Vibration monitoring is becoming increasingly important in machines and systems. The increasing demands are necessitating corresponding safety components. TWK offers a range of certified SIL devices. They register rotational angles, inclination angles and also oscillations and vibrations - 'safely'.
Is there such a thing as good and bad acceleration? You may perhaps think that this is a strange question, but it is entirely justified. Shaking may well be a good thing for a cocktail, but it isn't so good in a system or machine.
The task is therefore to ascertain whether acceleration caused by oscillations and vibrations can still be tolerated or whether it is already so high that the application is being damaged and has to be stopped.
TWK has developed the SIL2 vibration sensor NVA with CANopen Safety interface and switching relay for precisely this purpose. It is able to measure dynamic acceleration in a frequency range from 0.1 Hz to 60 Hz. Band filters can be used to subdivide the frequency range into sub-areas so that, for instance, low frequencies of less than 5 Hz can be analysed more precisely and higher frequencies do not act as a disturbance, and vice versa.
Let us take a look at an example.
Wind turbines generate electricity. However, the electricity does not simply come out of the socket or – as in this case – the wind turbine. Such a turbine is a highly complex design that is now refined down to the very last detail, a genuine feat of engineering artistry. The demands made on all of the turbine's individual components, and ultimately also on the sensor system that is used, are equally high. However, sensors are not only required to operate such a wind turbine, i.e. to ensure its pure function. There is also a range of sensor system components whose task is to protect the turbine and safeguard it from damage caused by external influences or defective parts of the system. In many cases, this enables major damage to be avoided and the value of the turbine to be preserved. This is important to the operator, for which operation ultimately has to be profitable, without causing unnecessary costs due to avoidable damage and down times.
The oscillations and vibrations that occur during operation, primarily in the gondola and the mast, are important physical measured variables which have to be registered in order to protect the system. If the vibrations are excessively high, the entire system is affected. The acceleration forces which occur in the mast may lead to crack formation or even fractures. Regardless of why excessively high accelerations occur, the system has to be shut down if there is a likelihood of danger.
What are the possible causes? On one hand, they may be internal events. For instance, damage to the transmission or the bearings may lead to the occurrence of excessive main shaft vibrations. These vibrations lie in a frequency range from approx. 10 Hz to 50 Hz.
On the other hand, external influences may cause the system to vibrate. Amongst others, these influences include rotor blade icing or damage. These do not occur uniformly and therefore lead to rotor imbalance which can cause the entire system to vibrate. Or unfavourable wind conditions lead to excessive movements on the part of the gondola and therefore also the mast. The frequencies in this case typically lie between 0.2 Hz and 3 Hz. These vibrations have to be determined as part of vibration monitoring for a wind turbine in order to cause the control system to shut the system down if respective limit values are exceeded.
This is where the vibration sensor NVA115 as a SIL2 safety component or the NVA65 as a standard device now comes into play. The measured acceleration value is constantly compared with limit values. If these are exceeded, internal safety relays are shut off. The two safety switching contacts, each of which in turn consists of two individual relays connected in series, are switched in the system's safety chain and, due to their series connection, ensure that the electrical circuit is safely disconnected - even under unfavourable conditions in which an individual relay would perhaps stick.
One of the NVA115's special features is the fact that it can 'monitor' vibration behaviour over a longer period of time. If a system's vibrations do not exceed a permissible amplitude value, everything is fine. If the value is exceeded briefly, however, the system does not have to be stopped immediately. Moderate and higher vibration values which occur temporarily are permissible if the system's vibration values subsequently decrease again. The NVA115 registers and evaluates precisely this behaviour with its integral function: the safety chain is only interrupted when the system vibrates extensively for 'too long'. If suddenly occurring accelerations are too high, however, the NVA115 reacts immediately (safety shut-off function).
Another special feature is that the NVA can be set to stop the system in a specific vibration phase. This is achieved using a shut-off delay T and the NVA115's ability to recognise the monitored vibration's zero-axis crossing. The zero-axis crossing is the starting point for the adjustable time T.
Of course, standard settings such as x or y axis assignment, momentary or RMS value are also possible with a number of adjustable parameters. Two analogue signals are also optionally available for additional value output alongside CANopen Safety.
With the NVA vibration sensor from TWK, an application is therefore always in safe hands for minimising damage caused by interference acceleration and guaranteeing effective operation.
