GB2238611A

GB2238611A - Apparatus for checking the positioning accuracy of a mobile component

Info

Publication number: GB2238611A
Application number: GB8926420A
Authority: GB
Inventors: Koorosh Khodabandehloo
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 1989-11-22
Filing date: 1989-11-22
Publication date: 1991-06-05
Anticipated expiration: 2009-11-22
Also published as: GB8926420D0; GB2238611B

Abstract

The apparatus is for checking the positioning accuracy of a mobile component such as a robot arm whereby the arm (2, Fig. 1) is moved to a checking position (4A). A beam emitted by the laser 16 passes through a passage 22 in a target 20 on the gripper 4 and is reflected from a reflector 26 onto a detector 18. If the arm does not precisely reach the checking position, the beam from the laser will be attenuated or fail to reach the detector. Alternatively, the laser and detector are aligned on opposite sides of the target through which a straight passage extends. <IMAGE>

Description

POSITION CHECKING This invention relates to apparatus for checking a position of a mobile component, and is particularly, although not exclusively, concerned with apparatus for checking the position of an end effector of a robot arm.

It is essential that the end effector of a robot arm will move precisely to the positions dictated by the program which controls it. Any deviation from these positions will result in inaccuracies in the manipulations performed by the end effector. In order to monitor the accuracy of a robot, it is known to move the end effector, under the control of the robot program, to a predetermined position in space and then to use external means for checking the accuracy of that position. However, the known checking systems are highly sophisticated and consequently expensive, with the result that it is economical to use such checking systems in only a few applications.

According to the present invention there is provided apparatus for checking a position of a mobile component, the apparatus comprising a source of electromagnetic radiation, a detector responsive to that radiation, and a target, the target comprising an opening through which radiation can pass from the source to the detector when the mobile component is in a desired position.

The opening may be in the form of a passage extending through the target. The passage may simply be a straight passage, passing through the target from one side to the other. Thesource and detector would then be aligned with one another on opposite sides of the target. In a preferred embodiment, however, a reflector is provided at the end of the passage, so as to reflect the incident beam from the radiation source through, for example, 900. From the reflector, the beam may pass through an outlet channel in the target.

For greater sensitivity, more than one reflector may be provided. For example, a second reflector may reflect the incoming beam 8o that it emerges orthogonal to the plane in which lie the incident and reflected beam of the first reflector.

The passage may be connected to an air supply so that air can be introduced into the passage (and the outlet channel, if provided) in order to remove any dirt or other material which may find its way into the passage or outlet channel, so as to avoid blocking the path of the beam of radiation.

The source of electromagnetic radiation is preferably a laser, the passage then collimating the laser beam so that light from the laser beam will be blocked if the target is in an incorrect position, or is incorrectly oriented.

The source and the detector may be mounted on a common support which is fixed with respect to a predetermined frame of reference within which the mobile component moves.

In a preferred embodiment, the target is provided on a robot end effector, and the source and detector will be mounted in a stationary manner so that, when the end effector is in a predetermined position, and in a predetermined orientation, light from the electromagnetic source will pass through the passage to the detector. If the target is in an incorrect position, radiation from the source will be blocked, at least partially, by the target, and will not reach the detector at full strength, if at all.

For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a robot installation; and Figure 2 shows a position checking system used in the installation of Figure 1.

The installation of Figure 1 comprises a robot arm assembly 2 having an end effector 4 in the form of a gripper. The position and orientation of the gripper are 'controlled by servo motors 6, which are under the control of an operating program.

As illustrated, the robot arm 2 is positioned over a conveyor 8 along which travel articles 10 which are to be picked off the conveyor by the gripper 4 and placed in predetermined positions on trays 12.

The positions of the articles 10 on the conveyor are determined by means upstream of the station shown in Figure 1, and are input into a memory of the processor controlling the operation of the robot arm 2.

Similarly, the memory contains data representing the positions of vacant regions of the trays 12.

It will be appreciated that it is essential, for the correct operating of the installation shown in Figure 1, that the gripper 4 is moved precisely to the position of the article 10 which it is next to pick up and that, after picking up the article 10, it moves precisely to the predetermined location on the trays 12 at which the article 10 is to be deposited. Thus, when the operating program for the robot arm provides signals to the motors 6 to move the gripper 4 to a predetermined position, it is essential for the motors to move the gripper 4 to the exact required location.

However, the small errors in the movements of the robot arm 2 are unavoidable. Although these errors may, individually, not be sufficiently large to affect the operation of the installation, the cumulative effect of the errors over several cycles of operation may result in the gripper 4 failing to pick up an article correctly, or failing to deposit it accurately in the required tray 12.

Consequently, it is desirable, at intervals in the operation of the robot arm 2 (for example at the end of each cycle, or after a predetermined number of cycles), to check the positional accuracy of the gripper 4. In the embodiment shown, this is achieved by moving the gripper 4 to a checking position and orientation. This is indicated in Figure 1 in phantom outline at 4A.

Situated near this position, and mounted on a common support 14 (Figure 2), are a laser 16 and a laser detector 18. The gripper 4 carries a target 20 which, when the gripper 4 is in the position 4A, causes the beam emitted by the laser 16 to be reflected to the detector 18. However, if the gripper 4 fails to reach precisely the position 4A, or reaches that position but in an incorrect orientation, the beam from the laser 16 will be attenuated, or may fail to reach the detector 18 altogether. Consequently, an error signal can be generated and corrective action can be taken. For example, operation of the robot arm can be terminated until the positioning of the gripper 4 has been corrected. Suitable software may be provided to enable automatic position and orientation correction.

The position checking system is shown in greater detail in Figure 2. The target 20 is formed from a solid block of material and is fixed to the gripper 4 which is represented, in Figure 2, by its motor box.

A collimating passage 22 and an outlet channel 24 are formed within the target 20. The passage 22 and the channel 24 are inclined to one another at 900, and a mirror 26 is provided where they meet. An air inlet fitting 28 is provided to enable cleaning air to be admitted to the collimating passage 22, in order to avoid accumulation of dust or other debris. In operation, when the gripper 4 is in position 4A in Figure 1, a beam emitted from the laser 16 will enter the collimating passage 22 and be reflected by the mirror 26 to pass through the outlet channel 24 to the detector 18. The detection of the beam by the detector 18 will generate an "OR!' signal which can be displayed, for example, by indicator lights 30. The robot arm 2 can then continue its operating cycle.

If the gripper 4 fails to reach the checking position 4A sufficiently accurately, or in an incorrect orientation, the beam from the laser 16 will fail to enter the collimating passage 32, or, even if it does enter the passage 22, will fail to reach the mirror 26.

The detector 18 will therefore not receive the reflected laser beam, or will receive it only at reduced strength. In these circumstances, an error signal will be generated, which, again, can be indicated by means of the lights 30. Appropriate corrective action can then be taken, either by a human operator, or automatically by means of an appropriate software routine.

By way of specific example, the laser may be a helium/neon laser, and the collimator passage 22 may have a diameter of 1.5 millimetres.

The simplicity of the system means that it can be constructed relatively inexpensively, and consequently that it can be used in robot installations in which previously the additional cost of a position checking system could not be justified.

The sensitivity of the device can be adjusted either by adjusting the sensitivity of the detector 18, or by altering the diameter of the collimator passage 22. Also, there could, for example, be a further mirror in the channel 24, reflecting the beam in a direction perpendicular to the plane of Figure 2. The detector 18 would, of course, then have to be repositioned to be in the path of the emerging beam.

It will be appreciated that, although the position checking system has been described with reference to a robot arm 2, similar principle can be applied to position checking in applications other than robotics.

Claims

1. Apparatus for checking a position of a mobile component, the apparatus comprising a source of electromagnetic radiation, a detector responsive to that radiation, and a target, the target comprising an opening through which radiation can pass from the source to the detector when the mobile component is in a desired position.

2. Apparatus as claimed in claim 1 wherein the opening is in the form of a passage extending through the target.

3. Apparatus as claimed in claim 2 wherein the passage is a straight passage, passing through the target from one side to the other, whereby the source and detector are aligned with one another on opposite sides of the target.

4. Apparatus as claimed in claim 2 or 3 wherein a reflector is provided at the end of the passage, so as to reflect the incident beam from the radiation source through an angle.

5. Apparatus as claimed in claim 4 wherein the angle is 0 9or .

6. Apparatus as claimed in claim 4 or 5 wherein the arrangement is such that the beam passes from the reflector through an outlet channel in the target.

7. Apparatus as claimed in claim 6 wherein, for greater sensitivity, more than one reflector is provided.

8. Apparatus as claimed in claim 7 wherein there is provided a second reflector to reflect the incoming beam so that it emerges orthogonal to the plane in which lie the incident and reflected beam of the first reflector.

9. Apparatus as claimed in any one of claims 2 to 8 wherein there is provided an air supply coonected to the passage so that air can be introduced into the passage (and the outlet channel, if provided) in order to remove any dirt or other material which may find its way into the passage or outlet channel, so as to avoid blocking the path of the beam of radiation.

10. Apparatus as claimed in any preceding claim wherein the source of electromagnetic radiation is a laser, the opening being arranged with respect to the source so as to collimate the laser beam so that light from the laser beam will be blocked if the target is in an incorrect position, or is incorrectly oriented.

11. Apparatus as claimed in any preceding claim wherei the source and the detector are mounted on a common support which is fixed with respect to a predetermined frame of reference within which the mobile component moves.

12. Apparatus as claimed in any preceding claim wherein the target is provided on a robot end effector, and the source and detector will be mounted in a stationary manner so that, when the end effector is in a predetermined position, and in a predetermined orientation, light from the electromagnetic source will pass through the opening to the detector, whereby, if the target is in an incorrect position, radiation from the source will be blocked, at least partially, by the target, and will not reach the detector at full strength, if at all.

13. Apparatus for checking the position of a mobile component, substantially as hereinbefore described with reference with the accompanying drawings.