ZED Tunnel Guidance, located in London, U.K., specializes in development and manufacture of advanced guidance systems for Tunnel Boring Machines (TBMs).
With an expected operational life of more than five years, if properly maintained and serviced between projects, Zed’s tunnel guidance systems rely on the engineered aspects of precision, reliability and robustness. Sherborne Sensors’ 2-axis servo inclinometers allow TBM operators to accurately identify the pitch and roll of the TBM relative to gravity, and ensure that the specified designed tunnel alignment (DTA) is delivered. This ensures a highly accurate tunnel is driven, with increased speed and at lower cost.
Calibration is achieved via precision analogue circuitry and analogue to digital converter.
“With the design that we had, it was not easy calibrating the inclinometer correctly in conjunction with the electronics PCB,” recalls Mick Lowe, senior project engineer at Zed Tunnel Guidance. “The cost in terms of labor was high, so we wanted an inclinometer with a narrower range of scale factors that would allow us to calibrate all of our PCBs the same way, and enable any PCB to work with any inclinometer, rather than having to pair each individually for each target unit produced.”
Space at a premium
In its simplest configuration, Zed Tunnel Guidance systems employ a combined target unit that is mounted at a convenient location on the TBM and incorporates both optical (laser) and gravitational sensors (servo inclinometer); a processor display unit (PDU) functioning as the main display and computer for the TBM operator; a junction box controlling the exchange of data between the target unit and the PDU; and a small set of tools and test equipment. In order to establish the present position of the TBM, the PDU requires information from the target unit, the DTA (design tunnel alignment) table and the user. The DTA table plots the course the TBM must follow, and can include up to 20,000 reference points. During installation of the guidance system, measurements are entered manually into the PDU by the user, informing it of the position of the target unit relative to the axis of the TBM.
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.
“One of the deciding factors in our selection of Sherborne Sensors’ T233 was that by having two inclinometers housed in a single casing, we were able to locate both the inclinometer and the optical sensor within a single target device and avoid having a separate casing for each,” continues Lowe. “This made things easier for us as it saves on a lot of cabling, and reduces the ‘box count’ of the system, which in turn makes it more cost effective and less complicated to build. And if there is less cabling employed in the configuration of the system, then inherently there is more reliability.”
According to Lowe, despite the hostile environments in which they are employed, Sherborne Sensors’ T233 inclinometers are regularly providing more than the average two-years of functional life. He also confirms that the savings made relating to calibration have been substantial. “Although the cost of purchasing the inclinometers is similar to that of manufacturing our own, the savings we have realized are significant – approximately 50 percent – when factoring in the labor costs of the calibration we had to undertake previously,” he says. Lowe also highlights that the modularity of Zed Tunnel Guidance systems has been enhanced, since the transducer unit can be swapped-out within 30 minutes.
For More Information:
Sherborne Sensors: (877) 486-1766, www.sherbornesensors.com
ZED Tunnel Guidance: +44 (0) 1932 251440, www.zed-tg.co.uk