Abstract:
A method and apparatus for calculating distance in an assembly operation. The method includes the steps of providing an assembly comprising a first part, a second part and a driver coupled to the first part. The driver arranged to selectively move the first part relative to the second part. The method also includes actuating the driver to selectively move the first part between a first known position and a second position and registering speed and/or acceleration data of the driver between the first known position and the second position. The time interval for the first part to move between the first known position and the second position is measured. The distance moved by the first part between the first known position and the second position using the measured time interval and the data registered from the driver can then be calculated.

Description:
RELATED APPLICATIONS 
     This Application is the U.S. National Phase Application of PCT International Application No PCT/GB2006/001715 filed May 10, 2006. 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for calculating distance in an assembly operation. The invention also provides a method for re-adjusting the distance for the next or subsequent assembly operations following calculation of the distance for a previous assembly operation. In particular, the assembly operation can include pick-up and placement operations performed by assembly machines. 
     2. Description of the Related Art 
     Assembly machines are often used to pick-up and place components in order to assemble mechanical or electrical parts. Typically, the machines are capable of three-dimensional movement. It is often necessary to accurately control this movement in the X and Y directions to ensure that components are picked up from or placed in the correct location. It is also desirable to monitor the height of the pick up or placement operation in the Z direction. However, for many assembly machines, height in the Z direction is often the least known co-ordinate, which can have adverse consequences for pick-up and placement operations, leading to longer assembly times and a lower reliability of assembly operations. 
     There may be several reasons why the Z co-ordinate is less well known than the X and Y coordinates. For example, the assembly machine structure may not be sufficiently precise or stable in the Z direction and the increased cost required to improve control in the Z direction may not be justified. Additionally, the assembly machines can be affected by the components which they manipulate. For example, the surface from which the components are picked up, or on which they are placed can be unpredictable; for instance the surface may be tape, plastic sticks or the like. As a result the accuracy of the machine in the Z direction may be reduced. Furthermore, there is likely to be variation in the pick-up and placement planar surface, which therefore means that an absolute value for the Z height for each operation is not appropriate. 
     In order to alleviate the affects of variable Z height, some handling heads on assembly machines are fitted with a retractable tool to compensate for some Z imprecision. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method for calculating distance in an assembly operation, the method comprising the steps of:
         providing an assembly comprising a first part, a second part and driver means coupled to the first part, wherein the driver means are arranged to selectively move the first part relative to the second part;   actuating the driver means to selectively move the first part from a first known position into a second position;   registering speed and/or acceleration data of the driver means between the first known position and the second position;   measuring the time interval for the first part to move between the first known position and the second position; and   calculating the distance moved by the first part between the first known position and the second position using the measured time interval and the data registered from the driver means.       

     According to the first aspect of the present invention, there is provided apparatus for calculating distance in an assembly operation, the apparatus comprising a first part, a second part, measuring means and a driver means coupled to the first part, wherein the driver means is arranged to selectively move the first part relative to the second part and wherein the driver means is actuable to move the first part between a first known position and a second position at a registered speed and/or acceleration, and wherein the measuring means are arranged to measure the time interval taken for the first part to move between the first known position and the second position to thereby enable calculation of the distance between the first known position and the second position. 
     The assembly operation can be a pick up operation or a placement operation. The first part can be engagement means, arranged to selectively engage a component. The second part can be a component receiving member arranged to selectively accommodate a component. Thus, the method and apparatus are suitable for use with so-called “pick and place” machines. The engagement means can pick-up a component from a component receiving member in a pick-up operation and can also deposit the component on a different component receiving member in a placement operation. 
     The calculated distance is preferably in a Z (vertical) direction. 
     A sensor means can be coupled to the assembly or apparatus. The first known position can be detected using sensor means. The sensor means can be operable in at least two states and can be arranged such that a transition from one state to the other occurs at the first known position. 
     The first known position can be in the region of a transitional area where the first part and the second part move from being spaced relative to one another to being in contact with one another. The first part and the second part can be considered to be in contact with one another when they are in indirect contact with one another, for example, when a component is positioned therebetween and the first part is in contact with the component as well as the second part being in contact with the component. 
     The second position can correspond to a position in which the first part and the second part are in pressed engagement with one another. As described above, the first and second part can be in indirect pressed engagement with one another. 
     The method and apparatus of the present invention are particularly useful for assembly operations where the second height is variable or unknown. For example, this may occur where there are dimensional variations of each second part. 
     The first part can be provided with resilient means arranged to at least partially deform as the first part moves between the first known position and the second position. Preferably the resilient means comprises a spring means. Where movement of the first known position results in deformation of the resilient means, the distance calculated can be used to evaluate the amount of deformation of the resilient means. 
     Preferably, the time interval is calculated as the first part is moved from the second position to the first known position. The first part can be stationary at the second position. The data can be registered from the driver means on actuation thereof, which actuation causes the first part to move from being stationary at the second position to the first known position. The first known position can be detected on operation of the sensor means. The sensor means can be coupled to the driver means in order to register the end point of the time interval and therefore the relevant registered data. Thus the driver means can hold the first part stationary when the first part is in pressed engagement with the second part. 
     The method can further include the step of providing a programmable driver means. The driver means can be programmed in response to the distance calculation. 
     According to a second aspect of the present invention, there is provided a method for calculating and readjusting distance in an assembly operation, comprising the steps of:
         calculating distance in an assembly operation according to the first aspect of the invention;   evaluating the difference between a theoretical distance and the calculated distance, between the first known position and the second position;   determining a correction factor based on the evaluated difference;   re-evaluating a theoretical optimum distance of the second position relative to the first known position using the correction factor; and   programming the driver means, such that on actuation thereof for a subsequent assembly operation, the first part is moved relative to the first known position by the theoretical optimum distance evaluated for the second position.       

     The method steps of the second aspect of the invention can be repeated in order to make the calculating and readjusting distance a continuous process. Thus, real time calculations of the distance can be used to readjust and optimize the second position based on theoretical and calculated values, thereby optimising assembly time and decreasing the risk of failure of each assembly operation. 
     The correction factor can be such that the theoretical optimum distance is equal to the calculated distance for the previous operation. Alternatively, the theoretical optimum distance can be a certain proportion of the evaluated difference between the initial theoretical distance and the calculated difference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described with reference to and as shown in the accompanying drawings, in which:— 
         FIG. 1  is a schematic side view of an assembly head apparatus in accordance with the present invention; 
         FIG. 2  is a schematic side view of the apparatus of  FIG. 1 ; 
         FIG. 3  is a graph of height of a support of the apparatus of  FIG. 1  in the Z direction versus time; and 
         FIG. 4  is a grid with letters denoting different placement areas for the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Part of an assembly head apparatus is shown generally at  24  in  FIGS. 1 and 2 . The assembly head apparatus  24  includes a pick up tool  10  at the lower end thereof. The pick-up tool  10  comprises a shaft  12 , a head  14  and a tip  16 . The longitudinal axis of the shaft  12  defines a z-axis. 
     The tip  16  is arranged to selectively engage a component  20  such as an electronic component to be placed onto a target member  70  such as a printed circuit board during assembly thereof.  FIGS. 1 and 2  show the tip  16  engaging the component  20 , which is maintained in position on the tip  16  through the action of a vacuum system within the tool head  14 . In an alternative embodiment, a mechanical grip incorporated into the tip  16  can be used to secure the component  20  in position. 
     The shaft  12  is slidably accommodated within a support  40 . A spring  50  is provided surrounding the shaft  12  and acts between the upper end of the tool head  14  and the lowermost face of the support  40 . The tool  10  is maintained in a first position shown in  FIG. 1  in which the support  40  abuts a stop member  30 , as a result of the bias of the spring  50 . The support  40  is coupled to and driven by a programmable motor (not shown). The programmable motor can be a servo motor or a stepper motor. Alternatively, another type of actuator can be employed such as an electromagnet or an air cylinder with a suitable encoder which can register the required data. The programmable motor is arranged to drive the support  40  along the Z axis. 
     A position sensor  60  is secured to the support  40  and is movable therewith. The position sensor  60  can be any suitable sensor such as an optical, magnetic or electronic sensor or a mechanical switch. In the present embodiment, the sensor  60  is operable in two states (such as ‘on’ and ‘off’) The sensor  60  can determine the distance between the support  40  with reference to the stop member  30  in the Z direction and is arranged to switch between one state (such as ‘on’) and the other (such as ‘off’) when the sensor is at a predetermined position in relation to the stop member  30 . 
       FIG. 2  shows the component  20  in contact with the target member  70 . The target member  70  is a printed circuit board and therefore  FIG. 2  is representative of a placement operation, in which the tip  16  with attached component  20  is brought into contact with the target member  70  in order to deposit the component  20  thereon. Alternatively, the target member  70  can be component packaging during a pick up operation, since the tool  10  occupies similar relative positions with respect to the remaining parts of the assembly head  24  during both the pick up and the placement operation. For example, during the pick up operation, the tip  16  moves towards the target member  70  in order to pick up a component  20 . 
     Before performing the placement operation, the assembly head  24  performs a pick up operation in order to couple the component  20  and the tip  16 , as shown in  FIG. 1 . The assembly head  24  with the component  20  coupled thereto is then moved into position over the target member  70 . Once the component  20  and the target member  70  are in close relation, the programmable motor is actuated to move the support  40  downwards in the Z direction, against the bias of the spring  50  and away from the stop member  30 . This action causes relative movement of the shaft  12  within the support  40  and compression of the spring  50 . Once the support  40  and coupled sensor  60  reach the predetermined distance from the stop member  30 , the sensor  60  switches from one state to the other. 
     In order to complete the placement operation the component  20  needs to be deposited on the target member  70  with sufficient pressure to secure the component  20  to the target member  70 . Therefore, once the sensor  60  has switched from one state to the other, there should be continued downward movement of the support  40  in the Z direction in order to press the component  20  and the target member  70  into closer contact with one another. The additional pressure ensures that the component  20  attaches to the target member  70  whilst the spring compensates by absorbing some of the force when the additional pressure applied via the continued downward movement of the support  40  is too great. 
       FIG. 3  is a graphical representation of height in the Z direction of the support  40  over a period of time. Line A on the graph represents movement of the support  40  from a nominal height  88  (that shown in  FIG. 1 ) to a final placement height  110  (that shown in  FIG. 2 ), followed by a retraction of the support  40  away from the final placement height  110 . 
     The portion of the graph labelled  80  represents downward movement of the support  40  driven by the programmable motor from the start height  88  to the placement height  110 . Movement of the support  40  will cause an approximately corresponding movement of the tip  16  and coupled component  20  down to the Z height  100 . Z height  100  is the height at which the sensor  60  will register the predetermined distance between the stop member  30  and the support  40  and will thus change state. Beyond the Z height  100  continued downward movement of the support  40  pushes against the bias of the spring  50  to ensure that the component  20  is firmly pressed into engagement with the target member  70 . 
     The support  40  reaches the lowest Z height  110  at time t 2 . The support  40  is held at a constant Z height during a portion of the graph labelled  76  between time t 2  and time t 0 . During the portion  76 , the vacuum in the tool head  14  is stopped so that the component  20  is no longer secured to the tip  16 . Thus, the component  20  is deposited onto the target member  70 . 
     Once this placement operation is complete, the programmable motor actuates the support  40 , in order to move the support  40  from being stationary at t 0  in an upward direction, thereby relieving the pressure forcing the tip  16  into contact with the target member  70 . A portion of the graph  90  represents the upward movement of the support  40  in the Z direction which begins from the placement height  110  at time t 0 . Acceleration and velocity of the support  40  are registered by the programmable motor when the support  40  is moved upwardly at time t 0 . As shown in the portion  90 , the sensor  60  switches state again at time t 1  when the sensor  60  detects that the support  40  is at the predetermined distance from the stop member  30 . The time interval between initial movement of the support  40  at time t 0  and when the sensor  60  changes state at time t 1  is recorded. 
     Providing maximum speed is not reached in the Z direction and the initial speed and acceleration along with the time interval from t 0  to t 1  is known, the distance s between Z height  110  and  100  can be calculated as follows:
 
 s= 0.5×( Z  acceleration)×( t 1 −t 0) 2  
 
     Thus parameters of acceleration and velocity registered by the motor and the measured time interval allow the distance s between Z heights  100  and  110  to be calculated. Since Z height  100  is the point at which the sensor  60  changes state and should be known, the height  110  in the Z direction, which corresponds to the height of the upper surface of the target member  70  can be calculated with precision. The height difference between  110  and  100  is proportional to and therefore gives an indication of the amount of spring  50  compression. Thus, the actual pressure applied by the tool  10  on the target member  70  can be determined with reference to the optimum pressure. 
     For the first assembly operation, an estimate is made of the Z coordinate at which the target member is predicted to be positioned. Thus, the programmable motor drives the support  40  until time t 2  to a theoretical height  110  for optimum spring crushing. For repeated pick up or placement operations calculated data for the actual Z height  110  can be fed into the programmable motor and a new estimated Z height  110  can be determined. 
     Usually, consecutive components  20  are placed in different locations during the placement operation. However, where each consecutive component  20  is placed adjacent the previous component  20 , the Z height in the placement operation can be readjusted wholly or partially in response to the previously calculated Z height  110 . 
       FIG. 4  shows a target member  70  divided into notional lettered portions a-x. Once a measurement for the Z height  110  and spring  50  compression has been taken for the placement operation in one portion, a suitable correction factor can be used for all placement operations within that portion. In the event that one of the spring  50  crushing measurements varies from the optimum spring  50  crushing in one of the lettered portions, a partial correction can be used for adjoining portions. For example, if a significant correction is required as a result of measured spring  50  crushing in portion ‘i’, a partial correction will be required for adjoining portions: b, c, d, h, j, n, o, p. 
     The same procedure can be employed for the pick up operation. Each component  20  can be picked up from a magazine at the same X-Y position. The correction of the Z pick up height  110  for each magazine can be determined by taking into account the measured spring  50  crushing during pick up of the previous component  20  in the same magazine. 
     The method allows real time calculations to be made by measuring the time difference between t 0  and t 1  from when the support  40  begins its upward movement until the sensor  60  changes state. The real time calculation of spring  50  compression enables the motor to be reprogrammed in real time to adjust Z height for future operations. Several commercial benefits are associated with this method including the fact that assembly time can be optimised per operation and therefore improved assembly rates can be obtained. 
     Modifications and alterations can be made without departing from the scope of the invention. Although in the described embodiment the assembly head  24  moves with respect to the target member  70 , the relative movement to bring part of the assembly head into contact with the target member  70  could occur due to motion of the target member  70  or due to motion of both the target member  70  and the assembly head  24 . The method and apparatus as described herein is also suitable for use with assembly machines in fields other than electronic printed circuit boards.