Page 116 - Instrumentation Reference Book 3E

Sensor practice 101
The sensors were adopted in early seismology compliance are fixed. Consideration of the
studies because of their inherently high output at F = ria . a law and spring compliance shows that
relatively low velocities. The coil impedance will displacement of the mass relative to the sensor
generally be low, enabling signals with good sig- case is proportional to the acceleration of the
nal-to-noise ratios to be generated along with case. This means that the curves, plotted in Figure
reduction of error caused by variations in lead 6.6 (for sinusoidal input of force to a second-order
length and type. They are, however, large with system), are also applicable as output response
resultant mass and rigidity problems. They tend curves of accelerometers using displacement sen-
to have relatively low resonant frequencies (tens sing. In this use the vertical axis is interpreted as
of hertz), which restricts use to the lower frequen- the relative displacement of the mass from the
cies. Output tends to be small at the higher fre- case for a given acceleration of the case.
quencies. It will be apparent that these sensors The curves show that a seismic sensor will pro-
cannot produce signals at zero velocity because vide a constant sensitivity output representing
no relative movement occurs to generate flux- sensor acceleration from very low frequencies to
cutting. near the natural frequency of the spring-mass
A second variation of the self-generating velo- arrangement used. Again. the damping ratio can
city sensor, the variable-reluctance method, uses be optimized at around 0.5-0.6 and electronic
a series magnetic circuit containing a permanent compensation added (if needed) to raise the upper
magnet to provide permanent magnetic bias. A limit a little further than the resonance point.
part of this circuit, the armature: is made so that At first sight it might, therefore, appear that a
the effective air-gap is varied by the motion to be single, general-purpose design could be made
monitored. Around the magnetic circuit is placed having a very high resonant frequency. This,
a pick-up coil. As the armature moves the result- however, is not the case for the deflection of the
ing flux variation cuts the coil, generating a signal spring (which is a major factor deciding the sys-
that can be tailored by appropriate design to be tem output sensitivity) is proportional to ULO';.
linear with vibration amplitude. This form of In practice this means that as the upper useful
design has the advantage that the armature can frequency limit is extended the sensor sensitivity
readily be made as part of the structure to be falls off. Electronic amplification allows low
monitored, as shown in Figure 6.15(b). This ver- signal output to be used but with additional cost
sion is not particularly sensitive, for the air-gap to the total measuring system.
must be at least as large as the vibration ampli- At the low-frequency end of the accelerometer
tude. As an example a unit of around 12mm response the transducers become ineffective
diameter, when used with a high magnetic per- because the accelerations produce too small a
meability moving disc set at 2mm distance, will displacement to be observed against the back-
produce an output of around 150 m Vim s-l. ground noise level.
A third method uses a permanent magnet as
the mass supported on springs. One example is 6.3.4. I Typical sensors
shown in Figure 6.15(c). Vibration causes the
magnet .to move relatively to the fixed coil As a guide to the ranges of capability available.
thereby generating a velocity signal. This form one major manufacturer's catalogue offers accel-
can produce high outputs, one make having a erometers with sensitivities ranging from a small
sensitivity of around 5 V/m s-'. 30 pV/m-' through to I Vims-* with corres-
Where a fixed reference cannot be used this ponding sensor weights of 3 g and 500 g and use-
form of sensor, instead of a displacement sensor, ful frequency ranges of 1-60 000 Hz and
can be built into the seismic sensor arrangement. 0.2-1000 Hz. Sensors have been constructed for
In such cases the vibrating seismic sensor will even higher frequencies but these must be regarded
then directly produce velocity signals. These will as special designs. A selection is shown in Figure
follow the general responses given in Figure 6.14. 6.16.
From those curves it can be seen that there is The many constraints placed upon the various
a reasonably Rat response above the natural performance parameters of a particular seismic
frequency which is inherently quite low. sensor can be shown on a single chart such as
Figure 6.17, Harris and Crede (1961).
As the accelerometer spring is often required to
be stiff compared with that of the seismic displace-
6.3.4 Acceleration measurement ment sensor it will not always need to make use of
The fixed-reference method of measuring acceler- coiling, a device for decreasing the inherent
ation is rarely used. most determinations being spring constant of a material. Accelerometer
made with the seismic form of sensor. For the springs may occur as stamped rigid plates, as flat
seismic sensor system the mass and the spring cusped spring washers, or as a sufficiently com-