Intelligent transceiver for wireless measurement of mechanical stress

Transcription

1 Intelligent transceiver for wireless measurement of mechanical stress Jerzy Ko1ktaj (1) Robert Koll4taj (2) (1) Bialystok Technical University, Chair oftelecommunication and Electronic Equipment 1 1/15 Grunwaldzka Str., PL Bialystok, Poland, tel (2) AMEX Research Corporation Technologies PL Bialystok, 1 Modliñska Str., Poland, tel/fax: ABSTRACT The present paper deals with technologically advanced, programmable, intelligent transceiver for the wireless measurement of the strain, static and dynamic stresses and forces using the strain gauges. The intelligence of the measurement modules enables the long time operation (battery life is Ca. 12 months). The system zero-scale calibration, span and offset limits can be set remotely. Digital filters are used for the optimum frequency response of the measuring signal. The transceiver is also equipped with the input for the latest generation of the Dallas digital temperature sensors. Keywords: Intelligent transceiver, Wireless measurement, Mechanical stress, Strain gauge 1. INTRODUCTION Bidirectional wireless communication between measurement points may be carried out by a transceiver (transmitterreceiver). Transceivers however are usually not equipped with a communication protocol suitable for network operation. They also do not contain measurement circuits, These devices contain specialised circuit of transmitter and receiver operating in UHF or VHF band. Depending on the needs the operation of the device with associated systems depends on particular applications. Transmission protocols must be created by a programmer with significant experience in cooperation with the equipment. Several companies produce separate transmitter and receiver circuits which can be used for bi-directional communication. Despite their sometimes attractive price, designing of transceivers for bidirectional radio communication is too complicated task and not advantageous in practice. Design problems associated with high frequency technology make it more reasonable to cooperate with experienced companies and use their products which have already been verified in practice. With this aim cooperation was commenced with British LPRS company in the area of using BiM transceiver by Radiometrix company for design of own, specialised telemetric measurement modules. Further part of the paper contains description of the transceiver which operates with strain gauge measurement circuit. Due to the fact that the measurements are performed over the radio, the transceiver had to be equipped with a number of functions, which are quite troublesome in traditional stress measurements with use of strain gauge technology (zeroing, scaling, calibration, etc.). In case of multichannel measurements, frequent implementation of these functions (separately for each channel) causes substantial difficulties of metrological and organisational nature. Block circuit diagram of transceiver is presented in Fig DESCRIPTION OF TRANSCEIVER MODULE Optoelectronic and Electronic Sensors IV, Jerzy Fraczek, Editor, Proceedings of SPIE Vol (2001) 2001 SPIE X/01/$

2 Figure 1. Transceiver Block Diagram 2.1. Radiomodem Radiomodem (Fig. 2) consists of high frequency module and transmission controller cooperating with measurement module. High frequency module contains a low power (10 mw) transmitter working in UHF band (433,92 MHz) with frequency modulation and a compatible superheterodyne receiver with double frequency conversion and sensivity of -107 dbm. The transmitter and receiver set provides bidirectional communication [4]. Very important feature of the receiver is very short time of switching from transmit to receive function. Internal switching circuits were designed to provide the receiver ready status within less than 1 ms from switching the power on. This crucial feature (uncommon in other solutions) provides significant saving in power consumption. Battery life time may thus be significanfly extended. For example, average current consumption by high frequency module is 15 jia with active time (reception on) of 1 ms and break time (off-state) of 1 sec. For comparison typical transceivers have the receiver readiness time of ms. Battery supply together with small size of the entire module is impossible with such long readiness time. Another important feature of the high frequency module is its high data transmission rate (up to 40 kbit/s). All the above mentioned properties are important in building of network measurement system. 88 Proc. SPIE Vol. 4516

3 RADIOMODEM CPU DO 4 -p Dl I -P Antenne 4 D2 D3 1xp Transmission Controller RX High Frequency Measurement 1XR Module System 92 MHz TXA a_.; PXR I P.XA RESET p 1XD Figure 2. Radiomodem block diagram Transmission controller (Fig. 4) provides data exchange between the main measurement microcontroller and the high frequency module. Data packet (1-27 bytes) from the measurement microcontroller is being sent to the buffer of transceiver operating in receiver mode. The received packet is decoded and delivered to the measurement module controller. Received block of data from measurement is preceded by data which identify the transceiver operating in transmitter mode (preamble), start byte and control sum. In order to avoid clashes in case of other transmitters operation,,listen before TX" method is used. if the transceiver is not in TX mode its reception circuits are attempting to catch the identification data (preamble). After decoding such data, the transmission controller synchronises itself with subsequent input data bytes, which are then decoded and control sum is checked. In the final phase the complete packet of measurement data is received. The initial byte and the control sum are practically invisible for the user. Additional feature of the transmission controller is programmable,,transceiver sleep" function which allows for reduction of average supply current to the level of 100,tA. When this function is activated, the transceiver enters the reception mode for a period of 5 ms. if within that time no identification packet is received, the transceiver enters the "sleep" mode. On- time is from 1 to 5 ms, while the,,sleep" time is programmable within range from 22 ms to 2,9 s. Proc. SPIE Vol

4 TXD Audio RXD CD RXselect TXselect Figure 3. Block diagram of high frequency circuit Reset RXA RXR TXA PD TXR RX TX D3 D2 Dl DO Figure 4. Block diagram of transmission controller 90 Proc. SPIE Vol. 4516

5 Transmission protocol is fully asynchronous, thus the main measurement controller (or a PC computer) may serve other interrupts resulting from measurement functions and then continue the transmission itself Typical time assumed for transmission of a 27-byte packet of data between the test circuit and the transmission controller is less than 1 ms. Time of radio transmission between two transceivers for a full packet of 27 data bytes together with identification bytes and control sum and the frame is 13,8 ms (40 kb/s +5 ms for identification data). Function of clash prevention (during operation of e.g. other transceivers within a adjacent radio frequency band) may be activate in order to protect the system against loose of relayed data (Listen Before Transmit LBT mode). Built-in internal software may decide itself if extended identification packet will be sent in case of stating that the receiving transceiver is in "sleep" mode. Length of the identification packet can be changed in the transmission controller BEPROM memory. Above described properties are part of the transceiver intelligence, which facilitates communication between different transceivers. In particular these features are welcomed during network operation in different situations, like e.g. autoidentification of various measurement points, seeking the shortest path of data transmission as well as automatic searching for alternative path of information transfer in case of e.g. defect of any of the measurement transceivers (measurement station). Also the necessity of saving the current consumption in order to extend the powering batteries life time, obstacles reducing the range, interference from other transmitters etc. requires the use of intelligent functions, particularly during creation of complex algorithms of data exchange within the data exchange in the entire measurement network. 3. MEASUREMENT MODULE Measurement module is a complete system based upon the latest circuit AD7730L by Analog Devices Fl]. Its block diagram is presented in Fig. 5. The circuit was specially designed for operation with low level signals in strain gauge circuits. The circuit accepts signals from both DC and from AC bridges [2,3]. Serial, digital output of the signal makes it easier to deliver it to the microprocessor system. Figure 5. Block diagram of the strain gauge measurement circuit Proc. SPIE Vol

6 With the strain gauge output voltage from the bridge of + 10 mv, it has noise voltage of 40 nv rms (240 nv p-p), the amplification precision 1 jiv, voltage drift below 1 'iv, temperature drift of 0, 5 jiv approx. With ambient temperatures ranging from -20 C to +85 C there is a possibility of remote, automatic calibration of strain gauge bridge, correction and background bias. Parameters of the procedures are stored in the measurement module memory. Built-in memory gives a possibility of performing dynamic measurements. Built in digital filters enable shaping of the recorded signal. Reading of results is performed by means of a notebook or palmtop computer working with the radio receiver station. The transceiver is equipped with standard input for the latest generation of Dallas miniature digital temperature sensors. Due to difficult environmental conditions of operation (wide range of temperature changes, including below-freezing temperatures, high humidity, dust etc.) the transceiver is mounted in tight metal housing of 1P67 class. Electronic circuitry is assembled in SMD technology. 4. CONCLUSIONS 1. Built protolype of intelligent measurement transceiver, after its initial testing (with bidirectional communication), allows for measurements of mechanical stress using strain gauge technology. Inherent,,intelligence" features provides more credible results, which are particularly required in field conditions. 2. Multipoint mechanical stress measurements will be performed in the further stage, after testing of newly developed telemetric measurement network (being subject of a separate project). 3. Transceiver prototype allows for long term, uninterrupted operation over 9 to 12 months. Further works will be performed to extend the batteries operation time to 24 months. 4. After performing of tests on selected real objects (bridges, silos), production of telemetric measurement systems is forecasted at AMEX Research Corporation Technologies, basing on the built prototype device. 5. ACKNOWLEDGEMENTS Realization of the present paper was possible thanks to financing provided by The Scientific Research Committee (Komitet Badañ Naukowych) for implementation of research work No. S/WE/2/OO. 6. REFERENCES 1. AD7730L Data Sheet. Analog Devices, 2. Kester W., Bryant J., Buxton J.: ADC 'sfor signal conditioning. Practical Design Techniques for Sensor Signal Conditioning. Analog Devices Inc., Kester W.: Bridge Circuits. Practical Design Techniques for Sensor Signal Conditioning. Analog Devices Inc., Low Power UHF Data Transceiver Module. Radiometrix Ltd, BiM-TJIHIF Data Sheet, Issue 3, September Proc. SPIE Vol. 4516