Transducers accelerometers

The resulting deflections are measured at the centre of the applied load (through a hole in the centre of the loading plate) and at various distances away from the load with sensors. The sensors may be displacement transducers, velocity transducers, accelerometers, geophones or even seismometers. 

Piezoelectric materials are pressure-sensitive materials. When subject to pressure (stress) they become polarized and produce an electric field. The prefix piezo- comes from the Greek word pressure. The reverse is true when these materials are subject to an electric field the result is a mechanical strain and displacement. These materials have many industrial applications as transducers, accelerometers, and sensors, and have found use in robotic and biomedical applications. Composites of these materials are also known as smart materials and are used in adaptive structures. 

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). 

Since acceleration is the second derivative of displacement, a piezoelectric accelerometer sensor with an integrator becomes a velocity transducer. This arrangement is gradually superseding the self-generating mo ing-coil velocity sensor (where a coil of wire moves relative to a magnetic field). 

This aspect is not included here, but is related to optical flow diagnostics. It is based again on the principle of the optical Doppler effect. Multifunctional equipment is available for noncontact measurements of flow-induced vibration on surfaces of structural elements, for acoustic measurements, and for calibration of accelerometers and vibration transducers. 

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. 

Three basic types of vibration transducers that can be used for monitoring the mechanical condition of plant machinery are displacement probes, velocity transducers, and accelerometers. Each has limitations and specific applications for which its use is appropriate. 

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... 

The best method of ensuring that the point of measurement, its orientation, and the compressive load are exactly the same each time is to permanently or hard mount the transducers, which is illustrated in Figure 43.26. This guarantees accuracy and repeatability of acquired data. However, it also increases the initial cost of the program. The average cost of installing a general-purpose accelerometer is about 300 per measurement point or 3000 for a typical machine-train. 

Cantilever bimorphs (see Section 6.5.2 below) with small masses attached to their free end can be used as accelerometers provided that the frequency of the vibrations to be detected is well below the resonance frequency of the transducer. 

Strain gauges and pressure transducers were attached at (k) 15.25 m (ft) intervals along the pipe and an accelerometer at (1) 29.3 m (ft) below the surface. The barge and pipe system, weighing a total of (m) 81,648 kg (ton) was anchored by (n) 22,680 kg (lb) concrete blocks on either side. A cable system held the barge in place. 

The complex mechanical impedance can be obtained by measuring force, velocity, and their phase difference using probes or pickups attached to the transducer at suitable points [144], This method is very convenient with solids [52] (sometimes the most accurate), but not with liquids, although in this case it can be a convenient method of checking the performance of a transducer. The output from an accelerometer or strain-sensitive pickup gives information on resonant frequency and vibrational amplitude. However subsequent correlation with the power transmitted to the sonicated medium is not straightforward. 

K. Chau, S. Lewis, Y. Zhao, R. Howe, S. Bart, R. Marcheselli, An integrated force-balanced capacitive accelerometer for low-g applications, in Tech. Dig. 8th Ini. Conf. Solid-State Sensor and Actuators (Transducers 95), Stockholm, Sweden, June 1995, 593-596. 

J.C. Cole, A new sense element technology for accelerometer subsystems, in Tech. Dig. 6th Int. Conf. Solid-State Sensors and Actuators (Transducers 91), San Francisco, CA, June 1991, 93-96. 

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. 

Detailed direct experimental evidence for the frictional behavior during the slip part of a stick-slip cycle is found in the work reported by Ko and Brockley [17]. The rider and its elastic coupling were fitted with the following transducer devices (a) a strain gage to measure the displacement of the rider from its null position (b) an electromagnetic transducer to measure the velocity of the rider during vibration or transport and (c) an accelerometer to measure the acceleration of the rider. The data obtained with this instrumentation were used in the equation for stick-slip (c(S. Eqn 8-18) ... 

Figure 7-9. Spring and lever apparatus for friction. A Disk. B, B Rider. C Cantilever beam. D Strain gage. E Accelerometer. F Velocity transducer. G Speed reducer and turntable. H Backlash take-up W Loading weights. After Ko and Broc kley [8 ]. 

Test and measurement sensors, pressure transducers, torque sensors, instrumentation, accelerometers, and displacement sensors An extensive array of instruments and systems for applications ranging from automotive R D, process and environmental monitoring, in vitro medical diagnostics Surface plasmon resonance biosensors for the determination of the affinity and kinetics of biomolecular interactions... 

Multiaxis accelerometers can be employed to measure both linear and angular accelerations (if multiple transducers are properly configured). Velocity and position data may then be derived through numerical integration although care must be taken with respect to the selection of initial conditions and the handling... 

In order to construct a magnitude and phase vs. frequency plot of the transfer function, the nondimensional time will be converted back to real time for use on the frequency axis. For the conversion to real time the following physical variables will be used po = 1350 kg/m, b = 15 p.m, and /Hf = 0.85 mPa/sec. The general frequency response is shown in Figure 64.4. The flat response from DC up to the first corner frequency establishes this system as an accelerometer. This is the range of motion frequencies encountered in normal motion environments where this transducer is expected to function. 

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... 

The strain transducers and accelerometers and a pile driving analyzer is shown in Figure 11.7. The pile driving analyzer monitors the output from the strain transducers and accelerometers as the pile is being driven, and evaluates this data as follows ... 

The accelerometer is a transducer designed to measure vertical acceleration and used to establish the inertial reference and effectively the response of the vehicle to the road surface (instant height of accelerometer in the host vehicle). The vertical distance between the accelerometer and the travelled surface is measured by displacement transducers or sensors (laser, acoustic or infrared sensors) and the distance travelled by distance transducers. Eigure 16.29 shows an accelerometer and a single sensor mid-mount on a vehicle. 

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