Signal accelerometers

Fig.l shows the layout of the SPATE 9000 system. It basically consists of a scan unit connected to a signal amplifier. The signals are then correlated with a reference signal derived from a load transducer (e.g. strain gauge, load cell, accelerometer, or function generator). 

It is advisable in most of these cases to use accelerometers. Displacement probes will not give the high-frequency signals and velocity probes because their mechanical design is very directional and prone to deterioration. Figure 10-64 shows me signal from the various types of probes. 

Every accelerometer has a response curve of the type shown schematically in Figure 4-222. Instead of having an ideal linear response, a nonlinear response is generally obtained with a skewed acceleration for zero current, a scale factor error and a nonlinearity error. In addition, the skew and the errors vary with temperature. If the skew and all the errors are small or compensated in the accelerometer s electronic circuits, the signal read is an ideal response and can be used directly to calculate the borehole inclination. If not, modeling must be resorted to, i.e., making a correction with a computer, generally placed at the surface, to find the ideal response. This correction takes account of the skew,... 

Downhole Shocks Measurements. An accelerometer in the MWD telemetry tool measures transverse accelerations, or shocks, that may be damaging for the bottomhole assemblies. When acceleration exceeds a certain threshold, the event is signaled to the surface as being a shock. These events versus time or depth are displayed as shock count. This information is used as a warning against excessive downhole vibrations and to alert the driller to change the rpm or weight on the bit [106]. 

Sunstrand, Q-flex accelerometers, brochure, Sundstrand Inc., Redmond, WA, 1985. (Now, a division of Allied Signal Inc., Redmond, WA.)... 

The transducer most commonly used to obtain vibration measurements is an accelerometer. It incorporates piezoelectric (i.e., pressure-sensitive) films to convert mechanical energy into electrical signals. The device generally incorporates a weight suspended between two piezoelectric films. The weight moves in response to vibration and squeezes the piezoelectric films, which sends an electrical signal each time the weight squeezes it. 

Acceleration is perhaps the best method of determining the force resulting from machine vibration. Accelerometers use piezoelectric crystals or films to convert mechanical energy into electrical signals and Figure 43.23 is a schematic of such a device. Data acquired with this type of transducer are relative... 

Accelerometers use a piezoelectric crystal to convert mechanical energy into electrical signals. 

Observation of the accelerometer signals shows that the support motion, x, is considerably smaller in amplitude than the shear displacement, y. In addition, the driven block mass, correction term, w m, is small in comparison with the ratio F/Y for tests at frequencies well below resonance of the sample. 

In addition to the desired dependence on AC, it has a matching-dependent offset and gain that depends on parasitics. Any deviation of the reference capacitor Cref from the nominal value of the sense capacitance Cs appears as offset. Since in many applications AC is much smaller than C0, this offset often exceeds the signal. Offset cancellation should therefore occur early to minimize the dynamic range of the readout electronics. Care should also be taken for the trimming not to introduce a poor temperature coefficient. One solution fabricates the reference with the same process and in close proximity to the sense capacitor. The z axis accelerometer shown in Fig. 6.1.3 [7] utilizes two rnicromachined structures for the sense and reference. The suspension of the reference structure has been made intentionally stiff. 

Fig. 7.1.1 shows an example of variations in system partitioning and integration on very similar products. The Analog Devices single-chip accelerometer is in Fig. 7.1.1a and the Motorola or Bosch two-chip accelerometer is in Fig. 7.1.1b. Each has particular advantages. For example, the single chip solution may improve the signal-to-noise ratio and reduce the number of interconnects. Whereas the multiple chip system may have a better time-to-market as a result of the flexibility afforded by the separation of the MEMS processing from the circuit fabrica-...

As airbags gained more field experience it became apparent that the force and timing of deployments needed to be controlled for a wider range of crash types in order to avoid possible injury to passengers. This requirement increased the need to monitor the accelerometer signal for longer periods of time than previously required. 

With the advent of acceleration detection over longer timeframes or satellite sensors near the perimeter of the vehicle, problems began to surface with signal distortion from the accelerometers. These occurred due to several specific sources. 

When subjected to such high-g, high-frequency shocks the output of the accelerometer would be driven to full scale (or saturated) if the 400 Hz bandpass filter was not present. In theory the 400 Hz bandpass filter would suppress these signals so that they would contribute little to the output signal. But, as will be discussed, such ideal accelerometers are not easily produced. 

Noise within an accelerometer is created from several sources. The noise comes from the switched capacitor design, inherent thermal noise within any devices, the flickeT noise of transistors, and the Brownian noise in the g-cell transducer due to random motion of atoms (see equation 5). Signal-to-noise ratio of 60 dB is common, but lower g devices with higher gains may be worse. 

Government safety regulations have been the compelling market driver for crash-detection accelerometers. Initially, mechanical sensors were used at several locations within the car to obtain the crash signal. The MEMS accelerometer enabled... 

Quartz and piezoelectric ceramic crystals have more temperature independent constants than PVDF, so they are used for force and acceleration transducers. However, PVDF films can be used for large area flexible transducers. Their sensitivity to stress or strain allows the construction of pressure sensors (using the J33 coefficient), and accelerometers by mounting a seismic mass on the film. PVDF electrets are particularly suited for large area hydrophones (Fig. 12.21) that detect underwater signals. Their... 

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