Abstract:
An assembly for releasably connecting an end effector in the form of a robotic tool or component to a robotic arm is disclosed. The connection is manually operated and formed of a first and second joint member including a cylindrical body, a locking collar, and a locking wall extending from the cylindrical body. The locking collar is coaxially aligned with and rotatably connected to the first joint member. The second joint member has a cylindrical mating body and a coupler, and engages the first joint member. The coupler also includes key pins, the pins being engageable in keyed relationship with the locking wall, the coupler and locking collar further includes intervening circumferentially spaced teeth, wherein the collar is rotatable to releasably engage the first joint member with the second joint member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on U.S. Provisional Patent Application No. 61/268,085, filed Jun. 8, 2009, on which priority of this patent application is based and which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Manipulators on mobile robots often require specialized end effectors (tools/components) in order to accomplish particular missions. Currently, deployed systems have end effectors designed, built, and installed at the factory. Factory installed tools can only be repaired or replaced in a factory. This limits the effectiveness of the robot to those missions which can be achieved with a single tool. Heretofore, when a new candidate task is identified, the typical response has been to design and build a new robot intended to perform the specific task. Sometimes existing unmanned ground vehicles (UGV) platforms are used, but just as often, a new robot is created to specifically address the task. This has resulted in a proliferation of small UGVs, each performing admirably on tasks within each of its subset of core competencies, but is generally unsuitable for tasks that vary too widely from its essential purpose. It is impractical to expect field teams to carry multiple UGVs, each suited for a specific task. In addition to the strain on the physical resources of the field team (e.g., transportation and maintenance), different robots come with different control schemes. This reduces the ability of the operator to capitalize on the experience and intuition gained from operating previous robots, because the operator cannot rely on the trained reflexes developed while controlling previous robots. In fact, these differing control schemes lead to operator errors and inefficient control. 
     Another approach has been to design new, more capable robots, but this approach has drawbacks because even if a robot were designed and built to perform all of the tasks currently assigned to UGVs, it would quickly become outdated as new tasks and jobs are identified. Additionally, external variables, such as physical environment, make UGVs designed for one environment wholly impractical for use in another environment, meaning a number of new robot types would need to be designed, tested, and built. Systems with replaceable end effectors are also ineffective because they require a technician and possibly a number of specialty tools. Generally, these changes would require a technician to remove the current tool and to attach its replacement. This may involve physically disconnecting the tool, disconnecting electrical connections, physically attaching the new tool, and hooking up its electrical connections. The system may also require reconfiguring the control software for each specialized tool. Particularly, in time critical applications, such as military or civilian Explosives Ordinance Disposal (EOD), this process is too slow and interferes with missions. 
     Military and law enforcement groups are increasingly relying on UGVs to perform life-threatening tasks ranging from under car inspection to EOD. As small UGVs, such as Omni-Directional Inspection Systems (ODIS), Talon and Packbot have gained acceptance, the variety of tasks they have been required to perform has increased. 
     In addition, unlike industrial robots, these systems are deployed in uncontrolled environments. They must have a robust design to survive the normal working environment they will encounter, both during deployment on the mobile robot and when the manipulator and tools are being stored or transported. The mechanical connection must be resilient to minor variations in tolerances of mating components, such as might occur when a tool is dropped or bumps against another tool in the toolbox, or such as might be caused by the presence of debris, such as dirt and sand, from the working environment. 
     Robotic arms often require specialized configurations to accomplish their particular mission, requiring change in the length of a link in the arm or attaching a different end effector or tool. 
     Tools that attach to links of the robotic arm that are pivoting or rotating must be able to withstand the large bending movements and torques that result from this. 
     An object of the present invention is to provide a quick-release assembly for separating robotic end effectors mechanically from their manipulator arms, thus allowing unhindered integration of end effectors as the complexity of the system is contained in the manipulator arms. A further object is to make the end effectors replaceable units that can be replaced by hand when they fail. 
     SUMMARY OF THE INVENTION 
     The present invention is an assembly for releasably connecting an end effector to a robotic arm comprising a first joint member having a cylindrical body, a collar, and a locking wall extending from said cylindrical body. The collar is coaxially aligned with and rotatably connected to the locking wall. A second joint member has a cylindrical mating body and a coupler, the cylindrical body of the first joint member being engageable with the mating body of the second joint member. The coupler being engageable in keyed relationship with the locking wall, the coupler and locking collar further includes intervening circumferentially spaced teeth, wherein the collar is rotatable to releasably engage the first joint member with the second joint member. The assembly further includes a locking pin extending axially outward from the collar. An engaging hole is included in the locking wall and a pin in the coupler, wherein the pin is receivable in the engaging hole. Displacement of the first joint member into the second joint member causes the pin to move adjacent the wall, and further displacement of the first joint member into the second joint member is terminated by the wall until alignment of the pin with the engaging hole occurs. The termination of displacement of the first joint member into the second joint member is offset by the length of the pin, wherein the length is within a range to terminate displacement before connections are made between the first joint member and second joint member. The second joint member has electrical connector terminals and the first joint member has engaging holes, whereby engagement causes the electrical connector terminals joined to the engaging holes to form a connection. The pin length terminates displacement before electrical connector terminals and engaging holes. 
     The teeth of the locking collar and coupler have chamfered edges. The locking collar rotation forces the chamfered edges of locking collar teeth to slide over the chamfered edges of the coupler teeth, wherein the chamfered edges facilitate engagement of the teeth. 
     The assembly can include a flexible ring placed between a lip of the collar and an end of the first joint member, wherein the ring is compressed between the collar and the first joint member when the collar is releasably engaged to the coupler. The locking wall of the first joint member includes a notched surface for engagement with a pin of the locking collar. The locking pin is a spring loaded retaining pin. The alignment ring can be aligned coaxially within the coupler for receiving said first joint member. The second joint member and first joint member can engage to form an electrical connection operative to transmit images, control signals, activators, identification information, video, USB, TCP/IP, UDP, and CanBus. 
     A non-limiting list of components of the present invention can include a manipulator arm, a boom arm, a stick arm, a gripper, a gimble grip, a flexible joint, a tilt table, a dozer, a shovel, a plow, a pan tilt table, or a digger. 
     The assembly can withstand large forces, in one embodiment, up to 3,000 pounds with the application of a 3 ft-lbs torque to the locking collar by hand. 
     The quick-release assembly can be connected to a robot arm. A robot end effector quick-release arrangement comprises a first joint member having a cylindrical body, a collar, and a locking wall extending from said cylindrical body, the collar being coaxially aligned with and rotatably connected to said locking wall, and a second joint member having a cylindrical mating body and a coupler, the cylindrical body of the first joint member being engageable with the mating body of said second joint member. The coupler is engageable in keyed relationship with the locking wall, the coupler and locking collar further including intervening circumferentially spaced teeth, wherein the collar is rotatable to releasably engage said first joint member with the second joint member. A robot arm attached to the first joint member with a chip embedded in said robot component and a connection from the component to a control unit with an identification signal, wherein the embedded chip transmits an identification of the component to a control unit through said connection is provided. 
     Also provided with the present invention is a robot end effector quick-release assembly, comprising a first joint member mounted on a robot component, and a locking collar for attaching to a coupler of a second joint member. 
     The present invention also teaches a method for connecting a robotic tool to a robotic arm, comprising providing a first joint member having a cylindrical body, a collar, and a locking wall extending from said cylindrical body, a second joint member having a cylindrical mating body and a coupler, displacing the first joint member into the second joint member, aligning the coupler to the first joint member by rotating the coupler having intervening teeth extending radially outward circumferentially spaced on a second end of the coupler from an end of the coupler, at least one of the teeth having a pin therethrough extending axially outward, until the pin mates to an engaging hole of the first joint coupler, whereby the pin of the coupler is received by the engaging hole, causing alignment of the second and first joint member. The intervening teeth of the coupler is rotated into engagement with teeth located circumferentially about the locking collar, wherein the locking collar rotation forces the teeth of locking collar to slide over the teeth of coupler, further wherein the coupler is clamped into engagement with the first joint member; and engaging a retaining pin to lock the collar to the first joint member. The method further includes the step of terminating displacement of the first joint member into second joint member when the pin engages the locking wall. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top-perspective view of the male and female coupler components of the quick-release assembly of the present invention; 
         FIG. 2  is a top-perspective view of the mechanical coupler components of the quick-release assembly of the present invention; 
         FIG. 3A  is a cross-sectional view of the male and female couplers showing the teeth when the quick-release unit is disengaged; 
         FIG. 3B  is a cross-sectional view of the male and female coupler components of the connector showing the male and female teeth in the engaged position of the present invention; 
         FIG. 4  is a cross-sectional side view of the male coupler unit of the present invention; 
         FIG. 5  is a portion of the view shown in  FIG. 4  taken along the broken lines of an O-ring in the male coupler enlarged for magnification purposes; 
         FIG. 6A  is a front view showing a male and female coupler engaged; 
         FIG. 6B  is a cross-sectional view of the object depicted in  FIG. 6A  taken along the broken lines, marked  6 B with the arrows indicating the direction of sight; 
         FIG. 6C  is a portion of the view shown in  FIG. 6B  enlarged for magnification purposes; 
         FIG. 7A  is an exploded view of the coupler components of the present invention; 
         FIG. 7B  is a cross-sectional view of the engaged coupler components of the present invention; 
         FIG. 8  is a top-perspective view of the mated coupler components in the unlocked position of the present invention; 
         FIG. 9  is a top-perspective view of the engaged coupler components in the locked position of the present invention; 
         FIG. 10  is a side view of a locking collar having a power transfer device; 
         FIG. 11  is a block diagram showing the method steps in accordance with the present invention; 
         FIG. 12  is a front view of male coupler in engagement position; and 
         FIG. 13  is a front view of male coupler in disengaged position. 
         FIG. 14A  is a perspective view showing a robot arm, a robot component that is attached to the end of the robot arm, and a system control unit in communication with the robot arm; 
         FIGS. 14B-14Q  are perspective views showing different robot components that are attachable to the end of the robot arm; and 
         FIG. 15  is a schematic drawing depicting the communication between an embedded chip in a robot component and a system control unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     These and other features and characteristics of the present invention, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and the claims, the singular form of “a”, “an”, and “the” include plural reference unless the context clearly dictates otherwise. 
     As shown in  FIG. 14A , the quick-release assembly can be connected to a robot arm  62 . The quick-release assembly provides a light-weight mechanical coupler to change-out tools to a robotic manipulator. The mechanical coupler can rigidly connect an end effector to a robotic manipulator and can include an electrical connection to pass power and signals between the end effector and a manipulator. The connection can also have full pass through power, electrical, and signal capabilities. As shown in  FIGS. 14B-14Q , end effectors that can be attached using the quick-release assembly can include components  60  such as a retrievable delivery device, gripper  60   a , gimble grip  60   b , dozer  60   c , shovel/digging tools  60   d , tilt table  60   e , drills  60   f , saws  60   g , cutters  60   h , grinders  60   i , sensors  60   j , camera  60   k , disruptor  60   l , arm extenders  60   m , arm linkages  60   n ,  60   o , and pan-tilt table  60   q . One skilled in the art will recognize that the use of other types of components with the quick-release mechanism of the present invention is possible. 
     A further object of the invention is adaptability. End effectors can operate seamlessly since they can be plug-n-play. In one embodiment, as shown in  FIG. 15 , an operator control unit  64  can identify the current end effector and the current controller by reading an embedded chip  66  in the end effector and can pass electrical signals to control the end effector through the quick-release assembly of the present invention. The embedded chip  66  can contain a unique identifier  68  for the particular end effector. Therefore, when a new end effector is attached using the quick-release assembly of the present invention, a unique identifier  68  for the tool can be read and passed to an onboard or external computer system that can analyze the signal  68  to identify the present end effector. The information can be used in programming instructions on an operator control unit  64  to operate the end effector accordingly. Alternatively, other types of electronic components can be used to produce an identifier signal, such as a jumper or resistor operative in the end effector to send a valve that can identify a component. 
     With reference to  FIG. 1 , a quick-release assembly of the present invention is shown including a first joint member and a second joint member coupled together to form a connection between a robotic manipulator of a robot. The first joint member can be a male coupler  2  and the second joint member can be a female coupler  4 . Male coupler  2  includes a locking collar  6 , a support tube  8 , and electrical housing  10 . The female coupler unit  4  includes a retaining tube  12  having a cylindrical cavity  14  formed therein for receiving the electrical connector housing  10  and support tube  8  of the male coupler unit  2 . The locking collar  6  can be a substantially cylindrical body rotatable about a circular locking wall  28  positioned on the first end  18  of the male coupler unit  2 . The locking collar  6  can further include engaging holes  20  and  22 , which can be mated to keying pins  24  and  26  of the female coupler unit  4  when the support tube  8  of male coupler unit  2  is inserted into cavity  14  of the female coupler unit  4 . During engagement of the coupler units, the female coupler unit  4  receives the male coupler unit  2 , the keying pins  24  and  26  of female coupler  4  are pressed up against first wall  28  at first end  18  of male coupler  2 . When the keying pins  24  and  26  are positioned against wall  28 , they will stop the displacement of the male coupler unit  2  into the female coupler unit  4 . At this point in the engagement, the female coupler unit  4  will not advance until the key pins  24  and  26  are aligned with the holes  20  and  22 . The female coupler or male coupler  2  can be rotated and the keying pins  24  and  26  being pressed against wall  28  will not be allowed into further vertical movement until they eventually mate with the engagement holes  20  and  22  of the locking collar  6 . After the keying pins  24  and  26  are aligned with the keying pin holes  20  and  22 , the keying pins  24  and  26  slide into the holes  20  and  22  and the male coupler  2  is further displaced into female coupler  4 . The displacement of the male coupler unit  2  into the female coupler unit  4  can continue until the units are engaged. 
     One object of the key pins  24  and  26  is to facilitate the mating of the internal components of male coupler  2  and female coupler  4 . As shown in  FIG. 2 , the electrical connector  100  engages an electrical connector unit  102  of female coupler  4  (shown in  FIG. 7 ). It is important that the electrical connector units of electrical connector  100  shown in  FIG. 2  are aligned properly with the electrical connection receivers  102  of the female coupler unit  4  before engagement. The keying pins  24  and  26  can be designed to only mate with the respective correct keying pin holes  20  and  22 . In addition, the pins  24  and  26  can be provided with a length which is sufficient to stop engagement of the internal components until alignment is correct. In other words, no internal parts can be connected until the keying pins  24  and  26  are aligned with the proper keying pin holes  20  and  22  at which time the engagement process can continue. One skilled in the art can recognize the combination of pin orientation and pin length can vary according to the specific placement of component parts. 
     With continuing reference to  FIG. 2 , as the engagement process continues, the female coupler unit  4  further includes at one end  16  a set of radial teeth  30   a ,  30   b ,  30   c , and  30   d . The teeth  30   a - 30   d  are equally spaced circumferentially about the outer surface of female coupler unit  4  and facing radially outward having engaging surfaces  32   a ,  32   b ,  32   c , and  32   d  on the interior wall of respective teeth  30   a - 30   d . The male coupler unit  2  also has radial members, formed of a set of teeth  36   a ,  36   b ,  36   c , and  36   d  spaced circumferentially radially inward about the axis of male coupler unit  2 . The teeth  36   a - 36   d  have engaging surfaces  38   a ,  38   b ,  38   c , and  38   d  on interior wall of respective teeth  36   a - 36   d . When the male coupler unit  2  is engaged with the female coupler unit  4 , the engaging surface  32   a - 32   d  of the teeth  30   a - 30   d  are mated with the engagement surfaces  38   a - 38   d  of teeth  36   a - 36   d.    
     With reference to  FIGS. 3A and 3B , the teeth  36   a - 36   d  of the male coupler unit  2  and  30   a - 30   d  of the female coupler unit  4  are shown in an open position in  FIG. 3A . In an embodiment of the present invention, surfaces  38   a - 38   d  and surfaces  32   a - 32   d  can be chamfered to facilitate the mating of the surfaces. As the female coupler unit  4  is rotated counter clockwise, the chamfered edged surfaces  32   a - 32   d  of the teeth  30   a - 30   d  will mate with surfaces  38   a - 38   d . When displaced together, the surfaces  38   a - 38   d  of the male teeth  36   a - 36   d  slide past the female surfaces  32   a - 32   d  and mate the teeth  30   a - 30   d  and  36   a - 36   d . The female coupler unit  4  and the male coupler unit  2  are mated by rotating one or the other, or both, causing the teeth  30   a - 30   d  and  36   a - 36   d  to become engaged as shown in  FIG. 3B . When the quick-release assembly is closed, the teeth are adjacent and mated. To achieve the closed position, as shown in  FIG. 3B , the locking collar  6  can be used to rotate the male coupler unit  2 , thereby forcing the chamfered ramps on the teeth surfaces  32   a - 32   d  and  38   a - 38   d  to slide onto and past each other. As the locking collar  6  is further rotated, any excess space between the teeth  30   a - 30   d  and  36   a - 36   d  is displaced and they are brought into tight contact with each other. Rotation of the locking collar  6  causes a clamping action between the teeth  30   a - 30   d  and  36   a - 36   d , thereby forming a tight fit. When the unit is fully engaged, the locking collar  6  will hit a mechanical stop and further rotation is halted. 
     With reference to  FIG. 4 , the locking collar  6  is shown with retaining pin  40  locked. When locking collar  6  is in an open state, the retaining pin  40  is positioned about the locking surface  44  and presses into the male coupler unit  2 . The retaining pin  40  engages locking hole  42  to form a detent such that the retaining pin  40  can be released by pulling on the head of the retaining pin  40  until the force applied withdraws the retaining pin  40  form the locking hole  42  and the locking collar  6  is thereby free to rotate about the locking surface  44  of the male coupler unit  2 . 
     With continued reference to  FIG. 4 , the male coupler unit  2  is shown further including an O-ring  46 , positioned between the locking surface  44  of the locking collar  6 , and washer  48  positioned adjacent a wall  27  at one end of male coupler  2 . 
     With reference to  FIG. 5 , the O-ring  46  is shown uncompressed. The O-ring  46  is resting between the unlocked locking collar  6 , washer  48 , and the locking surface  44  of male coupler unit  2 . 
     As shown in  FIGS. 6A and 6B , the locking collar  6  is now in an engaged position and couplers  2  and  4  forming a compressed and complete engagement.  FIG. 6C  is an enlarged view of the surfaces shown in  6 A and  6 B showing the locking surface  44  being forced toward the washer  48  and surrounding wall  27 , causing the compression of O-ring  46  as the locking collar  6  is rotated therebetween. As force is applied and the O-ring  46  is compressed, a resistance is formed between the locking surface  44  of the locking collar  6  and the locking surface  27  of male coupler unit  2 . As shown in  FIG. 6C , the positioning of the O-ring  46  enables the device to provide slack between coupler  2  and coupler  4  and, therefore, allows the quick-release assembly to make a rigid connection. It also reduces the need for adjustment of the couplers to form a tight fit. 
     With reference to  FIG. 7 , the female coupler unit  4  can further include a female electrical connector  102  and secondary alignment ring  104 . Electrical connector  102  and secondary alignment ring  104  are positioned internal to the retaining tube  12  of the female coupler unit  4 . As discussed previously, the female electrical connector  102  mates with the male electrical connector  100 , as seen in  FIG. 2 , inside the male coupler unit  2  as the male coupler unit  2  is displaced into the female coupler unit  4 . 
     With reference to  FIG. 8 , the male coupler unit  2  and female coupler unit  4  are not locked and the teeth  30   a - 30   d  and  36   a - 36   d  are shown in the unaligned position. 
     With reference to  FIG. 9 , the male coupler unit  2  and female coupler unit  4  are shown with the locking collar  6  engaged and the teeth  30   a - 30   d  and  36   a - 36   d  aligned. 
     With reference to  FIG. 10 , male and female coupler units are shown having a power transfer unit. To allow for inexpensive tools (by removing the motor and motor controllers), tools may require a source of mechanical power to drive the tool, such as a rotating shaft. However, some tools may require additional motors, processors, or sensors so connections for electrical power and electrical control signals are also required. The control software may require electrical connections from the tool to convey sensor information from the tool, as well as information which identifies the tool that is currently attached, United States Application Publication No. 2009/0044655, filed Jul. 3, 2008, is incorporated herein, showing an example of such an arrangement. In addition, feedback information can propogate back to the end effector. The electrical connector can transmit additional information including images, control signals, activators, identifiers, video, Universal Serial Bus (USB), Transmission Control Protocol/Internet Protocol (TCP/IP), User Datagram Protocol (UDP), CanBus, or jumper pin identifiers. 
     With reference to  FIG. 11 , a method of opening a locking collar  6  begins at block  200 . At conditional block  204 , if the locking retaining pin  40  is released, the locking collar  6  can be turned. If not, the locking collar  6  is in a locked position. At block  206 , the male coupler unit  2  is displaced into the female coupler unit  4 . At block  208 , if the male coupler unit  2  has been sufficiently displaced into the female coupler unit  4 , the pins  24  and  26  of the female coupler unit  4  will meet the locking wall surface  28  of male coupler unit  2 . At block  210 , after the pins  24  and  26  meet the wall  28 , the female coupler unit  4  can be rotated until the pins  24  and  26  engage the holes  20  and  22 . 
     Alternatively, the male coupler unit  2  can be rotated until the female coupler unit  4  mating pins  24  and  26  engage with the mating holes  20  and  22  of the male coupler unit  2 . 
     At conditional block  212 , when the pins  24  and  26  are aligned, the method can continue because internal components are aligned. At block  214 , the pins  24  and  26  are further displaced into the mating holes  20  and  22  and the internal components such as electrical receivers  102  of the female coupler unit  4  are coupled with the internal components such as electrical connectors  100  of the male coupler unit  2  and further, an alignment ring  104  aligns the male coupler unit  2  as it is displaced into the female coupler unit  4 . If the pins  24  and  26  are displaced fully into the mating holes  20  and  22 , the locking collar  6  can be rotated. Rotating the locking collar  6 , at block  218 , causes the engagement of teeth  30   a - 30   d  of the female coupler unit  4  with the teeth  36   a - 36   d  of the male coupler unit  2 . Further rotation of the locking collar  6  forces the male teeth  36   a - 36   d  to slide outside of the female teeth  30   a - 30   d  and pull the teeth  30   a - 30   d  of female coupler unit  4  toward the wall  28 . This movement of the teeth together causes movement of the locking surface  44  toward washer  48  and surrounding wall  27 , thereby compressing the O-ring  46  positioned between. 
     At block  222 , locking collar  6  rotation continues until the locking collar  6  locks to the couplers  2  and  4 . In one embodiment, the rotation can be a ⅛ rotation. After the locking collar  6  rotation stops at block  222 , the wall detent  42  in the locking retaining pin  40  slides into a corresponding hole. The engagement of the locking collar can be achieved with little relative force compared to the amount of force in the assembly. In one embodiment force of up to 3,000 pounds is achieved with under 3 ft-lbs of actuation torque applied by hand to locking collar  6 . Although aluminum is used in the preferred embodiment, other types of materials can be used to achieve strength or to effect weight. These materials include steel, titanium, stainless steel, brass, carbon composite, acetal resin, fiber glass composite, polyethelyne, or plastic. 
     To disengage the quick-release assembly, first at block  300 , disengagement begins. At block  302  the detent is disengaged by pulling the head of the locking retaining pin  40 . At block  304 , the locking collar  6  is rotated, in this case counter clockwise, until the locking collar  6  disengages. At block  306 , as the locking collar  6  is rotated counter clockwise, the teeth  30   a - 30   d  of the female coupler unit  4  and  36   a - 36   d  of the male coupler unit  2  are disengaged and slide away from each other forming an open engagement. At block  308 , the female coupler unit  4  and male coupler unit  2  can be pulled apart freely. 
     With reference to  FIG. 12 , in one embodiment, the mechanical stop is formed of a slotted surface  50 , defined by a first lip  52  and a second lip  54  of the locking collar  6 , where the slotted surface interacts with a dowel pin  56  on surface  27 . When the locking collar  6  is rotated to open, the locking collar  6  moves in relation to dowel pin  56 , as seen in  FIG. 12 . 
     With reference to  FIG. 13 , when locking collar  5  is moved to engagement position, the locking collar  6  moves in relation to dowel pin  56  until lip  52  engages dowel pin  56 , causing the locking collar  6  to stop rotation. The movement of the slotted surface  50  of locking collar  6  about the dowel pin  56  therefore forms the mechanical stop. One skilled in the art will recognize that other mechanical stop techniques used to stop rotation of the locking collar  6  are possible.