You always need a good initial guess when you try a curve fitting.
import numpy as np
import pylab as pl
import csv
from scipy.optimize import curve_fit
def sinfunc(x,a,b,c):
return a*np.sin((x-c)*(2.0*np.pi/b))
def findamp(x):
return (max(x)-min(x))/2.0
def find0cross(x):
n=0
while x[n] >= 0: # skip while positive
n+=1
while x[n] < 0: # skip while negative
n+=1
n1=n # 1st neg to pos transition
n+=10 # skip a little bit
while x[n] >= 0: # skip while positive
n+=1
while x[n] < 0: # skip while negative
n+=1 # 2nd neg to pos transition
n2=n
return [n1, n2]
t, v1, v2 = np.loadtxt('OwonData1.csv', unpack=True, delimiter=',')
#v1=3500*sin(t*(2.0*np.pi/400)+1.2)
#v2=4000*cos(t*(2.0*np.pi/400))
plt.plot(t,v1,'g--',lw=5)
plt.plot(t,v2,'b--',lw=5)
a1=findamp(v1)
a2=findamp(v2)
n11, n12 = find0cross(v1)
n21, n22 = find0cross(v2)
p1=n12-n11
p2=n22-n21
pave=(p1+p2)/2
print n11,n12,p1
print n21,n22,p2
guess1=[a1,pave,n11]
guess2=[a2,pave,n21]
fitpars1, covmat1 = curve_fit(sinfunc,t,v1,guess1)
fitpars2, covmat2 = curve_fit(sinfunc,t,v2,guess2)
print guess1, fitpars1
print guess2, fitpars2
pl.plot(t,sinfunc(t, *fitpars1),'r')
pl.plot(t,sinfunc(t, *fitpars2),'r')
phase2deg=360.0*((fitpars2[2]-fitpars1[2])/((fitpars1[1]+fitpars2[1])/2))
print phase2deg, "(negative value is in advance)"
The first and the second zero-crossings, from negative to positive, are used for estimating the period and the offset of a sine wave.
