Multistage Robotic Automation System To Minimise Tact Time

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Multistage Robotic Automation System To Minimise Tact Time Ranshinge Aditya 1, Dr. M. S. Patil 2 1,2 Electronics & Telecommunication Engg., R.I.T. Islampur Abstract Automobile industries are using automation systems to increase the production and improve the quality of the product.

1 Multistage Robotic Automation System To Minimise Tact Time Ranshinge Aditya 1, Dr. M. S. Patil 2 1,2 Electronics & Telecommunication Engg., R.I.T. Islampur Abstract Automobile industries are using automation systems to increase the production and improve the quality of the product. As the complexity of welding operation is increased, manual welding of complex points has become time consuming. To overcome these drawbacks an automated system is designed. This automation system is made up of mechanical fixtures to hold the body, small turn table to increase ease of reach of welding point, big turn table to separate loading / unloading time from auto cycle to get minimum cycle time and gantry to pick and place body from one stage to another stage. PLC is the central control unit of all these operations. Servo motor is used to control the movements of turn table which gives maximum accuracy during rotation. Movement of gantry is controlled using induction motors. For X travel and Z travel selection of different induction motors is necessary. For clamping and de-clamping operation pneumatic cylinders are selected, which are operated with the help of solenoid valves. The data through Human Machine Interface is intuitively feedback to the user. By controlling entire assembly with the help of PLC tact time minimization is possible. Keywords Mechanical Fixtures, Turn Table, Gantry, Solenoid Valves, Human Machine Interface. I. INTRODUCTION Today s automobile industries have exponentially growing market, so all these industries want their product in the market as early as possible. As the design of any automobile part is becoming complex day by day, it has become difficult for them to reduce the time to market of that product. To have proper design and to improve the mechanical strength of product the welding quality should be as high as possible. We can reduce the time to market of the product using automation with optimized stages. Robots are mainly used in flexible and programmable environment. A robotic welding produces high quality products, in less time and at lowest total cost. So to reduce production time of the product, industries are using robotic automation. So to get optimized design and manufacturing time, process is divided into six stages. Turn table is used to minimize cycle time of two stages. Slider is used to move fixture from small turn table to large turn table and vice versa. Movement of slider is controlled by two pneumatic cylinders. Position control of turn table is done using servo drive and servo motor [3]. Gantry is used to move the object between two stages, speed of gantry is controlled by variable frequency drive as per the load attached [5]. Mechanical fixtures are used to clamp and de-clamp the body with the help of pneumatic cylinders. Robot teaching is carried out at each stage with the help of pendant and robot controller. All these actions are controlled by PLC. GX works 2 software is used for the PLC programming and GT designer software is used for the HMI programming. II. BLOCK DIAGRAM Automation system needs precise interfacing of components with PLC. Induction motors are interfaced with the help of PLC by variable frequency drives. Motion of the turn table is controlled with the help of servo motor, which are interfaced to the PLC with the help of servo drive. Mechanical clamping is controlled with solenoid valves and these valves are interfaced to the PLC with the help of solenoid coils. Interfacing of robots and PLC is done with the help of robot controller. Device-net communication protocol is used to communicate 43 nodes with PLC and QJ71DN91 device-net module is used for configuration. Each node is assigned with predefined All Rights Reserved 519

2 bytes and output bytes. For each node of robot controller we have used 6 bytes for input and 6 bytes for output. HMI is connected to the PLC with the help of Ethernet cable. Position sensors and reed sensors are connected to the PLC through remote I/O panel. Fig. 1 shows the block diagram of proposed system. Fig. 1: Block diagram of proposed system Entire design is divided into six stages. The first and fourth stage have one big turn table and two small turn tables for fixture rotation and two robots for spot welding. Second and third stage has one small turn table each and one & two robots for each respectively. Gantry is used for picking and placing body from one stage to another stage. III. HARDWARE DESIGN 3.1 Interfacing of PLC and Robot As there are more than 40 nodes to communicate through device-net and the programming rungs are more than 50,000, Q series PLC is selected. PLC is central control unit of an automation system which requires single phase power supply and it communicates with other devices with the help of remote I/O cards, input-output cards and device-net node connection. Robots are connected to the PLC with device-net node connection and the other components are connected with the help of remote I/O cards and input-output cards. Remote I/O cards are spaced away from PLC and communication between the two is done with the help of device-net which prevents the signal loss [2]. Robot controller is a part of the robot which helps to communicate with PLC. Each robot controller has different node addresses of device-net communication which is assigned in PLC configuration of device-net module QJ71DN91. Robot controller is as shown in Fig. 2. Board 10 and board 11 is the separate hardware which is used to interface torch cleaning, tower lamp and welding source with robot. Both of them have different device-net node addresses but it is not necessary to configure these addresses through PLC. Digital input (DI) of robot controller is the output of PLC and digital output (DO) of robot controller which is the input of PLC. Using this digital output, PLC gets the current status of robot and using digital input, robot gets the current status of PLC All Rights Reserved 520

Hence remote I/O cards are preferred to have communication between these devices and PLC. Device-net is used to interface remote I/O cards with PLC which gives zero signal loss.

3 Fig. 2: Robot controller 3.2 Interfacing of turn table, gantry and mechanical fixtures with PLC In an automation system there are devices which are far away from controller panel but they must communicate with PLC. Hence remote I/O cards are preferred to have communication between these devices and PLC. Device-net is used to interface remote I/O cards with PLC which gives zero signal loss. For each group of remote I/O cards separate node address is assigned which is set on base card. There is provision to connect three more cards to a single base card. So as per the requirement of signals predefined numbers of remote input cards or remote output cards are connected. The remote I/O card connection is as show in Fig. 3. Fig. 3: Remote I/O card connection Interfacing of PLC with turn table, gantry, mechanical fixtures and indexer is done with the help of remote I/O cards. Each card has sixteen signals so each node can have maximum 64 signals as there is provision to connect maximum four cards to each node. Sending and receiving of 24V signal is done through hard wiring from remote I/O card to different devices Turn table Assembly of turn table is made up of servo motors which are controlled by the servo drives. Absolute encoders are used to get precise knowledge of current position. Encoder has connection from servo drive to servo motor. Servo drive provides count of pulses through encoder to PLC during motion. For a specific location encoder will provide count of pulses which is stored in buffer memory of PLC. This procedure is known as Jogging. Motion of servo motor will start from home position and it will continue until the predefined number of pulses from buffer memory is counted All Rights Reserved 521

On big turn table, two small turn tables are placed which holds the mechanical fixtures. This is as shown in Fig. 4. 3.2.

4 Fig. 4: Small turn tables on big turn table Servo motor will move forward when it gets forward command from PLC. PLC will latch this command until turn table reaches to 180 degree position with the help of encoder pulse count of buffer memory. On big turn table, two small turn tables are placed which holds the mechanical fixtures. This is as shown in Fig Gantry Induction motors are used to move gantry from one location to another location. X travel and Z travel of gantry is controlled using different motors which are interfaced to the PLC with the help of variable frequency drives. Forward, reverse and error reset commands are given to the variable frequency drive (VFD) from PLC [1]. VFD will control the speed, acceleration, decceleration and break release parameter of the induction motor. Two proximity sensors are used to provide the start point and end point of gantry travel. Induction motor does not have the precise movement and gives VFD an error for sudden stopage. To overcome these drawbacks two more proximity sensors are provided to slow down induction motor before sudden stop [4]. Fig. 5 shows the entire gantry line with six vertical motion shuttle and horizontal motion assemby. Vertical motion shuttle have two pneumatic cylinders at the end to hold the body and place it to another location. Fig. 5: Gantry line Mechanical fixtures The body on which welding is performed need to be hold properly so that welding spots will not get shifted during running cycle. Fixture holds the body by clamping & declamping operation of pneumatic cylinders and robot performs welding operation on this fixture. This is as shown in Fig. All Rights Reserved 522

5 Fig. 6: Mechanical fixtures Solenoid valves and solenoid coils are used to clamp and declamp these cylinders. During running cycle it is required to get the current clamping and declamping status of each cylinder. Reed switch gives current clamp declamp status of pneumatic magnetic cylinder. IV. TESTING AND ANALYSIS All signals are tested before taking into the auto cycle. Initially testing of device-net communication is done. Device-net requires 24V at all nodes. This 24 V is provided from separate SMPS at main panel. As there is a signal loss at each node, communication error occurs after 15 nodes. So separate 24 V signal is provided after an interval of 15 nodes. All signals of remote I/O panels are forcefully turned ON and their status is tested online. Parameters of variable frequency drive are set as per speed requirement. Acceleration & deceleration parameter should be increased with increase in frequency. If acceleration & deceleration parameter are not precise then VFD will go into the error because of sudden stoppage and overload of motor. As two frequencies are used during travel, second acceleration & deceleration parameters are set for second frequency. First & second frequency for X travel is 50Hz and 18Hz respectively. Acceleration & deceleration parameter for 18Hz frequency are 0.1s & 0.2s respectively. Fig. 9 & Fig. 10 show the acceleration and deceleration parameter selection graph for all frequencies. Analysis shows that if frequency is increased then acceleration & deceleration parameter should also increase to prevent VFD from error occurrence. Fig. 7: Frequency vs Acceleration graph for VFD parameter All Rights Reserved 523

6 Fig. 8: Frequency vs Deceleration graph for VFD parameter selection Acceleration & deceleration parameter for 18Hz frequency are 0.1s & 0.2s respectively. Fig. 9 & Fig. 10 show the acceleration and deceleration parameter selection graph for all frequencies. Analysis shows that if frequency is increased then acceleration & deceleration parameter should also increase to prevent VFD from error occurrence. Current, number of cycles and hold time are welding parameters of robot which varies with thickness of the body. One cycle is equal to 20 ms and number of cycles will decide the time for which current is passed through the spot gun. Metal plates of different thickness are joined together by welding. These plates are of 0.8, 1 and 1.2 mm thickness. Table 1 and 2 shows the parameter selection with varying thickness of 2 layer and 3 layer plates respectively. Table 1: Welding parameters of 2T thickness body Thickness Current Cycle Current Cycle Hold time 1.6 mm 5.5 KA KA 14 4 ms 1.8 mm 5.5 KA KA 14 4ms 2.2 mm 5.5 KA 14 7 KA 14 5ms 2.4 mm 6 KA 13 7 KA 12 6ms Table 2: Welding parameters of 3T thickness body Thickness Current Cycle Current Cycle Hold time 2.6 mm 6 KA 15 7 KA 17 4 ms 3 mm 6.5 KA KA 17 4ms 3.2 mm 7 KA 14 8 KA 15 5ms 4.8 mm 8 KA KA 14 6ms Analysis shows that, if thickness of welding point is increased then current should increase, number of cycles should decrease and hold time should increase. Larger thickness requires higher current for less time to get proper All Rights Reserved 524

7 IV. CONCLUSIONS As a novel technology of mechatronics, automation system has been applied in various automobile industries, which also processes a great strategic meaning in overall improving the modernize level of precision welding automation. Robotic welding is one of the key technologies in welding automation. In the multistage welding automation system, gantry is completing the pick and place operation, turn table is minimizing loading and unloading time, mechanical fixtures are holding the body and robots are welding spots with precise movement. At the end of paper, software program development of robot, PLC and HMI is explained with few steps. So the automation system with PLC as a central controlling unit is minimizing tact time and improving the quality of welding production. REFERENCES 1. M. L. Walker and G. Diana, Test Bed System to Evaluate the Efficiency of Variable Speed Drives Under Varying Load Conditions, 1998 IEEE. 2. Chan Woo Moon and M. S. Kim, PLC Based Coordination Schemes for a Multi-robot System, Proceedings of the 2001 IEEE Seoul, Korea, May 21-26, 2001 International Conference on Robotics & Automation. 3. Chen Xian-feng, HE Ya-fei, Shen Xin-di, Chen Qingzhang, Development of A Pulse-Type Servo Drive System and the Application on a 3-Axises Robot, nd International Asia Conference on Informatics in Control, Automation and Robotics, IEEE. 4. Sowmiya, Monitoring and Control of a PLC Based VFD Fed Three Phase Induction Motor for Powder Compacting Press Machine, Proceedings of 7th International Conference on Intelligent Systems and Control (ISCO 2013). 5. Fernando J. T. E. Ferreira, Joao A. C. Fong, and Anibal T. de Almeida, Eco-analysis of Variable-Speed Drives for Flow Regulation in Pumping Systems, IEEE Transactions on Industrial Electronics, VOL. 58, NO. 6, JUNE Xuewu Wang, Minghao Li, Lika Xue, Dongyan Ding, Xingsheng Guin, Welding Robot Path Optimization Based on Hybrid Discrete PSO, 2014 Seventh International Symposium on Computational Intelligence and Design. 7. All Rights Reserved 525

INTERNATIONAL JOURNAL OF DESIGN AND MANUFACTURING TECHNOLOGY (IJDMT)

INTERNATIONAL JOURNAL OF DESIGN AND MANUFACTURING TECHNOLOGY (IJDMT) International Journal of Design and Manufacturing Technology (IJDMT), ISSN 0976 6995(Print), ISSN 0976 6995 (Print) ISSN 0976 7002 (Online)