Designing ways to dig through miles of rock formations to create tunnels for transportation and other needs is one of the tallest orders faced by civil engineers. Construction crews must use extremely rugged equipment such as tunnel-boring machines (TBMs). Specialized guidance systems that employ advanced sensors for directional feedback keep TBMs on their specified course as they bore through layers of rock.

One company with expertise in the manufacture of advanced guidance systems for TBMs is Zed Tunnel Guidance of Surrey, U.K. The company's state-of-the-art systems employ dual-axis servo inclinometers from Sherborne Sensors Inc., Wyckoff, N.J., to help operators accurately identify a TBM's pitch and roll relative to gravity — and follow the specified designed tunnel alignment.

Tunnel guidance in action

The ZED tunnel guidance system is used to accurately monitor the position of a tunnel-boring machine when digging or driving a tunnel. By continuously confirming machine position, the guidance system provides operators with details of any deviation from the plotted course, so that corrections can quickly be made. This ensures that a highly accurate tunnel is driven, saving both time and costs compared with less sophisticated guiding methods.

Zed's guidance systems have been used by civil construction firms and TBM manufacturers in many high-profile civil engineering ventures, including construction of the Channel Tunnel in Europe and China's Yellow River Diversion Project. Zed originally manufactured its own inclinometer design, which is combined with an electronics package to make a complete transducer that converts physical parameters such as pitch and roll into electronic measurements. However, the company was selling just 80 units per year, either as part of complete guidance systems or as standalone units. Zed wanted to reduce the size of its units, as well as the time and cost associated with calibrating each inclinometer to a printed circuit board (PCB).

“Correctly calibrating our inclinometer with the electronics PCB wasn't easy,” explains Mick Lowe, Zed's senior project engineer. “The labor cost was high, so we wanted an inclinometer with a narrower range of scale factors. This allows us to calibrate all PCBs the same way, and enables any PCB to work with any inclinometer, rather than having to pair each one individually.”

Space at a premium

With space at the front of the TBM at a premium, the target unit must be as small as possible. Zed systems originally employed two separate transducer units to create the target — an optical sensor and a gravitational sensor — but these were bulky and required additional cabling.

In its simplest configuration, the new Zed Tunnel Guidance system employs a unit mounted on the TBM that incorporates both optical and gravitational sensors, a processor display unit for the operator, a junction box controlling data exchange between the target unit and display, plus a small set of tools and test equipment. To establish the present position of the TBM, the display requires information from the target unit, tunnel-alignment table, and user. The alignment table plots the course the TBM must follow and can include up to 20,000 reference points. During guidance system installation, measurements are entered manually into the operator's display pad, informing it of the target unit's position relative to the TBM's axis.

A laser is then affixed to the tunnel wall, providing a reference typically 50 to 100 m to the TBM's rear and projecting a forward beam to hit the target unit's screen. The latter is mounted on the TBM and measures any displacement of the laser beam from the target's center, including vertical and horizontal displacement, in addition to pitch, roll, and yaw.

“Given that the TBM is effectively a cylinder with cutters on 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 position,” says Lowe. “Most TBMs incorporate some type of 3D laser window within the tunnelling shield and 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's possible to calculate where the front of the machine is.”

Honing the guidance system

After evaluating several inclinometers, Zed specified the T233 from Sherborne Sensors. The T233 is a dc, closed loop, force-balance tilt sensor that offers accuracy and reliability several orders of magnitude greater than open-loop designs where system variations are not corrected. Its flexure-supported torque-balance system and fluid damping protect the sensor against severe shock and vibration, while maintaining accuracy. In addition, the electronics and dual-axis sensor are precisely aligned orthogonally and encased in a sealed housing, thereby permitting operation in hostile environments and enabling angular-tilt measurements in reference to gravity.

According to Lowe, despite the hostile environments in which they are employed, the inclinometers are lasting longer than expected for this sensor type. Lowe also notes that the modular transducer unit can be swapped out within 30 minutes.

This month's case study provided by Sherborne Sensors Inc. For more information, contact (877) 486-1766 or sherbornesensors.com.