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Automation Technologies 5/2014

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Automation Technologies 5/2014

Sensors and Measurement

Sensors and Measurement Quo vadis pressure switch – mechanical or electronic? Enrico Bossart For decades, many manufacturers and users in the machinebuilding industry have trusted the proven mechanical pressure switch technology for the control of fluid circuits. Although the technically superior electronic pressure switch has been winning market share for many years, the extinction of the mechanical pressure switch is still not in sight. This article attempts to unscramble this paradox and explain why even today one cannot imagine the market without mechanical pressure switches. Author: Enrico Bossart, Dipl.-Ing. (BA), Product Manager, Wika Alexander Wiegand SE & Co. KG, Klingenberg/Germany AUTOMATION TECHNOLOGIES 5/2014

Sensors and Measurement next page Pressure switches have always had an important role in the control of fluid circuits, especially those containing hydraulic, pneumatic or cooling fluids. But what is meant by pressure switches and why are they so essential for the control of fluid circuits? In its simplest definition, a pressure switch is a measuring device that converts a measured system pressure into an electrical switching signal which can then be further processed by a control system (PLC). Independently of whether it is a mechanical or an electronic switch, a pressure switch always outputs a binary signal. In practice, a pressure switch is inherently inactive until a set switch point has been reached (e.g. pressure = 10 bar), at which the pressure switch outputs the switch signal and thus changes its switching status. Put simply, a pressure switch opens or closes an electrical circuit (depending on its design and/or setting) when it reaches a defined switching point. How do mechanical pressure switches work? Despite their identical mode of operation (opening/ closing an electrical circuit), mechanical and electronic pressure switches differ significantly in their design and how the switch points and switching functions can be set. Mechanical pressure switches convert the process pressure, via a mechanical motion, into an electrical signal. In the design of a mechanical pressure switch it can be seen that the process pressure mechanically transmits the motion of a measuring element to a plunger, thereby actuating a switch contact. The counter-force of a spring (adjustable via a grub screw) acts against this process pressure, respectively against the force of the process pressure and thereby against the plunger movement. This counter-force corresponds to the set pressure limit or the switch point at which the plunger mechanically opens or closes the contact and thus activates the defined switching signal. 01 Mechanical OEM pressure switch (model PSM01) (left); design of a mechanical pressure switch, simplified (right) Functioning of electronic switches Electronic pressure switches, however, do not require any mechanical motion by a contact to generate a switching signal. The process pressure mechanically deforms a diaphragm, and with it a resistive sensor element, within the pressure connection of the electronic pressure switch. The sensor element generates a signal from the deformation which is electronically measured, processed in a microcontroller and then converted into an electrical switching signal. Switch point, reset point and various other switching functions can be set flexibly on the pressure switch itself through buttons and a display, through external software or through a configuration module. An electronic switch is, therefore, more flexible in operation than a mechanical pressure switch and can be specifically matched to the respective application. AUTOMATION TECHNOLOGIES 5/2014

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