UK sensors keep tunneling machines on target

A dual-axis inclinometer from Sherborne Sensors has enabled Zed to take its tunnel-boring targeting systems to the next level

Tunnelling is an exacting business. With few reference points deep beneath the ground, ensuring a tunnel-boring machine (TBM) stays on course is a major engineering challenge, and even the tiniest deviation from the planned route can cause major problems.

In the early days of tunnelling, when engineers relied on little more than a pen and paper, successful projects were hailed as near-miraculous. But techniques have advanced considerably in recent years and thanks to the advent of advanced guidance systems, today’s tunnellers can be reasonably confident that their plans will be accurately executed. But the technology is continuing to evolve.

Recently, one of the leading developers of TBM guidance systems, Zed Tunnel Guidance, announced that it had taken the technology a step further by incorporating innovative inclinometers developed by UK company Sherborne Sensors.

Based in London, Zed has developed the guidance systems for the equipment used in many of the world’s most challenging tunnelling projects – from construction of the Channel Tunnel in Europe to China’s Yellow River Diversion Project.

Initial versions of the system incorporated Zed’s own inclinometer design that was combined with an associated electronics package to become a complete transducer, converting physical parameters such as pitch and roll into electronic measurements.

However, driven by a desire to reduce the size of these targeting units, as well as the time and cost associated with calibrating each inclinometer to a printed circuit board (PCB) Zed looked elsewhere for a solution.

’With the design that we had, it was not easy calibrating the inclinometer correctly in conjunction with the electronics PCB,’ recalled Mick Lowe, senior project engineer at Zed. ’The cost in terms of labour 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.’

A sealed casing allows the T233 to operate in hostile environments

In its simplest configuration, a Zed guidance system uses a combined target unit that’s mounted 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.

To establish the position of the TBM, the PDU requires information from the target unit, the design tunnel alignment (DTA) table and the user. The DTA table plots the course that 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.

The DTA table plots the course theTBM must follow and can include up to 20,000 individual reference points

A standard tunnelling laser is then affixed to the tunnel wall, providing a reference typically 50-100m to the rear of the TBM and projecting a beam travelling forwards to hit the screen of the target unit. This measures any displacement of the laser beam from the target centre, including vertical and horizontal displacement, as well as pitch, roll and yaw.

With the space at the front of the TBM at a premium, the target unit must be as small as possible. Zed’s 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.

Having evaluated a number of inclinometer products from various manufacturers, Zed specified Sherborne’s T233. This is a DC, closed-loop, force-balance tilt sensor said to have accuracy, stability and reliability several orders of magnitude greater than open-loop types. Its flexure-supported torque-balance system and fluid damping ensures that the T233 is rugged enough to withstand severe shock and vibration while 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 that permits 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,’ said 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, the sensors are regularly providing more than the average two years of functional life. He also claimed that the savings made relating to calibration have been substantial. ’The savings we have realised are significant – approximately 50 per cent – when factoring in the labour costs of the calibration we had to undertake previously,’ he said.