Confiabilidad

Velocity sensing devices are either seismic pickups or fixed reference instruments including electrodynamic transducers. In an electrodynamic transducer, a coil moves through the magnetic field produced by a stationary permanent magnet. The transducer can also be designed with a stationary coil and the permanent magnet core moving within the coil. The principle of operation is the same.

When the core moves, magnetic lines of the field created by the core cross the turns. The electromotive force induced in the turns is proportional to the speed of the core. The unit thus produces a signal directly proportional to vibration

velocity. It is self-generating and needs no conditioning electronics in order to operate, and it has relatively low electrical output impedance making it fairly insensitive to noise induction.

A vibration pickup consists of a seismic mass supported by two membranes, so that part of the mass lies within the air gap of a magnetic circuit.

The velocity pickup is a seismic instrument fastened to a vibrating structure. At frequencies above the resonance of the mass-spring system, the relative motion between the mass and casing sensed by the transducer is essentially the same as the motion of the structure under test. The seismic mass and the pickup casing vibrate 180 out of phase. Relative to a fixed (inertial) reference frame, the mass remains 0

nearly stationary (becomes a fixed point) and the casing motion is measured with respect to it. The amplitude of the e.m.f. induced into the measuring coil is proportional to the velocity of the relative motion and hence to the vibration velocity of the structure under test.

Fig. 10.21 (from [10.19])

The measuring coil, the damping cylinder and the additional damping coil are supported in the air gap. The damping cylinder reduces the influence of the transducer’s natural frequency on the measuring signal. The additional damping

coil can be energized to compensate for possible reductions in damping at high temperatures or to compensate for static sag if the transducer is used in a vertical attitude.

A correction coil, wound round the magnetic flux source, i.e. permanent magnet, eliminates the influence of eddy-current damping on the flux. Limit stops are fitted to prevent excessive movement of the seismic mass.

The electrodynamic velocity pickup PR 9266 made by Philips is shown in Figure 10.21 where: 1 – permanent magnet, 2 – correction coil, 3 – measuring coil, 4 – additional damping coil, 5 – damping cylinder, 6 and 7 – membranes, 8 – casing, 9 – output leads, 10 – three-core screened cable, 11 and 12 – limit stops.

The frequency range is 10 to 1000 Hz, for displacement amplitudes up to 1 mm and accelerations up to 10 g. The undamped natural frequency is 12 Hz. The mass without cable is about 0.5 kg. The sensitivity is 30mV_pp mm s at 110 Hz.

Another type of velocity transducer consists of an accelerometer with a built-in electronic integrator. This unit is called a "velometer", and is by all accounts superior to the classic seismic velocity probe

Fig. 10.22

In spite of these advantages, the velocity pickup has many disadvantages that make it nearly obsolete for new installations, although there are many thousands of them still in use today. It is relatively heavy and complex and thus expensive, and it has poor frequency response, extending from about 10 Hz to 1000 Hz. The spring and the magnet make up a low-frequency resonant system with a natural frequency of about several Hz (Fig. 10.22). This resonance needs to be highly damped to avoid a large peak in the response at this frequency. The problem is that the damping in any practical design is temperature sensitive, and this causes the frequency response and phase response to be temperature dependent.

10.4.4 Accelerometers

Due to their advantages – light-weight, ruggedness, wide frequency response, good temperature resistance and moderate pricing – piezoelectric accelerometers are the most often used vibration sensing instruments. They are made in several different configurations, but the compression-type, illustrated in Fig. 10.23, serves to describe the principle of operation. This accelerometer is a seismic pickup in which the sensing piezoelectric ceramic discs form the elastic element of the spring-mass system.

Fig. 10.23 (after [10.20])

The seismic mass is clamped to the base by an axial bolt bearing down on a circular spring. The piezoelectric element is squeezed between the mass and the base. When the accelerometer is subjected to vibrations, the mass will exert a variable force on the piezoelectric discs. The charge developed across the piezoelectric discs is proportional to the applied force, which in turn is proportional to the acceleration of the mass. For frequencies much lower than the resonance frequency of the accelerometer assembly, the acceleration of the seismic mass is equal to the acceleration of the whole pickup.

Accelerometers have a very large dynamic range. The smallest acceleration levels they can sense are determined only by the electrical noise of the electronics, and the highest levels are limited only by the destruction of the piezo element itself. Acceleration levels can span an amplitude range of about 10 , which is 160 ⁸ dB.

The frequency range of the accelerometer is very wide, extending from very low frequencies in some units to several tens of kilohertz. The high-frequency

response is limited by the resonance of the seismic mass coupled to the springiness of the piezo element. This resonance produces a very high peak in the response at the natural frequency of the transducer, and this is usually somewhere near 30 kHz for commonly used accelerometers.

A rule of thumb is that an accelerometer is usable up to about 1/3 of its natural frequency. Data above this frequency will be accentuated by the resonant response, but may be used if the effect is taken into consideration. The lower limit is determined by cable and preamplifier. The frequency response curve of an accelerometer is presented in Fig. 10.24.

Fig. 10.24 (from [10.20])

Most accelerometers used in industry today are of the "ICP" type, meaning they have in internal integrated circuit preamplifier. This preamp is powered by a dc polarization of the signal lead itself, so no extra wiring is needed. The device the accelerometer is connected to needs to have this d.c. power available to this type of transducer. The ICP accelerometer will have a low-frequency roll-off due to the amplifier itself, and this is usually at 1 Hz for most generally available ICP units.

There are some that are specially designed to go down to 0.1 Hz if very low frequency data is required.

The resonant frequency of an accelerometer is strongly dependent on its mounting. The best type of mounting is always the stud mount - anything else will reduce the effective frequency range of the unit.

When mounting an accelerometer, it is important that the vibration path from the source to the accelerometer is as short as possible, especially if rolling element bearing vibration is being measured. If an accelerometer is mounted on a surface that is being strained (bent), the output will be altered. This is known as base strain, and thick accelerometer bases are used to minimize this effect. Shear-type accelerometers are less sensitive because the piezoelectric crystals are mounted to a center post and not to the base.