Manufacturing Trend 2011/05

Reliable machine protection with intelligent vibration switch, bearing monitoring transmitter, or knock sensor

One effective method for protecting rotating and reciprocating machines is to monitor the vibrations generated by the equipment, preventing the development of excessive vibration levels that endanger the machine, its related equipment, and the environment. Cost-effective, programmable electronic vibration switches serve the simple yet reliable execution of this task.

Application and operation of vibration switches

Various production equipment such as motors, fans, pumps, compressors, etc., generate vibrations. The vibration level produced by the equipment is – with few exceptions – proportional to the machine's condition. In this case, it is advisable to use vibration switches for basic protection of production, the machine, and its environment to prevent major damages. Depending on its function, the vibration switch measures the vibration of the device to be protected and classifies it according to a set threshold value. Upon exceeding the threshold value, the vibration switch provides a definite, two-state output signal to initiate the machine protection function. This seemingly simple task can be realized using mechanical or electronic components.

Precise electronic vibration switches instead of mechanical ones

Mechanical vibration switches are generally less adaptable to specific applications due to their simple structure. The sensitivity of the mechanical structure strongly depends on frequency and may not always match the characteristics of the vibrations generated by the machine to be protected, resulting in either false alarms or failure of the protective function. Due to its design, mechanical switches do not allow features such as remote acknowledgment/arming, start-up and alarm delay, or threshold value adjustment without external devices, limiting their application in protecting variable or automatic operation equipment. Electronic vibration switches aim to eliminate the disadvantages of mechanical structures. The sensor element in these devices is typically a piezoelectric vibration sensor, providing well-specified frequency range, linearity, and transmission characteristics for vibration measurement.

New generation of vibration switches – electronic sensor, signal processor, evaluator

With the reduction in size and cost of electronic signal processors, these elements are now part of vibration switches, offering numerous adjustable parameters. Setting the threshold value for vibration level is a fundamental function, with a significant difference compared to mechanical devices being accuracy and the range of adjustment. Electronic processing allows for the easy implementation of special features such as switching hysteresis, flexible threshold switching, activation based on environmental vibration levels, and even more complex evaluation or operating modes thanks to microprocessor-based signal processing.

The quality of vibration information provided by built-in vibration sensors can ensure comparison with standard evaluation, precise calibration of the device, which can be particularly useful for meeting warranty conditions, for example. Some vibration switches allow the raw vibration signal to be output. These outputs can serve calibration, operation verification, and connection to external vibration analysis tools or condition monitoring systems.

Integration of vibration switches – from direct control to PLC connection

Some versions of electronic vibration switches have visible on-site LED indication and provide numerous acknowledgment and arming options, making on-site operation and integration easier. Among the "intelligent," microprocessor-controlled vibration switches, electronic programming via, for example, serial communication ports (RS-485, USB) is possible. Electronic adjustment offers many advantages: the vibration switch can be quickly adjusted, possibly remotely, with precise values due to digital data transmission. The complete configuration of a specific vibration switch can be saved in electronic form, detailed documentation of the settings can be done, and it can be checked independently of the device. With digital signal processing, the wiring and application of vibration switches remain simple, as devices with normal relay outputs can be connected with minimal modifications and can be programmed for relay output modes (closing/opening, latching) as needed. Electronic vibration switches can perform well under various conditions, can be used in potentially explosive atmospheres with appropriate enclosures, are operational over a wide temperature range, and have virtually no maintenance requirements. Compared to sensor-signal processor configurations, electronic vibration switches are considered more reliable due to fewer wiring and connection errors. Due to the closed unit, there is less chance of configuration errors due to integration (e.g., sensitivity adjustment), and due to the compact design, they may be more resistant to mechanical and electrical influences. Additionally, the configuration with fewer elements can significantly reduce the overall cost. Some versions of electronic vibration switches meet strict criteria for chemical and oil industry facilities, allowing their use in potentially explosive environments.

PCB 686B Smart – configuration of "intelligent" vibration switches

The PCB 686B Smart electronic vibration switch family has a wide range of configurable parameters, yet their setup is simple, thanks to the USB interface and configuration program. In the PCB 686B Smart vibration switches, adjustable parameters include warning and alarm levels, switch-on and alarm delay, hysteresis, start level or blocking, alarm output mode, and the environmental vibration level that triggers the start. These parameters allow for very precise adjustments, enabling the well-defined definition of the machine's operational characteristics for effective machine protection. The PCB 686B Smart family offers multiple options against environmental influences, from armored cabling to explosion-proof enclosures. Despite the numerous functions and options, the PCB 686Bxx Smart family is competitively priced.

PCB 649Ax1 piston machine protection knock sensor transmitter

The operation of various elements of piston machines and their faults are related to impact-type vibration phenomena. These impacts generally have very high amplitudes but very short durations. The impulses can easily drive traditional vibration sensors into resonance or saturation, leading to false alarms, while mechanical switches may be triggered by lower-frequency vibrations sooner. Therefore, a different sensor, signal content, and more efficient evaluation method are required compared to the usual analysis of rotating machines. The PCB 649A series sensors are designed to provide protection for piston machines based on these considerations.

The PCB 649A series piston machine protection sensor transmitter is a sophisticated device. It employs a high-frequency piezoelectric vibration sensor for precise impulse detection, while filtering out low-frequency interfering vibrations with built-in filters. It monitors impacts, loosening, and knocks through microprocessor processing, allowing for adjustable transmitter output modes. The knock sensor transmitter can be set to provide an output signal that is proportional to the peak value of vibration impulses below the threshold and/or the number of impulses above the threshold indicating a fault. This method better reflects changes in knocking elements, such as valves and crossheads, enabling more effective protection for piston machines.

In summary, the application of electronic vibration switches and transmitters not only offers technical advantages but also represents a cost-effective alternative to both mechanical switches and sensor-signal processing configurations.

Rahne Eric (PIM Ltd.) pim-kft.hu, gepszakerto.hu

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