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From bespoke project to commercial spin-off

14 September 2012

A vibration monitoring system, originally developed as a bespoke product to protect the twin drive trains of a hovercraft, has led to the launch of a versatile commercial system that can be easily adapted to meet a variety of condition monitoring needs. Andy Anthony explains how it all came about.

Based in Southampton, Griffon Hoverwork has been designing, building and operating hovercraft for more than 40 years. The company has hovercraft in service in more than 40 countries, ranging from the jungles of South America to the frozen Arctic wastes.

Among its recent orders is one from the Indian Coast Guard for 12 Griffon 8000TD hovercraft, offering a top speed of 45 knots, a 4-tonne payload and the ability to traverse both land and water. These will be used for anti-smuggling and anti-infiltration operations, particularly in the shallow waters around India’s extensive coastline and offshore islands. Two hovercraft have now been delivered, and the remaining ten are scheduled for delivery during the next 18 months.

Drive train integrity
A hovercraft’s drive train experiences a wide range of loads during operation and tends to be worked hard when a crew is responding to an emergency. Drive train integrity can be compromised if it is overworked for long periods, while working some components above their recommended limits will typically accelerate their wear, making it difficult to establish an optimum service schedule. These conditions can ultimately lead to component failure.

With these concerns in mind, the Indian Coast Guard specified that each hovercraft be fitted with an integrated engine and gearbox vibration monitoring system. Specifically, this system had to flag to the pilot if the drive train experienced any ‘abnormal’ vibrations, discounting short periods of high vibration expected during certain manoeuvres, which might raise false alarms.

The customer also wanted maintenance engineers to be able to view the vibration levels on any given sensor in order to identify which component(s) might be wearing or developing faults. In addition, the system had to be lightweight, compact, compliant with marine regulations and itself tolerant to high vibration levels. At this point, vibrations sensor specialist, Monitran was called in to help.

The 8000TD is a twin-engine hovercraft and has two drive trains, one powering lift fans beneath the skirt and the other, a variable-pitch propeller at the rear of the craft. The vibration sensing part of the monitoring system features 14 Monitran MTN/1100W general purpose, constant-current analysis sensors with ac outputs, the ‘W’ denoting submersible IP68 sealing.

The cables feed into a bulkhead-mounted waterproof cabinet that contains 14 Monitran MTN/8066 g-mac signal conditioning units. Each g-mac has a buffered native accelerometer output with a BNC connector, to which maintenance engineers can connect an oscilloscope or spectrum analyser to view the raw signal.

The units also provide ac outputs proportional to velocity and peak g. These outputs feed into a PCB-mounted micro-controller that drives a touch-screen display on the front of the cabinet. Several screen views are available but the most useful show the current mm/s values of all sensors; the mm/s values of all sensors shown as a bar chart, with one bar per sensor showing ‘Current Value’, ‘Threshold’ and ‘Maximum’ value recorded; and the mm/s values of all sensors as lines on a 360 degree dial.

All of the above views include some form of status indicator. For the vibration levels, the status can be shown numerically, as bars or lines, presented in green to denote below-threshold and red to denote above-threshold. The cabinet also feeds a status indicator in the hovercraft’s cockpit to give the pilot an indication of vibration levels.

While the g-mac units within Griffon Hoverwork’s system are set up for accelerometer outputs they can also be used with the other transducer types, and the system software can be re-written to display virtually any parameter, such as temperature, pressure, voltage and so on. And while only 14 channels were required for this particular drive train monitoring project, the system architecture can accommodate up to 16.

Based on this work, Monitran decided to develop a commercial version of the system, and this year’s Southern Manufacturing & Electronics Show in Farnborough saw the formal launch of the MTN/5000-16. This is a made-to-order system, whereby customers can specify how many channels they want the system to contain (from one to a maximum of 16), the parameters to be monitored and the levels of control required over each channel. 

Customers can also specify what power source the system should utilise and whether or not it should carry out any direct switching for emergency shutdowns. The MTN/5000-16 also has 20 digital I/O channels, facilitating integration with other systems.

Though not strictly an ‘off-the-shelf’ system, the MTN/5000-16, with its mix of standard products and customisation services, can form the basis of a flexible monitoring system that is adaptable in terms of the physical parameters to be measured and the way the data are eventually presented to provide meaningful and useful information.

Andy Anthony, is managing director of Monitran

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