Sherborne Sensors case study: ZED Tunnel Guidance




Zed Tunnel Guidance specialises in development and manufacture of advanced guidance systems for Tunnel Boring Machines (TBMs). Combining leading-edge technologies with a broad base of opto-mechanical, electronic and software engineering principles, the company’s core strength is its ability to create innovative, modular, and highly reliable guidance systems for application in the toughest of industrial environments underground.

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. They employ Sherborne Sensors’ 2-axis servo inclinometers to 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.


·         Outsource manufacture of inclinometers for combined target units

·         Reduce time and cost of calibrating inclinometers and PCBs

·         Consolidate hardware, simplify operation, and meet modular configuration requirements


Sherborne Sensors’ T233 DC-operated 2-axis, fluid-filled, gravity-referenced servo inclinometer


·         Dual axis unit delivering high precision angular results in a rugged working environment

·         Cost of installation of inclinometers lowered by 50 percent

·         Single target unit enables supply of smaller, simpler, and more cost-effective tunnel guidance systems

Body copy

The ZED tunnel guidance system is used to accurately monitor the position of a tunnel-boring machine (TBM) when driving (digging) a tunnel. By continuously confirming the position of the TBM, the guidance system provides users with details of any deviation from the plotted course, so that corrections can quickly be made. This ensures a highly-accurate tunnel is driven, with increased speed and at lower cost.

Serving civil construction firms and TBM manufacturers, Zed’s guidance systems have been employed in high-profile civil developments, including construction of the Channel Tunnel in Europe, and China’s Yellow River Diversion Project. It originally manufactured its own inclinometer design that is combined with an associated electronics package to become a complete transducer, converting physical parameters such as pitch and roll into electronic measurements. Shipping up to 80 units per year, in a combination of complete systems and supplying to 3rd parties standalone target units, Zed had a relatively low run-rate for manufacture of its inclinometers, and also wanted to reduce the size of its target units, as well as the time and cost associated with calibrating each inclinometer to a printed circuit board (PCB).

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 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.”

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 tunnelling laser is then affixed to the tunnel wall, providing a reference (datum) typically 50-100 metres to the rear of the TBM, and projecting a beam travelling 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 tunnelling 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 should 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 realised are significant – approximately 50 percent – when factoring in the labour 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.

“We had looked at several inclinometer products but none were as technically sound, or as accurate, as Sherborne Sensors’ T233,” he adds. “Although we are not doing anything we couldn’t do before, we are now doing it in a much more efficient manner and, because we have been able to realise a smaller and better networked system, it is more cost effective. Our end users are also realising the benefits of this approach.”

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