The present invention relates to a motion analysis and/or tracking system including at least one camera unit and trackable markers. The camera unit includes emission detection means and being arranged for generating image frames substantially including activated marker, said markers comprise means for generating a response signal in response to a trigger signal detectable by said marker.
The invention also refers to method and an arrangement used for motion analysis and/or tracking.
Motion analysis is now a well-known method, using a camera unit and computer aid to analyse, e.g. bio-mechanics of human, animals or motions of a robot arm etc.
In a simple system markers are attached to the object to be analysed. In the past the object provided with the markers was first filmed and then manually analysed and digitalised to determine the correct position of the markers. This was a time-consuming procedure.
Presently, cameras equipped with so-called CCD plates are used. CCD plate, which is a light sensor, is generally arranged in communication with necessary optical elements. A CCD plate, consisting of lines of charged coupled sensors arranged as a matrix, i.e. arranged in an X and Y coordinate system, for one or several colours, converts the light (from the optical element) projected on it, by electronically scanning in Y direction each line of X sensors and producing a television (video) signal. Then, the signals may be analysed in different ways to detect the position of the markers attached to the object.
In many applications a body to be analysed is provided with several markers. One difficulty in these applications is to follow the motion of each individual marker.
Several suggestions have been made to solve this problem, for example by using markers having different colours. U.S. Pat. No. 5,459,793, teaches a system using several colour markers attached to the object to be analysed. Each marker has a high directive reflective member on a surface thereon, which reflects light having particular wave length. A colour TV camera receives the light reflected from the markers and outputs a colour image. A colour extraction unit is used, connected to the TV camera, extracts only a particular colour from the colour image. An area calculation unit connected to the extraction unit, calculates the area of the particular colour extracted by the colour extraction unit. A centre of gravity calculation unit calculates the position of gravity of the particular colour based on the area calculated by the area calculation unit.
Yet another arrangement is known through GB 2237699. This document describes a system of beacons and/or reflectors and cameras having CCD units. In case of coded beacons, identification coded bursts (ID code) of light emanating from a target are focussed onto the imagining portion of a CCD imager, which is initially operated in a capture mode wherein photo-generated charge pockets are serially scanned and transferred into a store section of the CCD. Each burst consists of an identification code followed by a tracking pulse. The store section contents of the CCD are read out to a processing unit looking for the presence of ID code burst. When an ID code is found, the CCD is operated in tracking mode in which only that photo-generated charge generated during the tracking pulse interval is transferred to the store section and charges generated during other times are dumped. The arrangement according to this invention uses several phases for tracking, i.e. target identification, image integration, frame transfer and image readout. A main drawback is that the image collection frequency is slowed down due to the processing phases. Another drawback is that if the photo-generated charges, e.g. from two different beacons bursting at same time are in same readout column or row (i.e. beacons are situated above each other or side-by-side) the ID code readout from the CCD will not be correct, which results in a defective tracking.
It is an object of the present invention to present a system that overcomes the problems related to tracking markers in real time in an inexpensive way. Yet another object of the invention is to provide a system in which several markers can be tracked and identified substantially simultaneously.
An advantage with the present invention is that camera exposure and marker identification can be carried out substantially simultaneously during same phase.
The objects are achieved by means of system according to the preamble, wherein each response signal has at least a first and a second state and the system further comprises means for detecting the positions of the response signals in each image frame, means for arranging said positions in a sorted set, means for processing the set of lists, and means for generating a unique identification code for each active marker based on the position of the response signals and state of the response signals.
Moreover, the invention relates to a method for tracking a marker device provided with a unique identity, in a motion analysis and/or tracking system including at least one camera unit, the method includes the steps of generating one or more trigger signals, detecting said trigger signal by said marker, generate a response signal by each marker according to instructions stored in each marker, detecting said response signal by the camera unit, generating a (sequential) code for each marker in respect of the response signals and positions of the same, and using said code for identifying each marker.
Other advantageous embodiments are characterized in the depending claims.