Picture 1

 Picture 2

 Picture 3

Bridge project at NPL

Techni Measure, along with several other companies, have been involved in an interesting project at the National Physics Laboratory site in Teddington, where a disused footbridge has been subjected to all kinds of tests, in order to discover modes of failure and enable testing of various sensor systems in the field.

With MicroStrain now having the ability to offer Cloud Computing for a range of their products, using their WSDA Data Aggregator as the link, it was agreed that we should try to set up a wireless modem link to the Cloud, to reflect what might be necessary out in the real world of large structure monitoring.

The aim with this system was to allow MicroStrain, Techni Measure and NPL to monitor on-line what the sensors are doing, via the Cloud, from wherever they were. The sensors in this case were two wireless 2g range triaxial accelerometers (G-Link) in waterproof boxes, which were mounted on the bridge surface at the centre point and at the far end (Pictures 1 & 2).

There was also a multi-channel wireless strain system (V-Link), that has three channels set up to monitor the strain from three strain gauges mounted near the end of two 4m long carbon fibre reinforced polymer (CFRP) panels installed on the top deck. (Picture 3). This was also mounted in a waterproof box. All three boxes contained enough battery power to allow the sensors to monitor and transmit data on a predetermined schedule, for at least 2-3 months.

The three nodes were set up to operate in a synchronised mode taking data at 64Hz for 70 seconds every 5 minutes, and transmitting this wirelessly to a base station (WSDA) that acts as a computerised link and control module, and situated in a nearby monitoring building. This in turn was connected to a wireless modem operating with a SIM card, allowing access to the Internet.

Although MicroStrain has set up such systems successfully in the USA, it was the first time this has been tried in this way overseas, and there were several initial problems in getting all the system to operate properly. Consequently the start of monitoring was delayed until these problems were sorted out, and the vibration testing of the bridge was missed, however eventually live data has been streamed successfully to the Cloud for over two months.

There has not been much change in the accelerometer signals, but there is a lot of variation in strain output during each 24 hour period, which is probably largely related to temperature variations in the composite reinforcing panels. There are also some positive strain readings (tension) that show excursions of about 140 microstrain on the two days of testing, when large weights were suspended from the end of the bridge (picture 4). Each tank weighed about 1200 kg when full. Please ask for further details of this project.

August 2012

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