Method measuring axial force of bolt

1. A method of measuring an axial force of a fixing member, comprising:

fastening a joint using a fastener;

receiving a vibration signal of the fixing member mounted to the joint, by a detector, wherein the vibration signal reflects a fastening force of the fixing member;

transforming, by a controller, the received vibration signal into frequency domain data having a plurality of frames;

analyzing, by the controller, the signal transformed into frequency domain data; and

predicting, by the controller, an axial force of the fixing member and indicating a predictive value of the axial force in a display.

2. The method of claim 1 , wherein, in the transforming the received vibration signal into frequency domain data, the received vibration signal undergoes fast Fourier transform while having a predetermined overlap ratio with respect to a predetermined time.

3. The method of claim 2 , wherein the predetermined time is 100 ms, and the predetermined overlap ratio is 50% to 90%.

4. The method of claim 1 , wherein the analyzing the signal transformed into the frequency domain data includes:

deriving a cepstrum coefficient;

determining a reference value; and

determining a Euclidean distance.

5. The method of claim 4 , wherein the deriving the cepstrum coefficient includes:

applying a high-pass filter to a frequency domain data pair to include only a high-frequency band therein;

determining a magnitude value a high-frequency section as a representative value for each frame; and

deriving the cepstrum coefficient by the data pair with a plurality of coefficients.

6. The method of claim 5 , wherein, in the determining the magnitude value in the high-frequency section as the representative value, the representative value is determined by dividing a configured frequency bandwidth by multiples of harmonic components.

7. The method of claim 4 , wherein, in the determining the reference value, the cepstrum coefficient, which is determined for a signal having a largest fastening force value of the axial three among predetermined vibration signals, is determined as the reference value.

8. The method of claim 4 , wherein, in the determining the Euclidean distance, the Euclidean distance between a centroid of a reference value data and a centroid of each fastening force data sample is determined.

9. The method of claim 8 , wherein, in the predicting the axial force and indicating the predictive value thereof, Euclidean distance values determined for each fastening force are fitted into a linear or quadratic polynomial function to determine the axial three by a differentiable equation, and the Euclidean distance of the determined vibration signal is substituted into an equation to predict the axial force.

10. A method of measuring an axial force of a fixing member, comprising:

fastening a joint using a fastener;

receiving a vibration signal of the fixing member mounted to the joint, by a detector wherein the vibration signal reflects a fastening force of the fixing member;

determining, by a controller, a torque at a predetermined time by synthesizing vibration signals generated during fastening; and

predicting, by the controller, an axial force of the fixing member and indicating a predictive value of the axial force in a display by transforming the received vibration signal into frequency domain data having a plurality of frames, deriving revolutions per minute (RPM) and a fastening time, and determining the derived RPM and the derived fastening time with the determined torque.

11. The method of claim 10 , wherein the determining the torque includes:

selectively synthesizing modes according to the fixing member, characteristics through empirical mode decomposition of the received vibration signal; and

determining the torque by deriving a cepstrum coefficient, setting a reference value, and then determining a Euclidean distance.

12. The method of claim 11 , wherein the predicting the axial force and indicating the predictive value thereof includes:

transforming the received vibration signal into the frequency domain data;

deriving the RPM to form a k value determined according to the torque and the RPM; and

deriving the fastening time to predict and indicate the axial force using the k value and the fastening time.

13. The method of claim 12 , wherein the RPM is specified by modulation derived from the vibration signal using frequency transform.

14. The method of claim 12 , wherein the fastening time is a difference between a first time (t 1 ) at which a predetermined torque is derived and a second time (t 2 ) at which a largest signal is generated.

15. A method of measuring an axial force of a fixing member, comprising:

fastening a joint using a fastener;

receiving a vibration signal of the fixing member mounted to the joint, by detector wherein the vibration signal reflects a fastening force of the fixing member;

deriving, by a controller, a cepstrum coefficient;

transforming, by the controller, the received vibration signal into frequency domain data having a plurality of frames;

determining, by the controller, a weight vector matrix using the cepstrum coefficient, the frequency domain data, and an artificial neural network; and

predicting, by the controller, an axial force of the fixing member and indicating a predictive value of the axial force in a display through a signal to be determined and the determined weight vector matrix.

16. The method of claim 15 , wherein the determining the weight vector matrix includes:

inputting the cepstrum coefficient and the frequency domain data as a supervised signal which is a vibration signal having a predetermined axial force value as an index;

forming a multilayered weight vector matrix by a gradient descent method using a cost function; and

learning and optimizing a plurality of input values.