BALL SPIN RATE MEASUREMENT
Spin rate measurement utilizing periodic phase modulation of the radar Doppler signal reflected from a golf ball is described. Some embodiments provide a method to directly measure the ball spin rate caused by the symmetrical structure of the golf ball seam, using the dielectric lens effect of a practical sports ball to magnify the modulating effect of a non-homogeneous feature on the ball, and utilizing the specific pattern of modulation caused in this manner.
1 . A method of determining a spin rate of a projectile, the method comprising:
receiving a reflected signal of microwave radiation incident on the projectile during a rotational trajectory of the projectile, the projectile including a non-homogeneous feature at or near a surface of the projectile;
detecting, by at least one processor, modulation of the reflected signal attributable to the feature; and
calculating the spin rate of the projectile based on the detected modulation.
2 . The method of claim 1 , wherein the reflected signal comprises a Doppler radar reflection signal.
3 . The method of claim 1 , wherein the projectile comprises a sport ball.
4 . The method of claim 1 , wherein the projectile comprises a golf ball.
5 . The method of claim 1 , wherein the feature comprises a seam, a signage, or a manufacturing occurring feature.
6 . The method of claim 1 , wherein the detection of the modulation comprises detecting an increase in an amplitude of the reflected signal at a first time during a period of rotation of the projectile and a decrease in the amplitude of the reflected signal at a second time during the period of rotation of the projectile.
7 . The method of claim 1 , wherein the detection of the modulation comprises detecting a change in a phase of the reflected signal at a first time during a period of rotation of the projectile and an advance in the phase of the reflected signal at a second time during the period of rotation of the projectile.
8 . The method of claim 6 , wherein a time period defined by the first and second times corresponds to the feature being located on a far side of the projectile relative to a source of the microwave radiation.
9 . The method of claim 8 , wherein the time period is less than half the period of rotation of the projectile.
10 . The method of claim 1 , wherein the spin rate is a function of a refraction index of a material included in the projectile.
11 . The method of claim 10 , wherein the material comprises a dielectric material.
12 . The method of claim 10 , wherein the material comprises a core material of the projectile.
13 . The method of claim 10 , wherein the detection of the modulation comprises detecting a magnifying effect of the feature by the material behaving as an optical lens.
14 . The method of claim 1 , wherein the calculating of the spin rate comprises using integer multiples of a periodic modulation frequency of the detected modulation.
15 . The method of claim 1 , wherein the detecting of the modulation of the reflected signal produces a first demodulated signal, and further comprising filtering the first demodulated signal to produce a filtered signal prior to the calculating of the spin rate.
16 . The method of claim 15 , wherein the calculating of the spin rate is based on the filtered signal.
17 . The method of claim 15 , further comprising demodulating the filtered signal to produce a second demodulated signal, wherein the calculating of the spin rate is based on the second demodulated signal.
18 . The method of claim 15 , wherein the filtering of the first demodulated signal comprises at least one of signal sampling, band-pass filtering, band-limited filtering, correlation processing, frequency transforms, pulse transforms, or wavelet transforms.
19 . The method of claim 1 , wherein the calculating of the spin rate comprises frequency analysis.
20 . A system, comprising:
a Doppler radar configured to receive a reflected signal of microwave radiation incident on a projectile during a rotational trajectory of the projectile, the projectile including a non-homogeneous feature at or near a surface of the projectile;
a demodulator in communication with the Doppler radar and configured to process the reflected signal to identify modulation of the reflected signal attributable to the feature; and
at least one processor in communication with the demodulator and configured to calculate a spin rate of the projectile based on the identified modulation.
21 . The system of claim 20 , further comprising a filter to receive a first demodulated signal from the demodulator and configured to filter the first demodulated signal to output a filtered signal.
22 . The system of claim 21 , wherein the at least one processor is configured to calculate the spin rate using the filtered signal.
23 . The system of claim 21 , further comprising a second demodulator configured to output a second demodulated signal based on the filtered signal.
24 . The system of claim 23 , wherein the at least one processor is configured to calculate the spin rate suing the second demodulated signal.
25 . The system of any of claim 20 , further comprising an output device in communication with the at least one processor, the output device configured to store or display the calculated spin rate.
26 . The system of claim 25 , wherein the output device is in wired communication with the at least one processor.
27 . The system of claim 25 , wherein the output device is in wireless communication with the at least one processor.