Advanced Guidance Systems and Inclinometers

Zed Tunnel Guidance specializes in the manufacture and development of advanced guidance systems for Tunnel Boring Machines (TBMs). The company combines cutting-edge technologies with a broad base of electronic, software and opto-mechanical engineering principles.

Its core strength is the ability to create modular, highly reliable, and innovative guidance systems for application in the toughest of industrial environments underground.

Tunnel guidance systems from Zed have an expected operational life of more than five years if properly maintained and serviced between projects and rely on the engineered aspects of robustness, precision and reliability. Sherborne Sensors’ 2-axis servo inclinometers are employed to allow TBM operators to accurately identify the roll and pitch of the TBM relative to gravity, and ensure that the specified designed tunnel alignment (DTA) is delivered.


  • Consolidate hardware, simplify operation, and meet modular configuration requirements
  • Outsource manufacture of inclinometers for combined target units
  • Reduce time and cost of calibrating inclinometers and PCBs


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


  • Single target unit enables supply of simpler, smaller, and more cost-effective tunnel guidance
  • Dual axis unit delivering high precision angular results in a rugged working environment
  • Cost of installation of inclinometers lowered by 50 percent

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

Guidance systems from Zed serve civil construction firms and TBM manufacturers and have been employed in high-profile civil developments, including China’s Yellow River Diversion Project and the construction of the Channel Tunnel in Europe. Originally, Zed manufactured its own inclinometer design that is combined with an associated electronics package to become a complete transducer, converting physical parameters such as roll and pitch into electronic measurements.

Zed shipped up to 80 units per year, in a combination of complete systems and supplying to 3rd parties standalone target units and had a relatively low run-rate for manufacture of its inclinometers. It also wanted to reduce the size of its target units and additionally reduce the time and cost associated with the calibration of each inclinometer to a printed circuit board (PCB).

Calibration is achieved via analogs to digital converter and precision analog circuitry.

With the design that we had, it was not easy calibrating the inclinometer correctly in conjunction with the electronics PCB.

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.

Mick Lowe, Senior Project Engineer, Zed Tunnel Guidance.

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 includes a processor display unit (PDU), both gravitational (servo inclinometer) and optical (laser) sensors, a small set of tools and test equipment, and a junction box controlling the exchange of data between the target unit and the PDU.

To establish the current position of the TBM, the PDU needs information from the target unit, the DTA (design tunnel alignment) table, and the user. The DTA can include up to 20,000 reference points and plots the course the TBM must follow. Measurements are entered manually into the PDU by the user during installation of the guidance system, and this informs it of the position of the target unit relative to the axis of the TBM.

Next, a standard tunneling laser is affixed to the tunnel wall which provides a reference (datum), typically 50-100 meters to the rear of the TBM, and also projecting a beam traveling forwards to hit the target unit’s screen.

The target unit is mounted on the TBM and measures any displacement of the laser beam from the target center, including both horizontal and vertical displacement, and also roll (clockwise/anticlockwise), yaw (heading), and pitch (up/down).

Given that the TBM 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.

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.

Mick Lowe, Senior Project Engineer, Zed Tunnel Guidance.

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

Honing the Guidance System

Zed Tunnel Guidance evaluated a number of inclinometer products from various manufacturers and specified the T233 from Sherborne Sensors. The T233 is a dc, closed loop, force balance tilt sensor with reliability, stability and accuracy several orders of magnitude greater than open loop types (e.g. where system variations are not detected or corrected). Its fluid dampening and flexure supported torque-balance system ensure that the T233 is rugged enough to withstand severe vibration and shock and still maintain its high level of accuracy.

Additionally, the dual-axis sensor with each axis precisely aligned orthogonally and electronics, are encased within a compact sealed housing. This permits operation in hostile environments and enables 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.

This is made things easier for us as it saves 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.

Mick Lowe, Senior Project Engineer, Zed Tunnel Guidance.

Sherborne Sensors’ T233 inclinometers are regularly providing more than the average two-years of functional life despite the hostile environments, according to Lowe. The savings made relating to calibration have been substantial he confirms. “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 the labour costs of the calibration we had to undertake previously,” he explains. He also explains that the modularity of Zed Tunnel Guidance systems has been improved because 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.

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 our approach.

Mick Lowe, Senior Project Engineer, Zed Tunnel Guidance.

This information has been sourced, reviewed and adapted from materials provided by Sherborne Sensors.

For more information on this source, please visit Sherborne Sensors.

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