1 Introduction
With the development of modern science and technology, PLCs have been widely used in industrial control microcomputers.
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.
2 Industrial robot joint DC servo system
The joints of industrial robots are driven by DC servo motors. The forward and reverse rotation of the motors is controlled by a circulating reversible speed regulation system to achieve servo control of the joints of industrial robots.
2.1 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 1.
Figure 1 Schematic diagram of the DC servo system for robot joints
2.2 System Working Principle
The system principle is shown in Figure 2. 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 the 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.
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 (see Figure 2). The synchronization signals of the two triggers are taken from the synchronization transformers corresponding to the rectifier transformers.
Figure 2 Schematic diagram of DC servo system for industrial robot joints
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 invertible 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.
3 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 match, 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 3.
Figure 3 System flowchart
4. Conclusion
The DC servo system developed based on PLC utilizes the strong expansion capability of PLC and adds a manual input/output device to realize visual operation of the DC servo system of industrial robot joints. Its advantages are: (1) the forward and reverse rotation of the motor can be controlled by the program without changing the circuit structure; (2) the motor can be made to rotate in the opposite direction immediately without waiting to stop rotating; (3) the motor can be stopped suddenly to avoid inertial rotation; (4) programming and maintenance are convenient.
