Wireless Vibration Exploration

Transcription

1 Wireless Vibration Exploration By Jean Louis Rouvet M.C.E. Commercialise Mécaptélec ABSTRACT : For in-situ experiment, what kind of wireless transmission may be used successfully to transmit wireless wise vibration measurements : the approach of analogical double frequency modulation, the use of Bluetooth or WiFi means and potential operations with GSM/GPRS should be considered. Following discussion explain benefit for vibration expert in using new communication technologies with respect to vibration analyses. 1. INTRODUCTION The basic experience and expertise in different fields of application from military to industrial through field and structures testing pushed M.C.E. to extend its capabilities and enter developments of new products and services. A strong back ground in vibration sensors based on collaboration with synthetics or natural quartz sensors manufacturers, collaboration also with piezoelectric plastics sensors manufacturers for dynamic motion sensors was of a big help. Beside this a knowledge and collaborations with piezo-resistive sensors manufacturers ( strain gages ), years of operation with force balances devices and the recent involvement with MEM s covers largest part of vibration technologies bases. A twenty years of telemetry designing, manufacturing and implementing, added to the competences for developing a tool dedicated to wireless vibration measurement systems applied to civil engineering structures. 1.1 The concept Physical world is the behaviour of the structure and first part of the job is to make the relationship in between this behaviour and exploitable measurement. The target was to create a box able to pick up

2 the measurement and to supply it to the user. As more control systems become computerized, there is an increase need to provide a digitalized signal. As soon as signal has been digitalized the user, typically expect to use it on any processing device as P.C. s or P.D.A. s. As cables routing, implementation, and so on, generates lots of problems, the radio transmission has been requested. From there, M.C.E. s engineers thought of all requests from time consuming installation to expensive processing. It became obvious for engineers the solution had to be a non contact, fully digital, autonomous, ruggerized and easy to use unit integrating all indispensable stages to condition and transmit and receive a digital data corresponding to vibration measurement. On an other hand, the field unit should be compatible with most of the available processing hosts and therefore compatible with Windows compatibles Short specification From discussions and meetings, one can write a short specification. Radio transmission, fully digital, ruggerized, autonomous and compatible with most of the available P.C. s and others was the first part of it. Very soon, words like stand alone, battery powered, accurate and low cost came up. Few new demands had to be considered as the transmission range or global bandwidth for the measurement. As possibilities sounded available, the demand increased with cascade mode possibilities, simultaneous synchronized measurements Internet connections and others. A short specification was established : - Single or multiple channels - Accelerometer input from DC to 2000 KHz bandwidth - Non contact transmission - Allowed almost everywhere without specific permission - Range from 100 to 300 meters - Low power consumption - Possibility to create networks or cascades - Compatible with computer technologies - Compatible with internet and Ethernet - Low cost - Very flexible 2. BASIC ANSWER FM/FM technologies background tells this analogical solution, efficient with rotating machines or short ranges telemetries, can handle up to 20 KHz bandwidth but is subjected, for distances to restrictions with only a maximum of 10 mw power at the transmitter inducing licenses problems over and therefore limiting distances below 50 meters. Beside this, this technology, fully analogical will not address the requested specification. Digital technologies used frequently in telemetry as PCM or PAM encoding would face a similar problem concerning transmitting frequencies and, beside this, will never be cost effective to match the specification. The answer was obviously into new technologies as Bluetooth, WiFi or GSM/GPRS. These 3 technologies are low cost because dedicated to large production products ranges. Bluetooth and WiFi being free in terms of fees but having limitation in distances to be covered where GSM/GPRS covering potentially long distances is subjected to subscription rate.

3 Bluetooth and WiFi can cover 200 meters in free view for the first one and 300 meters for second one. Chipsets exists and are low cost, power consumption is far over what is usually encountered with FM/FM telemetries but most of the specification can be fulfilled. GSM/GPRS has a subscription cost but depending upon coverage can operate long distances, bandwidth is reduced but some features embedded are interesting. If all 3 technologies are very similar, a choice had to be done in order to start developing. M.C.E. made the choice of Bluetooth technology to start with because of her early involvement as a partner of Bluetooth team in early Technology and architecture The story remain almost the same for the 3 technologies, for both Bluetooth and WiFi the transmitting frequency is from 2.4 to 2.45 GHz ( DECT technology very similar is from 1.88 to 1.99 GHz ). GSM/GPRS is located in 900 MHz ( Class 4 ) and 1800 MHz ( Class 1 ). All of them are approved. UART interface is included in the 3 technologies offering serial high speed interface. Transmission speed is 732 Kbps for Bluetooth, 11 Mbps for WiFi and 115 Kbps for GSM/GPRS. In both, Bluetooth and WiFi, embedded protocol management is connected to either Bluetooth or WiFi controller for sending, one with GFSK modulation method, the other one with CCK, DQPSK, DBPSK. The choice is then depending upon the application but remain quite the same for surrounding functions and electronics. Then the heart of the architecture of a system is the Bluetooth or WiFi or GM/GPRS layer. An analog to digital converter is used at the output of a conditioner. Conditioner is dedicated to the sensor and experience with industrial FM/FM analogical telemetry simplifies this part of processing as most of the conditioners are already existing in rugged miniaturized form. Then, the puzzle had to be assembled and tested. This was done and, experiments shows the system is operating in urban or industrial environment with obstacles in between both transmitting and receiving antenna up to 120 meters in most of cases, up to 20 meters in building with reinforced concrete walls. The reached bandwidth was causing problems when using PCMCIA or serial port receiver but it was improved with the use of USB receivers. Today a DC to 500 Hz bandwidth is feasible for a single channel unit but most of the applications requests only 300 Hz bandwidth. Tests with 1 KHz bandwidth were performed demonstrating both feasibility and problems. Future Bluetooth generation will be available with instead of 732 KBps a 3.3 MBps opening the door to enlarge bandwidth and extend applications. 2.2 Field operable unit As the electronic architecture operates properly, the next step needs to integrate the components in an easy to use housing. Let s consider measuring point first. Whatever the transmitting technology is, the complete electronic package remain small and light weight. For field applications, electronics are

4 encapsulated into a metal housing with appropriate input connectors for sensors and either integrated antenna or connector for an external antenna. The full package is IP 67. Increasing housing size, batteries can fit in or a remote power supply may be used. For some applications in industries like gas or chemicals, an explosion proof housing exists and will, upon user request be qualified. The box weight 1,2 Kg is included in a volume of 138 x 257 x 150 mm with two threaded holes at the bottom and the complete unit can be certified Eexd IIB T4/T6/IP 65-66/ATEX ex II 2 G. Moreover some other applications needs withstanding to shocks and vibrations and experience in potting was used to maintain all components in metal housing for either regular unit or explosion proof device. Vibrations up to 2000 g s and shocks up to 5000 g s hav e no effect on the electronics but batteries must be selected properly and in such cases Lithium batteries will be used. 2.3 Receiving units Receiver gives a compatibility in between industrial Bluetooth or WiFi and computer technology. The simplest receiver consist of a PCMCIA card. But, as explained earlier this kind of receiver is not the best to use with the technologies, therefore the USB key, preferable, has to be selected. There, again, depending upon the application, both types can be used as of the shelves units or some specific housing can be manufactured. One has to understand in case of mobile receiver and processing unit there is no need for ruggerized receiver. 3. FULL SYSTEM DEDICATED TO THE APPLICATION Accelerometers, single and multi-axes are available as encapsulated units with connectors, mounting brackets and so on, but they are also available as chips to be hand soldered on printed circuit boards. Again, depending upon applications, piezo-electrics or MEM s have to be selected. Th e picture shows a single or 3 axes accelerometer based on piezo-electric technology and supplying a fairly interesting voltage for very low power consumption ( piezo-electric being auto generator ). Another possibility is to use capacitive MEM s but there, power consumption is much higher when, on another hand the response comes from Dc up to 500 Hz. Either the sensor may be integrated into the transmitter housing either it can be placed on any location and wire connected to the transmitter. Sensor can be used to measure direct vibration in any axes or can be used in g projection to measure the angle via the acceleration applied on any horizontal axes. Even if more than 90 angular degrees

5 have to be sensed, M.C.E. has a 360 angular degree solution for applications like mobile arm of shovels. To save energy, transmitter may be put on sleep mode where conditioner and sensor will remain operating. In such case, a value over a setted threshold will back up transmitter in operating mode for transmission and data acquisition. It is even possible to use an extra sensor, very low cost and with no power consumption to wake up the transmitter at a shock or vibration amplitude value. The transmitter can integrate thresholds, small data acquisition to prevent losses of measurements due to whatever event. The usual use of Bluetooth WiFi is a point to point link from measurement location to accessible data acquisition or processing place. Received Bluetooth or WiFi signal will feed a PC or a Palm Pilote for data acquisition and processing. USB Key fits in both which may receive signal simultaneously ( only WiFi ) if and only if the proper USB receiver with appropriate code is connected onto it. Distance remain in range of 130 meters in between both locations. To extend the distance a cascade possibility exist. There is a layer allowing a Bluetooth ( specific additional electronics ) and WiFi ( integrated ) transmitter to be relayed by a second transmitter with a 120 meters distance, this can be mounted as a chain of n transmitter to extend distances of n time 120 meters. In counter part, the bandwidth of each transmitter will be reduced and become the full bandwidth available divided by number of transmitters.

6 The most common architecture remain point to point, eventually through a node receiving simultaneously up to 5 or 7 transmitters. The node may be an host PC type or a stand alone unit able to transmit its signal to an end of line host using Bluetooth, Wifi or any other technology including Internet. It remains the basic link includes a sensor the Bluetooth or WiFi transmitters and the Key receiver connected to a computer. Other arrangements will just make possible several field applications. 4. APPLICATIONS Consider the need to measure vibrations on a bridge pole in order to modelize with generated low vibrations what could be the impact of an earth quake on the structure or since we have to imagine, consider the dynamic modelisation of vehicle flow on same bridge before opening it to the public. In both cases, vibrations are to be measured in several places and there are few chances the distance from first point to data acquisition system will remain below 120 meters. Over listed solution listed in precedent paragraph may be used simultaneously. Vibration sensors located on the pole or on bridge slings will be, depending upon on bandwidth requested be transmitted either in Bluetooth or WiFi technology. There are no reason for shorter distances than 120 meters, the cascade mode will then be chosen with respect to global bandwidth for routing measurements from top of the pole or middle of the sling attached to central pole down to the computer. Computer? As there is no reason why each pole or each sling could not be considered as independent cells, the use of splitted centralisations will consequently suppose the use of nodes to collect all data and centralize on a master acquisition and processing spot.

7 This is the basic architecture and some engineers will need to know in real time measurements while inspecting the structure, therefore the catch of measurement without interfering with main acquisition will have to be performed with PDA. In mean time the main contractor wants to get measurements in order to confirm, verify, control or analyse the behaviour. All nodes collected to main in-situ data acquisition will be easily connected via telephone, either GSM or fixed line to main facility located in Paris when the bridge is located in Toulouse. Use of Bluetooth and WiFi technology takes there its full explanation. Another interesting thing would be if Laboratoire Central des Ponts et Chaussées worries about an impact of the construction on the cliff surrounding the site. Then, implementation of vibration sensors will help alarming just in case of problems. An accelerometer with a threshold can start a GSM/GPRS transmission any time just in case. As L.C.P.C. s engineers expect to get few data punctually they will make possible a remote wake up of GSM/GPRS and download of measurements As one can tell all these possibilities are covered with the 3 technologies associated to existing networks. 5. SYNCHRONISATION Most of the answers to the initial request are listed in the over written discussion but two important points are missing. One is synchronized measurements and the other one is measurement localisation. To obtain synchronized measurements, the best solution would be to send a simultaneous signal at a t 0 time on all of the transmitters. Such a trick suppose to wire physically all the transmitters and kills the benefit of wireless transmission. The other option is to use an accurate clock connected to each transmitter. One knows GPS is providing accurate timing. Then, the digital signal out of the GPS will be mixed with measurement in digital format and forwarded to UART serial input port. Then as real time is received with measurements there is a possibility to get several different measurement without de-phasing. 6. VOICE CHANNEL AND VIDEO Bluetooth and WiFi have an embedded analogical bi-directional channel for vocal exchanges. This can be used by operators, for instance when installing sensors or during periodical survey. There is also a possibility for video transmission through compression via Jpeg 2000 and transmission with Bluetooth or Wfi. 7. CONCLUSIONS Bluetooth and WiFi can be used easily for vibration explorations on civil engineering structures up to 120 meters. In case of bandwidth request WiFi may be better but remain a little less secured ( for transmission ) than Bluetooth. Bluetooth is used as slave when host will me master when WiFi can be as well master and slave offering more than pico-net.

8 Both solutions are cost effective and reliable compared to usual analogical telemetries. GSM/GPRS will offer distance coverage but will limit bandwidth and will have a subscription cost. On another hand, GSM/GPRS can get embedded GPS option allowing positioning of the stand alone measurement point. The choice is depending upon application and all these solutions will last in time as the technologies are used in growing markets. Bluetooth has been implemented as standard in Windows stacks, in PDA s, mobile phones as WiFi is used to create networks in a lot of industries and is compatible with Windows. GSM/GPRS is already used for alarms, meteorological applications and others M.C.E. has been concentrating on Bluetooth and exploring WiFi possibilities not polarizing on GSM/GPRS easier ton handle by developers and therefore more common. Quite a lot of tests were performed, particularly for bridges applications. M.C.E. has been performing tests and demonstrations in urban locations, inside buildings from floor to floor and across floors of towers to validate the use of Bluetooth and WiFi in worst conditions. As well, several campaigns offered the opportunity to validate transmissions with dense metal surrounding conditions as with gratings../ April For Bordeaux October Conference