Wireless Sensor Communication

Wireless Sensor Communication and Powering System for Real Time Industrial Applications Dacfey Dzung ABB Corporate Research CH-5405 Baden/Switzerland [email protected]

Christoffer Apneseth ABB Corporate Research Billingstad/Norway [email protected]

Guntram Scheible ABB Forschungszentrum D-68526 Ladenburg/Germany [email protected]

Wolfgang Zimmermann ABB Stotz-Kontakt GmbH D-69123 Heidelberg/Germany [email protected]

Abstract

In operation cables are the main causes for sensor faults and machine downtimes. As a consequence there are long periods of troubleshooting before the machine operation can be continued. Today the machine designer finds various technical solutions for increasing the reliability of electrical connections across moving machine components. Most of these solutions are only suitable for special applications like slip ring transmitters for example. A more general solution is the target of the completely new system of wireless proximity switches described in this paper.

For industrial automation applications such as production machines and robotic installations, truly wireless systems offer advantages in cost and flexibility. However, requirements on power supply and real time communication are highly demanding. A novel system has been developed which provides both wireless communication and wireless power supply. Power supply is based on magnetic coupling. Real time wireless communication uses a protocol optimized for reliable and low delay transmission.

3. System Description 1. Introduction Production machines and robotic installations are characterized by large numbers of cables to sensors and actuators. These cables are costly to install, and they represent a common source of failures that lead to production stops. This motivated the development of a novel system for wireless sensors that completely eliminates the need for cables.

2. Wireless Sensors Proximity switches are the most widely used position sensors for the control of numerous types of machines. However the main problem affecting their reliability are the cables between the sensors and the control system. Especially the cable connection between machine components which are moving against each other is very critical. Flexible cable ducts and high flexible cables are one option to increase the reliability of those connections.

The system consists of three main parts. These are (i) the wireless communication system, (ii) the wireless power supply and (iii) the low-power sensing. Each of the three parts are presented in separate sections below. The overall architecture of the system is shown in Figure 1: For the wireless power supply, four primary loops are installed around the manufacturing cell. These loops are fed by two power supplies that set up an alternating current in the loops. The alternating currents in the loops result in a magnetic field distributed throughout the cell. Within the cell, a number of wireless proximity switches are placed. The sensors include small coils that pick up the energy from the magnetic field and convert it to electric power. The sensors also include small radio transceivers and low-power electronics that take care of the wireless communication link. The sensors communicate with an input module through antennas mounted in the cell. The input module acts in much the same way as a traditional, wired input module. It can handle up to 60 wireless links simultaneously, and it is connected to the control system via a field bus plug. To

be able to serve up to 300 sensors in one manufacturing cell, five input modules can co-exist within the same area (Figure 2).

4. Wireless Power Supply The design of the wireless power supply posed a fundamental challenge. Whereas battery technology may have improved over the last decade, no batteries could provide the minimum of ten years maintenancefree operation as required by the system. A number of technologies were investigated, including thermocoupling, photovoltaic cells, fuel cells and more. Unlike as in wireless consumer devices, regular charging or change of batteries can not be accepted in an industrial application, especially if there are hundreds of sensors often mounted in partly not easily accessible locations and in machines running three shifts, 24h a day. Therefore a key feature is an energy autonomous device or a wireless power supply [1]. After a thorough evaluation of the various options it was concluded that the only viable solution is based on magnetic coupling. The basic principle of a wireless supply via magnetic fields can be described by the well known transformer model with parasitic elements. The primary winding is a large coil around the volume to be supplied, the secondary side are then a number of small receiver coils, e.g. with a small ferrite core to increase the flux collected by the coil. The parasitic elements dominate by far, the main coupling quantity is the magnetic field strength at the secondary position. If the primary coils are setup in an Helmholtz - like arrangement, this quantity is fairly constant over a large volume. – Though people will not work continuously in those cells, the allowed field strength values regarding operator safety were a main design criteria to keep the field strength within all international occupational regulations and recommendations [2]. Such a “transformer” can only be operated conveniently in a resonant mode, where the large (leakage) inductance is compensated. Then an operation with relatively low voltage and current is possible. Such a supply should also be able to cope with changes in the environment, e.g. large and moving metallic obstacles (e.g. robots) and with large “load” variations, like different sized primary coils and different amount of losses e.g. by eddy currents in adjacent metallic obstacles. The primary supply therefore needs an automatic and highly accurate control to maintain a perfect tuning for the resonant system to the fixed power frequency of 120kHz.

To achieve significant power at the receiver side, also the receiver side coils have to be operated in a resonant mode. To obtain constant power output regardless of orientation in relation to the primary field vector, an orthogonal setup of three coils has been chosen, to achieve a fairly simple tunable resonant system suited for industrial mass production (Figure 3). Obviously all types of metal will be present in the target application (production machines, robots, ...) therefore a unidirectional field vector of the magnetic field can be unintentionally shielded. Therefore a rotating field is used increasing the shielding tolerance. Considering a worst case minimal field strength, also under shielding conditions, achievable power levels on the secondary coils mainly depend on the size of the coil and core size/shape used. A typical specific worst case value achieved in the first prototypes is 1mW/cm³ at 6A/m. The power supply system should be able to cover systems sizes matching typical manufacturing cell sizes, e.g. up to 6x3x3m³. The system is modular, so cells can be connected to cover larger areas with several cells.

5. Wireless Communications The wireless communication subsystem transmits the sensor signals to the input module (wireless base station). The system must satisfy the demanding requirements for communications that exist in an industrial environment. These requirements are even more demanding than for wireless communication systems for office or home use: The system must have very fast response times (generally much less than 1015ms for real time applications), serve a large number of sensors located in a cell of several meters radius, and guarantee a high reliability of data transmission even in the unfriendly environment of factories, where radio propagation may be affected by many obstacles and where various source of interfering signals must be expected. An extensive study was undertaken to evaluate technologies and standards for their performance and applicability to the wireless sensor system. None of the existing systems satisfied all industrial requirements. Passive electronic tagging systems as used for example in department stores for electronic article surveillance do not have sufficient range and flexibility. Wireless local area networks (WLANs) or short range wireless links such as Bluetooth do not support a high number of sensors. Hence it was decided to design a new system tailored to the needs of the wireless sensor, while re-using as many of the available standard low cost components as possible. The resulting system operates in the 2.4 GHz radio band allocated to ISM (Industrial, Scientific and

Medical) systems. A highly sophisticated input module has been designed so that the greatest complexity resides on the input module side in order to keep the sensor design as simple as possible. One such input module wirelessly connects to and controls up to 60 sensors. Although similar to a wireless local area network (WLAN) base station in many respects, the design has some features that set it apart. These include: Simultaneous transmission and reception of radio signals. Such full-duplex operation is not possible with Bluetooth and WLAN radios. Simultaneous reception of the strongest and weakest signals. The difference in power between a strong signal and a weak one may be as much as 60 dB. Interference suppression. Reception of a very weak sensor message/signal is possible even though a large interfering signal may exist at some adjacent frequency. In addition, transmit and receive antennas at the base stations may be periodically switched, to provide diversity radio propagation paths as a protection against fading and shadowing effects. The sensor communication hardware is based on a standard Bluetooth transceiver (radio) in order to benefit from the economies of scale (low cost), component integration (small size) and low powerconsumption. In particular, the communication antenna on the sensor communication module must be carefully chosen. Its radiation characteristics should be omnidirectional, in order to achieve uniform transmission performance irrespective to the orientation of the sensor with respect to the input module antenna. However, a sophisticated input module and optimised sensor hardware are not sufficient when other requirements such as the high reliability, short message delays and provision for a large number of sensors are to be met. A tailor-made communication protocol was designed and implemented. This protocol provides sensors with collision-free air access by allocating each sensor a specific time/frequency slot for its messages. The parameters of this time division multiple access and frequency duplex multiplexing scheme (TDMA/FDM) are chosen to satisfy the requirements on the number of sensors and the response time, and to make full use of the available radio band (Figure 4). A novel frequency hopping scheme, combined with error detection and automatic message retransmission in case of failure, ensure that the messages from the sensors are reliably delivered, even in the presence of interfering systems such as Bluetooth, WLANs, microwave ovens, electronic tagging systems, and indeed adjacent wireless sensor systems on the same factory floor.

To ensure that power consumption is kept as low as possible, the sensor communication module hibernates until a change in the actual sensor state occurs. When an event takes place at the sensor, the sensor quickly establishes the radio link by use of a pilot signal from the input module, before transmitting the message. Typically this takes 2ms, with worst-case scenarios of 10-15ms if the message must be re-transmitted several times. For system diagnostic purposes, the sensors also transmit an “I’m Alive” message twice per second.

6. Low Power Sensing For the integration of sensors in the wireless sensor system power consumption must be kept as low as possible. This can be achieved by ultra low power electronics and by reducing the duty cycle of the sensing: Instead of continuously measuring, only pulsed operation is done, whereby the sensor is turned on and off rapidly, at a rate which balances the power consumption and the speed of measurement.

7. Conclusions At the Hannover Fair 2002, Germany, ABB StotzKontakt presented the first pre-production samples of wireless proximity switches in a life demonstration (Figure 5). Sales start will be in the second quarter of 2003. With the introduction of wireless proximity sensors, a big leap has been taken towards a future of wireless automation. Fundamental challenges in wireless for automation have been the power distribution and the reliability and delay of the communication. This paper has shown that these problems can be successfully solved. While the system described here is for wireless proximity switches, the technology could be easily extended to other sensors and actuators. The generic power supply and communication technology could find the way into many new applications yet to be imagined. References [1]

[2]

Guntram Scheible: Wireless Energy Automation Systems: Industrial Use ? Sensoren und Messysteme VDE/IEEE Conference, Ludwigsburg, Germany, March 11th – 12th 2002. International Commission on Non-Ionizing Radiation Protection (ICNIRP): Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz), Health Physics Vol. 74, No 4, pp 494-522, 1998.

Figure 3. Communication module: omnidirectional receiver coil for the power supply; integrated antenna (in front, orthogonal) for the radio communication.

Figure 1. Assembly line cell: Wireless proximity switches clustered at the robot grip, primary loops, power supplies in the middle of the lower left and lower front guard, two antennas at the upper right guard, one input module in the cubicle.

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Figure 2. Wireless sensor communication: The information is transmitted by the sensors (S) over wireless links to the relevant input module (IM) and then via a field bus (FB) to the machine controller (PLC).

Figure 4. Frequency Hopping scheme of wireless sensors (dark grey). Also shown are frequency occupation of wireless LANs and Bluetooth.

Figure 5. Wireless proximity switch with two parts: (i) Communication module with foil switch and LED´s and (ii) cylindric sensor head.