Force Panel
IDENTIFICATION OF HUMAN
TRANSFER FUNCTION
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
2
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Human Transfer Function
Reference
position (to be
followed by
the finger)
H(w)
3
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Modelling
Actual finger
position
The considered HTF to find are:
1)
1
1

2)
iw
wP
iw
wZ
1

e  iw
iw
iw
1
1
w P1
w P2
1
3)

1
 e 
iw
iw
wZ
1
 iw

e
2
iw


2i
w
i
w
1
w P 1 w  w 
 N 
N
M. De Cecco - Lucidi del corso di Measurement Systems and Applications

Class work
Identification in frequency
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
1th work in class:
filter outliers due
to low finger
pressure
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1 filter outliers
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Show and run the file “PROPEDEUTICO_Filtra
Outliers.m”
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.a Filter outliers with heuristics
Possible ideas to identify
outliers:
- Identify them with an upper
value [use ‘find’]
- identify them with the
derivative [use ‘diff’]
To replace the outliers values:
- With the previous value
- with the mean value between
the previous and the following
- interpolating
After outliers compensation
compare the FFT of the original
and the filtered signal [use ‘fft’]
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.a Filter outliers with heuristics
A stationary signal
x(t)=cos(2*pi*10*t)+cos(2*pi*25*t)+cos(2*pi*50*t)+cos(2*pi*100*t)
http://users.rowan.edu/~polikar/WAVELETS/WTpart1.html
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
Why wavelets? They are extremely useful in non-stationary signals
a signal with four different frequency components at four different time
intervals, hence a non-stationary signal. The interval 0 to 300 ms has a
100 Hz sinusoid, the interval 300 to 600 ms has a 50 Hz sinusoid, the
interval 600 to 800 ms has a 25 Hz sinusoid, and finally the interval 800
to 1000 ms has a 10 Hz sinusoid.
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
Why wavelets? They are extremely useful in non-stationary signals
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
How can appear a wavelet:
the Morlet wavelet shown is defined as the product of a complex
exponential wave and a Gaussian envelope:
http://paos.colorado.edu/research/wavelets/wavelet2.html
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
we now need some way to change the overall size as well as slide the entire
wavelet along in time. We thus define the "scaled wavelets" as:
We are given a time series X, with values of xn, at time index n. Each value
is separated in time by a constant time interval dt. The wavelet transform
Wn(s) is just the inner product (or convolution) of the wavelet function with
our original timeseries:
W(s, n)
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
scale
The above integral can be evaluated for various values of the scale s (usually
taken to be multiples of the lowest possible frequency), as well as all values of
n between the start and end dates. A two-dimensional picture of the variability
can then be constructed by plotting the wavelet amplitude and phase.
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
The wavelets allow to decompose a signal as follows: the DWT (Discrete
Wavelet Transform)
The DWT analyzes the signal at different frequency bands with different
resolutions by decomposing the signal into a coarse approximation and
detail information. DWT employs two sets of functions, called scaling
functions and wavelet functions, which are associated with low pass and
highpass filters, respectively. The decomposition of the signal into different
frequency bands is simply obtained by successive highpass and lowpass
filtering of the time domain signal.
The original signal x[n] is first passed through a halfband highpass filter g[n]
and a lowpass filter h[n]. After the filtering, half of the samples can be
eliminated according to the Nyquist's rule, since the signal now has a
highest frequency of p /2 radians instead of p . The signal can therefore be
subsampled by 2, simply by discarding every other sample.
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
This constitutes one level of decomposition and can mathematically be
expressed as follows:
(where yhigh[k] and ylow[k] are the outputs of the highpass and lowpass
filters, respectively, after subsampling by 2)
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
The above procedure, which is also known as the subband coding, can be
repeated for further decomposition. At every level, the filtering and
subsampling will result in half the number of samples (and hence half the
time resolution) and half the frequency band spanned (and hence double
the frequency resolution).
Detail of level 1
Detail of level 2
Detail of level 3
Approximation
of level 3
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
One important property of the discrete wavelet transform is the relationship
between the impulse responses of the highpass and lowpass filters. The
highpass and lowpass filters are not independent of each other, and they are
related by
where g[n] is the highpass, h[n] is the lowpass filter, and L is the filter length
(in number of points). Note that the two filters are odd index alternated
reversed versions of each other. Lowpass to highpass conversion is provided
by the (-1)n term. Filters satisfying this condition are commonly used in signal
processing, and they are known as the Quadrature Mirror Filters (QMF).
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
a quadrature mirror filter is a filter whose magnitude response is mirror
image about p/2 of that of another filter. Together these filters are known as
the Quadrature Mirror Filter pair.
The filter responses are symmetric about
:
The analysis filters are often related by the following formulae in addition to
quadrate mirror property:
In this way they are also complemetary and there fore the signal is not
distorted!!!
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
Show an example with wavemenu
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 1.b Filter outliers with Wavelets
2th work in class:
filter the frequency
ratio (that is too
noisy to be used
as it is)
zoom
Zoom of the zoom
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Main steps – 2 filter the FFT ratio
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 2 filter the FFT ratio
3th work in class:
fit the modulus
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Main steps – 3 fit the modulus of the HTF
4th work in class:
find the phase
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Main steps – 4 find the HTF delay ()
% program to complete in class
% [to do between square brackets]
% so, find '[]' to localize where to write code (or functions)
clear all
close all;
clc;
%% Load DATA
% [1.]
% [copy the filename to elaborate and its directory]
im = strcat('Dati/Segnale_5/fp_acq_03_04_20111004115703.txt');
[header, hh, dd] = readColData(im,8,7,1);
tempo = dd(:,1);
x = dd(:,2);
y = dd(:,3);
f1 = dd(:,4);
f2 = dd(:,5);
f3 = dd(:,6);
x_segnale = dd(:,7);
x_touch = x * 1.19 - 108.55 ;
ID = dd (:, 8 ) ;
% time
% x finger position red by arduino [ bit ]
% y finger position red by arduino [ bit ]
% first load cell red by arduino [ bit ]
% second load cell red by arduino [ bit ]
% third load cell red by arduino [ bit ]
% x reference of the vertical line [ pixel ]
% x finger position with respect to LCD (calibration) [ pixel ]
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Class work
% extract time
tempo = tempo - tempo(1) ;
Tc = tempo(end) / length(tempo) ;
tempo = 0 : (length(tempo) - 1) ;
tempo = tempo * Tc ;
% filter repeated data on the ARDUINO ID
IIf = find( [ 1; diff(ID)] ) ;
data_finger = x_touch( IIf ) ;
% x finger position filtered
data_input = x_segnale( IIf ) ;
% x reference filtered
tempo = tempo( IIf ) ;
% time filtered
tempo = tempo - tempo(1) ;
figure, plot( tempo/1000, data_finger, 'b', tempo/1000, data_input, 'r' )
title('Reference and finger position'), grid on
%% 1. Filtering outliers
% [filtrare i salti dovuti a scarsa pressione del dito]
meanF = mean (f1(IIf) + f2(IIf) + f3(IIf)) ; % mean of the load
figure, plot( tempo/1000, data_finger, 'b', tempo/1000, (f1(IIf) + f2(IIf) + f3(IIf) - meanF) *10 + 300, 'r' )
title('Finger position and applied force'), grid on
% [1.]
soglia_Dx = 100 ;
data_finger = FiltraOutliers_CELLE(data_finger, data_input, soglia_Dx) ;
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Class work
%% 2. Trasfer Function estimation
data_input = data_input - mean(data_input) ;
data_finger = data_finger - mean(data_finger) ;
% FFT computation
F_Iinput = fft(data_input) / length(data_input) ;
F_Ooutput = fft(data_finger) / length(data_finger) ;
% plot of the transform ratio
Hs = F_Ooutput ./ F_Iinput ;
Hs(1) = 1 ; % (to recover the fact that the mean values were just eliminated)
ffTF = 1000 * ( 0:(length(data_input)-1) ) / tempo(end) ; % Frequency vector [Hz]
figure, plot(ffTF, abs(F_Iinput), 'r'), hold on
plot(ffTF, abs(F_Ooutput))
plot(ffTF, abs(Hs), 'c')
legend('Input','Output','Hs')
xlabel ('Frequency [Hz]')
% []
% [to filter the experimental transfer function 'Hs' just obtained by FFT ratio]
plotta = 1 ;
Hs = Filtro_Hs( Hs, F_Iinput, F_Ooutput, ffTF, plotta ) ;
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Class work
%% just rename the variables:
Iinput = data_input ;
Ooutput = data_finger ;
%% 3. FITTING modulus with lsqnonlin
% []
% write a function 'FittingFrequenza'
% par_ini = [0 2*pi*0.2 2*pi*0.1 2*pi*2] % solo primo ordine, 4 par
par_ini = [0 2*pi*0.2 2*pi*0.1 0.7 2*pi*2] % anche secondo ordine, 5 par
if length(par_ini) == 4 % 1th order
par_ott = lsqnonlin(@(par) FittingFrequenza(par, Hs, ffTF, 0), par_ini, ...
[0 0 0 0], [100 100 100 100]) ;
elseif length(par_ini) == 5 % 2nd order
par_ott = lsqnonlin(@(par) FittingFrequenza(par, Hs, ffTF, 0), par_ini, ...
[0 0 0 0 0], [100 100 100 100 100]) ; % con [0 100 90 0 0] si fa 2d ordine
end
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Class work
%__________________________________
%
% 4. estimate the parameter delay
%__________________________________
% []
% write a function simulink 'CompDinamica' that simulates the model without delay
% use the function 'corr' to estimate the delay
% 0. neglect the mean values
Iinput = Iinput - mean(Iinput) ;
Ooutput = Ooutput - mean(Ooutput) ;
% 1. dynamical simulation without delay
par = par_ott ;
if length(par) == 4 % solo primi ordini
sim('CompDinamica_1ord')
elseif length(par) == 5 % secondo ordine sottosmorzato
sim('CompDinamica_2ord')
end
Comp_Iinput = interp1(time, Comp_Iinput, tempo/1000)' ;
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Class work
% 2. correlation of the output with the simulated output (without delay)
% []
% [use the function 'corr']
tau = 0 ;
par_ott(1) = tau ;
% plot results
FittingFrequenza(par_ott, Hs, ffTF, 1) ; % !!!!!!!!!!!!!!!!!!!!!!!!!
ModelloHTF(par_ott, ffTF, 1) ;
%% SIMULATION OF A STEP INPUT WITH THE HTF
par = par_ott ;
if length(par) == 4 % 1th order
sim('simulazione_1ord')
elseif length(par) == 5 % 2nd order
sim('simulazione_2ord')
end
figure, plot(time, step, time, step_out), title('Step input simulation')
M. De Cecco - Lucidi del corso di Measurement Systems and Applications
Class work
M. De Cecco - Lucidi del corso di Measurement Systems and Applications