Predictive maintenance methods depend heavily on monitoring systems that have the ability to accurately sense and report one or more key equipment condition indicators. Most monitoring systems have several distinct components (see Figure 2.1). The intent of the monitoring system is to take a physical event and convey that change so it can be observed over time; a decision can then be made as to the proper action to take.
1.They all have some type of sensor that detects and transmits a signal, usually a current or voltage, to the signal processor.
2.Next is a signal processor which receives the signal and converts it to a usable output signal. Signal processing can include filtering out unwanted portions of the input signal, converting the signal to a digital set of values, or calculating the average, maximum, or minimum value of a series of inputs. The design of signal processors are numerous and varied in purpose. In the end, you want the output of the signal processor to provide a useful output that can be displayed or used in a protection system or scheme.
Figure 2.1 Monitoring system schematic
Signal processors are designed to handle static and/or dynamic signals. An example of a static signal is temperature. If you plot a temperature over time, you typically get a gradually changing series of points that can visually be studied and analyzed. Static signals do not carry any rapidly changing, i.e. dynamic, components. On the other hand, dynamic signals can vary rapidly with time, as seen in Figure 2.2. Dynamic signals, also called dynamic waveforms, require more complex signal processing to determine their properties. Processing speed is critical when high speeds or high frequencies are involved. Typical waveform properties are frequency, peak amplitude, root mean square amplitude, and phase. In some cases, both static and dynamic information are extracted from the raw sensor data.
In Figure 2.2, Waveform A is the complex wave detected by the sensor that is to be processed. Waveforms B1, B2, and B3 are the fundamental sine wave components of the original complex wave that are determined by the signal analysis process.
Figure 2.2 Complex dynamic waveform decomposed into sine wave components
3.The signal processor then sends an output signal to the display or monitor that receives the output from the signal processor and displays in a way that is easy to interpret, as seen in the heart monitor in Figure 2.4. Displays can use dials, scales, simple numerical displays, or waveforms to communicate the status of variable being measured.
Sophisticated monitoring systems can also have internal storage capabilities in order to provide a means of trending and comparing the present status with the past. This capability is a must in critical applications.
Data presentation
Figure 2.3 illustrates two trend plot examples. One plot is that of a gradually increasing value whereas the other shows a step change in a measured value. Trend plots are useful because they provide visual representations of the measured parameter over time; this representation can help in the troubleshooting process. Suppose a step change occurred at the same time as a change in the process; there may be a correlation between the two events that should be investigated. A gradually increasing trend plot may indicate either a deteriorating internal or external component.
Figure 2.3 Trend Examples
4.Critical monitoring systems may also have built-in protection schemes, which can be readily programmed. These systems can provide either a remote or local alarm or alert whenever an undesirable condition has been detected.
5.Finally, for a monitoring system to be complete, assessment criteria are required to determine when the machine owner should be concerned or if action must be taken immediately. Without assessment criteria, there would be no need for monitoring systems because their outputs would be meaningless. If assessment criteria are set too low, then time and money are wasted. However, if assessment criteria are set too high, then human health, environment, and equipment are placed in jeopardy. One of the primary goals of this book is to provide proven and accepted assessment criteria to owners of process machinery.
Let’s put our newly acquired information about machinery monitoring knowledge to use by studying a hospital heart monitor (see Figure 2.4). Many of us have seen them in hospitals beeping and flashing incessantly. Their purpose is to continuously monitor key health parameters of patients in serious or critical conditions. If any of the key indicators are found to be outside normal values, an alarm will sound at a nurse’s station.
Figure 2.4 Typical heart monitor
Here, the patient is analogous to a machine; the heart monitoring system is analogous to a machine’s monitoring system, complete with remote alarms. Because this application is considered a critical monitoring one, i.e., a seriously ill human, continuous monitoring is employed with local and remote alarming capabilities.
Heart monitors typically include the following functions:
•Heart rate (dynamic data)—A heart monitor is a device that measures the heartbeat rate of a patient in real time. A pad with electrodes is placed on the patient’s chest. These electrodes must contact the skin directly in order to monitor the heart’s electrical voltages. Sensors detect a series of heart beats that are sent as raw signals to a signal processor, which processes the information and calculates the beat frequency. This heartbeat rate and digital waveform information are then sent to the monitor for display and storage.
•Blood pressure (dynamic data)—A blood pressure monitor must measure and report two key values that reflect blood pressure in a fractional form; one number on top and one on the bottom, e.g., 128/82. The number on top is called the systolic pressure, which is the pressure inside your blood vessels at the moment your heart beats. The number on the bottom is your diastolic pressure, which is the pressure in your blood vessels between heartbeats, when your heart is resting. The signal processor must be capable of detecting the maximum and minimum pressure in pressure waveform, storing the data, and then displaying on the monitor.
•Temperature (static data)—Temperature is probably the oldest indicator of