Optical inspection probe

• US 5,319,442 A
• Filed: 08/11/1992
• Issued: 06/07/1994
• Est. Priority Date: 08/20/1991
• Status: Expired due to Fees

• Current Assignee: Renishaw Metrology Limited (Renishaw PLC)
• Original Assignee: Renishaw Metrology Limited (Renishaw PLC)
• Inventors: Rosser, John I.
• Primary Examiner(s): Evans, F. L.
• Application Number:
  US07/927,960
• Time in Patent Office: 665 Days
• Field of Search: 356/375, 356/373, 358/107, 250/561, 901/47
• US Class Current: 356/615
• CPC Class Codes: G01B 11/00 Measuring arrangements characterised by the use of optical techniques
• CPC Class Codes: G01B 11/024 by means of diode-array scanning

Abstract

An optical inspection probe, used on a coordinate measuring machine, includes a sensing module having a CCD array and an imaging module releasably mountable thereon. The position of a feature on the surface is measured using the probe by calculating the instant of time at which an image of the feature will pass across a predesignated pixel Q_c of the array. The calculation is performed on the basis of the relative velocity of the probe and the surface as determined from two consecutively scanned image frames of the CCD array. The probe may be used to determine, for example, the position of an edge of a surface by moving the probe laterally with respect to the surface or, may be used to measure the position of a predetermined height of a surface by projecting a light beam onto the surface at an angle to the optical axis and measuring the position of the resulting spot on the surface.

Claims

1. An optical inspection probe for use on a movable arm of a coordinate positioning machine to determine a position of a feature of a surface, the probe comprising:

a sensing device having a light sensitive array which includes a plurality of discrete light sensitive cells, each of which outputs a signal corresponding to the intensity of light incident thereon;

an imaging device having one or more optical components for imaging said feature on said array;

processing means for

a) analysing said image by, inter alia, sequentially scanning each cell of said array to determine the value of each said signal, a single scan of said array constituting an image frame; and

b) determining in advance, from two or more said image frames, an instant of time at which the image of said feature will reach a predetermined cell of said array, and for generating at said instant a trigger signal for instructing a control system of said machine to determine a position of the movable arm relative to said feature of said surface.

2. An optical inspection probe according to claim 1 wherein the processing means includes means for determining, from at least to of said image frames, the speed and direction of relative travel of the probe and the surface, and calculating from speed and direction a time interval between the second of said image frames and said instant of time.

3. An optical inspection probe according to claim 2 wherein the processing means includes means for determining the processing time required to calculate said time interval, and for comparing said time interval with said processing time in order to determine whether a further calculation of said time interval may be made before said instant of time, and where sufficient time remains, performing a further calculation of said time interval.

4. An optical inspection probe according to claim 3, wherein said processing means includes means for generating a corrected value of said time interval by subtracting the processing time therefrom.

5. An optical inspection probe according to claim 1, wherein the probe has an optical axis extending orthogonal to said array and illuminating means for generating a light beam to illuminate an area of said surface, wherein said light beam extends at an angel to the optical axis.

6. An optical inspection probe according to claim 1, wherein said sensing device and said imaging device are provided in the form of a sensing module and an imaging module and means are provided for releasably and repeatably retaining said imaging module on said sensing module.

7. A method of inspecting a surface to determine a position of a feature thereon, using a coordinate positioning machine having an optical inspection probe connected to a movable arm of the machine, the optical inspection probe including an array of light sensitive elements each of which outputs a signal corresponding to an intensity of light incident thereon, and one or more optical elements for imaging said feature on said array, the method comprising the steps of:

analysing said image by, inter alia, sequentially scanning each light sensitive element of said array to determine a value of each said signal, a single scan of said array constituting an image frame;

determining in advance, from two or more said image frames, an instant of time at which an image of said feature will reach a predetermined cell of said array, and generating at said instant a trigger signal for instructing a control system of said machine to determine a position of the movable arm relative to said feature of said surface.

8. The method according to claim 7, further comprising the step of generating , from a light source in said probe, a bema of light which is incident upon siad surface, and extends in a direction non-parallel to a direction of relative movement of said surface and said arm.

9. A method of operating an optical inspection probe for use on a movable arm of a coordinate positioning machine in determining a position of a feature of a surface, the probe comprising:

a light sensitive array including a plurality of discrete light sensitive cells, each cell outputting a signal corresponding to an intensity of light incident thereon, and an imaging device having at least one optical component for imaging said feature on said array, the method comprising the steps of;

analysing said image by sequentially scanning each cell of said array to determine a value of each said signal, a single scan of said array constituting an image frame; and

determining in advance, from two or more of said image frames, an instant of time at which an image of said feature will reach a predetermined cell of said array, and generating at said instant a trigger signal for instructing a control system of said coordinate positioning machine to determine a position of the movable arm relative to said feature of said surface.

10. The method according to claim 9, further comprising the steps of determining, from at least two of said image frames, a speed and direction of relative travel of the probe and the surface, and calculating from said speed and direction a time interval between the second of said image frames and said instant of time.

11. The method according to claim 10, further comprising the steps of determining the processing time required to calculate said time interval and comparing said time interval with said processing time in order to determine whether a further calculation of said time interval may be made before said instant of time, and where sufficient time remains, performing a further calculation of said time interval.

12. The method according to claim 11, further comprising the step of generating a corrected value of said time interval by subtracting the processing time therefrom.