Proximity probes or eddy current probes have proven to be one of the best transducers in rotating shaft vibration monitoring systems. These non-contact sensors are used to measure shaft vibrations or movements in systems that diagnose the condition of the machinery or shaft, and in dynamically balancing the shaft. The proximity probes measure the spacing between the probe mounted in the case housing and the shaft during operational shaft rotation. Changes in this spacing reflect vibration in the shaft, which if excessive can seriously damage the machinery. It is the detection of this vibration signal that is a prime object of shaft vibration monitoring.
However, eddy current probes suffer the limitation of runout signals. Runout is the mechanical imperfection in machine shafts. It is also been adopted as the name for the output signal of a vibration pick-up that does not represent shaft vibratory motion, but is caused by the eccentricity of the shaft, surface irregularities, and by properties of the shaft material that causes the vibration probe to give an incorrect vibration signal. This incorrect signal is called "electrical runout" or "runout signals," and it is this runout that presents the machinery manufacturers with major difficulties since the electrical runout signal often exceed the magnitude limit acceptable in determining the magnitude of the vibration signal. While mechanical runout can be reduced by proper finishing of the surface that the probe is going to observe, and methods such as shaft-peening and burnishing have been developed to reduce electrical runout, these techniques have only been partially successful. It has been suggested to derive an electrical signal that corresponds to the "electrical runout" and then subtract this electrical runout signal from the composite runout signal and vibration signal. However, such systems have not left the laboratories stage and have not proven effective in actual use in rotating equipment environments and do not use or employ circuits that provide simple, inexpensive and reliable performance.
It is therefore advantageous to have a circuit for reducing the electrical runout from a composite runout and vibration signal generated in a proximity probe circuit, that has a selective memory containing subtracting runout signals corresponding to a given rotating shaft, which memory circuit is an IC or module insertable into circuit components that are mounted adjacent the proximity probe, and provides a vibration signal output with reduced electrical runout and that is effectively processed to determine with a higher degree of accuracy the vibration movement of the rotating shaft being monitored.