Method for reducing error in rotor speed measurements

A method for measuring rotor speed, comprising:

• positioning a sensor opposite a target, said target including a plurality of features wherein said features include at least one geometric imperfection;
• measuring a period corresponding to time between each of said plurality of features on said target passing said sensor, the period between each pair of adjacent features of said plurality of features being measured from a first point of one of said plurality features to a same second point of an adjacent feature of said plurality of features;
• establishing an array of said periods, said array including up to a preselected number of said periods;
• removing old periods from said array when new periods are added and said preselected number of periods is reached;
• calculating an average period from a subset of said periods in said array, said average period corresponding to one of a complete revolution or a multiple of complete revolutions, wherein the average period is calculated utilizing a shortened period associated with the feature having said at least one geometric imperfection and a lengthened period associated with the feature having said at least one geometric imperfection, wherein the shortened period is a shorter time period and the lengthen period is a greater time period than normal periods of said array associated with features of the plurality of features not having said at least one geometric imperfection;
• and
• calculating rotational speed of a rotor from said average period to compensate for said at least one geometric imperfection.

A measurement system for measuring a rotor speed of a rotor, the system comprising:

• a sensor operable to sense a target of the rotor, said target including a plurality of features, wherein at least one of said plurality of features has a geometric imperfection relative to the other features of said plurality of features;
• and
• a controller communicatively coupled with the sensor, the controller configured to:
• receive, from the sensor, a signal indicative of the rotor speed of the rotor;
• determine a period corresponding to a measure of time from a single point of one feature of the plurality of features to a same second point of an adjacent feature of the plurality of features based at least in part on the received signal;
• update an array having a preselected number of measured periods, wherein updating the array comprises adding the measured period to the array and removing an old measured period of the array, the old measured period being the oldest measured period of the array;
• calculate an average period over one or more complete revolutions of the target based at least in part on the measured periods of the array, wherein the average period is calculated utilizing a shortened period associated with the feature having the geometric imperfection and a lengthened period associated with the feature having the geometric imperfection, wherein the shortened period is less than and the lengthen period is greater than normal periods of the measured periods of the array associated with features of the plurality of features not having the geometric imperfection;
• calculate a feature frequency based at least in part on the calculated average period;
• and
• calculate the rotor speed of the rotor based at least in part on the calculated feature frequency.