Abstract:
An electrically driven gripper has a housing, a gearmotor attached to the housing, a cam engaged to the gearmotor, a pair of opposing jaws slidingly mounted to the housing, a cam follower secured to one of the jaws and in sliding contact with the cam; and a spring configured to impart a force to the jaws such that the jaws are pulled toward each other. The gearmotor drives the cam to force the jaws to a maximum open position. Continued rotation of the cam allows the spring to pull the jaws closed to grasp an object with the jaws. Sensors mounted to the housing determine whether the jaws are in an open or closed state. A simple circuit on a printed circuit board mounted to the housing is used to control the electric gearmotor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention pertains to robotic grippers and more particularly to grippers driven by electric motors. Grippers are used to grasp an object so the object can be held or moved to a desired location and orientation.  
           [0003]    2. Description of Prior Art  
           [0004]    The robotics and automation industry heavily relies on robotic grippers for grasping objects such as mechanical or electrical components so those components can be moved from one place to another or held in a particular orientation. Grippers of various sizes, shapes, and configurations have been used to handle objects ranging in size from as small as electronic components to as large as satellites deployed in or retrieved from low-earth orbit. Grippers can be opposing jaws, ensnaring wires that wrap around a grappling pin, anthropomorphic, hand-like designs, as well as many other specialized shapes.  
           [0005]    Grippers may be mounted on highly articulated robotic arms having multiple degrees of freedom, or simple automation devices that may have only one or two degrees of freedom. Generally, highly articulated grippers and robotic arms require complicated control and power systems. The majority of grippers currently used in the automation industry are pneumatically powered. This is primarily due to the significantly greater power obtainable from a pneumatically driven gripper compared to an electrically driven gripper of similar size. Disadvantages of prior versions of electrical grippers include being large, complex, delicate, or expensive.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention uses an innovative design to produce an electrically driven gripper with high gripping power in a small and relatively inexpensive package. The gripper of the present invention comprises an electrically driven gripper having a housing, a motor attached to the housing, a cam engaged to the motor, a pair of opposing jaws slidingly mounted to the housing, a cam follower secured to one of the jaws and in sliding contact with the cam; and a spring configured to impart a force to the jaws such that the jaws are pulled toward each other. The motor drives the cam to force the jaws to a maximum open position. Continued rotation of the cam allows the spring to pull the jaws closed to grasp an object with the jaws. Sensors mounted to the housing determine whether the jaws are in an open or closed state. A simple circuit on a printed circuit board mounted to the housing is used to control the electric motor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    So that the manner in which the described features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical preferred embodiments of the invention and are therefore not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
         [0008]    In the drawings:  
         [0009]    [0009]FIG. 1 is a side view of an electric gripper constructed in accordance with the present invention showing the gripper in its maximum closed position.  
         [0010]    [0010]FIG. 2 is a side view of the gripper of FIG. 1, but showing the gripper in its maximum open position.  
         [0011]    [0011]FIG. 3 is a top partial cross section view of the gripper of FIG. 1.  
         [0012]    [0012]FIG. 4 is an end view of the gripper of FIG. 1.  
         [0013]    [0013]FIG. 5 is a side view of a first alternative embodiment of an electric gripper constructed in accordance with the present invention showing the gripper in its maximum open position.  
         [0014]    [0014]FIG. 6 is a side view of the gripper of FIG. 5, but showing the gripper in a closed position.  
         [0015]    [0015]FIG. 7 is a side view of a second alternative embodiment of an electric gripper constructed in accordance with the present invention showing the gripper in its maximum open position.  
         [0016]    [0016]FIG. 8 is a side view of the gripper of FIG. 7, but showing the gripper in a closed position.  
         [0017]    [0017]FIG. 9 is a top view of the cam of FIG. 7.  
         [0018]    [0018]FIG. 10 is a side view of a third alternative embodiment of an electric gripper constructed in accordance with the present invention showing the gripper in its maximum open position.  
         [0019]    [0019]FIG. 11 is a side view of the gripper of FIG. 10, but showing the gripper in a closed position.  
         [0020]    [0020]FIG. 12 is a top view of the cam of FIG. 10.  
         [0021]    [0021]FIG. 13 is a side view of a fourth alternative embodiment of an electric gripper constructed in accordance with the present invention showing the gripper in its maximum open position.  
         [0022]    [0022]FIG. 14 is a side view of the gripper of FIG. 13, but showing the gripper in a closed position.  
         [0023]    [0023]FIG. 15 is a schematic diagram of a control circuit constructed in accordance with the present invention.  
         [0024]    [0024]FIG. 16 is a graph showing the displacement of a cam follower as a function of the rotational angle of the cam for a cam having a first profile constructed in accordance with the gripper of FIG. 1.  
         [0025]    [0025]FIG. 17 is a graph showing the displacement of a cam follower as a function of the rotational angle of the cam for a cam having a second profile constructed in accordance with the gripper of FIG. 1.  
         [0026]    [0026]FIG. 18 is a side view of an electric gripper constructed in accordance with the present invention showing a gripper designed to grip from within the interior region of an object in its maximum open position.  
         [0027]    [0027]FIG. 19 is a side view of the gripper of FIG. 18, but showing the gripper in its maximum closed position. 
     
    
     DETAILED DESCRIPTION  
       [0028]    [0028]FIGS. 1 and 2 illustrate an electric gripper  10  comprising housing  12 , gearmotor  14 , cam  16 , jaws  18 , and cam follower  20 . Housing  12  can be made of any durable, lightweight material, but is preferably metal or another conductive material that can be electrically grounded. Housing  12  serves as a base on and inside of which other structural elements are mounted. The housing  12  also protects the housed components. It is desirable that housing  12  be easily formed into complex shapes to allow for space-efficient integration of various components.  
         [0029]    Gearmotor  14  is a conventional electrically driven motor. Gearmotor  14  mounts to housing  12  and serves to drive cam  16 . The gearmotor  14  can be replaced by an electric motor and gearbox (e.g., FIG. 10), but better efficiencies and economy of scale are usually achieved in the combined gearmotor  14 . The term “gearmotor”, as used herein, shall mean either configuration or any type of suitable power source, including a pneumatically driven power source. The motor portion of gearmotor  14  can be virtually any type of electric motor. Different applications may dictate whether the motor is preferably an ac or dc motor, a stepper motor, an induction motor, a brushless motor, or other less common motor type. A dc motor offers the advantages of low cost and simple control requirements, but other requirements may dictate other motor types. Larger motors are generally required for larger grippers.  
         [0030]    The gear ratios in the gearbox portion of gearmotor  14  can be chosen to produce a desired rotational rate for the gearmotor output shaft  17 . The rotational rate of output shaft  17  can be chosen in conjunction with the size and profile of cam  16  to produce a particular opening or closing rate for jaws  18 , as will be further explained below.  
         [0031]    In the preferred embodiment of FIG. 1, cam  16  is rotationally joined to output shaft  17  by thrust bearing  22 . Thrust bearing  22  serves to isolate the relatively weak bearings of gearmotor  14  from loads transmitted through cam  16 . Cam  16  undergoes heavy loading while opening jaws  18 .  
         [0032]    Cam  16  is variously shaped depending on application and particular embodiment chosen. Again referring to the preferred embodiment of FIG. 1, cam  16  is a circular disk with a canted profile on its end face opposite thrust bearing  22 . FIG. 1 shows cam  16  having a narrow, flat portion  24 , a tapered portion  26 , and a wide, flat portion  28 .  
         [0033]    As cam  16  is rotated, cam follower  20  tracks the profile of cam  16 . Cam follower  20  is in constant sliding or rolling contact with cam  16  unless an object is grasped by jaws  18 . In such event, a gap may develop between cam  16  and cam follower  20 , as explained below. Cam follower  20  is preferably a roller attached to one of the jaws  18 A.  
         [0034]    The jaws  18  of FIG. 1 are linked by centering linkages  30 , illustrated in FIG. 3. Thus, jaws  18  move in oppositely directed, but synchronized motion. Because cam follower  20  is attached to jaw  18 A, cam follower  20  and jaw  18 A move as one body. Through linkages  30 , the force applied to jaw  18 A is transmitted to jaw  18 B, but in an opposite sense. Thus, jaw  18 B moves in an exactly opposite manner from jaw  18 A.  
         [0035]    [0035]FIG. 4 shows an end view of springs  32  and FIG. 7 shows the conceptual equivalent to springs  32 . Springs  32  attach to jaw  18 A and jaw  18 B at the respective ends of springs  32 . That is, the springs  32  attach to jaws  18  such that jaws  18  are pulled toward each other as springs  32  seek to return to their natural (unstretched) length. The spring force from springs  32  acts as a closing force, pulling jaws  18  closed so the jaws  18  can grasp an object.  
         [0036]    In the preferred embodiment of FIG. 2, springs  32  (not shown) alone supply the closing force to allow jaws  18  to grasp an object. In alternative embodiments described below, springs  32  may be replaced by alternative structure or functional arrangement. However, an advantage offered by springs  32  providing the closing force is cam  16  can be rotated to a particular position regardless of whether jaws  18  are grasping an object.  
         [0037]    While other structural elements can be added, the elements described above permit a description of the operation of the preferred embodiment of gripper  10 . FIG. 1 shows gripper  10  with jaws  18  fully closed. Springs  32  (not shown) hold jaws  18  closed, the spring force being at a minimum, but greater than zero. That is, springs  32  are slightly stretched even when jaws  18  are in their fully closed position.  
         [0038]    Note the position of cam  16  and cam follower  20  shown in FIG. 1. Cam  16  is positioned so that its narrow portion  24  is in contact with cam follower  20 . This configuration permits the jaws  18  to fully close if no object is being held by jaws  18 . To open jaws  18 , cam  16  is rotated by gearmotor  14 . Such rotation brings tapered portion  26  of cam  16  to bear against cam follower  20 . Tapered portion  26  displaces cam follower  20  and jaw  18 A, and thus jaw  18 B as well, away from the closed position. Recall jaw  18 B mirrors the motion of jaw  18 A because of linkages  30 . Jaws  18  are forced open by further rotation of cam  16  until cam follower  20  contacts the wide portion  28  of cam  16 , as shown in FIG. 2. Jaws  18  are fully open in this configuration and springs  32  are in their most stretched position. Thus, to open jaws  18 , gearmotor  14  must be able to rotate cam  16 , overcoming the spring force tending to close jaws  18 .  
         [0039]    To grasp an object, jaws  18  are initially set to their fully open position, as in FIG. 2. In that configuration, cam  16  is oriented such that cam follower  20  is precisely centered in wide portion  28 . Once the object is between jaws  18 , cam  16  is rotated so tapered portion  26  comes to bear against cam follower  20 . Further rotation of cam  16  presents a narrowing profile to cam follower  20 , and springs  32  pull jaws  18  together, keeping cam follower  20  in contact with cam  16 . When jaws  18  contact the object, further closing of jaws  18  is restricted by the object itself. Springs  32  holdjaws  18  in place, gripping the object. Motion of cam  16 , however, is not impeded and such rotation continues until cam  16  reaches the precise orientation corresponding to the fully closed position of jaws  18 . Thus, cam follower  20  ceases to be in contact with cam  16  once jaws  18  contact the object and cam  16  is rotated to the fully closed orientation.  
         [0040]    To release the object, cam  16  is rotated in the same direction as when closing jaws  18 . There is essentially no load on cam  16  until it engages cam follower  20 . At that point, cam follower  20  again begins to track cam  16  and jaws  18  are displaced by cam  16 . Thus, the object is released and cam  16  is rotated until jaws  18  reach their fully open position.  
         [0041]    Both narrow portion  24  of cam  16  and wide portion  28  of cam  16  have flat areas in which the fully closed and fully open positions are centered, respectively. The flat areas eliminate any torque being applied from cam follower  20  onto cam  16 . This allows gearmotor  14  to be turned off while cam  16  is in the fully open or fully closed configuration.  
         [0042]    The particular cant of cam  16 , in conjunction with the gearmotor output rotational rate, determines the amount and rate of displacement of jaws  18 . The cant and spring constant can be varied to achieve a desired closing force. The cant profile can be symmetrical or asymmetrical. FIG. 16 shows the displacement of cam follower  20  as a function of rotation of cam  16  for a symmetrical cant profile. The angular position of zero degrees corresponds to the fully closed position of jaws  18 . As cam  16  is initially rotated, there is no displacement because of the flat area on the narrow portion  24  of cam  16 . Once cam  16  rotates enough to bring tapered portion  26  to bear on cam follower  20 , cam follower  20  is displaced. The displacement increases until cam  16  is rotated nearly 180 degrees. The displacement reaches its maximum just prior to 180 degrees and remains constant until rotated slightly past 180 degrees because of the flat area on wide portion  28  of cam  16 . Jaws  18  are fully open while cam follower  20  is on the flat area in the neighborhood of 180 degrees. The displacement decreases with further rotation of cam  16  as cam follower  20  tracks tapered portion  26  until cam follower  20  again encounters the flat area on the narrow portion  24  of cam  16 . At that point, there is no further displacement as cam  16  is rotated to 360 degrees, returning jaws  18  to their fully closed position.  
         [0043]    A symmetrical cant profile tends to waste motor power because much more power is necessary to open jaws  18  than to close them. The force of springs  32  must be overcome to open jaws  18 . However, the force to close jaws  18  comes from springs  32 ; gearmotor  14  essentially coasts during that portion of the operational cycle. Thus, a more power-efficient cant profile is an asymmetrical one. That is, efficiency is gained by using a cant profile in which a greater percentage of circumference is dedicated to opening jaws  18  than to their closing. FIG. 17 shows how such an arrangement skews the displacement versus rotation curve. The mechanical advantage gained by using a longer ramp allows a less powerful gearmotor to be used.  
         [0044]    The above description for an asymmetrical cant profile applies to embodiments using springs  32  to close jaws  18 . Some alternative embodiments, such as those described below, do not use springs  32  to close jaws  18 . However, it may still be desirable to use an asymmetrical profile. In those embodiments, the mechanical advantage is analogous to that of a screw. The cant, analogous to the pitch (or lead), can be varied to produce a greater closing force than opening force.  
         [0045]    The above-described structure and operation describe a preferred embodiment of gripper  10  in a basic form. Gripper  10  can be made “smarter” and more user-friendly by including sensors, indicator lights, and control circuitry. Cam orientation sensor  34  can sense when cam  16  is rotated to the fully open or fully closed orientation. A part presence proximity sensor (not shown) can be included to sense whether gripper  10  successfully grasped an object or if it missed. Light emitting diodes can be mounted to a circuit board to indicate conditions such as the presence of power or motor movement.  
         [0046]    [0046]FIG. 15 shows a schematic diagram of an inexpensive, reliable circuit to control gearmotor  14 . The user can send a control signal to command gearmotor  14  to position cam  16  in the open or closed position. Control logic determines whether to allow power to flow to gearmotor  14  based on the user control signal and current status of cam orientation sensors  34 . When cam  16  reaches the desired orientation, gearmotor  14  serves as a brake by temporarily becoming a generator. This is an important feature because it eliminates cam overshoot. If cam  16  were to overshoot the desired orientation, the control logic would continue to seek the desired orientation angle indefinitely.  
         [0047]    Several alternative embodiments of the present invention are readily conceived. To a large extent, the essential difference between the alternative embodiments is the cam. FIGS. 5 and 6 show a variation in which a cam slot  38  is machined into cam  36 . Cam follower  40  is a pin captured by cam slot  38  so that cam follower  40  never loses contact with cam  36 . As cam  36  is rotated, cam follower  40  is displaced The motion imparted to cam follower  40  is transferred to jaws  18  as before. In this embodiment, however, there is no need for springs  32 . Jaws  18  are opened and closed solely by gearmotor  14 . Cam slot  38  can be designed so that more of the cam rotation is used to close jaws  18  than to open them. This allows the maximum closing force to be supplied to jaws  18 . However, because cam follower  40  never loses contact with cam  36 , gearmotor  14  will stall when an object is grasped. Therefore, appropriate current detection circuitry is required to limit current to gearmotor  14  to prevent gearmotor  14  from overheating or self-destructing. To release an object, the drive direction of gearmotor  14  must be reversed.  
         [0048]    [0048]FIGS. 7 and 8 show an embodiment in which linkages  30  are eliminated and jaw  18 B has its own cam follower  42 . FIG. 9 shows a cam  44  having a variable diameter. In this embodiment, cam  44  has an axis of rotation that is perpendicular to the displacement of cam followers  40  and  42 . The cam profile can be varied to achieve different objectives such as increased opening force or rapid closing of jaws  18 . FIG. 7 shows gripper  10  in its fully opened position. This is achieved by rotating cam  44  so that cam followers  40  and  42  are separated by the maximum diameter “D” of cam  44 . The fully closed position would correspond to cam  44  being rotated so that cam followers  40  and  42  are separated by the minimum diameter “d”.  
         [0049]    [0049]FIG. 8 shows an object being grasped by this embodiment. Similar to the embodiment of FIG. 1, springs  32  supply the closing force to jaws  18 . Because cam followers  40  and  42  are not captured in a cam slot, they lose contact with cam  44  as soon as jaws  18  contact the object. Thus, this embodiment has operational features similar to the embodiment of FIG. 1, such as the ability to drive gearmotor  14  in only one direction to open and close jaws  18 . However, the embodiment of FIG. 7 could be modified so that cam  44  captures cam followers  40  and  42 . This would eliminate the need for springs  32  and more closely resemble the operational features of the embodiment of FIG. 5.  
         [0050]    [0050]FIGS. 10 and 11 show an arrangement in which output shaft  17  is at a right angle to the motor shaft (not shown). This may be useful to reduce the overall length of gripper  10 . A single cam follower  40  is used, along with linkages  30  and springs  32 . This embodiment is operationally similar to the embodiment of FIG. 1. Springs  32  serve to close jaws  18  and linkages  30 , in conjunction with the displacement of cam follower  40 , serve to move the jaws in opposing directions. Jaws  18  are opened by cam  46  as it is rotated by output shaft  17 .  
         [0051]    [0051]FIG. 12 shows cam  46  as having a variable diameter perpendicular to its axis of rotation, similar to that of FIG. 9, but with an elliptical profile. The elliptical profile is yet another example of variability of cam profile to achieve different objectives. The symmetrical profile yields a correspondingly symmetrical displacement of jaws  18 . The major and minor axes can be varied to modify the throw or opening/closing rate of jaws  18 . Recall, an advantage to configurations using springs  32  to supply the closing force, and that allow cam follower  40  to lose contact with cam  46  upon contact by jaws  18  with an object, is cam  46  can always be driven to a precise orientation, such as fully open or fully closed, thus simplifying the control system. However, as before, the design can be modified so that cam follower  40  is captured by cam  46 .  
         [0052]    [0052]FIGS. 13 and 14 show an embodiment that is operationally similar to that of FIG. 5. The embodiment of FIG. 13, however, uses a second cam  48  with a second cam slot  50  and second cam follower  52 , eliminating linkages  30  and springs  32 . The embodiment of FIG. 13 also shows a measurement device  54 . Measurement device  54  can be, for example, a micrometer, a magnetorestrictive position sensor, an encoder, or potentiometer. A measurement could be made while the object is being moved to a drop off position. Additionally, measurement device  54  could be embedded in jaws  18  to measure physical attributes such as pH, color, or temperature.  
         [0053]    Any of the above-described embodiments can be modified so jaws  18  move apart to grasp an object from within the object&#39;s interior region, such as along an inner diameter or the inside walls of a channel. FIGS. 18 and 19 illustrate a representative embodiment for such modified versions. FIG. 18 shows cam  56  displacing cam follower  58  in a manner that compresses spring  60  and moves jaws  18  together. As before, linkages  62 ,  64  coordinate a reciprocal motion between jaws  18 A and  18 B. In that configuration, jaws  18  can be inserted into the interior region of an object.  
         [0054]    [0054]FIG. 19 shows cam  56  rotated to allow spring  60  to separate jaws  18 . Jaws  18  separate until contacting the interior walls of channel  66 . Cam follower  58  loses contact with cam  56  as cam  56  rotates to the “fully closed” position. Spring  60  is compressed throughout the entire operational cycle of this embodiment.  
         [0055]    The present invention offers many advantages over the prior art. For example, for those embodiments using springs to close jaws  18 , there is less chance of gears being stripped due to excessive motor force. Those embodiments also allow a very simple controller, or even no controller, to be used. The motor need be driven in only one direction to open and close jaws  18 , and can be driven to precise orientation without regard to whether an object is grasped by jaws  18 . Those embodiments are simple, inexpensive, fast, and powerful.  
         [0056]    Those embodiments that do not use springs offer advantages as well. Gripping force of jaws  18  can be actively controlled by the motor. Also, different cant profiles are available because there is no spring force to overcome.  
         [0057]    While the invention has been particularly shown and described with reference to a preferred and alternative embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.