How programmable logic controllers are used in vibration monitoring of a rotating machine

The condition of a machine dictates its maintenance.To know the condition of a machine, you need to analyse the machine’s vibration data. Most industries gather vibration data by implementing vibration monitoring techniques. These methods need the use of a portable data collector and a predetermined route of data collection points.

The collected vibration data is reviewed and compared with the trend data to determine any anomalies or machine failure. This process, however, wastes time and resources if the machines work as expected.

Vibration monitoring can be hard in large manufacturing plants with several rotating machines. It can be challenging to determine the data collection routes and frequency. These issues get compounded if different machines in a plant have different failure rates. Many plant managers thus tend to investigate multiple continuous vibration monitoring solutions. Such investigations often reveal that most permanent vibration monitoring sensors are incompatible with existing plant monitoring instrumentation systems such as programmable logic controllers (PLCs).'

The use of a vibration sensor and retro-reflective photoelectric sensor with a condition monitoring system  (CMS) allows vibration acceleration, speed data, and alarms to be available directly onto the PLC system in real-time. In this way, the PLC can react instantaneously when a warning condition presents itself, whether by sending an alert on an HMI or giving the command to VFD to stop the machine from running altogether until the defective component is fixed or replaced.

Figure 1: Vibration monitoring of motor using condition monitoring system (CMS) and PLC

Monitoring and diagnostic vibrations

Figure 1 includes a PLC, a CMS, a HMI, and sensors to measure and calculate the desired machine parameters. Any violation of limits results in the generation of corresponding messages and the execution of the parameterized response. The control program can access these messages through a function block in the software. The measured variables are cyclically transmitted to the controller and recorded as a trend curve in the CMS. The trend curves can be displayed via the integrated web server.

The CMS enables you to calculate the following characteristic values:

• vRMS (root mean square velocity) is calculated based on the interval RMS value of vibration velocity.
• aRMS (root mean square acceleration) is calculated based on the interval RMS value of vibration acceleration.

Setting the warning and alarm limits

We will use an example to show how to determine the warning and alarm limits.

According to DIN ISO 10816-3, the following guide values apply for this type of machine:

Figure 2: Guide values according to DIN ISO 10816-3

Warning limit vRMS: The warning limit indicates that a significant change has already occurred, but the operation can continue. It is necessary to investigate the reasons for the changed vibration condition and eliminate it if necessary. According to DIN ISO 10816-3, if the vibration quantity increase (or decrease) exceeds 25 percent of the upper limit value of the corresponding zone B, the changes must be considered essential, particularly when they suddenly happen. We, therefore, recommend setting the warning limit to 25 percent of the upper limit value of the corresponding zone B higher than the basic value (the basic value is obtained from past operational experiences at this measuring point). The limit should not exceed 1.25 times the upper limit of zone B. As no experience values are available at the beginning, the reference value measured when determining the normal operating state is taken as the basic value.

In this example, the warning limit is defined as follows:
Warning limit vRMS = basic value + 0.25 × upper limit zone B
Warning limit vRMS = 0.8 mm/sec + (0.25 × 4.5 mm/
Warning limit vRMS = 1.925 mm/sec

Alarm limit vRMS:  The alarm limit intends to indicate that further operation may cause machine damage. If this limit is exceeded, immediate measures should be taken to reduce vibrations, or the machine must be shut down. The DIN ISO 10816-3 recommends the limit to be located within zones C or D. Generally, the limit must not break 1.25 times the upper limit of zone C.
In this example, the upper limit of zone C is used as alarm limit:
Alarm limit vRMS = 7.1 mm/sec

Warning limit aRMS:  The operator can use the value of the aRMS vibration acceleration averaged over a frequency range between 1 kHz and 10 kHz as bearing status monitoring. The suggested warning and alarm limits are based on practical experiences. There is no normative specification for limits. To determine the warning limit, 1 m/sec² is added to the measured reference value in the normal operating condition of the machine.
A reference value of 0.8 m/sec² is measured for the machine in the example.
Warning limit vRMS = basic value + 1m/sec2
Warning limit aRMS = 1.8m/sec2

Alarm limit aRMS:  To determine the alarm limit, 2 m/sec² is added to the measured reference value in normal operating condition of the machine.
Alarm limit aRMS = basic value + 2 m/sec2
Alarm limit aRMS = basic value + 2.8 m/sec2