Acoustic emission sensor enables early stage fault detection
We all remember stories of the engineers who could hold a screwdriver to their ear and press it against a machine to judge the condition of the rotating machinery inside. To the well-trained ear, those vibrations tell a story.
This principle has been taken up by West Sussex-based Kittiwake, which has developed a Machinery Health Checker (MHC) sensor. The sensor is used to capture the acoustic emissions from bearings and assess their condition.
A special acoustic emission sensor allows the user to listen to the sound signals being generated on headphones where special audio circuitry filters out normal vibrations and audible signals to let engineers clearly hear rubs, scuffing and impacts as they happen.
"It works using a piezo-electric transducer," says Dr Steve Dye, business development manager at Kittiwake. "Traditional vibration sensors tend to have a relatively large weight inside that shakes and this is what is measured and gives a reading. But, if you have high shock instances such as drilling in harsh environments, it can actually break the sensor. Our sensor has much less mass so doesn't get these issues."
Vibration measurements often need a lot of sampling, analysis and diagnostics but Kittiwake wanted an almost instantaneous reading that tells engineers what they need to know. The sensor measures dB – essentially how noisy the bearing is. The other parameter is 'Distress'. This is an indication of how many high frequency impacts there are as the bearing goes round.
'Spikes' in the normal operation of the bearings acoustic emission spectrum are generated by deterioration inside the rotating machinery. This can be shards of metal, corrosion, or an increase in friction.
As the mechanical condition of machinery deteriorates impacts, friction and crushing generates sound waves that span a broad range of frequencies. By detecting only the high frequency part of the signal it is possible to detect miniscule amounts of activity from the slightest of rubs to the crushing of particulates in the lubricant.
"The beauty of this is that it is bearing independent," says Dr Dye. "Normally, when you do a vibration measurement you have to know the bearing type, the size of it and the speed of rotation.
"By coming up with a dB and Distress value straight away it gives you a very quick way of determining the condition of the bearing prior to saying there is a problem. Although it does have the ability to take a spectral waveform to do Fast Fourier Transform for analysis, which is very akin to standard vibration measurements, generally you want to know if the bearing is working well or not. You don't replace the inner race, outer race or the ball bearing if there is pitting, normally you take the whole bearing out and put a new one in.
"This allows engineers to very quickly determine if you have a problem or not. If the bearing is noisy and the Distress level is greater than 10 then it may just need to be re-greased. If it remains high then it is advisable to do some further diagnostic analysis."
The sensors are available in a handheld, standalone version and also as Smart sensors which can be integrated on to machines to take continuous readings that feed back data to a PLC or SCADA. To facilitate sensor coupling a variety of mounting methods are available including magnetic front face, adhesive bonding, bolt-on and screw in. The MHC sensor has two modes, Standard and Super-slo mode which together allow measurements to be taken between 0.25 to 2500rpm.
The Smart sensors are currently being used on the London Eye to monitor the condition of the bearings and have also been used at the other extreme monitoring the wheel bearings of the current land speed record-holding car; Thrust SSC.
