Vibration monitoring for machines

Inside Machines: Vibration sensors are designed to ensure that machines and systems operate within safe operational parameters and to help users find imbalances caused by internal and external forces.

06/05/2017


A screenshot showing an example of a CANopen Safety CRC calculation. Courtesy: TWKAsking if there is there such a thing as good and bad acceleration in manufacturing may seem like an odd question, but it is justified. Shaking may be good 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 still can be tolerated or whether it is already so high that the application is being damaged and has to be stopped.

Vibration monitoring is becoming increasingly important in machines and systems and it requires corresponding safety components to register vibrations, oscillations, inclination angles, and rotational angles safely. Vibration sensors with a CANopen safety interface and switching relay are designed to measure dynamic acceleration with a frequency range from 0.1 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 analyzed more precisely and higher frequencies do not act as a disturbance, and vice versa.

The CANopen safety interface is implemented according to the following profiles: CiA 301 version 4.2 (application layer) and EN 50325-5 (safety protocol) as well as CiA 401-1 version 3.1 (Profile for I/O devices — Part 1: Generic I/O modules). The vibration sensor can be extensively parameterized using this interface and specific procedures have to be taken into consideration during safe communication and parameterization by means of this interface. It is important that safety-relevant parameters are not changed and the safe functions are not impaired as a result. 

Parameterization for safe vibration sensor operation

With parameterization, special CANopen objects are used to write new values, which allow the user to change the parameters to meet specific needs. However, checksums additionally have to be transferred for each parameter change and these checksums have to be calculated in advance by the user for each parameter to be changed so that each can be transferred to the sensor. The parameters for the sensor to operate are entered in this tool, and the checksum is displayed in the result window.

However, this calculation also can be carried out independently using the underlying calculation polynomial (CRC-CCITT: x16+x12+x5+1), or a calculation tool to be programmed by the user can be stored in the control system. Before any change can be made to the sensor, the valid flag, which is a type of sensor lock mechanism, must be deactivated by writing a "0" into this object.

The object structure of CANopen—in this case, CANopen Safety—permits this safe parameterization and simplifies handling. No special device programs have to be used to carry out the changes. Unintentionally changing the parameters is out of the question. 

Vibration testing at a wind turbine

TWK’s NVA115 vibration sensor is a SIL2 Safety component. Courtesy: TWKWind turbines are designed to generate electricity. However, the electricity does not simply come out of the socket or—in this case—the wind turbine. A wind turbine is a complex and extremely detailed piece of construction that has to be protected from potential damage. The oscillations and vibrations, which occur during operation, primarily in the gondola and the mast, are important physical measured variables. They have to be registered 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. Possible internal event causes include damage to the transmission or the bearings that may lead to the occurrence of excessive main shaft vibrations. These vibrations lie in a frequency from 10 to 50 Hz.

On the other hand, external influences may cause the system to vibrate. Among 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. Unfavorable wind conditions leading to excessive movements on the part of the gondola and the mast are another potential concern. The frequencies typically lie between 0.2 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. 

Vibration sensors, machine safety

This is where the sensor comes into play as a safety component. The measured acceleration value is constantly compared with limit values. If these are exceeded, the internal safety relays are shut off. The two safety switching contacts, each of which consists of two individual relays connected in series, are switched in the system's safety chain. Their series connection helps ensure that the electrical circuit is safely disconnected-even in unfavorable or extreme conditions in which an individual relay would perhaps stick.

The sensor also can monitor vibration behavior over a longer period of time and ensure the vibrations do not exceed a permissible amplitude value. If the value is exceeded briefly, 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 safety chain is interrupted only when the system vibrates extensively for too long. If the user wishes, the vibration data can be transferred to the control system to ascertain what vibrations are present in the system.

The configurable safety switching contacts with the respective limit value comparison always react to a filter's output main value. Another special feature is that the sensor can be set to stop the system in a specific vibration phase. This is achieved using a shut-off delay and the vibration sensor's ability to recognize the monitored vibration's zero-axis crossing. The positive zero-axis crossing is the starting point for the adjustable time if the shut-off criterion was met beforehand. Standard settings such as x- or y-axis assignment, momentary, or root mean square (RMS) value also are possible with a number of adjustable parameters. Two analog signals also are available optionally for additional value output alongside CANopen Safety.

Sometimes it is occasionally necessary to know whether the two switching contacts' relays are still operating reliably. It may be that they have not had to switch for a number of weeks or months. Despite robust relays and durable electronics, it makes sense to test them occasionally. To do this, it is possible to initialize a self-test lasting a few seconds via object 32FD. Depending on which value is transferred to this object, either switching contact 1 or 2 is tested, or both. However, the sensor remains in operational status during the test so the control system still can see all of the current vibration data.

The self-test can be used to check if the switching contacts really open, even if they are in the safety chain, so the user can check the relay status from the point of view of the vibration sensor and can read out this switching status. Incidentally, this can be done at any time, not only during the self-test. This is important so that the control system can detect which device in the safety chain was triggered. The vibration sensor helps ensure that an application is in safe hands for minimizing damage caused by interference acceleration.

 

Achim Albertini, TWK Elektronik. This article originally appeared in the CAN in Automation (CAN) Newsletter magazine. CiA is a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

MORE ADVICE

Key concepts

Vibration sensors are designed to examine internal and external causes of vibration and oscillation for machines.

Parameterization allows users to change the settings to meet their specific needs.

Vibration sensors also can be used as part of a safety test and can help ensure that a machine does not exceed a certain limit.

Consider this

What other factors should be considered when testing for vibration in machines?



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