Today a variety of machines is present in the industries which serve an extremely wide range of purposes. These machines range from commonly used ones to special-purpose ones which may be for crucial or special industrial operations. For enhanced productivity, it is extremely vital for the machines to be properly maintained so that it can deliver the required functions efficiently over their life period. There are three different types of maintenance schemes that are followed in the industries as listed below
- Reactive Maintenance�
- Preventive Maintenance
- Predictive Maintenance
In this type of scheme, no maintenance is done on the machine until it fails. The entire machine or the defective component is replaced with a new one after the failure occurs. This scheme is generally followed for the least important machines.
In this type of scheme, the maintenance is scheduled in a regular periodic manner as prescribed by the manufacturer. This results in the machine always being in perfect working condition. The downside is that sometimes components of the machine may be replaced in spite of it being functional. This may lead to added costs.
In this type of scheme, the maintenance is scheduled depending on the current condition of the machine. Unlike reactive maintenance where the machine is run till breakdown and preventive maintenance where the machine may be inspected even when there is no particular necessity to do so, the maintenance is carried out when the running condition of the machine warrants an inspection. Predictive maintenance, popularly known as condition-based maintenance (CBM) is the topic of discussion of this article.��
Overview of Condition Based Maintenance (CBM)
As mentioned, in CBM we need to know the running condition of the machine based on which the maintenance is scheduled. This necessitates the collection of information about the machine's running condition through the measurement of appropriate parameters that can help us decide on the status of the machine's health. The machine needs to be attached with sensors to measure the parameters. The common parameters that are used to detect machine health include vibration amplitude, acoustic emission, temperature, lubricant condition, etc. The instrumentation for the collection of information about the machine condition is done during the installation of the machine. Thus, there are added expenses involved in CBM. However, in the long run, it has been found out that CBM is more economical than preventive maintenance. It also provides the added benefits that the condition of the machine is known to the user at any time. A majority of CBM methods rely on vibration information to detect the presence of damages in the machinery. Some CBM techniques make use of lubricant analysis in which the presence of debris in the lubricant, its size and shape, and the chemical composition of the lubricant are analyzed to infer the presence of damage. Also, non-destructive techniques such as ultrasound are available to detect the damage but these techniques require the machine to be stopped for inspection and are often expensive. Vibration-based techniques are extremely popular among the CBM fraternity. The reasons being convenience in measuring the vibration parameters and that the vibration response is always indicative of the physical operation of the machine. For example, in a 4-stroke engine, combustion occurs once every two complete revolutions of the crank and this is reflected in the vibration response of the engine. In a similar way, damages occurring in the machines have certain frequencies associated with them.�
Modules of CBM system
Generally, a CBM system can be thought to have three units which are as follows
Sensors and Transducers
These are attached to the physical system that is being monitored (i.e. the machine) to measure the parameters of interest. The most commonly measured parameters for a mechanical system are position, velocity, acceleration, temperature, and lubricant condition. Since the interpretation of the signals recorded by the sensors and transducers will be the basis for determining the machine condition, an appropriate selection and proper installation are of paramount importance.
Fig. 1: Piezoelectric accelerometer
Signal Processing and Conditioning Unit
The signal recorded from the sensors may contain unwanted noise which needs to be filtered out before further analyzing the signal. This is done by the signal conditioning unit. The filtered signal needs to be processed to extract useful information from it. This is done by the signal processing unit. Nowadays most signal processing units work on digital signals and hence it may be necessary to convert the analog signal recorded by the sensors to a digital signal using analog to digital converters.
Fig. 2: Filtering noise from a signal .
This unit displays the signals for interpretation by the user as well as provides an interface for the user to interact with the CBM system.�
In this article, we have introduced the topic of condition-based maintenance which is a widely adopted maintenance scheme in most industries. CBM has also attracted great interest among the research community and one can find plenty of research papers dedicated to the topic published to date.��
- A.R. Mohanty, "Machinery Condition Monitoring: Principles and Practices", CRC Press.
- R.B. Randall, "Vibration-based Condition Monitoring", Wiley.