Development of a Controller for a Reconfigurable Manufacturing Cell

Transcription

1 Development of a Controller for a Reconfigurable Manufacturing Cell T.P. FERREIRA, I.A. GORLACH Department of Mechatronics Nelson Mandela Metropolitan University (NMMU) University Way, Summerstrand, Port Elizabeth SOUTH AFRICA Tremaine.Ferreira@live.nmmu.ac.za, Igor.Gorlach@nmmu.ac.za Abstract: - Reconfigurable Manufacturing Systems (RMSs) are widely used in modern production. The important features of RMSs, such as modularity, integrability and diagnosability, require open-architecture controllers. However, sub-systems from different vendors often use different communication protocols and interfaces. Hence, a RMS controller must be capable to overcome this drawback and allow easy control and integration of various types of controllers, devices and machines. This paper presents the development of a personal computer based cell controller, which is used for the control of a reconfigurable manufacturing cell. The SCADA control platform, developed with Movicon software, uses standard and special communication protocols. Keywords: - SCADA, Controller, Movicon, Reconfigurable manufacturing system. 1 Introduction Highly competitive markets demand rapid changes of manufacturing processes and systems to produce products at lower costs and shorter delivery times. Reconfigurable Manufacturing Systems (RMSs) are capable of rapid adjustment of their structure, modularity and functionality in response to new product type and production quantities, [1]. The core of an RMS is a computer numerical control (CNC) based reconfigurable machining centre, [2]. However, technological advantages of RMSs can only be achieved when all elements and subsystems of a RMS, including control systems, are seamlessly interfaced. Therefore, it is important to design controllers in such a way that they provide efficient control of various types and complexities of equipment, manufacturing systems and technological processes. Controllers of modern discrete manufacturing systems are based on distributed control via networks with industrial PCs and PLCs as hardware on different control levels, which utilise Supervisory Control and Data Acquisition (SCADA) as software. SCADA is the technology that enables data to be collected from distant facilities and/or control instructions to be sent to them, thus eliminating the need to be physically present at the manufacturing system to observe or alter its operation [3]. Common SCADA development packages include Movicon, InduSoft Web Studio and Phocus [4]. In this research, a reconfigurable manufacturing cell is used as a hardware platform [5-8]. 2 Description of the Reconfigurable Manufacturing Cell Figure 1 shows a model of the Reconfigurable Manufacturing Cell, which consists of a Reconfigurable Machining Centre (RMC), a Robotised Automated Guided Vehicle (R-AGV) and Automated Storage and Automated Retrieval (AS/AR) stations. Reconfigurable Machining Centre AGV Track Robotised AGV AS/AR loading/unloading stations Fig. 1 CAD model of the Reconfigurable Manufacturing Cell Individual controllers of the sub-systems are integrated with the system cell controller. The subsystem controllers are described below. 2.1 Reconfigurable Machining Centre The RMC is a 3-axis CNC machine capable of performing milling and electric-discharge machining in a single setup [8] (Fig 2). The RMC is ISBN:

2 enclosed for safety reasons, with the enclosure door capable of opening and closing automatically. The RMC is controlled by a PC using Artsoft Mach3 TM CNC software. Mach3 TM supports the following industrial protocols for data communication: ModBus Serial RTU and ModBus TCP/IP. Mach3 TM has a Brains function, which allows the selection of data to be sent or received via the ModBus protocol and can be located under the Operator tab in the Mach3 TM environment. Since Mach3 TM can only act as a master device, it is important that a SCADA development package can support the ModBus protocol as a slave device. Movicon software was selected as the development software for Supervisory Control and Data Acquisition (SCADA) of the cell controller. Movicon includes standard drivers for common industrial communication and devices, such as ModBus, ProfiBus and CANBus [4]. Furthermore, Movicon has a Visual Basic for Applications (VBA) editor and allows OPC connectivity and tag importation. ModBus serial and TCP/IP protocols via serial and wireless communication were chosen for the cell controller. Automation control of the cell is achieved by means of instruction list logic on the cell controller, which eliminates the need to use a physical PLC to handle logic operations. The interface selection and configuration for each sub-system is explained below. 3.2 Control Interface with the RMC Fig 2. Photograph of the RMC 2.2 Robotised Automated Guided Vehicle The R-AGV is a mobile part handling system, which is used for loading/unloading of the RMC and the AS/AR stations. The R-AGV consists of a mobile platform and an articulated robotic arm. The R-AGV is controlled by a laptop running RoboRealm software. RoboRealm is an application that is used for computer vision and image analysis [9] and is used for line following based on computer vision. The utilization of computer vision allows the R-AGV to distinguish between different colored lines when performing routing operations and docking. RoboRealm allows data communication through either the serial port or ModBus TCP/IP (Slave) protocol. The cell controller is required to communicate with the RMC controller in order to control the cycle start/stop and other auxiliary functions. This is achieved with serial communication and the ModBus serial protocol. The main advantage of using the ModBus protocol in Mach3 TM is that it does not require any additional interfaces to be able to communicate with the cell controller. The two computers are connected to one another using a null modem". Furthermore, Movicon has builtin drivers for the ModBus serial protocol that allows simple data transfer between ModBus devices. Therefore, it reduces implementation time, since no addition programming is required. ModBus TCP/IP was not implemented, since the Movicon package does not support ModBus TCP/IP as a slave. Figure 3 illustrates a simple ModBus configuration in Mach3 TM that assigns three different variable addresses. 3 Development of the Cell Controller 3.1 Development Software The requirement of open-architecture of the cell controller is achieved with PC-based software and widely used industrial communication protocols. Fig. 3 Setup serial ModBus control interface Figure 4 shows a brain created in Mach3 TM, which monitors the status of the RUN and STOP LED s in the Mach3 TM environment. If, ISBN:

3 for example, the RUN LED is active, then cfg #0 will be used to transmit the information to ModBus address 5 as a 16 bit holding register. Fig.4 Mach3 TM Brain screenshot 3.3 Control Interface with the R-AGV The cell controller is required to transmit instructions with regard to operations and receive feedback from the R-AGV controller. The instructions include the required docking position information, pick-and-place coordinates and sequences of tasks to complete (load/unload). Communication between the cell and R-AGV controllers can only be wireless since the R-AGV is a mobile system. The XBee Pro S2B wireless module was selected for communication between the cell controller and the R-AGV because of its low cost and a large range. The XBee modules are configured with X-CTU software and run 9600 baud. RoboRealm was configured to transmit and receive serial data in the following format and order: [Status]/[MarkerPos]/[X]/[Y]/[Z]/[Theta]/[Dock] The first value which is transmitted represents the [Status] variable used in RoboRealm. Usage of the / character serves as a separator between variables and is used by the cell controller when reading data transmitted by the R-AGV controller. 3.4 Control Interface with the Safety System The cell controller is required to monitor safety of the RMC during its operation and to control the sliding door. The machine enclosure, with a number of micro switches at the doors, provides safety during machine operation. The sliding door of the RMC enclosure is used when delivering and removing workpieces by the R-AGV. The door needs to be controlled manually or automatically, i.e. during a cycle. The sliding door is actuated by a pneumatic cylinder via a double solenoid 3/2 control valve. An Arduino Mega 2560 microcontroller was selected as the controller of the safety system and for the sliding door actuation. The valve, micro switches and E-stop button are connected to digital pins on the Arduino board and configured as I/Os. The cell controller communicates with the Arduino microcontroller via a serial link. The VBA editor in Movicon was used to create a program that sends and receives serial data since there are no standard communication drivers available for the Arduino microcontroller. The microcontroller analyses the signal received from the cell controller and switches an output accordingly. For example, if the cell controller transmits an O the microcontroller will set output pin 51 high, which will activate the solenoid valve that will open the sliding door. 3.5 Overall Control Interface with the Manufacturing System The overall controller interface architecture is shown in Fig. 5. The cell control hardware is a PC, while the control software is Movicon, which is used for the development of SCADA. 3.6 SCADA In conjunction with the SCADA control functionality, interactive screens were designed as a human-machine interface (HMI) for cell control and communication. All sub-systems can be controlled in manual or automatic control modes from this interface. Fig.6 shows the manual control mode of the RMC. The interactive screens allows a user to control and/or monitor all aspects of the manufacturing cell by means of buttons, text display and status LEDs. With the SCADA system, a user gains access to either a single component of the cell, or the complete system, by selecting the appropriate option in the home screen. Tag values (such as RMC status and R-AGV position) are updated in real-time, regardless of the selected screen. Each screen is also equipped with a large E-stop button that enables the user to stop the process at any time during the cycle. New tags and screens can be created or defined by a developer. The developer can then either configure the built-in protocols (ModBus, CANbus etc.) to transmit the status of a tag, or create custom VBA scripts to establish communication. ISBN:

4 Figure 5. Diagram of the cell controller interfaces Fig. 6 RMC screen (manual mode of operation) This allows devices from various vendors to be added to the system if such a need may arise. The VBA script editor also caters for the use of custom built products or products not commonly encountered. Flexibility and expandability of the system is therefore fully accounted for by Movicon. User accounts are created to assign rights, as determined by the developer, to various operators or users of the system. This prevents unauthorised access and enables logging of system users by capturing usernames and recording login/logout times. An additional feature developed for the system is the capability of remote access to the SCADA system. This is made possible with the Movicon software by exporting the SCADA solution into hyper text markup language (HTML) format. By appropriate configuration of the internet information services (IIS) on the SCADA computer, access to the SCADA system is possible from any computer connected to the network by means of a web browser. The cell controller is accessed remotely by entering the local host uniform resource locator (URL) into the address bar of the web browser. The HTML format can also be uploaded to the internet, permitting access to the system from devices such as smartphones, tablets and laptops. Web hosting is required to upload the website onto the world wide web, which will render the system accessible from anywhere in the world where a internet connection is available, and not only the local area network of the cell controller. The previously mentioned capability was not implemented due to controller requirements. ISBN:

5 4 Experiments 4.1. Control of R-AGV The R-AGV is controlled by RoboRealm, which allows for communication by either ModBus TCP/IP (Slave) or traditional serial communication via the USB port. The selected wireless modules do not support Ethernet communication. However, a simple experiment was conducted whereby an Ethernet cable was used to physically connect the R-AGV to the controller. By appropriate configuration of the ModBus protocol, communication between the R- AGV and the cell controller was established. The observations made are summarized in Table 1. ID # Result No noticeable difference in execution time for the chosen application ModBus communication via TCP/IP resulted in less complex implementation and significantly lower setup time. No additional programming required for TCP/IP ModBus, whereas complex programming is required to send both positive and negative numbers by means of USB (Serial) Table 1. TCP/IP vs. serial communication Using Ethernet communication it was observed that changes made within the SCADA program were reflected on RoboRealm, but not vice versa. Therefore, control via ModBus TCP/IP was possible, but due to some unidentified error, monitoring was not. Nevertheless, the control of the R-AGV was successfully implemented using serial communication. The use of serial communication was furthermore evaluated by using the cell controller to operate the R-AGV in both manual and automatic mode of operation. The sequence of tasks performed by the R-AGV, during evaluation in automatic mode, was as follows: Initialize system to home positions Drive R-AGV to automated loading station Load material Drive R-AGV to RMC Place material for machining (Achieved by commanding the manipulator to specified coordinates) Wait for machining operations to complete Retrieve machined workpiece once the safety door is opened. The control results obtained during experimentation are summarized in Table 2. Task Start/Stop of R-AGV Result Capable of successfully starting or stopping R-AGV motion Capable of successfully commanding Manipulator Control the articulated manipulator to various positions in a three dimensional coordinate system. E-Stop Successfully terminates R-AGV operation until released. Forced status changes on R-AGV R-AGV Status (such as E-Stop activation) were Monitoring successfully reflected on Cell Controller. Automatic Control The R-AGV successfully performed a sequence of tasks without human intervention. Table 2. Results for R-AGV communication 4.2. RMC and Safety Control The aim of the Mach3 communication experiment was to determine the reliability of the chosen communication strategy, as well as to identify any limitations when using brains. The following topics were addressed during experimentation: Control operations at various baud rates Evaluation of tool centre point monitoring E-Stop functionality General control functionality in manual and automatic mode of operation The sequence of tasks performed during testing in automatic mode is as follows: Open safety door Wait for material delivery Close safety door once it is safe to do so Start execution of machining sequence Open safety door for material collection once machining sequence has been completed The results obtained during experimentation are shown in Table 3. ISBN:

6 Task Result Start/Stop Capable of successfully starting or stopping the machining sequence TCP Monitoring Positional changes of the tool centre point were successfully reflected on both Mach3 and the Cell Controller. E-Stop Successfully terminates RMC operation until released. Automatic Control The RMC successfully performed a sequence of tasks without human intervention. Baud Rate Optimal reliability and performance at 9600 baud Since the Mach3 ModBus driver can only transmit a maximum of 32 bits of Accuracy data (double word) at the time, it limits the accuracy. Combining multiple double words was not achieved. Table 3. Results for RMC communication The encoder positions, as displayed on the cell controller, are used for monitoring of the system and not for critical control functions. The accuracy limitation does therefore not affect the functionality of the controller. 5 Conclusion In this paper, the development of a cell controller for a Reconfigurable Manufacturing Cell is presented. The cell controller uses a PC-based platform with SCADA software as HMI, which was developed on the Movicon software platform. The controller uses ModBus and serial protocols, implemented using both hardwired and wireless networks. The cell controller was successfully interfaced with the PC controllers of the reconfigurable machining centre and the robotised mobile platform, together with the Arduino microcontroller used for the auxiliary equipment and safety system. The developed cell controller is capable of controlling the sub-systems and devices from different vendors. The controller exhibits all the features of reconfigurable systems. It can be easily expanded on the lower control level to include additional manufacturing sub-systems and devices. The controller can also be integrated into higher level control systems via networks due to its highly flexible and compatible software platform. 6 Acknowledgments The authors would like to acknowledge financial assistance and support provided by General Motors South Africa, through the GMSA Chair of Mechatronics, the National Research Foundation, and the Technology Innovation Agency (TIA). 7 References [1] Y. Koren, M. Shiptalni, Design of Reconfigurable Manufacturing Systems, Journal Of Manufacturing Systems, Volume 26, 2010, pp [2] Z.M. Bi, S.Y.T. Lang, M. Verner, P. Orban, Development of Reconfigurable Machines, International Journal of Advanced Manufacturing Technology, Volume 39, 2008, pp [3] S.A. Boyer, 1993, SCADA: Supervisory Control and Data Acquisition, North Carolina: Instrument Society of America, [4] Scadaworld, SCADA Products, [5] M. Simpson, I.A. Gorlach, Development of a Reconfigurable Machine Tool, International Conference on Competitive Manufacturing, COMA 10, 2010, Stellenbosch, South Africa. [6] W. Estment, I.A. Gorlach, G.J. Wiens, Design of sub-systems of a reconfigurable machine tool, CAD/CAM Robotics and Factories of the Futures Conference, July 2010, Pretoria, South Africa. [7] I.A. Gorlach, B.H. Roberts, Conceptual design of a reconfigurable manufacturing system for the tooling industry, Proc. 26 th Conf. on International Conference of CAD/CAM, Robotics & Factories of the Future, Kuala Lumpur, Malaysia, [8] M. Simpson, T. Light, I.A. Gorlach, B. Roberts, Development of a Model of a Special Purpose Machine, Proc. The 20 th IASTED International Conference on Applied Simulation and Modelling, ASM, Napoli, Italy, [9] RoboRealm, RoboRealm: Vision for Machines, ISBN: