PCB Test & Measurement Seite 160

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PCB PIEZOTRONICS INC 716 684 0001 Fax 716 685 3886 For Additional Specification Information Visit www pcb com 158 Technical Information Force ICP force sensors incorporate a built in MOSFET microelectronic amplifier This serves to convert the high impedance charge output into a low impedance voltage signal for analysis or recording ICP sensors powered from a separate constant current source operate over long ordinary coaxial or ribbon cable without signal degradation The low impedance voltage signal is not affected by triboelectric cable noise or environmental contaminants Power to operate ICP sensors is generally in the form of a low cost 24 to 27 VDC 2 to 20 mA constant current supply Page 144 schematically illustrates a typical ICP sensor system PCB offers a number of AC or battery powered single or multi channel power signal conditioners with or without gain capabilities for use with force sensors see Signal Conditioners Section of this catalog for available models In addition many data acquisition systems now incorporate constant current power for directly powering ICP sensors Because static calibration or quasi static short term response lasting up to a few seconds is often required PCB also manufactures signal conditioners that provide DC coupling Page 145 summarizes a complete 2 wire ICP system configuration In addition ICP Low Impedance Quartz Force Sensors to ease of operation ICP force sensors offer significant advantages over charge output types Because of the low impedance output and solid state hermetic construction ICP force sensors are well suited for continuous unattended force monitoring in harsh factory environments Also ICP sensor cost per channel is substantially lower since they operate through standard low cost coaxial cable and do not require expensive charge amplifiers Polarity The output voltage polarity of ICP force sensors is positive for compression and negative for tension force measurements ICP strain sensors have the opposite polarity The polarity of PCB charge output force sensors is the opposite negative for compression and positive for tension This is because charge output sensors are usually used with external charge amplifiers that exhibit an inverting characteristic Therefore the resulting system output polarity of the charge amplifier system is positive for compression and negative for tension same as for an ICP sensor system reverse polarity sensors are also available Why Can Only Dynamic Force be Measured with Piezoelectric Force Sensors The quartz crystals of a piezoelectric force sensor generate an electrostatic charge only when force is applied to or removed from them However even though the electrical insulation resistance is quite large the electrostatic charge will eventually leak to zero through the lowest resistance path In effect if you apply a static force to a piezoelectric force sensor the electrostatic charge output initially generated will eventually leak back to zero The rate at which the charge leaks back to zero is dependent on the lowest insulation resistance path in the sensor cable and the electrical resistance capacitance of the amplifier used In a charge output force sensor the leakage rate is usually fixed by values of capacitance and resistance in the low noise cable and external charge or source follower amplifier used In an ICP force sensor with built in electronics the resistance and capacitance of the built in circuitry normally determines the leakage rate When a rapid dynamic force is applied to a piezoelectric force sensor the electrostatic charge is generated quickly and with an adequate discharge time constant does not leak back to zero However there is a point at which a slow speed dynamic force becomes quasi static and the leakage is faster than the rate of the changing force Where is the point at which the force is too slow for the piezoelectric force sensor to make the measurement See the next section on Discharge Time Constant for the answer Discharge Time Constant DTC When leakage of a charge or voltage occurs in a resistive capacitive circuit the leakage follows an exponential decay A piezoelectric force sensor system behaves similarly in that the leakage of the electrostatic charge through the lowest resistance also occurs at an exponential rate The value of the electrical capacitance of the system in farads multiplied by the value of the lowest electrical resistance in ohm is called the Discharge Time Constant in seconds DTC is defined as the time required for a sensor or measuring system to discharge its signal to 37 of the original value from a step change of measurand This is true of any piezoelectric sensor whether the operation be force pressure or vibration monitoring The DTC of a system directly relates to the low frequency monitoring capabilities of a system and in the case of force monitoring becomes very important as it is often desired to perform quasi static measurements DTC Charge Output System In a charge output system the sensors do not contain built in amplifiers therefore the DTC is usually determined by the settings on an external charge amplifier A feedback resistor working together with a capacitor on the operational amplifier determines the time constant PCB s laboratory style charge amplifiers feature short medium and long time constant selections It is assumed that the electrical insulation resistance of the force sensor and cable connecting to the charge amplifier are larger than that of the feedback resistor in the charge amplifier otherwise drift will occur Therefore to assure this the force sensor connection point and cable must be kept clean and dry Low Frequency Response of ICP Systems With ICP force sensors there are two factors which must be considered when making low frequency measurements These are 1 The discharge time constant characteristic of the ICP force sensor 2 The discharge time constant of the AC coupling circuit used in the signal conditioner if DC coupling is used only 1 above needs to be considered It is important that both factors be readily understood by the user to assure accurate low frequency measurements 9 T Mcatalog 2011 Seite 125 166 SYN G500 21 04 11 18 04 Seite 164