Method and arrangement for determining the position of an object

• US 6,415,043 B1
• Filed: 07/09/1999
• Issued: 07/02/2002
• Est. Priority Date: 01/13/1997
• Status: Expired due to Fees

• Current Assignee: Optomatrix Technology AB
• Original Assignee: Qualisys AB (Qualisys Holding AB)
• Inventors: Josefsson, Thorleif
• Primary Examiner(s): Patel, Jayanti K.
• Application Number:
  US09/350,883
• Time in Patent Office: 1,089 Days
• Field of Search: 382/291, 382/117, 382/100, 382/103, 382/107, 382/199, 382/203, 382/218, 382/219, 382/286, 382/300, 382/312, 345/641, 345/7, 348/135, 348/172, 348/157, 348/141, 250/221, 396/51, 396/89
• US Class Current: 382/103
• CPC Class Codes: A61B 5/1127 using markers
• CPC Class Codes: G06T 7/73 using feature-based methods

Abstract

A method for determining a position of a body having at least one marker object with a known dimension parameter. The method utilizes at least one camera unit having a sensor device and an optical element a determinable distance from the sensor device. The at least one camera unit includes an element for retrieving and calculation. The method includes the steps of computing the coordinates of an image of the object reproduced on the sensor device, computing a dimension parameter of the object corresponding to the known dimension parameter of the at least one marker object and calculating the proportion of parameters obtained to determine the position of the object.

Claims

1. A method for determining X, Y and Z-axis coordinates of a marker comprising:

placing said marker having a predetermined dimension on a body;

providing a camera including an optical element and an optical sensing device having a center point, said camera being adapted for projecting an image of said marker onto said optical sensing device, wherein a distance between said optical element and said optical sensing device is known;

projecting the image of said marker onto said optical sensing device;

determining the center point of said projected image relative to the center point of said optical sensing device, the center point of said projected image including both X and Y-axis coordinates relative to the center point of said optical sensing device;

determining a dimension parameter of the image projected onto said optical sensing device; and

using the predetermined dimension parameter of said marker, the determined dimension parameter of said projected image, the determined center point of said projected image, and the known distance between said optical element and said optical sensing device to determine the X, Y, and Z-axis coordinates of said marker.

2. The method according to claim 1,

wherein a distance to said object is determined using the proportionality between the distance to said object and the determinable distance between the optical element and sensor plane, and the dimension parameters of the image of the object on said sensor device and the dimension parameter of the object.

3. The method according to claim 1, wherein the center coordinate of said image is the distance between a center point of the image and a center point on said sensor device.

4. The method according to claim 3, further comprising the step of calculating the center point of said image on said sensor device

the calculating step including the steps of interpolating a signal from the sensor device containing pixel data for the image of the object on the sensor device and calculating a center point for a length of a segment representing a section of said image;

and

using the calculated center point and the length of the segment to determine the centroid coordinate by using the formula;

$\stackrel{.}{y}=\frac{\sum <br><br>\left(l_k^n·<br><br>S\right)}{\sum <br><br>l_k^n},<br><br>\stackrel{.}{x}=\frac{\sum <br><br>\left(l_k^n·<br><br>{\stackrel{\_}{x}}_k\right)}{\sum <br><br>l_k^n}.$

5. The method according to claim 3, wherein said start or end point (x_k) is calculated using the formula:

$X_k=p_m+\frac{T-V_1}{V_2-V_1}$

Where T is a predetermined threshold value V₁ and V₂ are signal levels of preceding and succeeding pixels, respectively; and

p is the number of the pixel m.

6. The method according to claim 1, further comprising the steps of:

interpolating signal containing pixel data for said image and calculating a length of a segment representing a section of said image; and

using the calculated length of the segment to determine the area (A) of the image using the formula A=Σ

l_k.

7. The method according to claim 1, wherein said marking object is substantially spherical.

8. The method according to claim 1, wherein said known dimension parameter is the diameter of the object.

9. An arrangement for determining X, Y and Z-axis coordinates of a marker having a predetermined dimension said arrangement comprising:

a camera including an optical element and an optical sensing device having a center point, the distance between said optical element and said optical sensing device being predetermined, wherein said camera is adapted for projecting an image of said marker onto said optical sensing device; and

a processor for determining the center point of said projected image relative to the center point of said optical sensing device, the center point of said projected image including both X and Y-axis coordinates relative to the center point of said optical sensing device, determining a dimension parameter of the image projected onto said optical sensing device, wherein said processor uses the predetermined dimension parameter of said marker, the determined dimension parameter of said projected image, the determined center point of said projected image, and the known distance between said optical element and said optical sensing device for determining the X, Y, and Z-axis coordinates of said marker.

10. The arrangement according to claim 9, further comprising:

means for collecting image data for said body;

converting means for producing a digital signal containing pixel data for said reproduced image;

a comparator unit for generating a difference between a pixel signal level and a threshold value;

a computation unit for calculating a center point for a length of each segment representing a section of said image; and

means for calculating a centroid coordinate and/or an area of said image and/or a radius of said image.

11. The arrangement according to claim 9, wherein said sensor device includes a CCD plate.