PCB Test & Measurement Seite 148

Hinweis: Dies ist eine maschinenlesbare No-Flash Ansicht.
Klicken Sie hier um zur Online-Version zu gelangen.

Inhalt

PCB PIEZOTRONICS INC 716 684 0001 Fax 716 685 3886 For Additional Specification Information Visit www pcb com 146 Introduction to Piezoelectric Sensors Upon removal of the measurand the output signal will dip below the sensor bias voltage by the same amount that it has discharged Then it will charge toward the sensor bias voltage level until reaching a steady state For a minimum 1 measurement accuracy the discharge time constant should be at least 100 times the duration of a square wave event 50 times the duration of a half ramp and 25 times the duration for a half sine pulse Longer time constants will improve measurement accuracy Effect of Coupling on Low frequency Response As previously mentioned if the constant current signal conditioner shown in Figure 5 is DC coupled the low frequency response of the system is determined only by the sensor DTC However since many signal conditioners are AC coupled the total coupling DTC may be the limiting factor for low frequency measurements For example Figure 7 illustrates typical AC coupling through a 10 µF coupling capacitor built into many constant current signal conditioners Assuming a 1 megohm input impedance on the readout instrument not shown the coupling time constant simply equals R times C or 10 seconds This also assumes a sensor output impedance of 100 ohms As a general rule keep the coupling time constant at least 10 times larger than the sensor time constant When acquiring low frequency measurements low input impedance tape recorders and other instruments will reduce the coupling time constant significantly For such cases use a signal conditioner which incorporates DC coupling or a buffered output Methods of DC Coupling To take full advantage of the sensor DTC especially during static calibration it is often essential to DC couple the output signal The simplest method is to use a signal conditioner which incorporates a DC coupling switch However standard signal conditioners may also be adapted for DC coupling by using a T connector as in Figure 17 The important thing to keep in mind is that the readout instrument must have a zero offset capability to remove the sensor bias voltage If the readout is unable to remove all or a portion of the bias voltage a current limited bucking battery or variable DC power supply placed in line with the signal may be used to accomplish this task It is imperative that any opposing voltage be current limited to avoid potential damage to the sensor s built in circuitry For convenience several constant current signal conditioners manufactured by PCB incorporate level shifting circuits to allow DC coupling with zero volts output bias Most of these units also feature an AC coupling mode for drift free dynamic operation Cautions These precautionary measures should be followed to reduce risk of damage or failure in ICP sensors 1 Do not apply more than 20 mA constant current to ICP sensors or in line amplifiers 2 Do not exceed 30 VDC supply voltage 3 Do not apply voltage without constant current protection Constant current is required for proper operation of ICP sensors 4 Do not subject standard ICP sensors to temperatures above 250 F 121 C Consult a PCB Applications Engineer to discuss testing requirements in higher temperature environments 5 Most ICP sensors have an all welded hermetic housing However due to certain design parameters certain models are epoxy sealed In such cases high humidity or moist environments may contaminate the internal electronics In such cases bake the sensors at 250 F 121 C for one or two hours to evaporate any contaminants 6 Many ICP sensors are not shock protected For this reason care must be taken to ensure the amplifier is not damaged due to high mechanical shocks Handle such sensors with care so as not to exceed the maximum shock limit indicated on the specification sheet 9 T Mcatalog 2011 Seite 125 166 SYN G500 21 04 11 18 04 Seite 152