MATLAB: Removing spikey data from a sensor readout.

data processingfiltersavitzky-golay filtersensorsSignal Processing Toolboxsmootheningspikes

I have a loop which reads out a sensor, once in a while the data is completely off, and gives a spike. I would like to remove those spikes.

The data given moves from -180 to 180 degrees, so when my measurement moves from -180 to 180 this should not be filtered (as this is normal).

However, if it moves from -180 to 90 then this counts as a spike.

It's not possible to do data processing after getting every readout, ideally there should be a filter of some sort in the loop itself.

Any ideas?

Best Answer

Do you have the Signal Processing Toolbox? If so, you could use the median filter. If you want to replace spikes with the median value around them, then compute the median and subtract it from the original and take the absolute value. Then replace those elements with a high value with the median.

medianSignal = medfilt1(signal, 7);
diffSignal = abs(signal - medianSignal);
spikes = diffSignal > 100; % Whatever...
% Replace
fixedSignal = signal; % Initialize
fixedSignal(spikes) = medianSignal(spikes);

Or you can use the Savitzky-Golay filter, sgolayfilt(). A demo is attached.

If this is not right for you, then post your data - both a picture and the actual data in a txt or .mat file.

Related Solutions

MATLAB: Dear all, how to select the values for k and f, of savitzky-golay filter

You pick them to give the signal you want. There is no one right value for all situations. It's a judgment call. A longer frame width will take more points into consideration when fitting the polynomial and give a smoother curve. A higher polynomial will more accurately "hug" the actual/noisy training data, while a lower polynomial order will give a smoother, less "huggy" fit.

MATLAB: Adding Noise to the Image

Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
%===============================================================================
% Read in a standard MATLAB gray scale demo image.
folder = fileparts(which('cameraman.tif')); % Determine where demo folder is (works with all versions).
baseFileName = 'cameraman.tif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- didn't find it there.  Check the search path for it.
  fullFileNameOnSearchPath = baseFileName; % No path this time.
  if ~exist(fullFileNameOnSearchPath, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
grayImage = imread(fullFileName);
% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(grayImage);
if numberOfColorBands > 1
  % It's not really gray scale like we expected - it's color.
  % Convert it to gray scale by taking only the green channel.
  grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the original gray scale image.
subplot(1, 2, 1);
imshow(grayImage, []);
title('Original Grayscale Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 
% Initialize an output image.
noisyImage = grayImage; % Initialize
% Get 5,000 random locations
noiseIndexes = randperm(numel(grayImage), 5000);
% Add noise
noisyImage(noiseIndexes) = 255;
% Display the noisy image.
subplot(1, 2, 2);
imshow(noisyImage, []);
title('Noisy Image', 'FontSize', fontSize);
axis on;

Best Answer

Related Solutions

MATLAB: Dear all, how to select the values for k and f, of savitzky-golay filter

MATLAB: Adding Noise to the Image

Related Question