Voltages, accelerometers

A simple form of accelerometer is shown in Fig. 6.23(a). During acceleration in the direction of poling the mass exerts a force, F (= M x ace11) on the piezoceramic disc and therefore a stress of F/A where A is the area of the disc. The measured voltage output is proportional to the acceleration. 

Most silicon accelerometers are based on a micromachined variable capacitance element (g-cell) that is converted to a voltage using a C-V converter and then amplified, filtered, and buffered to provide an analog output as shown in Fig. 7.1.4. To date, open-loop implementations for capacitive read-out circuits are more widely employed than closed-loop systems, primarily as a result of the stability of such systems [16]. Interface electronics for micromachined sensors depend not only upon the transduction technique (input specification) and the product requirements (output specification) but also on the packaging approach, as parasit-ics are introduced when a multiple-die packaging technique is used. 

To achieve vibration damping, the piezo actuator can be combined with an accelerometer [18]. A first solution consists of using a piezoelectric actuated proof mass damper (Fig. 6.25), in which the compliance of the proof mass corresponds to the piezoelectric compliance. The force provided by the piezo actuator is F = N V, where N is the force factor and V the applied voltage. This method is generally adapted to high frequency mode (e. g. 100. .. 400 Hz), as it remains difficult to build a piezo proof mass (PPM) at low frequency. 

Currently, quartz is often utilized in accelerometers. Due to their high piezoelectric voltage coefficient lithium sulfate and tourmaline are often applied in commercial hydrophones especially to measure shock and pressure waves. Rochelle salt can be found in acoustic pickups and special devices to measure acoustic pressure. Due to their long-term stable piezoelectric properties natural crystals are in particular perfect for sensor applications where the monitoring of a quantity has to be made over long periods [85]. 

The major advantages of MEMS sensors for measurement of stmcture seismic response are their small size, low power consumption, and low price in contrast with their high accuracy in measurement. For example, a typical price of MEMS accelerometers is less than 10 USD. In spite of this low cost, a MEMS accelerometer with 3 axes, 2 g full scale, 5 mm x 5 mm x 2 mm size, 1.1 mA current consumption at the input voltage of 2.7 V, and noise density of 175 fig/v/Hz is available off the shelf. 

The mass motion has to be measured and recorded by some device. In early accelerometers, a light beam was reflected in a mirror which rotated with the mass motion. Presently, almost all seismic accelerometers use a capacitive transducer, which gives a voltage output proportional to the mass displacement. This type of transducer is very sensitive - it can resolve displacements of the order of pm (10 mm). 

The output of a seismic sensor, a seismometer or an accelerometer, is a time-varying voltage, which is related to the ground motion by a differential equation in the time domain or by a transfer function in the frequency domain. This transfer function or response function is characterized by a number of parameters, which are assumed to be constant, at least in the short term. 

Output signals from the shear force sensors and accelerometers were digitized using a DAQ system (Model PCI-3101 board connected to a Model STA-300 screw terminal accessory using a CAB 305 cable, Keithley Instruments, Inc.). The sample rate of the equipment is 4000 samples s which, at a frequency of 208.4 Hz, corresponds to approximately 20 samples cycle. A Microsoft Visual Basic program was used to process and convert the acceleration and force output signals in voltages (V) to units of acceleration (G) and Newtons (N), respectively. 

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