed and related to the parameters of the motor's built-in encoder.
It can be observed that there are two types of line counts: one is similar to 2500 lines, 1600 lines, etc., and the other is 17 bits (17B), 20 bits (20B), etc. The former is the line count of an incremental encoder, and the latter is the line count of an absolute encoder. 17B means that the number of pulses required for one revolution is 2^17, or 131072 pulses.
IV. Servo Driver Control Principle
Motion servo systems are generally three-loop control systems, consisting of a current loop, a speed loop, and a position loop from the inside out.
There are three types of servo control: position control, speed control, and torque control.
1. Torque Control (Current Loop/Single Loop Control): Torque control sets the output torque of the motor shaft by inputting an external analog signal or directly assigning a value to an address. The torque can be changed in real-time by altering the analog signal setting, or by changing the corresponding address value via communication. It is primarily used in applications requiring strict torque control. In torque mode, the driver's computation is minimized, resulting in the fastest dynamic response.
Single-loop control is difficult to meet the dynamic requirements of servo systems and is generally not used.
2. Speed Control (Speed Loop, Current Loop/Dual Loop Control): Rotational speed can be controlled via analog input or pulse frequency. Speed control includes a speed loop and a current loop. A current loop must be used in any mode; the current loop is the foundation of the control.
3. Position Control (Three-Loop Control): The most commonly used control method in servo motors. Position control typically determines the rotational speed by the frequency of externally input pulses and the rotation angle by the number of pulses (similar to a stepper motor). Some servos can also directly assign speed and displacement values via communication (external analog input). Because position control allows for very strict control over both speed and position, it is generally used in positioning devices.
In position control mode, the system performs calculations on all three loops, which results in the highest computational load and the slowest dynamic response speed.
Torque control: refers to the servo driver controlling only the torque of the motor.
Speed control: refers to the driver controlling only the speed and torque of the motor.
Position control: refers to the driver controlling the speed, angle, and torque of the motor.
APR – Position Regulator; ASR – Speed Regulator; ACR – Current Regulator
4. The three-loop system consists of three closed-loop negative feedback PID control systems.
The first loop is the current loop, the innermost loop. This loop operates entirely within the servo drive, and its PID constants are pre-set and do not require modification. The input to the current loop is the output after PID adjustment from the speed loop, and the output of the current loop is the phase current of each phase of the motor. The function of the current loop is to perform PD/PID adjustment on the difference between the input value and the feedback value. The feedback of the current loop comes from the Hall elements of each phase inside the drive. Current closed-loop control can suppress starting and braking currents and accelerate the current response process.
The second loop is the speed loop, also known as the intermediate loop. The speed loop's input consists of the output from the position loop's PID control and the feedforward value for the position setting. The current loop's function is to perform PI control on the difference between the input value and the speed loop's feedback value. The speed loop's feedback comes from the encoder's feedback value, calculated by the "speed calculator."
The third loop is the position loop, the outermost loop. The input to the position loop is the external pulse. The function of the position loop is to adjust the difference between the input value and the position loop feedback value using P-regulation. The feedback of the position loop comes from the pulse signal fed back by the encoder, calculated by the "deviation counter". The output limit of the position regulator APR is the maximum current, which determines the maximum speed of the motor.
There are no fixed values for adjusting the parameters of the position loop and speed loop; they are determined by many factors.
The design method for controllers in multi-loop control systems involves designing controllers for each loop sequentially, from the inner loop to the outer loop. This ensures the stability of each control loop, thereby guaranteeing the stability of the entire control system. Each loop has its own controlled object, with clear division of labor and ease of adjustment. The disadvantage of this design is that the response to the outermost loop's control action is not very fast.
5. Gain parameters of the servo system
After selecting the appropriate control mode based on equipment requirements, the servo gain parameters need to be adjusted appropriately. This ensures the servo driver can quickly and accurately drive the motor, maximizing mechanical performance. Servo gain is adjusted through multiple parameters, which influence each other.
1. Position proportional gain: The larger the setting value, the higher the gain, the greater the stiffness, and the smaller the position hysteresis under the same frequency command pulse conditions. However, too large a value may cause oscillation or overshoot;
2. Position feedforward gain: A large feedforward gain in the position loop improves the high-speed response characteristics of the control system, but it can make the system's position unstable and prone to oscillation.
3. Speed proportional gain: The higher the setting value, the higher the gain, the greater the stiffness, and the smaller the speed lag under the same frequency command pulse conditions. However, too large a value may cause oscillation or overshoot;
4. Speed integral time constant: The smaller the setting value, the faster the integral speed.
5. Speed feedback filter factor: The larger the value, the lower the cutoff frequency and the less noise the motor generates; the smaller the value, the higher the cutoff frequency and the faster the speed feedback response.
6. Maximum output torque setting
V. Servo System Design
Based on the type of servo motor, servo systems can be divided into two main categories: DC and AC. After adopting current closed-loop control, both types have the same mathematical model of the controlled object. Therefore, the same method can be used to design AC or DC servo systems. For closed-loop servo control systems, series or parallel correction methods are commonly used to adjust dynamic performance. Correction methods where the correction device is connected in series in the forward channel are called series correction, and the series correction unit is generally called a regulator, hence it is also called regulator correction; if the correction device is parallel to the forward channel, it is called parallel correction.
Regulator calibration: Commonly used regulators include PD regulators, PI regulators, and PID regulators. The appropriate regulator is selected based on the characteristics of the actual servo system during the design process.
VI. System Wiring and Panel Setup
This is just an overview.
System wiring
Panel Settings
VII. Distinguishing Servo Motors from Other Motors
1. The difference between a servo motor and a regular motor
1. Ordinary electric motors (brushed) mostly operate under open-loop control, while servo motors operate under closed-loop control.
2. Servo motor has high dynamic performance
3. Servo motors have high starting torque and wide speed range.
4. The servo motor has a compact structure.
5. The servo motor stator has convenient heat dissipation.
2. The difference between a servo motor and a servo motor
A servo motor is essentially a simplified version of a complete servo system.
Servo motors are all controlled by three loops: current loop, speed loop, and position loop; servo motors only detect the position loop (usually using a potentiometer).
3. The difference between servo motors and stepper motors
1. Stepper motors mostly operate under open-loop control, while servo motors operate under closed-loop control. (In applications using stepper motors, either position feedback is not required, or position feedback is provided on other equipment.)
2. Servo motors offer higher control precision and positioning than stepper motors.
3. Servo motors have good low-frequency characteristics, high overload capacity, and short response time.
4. The speed range of a servo motor is greater than that of a stepper motor.
5. Stepper motors can only accept pulse signals, while servo motors can accept analog signals, pulse signals, and bus communication signals.
Servo motors and stepper motors are often confused. They look similar, but the difference lies in the feedback device at the tail of the servo motor. In addition, stepper motors generally have one lead-out terminal, while servo motors have two lead-out terminals (encoder line and power line) because they have an encoder.
