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Andy Anthony, Managing Director of Monitran, explains how a bespoke vibration monitoring system developed for protecting the twin drivetrains of a hovercraft led to the launch of a versatile commercial solution.

Based in Southampton, Griffon Hoverwork is an Original Equipment Manufacturer (OEM) of hovercraft; having been involved in their design, build and operation for more than 40 years. In addition, the company has hovercraft in service in more than 40 countries, ranging from the jungles of South America to the freezing conditions of the Arctic.

Griffon Hoverwork recently received an order from the Indian Coast Guard for 12 Griffon 8000TD hovercraft. Of great appeal is the 8000TD’s impressive capabilities; including a top speed of 45 knots and its ability to carry a 4 tonne payload, as well as the craft’s ability to traverse land and water. The Indian Coast Guard intends to use the 8000TDs for anti-smuggling and anti-infiltration operations, particularly in the shallow waters around India’s extensive coastline and offshore islands.

Monitoring loads

A hovercraft’s drivetrain experiences a wide range of loads (in terms of torque and RPM) during operation and tends to be worked hard when a crew is responding to an emergency. The integrity of the drivetrain can be compromised if it is overworked for long periods and working some components above their recommended limits will typically accelerate their wear, making it difficult to establish an optimum service schedule. It can also result in component failure.

The Indian Coast Guard therefore, requested that each hovercraft be fitted with an integrated engine and gearbox vibration monitoring system, as an increase in vibration levels is frequently the tell-tale sign of an impending failure. Specifically, they wanted to be able to flag to the pilot if the drivetrain experienced any abnormal vibrations, noting that short periods of high vibrations are to be expected during certain manoeuvres, and should not 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.

Griffon Hoverwork turned to Monitran to develop the monitoring system as, in addition to being an OEM of sensors and transducers for the measurement of vibration, displacement and proximity, the company has been undertaking a variety of turn-key projects during recent years.

The 8000TD is a twin engine hovercraft and has two drivetrains. Each drivetrain powers lift fans beneath the skirt and a variable-pitch propeller at the rear of the craft. The vibration sensing part of the monitoring system was fulfilled using 14 MTN/1100W general purpose, constant-current analysis sensors with AC outputs. The ‘W’ denotes that the sensors are submersible and sealed to IP68 and for this project they were supplied with marine-approved cables.

The cables feed into a bulkhead-mounted waterproof cabinet that contains 14 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 microcontroller 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^o 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.

Made-to-order

The Indian Coast Guard has already taken delivery of two hovercraft and the remaining 10 are scheduled for delivery during the next 18 months.

While the g-mac units within the system developed for Griffon Hoverwork interface with accelerometers they can also be used with the other transducer types, and the system software can be re-written to display virtually any parameter, e.g. temperature, pressure, light, voltage etc.

In addition, though only 14 channels were required for the drivetrain monitoring project, the system architecture can accommodate up to 16.

Monitran therefore 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. It is a made-to-order system, whereby customers can specify how many channels they want the system to contain (1 to 16), the conditions/parameters to be monitored and what levels of control they wish to have over each channel.

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

Though not strictly an ‘off-the-shelf system’, the MTN/5000-16 is a mix of products and customisation services used to build a monitoring solution from the physical parameters to be measured all the way up to meaningful and useful information.

Monitran

www.monitran.com