introduction
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, threatening the obsolescence of traditional DC servos. 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. Currently, the main problems restricting the industrial application of PLC-based control servo systems include: increased initial investment due to intelligent equipment supporting PLC control servo systems; changes in system structure and how to construct a PLC-based servo control system; communication reliability; and limitations in equipment selection, among other user concerns.
In recent years, with the development of robotics and control technology, robots have been widely used in daily life and industrial and agricultural production. A robot is a nonlinear, strongly coupled, multivariable system. During its motion, due to uncertainties such as friction and load changes, it is also a time-varying system. Traditional robot control technologies are mostly model-based control methods, which cannot achieve satisfactory trajectory tracking results. The development of artificial intelligence, such as fuzzy control and neural networks, has provided new ideas for solving the robot trajectory tracking problem. The control rules of ordinary fuzzy control are mostly summaries of human experience. They lack self-learning and adaptive capabilities and are often influenced by human subjectivity. Therefore, they cannot effectively control time-varying and uncertain systems.
Currently, industrial robot joints are mainly controlled by AC servo systems. This study applies the mature, easy-to-program, highly reliable, and small-sized SIEMENSS-200 programmable controller to a controllable circulating current reversible system to develop a DC servo system for robot joints, which is used for servo control of industrial robot joints.
1. Establishment of robot servo control system
In this system, the stereo positioning system serves as the primary data input channel, accurately acquiring the precise relative position between the target location and the robot. This real-time spatial information is then integrated into the previously established spatial model. During this process, it is necessary to determine the transformation relationship between the previous model and the actual 3D space, i.e., registration.
Then, the robot performs motion operations according to the motion plan formulated by the computer-aided system. During the motion, the stereo positioning system continuously collects the spatial position of the robot relative to the target, and performs visual control in conjunction with the robot's multi-axis controller. The robot control system is shown in Figure 1. In the block diagram, the input is the feedback current of the robot's walking drive servo motor, and the output is the robot's walking speed, which is achieved by servo speed regulation.
Figure 1 Robot Control System
This paper designs a six-degree-of-freedom (DOF) robot: three rotational and three translational. The robot's six DEFs work together to complete spatial motion. Considering the robot's small size, the goal is to minimize weight. This necessitates limiting the overall load on the mechanism due to reduced stiffness, while also considering stability during high-speed motion. Furthermore, the stiffness design of this multi-DOF mechanism depends on the speed and direction of motion.
1.1 Control System Structure
The system uses a SIEMENS 7-200 PLC with an external D/A converter module to convert the PLC's digital signals into analog signals. These analog signals are then driven by a BT-I variable current speed control system (mainly composed of a speed regulator ASR, a current regulator ACR, a circulating current regulator ARR, a positive trigger GTD, a negative trigger GTS, and a current feedback circuit TCV) to drive the DC motor, which in turn drives the robot joints to perform actions according to control requirements. The system structure is shown in Figure 2.
Figure 2. Schematic diagram of robot servo control system structure
1.2 System Working Principle
The control system principle is shown in Figure 3. The main circuit of the controllable circulating current reversible speed regulation system adopts a cross-connection method. One secondary winding of the rectifier transformer is connected in a Y-shape, and the other is connected in a Δ-shape. The phases of the two AC power supplies are staggered by 30°, and the frequency of its circulating current voltage is 12 times the power frequency. In order to suppress the AC circulating current, two balancing reactors are connected between the two sets of controllable rectifier bridges, and a smoothing reactor is still retained in the armature circuit.
Figure 3. Schematic diagram of DC servo system for industrial robot
The control circuit mainly consists of a speed regulator ASR, a current regulator ACR, a circulating current regulator ARR, a positive trigger GTD, an inverse trigger GTS, and a current feedback TCV, as shown in Figure 3. The synchronization signals of the two triggers are taken from the synchronization transformers corresponding to the rectifier transformers.
When the system setpoint is zero, the speed regulator ASR and current regulator ACR are locked to zero by the zero-speed block signal. At this time, the system mainly consists of a cross-feedback constant current system composed of the circulating current regulator ARR. Due to the influence of the circulating current setpoint, both sets of thyristors are in rectification mode, with equal output voltages but opposite polarities. The DC motor armature voltage is zero, the motor stops, and the output current flows through the two sets of thyristors to form a circulating current. The circulating current should not be too large, generally limited to about 5% of the motor's rated current. During forward start-up, as the speed signal Ugn increases, the block signal is released, the speed regulator ASR outputs positive, and the motor runs in the forward direction. At this time, the positive group current feedback voltage +Ufi2 reflects the sum of the motor armature current and the circulating current; the negative group current feedback voltage -Uril reflects the armature current, thus allowing for the regulation of the main current. The circulating current setpoint signal -Ugih and the cross-current feedback signal -Ufil applied to the input of the positive group circulating current regulator have minimal impact on this regulation process. The input voltage of the reverse-current circulating regulator is (+Uk) + (-Ugih) + (Ufi2). As the armature current increases, the circulating current automatically disappears when it reaches a certain level, and the reverse-current thyristor enters the inverting state. The situation is reversed during reverse start-up. Furthermore, the controllable circulating current reversible speed regulation system still exhibits processes such as local bridge inversion, reverse braking, and feedback braking during braking. Since the starting process is also a process of gradually decreasing circulating current, the circulating current reaches its maximum value when the motor stops. The circulating current helps the system overcome the switching dead zone and improves transient characteristics. 1.3 Servo Control System Speed Regulation Control System and its Principle
A servo motor speed control system consists of a servo driver, a motor, and its control system. The servo speed control system changes the synchronous speed of the asynchronous motor by altering the power supply frequency to the stator. Its speed control characteristics largely maintain the inherent mechanical characteristics of a servo motor, such as high mechanical stiffness and low slip, while also offering advantages such as high efficiency, wide speed range, high precision, and smooth speed control. The working principle diagram of servo speed control is shown in Figure 3.
(a) Schematic diagram of servo driver working principle (b) Block diagram of servo speed regulation working principle
Figure 3. Working principle diagram of servo speed regulation
Changing the motor's frequency and number of poles can both alter its speed. Therefore, changing the motor's frequency allows for speed regulation.
The main component of a servo drive system is the servo drive that provides variable frequency power. Servo drives can be divided into two main categories: AC-DC-AC drives and DC-AC frequency converters. Currently, most domestic systems use AC-DC-AC drives. Their advantages include high efficiency, no additional losses during speed regulation, wide application range, large speed range, and high precision.
Changing the stator power supply frequency can change the synchronous speed and the motor speed. Furthermore, according to the electric potential formula for a motor, the applied voltage is approximately proportional to the product of the frequency and the magnetic flux.
As shown in the above formula, if the applied voltage remains constant, the magnetic flux changes with the frequency. Generally, in the design of motors, the magnetic flux Φ is selected close to the magnetic saturation value to fully utilize the core material. Therefore, if the frequency is reduced from the rated value, the magnetic flux will increase, causing magnetic circuit oversaturation, increased excitation current, and core overheating, which is unacceptable. Therefore, we need to reduce the voltage while reducing the frequency, which requires coordinated control of frequency and voltage.
2. System Programming
The program design involves manually inputting an angle value to rotate the motor. A photoelectric encoder connected to the motor detects the rotation angle and converts it into a pulse signal. Because the motor rotates very quickly, only the pulse signal can be sent to the PLC's high-speed counter. The counter's pulse record is then compared with the manually input value. If they are equal, the motor has reached the specified angle position; otherwise, the correction process continues. It is important to note that the motor has some inertia when it suddenly stops rotating, so a certain margin of error should be allowed during signal comparison; otherwise, the motor will remain in the correction position indefinitely. The system flowchart is shown in Figure 4.
Figure 4 System Program Flowchart
3. Conclusion
A DC servo system based on a PLC is developed. Leveraging the PLC's strong expandability, a manual input/output device is added to enable visual operation of the industrial robot's joint DC servo system. Its advantages are:
(1) The forward and reverse rotation of the motor can be controlled by the program without changing the circuit structure;
(2) It enables the motor to rotate in the opposite direction immediately without waiting for it to stop rotating;
(3) It can bring the motor to an emergency stop, preventing the motor from rotating due to inertia;
(4) Easy to program and maintain.
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