FREQUENCY SELECTION METHOD AND HARDENING DEPTH MEASUREMENT METHOD OF EDDY CURRENT MEASUREMENT

• US 20120126802A1
• Filed: 10/16/2009
• Published: 05/24/2012
• Est. Priority Date: 10/28/2008
• Status: Active Grant

• Current Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Original Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota Motor Corporation)
• Inventors: Yamamoto, Takanari
• Granted Patent: US 8,432,161 B2
• Time in Patent Office: Days
• US Class Current: 324/239
• CPC Class Codes: G01N 27/725 by using magneto-acoustical effects or the Barkhausen effect

Abstract

Disclosed is a method of appropriately and easily selecting a combination of two different frequencies of an AC exciting signal used for eddy current measurement while decreasing or removing the dependence of the measurement accuracy on temperature. Three or more AC exciting signals of different frequencies are applied to an object under measurement by an exciting coil, and an induced current is produced. Detection signals corresponding to the frequencies are detected by a detection coil, and the phase difference between each detection signal and the AC exciting signal corresponding to each frequency is calculated from the detection signals corresponding to the frequencies. From the phase differences, a combination of two frequencies the phase differences of which are within a predetermined range is extracted. The lower frequency of the extracted combination is used for a first frequency, and the higher one is used as a second frequency. Thus, the combination of two frequencies are selected.

Claims

1. A frequency selection method of eddy current measurement, wherein

a magnetization coil applies alternating current magnetization signals of a first frequency and a second frequency different from the first frequency on an measurement object so as to generate induced currents including eddy currents respectively corresponding to the first frequency and the second frequency in the measurement object,

a detection coil detects detection signals caused by the induced currents respectively corresponding to the first frequency and the second frequency,

a phase difference D shown in below formula 1 is calculated based on an amplitude Y1 of the detection signal corresponding to the first frequency, a phase difference X1 of the alternating current magnetization signal and the detection signal corresponding to the first frequency, an amplitude Y2 of the detection signal corresponding to the second frequency, and a phase difference X2 between the alternating current magnetization signal and the detection signal corresponding to the second frequency, and

the measurement object (the hardening depth thereof) is measured based on the phase difference D,

the method comprising:

a results of the magnetization and detection process for frequency selection that the magnetization coil applies alternating current magnetization signals of three or more different frequencies on the measurement object so as to generate induced currents respectively corresponding to the three or more different frequencies in the measurement object and the detection coil detects the detection signals caused by the induced currents respectively corresponding to the three or more different frequencies;

a phase difference calculation process that phase differences between the alternating current magnetization signals and the detection signals respectively corresponding to the three or more different frequencies are calculated based on the detection signals caused by the induced currents respectively corresponding to the three or more different frequencies detected in the magnetization and detection process for frequency selection; and

a frequency sampling process that a combination of two frequencies which make a difference of the phase differences within a predetermined range is extracted from the three or more different frequencies calculated in the phase difference calculation process, and lower one of the two frequencies of the combination is referred to as the first frequency and higher one thereof is referred to as the second frequency.

2. The frequency selection method according to claim 1,

wherein the phase difference D is substituted for a relational expression of the difference D, a hardening depth H of the measurement object, and constants A and B shown in below formula 2 so as to calculate the hardening depth H of the measurement object.

D=A×

H³+B 

 

[Formula 2]

3. A hardening depth measurement method comprising:

a magnetization and detection process that a magnetization coil applies alternating current magnetization signals of a first frequency and a second frequency different from the first frequency on a measurement object so as to generate induced currents including eddy currents respectively corresponding to the first frequency and the second frequency in the measurement object, and a detection coil detects detection signals caused by the induced currents respectively corresponding to the first frequency and the second frequency;

a difference calculation process that a phase difference D shown in below formula 1 is calculated based on an amplitude Y1 of the detection signal corresponding to the first frequency, a phase difference X1 of the alternating current magnetization signal and the detection signal corresponding to the first frequency, an amplitude Y2 of the detection signal corresponding to the second frequency, and a phase difference X2 between the alternating current magnetization signal and the detection signal corresponding to the second frequency; and

a hardening depth calculation process that the difference D calculated in the difference calculation process is substituted for a relational expression of the difference D, a hardening depth H of the measurement object, and constants A and B shown in below formula 2 so as to calculate the hardening depth H of the measurement object.