Feature Condition monitoring
vibration levels, these can be shown numerically, as bars or as lines; each presented in green to denote below-threshold and red to denote above-threshold. The cabinet also feeds a status indicator in the hover- craft’s cockpit to give the pilot an indication of vibration levels.
Monitoring vibrations CM in action
Andy Anthony, managing director of Monitran, recounts how two very different condition monitoring projects went far beyond the provision of just vibration sensors
A
s a vibration sensor manufac- turer we’re always keen to ensure the sensors we supply give the best results for any given application. Increasingly, how- ever, we are also developing and installing complete conditioning mon- itoring systems.
The provision of such turnkey solu- tions means we need to draw not only on our understanding of how vibration energy should be acquired (i.e. types of sensor, sensitivity, range and so on) but also how the sensors’ outputs should be conditioned, sampled, inter- preted and presented.
Drivetrain monitoring
Monitran recently developed a 14- channel monitoring system for Griffon Hoverwork, a UK-based hovercraft OEM. During recent years the company has received orders for 12 Griffon 8000TDs – twin-engine, medium-lift hovercraft capable of speeds of up to 45 knots – from the Indian Coast Guard. A hovercraft’s drivetrain experi- ences a wide range of loads (in terms of torque and RPM) during operation and they tend to be worked hard when crews are responding to emergencies. The Indian Coast Guard therefore requested that each hovercraft be fitted with an integrated engine and gearbox vibration monitoring system. Specifically, they wanted to be able to: • Flag to the pilot if the drivetrain experienced any abnormal vibra- tions (noting that short periods of high vibrations are to be expected during certain manoeuvres, and should not raise false alarms);
• Allow maintenance engineers to 8
view the vibration levels on any given sensor in order to identify which component(s) might be wear- ing or developing faults.
In addition, the system had to be lightweight, compact, tolerant to high vibration levels, and compliant with marine regulations.
The sensing part of the project was fulfilled using 14 accelerometers: MTN1100W general purpose, constant- current analysis sensors with AC outputs. The ‘W’ denotes they are sealed to IP68 (submersible) 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 main- tenance engineers can connect an oscilloscope or spectrum analyser to view the raw signal.
The units also provide analogue (AC) outputs proportional to velocity and peak g. These outputs feed into a PCB-mounted microcontroller programmed by Monitran to drive 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 sensor shown as a bar chart. One bar per sensor, and with each showing current value, threshold and maximum value recorded;
• The mm/s values of all sensors as lines on a 360˚ dial.
All of the above include some form of status indicator. As with the
Each Griffon
hovercraft is fitted with an integrated engine and gearbox vibration
monitoring system
In a very different example, Monitran was asked to develop a system for con- tinuously monitoring the vibrations caused during the construction of an extension to a girls’ school in London. The construction work required some very deep piling close to a row of old houses and their boundary walls, so the project needed to: • Record and log the vibrations (in three axes) at seven locations during site working hours and in all weather conditions;
• Activate a visual alarm when vibra- tions exceed threshold levels;
• Log vibration events in case of com- plaints from residents.
For this project, the MTN1100 accelerometer was once again employed, but not the waterproof variant. Here, three standard sensors were mounted, at 90˚ to each other, along with three MTN8006 signal conditioning circuit boards into a waterproof box.
Seven boxes are positioned around the site and feed signals (proportional to velocity) back to a microcontroller- based monitoring and data logging system developed by Monitran. Note: unlike the hovercraft project, where machinery is being monitored, here the vibrations are typically low fre- quency and of unpredictable duration. The system incorporates an indus- trial touch screen to input commands and display the outputs while contin- uously logging any out of range vibra- tions with time and magnitude. Vibration levels can be viewed as numerical values, current RMS and current peak, plus highest peak (that day) and peak since recording started. Also, in real time, any sustained high level vibrations will trigger xenon alarm beacons to warn the site opera- tor to suspend or modify operations. Vibration levels from the monitoring system can also be layered on to a digital site map.
Turnkey solutions
Whilst very different applications, the above two projects have much in common. Both were turnkey solutions, developed from the sensor up. They also exemplify the versatility of a ‘gen- eral purpose’ sensor (the MTN1100) and how it can be employed to monitor very different types of vibration.
Monitran
www.monitran.com
measurement & sensors directory 2012-2013
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