Determination of RPM from vibration spectral plots
A vibration analyzer for use in determining a rotational speed. The vibration analyzer includes: a) an input for sensing vibration data, b) a memory for storing the vibrational data, and c) a processor. The processor 1) produces a spectral plot of the vibrational data, 2) locates peaks in the spectral plot, 3) inputs a rotational speed, and 4) scans the spectral plot in predetermined rotational speed increments to provide a candidate rotational speeds. For each candidate rotational speed i) a number of associated harmonics is identified, ii) closest peaks in the spectral plot to the candidate rotational speed and its harmonics are located, iii) gaps between the closest peaks and the candidate rotational speed and its harmonics are measured, iv) summed, and a sum of the gaps is recorded. In step (5) The candidate rotational speed that is associated with a minimum sum is selected as the nominal rotational speed.
1. A vibration analyzer for use in determining a nominal rotational speed of a rotating shaft, comprising:
an input for sensing vibration signal data at an unknown rotational speed of the shaft,
a memory for storing the vibrational signal data,
a processor for:
producing a spectral plot of peaks in the vibrational data,
receiving an estimated rotational speed,
receiving an accuracy value relative to the estimated rotational speed,
identifying a predetermined number (k) of largest peaks in the spectral plot, and recording a located amplitude and associated frequency for each peak,
creating a set of candidate rotational speeds by,
dividing the located frequencies of each of the largest peaks by integer values of 1 through N, wherein N is a number of harmonics evaluated in order to calculate a rotational speed, to produce divided values for each of the located frequencies, and
designating as candidate rotational speeds the divided values that fall within the accuracy value relative to the estimated rotational speed, wherein each candidate rotational speed represents a harmonic family,
scoring each candidate rotational speed based on a proximity of the candidate rotational speed and harmonic multiples of the candidate rotational speed to each of the largest peaks, and
selecting as the nominal rotational speed the candidate rotational speed that has the highest score.
2. The vibration analyzer of claim 1 , wherein the estimated rotational speed is a nameplate speed of the rotating shaft.
3. The vibration analyzer of claim 1 , wherein the accuracy value ranges from about 0.5 percent to about 2 percent.
4. The vibration analyzer of claim 1 , wherein the predetermined number (k) of largest peaks in the spectral plot ranges from 1 to K, wherein K is less than or equal to LOR/4, wherein LOR is lines of resolution of the spectral plot.
5. The vibration analyzer of claim 1 , wherein N ranges from about 6 to no more than Fmax/(estimated rotational speed), wherein Fmax is a maximum frequency evaluated for a particular spectral plot.
6. The vibration analyzer of claim 1 , wherein the total score for each harmonic family is calculated by to the formula:
Total Score=2−(Located Frequency ( k )/( n *candidate rotational speed( j ))),
wherein j is the number of number of candidate rotational speeds and n is the number of harmonic families considered.
7. The vibration analyzer of claim 6 , wherein the rotational speed is equivalent to the candidate rotational speed having the highest score.
8. The vibration analyzer of claim 7 , where if two or more candidate rotational speeds have the same score, the rotational speed is determined as the candidate rotational speed with the largest score and having the largest located amplitude peak as one of the members of the harmonic family.