BACKGROUND OF THE INVENTION 1) FIELD OF THE INVENTION [0002] The present invention relates to gripper devices associated with robotic arms and, more particularly, to an error detection mechanism associated with a robotic gripper for signalling an error during the gripping function.

2) DESCRIPTION OF THE PRIOR ART 100031 Material handling devices such as robots and laboratory movers are normally provided with a gripping device for grasping their payload. An example of such a gripping (or grasping) device is provided in US patent number 5,588, 688.

(0004] A common strategy in material handling system design incorporating gripping devices as indicated above, is to include some form of sensor to verify that the robot or mover successfully grasps the desired payload when commanded. The sensor is usually a proximity switch or optical sensor. Typically, the sensor is placed on the gripper and wiring is routed from the gripper to the body of the robot or mover.

10005] It is commonly known that wiring between the gripper and robot body continually flexes as the robot moves and, therefore, is susceptible to fatigue over time. To mitigate the risk of wire failure, careful routing and generous bend radii are required to prevent fatigue of the wire conductors.

D06] Various systems and mechanisms have been proposed to sense positioning and Lpplied force of gripping devices, examples of systems for sensing posilionmn no : rce are

provided in US patent numbers 4,682, 805 and 4,600, 357, respectively. One of the drawbacks with these known systems is the lack of an efficient grip error detection mechanism that does not require wiring as described above. Grip error occurs where the gripping device fails to grasp the desired article, which may be, for example, a container or a microtitre plate etc.

[0007] When a grip status detection sensor is placed on the gripper itself, traditional methods of routing electrical signals back to the robot or mover body include: use of a slip ring; and, direct wiring designed and routed for dynamic motion.

[0008] Slip rings tend to be expensive, sometimes large, and must be placed in the center of the joint of the robotic arm. Dynamic wiring must have adequate space available to bend or flex without fatiguing significantly. The larger the range of motion of the rotating gripper, the more space that is required. Furthermore, the sliding contact of brush type slip rings produces electrical noise, and can result in false readings. Wet contact slip rings have fewer noise problems, but are large and expensive. If the gripper rotate joint does not allow for inclusion of a slip ring or direct wiring because of space constraints, an alternate method of signal passing is necessary.

[0009] In developing a mechanism to pass grip status information to the associated sensor, it is necessary to maximize reliability and Mean Time Between Failure. It is of no value to eliminate the wire fatigue risk in the flexing sensor cable and replace it with similar risk in the new mechanism.

[0010] Apart from the disadvantage of prior art methods using wiring, another problem with known grip detection methods involves the inability to detect the grip signal at any position in the gripper joint rotation. For example, PCT publication number WO/03/037574 (the contents of which are incorporated herein by reference), which shares a common assignee with the present application, describes a robotic arm having a gripping device. The gripping device taught in this reference is provided with a stationary grip status sensor that avoids much of the wiring etc. as described above. However, the grip status detection of this svstcm is able to detect the grip position status at only four specific locations 1-. 1 the of unopcr travct.

[0011] Thus, there exists a need for a more efficient and accurate grip status sensor for robotic devices.

SUMMARY OF THE INVENTION [0012] In one aspect, the present invention provides an apparatus for signalling an error condition, the apparatus being provided on a robotic arm having a gripping device attached thereto, the gripping device having two or more gripping members adapted to engage an object, the error condition arising from the gripping device failing to engage the object, the apparatus comprising: - a sensor attached to the robotic arm for signalling the error condition; - a trigger attached to the gripping device or the robotic arm, the trigger being adapted to activate the sensor in the event of an error condition; - at least one switch provided on the gripping device for contacting the trigger in the event of an error condition and urging the trigger to activate the sensor; - the at least one switch being adapted to cooperate with the gripping members and being movable to contact the trigger when the gripping device fails to engage the object.

[0013] In another aspect, the at least one switch comprises a cam, the cam being pivotally attached to the gripping device and having a first end cooperating with at least one of the gripping members and a second end for contacting the trigger.

[0014] In another aspect, the gripping device comprises at least two members moveable towards each other for engaging the object, and each of the members having an associated cam.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] These and other features of the preferred embodiments of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: T0ûE 6 ! Figure 1 is a front perspective view of a gripping device associated with a rot7otic arm.

[0017] Figure 2 is a rear perspective view of a gripping device associated with a robotic arm.

[0018] Figure 3 is a detail of the device of Figure 1.

[0019] Figure 4 is a schematic diagram illustrating the sensor activating mechanism of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS [0020] In one aspect, the present invention provides a grip status sensing device for a robotic gripping device in a location on the robot, or mover, where wiring to the sensor remains static as the gripper moves, and provide a mechanism to pass grip status information to the sensor by some other means.

[0021] The present invention will now be described with reference to a specific example of its application. The invention is described herein in conjunction with a two jaw parallel pneumatic gripper as is known in the art and as illustrated in the figures contained herein.

Such grippers are commonly used in systems involving robotic transport of microtitre plates, for example. It will be appreciated by persons skilled in the art that this particular example is used purely for the purpose of illustrating the present invention and is not intended to limit the scope of uses of the invention in any way.

[0022] Figures 1 to 3 illustrate a typical robotic arm 10 having at its terminal end, a typical two jaw parallel pneumatic gripper 12, designed for grasping a microtitre plate 11.

The gripper 12 includes a pair of jaws 14 and 16, each having an associated gripper finger 18, 20, respectively. The gripper fingers are used to grip the payload 11, for example, a microtitre plate. As is known in the art, gripper fingers are customized for each particular payload and, as such, various other types of gripper fingers can be used. On the inside, or facing surface of each jaw, 14,16, is provided an activator 22 and 24, respectively, which are used to start the chain of motion of the mechanism of the present invention. The activators 22,24, are oriented such that each points outward (each in an opposite direction) from the side of the each gripper jaw. Activators 22 and 24 are designed to bear upon respective cams 26 and 28, which convert the motion applied horizontallv by the jawiactivator to a vcrnca : notion, as wo I1 he clescnbed further below.

[0023] Cams 26 and 28 are pivotally secured to the gripper device at pivot points 30 aloud 32, respectively. The cams are designed to rotate in opposite directions when activated, as will be described further below. The cams in turn are designed to bear upon on a preferably circular sensor trigger 34 provided at the joint of the gripping device and the robotic arm 10.

Such joints are generally designed to allow rotation of the gripping device. The trigger 34 may be attached to either the gripping device or the robotic arm. When the cams bear on the sensor trigger 34, the trigger is lifted vertically and activates the grip status sensor 36, which is mounted on the housing of the robotic arm 10.

[0024] The arrangement of the cams, activators and trigger 34 can be seen with reference to Figure 4 wherein the case of cam 26 is illustrated. As can be seen when the gripper arm 18 (not shown) is moved toward the opposing gripper arm 20 (also not shown), the activator 22 is moved in the direction of arrow A. If movement of the gripper finger 18 is not impeded, as would be the case where the gripper arms grasp the desired payload, the associated activator 22 moves to bear against an activator bearing surface 38 of cam 26. In the preferred embodiment, the cam is provided with a travel adjuster 40, which allows for the travel distance of the activator 22 to be adjustable. A similar travel adjuster 46 is provided on the opposing activator 24. As shown in Figure 3, the travel adjusters, for example 46, may optionally extend through the base of the cam and be associated with, for example, a set screw 50, which can adjust the length of the adjuster 40. Various other means of adjusting the length of the travel adjuster will be apparent to persons skilled in the art. Once activator 22 bears against the cam surface 38 (either directly of through the adjuster 40), the applied force causes the cam 26 to rotate about pivot point 30 in the direction of arrow B. This rotation of the cam in turn causes the cam trigger end 42 to move generally vertically in the direction shown by arrow C. In doing so, the cam trigger end 42 bears against the sensor trigger 34, which rises vertically and impinges on sensor 36, which is provided on the robotic arm 10 housing. At such time, an error condition is signalled. As indicated above, the activator 22 is provided on the jaw 14 of the associated finger 18. It will be understood that such arrangement minimizes the travel distance of the activator 22. It will also be appreciated that the cams 26 and 28 as well as the sensor trigger 34 are provided with snrings n'other such apparatus which serve to force each element into a"normal"settin2.

the sensor 36 is not activated. An example of such a spring, associated with the sensor trigger 34 is shown as elements 44a and 44b in Figures 1 to 3.

[0025] In a typical gripping sequence, the gripper fingers 18,20 close on the payload 11.

The gripper jaws 14,16 do not travel far enough to rotate the cams if the grip is successful.

Thus, the mechanism is not engaged so the sensor 36 does not receive a signal. If the gripper is commanded to close and there is no payload in place, the gripper jaws and activators travel farther. This time, the travel is far enough to contact the cams and rotate them as described above, lifting the circular sensor trigger. The trigger activates the grip status sensor 36, which may be an inductive proximity sensor as known in the art. When the gripper opens again, sensor trigger return springs 44 (shown as 44a and 44b in the figures) press on the sensor trigger 34, driving it away from the grip status sensor 36. This opens the sensing circuit, clearing the error condition. The sensor trigger 34 also pushes the cams back to their neutral position, ready for the next detection.

[0026] As described above, the sensor trigger 34 is preferably circular. As such, it will trigger the grip status sensor at any angle of gripper rotation. In other words, by having a circular trigger, the cams 26 and 28 can activate the trigger in any position of rotation of the gripping device. Persons skilled in the art will appreciate that various embodiments of the invention may be possible without a trigger 34 in the form of a continuous ring while still achieving the benefits of the present invention.

[0027] As indicated above, in one aspect of the invention, two cams 26 and 28 are provided. This type of arrangement serves to balance the force on the sensor trigger 34 at opposite sides of the ring, preventing binding or other such deformation.

[0028] The logic for detecting a failure according to the invention is as follows : - issue command for gripper to close: if no sensor triggered, grip is successful - issue command for gripper to close: if sensor triggered, no payload in gripper.

[0029] Using this invention, the gripper status detection sensor can be located on the section of the robot arm which supports the gripper rotate axis. As the gripper rotates, the sensor undergoes no motion, hence the sensor wiring remains static and would not be subjected to the strains as described above with respect to prior art devices.

[0030] Because the gripper status detection sensor is not located on the gripper, no provision needs to be made for moving wires. This allows for a more compact gripper rotate joint design.

[0031] The mechanism developed to pass grip status information to the sensor is designed such that it is only activated when a failed attempt to grasp a payload occurs. Because the frequency of failed grip attempts is typically fairly low, the actual operating time of the mechanism relative to the time of operation of the robot or mover is also fairly low. This increases the overall life of the mechanism.

[0032] Eventually, the mechanism may begin to show evidence of wear leading to improper activation. Provision is made in the design of the mechanism to adjust the activation range to compensate for wear. This feature is achieved, for example, by means of the travel adjusters 40 and 46 described above.

[0033] The mechanism of the present invention allows for detection of an error signal at any position in the gripper joint rotation. Because the invention is capable of detection at any position in the range of gripper travel, the mechanism design is very different.

[0034] The principal advantages and differences of the present invention over existing devices are as follows: ° no grip status sensor wire fatigue (due to the use of"static"wiring) ° more compact gripper rotate axis design ° can detect at any point in the rotation of the gripper axis ° adjustment can be made to compensate for wear of the mechanism

[0035] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.