IMI Industrial Vibration Sensors Katalog Seite 172

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Technical Information Pressure Sensors Typical Piezoelectric System Output Piezoelectric pressure sensors measure fast responding microsecond dynamic pressure events They are not suited for longer static events Dynamic pressure measurements including turbulence blast ballistics and engine combustion may require sensors with special capabilities Fast response ruggedness high stiffness extended ranges and the ability to also measure quasi static pressures are standard features associated with PCB quartz pressure sensors The following information presents some of the design and operating characteristics of PCB pressure sensors to help you better understand how they function which in turn helps you make better dynamic measurements Types of Pressure Sensors Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various applications IMI manufactures two types of piezoelectric sensors Charge mode pressure sensors generate a high impedance charge output ICP Integrated Circuit Piezoelectic voltage mode type sensors feature built in microelectronic amplifiers which convert the high impedance charge into a low impedance voltage output ICP is a registered trademark of PCB Group Inc Why Can Dynamic Pressure Only Be Measure with Piezoelectric Pressure Sensors The quartz crystals of a piezoelectric pressure sensor generate a charge when pressure is applied However even though the electrical insulation resistance is quite large the charge eventually leaks to zero The leakage rate is dependent on the electrical insulation resistance In a pressure sensor with built in ICP electronics the resistance and capacitance of the crystal and the built in ICP electronics normally determine the leakage rate In a charge mode pressure sensor used with a voltage amplifier the leakage rate is fixed by values of capacitance and resistance in the sensor by low noise cable and by the external source follower voltage amplifier used In the case of a charge mode pressure sensor used with a charge amplifier the leakage rate is fixed by the electrical feedback resistor and capacitor in the charge amplifier The output characteristic of piezoelectric pressure sensor systems is that of an AC coupled system where repetitive signals decay until there is an equal area above and below the original base line As magnitude levels of the monitored event fluctuate the output remains stabilized around the base line with the positive and negative areas of the curve remaining equal The graph right represents an AC signal following this curve Output from sensors operating in DC mode follow this same pattern but over an extended time frame associated with system discharge time constant values Assume that a 0 to 3 volt output signal is generated from an AC coupled pressure application with a one second steady state pulse rate and one second between pulses The frequency remains constant but the natural decay associated with a piezoelectric sensors will cause the signal to quickly decay negatively until the signal centers around the original base line Eventually the signal reaches an equilibrium point where the area above the baseline equals the area below the area A area B The original output signal remains the same though one is now reading a peak to peak output from 1 Volt to 1 Volt instead of an output from 0 to 3 Volts 3 1 1 3 Repetitive Pulse AC Signal Incorrect Correct Flush Mount Pressure Alignment Pressure Sensor Clamp Nut Off center Side Wall Strain Off center Side Wall Strain Pressure Sensor Apply Recommended Torque to Clamp Nut Technical Information visit us online at www imi sensors com 1 716 684 0003 169