PLC-based DC servo system for industrial robot joints

Abstract: By utilizing the high reliability, convenient programming and maintenance, and small size of the programmable controller (PLC) control system, this system is applied to the circulating reversible speed regulation system. A PLC-based industrial robot joint DC servo system is developed. The forward and reverse rotation of the motor is controlled by the circulating reversible speed regulation system to achieve servo control of the industrial robot joint. The advantage is that the circuit structure does not need to be changed when changing the forward and reverse rotation of the motor, making the servo control of the industrial robot joint simpler, more reliable, and more stable.

1 Introduction

With the development of modern science and technology, PLC has been widely used in industrial control microcomputers.

At present, industrial robot joints are mainly controlled by AC servo systems. This study applies the SIEMENS S-200 programmable controller, which has mature technology, convenient programming, high reliability and small size, to a controllable circulating reversible regulation system, and develops a robot joint DC servo system 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, and the servo control of the joints of industrial robots is achieved by controlling the forward and reverse rotation of the motor through a circulating reversible speed regulation system.

2.1 Control system structure

The system uses SIEMEN S7-200 PLC, plus D/A digital-to-analog conversion module, to convert PLC digital signal into analog signal, and drives DC motor to operate through BT-I variable current speed control system (mainly composed of speed regulator ASR, current regulator ACR, circulating current regulator ARR, positive group trigger GTD, negative group trigger GTS, current feedback device TCV), and drives robot joints to move according to control requirements. The system structure is shown in Figure 1.

Figure 1 Schematic diagram of the DC servo system structure of the robot joint

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 Y type, and the other is connected in △ type. The phases of the two AC power sources 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 groups of controllable rectifier bridges, and a smoothing reactor is still retained in the armature circuit.

The control circuit is mainly composed of a speed regulator ASR, a current regulator ACR, a circulating current regulator ARR, a positive group trigger GTD, a negative group trigger GTS, and a current feedback device TCV (see Figure 2), wherein the synchronization signals of the two groups of triggers are respectively taken from the synchronous transformer corresponding to the rectifier transformer.

Figure 2 Schematic diagram of DC servo system for industrial robot joints

When the system setting is zero, the speed regulator ASR and the current regulator ACR are locked to zero by the zero speed blocking signal. At this time, the system is mainly composed of the circulating current regulator ARR to form a cross-feedback constant current system. Due to the influence of the circulating current setting, the two groups of thyristors are in the rectification state, the output voltage is equal in magnitude and opposite in polarity, the DC motor armature voltage is zero, the motor stops, and the output current flows through the two groups of thyristors to form a circulating current. The circulating current should not be too large, and is generally limited to about 5% of the rated current of the motor. During forward starting, as the speed signal Ugn increases, the blocking signal is released, the speed regulator ASR inputs positive, and the motor runs forward. 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, so the main current can be adjusted. The circulating current setting signal -Ugih and the cross current feedback signal -Ufil added to the input end of the positive group circulating current regulator have little effect on this adjustment process. The input voltage of the reverse circulating current regulator is (+Uk)+(-Ugih)+(Ufi2). As the armature current increases, when it reaches a certain level, the circulating current automatically disappears, and the reverse thyristor enters the waiting state for inversion. The situation is the opposite during reverse starting. In addition, the controllable circulating current reversible speed regulation system still has the processes of local bridge inversion, reverse braking and feedback braking during braking. Since the starting process is also a process in which the circulating current gradually decreases, the circulating current of the system reaches the maximum value when the motor stops. The circulating current helps the system to cross the switching dead zone and improve the transition characteristics.

3 System Programming

The program design scheme is to manually input an angle value to make the motor rotate, and detect the angle of the motor through the photoelectric encoder connected to the motor, and convert the rotation angle into a pulse signal. Since the motor rotates very fast, the pulse signal can only be sent to the high-speed counter of the PLC. Then compare the pulse record of the counter with the manual input. If the two are equal, it means that the motor has reached the specified angle position, otherwise continue to correct. It is worth noting that since the motor will have a certain inertia when it suddenly changes from rotation to stop, a certain error should be allowed when comparing the signals, otherwise the motor will always be in the correction position state. The system program block diagram is shown in Figure 3.

Figure 3 System flowchart

4 Conclusion

The DC servo system developed based on PLC takes advantage of the strong expansion capability of PLC and adds a manual input device to realize the visual operation of the DC servo system of the industrial robot joint. Its advantages are: (1) the forward and reverse rotation of the motor can be controlled by program without changing the circuit structure; (2) the motor can be turned in the opposite direction immediately without waiting for it to stop; (3) the motor can be stopped urgently to avoid inertial rotation; (4) programming and maintenance are convenient.