Wireless multipoint communication for optical sensors in the industrial environment using the new Bluetooth standard Stephan Hussmann*a, Wing Yan Laua, Terry ChUa, Markus Grothofb aschool of Engineering, Department of Electrical and Electronic Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand blnstitut fuer Nachnchtenverarbeitung (INV), University of Siegen, Hölderlinstr. 3, 57068 Siegen, Germany ABSTRACT Traditionally, the measuring or monitoring system of manufacturing industries uses sensors, computers and screens for their quality control (Q.C.). The acquired information is fed back to the control room by wires, which - for obvious reason - are not suitable in many environments. This paper describes a method to solve this problem by employing the new Bluetooth technology to set up a complete new system, where a total wireless solution is made feasible. This new Q.C. system allows several line scan cameras to be connected at once to a graphical user interface (GUI) that can monitor the production process. There are many Bluetooth devices available on the market such as cell-phones, headsets, printers, PDA etc. However, the described application is a novel implementation in the industrial Q.C. area. This paper will contain more details about the Bluetooth standard and why it is used (network topologies, host controller interface, data rates etc.), the Bluetooth implementation in the microcontroller of the line scan camera, and the GUI and its features. Keywords: Bluetooth, CCD camera, industrial application, quality control, embedded system design 1. INTRODUCTION Optical sensors have been successfully used for Q.C. applications in industries, such as the sheet metal, paper, plastics and textiles industries [ 1, 2]. Their communications were based on wired connections between the sensing element and the data-acquisition electronics. However, there are situations where a wired connection may not be the best approach to a problem and the invention of a wireless solution becomes crucial. Under the following environments wired solution becomes infeasible to the task. For example, where the application's environment is harsh, such as extreme cold, hot, or filled with corrosive gas or liquid, or when fast and easy set-up test labs for large and complex experiments are required [3]. Another application area might be Q.C. sensor systems based on moving sensors, where due to the continual movement of the wires the wires lifetime is not very long. The wires have to be replaced at regular intervals and therefore the maintenance cost of the Q.C. systems is very high [4]. With a wireless solution, all these problems can be solved. This paper examines how to implement the new Bluetooth technology in a Q.C. system using high-speed line scan cameras. The optical line scan camera (model IZ1024, ASENTICS GmbH) used for this project is based on a high resolution CCD linear sensor with 1024 pixels, which means it can scan and divide a line into 1024 parts. The camera's accuracy depends upon how far the sensor is placed from the object and how many of them are used. The maximum frame rate is 2kHz. * s.hussmann@auckland.ac.nz; phone +649 373 7599 ext. 85979; fax +649 373 7461; http://www.ele.auckland.ac.nzi hussmann 84 Digital Wireless Communications V, Raghuveer M. Rao, Soheil A. Dianat, Michael D. Zoltowski, Editors, Proceedings of SPIE Vol. 5100 (2003) 2003 SPIE 0277-786X/03/$15.00
_ A connection overview of the actual system is shown in Figure 1. A measure object is placed in front of a light source. The object widths object shadow - is measured and send via Bluetooth to the data-acquisition electronics. An Ericsson Bluetooth module is used and connected via the serial interface (RS-232) to the camera's microcontroller. line scan camera I reobje Ericsson Bluetooth module 1 _light source Figure 1 : Connection overview between the line scan camera and the Bluetooth module The paper is organized as follows. In section two, a general description of Bluetooth is presented. After that, Host Controller Interface (HCI) commands, events and data packets are discussed in more detail. A system overview is given in section four. In section five the final implementation is presented and finally, concluding remarks are given in section six to summarize the work. 2. OVERVIEW OF THE BLUETOOTH STANDARD Bluetooth wireless technology is a low-cost, low-power, short-range radio link for voice and data transfer. It operates in the 2.4 GHZ ISM (Industrial, Scientific, Medical) band. Instead of modulating data at a specific frequency, Bluetooth employs frequency-hopping techniques. It is meant that the radio hops through the full spectrum of 79 1-MHZ channels using a pseudorandom hopping sequence. The hopping rate of 1600 hops per seconds provides a good immunity against other sources of interference in the 2.4 GHz band. The maximum link speed is 1 Mbps. It is achieved by using a Gaussian Frequency Shift Keying (GFSK) technique. Higher rates can be obtained by using a more complex modulation technique, but GFSK keeps the radio design simple and low cost. The expected range of a typical Bluetooth device is lom to loom and it is depended on the transmission power. The transmission power is between 1mW to 100mW. The Bluetooth system supports both point-to-point and point-to-multipoint connection. When two or more Bluetooth devices are connected together, a piconet is formed. There will be only one master and a maximum of 7 active slaves within a piconet. A scatternet is formed when there is an overlapping between different piconets [5]. 2.1 Throughput performance Bluetooth can support an asynchronous data channel, up to three simultaneous voice channels, or a channel that simultaneous support asynchronous data and synchronous voice. In this project, an asynchronous data link (ACL) connection is required for data transfer between the line scan cameras and the PC. A synchronous data link (SCO) connection can be ignored since there is no voice transmission among devices. When there is only one slave with an ACL connection, the data rate can reach 718 kps in one direction (slave to master! master to slave) and 57.6 kps in the other direction (master to slave! slave to master). However, as the number of slave devices increases, the data rate continues to decrease. When seven active slaves are in a piconet, the maximum data rate drops to 102 kps [6]. Proc. of SPIE Vol. 5100 85
2.2 Data packets ACL packets are used on asynchronous links. The information carried can be user data or control data. Seven ACL packets including the DM1 packet have been defined. Six of the ACL packets contain a CRC code and retransmission is applied if no acknowledgement of proper reception is received. The 7th ACL packet, the AUX1 packet, has no CRC and is not retransmitted. DM stands for Data Medium Rate. DH stands for Data High Rate. The difference between the DM packet and DH packets is the existence of FEC. All DM packets are FEC encoded. With FEC, parity bits are added to the payload for error correction and it will reduce the data rate. On the other hand, packets, which cover more time slots, will give a higher throughput. Both Dxl and AUX1 packet may cover up to a single time slot. Dx3 packet may cover three time slots and Dx5 packet may cover five time slots. A summary of the packets and their characteristic are shown in Table 1. Times Slot Occupied Throughput in Kps Packet In slave In Master In slave In Master Type..... S Direction Direction FEC CRC Direction Direction DM1 1 1 / J 108.8 108.8 DHJ 1 1 X / 172.8 172.8 DM3 3 1 4 / 387.2 54.4 DH3 3 1 X 585.6 86.4 DM5 5 1 / / 477.8 36.3 DH5 5 1 X / 723.2 57.6 AUX] 1 1 X X 185.6 185.6 Table 1: Data throughput of single ACL connection in Bluetooth [5] 3. INVESTIGATION ON HOST CONTROLLER INTERFACE Bluetooth has its own protocol layer, which is different from the well-known 051 layer. The layer, which this section will cover, is called the host controller interface (HCI) layer. This layer enables user to send different packets to the Bluetooth's hardware hence allowing a communication with other Bluetooth enabled devices. In order to develop the communication interface between the sensor and the Bluetooth module, the Bluetooth's communication protocol must be understood. From the undertaken research it is known that the HCI provides a uniform command interface to the baseband and the link manager (LM). This means that using the HCI command can control the Bluetooth module. Therefore, the investigation has been concentrated on the HCI layer. 3.1 HCI packets There are three different types of HCI packet:. HCI command packet - From host to Bluetooth module HCI. HCI event packet - From Bluetooth module HCI to host. HCI data packet - going both ways. In order to distinguish between these HCI packets, four different HCI command indicators are used. These indicators must be sent before the HCI packets. Table 2 shows the hexadecimal code of each HCI command indicator. 86 Proc. of SPIE Vol. 5100
HCI packet type HCI command packet HCI ACL data packet HCJ SCO data packet HCI event packet HClpacket indicator OxOl 0x02 0x03 0x04 Table 2: HCI packet Indicator [7] 3.2 HCI commands and events In order to communicate with the Bluetooth module, HCI commands are sent to the Bluetooth module through the serial port in a series of predefined bytes. The response from the serial port is then read. The response will be a series of bytes and is called the HCI event. The following Figure 2 helps to understand this more clearly. [ter Computer J [ Slave CompuJ. [11 Initialise Serial Communication ; ' Initialisation Succeeded [2] Setup HCI layer Setup Complete [3] Search for nearby devices Devices found, addresses are returned [4] Request Connection [1] Initialise Serial Communication Initialisation Succeeded [2] Setup HCI layer ' Setup Complete Connection established, connection handle returned ' Connection Connection Request received [3] Accept Connection established, connection handle returned U Figure 2: Connection procedure between master and slave devices At the beginning the serial communication is initialized by sending a reset command to setup the link between the host (master/slave) and the module. Therefore a serial of bytes, which represent the reset command, are sent. An event should be received after the transmission of this HCI command. If the reset command is successful the command Complete Event is returned. Afterwards the HCI layer will be setup by sending more commands to the module following the same handshake procedure (see Figure 5). Once the setup is completed the nearby devices are searched and if a device has been found its valid address is returned to the host. Now a connection can be requested and once accepted, a connection handle is returned and the connection is established. Proc. of SPIE Vol. 5100 87
3.3 HCI ACL data packets When a connection is established, a connection handle will be returned. Data should be able to be transmitted among those connected devices with this connection handle. The HCI ACL data packet is used for transmitting data when an ACL connection is established. The format of the ACL data packet is the same for both the sender and the receiver. If a sender has submitted this series of bytes to the module, the receiver end will receive the same series of bytes after the transmission1. 4. SYSTEM OVERVIEW The realized system consists of three main components: (1) PC 1, connected to a point-to-multipoint Bluetooth module (model ROK 101007, Ericsson); (2) a line scan camera (model 1Z1024, ASENTICS GmbH), connected to a point-topoint Bluetooth module (model ROK 101008, Ericsson) and (3) PC 2, which is also connected to a point-to-point Bluetooth module (model ROK 101008, Ericsson). The system overview is illustrated in Figure 3. Figure 3 : System overview of the wireless multipoint communication PC 1 is connected to the point-to-multipoint module, Module A. This module allows PC 1 to have up to 7 connections simultaneously. Hence, PC 1 can communicate with up to 7 other modules at the same time. Communication includes sending and receiving data or voice packets. However, in the current project, only data packets are used. Module B is connected to the optical sensor and Module C is connected to PC 2. With this configuration, the GUI allows the user to operate Sensor 1 remotely while chatting with PC 2 at the same time. Quality control systems often require more than one line sensor, for instance the length as well as the width of an object has to be measured. With the Bluetooth's Multipoint capability a PC here PC 1 - can act as a master, which communicates with more than one sensor (slave) at the same time. In this configuration, each sensor has a separate connection with the host PC and each of these connections is given a unique connection handle code. Therefore, it allows the master to identify incoming and outgoing packets for different sensors. This topology is called a piconet with a maximum number of seven sensors (slaves). 1 More information on the HCI can be found in the Bluetooth Specification V 1.1 PartH. This document can be downloaded from the Bluetooth official home page - http://www.bluetooth.com. 88 Proc. of SPIE Vol. 5100
5. SYSTEM IMPLEMENTATION 5.1 Mini Bluetooth stack V A mini Bluetooth stack is built in the sensor's microcontroller. With this Bluetooth stack, the sensor will be able to communicate with the Bluetooth module directly through the serial connection. This mini Bluetooth stack is actually the Host Controller Interface driver. Figure 4 provides an overview of the interaction between the sensor and the Bluetooth Module. With this HCI driver, the sensor has the capability to access the Bluetooth Hardware. The implemented mini Bluetooth stack provides 19 HCI commands for the sensor to communicate with the Bluetooth Module. 5.2 Setup between sensor and Bluetooth module Figure 4: Interaction between sensor and Bluetooth module )atfon. - --._c,>$ HI REAU.B..._..+ ICLHL )Si..C HC 4)31 r HC.f :FR9[. ER..SZE D U H i HC.,Wfii..5CO H ( W A I. HW..WRriE.M -- - * LwR1E,ENc t i ED W t U 1 RO. R I AC H5 NTCTr.)N -.. -..--.. Ht4WA1iEP UMEOUT.. : A. HCL (HAN3E (;AL...WAuE Figure 5: Linkage setup procedure between the sensor and the Bluetooth module Proc. of SPIE Vol. 5100 89
The following setup is carried out in order to establish the linkage between the sensor and the Bluetooth module. After the sensor has initialized its setting, those HCI functions that are provided by the mini Bluetooth stack will be called one by one to setup the linkage. When the linkage is created, the sensor can be called a Bluetooth enabled devices. Its Bluetooth address will be available to other Bluetooth devices. However, if the linkage is not created, the addresses will not be discovered. This is a reliable test to check whether the sensor has successfully created a linkage with the Bluetooth module or not. The required linkage setup procedure is shown in Figure 5. 5.3 Connection procedure Once the linkage between the sensor and the Bluetooth module is established, the sensor is ready to make a connection with other devices. In this project, the sensor acts as a slave within the piconet. Therefore, the sensor is only designed to accept connections. The sensor keeps listening to the serial port and searches for a series of bytes with the pattern of the Connection_Request_Event. When the Connection_Request_Event is received, a HCI_ACCEPT_CONECTION_REQUEST function is called to accept the connection. Afterwards, the connection handle can be retrieved from the Connection_Complete_Event. The connection is then created and ready for data transmission. 5.4 Graphical user interface The realized GUI is designed for a Q.C. monitoring system, which allows the user to communicate with 2 Bluetooth modules at the same time. One module is connected to the optical sensor and the other is connected to a PC (see Figure 3). The GUI consists of three main panels: (a) a connection panel, (b) chat panel and (c) sensor panel. The connection panel allows the user to search for Bluetooth enabled devices. If the modules connected to the sensor and the chat program have been found, a connection will be established. The sensor panel displays the processed senor data such as max/mm value, number of edges, edge positions etc. and the chat panel can be used to chat with PC 2. Figure 6 shows the GUI of the Q.C. monitoring system and demonstrates its multipoint communication capability. [chat panel sensor panel 1 Figure 6: GUI for a Q.C. monitoring system demonstrating the multipoint communication capability 90 Proc. of SPIE Vol. 5100
6. CONCLUSION In this paper a wireless multipoint communication for optical sensors using the new Bluetooth standard has been demonstrated. The system can employ up to seven sensors, which are connected to a central PC. The data rate of the sensors will decrease as the number of connections sensors - increases. Furthermore, different data packets will affect the data rate as well. The HCI and its commands are discussed and a mini Bluetooth stack based on these commands has been developed and implemented into the sensor's microcontroller. It provides 19 HCI commands for the sensor to communicate with the Bluetooth module. With this mini Bluetooth stack, the sensor can establish a connection with other Bluetooth devices and support the wireless data transmission. A GUI monitor system is developed to operate under the windows environment. This system can handle multiple connections. Furthermore, it can monitor and retrieve measurement results from the sensor wirelessly. REFERENCES 1. S. Hussmann, W. Kleuver, B. Gunther, J. Gröneweller and H. Rath, "Automatic measurement equipment for quality control in the loop slitting department", SPIE Proc. Advanced Photonic Sensors and Applications: Three- Dimensional Inspection, 3897, pp.328-334, 1999. 2. S. Hussmann, W. Kleuver, and J. Gröneweller, "High resolution linear sensor system for sheet metal width measurement", SPIE Proc. Optical Measurement Systemsfor Industrial Inspection: Industrial Applications Ii, 3824, pp.365-372, 1999. 3. M. Dunbar, "Plug and play sensors in wirless networks", IEEE Instrumentation & Measurement Magazine, 4 (1), pp.19-23, 2001. 4. S. Hussmann and A. P. Hu, "A novel low-cost contactless power supply for fast moving triangulation sensors", Applications of Electromagnetic Phenomena in Electrical and Mechanical Systems, JSAEM Studies in Applied Electromagnetics and Mechanics, 14, pp.189-196, 2003. 5. P. Bhagwat, "Bluetooth: technology for short-range wireless apps", Internet Computing, IEEE, 5 (3), pp.96-103, 2001. 6. T.J. Lee, K. Jang, H. Kang and J. Park, "Model and Performance Evaluation of a Piconet for Point-to-Multipoint Communications in Bluetooth", Proc. of IEEE VTC 2001,2, pp.1 144-1148, 2001. 7. K. Sung-Yuan, "The embedded Bluetooth CCD camera", Proc. of IEEE TENCON 2001, 1, pp.81-84, 2001. Proc. of SPIE Vol. 5100 91