function p_aver = priod(data,fs)   
    %以下兩種方法都能用
    Y=fft(data);
    N=length(data);
    power = abs(Y(1:floor(N/2))).^2;
    nyquist = 1/2;
    freq = (1:N/2)/(N/2)*nyquist*fs;
    period=1./freq;
    index=find(power==max(power));
    p_aver=period(index);

%     [Pxx,F]=pwelch(data,[],[],[],fs,'onesided') ;
%     a=sum(Pxx.*F);
%     b=sum(Pxx);
%     p_aver = b/a;