Measurements should be made to produce
the data needed to draw meaningful conclusions from the system under test. These data can
be used to minimize or eliminate the vibration and thus the resultant noise. There are
also examples where the noise is not the controlling parameter, but rather the quality of
the product produced by the system. For example, in process control equipment, excessive
vibration can damage the product, limit pro-cessing speeds, or even cause catastrophic
machine failure. The basic measurement system used for diagnostic analyses of vibrations
consists of the three system components shown in Figure 9.
Figure 9 Basic Vibration
In general, the transducers
employed in vibration analyses convert mechanical energy into electrical energy; that is,
they produce an electrical signal which is a function of mechanical vibration. In the
following section, both velocity pickups and accelerometers mounted or attached to the
vibrating surface will be studied.
(a) Velocity Pickups
The electrical output signal of a
velocity pickup is proportional to the velocity of the vibrating mechanism. Since the
velocity of a vibrating mechanism is cyclic in nature, the sensitivity of the pickup is
expressed in peak milli-volts/cm/s and thus is a measure of the voltage produced at the
point of maximum velocity. The devices have very low natural frequencies and are designed
to measure vibration frequencies that are greater than the natural frequency of the
Velocity pickups can be mounted in a
number of ways; for example, they can be stud-mounted or held magnetically to the
vibrating surface. However, the mounting technique can vastly affect the pickup's
performance. For example, the stud-mounting technique shown in Figure 10(a), in which the
pickup is mounted flush with the surface and silicone grease is applied to the contact
surfaces, is a good reliable method. The magnetically mounted pick-up, as shown in Figure
10(b), on the other hand, in general has a smaller usable frequency range than the
stud-mounted pickup. In addition, it is important to note that the magnetic mount, which
has both mass and spring like properties, is located between the velocity pickup and the
vibrating surface and thus will affect the measurements. This mounting technique is
viable, but caution must be employed when it is used.
Figure 10 Two Transducer Mounting
(a) Stud-Mount Pickup; (b) Magnetically
Held Velocity Pickup
The velocity pickup is a
useful transducer because it is sensitive and yet rugged enough to withstand extreme
industrial environments. In addition, velocity is perhaps the most frequently employed
measure of vibration severity. However, the device is relatively large and bulky, is
adversely affected by magnetic fields generated by large ac machines or ac current
carrying cables, and has somewhat limited amplitude and frequency characteristic.
The accelerometer generates an output
signal that is proportional to the acceleration of the vibrating mechanism. This device
is, perhaps, preferred over the velocity pickup, for a number of reasons. For example,
accelerometers have good sensitivity characteristics and a wide useful frequency range;
they are small in size and light in weight and thus are capable of measuring the vibration
at a specific point without, in general, loading the vibrating structure. In addition, the
devices can be used easily with electronic integrating networks to obtain a voltage
proportional to velocity or displacement. However, the accelerometer mounting, the
interconnection cable, and the instrumentation connections are critical factors in
measurements employing an accelerometer. The general comments made earlier concerning the
mounting of a velocity pickup also apply to accelerometers.
Some additional suggestions for
eliminating measurement errors when employing accelerometers for vibration measurements
are shown in Figure 11. Note that the accelerometer mounting employs an isolation stud and
an isolation washer. This is done so that the measurement system can be grounded at only
one point, preferably at the analyzer. An additional ground at the accelerometer will
provide a closed (ground) loop which may induce a noise signal that affects the
accelerometer output. The sealing compound applied at the cable entry into the
accelerometer protects the system from errors caused by moisture.
Figure 11 Mounting
Technique for Eliminating Selected Measurement Errors
The cable itself should be glued or
strapped to the vibrating mechanism immediately upon leaving the accelerometer, and the
other end of the cable, which is connected to the preamplifier, should leave the mechanism
under test at a point of minimum vibration. This procedure will eliminate or at least
minimize cable noise caused by dynamic bending, compression, or tension in the cable.
The second element in the
vibration measurement system is the preamplifier. This device, which may consist of one or
more stages, serves two very useful purposes: it amplifies the vibration pickup signal,
which is in general very weak, and it acts as an impedance transformer or isolation device
between the vibration pickup and the processing and display equipment.
Recall that the manufacturer provides
both charge and voltage sensitivities for accelerometers. Likewise, the preamplifier may
be designed as a voltage amplifier in which the output voltage is proportional to the
input voltage, or a charge amplifier in which the output voltage is proportional to the
input charge. The difference between these two types of preamplifiers is important for a
number of reasons. For example, changes in cable length (i.e., cable capacitance) between
the accelerometer and preamplifier are negligible when a charge amplifier is employed.
When a voltage amplifier is used however, the system is very sensitive to changes in cable
capacitance. In addition, because the input resistance of a voltage amplifier cannot in
general be neglected, the very low frequency response of the system may be affected.
Voltage amplifiers, on the other hand, are often less expensive and more reliable because
they contain fewer components and thus are easier to construct.
(iii) Processing and display
The instruments used for the
processing and display of vibration data are, with minor modifications, the same as those
described earlier for noise analyses. The processing equipment is typically some type of
spectrum analyzer. The analyzer may range from a very simple device which yields, for
example, the rms value of the vibration displacement, to one that yields an essentially
instantaneous analysis of the entire vibration frequency spectrum. As discussed earlier,
these analyzers, which are perhaps the most valuable tool in a vibration study, are
typically either a constant-bandwidth or constant-percentage-bandwidth type of device.
They normally come equipped with some form of graphical display, such as a cathode ray
tube, which provides detailed frequency data.