Automatic threshold detection for tachometer signals

• US 10,345,326 B2
• Filed: 02/20/2017
• Issued: 07/09/2019
• 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:
  US15/436,925
• Publication Number:
  US 20170160302A1
• Time in Patent Office: 869 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 peak amplitude values in a tachometer signal generated by a tachometer attached to a particular machine in order to calculate a threshold value for the tachometer signal and for determining associated statistical outlier values, the tachometer signal including pulses that are indicative of a rotational speed of a rotating component of a machine, wherein the 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) sensing the tachometer signal using the tachometer attached in proximity to a rotating component of the particular machine, the tachometer signal including some peak values associated with the tachometer pulses in the tachometer signal and some peak values associated with noise, 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, wherein due to variations in the peak values, the threshold value determined for the particular machine will be different than a threshold value determined for another machine;

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

(c) taking a running derivative of the filtered tachometer signal using a processor to generate a derivative signal that includes peak values associated with steep slopes in the filtered tachometer signal, which peak values indicate the tachometer pulses as opposed to peaks 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) and therefore easier to identify for use in determining the threshold value;

(d) the processor determining a set of the peak values in the derivative signal over a period of time and defining P₀ as the number of peak values in the set;

(e) the processor calculating a mean amplitude value μ and

a standard deviation amplitude value σ

of the peak values in the set;

(f) if

2. The method of claim 1 wherein x =0.1.

3. A computer-implemented method for determining a plurality of statistically significant peak values of a tachometer signal and excluding statistical outlier values prior to further processing of the tachometer signal, the tachometer signal including tachometer pulses that are indicative of a rotational speed of a rotating component of a 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 distinctive and have levels that vary from one machine to the next, the tachometer signal including some peak values associated with the tachometer pulses and some peak values associated with noise, the method comprising:

(a) determining a set of peak values in the signal and defining P₀ as the number of peak values, the set of peak values associated with steep slopes in the tachometer signal, which peak values indicate the tachometer pulses as opposed to the peak values associated with noise;

(b) calculating a mean amplitude value μ and

a standard deviation amplitude value σ

of the peak values in the set;

(c) if

4. The method of claim 3 wherein x =0.1.

5. The method of claim 3 wherein the first lower statistical boundary is μ

−

nσ and

the first upper statistical boundary is μ

+nσ

.

6. The method of claim 3 wherein the second lower statistical boundary is

7. The method of claim 3 wherein the third lower statistical boundary is (1−

x)×

μ and

the third upper statistical boundary is (1+x)×

μ

.

8. The method of claim 3 wherein the first, second, and third lower statistical boundaries are set to zero.

9. A computer-implemented method for determining a plurality of statistically significant amplitude values of a tachometer signal and excluding statistical outlier values prior to further processing of the tachometer signal, the tachometer signal including tachometer pulses that are indicative of a rotational speed of a rotating component of a 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 distinctive and have levels that vary from one machine to the next, the tachometer signal including some peak values associated with the tachometer pulses and some peak values associated with noise, the method comprising:

(a) determining a set of peak values in the signal and defining P₀ as the number of peak values, the set of peak values associated with steep slopes in the tachometer signal, which peak values indicate the tachometer pulses as opposed to the peak values associated with noise;

(b) calculating a mean amplitude value μ and

a standard deviation amplitude value σ

of the peak values in the set;

(c) if

10. The method of claim 9 wherein the lower statistical boundary is set to zero.

11. The method of claim 9 wherein x =0.1.