Page 33 - MetalForming November 2010

the length of time the valve remains open when the lubricant is released— referred to as pressure drop. Here, the sensor tells the controller that the valve has opened a certain amount, allowing a predetermined amount of lubricant pressure drop to occur.
Each pressure sensor is plumbed into the pressurized tank or line. The lubri- cant exerts a force against the sensor’s measuring element—typically of stain- less steel or ceramic. Ceramic proves to be very stable and consistent over time. If overpressure does occur, causing the sensor to reach its burst pressure, a ceramic sensor will simply fail rather than provide erroneous readings. This design ensures the integrity of the sen- sor’s results. Steel measuring elements, which typically have higher burst ratings than do ceramic elements, are more prone to zero and span offsets, which can lead to erroneous readings.
The measuring cell connects to the sensor’s internal electronics, which con-
vert the measured pressure into an elec- trical signal output. The press control system, interpreting the output, can either convey the error state on an external display for the operator to see, or it can halt operations if an error occurs and prevent further rejects and equipment damage while allowing operators to quickly identify and address the problem.
Along with increased efficiency and extended die life, stampers can rely on sensors to monitor static and dynamic lubricant pressure to overcome several common setup errors. For example, pres- sure sensors can identify if a valve is not opening for the correct duration (or if it’s opening at all); if the wrong lubri- cant or lubricant mixture is being used; or if the amount of lubricant being applied is inadequate for the application.
Sensor Selection
Users can select pressure sensors that use either a dual switching output
that provides simple on/off informa- tion, or an analog output for scalable results based on the continuous flow of information. Dual-setpoint sensors use one setpoint to monitor static pressure and the other to monitor dynamic pressure. Since analog sensors provide continuous output, one signal monitors both static and dynamic pressure.
For stamping-die applications, which locate the pressure sensor near the lubri- cant, metalformers typically specify a pressure sensor rated to IP67 to defend against lubricant ingress. An IP (ingress protection) number specifies the envi- ronmental protection capability of enclosures surrounding electronic equipment. The first number, “6,” iden- tifies the enclosure as offering total pro- tection from dust particles of any size; the second number, “7,” identifies the enclosure as protecting against the effect of immersion to a depth between 15 cm
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