What are the components of motion control for an industrial robot?
1. Servo Control Section
2. Stepper control section
3. Variable frequency control section
Let's answer these control points one by one.
Servo control
I. Working Principle of AC Servo Motors
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.
II. Composition and Classification of Servo Systems
1. Composition of a servo system:
A servo system is a general term for control systems that use position and angle as control variables. Systems that use velocity, angular velocity, acceleration, force, etc., which are related to position and angle as control variables are also included in the servo system.
2. Classification of servo systems:
(1) According to the control structure, it is divided into: open-loop and closed-loop.
(2) Classified by drive components as follows:
a. Stepper motor servo system.
b. DC motor servo system.
c. AC motor servo system.
III. Characteristics of Servo Motors (AC)
1) High positioning accuracy; ordinary servo motors can achieve 0.036 degrees.
2) Fast response time.
3) The control is convenient and flexible, and the control system is easy to implement.
4) There are many models available, and different types can be selected according to different application environments.
5) Provides full closed-loop control, enabling real-time monitoring of operating status and appropriate adjustments.
IV. Servo System Structure
V. Selection Steps for Servo Control Systems
1. Determine the mechanical specifications, load, rigidity, and other parameters.
2. Confirm the motion parameters, including movement speed, stroke, acceleration/deceleration time, cycle, and accuracy.
3. Select the motor inertia, load inertia, motor shaft rotation inertia, and rotor inertia.
4. Select the motor rotation speed.
5. Select the motor's rated torque. This includes load torque, acceleration/deceleration torque, instantaneous maximum torque, and effective torque.
6. Select the motor mechanical position resolution.
7. Select the motor model based on the above.
VI. Applications of Servo Control
VII. Commonly Used Servo Motor Brands
Japan Panasonic PANASONIC
Mitsubishi MITSUBISHI
Japan Sanyo SANYO
Taiwan Teco
Step control
I. Working Principle of Stepper Motors
A stepper motor is an actuator that converts electrical pulses into angular displacement. When a stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called the "step angle") in a set direction. Its rotation occurs step by step at fixed angles. The amount of angular displacement can be controlled by controlling the number of pulses, thus achieving accurate positioning; simultaneously, the speed and acceleration of the motor can be controlled by controlling the pulse frequency, thus achieving speed regulation. Stepper motors can be used as special motors for control applications, and due to their characteristic of having no accumulated error (100% accuracy), they are widely used in various open-loop control systems.
II. Classification of Stepper Motors
Commonly used stepper motors include reactive stepper motors (VR), permanent magnet stepper motors (PM), hybrid stepper motors (HB), and single-phase stepper motors.
● Permanent magnet stepper motors are generally two-phase, with smaller torque and size, and a step angle of 7.5 degrees or 15 degrees.
● Reactive stepper motors are generally three-phase, capable of high torque output, with a step angle typically of 1.5 degrees, but they also generate significant noise and vibration. The rotor winding of a reactive stepper motor is made of soft magnetic material, and the stator has multi-phase excitation windings, utilizing changes in magnetic permeability to generate torque.
● Hybrid stepper motors combine the advantages of permanent magnet and reactive stepper motors. They are further divided into two-phase and five-phase types: two-phase stepper motors typically have a step angle of 1.8 degrees, while five-phase stepper motors typically have a step angle of 0.72 degrees. This type of stepper motor is the most widely used.
III. Stepper Motor System
1. Static Specifications Terminology for Stepper Motors
a. Number of phases: The number of excitation coil pairs that generate different N and S magnetic fields. Commonly represented by m. b. Number of pulses: The number of pulses required to complete one periodic change in the magnetic field, or the number of conduction states, represented by n, or the number of pulses required for the motor to rotate through one tooth pitch angle.
c. Step angle: The angular displacement of the motor rotor corresponding to one pulse signal is represented by θ.
d. Locking torque: The locking torque of the motor rotor itself when the motor is not powered on (caused by harmonics of the magnetic field teeth and mechanical errors).
e. Static torque: The locking torque of the motor shaft when the motor is not rotating under rated static electric action.
2. Stepper Motor Dynamic Specifications and Terminology
a. Step angle accuracy: The error between the actual value and the theoretical value of each step angle rotated by the stepper motor.
b. Step loss: The number of steps the motor actually moves during operation is not equal to the theoretical number of steps. This is called step loss.
c. Misalignment angle: The angle by which the rotor tooth axis deviates from the stator tooth axis.
d. Maximum no-load starting frequency: The maximum frequency at which the motor can start directly without load under certain drive conditions, voltage and rated current.
e. Maximum no-load operating frequency: The highest speed frequency of the motor without load under a certain drive mode, voltage and rated current.
f. Operating torque-frequency characteristics: The curve showing the relationship between the output torque and frequency of a motor under certain test conditions is called the operating torque-frequency characteristics.
IV. Stepper Motor Selection
1. Selection of step angle: The step angle of the motor depends on the accuracy requirements of the load.
2. Selection of static torque: The selection of static torque is based on the load on the motor. Generally, the static torque should be 2-3 times that of the friction load.
3. Current Selection: Due to the significant differences in operating characteristics caused by different current parameters, the motor current can be determined based on the torque-frequency characteristic curve.
V. Some characteristics of stepper motors
1. The accuracy of a typical stepper motor is 3-5% of the step angle, and this accuracy does not accumulate.
2. The maximum allowable temperature for the surface of a stepper motor is generally above 130 degrees Celsius.
3. The torque of a stepper motor decreases as the rotational speed increases.
4. The stepper motor can operate normally at low speeds, but it cannot start if the speed exceeds a certain limit, and it is accompanied by a whistling sound.
5. Stepper motors are used in low-speed applications—the speed should not exceed 1000 revolutions per minute.
VI. Performance Comparison of Stepper Motors and AC Servo Motors
1. Different control precision
Five-phase hybrid stepper motors typically have step angles of 0.72° or 0.36°. The control accuracy of AC servo motors is determined by the rotary encoder at the rear end of the motor shaft.
The device guarantees that for a motor with a standard 2500-line encoder, its pulse equivalent is 360°/10000 = 0.036°, and the accuracy of a servo motor is higher than that of a stepper motor.
2. Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. AC servo motors, on the other hand, operate very smoothly and do not vibrate even at low speeds.
3. Different overload capacities
Stepper motors generally do not have overload capacity. AC servo motors, on the other hand, have a strong overload capacity.
4. Different operating performance
Stepper motors are controlled in an open-loop manner. If the starting frequency is too high or the load is too large, step loss or stalling may occur. If the speed is too high when stopping, overshoot may occur. AC servo drive systems are controlled in a closed-loop manner. The driver can directly sample the feedback signal from the motor encoder and internally form a position loop and a speed loop. Generally, step loss or overshoot of stepper motors will not occur, and the control performance is more reliable.
5. Different speed response performance
Stepper motors require 200-400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance; for example, the Panasonic MSMA400W AC servo motor accelerates from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
6. Different torque-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds, while an AC servo motor provides constant torque output.
In summary, AC servo systems outperform stepper motors in many aspects. However, stepper motors are often used as actuators in less demanding applications. Therefore, the design of a control system must comprehensively consider factors such as control requirements and cost to select an appropriate control motor.
VII. Commonly Used Stepper Motor Brands
Oriental Stepper Motor
Teco stepper motor
Sanyo stepper motor
Variable frequency control
General Motor Introduction
The three-phase squirrel-cage AC motor is the most common type of induction motor. Its structure and characteristics are as follows:
1) Schematic diagram of the structure of an induction motor
Schematic diagram of the motor structure
2) Characteristics of the motor:
frequency converter
1) Frequency Conversion Principle
A frequency converter is a control device that can easily and freely change the speed of an AC motor.
A frequency converter achieves speed regulation by changing the frequency of the AC motor power supply.
2) Composition of frequency converter
2.1 Converter (Rectifier): Diode bridge rectifiers are widely used, as shown in Figure 1, which converts mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate regeneratively due to its reversible power direction.
2.2 Smoothing Circuit: The DC voltage after rectification by the rectifier contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
2.3 Inverter: In contrast to rectifier, inverter converts DC power into AC power at the required frequency. By turning on and off 6 switching devices at a predetermined time, a 3-phase AC output can be obtained.
2.4 Braking Circuit: When the asynchronous motor is used in the regenerative braking region (with negative slip), the regenerative energy is stored in the smoothing circuit capacitor, causing the DC voltage to rise. Generally speaking, the energy accumulated by the inertia of the mechanical system (including the motor) is greater than the energy that the capacitor can store. When rapid braking is required, a reversible converter can be used to feed back to the power supply or a braking circuit (switch and resistor) can be set up to consume the regenerative power, so as to prevent the DC circuit voltage from rising.
2.4 Braking Circuit: When the asynchronous motor is used in the regenerative braking region (with negative slip), the regenerative energy is stored in the smoothing circuit capacitor, causing the DC voltage to rise. Generally speaking, the energy accumulated by the inertia of the mechanical system (including the motor) is greater than the energy that the capacitor can store. When rapid braking is required, a reversible converter can be used to feed back to the power supply or a braking circuit (switch and resistor) can be set up to consume the regenerative power, so as to prevent the DC circuit voltage from rising.
3) Application purpose and uses of frequency converters
A variable speed drive consisting of a frequency converter and an AC motor is called a frequency converter drive , and its functions and uses are as follows. These may be related to each other, and there is actually no clear classification; this table is for reference only.
4) Brand
The following brands are currently widely used in the market:
International brands include: Siemens (MM4 series including MM420, MM440, MM430, MM410, etc.), ABB (ACS series including ACS350, ACS510, ACS550, ACS800, etc.), FUJI, Hitachi, Danfoss, Schneider Electric, etc.
Domestic: Delta Electronics (China), Senlan
