Sherborne Sensors Keeps ZED Tunnel Guidance On Target

April 2011 Vol. 66 No. 4

A standard tunneling laser is then affixed to the tunnel wall, providing a reference (datum) typically 50-100 meters (165-330 feet) to the rear of the TBM, and projecting a beam traveling forwards to hit the screen of the target unit. The latter is mounted on the TBM and measures any displacement of the laser beam from the target centre, including vertical and horizontal displacement, as well as pitch (up/down), roll (clockwise/anti-clockwise) and yaw (heading).

“Given that the TDM is effectively a cylinder with cutters at the front, one must have the target in the back of the cylinder and facing backwards to receive the laser beam in order to establish the TBM’s position,” says Lowe. “Most TBMs incorporate some kind of ‘3D laser window’ within the tunneling shield and the backup gear to allow the laser beam to project onto the target unit from further back down the tunnel. By measuring where the laser beam hits the target unit, it is then possible to calculate where the front of the machine is.”

With space at the front of the TBM at a premium, the target unit must be as small as possible. Zed Tunnel Guidance systems originally employed two separate transducer units to create the target – an optical sensor, and a gravitational sensor, which were relatively bulky and required additional cabling. Furthermore, with modularity viewed as an inherent design benefit, and system configuration dependent on a number of variables including complexity of the DTA, and the costing restrictions associated with a project, ‘plug-and-play’ operation was an essential requirement – especially if any of the transducers need replacing.

Honing the guidance system
Having evaluated a number of inclinometer products from various manufacturers, Zed Tunnel Guidance specified the T233 from Sherborne Sensors. The T233 is a dc, closed loop, force balance tilt sensor with accuracy, stability and reliability several orders of magnitude greater than open loop types (i.e. where system variations are not detected or corrected). Its flexure supported torque-balance system and fluid damping ensures that the T233 is rugged enough to withstand severe shock and vibration whilst maintaining its high level of accuracy. In addition, the electronics and dual-axis sensor with each axis precisely aligned orthogonally are encased within a compact sealed housing, permitting operation in hostile environments, and enabling measurement of angular tilt in reference to gravity.

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