The Lord Mayor’s coach has been used in every Lord Mayor's parade since it was built in 1757, but historians had raised concerns that its continued use could threaten its structural integrity and that it might be in danger of serious damage. They were concerned that, without a better understanding of its behaviour the coach may have to be confined to a museum.
In a bid to discover just what forces and stresses the coach was being subjected to on its solitary annual outing, GB Geotechnics was contracted in 2006 to apply instrumentation and conduct a thorough analysis of the vehicle's performance. The object was to see if the machine was capable of continued use, or if it should be retired from active service.
GBG Structural Services, a division of GB Geotechnics, set about this unusual task by seeking the most reliable and portable data acquisition it could find. The coach would only be made available to the company's engineers for a limited time and, because of fears over its fragility, there would be no road testing except for the actual 2006 Lord Mayor's Parade. Stephen Kemp, manager of GBG Structural Services, explains the rationale behind the choice of USB data acquisition devices from Measurement Computing Corporation, supplied in the UK by Adept Scientific:
"We had a large number of instruments to connect and would only get one chance to get the data. We have used MCC devices for many years on any number of different projects, from pile testing to seismic tests, and we've always been more than satisfied with their performance. This was the natural choice for us."
GBG already owned a number of eight-channel USB-1608 data acquisition interfaces, but most of these were in use elsewhere. However, the instrumentation load on the Lord Mayor's Coach meant that at least three 1608s would be needed. The alternative was to use the similar, but newer USB-1616FS, offering 16 analogue input channels and USB output.
Instrumentation included a series of velocity transducers under and around the body of the cabin and a number of accelerometers mounted on the axles. The leather straps, which provide the rudimentary suspension, were fitted with extension sensors to monitor their elastic response, while critical parts of the coach structure were fitted with strain gauges. Altogether, 19 separate instruments were wired up via two data acquisition units (one 1608 and one 1616).
The advantages of the MCC USB units, according to Stephen Kemp, are their reliability, portability and flexibility. Their relatively small size proved a boon on a project of this type where their unobtrusiveness was more significant than their low space requirement. Data were collected via a standard laptop computer running GBG's in-house developed data acquisition software.
In the event, the data acquisition went without a hitch. Analysis of the data showed that the top part of the cabin moved significantly more than the lower part (perhaps not surprising, given the primitive suspension system employed), but most significantly there was little difference in performance over differing road surfaces. "The biggest movements we detected were when the horses stopped and started," observes Stephen Kemp.
As a result of this engineering analysis the Lord Mayor's Coach was able to take part in the 2007 Lord Mayor's Show as it had on every such event in the last 250 years.