Distributed Embedded Control System in Remote Labs

JAN BILEK, MARTIN ZOLDAK

Department of Control Engineering

Czech Technical University of Prague, Faculty of Electrical Engineering

Karlovo namesti 13, 121 35 Prague 2

CZECH REPUBLIC

http://dce.felk.cvut.cz/bilek

Abstract: - Request of a higher number of students at the technical university is reduced by labs capacity. The reduction of direct teaching hours is possible in seminars but not in laboratories being important for technical skills of students. Creating online laboratories is tool for removing high expenses, time-limited use of facilities and limited access for students to laboratories. This policy has brought the new requirements on the structure of laboratory models and their control systems. Among the supervisory system and the model of technology there is necessary to insert a lot of other systems, which mediate communication between the remote control and the monitoring of the laboratory model. Some of these systems may be hardware or software equipment and together they form the embedded system. This paper presents distributed embedded systems structure.

Key-Words: – embedded system, remote control, laboratory model, Internet, Intranet, local connection

1 Introduction

Laboratory exercises are important in technical education. They enable to obtain practical knowledge and understanding of solution of problems in the whole complexity. With the evolution of information technologies (IT) there are markedly increasing possibilities in remote controlling. This trend affected existing forms of education. Students get a chance of controlling of a technology model at a distance and at the same time they are studying without required sitting in the laboratory of the department. There has to be predefinition on what level will the distant control be used. We must decide whether the remote control refers to reading of technology parameters, visualization of choice parameters, recording of new parameters or developing of new algorithms for the control system. There on depends the structure and the cross connection of the distributed embedded system.

2 Embedded system structure

A very simple embedded system may be realized using one block between the control system and the technology. The major problem we must solve is a connection between the technology, embedded system and control system. The connection is usually performed by serial or parallel bus (Figure 1). In the case of a serial bus there is often used Profibus, which has in accordance with the specification three variants [1]. Older Profibus FMS (Fieldbus Message Specification) time responses are not suitable for real-time control. The second variant is Profibus DP (Decentralized Periphery), designated for communicating between a control system (Master) and distributed I/O (Slaves).

Control System

Parallel I/O bus

Embedded

System

Profibus PA

Technology

Fig. 1 A simple embedded system

The last variant is Profibus PA, which has the same protocol like Profibus DP but can be also used in an explosive environment. Obviously there are many other industrial buses suitable for real-time control.

This paper has been supported by Grant Agency of the Czech Republic, project number 102/02/1032 “Embedded Control Systems and their Inter-communication”.

2.1 Embedded system and the local connection

A connection of more than two stations via local bus requires the configuration of type master – master or master – slave. In the first case the access on bus is controlled by enforcement so–called as token-pass authorization to bus control. Another possibility is to determine one station – usually controller from the PLC family (Programmable Logical Controller with e.g. MPI – Multiple Point Interface) as fixed Master. Modern local control systems consist of a controller, communication processors, actuators and sensors, visualization module and also of programming device for the controller. Figure 2 shows four embedded systems (ES 1 to ES 4) in structure of a control system. Embedded systems serve as converters between

Control system

Controller Programmer Visualization

MPI MPI RS 485

ES 1 ES 2 ES 3

Profibus DP

ES 4

Profibus PA

Technology

Fig. 2 Embedded System with Local Net

different communication protocols. We can design one big embedded system for mutual connection of all parts that create the control system. But in this case the general embedded system will be more complicated and without standard structure. For any changes of the control system we will need a new design of this big embedded system and in other words such embedded system will not be an open system. The advantage of the distributed embedded system is evident. The most of small parts i.e. single embedded systems will be a standard commercial device. The set of simple embedded systems we may change as soon as we change the control system structure. Distributed embedded system is a very flexible one but there may be a specific problem with communication between its separate components. It appears that we may search compromise between a number of parts and a complexity of the embedded system. With the respect for the required simplification of structure of a control system it is necessary to speculate which simple embedded systems might be put together. We often join small embedded systems to the PC and so happens a concentration of several programs on the one computer. If the controlled technology isn't time–consuming any common fast PC will be sufficient but at all events it would be better to take into account bigger working memory storage (Figure 3). Also in this case we can meet with individually designed embedded systems. They usually use small microcontrollers or programmable logical fields as a type of FPGA.

Control system

Controller Programmer

+

Visualization

MPI

PCI bus

ES 1

PC

PCI bus

ES 2

Profibus PA

Technology

Fig. 3 Embedded System and PC

2.2 Embedded system and global the connection

Global connection of main parts in the control system supposes also a communication by means of the Internet. It results in using the Ethernet standard TCP/IP protocol [2]. This protocol offers sufficient degree of reliability but uncertain time response on data transmission. Therefore we always required that a control algorithm was ran in the controller directly connected to the technology. In contrary to running the algorithm in the controller next operations can be in progress on the remote workplace. It is concerned specially of monitoring of controlled parameters of the process, their tuning as well as the preparation of new programs and their downloading

WEB browser

Programmer + Visualization

Internet

Embedded System – WEB Server

Controller + Technology

Fig. 4 Remote control with a WEB browser

to the controller. As far as the development environment is licence free, the solution is simple (Figure 4). On the remote workplace we prepare a new version of the program and by means of web browsers we download it to the controller. The design of the new programs is more complicated in case that the utilize of the program environment is blocked by a licence. In the Microsoft environment it is necessary to use the client application Terminal Server Client (TSE). The operating system Windows 2000 Server allows to login of several users. For any user there is allocated his own remote desktop. For communication among server and client station the service TSE is using the Remote Desktop Protocol (RDP) for video transmission, mouse movement and for reading of keyboard. The RDP sends only changes of the picture thereby reducing time response on the transmission. User name and password allows the access only to a single client. Starting up the TSE Client application on the remote workplace, setting IP address or name server including domain, user name and password opens one of the 64000 communications channels. For restriction of the access only for one client there must be another software. Picture 5 shows connection with a remote terminal client.

3 Remote Lab

Control Systems laboratory contains laboratory scale model connected to the Internet with the possibility of modifying the program in the controller and a few different embedded systems. The basic solution of the problem shows schematic on Fig. 5 but actually the task is more complicated.

TSE Client

“terminal”

Internet

Windows 2000 Server

Controller + Technology

Fig. 5 Remote control with a Terminal

3.1 The controller

The sensors of the laboratory model connect with controller Simatic S7-315 DP directy [3], another parts of the model co-operates via embedded system SS (Safety System) Simatic S7-222 [4]. SS protects power solenoids from overloading and also evaluate the data from incoming from the serial

line. The configuration is shown on Figure 6. The programming environment STEP-7 is run on Windows 2000 Server.

Internet 2K Server

ES 1 Step - 7

RS 232 Profibus DP

PCI bus

I/O bus

M SS ES 2

o

d MPI

e I bus

l Controller S7-315-DP

Fig. 6 Controller connection

WEB Browser

Internet

Suite Voyager

ES

Industrial SQL Server

Factory Suite - InTouch

S7 I/O Server

Controller + model

Fig. 7 The visualization software hierarchy

3.2 The visualization

The visualization has two parts. The first supports displaying of a technological diagram and the second offers a real view of a task sorting of colored balls with different weight. The technological diagram is created in the environment Factory Suite made by the company Wonderware. For the remote control it must be completed with Industrial SQL Server and Suite Voyager web server. The number of accesses to the technological model is restricted by the Suite Voyager license. The software hierarchy is shown on Fig. 7.

The real view of the laboratory model mediates web camera [5] with the embedded system as interface to the Internet. The camera has its own IP address and the pictures on the Internet are freely accessible.

Fig. 8 View of the laboratory model

4 Conclusion

Here we presented a new architecture and results of our last work with lab model suitable for remote control [8]. Our previous model [6], [7] was finished before January 2002 and about seventy students it used during summer term in the academic years 2001/2002 and 2002/2003. The old model was simpler because its embedded system contained only one module for connection to the Internet. PLC Wago allowed without a license transmitting of the development environment on a remote workplace. The new laboratory model which photo is on Fig. 8 corresponds with block diagrams on the Fig. 6 and Fig. 7. Further will our work concentrate on design of a next group of models using embedded systems and new means of communication for remote control.

References:

[1] Profibus Technology and Application, System Description, PROFIBUS Nutzenorganisation, Germany. 2002

[2] W. R. Stevens, The Protocols (TCP/IP Illustrated, Volume 1),Addison - Wesley Pub Co, 1994

[3] SIMATIC, PLC S7/300 CPU Specifications, Reference Manual, Germany 2001

[4] SIMATIC, Programable Controller S7-200, Reference Manual, Brno 2002

[5] AXIS, Communications 2100 Network Camera, Axis 2002, http://www.axis.com

[6] J. BAYER, J. BILEK,. Small Embedded Control Systems, microCad’03 International conference, March 2003, Miskolc, Hungary

[7] J. BILEK, P. RUZICKA, Evolutionary Trends of Embedded Systems, IEEE ICIT 2003International Conference on Industrial Technology, December 2003, Maribor, Slovenia

[8] M. ZOLDAK, Remote Control of Technology, diploma thesis, CTU-FEE, January 2004, Prague