The Velocity Probe
Velocity Transducer
Some velocity transducers are made with a moving coil outside a stationary magnet. The principle of operation is the same. 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
The velocity probe was one of the first vibration transducers to be built. It consists of a coil of wire and a magnet so arranged that if the housing is moved, the magnet tends to remain stationary due to its inertia. The relative motion between the magnetic field and the coil induces a current that is proportional to the velocity of motion. 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 a relatively low electrical output impedance making it fairly insensitive to noise induction.
In spite of these advantages, the velocity transducer 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 10 Hz. 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.
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