Abstract:
An electrically powered clamp has a housing, a motor attached to the housing, a ball screw driven by the motor via gears, and a linkage driven at one end by the ball screw such that the linkage rotates an output shaft attached to the other end of the linkage. The motor and gears drive the ball screw to a fully extended position to rotate the shaft to a clamped position or to a fully retracted position to rotate the shaft to an unclamped position. A built-in computer monitors and controls the clamp. The clamp can also be controlled and monitored by a remote pendant. Indicator lights on the housing and remote pendant convey clamp status information. The clamp is programmable and can memorize the clamped and unclamped positions. The clamp uses velocity and position feedback to determine appropriate drive mode. Torque monitors and timers determine if the clamp becomes stuck.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to power clamps and more particularly to clamps driven by electric motors. Clamps are used to secure an object to aid assembly or to secure it during transport from one location to another. 
     2. Description of Prior Art 
     The robotics and automation industry heavily relies on power clamps for securing objects such as mechanical or electrical components so those components can be integrated into an assembly or moved from one assembly station to another. Clamps of various sizes, shapes, and configurations have been used to secure objects ranging in size from as small as electronic circuit boards to as large as entire automobile body panels. Clamps can be comprised of opposing members, but are more commonly mounted to a work surface and use one arm to pin the object against the work surface. 
     The majority of clamps currently used in the automation industry are pneumatically powered. This is primarily due to the significantly greater power obtainable from a pneumatically powered clamp compared to existing electrical clamps of similar size. Disadvantages of prior versions of electric clamps include being large, complex, delicate, or expensive. 
     SUMMARY OF THE INVENTION 
     The present invention uses an innovative design to produce an electric clamp with high clamping power in a small and relatively inexpensive package. The clamp of the present invention comprises an electrically powered clamp having a housing, a motor attached to the housing, a ball screw driven by the motor via gears, and a linkage driven at one end by the ball screw such that the linkage rotates an output shaft attached to the other end of the linkage. The motor and gears drive the ball screw to a fully extended position to rotate the shaft to a clamped position or to a fully retracted position to rotate the shaft to an unclamped position. A built-in computer monitors and controls the clamp. The clamp can also be controlled and monitored by a remote pendant. Indicator lights on the housing and remote pendant convey clamp status information. The clamp is programmable and can memorize the clamped and unclamped positions. The clamp uses velocity and position feedback to determine appropriate drive mode. Torque monitors and timers determine if the clamp becomes stuck. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
     In the drawings: 
     FIG. 1 is a side view of an electric clamp constructed in accordance with the present invention showing the clamp in its clamped position. 
     FIG. 2 is a side view of the clamp of FIG. 1, but showing the clamp in its unclamped position. 
     FIG. 3 is a section view along Section  3 — 3  of FIG.  2 . 
     FIG. 4 is a top view of the clamp of FIG. 1 with cover removed. 
     FIG. 5 is a top view of the clamp of FIG. 1 with cover on and remote pendant attached. 
     FIG. 6 is an end view of the clamp of FIG.  1 . 
     FIG. 7 is a schematic diagram of the electronics used in the clamp of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 illustrate an electric clamp  10 . Electric clamp  10  has a housing  12  that serves as a base on and inside of which other structural elements are mounted. Housing  12  protects the housed components. Housing  12  can be made of any durable, lightweight material, but is preferably metal or another conductive material that can be electrically grounded. It is desirable that housing  12  be easily formed into complex shapes to allow for space-efficient integration of various components. 
     Electric clamp  10  further comprises a motor  14 . Motor  14  is a conventional electrically driven motor that mounts to housing  12  and serves to drive motor gear  16 . The motor  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 clamps. 
     Motor gear  16  is on the output shaft  17  of motor  14  and engages ball nut gear  18  (FIG.  3 ). Ball nut gear  18  attaches to and drives ball nut hub  20  in response to motor gear  16 . Hub  20  attaches to and drives ball nut  22 . As ball nut  22  is rotated in place by hub  20 , ball screw  24 , a threaded shaft going through ball nut  22 , advances or retreats depending on the direction of rotation of ball nut  22 . The gear ratios for motor gear  16  and ball nut gear  18  can be chosen to produce a desired torque or rotational rate for ball nut  22 . That determines the power or rate of advance/retreat of ball screw  24 . 
     One end of ball screw  24  pivotally attaches to one end of link  26 . The opposite end of link  26  pivotally attaches to an end of link  28 . Clamp output shaft  30  is rigidly attached to the opposite end of link  28 . Clamp arm  31  (shown in phantom line) is mounted to clamp output shaft  30 . Clamp arms of various sizes can be attached, depending on a user&#39;s needs. 
     In the embodiment of FIG. 1, slave motor  32  is used to provide additional torque. Slave motor  32  is wired in parallel with motor  14  to assist motor  14 . The same voltage is applied to both motors. Slave motor  32 , through its output shaft  33 , drives motor gear  34 , which drives ball nut gear  18 , each identical in operation to motor  14 , output shaft  17 , and motor gear  16 , respectively. 
     In the basic operation of clamp  10  of FIG. 1, power is supplied to motors  14  and  32  to drive motor gears  16  and  34 . Those gears drive ball nut gear  18 , which drives hub  20 . Hub  20  rotates ball nut  22 . Ball nut  22  drives ball screw  24 , which drives links  26  and  28 , rotating clamp output shaft  30  to a fully clamped (FIG. 1) or fully released (FIG. 2) position, depending on the direction of rotation of ball nut  22 . 
     FIG. 2 shows an optional brake  37  attached to the motor shaft  33  of slave motor  32  that can be used to stop slave motor  32 , and therefore stop the motion of clamp  10 . Brake  37  may be required if large clamp arms having high rotational inertia or significant weight are used. In those situations, the inertia or moment may cause clamp  10  to move toward the clamped or unclamped position even though no power is applied. Brake  37  prevents such drift. 
     While the structural elements described above are sufficient to describe the basic configuration and operation of clamp  10 , there are many other elements that enhance its functionality. Encoder  38  mounts to motor  14 . The encoder  38  shown in FIG. 1 attaches to motor shaft  17  of motor  14 . Encoder  38  provides motor angle information for position feedback. The motor angle information tells how far motor  14  has rotated from the clamped or unclamped position, therefore determining the position of clamp arm  31 . An absolute or incremental encoder can be used, or another type of motor position sensor, such as a resolver, can be used. 
     Ball nut  22  is supported by thrust bearing  40 . Thrust bearing  40  mounts between housing  12  and ball nut  22  and carries the thrust load generated during the clamping process. Similarly, ball screw  24  is supported by support bearing  42 . Bearing  42  mounts between housing  12  and ball screw  24  and prevents lateral loads from being transferred to ball screw  24  during extreme loading conditions. Bearing  42 , in conjunction with retainer ring  44 , also acts as a barrier to prevent grease from moving from links  26 ,  28  into the vicinity of ball nut  22 . 
     Stop collar  46  is adjustably fixed to ball screw  24  and physically inhibits further retraction of ball screw  24  once stop collar  46  is pulled into contact with bearing  42 . This feature is useful to prevent clamp  10  from opening too far. The need for restriction commonly arises when objects in the vicinity of clamp  10  interfere with the full range of motion of clamp  10 , particularly when longer clamp arms are used. 
     FIG. 4 shows thumb wheel  48  attached to the motor shaft of slave motor  32 . Wheel  48  allows clamp  10  to be moved without electrical power. This is useful when no power is available, such as during initial setup, or when the drive control electronics (described below) are unavailable. This can occur when clamp  10  becomes extremely stuck or the electronics themselves fail. Wheel  48  is normal concealed and protected by access cover  50 , as shown in FIG.  5 . 
     FIG. 5 also shows clamp buttons  52  and  54 . Buttons  52 ,  54  allow a user to drive clamp  10  to a clamped or unclamped position, respectively. The motion produced is relatively slow in both directions and clamp  10  moves only while a button is depressed. Buttons  52 ,  54  are located in recesses  56  (FIG. 1) in cover plate  58 . Recesses  56  are covered to prevent infiltration of contaminates and to prevent inadvertent engagement of buttons  52 ,  54 . A pointed tool, such as a screwdriver, is needed to actuate buttons  52 ,  54 . 
     Also located on cover plate  58  are status lights  62 ,  64 . Clamped status light  62 , when lit, indicates clamp  10  is very close to the programmed clamped position. (The programmable aspects are discussed below.) Similarly, unclamped status light  64  lights up when clamp  10  is very close to the programmed unclamped position. In addition, there are indicator lights  66  (FIG. 6) on control circuit board  68  (FIG. 2) within housing  12 . Indicator lights  66  are viewed through window  70  (FIG. 1) and provide an operator information about the operational state of clamp  10 . 
     Electrical power is primarily supplied to clamp  10  through control cable  72  (FIG.  6 ), which fastens to cover plate  58  and electrically connects a wire bundle to electronics within housing  12 . Power could be dc, ac, 24 volts, or 48 volts—a preferred embodiment uses 24 volts dc. Higher voltages, such as 110 or 220 ac voltages, could be used, but are generally considered unacceptable because of safety concerns. Electrical power is typically provided by an external power supply with enough current capacity to service several clamps. 
     Other electrical signals, such as a command signal from the user or clamp status information, are also transmitted through control cable  72 . The electronics within housing  12  include control circuit board  68  (FIG.  1 ). Control board  68  has the circuitry necessary to control clamp  10 . 
     FIG. 7 shows conceptually the electronic components comprising control board  68 . Power conditioner  74  is used to provide clean 5 and 15 volts dc signal to control board  68 . A CPU  76  mounted to control board  68  controls all aspects of the operation of clamp  10 . CPU  76  comprises timers, counters, input and output portals, memory modules, and programmable instructions to regulate motion algorithms, error recovery, status messaging, test display, limit adjustment, and pushbutton control. Indicator lights  66  are connected to CPU  76 . 
     Clamp  10  has pushbuttons  79 ,  81 ,  83 ,  85  on the exterior of housing  12  to permit a user to adjust the position to which CPU  76  will command the motor to move upon receiving a clamp or unclamp command. There is also a pushbutton  78  allowing CPU  76  to learn and memorize the clamped position based on when the motor stalls. This is usually a quicker way to set the programmed clamp position than by using pushbuttons  79 ,  81 ,  83 ,  85 . All of those pushbuttons  78 ,  79 ,  81 ,  83 ,  85 , as well as clamp/unclamp buttons  52 ,  54 , are illustrated in FIG.  7 . 
     CPU  76  controls motor drive circuit  80  and enabling circuit  82 . Those circuits  80 ,  82  supply the drive current sent to slave motor  32  and motor  14 . Because motor drive circuit  80  is easily damaged by logically inconsistent electrical input, enabling circuit  82  is used to independently assure logically consistent input. If excess current is detected by current monitor  84 , such as may occur if clamp  10  is stalled or stuck, the output from motor drive circuit  80  is inhibited. A user may set an over-current threshold using over-current circuit  86 . 
     All user interfaces described above are also found on remote pendant  88  (FIG.  5 ). Thus, remote pendant  88  allows a user to operate clamp  10  some short distance from clamp  10 . This can be useful if clamp  10  is placed deeply within an automation tool, making the interfaces on housing  12  inaccessible. Lights  90  equivalent to indicator lights  66  are found on remote pendant  88 , so clamp status information can be observed. Remote pendant power supply  91  (FIG. 5) provides electrical power to clamp  10  through remote pendant  88  via connector  93  on cover plate  58 . This is useful if conventional power is unavailable, as is often the case in the early stages of building an automation system. Pushbuttons  92 ,  94 ,  96 ,  98 ,  100 ,  102 , and  104 , provide the same functionality as pushbuttons  78 ,  54 ,  52 ,  85 ,  83 ,  81 , and  79 , respectively, using remote pendant  88 . 
     Clamps used in the automation industry are commonly used in conjunction with hundreds of other clamps, each clamp performing a specific function in a carefully choreographed manner. Often the multitude of clamps is controlled by a central controller issuing commands to the various clamps at the proper time. Clamp  10  accepts such external control commands through interface  106  (FIG.  7 ). Clamp  10  is typically isolated from the external controller using optical isolators  108 , however simple lights or light emitting diodes (LEDs) may also be used. The lights or LEDs can convey essential status information such as clamped, unclamped, or a fault condition. This information can be passed to the central controller as well. 
     The present invention offers many advantages over the prior art. Housing the electronics controlling the clamp internally is a significant advantage. Using two motors in tandem is a new and useful arrangement for making a more powerful electric clamp while staying within industry size standards. The remote control provided by the remote pendant is another novel advantage, as is the ability to drive the clamp with power supplied through the remote pendant when normal power is unavailable. The use of an encoder rather than limit switches allows for more intelligent, and more easily modified control. Being able to manually move the clamp using the thumb wheel allows for quick remedy for stuck or defective control condition. The ability to program a clamped and an unclamped position is new and useful, as is the ability to use software to command the clamp to stop when an unrecoverable stuck condition is sensed. The clamp allows for automatic learning of the programmed clamp and unclamped positions, and allows a user to fine tune those positions, if desired. 
     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.