Automatic threshold detection for tachometer signals

• US 10,352,956 B2
• Filed: 06/05/2014
• Issued: 07/16/2019
• Est. Priority Date: 06/17/2013
• Status: Active Grant

• Current Assignee: Computational Systems Incorporated (Emerson Electric Company)
• Original Assignee: Computational Systems Incorporated (Emerson Electric Company)
• Inventors: Bowers, III, Stewart V.
• Primary Examiner(s): Rivera Vargas, Manuel A
• Assistant Examiner(s): Perez Bermudez, Yaritza H
• Application Number:
  US14/296,480
• Publication Number:
  US 20140372071A1
• Time in Patent Office: 1,867 Days
• CPC Class Codes: G01H 1/003 of rotating machines G01H1/10 takes precedence
• CPC Class Codes: G01P 3/481 of pulse signals

Abstract

A method is described for automatically determining a proper threshold for a tachometer signal in order to produce desired tachometer pulses necessary for analysis of machine vibration data. A tachometer signal is low-pass filtered to exclude high frequency noise and a running derivative of the filtered tachometer waveform is taken to create a derivative waveform. Another waveform is created that includes only positive values from the derivative waveform that correspond to positive values in the low-pass filtered tachometer waveform. In general, a tachometer signal has the greatest derivative value (slope) when a tachometer pulse is present. Based on this observation, a threshold value is determined using both the low-pass filtered tachometer waveform and the positive-value derivative waveform along with statistics from both waveforms.

Claims

1. A method for determining a threshold value for a tachometer signal generated by a tachometer attached to a particular machine, wherein the determined threshold value is used to reliably trigger tachometer pulses for the particular machine based on rising or falling edges of the tachometer signal the method comprising:

(a) attaching the tachometer in proximity to a rotating component of the particular machine, wherein due to a lack of reflective tape or other distinctive feature on the rotating component, the tachometer signal is noisy and the tachometer pulses are not distinct from the noise;

(b) the tachometer generating the tachometer signal that includes pulses which are indicative of a rotational speed of the rotating component, wherein the tachometer signal includes some peak values associated with the pulses and some peak values associated with noise, wherein due to the variations in the peak values, the threshold value determined for the particular machine will be different than a threshold value determined for another machine;

(c) a low-pass filter filtering the tachometer signal to remove high-frequency noise, thereby producing a filtered tachometer signal;

(d) a processor determining a characteristic of the filtered tachometer signal that indicates a relative amplitude of the peak values in the filtered tachometer signal compared to a mean value of the filtered tachometer signal;

(e) the processor taking a running derivative of the filtered tachometer signal to generate a derivative signal that includes peak values associated with steep slopes in the filtered tachometer signal, which peak values indicate the pulses in the tachometer signal as opposed to the peak values associated with noise, the peak values in the derivative signal thereby making the pulses more distinctive than they are in the tachometer signal generated in step (b) and therefore easier to identify for use in determining the threshold value;

(f) the processor determining a characteristic of the derivative signal that indicates a relative amplitude of the peak values in the derivative signal compared to a mean value of the derivative signal;

(g) the processor calculating a characteristic ratio, which is a ratio of the characteristic of the derivative signal to the characteristic of the filtered tachometer signal;

(h) the processor comparing the characteristic ratio to a predetermined value;

(i) based on the comparing of step (h), the processor;

(i1) using the filtered tachometer signal to derive the threshold value for the tachometer pulses in the tachometer signal if the characteristic ratio is less than the predetermined value;

or

(i2) using the derivative signal to derive the threshold value for the tachometer pulses generated from the derivative signal if the characteristic ratio not less than the predetermined value;

or

(i3) determining that a meaningful threshold value cannot be derived due to insufficient information in the tachometer signal;

(j) detecting tachometer pulses based on rising or falling edges of the tachometer signal that exceed the threshold value derived in step (i);

(k) determining a rotational speed of the particular machine based on the tachometer pulses detected in step (j); and

(l) analyzing vibration data collected from the particular machine using the rotational speed determined in step (k).

2. The method of claim 1 wherein the characteristic of the filtered tachometer signal is a crest factor of the tachometer signal, the characteristic of the derivative signal is a crest factor of the derivative signal, and the characteristic ratio is a ratio of the crest factor of the derivative signal to the crest factor of the filtered tachometer signal.

3. The method of claim 2 wherein the predetermined value is 3.

4. The method of claim 1 further comprising the processor performing the following steps after step (e) and before step (f):

(e1) determining a mean value and a standard deviation value of the derivative signal;

(e2) setting all negative values in the derivative signal to zero to generate an all-positive-value derivative signal;

(e3) finding N number of peaks in the all-positive-value derivative signal; and

(e4) discarding statistical outlier peaks from the N number of peaks in the all-positive-value derivative signal, wherein the statistical outlier peaks have an amplitude greater than a sum of the mean value and the standard deviation value,

wherein step (f) comprises the processor determining a characteristic of the all-positive-value derivative signal from which the outlier peaks have been discarded.

5. The method of claim 4 further comprising the processor determining that a threshold value cannot be derived from the derivative signal if statistical outlier peaks were discarded in step (e4).

6. A method for determining a threshold value for a tachometer signal generated by a tachometer attached to a machine, the tachometer signal including pulses that are indicative of a rotational speed of a rotating component of the machine, wherein due to a lack of reflective tape or other distinctive feature on the rotating component, the tachometer signal is noisy and the pulses within the tachometer signal are not distinctive, wherein the threshold value is for use in detecting the pulses in analysis of machine vibration data, the method comprising:

(a) the tachometer generating the tachometer signal that includes some peak values associated with the pulses in the tachometer signal and some peak values associated with noise;

(b) a low-pass filter filtering the tachometer signal to remove high-frequency noise, thereby producing a filtered tachometer signal;

(c) a processor taking a running derivative of the filtered tachometer signal to generate a derivative signal that includes peak values associated with steep slopes in the filtered tachometer signal, which peak values indicate the pulses in the tachometer signal as opposed to peak values associated with noise, the peak values in the derivative signal thereby making the pulses more distinctive than they are in the tachometer signal generated in step (a);

(d) the processor determining a mean value and a standard deviation value of the derivative signal;

(e) the processor setting all negative values in the derivative signal to zero to generate an all-positive-value derivative signal;

the processor finding N number of peaks in the all-positive-value derivative signal;

(g) the processor discarding statistical outlier peaks from the N number of peaks in the all-positive-value derivative signal, wherein the statistical outlier peaks have an amplitude greater than a sum of the mean value and the standard deviation value;

(h) the processor determining a characteristic of the all-positive-value derivative signal from which the statistical outlier peaks have been discarded;

(i) based on the characteristic, the processor determining that;

the filtered tachometer signal is to be used to derive a threshold value for tachometer pulses in the tachometer signal;

or

the derivative signal is to be used to derive a threshold value for tachometer pulses in the derivative signal;

(j) the processor using the filtered tachometer signal or the derivative signal as determined in step (i) to derive a threshold value for tachometer pulses in the tachometer signal; and

(k) the processor generating tachometer pulses based on the tachometer signal and the threshold value derived in step (j).

7. An apparatus for determining a tachometer threshold value for use in analysis of machine vibration data, the apparatus comprising:

a tachometer attached adjacent a rotational component of a machine, the tachometer for generating an analog tachometer signal including pulses that are indicative of rotational speed of the rotational component, wherein due to a lack of reflective tape or other distinctive feature on the rotating component, the tachometer signal is noisy and the pulses are not distinctive, wherein some peak values in the analog tachometer signal are associated with the pulses and some peak values in the analog tachometer signal are associated with noise, wherein the threshold value is for use in detecting the pulses in analysis of machine vibration data;

a vibration data collector comprising;

an analog-to-digital converter for converting the analog tachometer signal into digital tachometer data;

a low-pass filter for filtering the digital tachometer to remove high-frequency noise, thereby producing filtered tachometer data; and

memory for buffering the filtered tachometer data; and

a vibration data processor for executing operational instructions to;

determine a first crest factor based on the filtered tachometer data;

take a running derivative of the filtered tachometer data to generate derivative data that includes peak values associated with steep slopes in the filtered tachometer data, which peak values indicate the pulses in the tachometer signal as opposed to peak values associated with noise, the peak values in the derivative data thereby making the pulses more distinctive than they are in the filtered tachometer data;

determine a second crest factor based on the derivative data;

calculate a characteristic ratio, which is a ratio of the second crest factor to the first crest factor;

compare the characteristic ratio to a predetermined value;

if the characteristic ratio is less than the predetermined value, use the filtered tachometer data to derive the tachometer threshold value for tachometer pulses in the tachometer signal;

if the characteristic ratio is not less than the predetermined value, use the derivative data to derive the tachometer threshold value for tachometer pulses in the tachometer signal; and

generate tachometer pulses based on the tachometer signal and the tachometer threshold value.

8. The apparatus of claim 7 wherein the vibration data processor executes operational instructions to:

determine a mean value and a standard deviation value of the derivative data;

set all negative values in the derivative data to zero to generate all-positive-value derivative data;

find N number of peaks in the all-positive-value derivative data;

discard statistical outlier peaks from the N number of peaks in the all-positive-value derivative data, wherein the statistical outlier peaks have an amplitude greater than a sum of the mean value and the standard deviation value; and

determine the second crest factor based on the all-positive-value derivative data from which the outlier peaks have been discarded.

9. The apparatus of claim 7 wherein the vibration data collector comprises a digital data recorder.

10. The apparatus of claim 7 wherein the vibration data processor is a component of the vibration data collector.

11. A method for determining a threshold value for a tachometer signal generated by a tachometer attached to a machine, wherein the determined threshold value is used to detect tachometer pulses for the machine based on the tachometer signal, wherein the tachometer signal includes some peak values associated with the tachometer pulses and some peak values associated with noise, the method comprising:

(a) low-pass filtering the tachometer signal to remove high-frequency noise, thereby producing a filtered tachometer signal;

(b) using a processor, determining a characteristic of the filtered tachometer signal that indicates a relative amplitude of the peak values in the filtered tachometer signal compared to a mean value of the filtered tachometer signal;

(c) using the processor, taking a running derivative of the filtered tachometer signal to generate a derivative signal that includes peak values associated with steep slopes in the filtered tachometer signal, which peak values indicate the pulses in the tachometer signal as opposed to the peak values associated with noise;

(d) using the processor, determining a characteristic of the derivative signal that indicates a relative amplitude of the peak values in the derivative signal compared to a mean value of the derivative signal;

(e) using the processor, calculating a characteristic ratio, which is a ratio of the characteristic of the derivative signal to the characteristic of the filtered tachometer signal;

(f) using the processor, comparing the characteristic ratio to a predetermined value;

(g) based on the comparing of step (f), the processor;

(g1) using the filtered tachometer signal to derive the threshold value for the tachometer pulses in the filtered tachometer signal if the characteristic ratio is less than the predetermined value;

or

(g2) using the derivative signal to derive the threshold value for the tachometer pulses generated from the derivative signal if the characteristic ratio not less than the predetermined value;

(h) detecting tachometer pulses based on rising or falling edges of the tachometer signal that exceed the threshold value derived in step (g); and

(i) determining a rotational speed of the particular machine based on the tachometer pulses detected in step (h).