Method for determining a distortion angle in a textile material and an apparatus for use therein

A method of determining a distortion angle in a textile material comprising the steps of:

• feeding the material in a specific direction;
• providing a first detection means capable of generating a first output signal dependent on an angular position of the first detection means with respect to said specific feeding direction of the material and to the distortion angle;
• monitoring the material with said first detection means to provide said first output signal;
• providing a filtering means for the first output signal, which filtering means comprises a band-pass filter defining a band-pass curve which is adjustable with respect to at least one of its center bandpass frequency, its bandwidth, and its filter-curve shape;
• filtering the first output signal using said filtering means so that components of said first output signal which are at least statistically independent of the distortion angle are filtered out of said output signal;
• providing a second detection means capable of generating a second output signal dependent on the frequency characteristic of signal components in said first output signal that are at least statistically dependent on the distortion angle;
• inputting said second output signal to said filtering means so that said band-pass curve can be adjusted with respect to said distortion-angle-dependent signal components;
• and
• using an output signal from the filtering means to determine said distortion angle.