Sensor principles in Software Access Code 128 Code Set C in Software Sensor principles

Sensor principles generate, create barcode standards 128 none on software projects iPhone OS Voltage probes Planar electrode Figure 4.17 Piezoelectric transducer. Piezoelectric transd ucers Piezoelectric transducers, shown in Figure 4.17, transform a force into an electric polarization charge and a resulting measurable potential difference. The force in the x-direction Fx, acting on the electrodes creates a charge, Q, on the crystal (ceramic) electrodes.

A tensor determines the force strain relationship, which depends on the material properties. For this particular case (force in the x-direction, electrodes in the y z plane), the resulting charge Q due to force is Q d31 Fx ; (4:43). where d31 is the pie Software Code 128C zoelectric strain coefficient. The voltage signal, V, for force, Fx, applied to electrodes of area, A, and electrode separation, t, is given by V Q=C d31 Fx =C d31 Fx t=""0 A; (4:44). where " is the relat ive permittivity of the material and "0 is the permittivity of free space. For the typical piezoelectric material, lead-zirconate-titanate (PZT), d31%10 10 C/N. Also, the elastic modulus E % 1010 N/m2 determines the displacement of the material system for a given pressure.

Thus, this yields a displacement sensitivity of % 1 V/A. This displacement sensitivity enables high resolution for applications where extremely low compliance (high elastic modulus) is not a design limitation. Piezoelectric devices provide poor response at frequencies below 1 Hz since their material dielectric time constant is short, approximately 1 s.

Thus, low-frequency signals encounter a highpass filter and are not observed.. Example 4.7 Vibratio code 128 barcode for None n accelerometer Figure 4.18 depicts an example of a piezoelectric vibration accelerometer.

Here, a proof mass is supported by a piezoelectric element. A voltage signal appears across this element under applied acceleration (in the z-direction) since the element must supply a force to the proof mass, inducing a potential difference due to the piezoelectric effect. With a proof mass M 10 g, a typical geometry yields 100 nV signal for 10 6g acceleration.

The acceleration. 4.5 Motion and force sensors Proof mass Piezoelectric element z Inertial reference frame Figure 4.18 Vibration accelerometer. Capacitance 2 Suspension Capacitance 1 Figure 4.19 Cross-sectional view of capacitive measurement of proof mass displacement. response is flat wit h frequency above the low-frequency cutoff. Spring mass systems by contrast have a response that is frequency dependent..

Capacitive position code-128c for None sensor Capacitive position sensors measure the variation in the displacement current due to a change in capacitor gap. Consider Figure 4.19 in which an accelerometer is designed to measure accelerations in the z-direction.

Capacitive plates can be used to measure the gap value, D, directly. This method shows high-resolution measurement capability, but requires differential measurement architectures to ensure linearity and low drift. Additionally, in contrast to piezoresistive/piezoelectric methods one may design the system with independent control of position sensor sensitivity and structure compliance.

Thus in both the accelerometer and gyroscope examples considered earlier the resonant frequency of the proof mass system can be made independent of position measurement. There are, however, important scaling issues. First, the reduction in capacitance area with decreasing sensor scale results in the reduction of the capacitor displacement current, thus producing reduced responsivity and reduced sensitivity.

Second, the input capacitance of capacitance detection electronics limits responsivity. p For conventional (large-scale) capacitive detectors, 10 4 A= Hz sensitivity has been demonstrated for applications in large-scale pressure sensors, accelerometers, and seismometers. One such structure is illustrated in Figure 4.

20. In such systems, sensitivity (noise) is determined by the measurement front end. First, by operating at high frequency, 1/f flicker noise is eliminated from the measurement.

Second, using a transformer differential signal generator, linear response is obtained. Further, the feedback system results in an output always at null, eliminating gain errors..

Sensor principles Bias signal source ac coupled gain stage Output signal Phase sensitive detector Force rebalance network Figure 4.20 Blumlein capacitance bridge. t ac coupled gain st age Output signal Phase sensitive detector V Switched capacitor drive Output amplifier. Switched capacitor drive Figure 4.21 Integrated capacitance bridge. Additionally, the fo Software barcode standards 128 rce rebalance output is immune to oscillator amplitude and phase drift. The precision and stability rely only on maintaining transformer symmetry. Unfortunately, this requires a large, stable, shielded (primary secondary capacitive isolation) transformer.

An alternative is to employ an integrated capacitance bridge, as illustrated in Figure 4.21. This makes use of a switched capacitor drive, which modulates the desired signal away from dc, reducing 1/f noise.

Such circuits are compatible with complementary metal oxide semiconductor (CMOS) implementation and may be directly integrated with MEMS mechanical structures. Sensor system errors persist due to drift in drive amplitude and to electrostatic force imbalance induced in the measurement process. Careful use of feedback is also required for control of the switched capacitor drivers.

The resolution is defined by the ratio of the drive voltage to the amplifier front end noise. Resolutions approaching 106 have been demonstrated in MEMS devices. Stray capacitance to the substrate may limit the applicability of the capacitive position sensor, in general requiring careful attention to structural design.

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