Specifications for the ICP force sensors shown in this catalog list the DTC for each force sensor. The DTC is fixed by the components in the ICP sensors internal amplifier. 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. 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. 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. In a force sensor with built-in ICP electronics, the resistance and capacitance of the built-in ICP electronics normally determines the leakage rate. In a charge mode 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. 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 effect, if you apply a static force to a piezoelectric force sensor, the electrostatic charge output initially generated will eventually leak back to zero. However, even though the electrical insulation resistance is quite large, the electrostatic charge will eventually leak to zero through the lowest resistance path. The quartz crystals of a piezoelectric force sensor generate an electrostatic charge only when force is applied to or removed from them. "Low noise" cables have a special graphite lubricant between the dielectric shield which minimizes the triboelectric effect.įigures 2 and 3 show a typical charge amplifier system schematic including: sensor, low noise cable, and charge amplifier. This is referred to as "triboelectric noise" and cannot be distinguished from the sensor's crystal electrostatic output. Standard, two-wire or coaxial cable when flexed, generates an electrostatic charge between the conductors. Use of special "low noise" cable is required with charge mode force sensors. Environmental contaminants such as moisture, dirt, oil, or grease can all contribute to reduced insulation, resulting in signal drift and inconsistent results. Consequently, any connectors, cables or amplifiers used must also have a very high insulation resistance to maintain signal integrity. The internal components of the force sensor and the external electrical connector maintain a very high (typically 10el3 ohm) insulation resistance so that the electrostatic charge generated by the crystals does not "leak away". When considering the use of charge mode systems, remember that the output from the crystals is a pure electrostatic charge. Quartz charge mode force sensors with fallen insulators can be used at operating temperatures up to 400☏ (204☌). Miniature in-line amplifiers are generally of fixed range and frequency. PCB's "electrostatic" charge amplifiers have additional input adjustments for quasi static measurements, static calibration and drift-free dynamic operation. Laboratory charge amplifiers provide added versatility for signal normalization, ranging and filtering. The primary function of the charge or voltage amplifier is to convert the high impedance charge output to a usable low impedance voltage signal for recording purposes. Connection from the sensor directly to a readout device such as an oscilloscope is possible for high frequency impact indication, but is not suitable for most quantitative force measurements. This high impedance charge must be routed through a special "low noise" cable to an impedance converting amplifier such as a laboratory charge amplifier or source follower for recording purposes. A charge mode piezoelectric force sensor, when stressed, generates a high electrostatic charge from the crystals.
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