Monitoring Sound & Vibration for Process Control

In a production environment, one can always find two common themes for the successful application of acoustical or vibrational monitoring. Usually, workers judge the noise or vibration event as being the start or end of a particular process. Initiated by such an event, an automated control system can easily minimize any loss of production.

On the production floor, control of manufacturing processes have used continuous monitoring of sound and vibration for the past several years. For instance Brüel & Kjær had used their 2505 Multipurpose Monitor in the early 1980s to automatically monitor vibration signals. One could connect an accelerometer, a microphone, or other piezoelectric device to this monitor, and set limits for alerting the user whenever the levels exceeded them. They had filters to limit the signal bands, and detectors to average signals that fluctuated highly. On the output side, relays interfaced with the process control systems or other instrumentation. No other expensive analysis systems were necessary if the process control technician used this device to monitor acoustic or vibration levels automatically. People used these monitors also in the machine condition monitoring field as basic overall vibration detectors to switch off the machine if vibration levels exceeded the set limits.

Discrete analog circuit boards enclosed in weather proof enclosures made up these early monitors. The user had to select the circuit cards necessary for their specific application. Usually, a circuit card was capable of performing a specific function, such as RMS detector, amplifier or attenuator, high and/or low pass filter, and signal conditioner. The circuit cards worked together with the relays, alarm indicators, and the meter module. With very little dynamic range, users had to be very careful in selecting a circuit card for each application. One had to be knowledgeable about the transducer they employed and the particular measurement they were making. If conditions changed, they had to order additional circuit cards.

The above disadvantages of the analog system made Brüel & Kjær develop their digital signal processors replacing the monitors with modern electronics. They now had software controlling the functions of RMS detection, gain/attenuation, and filtering. End users found the application of the new monitors much simpler, as a monitor could be field-programmed for meeting the demands of the present task. The supplied software and its use in setting up and control of the unit allowed users to save time they earlier spent on analyzing the required settings before purchasing the monitor.

The new monitors use a PC interface for setting up and to display the results of their measurements. Users can store programmed data within the unit, so the monitor can operate even without the presence of the PC and retain measurements if the power fails. Digital signal processing within the unit allows the user to set up many low and high pass filters, true RMS, and peak-to-peak measurements. Users can set other built-in voltage references and test functions for set-ups related to new tests, including relays and indicators for system failure. In addition, the presence of electrical outputs for unconditioned and conditioned AC signals makes these new monitors ideal for real-time detection and control of acoustic and vibration events.