British engineering and sensor innovation
20 March 2015
In the early nineties, British engineer Jonathan May was involved in a project to develop a ship-borne weapon alignment system that would enable naval vessels to align their weapons more accurately and at lower cost.
At that time, the use of electronic sensors was being investigated by both the US Navy and the British Royal Navy for static alignment of weapons platforms.
Static alignment of weapons platforms is of critical importance to achieving weapons accuracy. A warship’s structure will bend and flex over the course of its lifetime. Exposure to rough seas, variations in temperature, and changes in loading cause varying stress on the ship’s frame; refits and accidents cause even more permanent changes. All of this precipitates variations in the accuracy and precision of the alignment of weapons and to the physical configuration of the ship.
The fundamental method for finding static alignment errors is the tilt test or roller path test. This involves measuring the relative tilt between platforms at a series of bearings. When these individual errors are plotted against the bearings, a sine curve results, which identifies both the magnitude of tilt and the bearing at which it occurs. To achieve the high performance demanded by modern weapons systems, these measurements have to be precise to within a few minutes of arc.
The conventional method of performing a tilt test was to use bubble clinometers to measure the errors between platforms. This required that the ship be secured in dock for the duration of the measurements, with maintenance activities on board halted to minimise any movement. Even under these conditions, the ship could flex with the wind, causing movement of the bubble and hindering measurement. With a bubble clinometer, the reading is only accurate when the bubble is centred, so an engineer had to be present on each platform to adjust the level. Manual adjustments would be made to try and centre the bubbles in all clinometers simultaneously, meaning communications between stations was also necessary.
For a ship equipped with many weapons, the conventional method could take several days to complete, and the cost of docking and downtime on the ship would be substantial. Moreover, a docked ship experiences different stresses to those experienced when fully floating. Consequently, there was also uncertainty as to how much the structure would flex when released from dock and how much error would be re-introduced.
Although a prototype system being evaluated by May was intended to automate the tilt test and remove the requirement for docking, he was convinced it was not commercially viable. It was too big, too heavy, and too expensive.
Digital signal processing
May remained convinced there was a better way to approach weapons system alignment, by combining the approaches of the US Navy and Royal Navy, and using electronics and signal processing. Electronic Measurement Systems (EMS) was created to pursue the concept further.
May designed a new tilt system employing higher ranged electronic sensors with more accurate custom electronics. Much lighter and more user-friendly than the system piloted by the Royal Navy, May’s Computerised Electronic Tilt Angle Measuring System (CETAMS) comprised a set of up to eight inclinometers linked via a single electronics module to a laptop PC.
The measurement process for the tilt system is as follows: The ship’s Master Level Datum located on the ship’s founding plate acts as the reference platform and a sensor is placed on it, with the remaining sensors secured on the other platforms. This allows simultaneous measurement of all the ship’s platforms, and calculation of the errors between them.
In order to meet the requirements of the design, the inclinometers were specially modified by Sherborne Sensors to enhance the accuracy of the device and provide a low output impedance drive suitable for driving the long cables without interference.
The concept of operation is essentially the same as the conventional tilt test technique described previously, but the measurement of each tilt takes just 10-15 seconds, with the system using a signal processing algorithm to calculate the tilt of each inclinometer. Tilt measurements are integrated and processed to effectively eliminate inaccuracies caused by the ship’s movement.
Accurate measurement of the tiny tilt differences between individual components allows mechanical compensation to be applied. More frequent correction factors can be entered into the ship’s Fire Control System and programmed to correct them. This improves gun fire control and increases weapons’ accuracy.
It is now possible to perform a test in under an hour using CETAMS, whereas previously a tilt test could take three or four days to complete. Crucially, as the system compensates for small movements during measurement, it enables tilt tests to be performed while the ship is alongside dock and without impacting on other activities taking place on board.
A significant upgrade was made to the system’s design that included new software and digital electronics. The purpose behind the upgrade was to further improve the accuracy of measurement, increase functionality and ensure compatibility with modern computer operating systems. This resulted in a complete new system design that was based around a dual axis sensor.
The key component of the system is the Sherborne Sensors’ sensor which formed the starting point of the new design. Early prototypes were made to incorporate two analogue sensor heads into a single unit with room to insert the digitising electronics. In addition to improved accuracy, it was essential to keep track of each inclinometer so that recorded scale factor and offset adjustments could be linked to the sensor itself. This required adding intelligence to the inclinometer and non-volatile storage on additional circuit boards. It was just after the first working prototypes of the new sensor assembly had been commissioned that the DSIC, a dual axis digital inclinometer became available.
The new DSIC had all the functionality required for the new sensor assembly, but in only two thirds of the space occupied by its prototype. However, EMS needed the best accuracy at extremely small angle increments optimised over a particular temperature range. It also needed to be able to cope with a ship’s motion and an exposed environment. While the standard DSIC was close to meeting these requirements, EMS discussed potential changes with Sherborne Sensors, who were able to customise the design accordingly.
Consequently the EMS systems can offer an improved measurement capability and greater functionality for a wider range of applications. The dual channel capability means that both standard tilt test measurements and accurate plane alignment, such as that used for mast alignment are supported. A further benefit of the DSIC is the internal non-volatile memory provided. Each sensor head has its own serial number that can be read remotely, which means that whenever an inclinometer is connected to the system, it can immediately be identified. This is particularly useful in compensating for offset errors that may arise over the life of the inclinometer.
When the sensors are being secured to weapons components and other metal structures on board ship, it is possible for the base to pick up grease and dirt, or be knocked causing roughened feet. This could result in a change to the zero tilt offset, introducing a fixed error. However, by measuring the offset of a device and storing its value linked to the serial number, such errors can be compensated for in the system software.
Four years on from the initial system upgrade and 18 months since adopting the DSIC, EMS has delivered a new CETAMS unit to the AWD Alliance for the Australian Navy, and also received orders from the Canadian Navy.
EMS is looking to collaborate further with military, naval, and shipyard customers on other innovative technologies that employ highly-customised, dual-axis inclinometers. There has also been an industry drive to recruit and train young people in the niche skills that firms such as EMS require. It is now hoped that together, the network of dynamic firms can build on their reputation as industry pioneers to nurture and grow the next generation of talent and technological discovery.
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