Method and circuit for providing electrical runout reduction in rotating shaft vibration detection systems

A method and circuit for reducing by electrical runout subtraction, the runout signal portion of a composite runout signal and vibration signal, which runout subtraction signal is provided by a pre-programmed digital memory circuit module, PROM, that is selectively inserted into the circuit for a given rotating shaft. The PROM is accessed by a phase lock loop, master dynamic clock, synchronized to the tachometer signal, which provides the subtract signal in digital form that is changed to an analog signal and then fed with the composite signal to a differential amplifier circuit that subtracts the PROM waveform from the composite signal. In one mode the subtract amplifier circuit is inhibited in operation when the tachometer signal falls below a given CPM, or when the tachometer signal is lost.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
In an exemplary embodiment of this invention, the method and circuit 
functions to reduce by a subtract runout signal the runout signal portion 
of a composite runout signal and vibration signal generated in a proximity 
probe circuit by the rotating shaft that is being monitored. The composite 
signal is synchronized with the rotating shaft tachometer signal. A 
pre-programmed digital memory circuit module, PROM, stores the subtract 
runout signal corresponding to the particular rotating shaft that is being 
monitored. This pre-programmed digital runout signal is synchronized with 
the particular tachometer signal for the given rotating shaft. So the 
particular subtract runout signal is generated for the particular rotating 
shaft, and is subtracted from the same electrical runout signal picked up 
by the probe circuit in rotation of the shaft. 
This digital runout signal in the PROM is accessed by a clock signal that 
is synchronized with the tachometer signal and provides a sequence of 
output counts that are fed to an address counter. The address counter 
accesses the PROM that contains the digital subtract runout signal. The 
digital runout signal thus accessed is fed to a digital to analog 
converter that converts the runout signal to an analog signal. This analog 
subtract runout signal is then fed along with the composite signal to a 
subtract amplifier circuit. This circuit generally comprises differential 
amplifiers that subtract the stored analog signal from the composite 
signal, reducing the runout signal portion of the composite signal and 
allowing the vibration signal to pass therethrough and be detected in the 
analyzing monitors. 
The use of PROM module circuits allows quick and easy programming for given 
rotating shafts, which modules are easily and quickly changed for 
different shafts, providing a flexible, operational system. 
The runout subtract circuit also includes means for detecting when the 
shaft rotation input to the circuit is below a given CPM, such as 300 CPM. 
At this low CPM the phase lock loop circuit of the count generator cannot 
hold the locked condition; and thus a misleading output occurs. So in this 
event, a frequency detector and lock logic circuit provides an output that 
inhibits the subtract amplifier circuits, in addition the lock logic will 
detect the total loss of the tachometer signal and inhibits the subtract 
amplifier circuits. Also, the subtract signal is fed through a gate to the 
subtract amplifier circuits. When the gate is closed, the composite signal 
passes through uncorrected, giving an indication through the monitor of 
the effectiveness of the removal of the runout signal portion of the 
composite signal by the method and circuit. 
It is therefore an object of this invention to provide a new and improved 
circuit for reducing the electrical runout signal portion of a composite 
runout signal and vibration signal generated in eddy current type 
proximity probe circuits used in monitoring rotating shafts.

Referring to FIG. 3, the rotating shaft 136 to be monitored rotates in 
either a clockwise or counterclockwise direction. The shaft has a slot 138 
that in cooperation with tach probe 140, provides a tach signal per 
revolution that is fed through line 70 to the runout subtractor circuit 
11. The eddy current proximity probe 142, is positioned immediately 
adjacent the outer surface of the rotating shaft 136. The nominal gap 
between the probe 142 and the rotating shaft 136 depends on the parameters 
of the two components, but it is generally in the order of 40 milliinches 
gap. The oscillator 146 normally generates an oscillating signal that may 
be in the order of 2 megahertz, that is fed through lines 144 and through 
a coil or the like in the known probe circuit 142. The shaft 136 induces 
into the coil in probe 142 an electrical signal that modifies the sine 
wave in the coil from the oscillator 146. The conductivity as well as the 
permeability of the shaft 136 and its surface condition and its spacing, 
have a distinct effect on the sine wave signal in the coil in 142, as 
reflected in the oscillating signal that is picked off by line 148 and fed 
to the detector circuit 150. The known detector circuit 150 provides a DC 
output having a magnitude commensurate with or proportional to the degree 
to which the sine wave in the oscillator probe circuit 142, 144, and 146 
is modified by the changing of the "gap" between transducer 142 and the 
surface of the shaft 136, during vibration of the shaft in rotation; and 
also by that which reflects the "electrical runout" signal. The DC output 
of the detector 150 is fed through line 10 to the runout subtractor 
circuit 11. The runout subtractor circuit 11 then provides an output 
through line 34 in a manner that will be described hereinafter. 
Referring now to FIG. 1, the DC output of detector 150 from the probe 
circuit is fed through line 10 to the buffer amplifier 12 and through line 
14 to the subtract amplifiers circuit 16. Simultaneously, the tach signal 
is fed through line 70 and through the limiter and buffer amplifier 
circuit 68 and through line 66 to the one-shot multi-vibrator circuit 64. 
The one-shot 64 provides correctly shaped output pulses that are fed 
through line 62 to the phase lock loop circuit 58. The phase lock loop 
circuit 58 functions as a 256 count multiplier circuit that provides 256 
counts in lines 52 and 54 for each tachometer input signal. Divider 
circuit 50 divides the 256 counts by 256, providing an out signal on line 
56 that is fed back through line 57 to the phase lock loop circuit 58 that 
is coherent with the input signal in line 62 when the phase lock loop 
circuit is in lock. The output of the one-shot 64 is also fed through the 
reset circuit 60 that resets to 0 count the address counter 48. Upon being 
reset, address counter 48 then receives and counts the 256 counts from the 
phase lock loop circuit 58 through line 52, and feeds these output counts 
through lines 46 to the programmable read only memory or PROM 44. 
The PROM 44 is a pre-programmed digital memory circuit module or IC that is 
an easily and quickly insertable and removably component from the runout 
subtractor circuit 11. The PROM circuit comprises 256, 8-bit words. Each 
of these 8-bit words are programmed to contain the digital equivalent of 
the analog representation of the electrical runout for the particular 
shaft being monitored. This PROM circuit 44 is pre-programmed in a 
programming circuit wherein the shaft 136 is slow rolled with the detected 
electrical runout wave form, see FIG. 4, synchronized to the tachometer, 
is changed to digital form and then correctly addressed and inserted into 
the memory slots of the PROM 44. As illustrated in FIG. 4, the 
representative electrical runout analog signal generated by slowly rolling 
the shaft 136, is waveform 224. The corresponding digital subtract runout 
signal stored in PROM 44 is waveform 226. Signal 226 comprises 256, 8-bit 
words that provides slices 232 in the complimenting waveform 226 versus 
224, that is used to balance out the electrical runout. The utilizing of 
the 8-bit word in the PROM provides 256 levels for establishing the 
magnitude of the particular sliced value 232 of electrical runout and the 
256 slices or segments of one revolution of the shaft 136. 
Thus, the address counter circuit 48 provides the 256 output counts that 
access the 256 memory slots of PROM 44 and provides the digital output 
through line 42 to the D to A converter 40 that changes the signal 226 in 
FIG. 4 to the analog signal 224 in line 38 that is fed through gate 30. 
Gate 30 is normally open and passes the analog subtract runout signal 
through line 32 to the subtract amplifiers circuit 16. The subtract 
amplifiers circuit 16 comprise differential amplifier circuits which may 
be as illustrated in FIG. 2. The operational amplifier 216 receives the 
composite signal from line 14 and the electrical runout subtract signal 
from line 32, which signals are fed through respective lines 222 and 220 
to the operational amplifier 216 that then subtracts runout signal of line 
220 from the larger composite signal in line 222. This provides the 
corrected signal waveform out the output line 34. This corrected output is 
then supplied to a known subsequent monitor or analyzer circuit or the 
like. 
The feedback signal in line 56 of the phase lock loop circuit 58 is also 
fed through line 59 to the one-shot circuit 28 that provides an output in 
line 26 for each tach signal fed through line 70. The signal in line 26 is 
fed to the frequency detector 24, that detects the RPM of the monitored 
shaft 136. When the shaft rotation is too slow, that is when the RPM or 
CPM is below that which will allow the phase lock loop 58 to maintain lock 
condition, or is below the RPM that the shaft was rotated in slow roll to 
provide the information in PROM 44, then the frequency detector either 
provides a signal through line 20 to the lock logic 22 that provides a 
corresponding output signal in line 18, or the phase lock loop circuit 56 
provides an out of lock signal in line 72 to the lock logic circuit 22 
that in turn provides a corresponding output signal in line 18. The 
corresponding output signal in line 18 is a relatively large voltage that 
drives the amplifier 216 to a large negative output in line 34, in the 
order for example 20 volts, which when received by the analyzing monitor 
is detected by the analyzing monitor as showing that the runout subtractor 
11 has become inoperable. Thus the user in viewing the output of the 
monitor is not mislead by the spurious signals that could otherwise be 
reflected in the monitor by the out of lock or low RPM condition in the 
runout subtractor 11. 
In operation of the circuit, the particular pre-programmed PROM circuit 
module 44 is inserted into the circuit 11 and then the shaft 136 is 
rotated at its given operational speed and is monitored in the normal 
manner with the runout subtractor circuit 11 removing the runout 
electrical signal from the composite signal in line 14. Gate 30 may be 
selectively closed by any electrical command or by closing a switch 
manually, or the like, which cuts off the runout signal to the subtract 
amplifier circuit 16. The user is thus able to monitor the actual signal 
generated by the probe and detector circuits 142 and 150, both as 
corrected by the subtractive electrical runout signal and in the 
uncorrected condition. 
In referring to FIG. 5, waveform 236 represents a given illustrative 
electrical runoff signal as generated in a slow rolling of the shaft 136, 
that would be fed to line 10 in FIG. 1. Waveform 238 is the direct probe 
signal at this low RPM of, for example 600 RPM, that shows that the direct 
probe signal has little or no vibration signal component at this low RPM 
and comprises almost wholly the runout electrical signal. 
Waveform 242 is the direct probe signal at 600 RPM while waveform 240 
illustrates the runout electrical signal portion as being corrected by the 
application of the subtract runout signal in line 32. The two signals, 224 
and 246, reflect that at 6000 RPM the resultant direct probe signal 246 
includes the runout electrical error signal 238, and the composite signal 
246 as corrected in waveform signal 244 in line 34.