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Calibration of a stereo system with points of unknown location
154
Abstracts-International Society of Biomechanics XIV Congress 1993
MOUNTlNG AND CALIBRATION OF STAIRS ON FORCE PLATFORMS Marcus P. Besser’; David L. Kowalk’; Christopher 1,. Vaughan” Departments of Biomedical Engineering* and Orthopaedicsd University of Virginia, Charlottesville, Virginia, USA An investigation of the problems inherent in mounting stairs on biomechanical force platforms was performed. Steps were mounted on two Kistler force plates with the intention of collecting data for both normal and pathological adult populations ascending and descending the stairs. Force components F,,F,, and F,, as well as moments M,, M,, and M,, may be calculated from the raw signals from the individual force transducers at the comers of each force plate, as would be done with no steps present. In calculating the point of application, however, the manufacturer of the force plates assumes that there are no tensile stressesimposed on the surface of the plate. This is true if nothing is bolted or glued to the top of the plates, but this condition is violated when stairs are attached to the plates, To accommodate for this, the platforms must be preloaded in the vertical direction. The literature was reviewed to ascertain typical loading values for shear forces encountered when ascending and descending stairs. It was determined that preloads of 360 N and 720 N for steps 22 cm and 44 cm high were adequate for a 115 kg individual. To assessthe accuracy of our results, a point force application jig was constructed to allow forces to be applied in the x, y and z directions at a point, either on the surface of the plate or on the stairs. A 5 cm x 5 cm grid was laid out on the steps; loads of 180 N vertically and 45 N horizontally were applied at points on the grid; and points of application were calculated from force platform outputs. Deviation from the known location of the point of application were well within the limits of accuracy reported by the plate manufacturer (+- 3mm). To demonstrate the effect of point of application inaccuracy on calculated joint moments, the coordinates of the point of application for a normal individual ascending the stairs were intentionally shifted 2 cm medially Calculated peak knee abductioniadduction moment errors of 35% were measured.
AT REST CHARACTERISATION OF MUSCLE SOUNDS Michel Boiteux*, Fayzile Boutella*, Marie-Anne. Mayoux-Benhamou**, Michel Revel”, Andre Sales,* Jean-Yves Soria* and Lincoln Travens* Laboratoire de Physique Biomedicale, Universite Rene Descartes, Paris, France. ** Hopital Cochin, Clinique de Rhumatologie, Paris, France. Muscles emit sounds, even at rest. AMG at rest is shown to be present even when blood circulation is stopped in the observed biceps brachii. The method of delays (borrowed from the theory of dynamic systems) is used to characterize the AMG signal. In particular, the histogram of pseudo-periods was computed. The histograms of a same biceps brachii under contraction and at rest are similar. We attribute these sounds to muscular tonus. l
CALIBRATION Or A STEREO SYSTI$$ WITH POINTS OF UNKNOWNLOCATION and P. Perona N.A. Borghese *Istituto Neuroscienze Bioimmagini CNR, Via Mario Bianco 9 20131 MILANO, I **California Institute of Technology, 116-81, PASADENA, CA 91125, USA A new method is proposed to estimate the spatial arrangement of a stereo system based on TVcameras (space resection). It is derived from the constraint of coplanarity that exists between a three-dimensional point and its projections on the target of the two sensors. It does not require any special distribution of control points like planar grids or points whose position is measured with very high accuracy. A set of points randomly distributed inside the working space and the knowledge of few distances between them are sufficient. Several simulations have been carried out by generating randomly a set of calibration points to which noise is added. The accuracy in the estimation of the parameters is indirectly assessed by the evaluation of the mean and rms error on the 3D coordinates of the points reconstructed with the estimated parameters. The results show that this method can give an accuracy in the estimation of the parameters that is comparable with the one obtained with classical methods. This method greatly simplifies the calibration procedure that can be carried out for example by moving a bar carrying two markers on its extremity across the calibrated volume.
Abstracts-International Society of Biomechanics XIV Congress 1993
MOUNTlNG AND CALIBRATION OF STAIRS ON FORCE PLATFORMS Marcus P. Besser’; David L. Kowalk’; Christopher 1,. Vaughan” Departments of Biomedical Engineering* and Orthopaedicsd University of Virginia, Charlottesville, Virginia, USA An investigation of the problems inherent in mounting stairs on biomechanical force platforms was performed. Steps were mounted on two Kistler force plates with the intention of collecting data for both normal and pathological adult populations ascending and descending the stairs. Force components F,,F,, and F,, as well as moments M,, M,, and M,, may be calculated from the raw signals from the individual force transducers at the comers of each force plate, as would be done with no steps present. In calculating the point of application, however, the manufacturer of the force plates assumes that there are no tensile stressesimposed on the surface of the plate. This is true if nothing is bolted or glued to the top of the plates, but this condition is violated when stairs are attached to the plates, To accommodate for this, the platforms must be preloaded in the vertical direction. The literature was reviewed to ascertain typical loading values for shear forces encountered when ascending and descending stairs. It was determined that preloads of 360 N and 720 N for steps 22 cm and 44 cm high were adequate for a 115 kg individual. To assessthe accuracy of our results, a point force application jig was constructed to allow forces to be applied in the x, y and z directions at a point, either on the surface of the plate or on the stairs. A 5 cm x 5 cm grid was laid out on the steps; loads of 180 N vertically and 45 N horizontally were applied at points on the grid; and points of application were calculated from force platform outputs. Deviation from the known location of the point of application were well within the limits of accuracy reported by the plate manufacturer (+- 3mm). To demonstrate the effect of point of application inaccuracy on calculated joint moments, the coordinates of the point of application for a normal individual ascending the stairs were intentionally shifted 2 cm medially Calculated peak knee abductioniadduction moment errors of 35% were measured.
AT REST CHARACTERISATION OF MUSCLE SOUNDS Michel Boiteux*, Fayzile Boutella*, Marie-Anne. Mayoux-Benhamou**, Michel Revel”, Andre Sales,* Jean-Yves Soria* and Lincoln Travens* Laboratoire de Physique Biomedicale, Universite Rene Descartes, Paris, France. ** Hopital Cochin, Clinique de Rhumatologie, Paris, France. Muscles emit sounds, even at rest. AMG at rest is shown to be present even when blood circulation is stopped in the observed biceps brachii. The method of delays (borrowed from the theory of dynamic systems) is used to characterize the AMG signal. In particular, the histogram of pseudo-periods was computed. The histograms of a same biceps brachii under contraction and at rest are similar. We attribute these sounds to muscular tonus. l
CALIBRATION Or A STEREO SYSTI$$ WITH POINTS OF UNKNOWNLOCATION and P. Perona N.A. Borghese *Istituto Neuroscienze Bioimmagini CNR, Via Mario Bianco 9 20131 MILANO, I **California Institute of Technology, 116-81, PASADENA, CA 91125, USA A new method is proposed to estimate the spatial arrangement of a stereo system based on TVcameras (space resection). It is derived from the constraint of coplanarity that exists between a three-dimensional point and its projections on the target of the two sensors. It does not require any special distribution of control points like planar grids or points whose position is measured with very high accuracy. A set of points randomly distributed inside the working space and the knowledge of few distances between them are sufficient. Several simulations have been carried out by generating randomly a set of calibration points to which noise is added. The accuracy in the estimation of the parameters is indirectly assessed by the evaluation of the mean and rms error on the 3D coordinates of the points reconstructed with the estimated parameters. The results show that this method can give an accuracy in the estimation of the parameters that is comparable with the one obtained with classical methods. This method greatly simplifies the calibration procedure that can be carried out for example by moving a bar carrying two markers on its extremity across the calibrated volume.