Abstract:
A single housing sensor includes a detector in the housing receiving light from a light source. A detector in the housing provides an output representative of a profile of a component which is rotatably mounted on a nozzle of the pick and place machine. The sensor also includes a receiving circuit in the housing coupled to an encoder in the pick and place machine, the encoder providing an output representative of the rotational position of the nozzle to the sensor. Computing circuitry in the sensor computes a correction signal representative of the pick-up offset of the component and of the orientation angle of the component. The correction signal is computed from the motor encoder data and from a plurality of images of the shadow of the component, each image collected at a rotational angle, θ, of the nozzle. A UART serially sends the correction signal to the pick and place machine, for use in placing the component precisely onto a printed circuit board.

Description:
PRIORITY CLAIM 
     The present application relates to a provisional patent application filed by the present assignee on Aug. 4, 1998 titled “Enhanced Sensor”, having a patent application Ser. No. 60/095,276. 
    
    
     COPYRIGHT RESERVATION 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent or the patent disclosure, as it appears in the Patent and Trademark Office, patent file or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     This invention relates to optical sensor systems for determining a location or orientation of an object, such as an electronic component, including such systems which report a physical condition of the object, such as reporting “lead tweeze” on an electronic component, the height of a particular feature on the object, the distance between features on the object, or the presence and absence of expected features on the object. 
     BACKGROUND OF THE INVENTION 
     General Background 
     There are many optical based approaches to identifying various parameters related to an object, such as an electronic component. The various parameters which may be reported include angular orientation of the component, location of features on the component (e.g., pick-up off-set), indication of the quality of the component, such as a bent lead (lead tweeze) in the case of a Quad Flat Pack (QFP) component, or a measurement of a feature located on the bottom of the component, such as height of a ball on a Ball Grid Array (BGA) component. The system for identifying these parameters is generally mounted in a pick and place machine which picks up the component and places the component onto a printed circuit board after determining proper orientation and pick-up offset of the component. All of these approaches rely on a quill in the pick and place machine to pick up the component and all generally utilize the motion control system in the pick and place machine in the measurement process. Some of the approaches use cameras to collect data representative of an image of the component and others collect data representative of an outline of the component. All have some sort of processor that analyzes the collected data in order to identify some specific portion of the component, such as the edges of the part or tips of the component leads. 
     The word “orientation” used with respect to an object, and forms thereof, will be used throughout this document to indicate the angular position of the object with respect to a reference. The word “location” used with respect to an object, and forms thereof, will be used throughout this document to indicate the (x,y) position of the object with respect to a nominal (x,y) reference location. 
     Different types of placement methodologies are now in use in pick and place machines as well. In general, pick and place machines can be characterized as being either “on-head” or “off-head”. On-head sensor systems (considered together with their host pick and place machines) sense and correct for the orientation and pick-up offset of the component while the component travels to the target circuit board. On-head systems are preferred as they minimize the amount of time to place a component on a board, thereby increasing the throughput of the pick and place machine. Off-head sensor systems perform the data collection in a fixed location on the pick and place machine. With an off-head system, the quill must first move the component to a dedicated station and after acquiring the image, move the component to the target circuit board for placement. Off-head systems typically reduce throughput of the machine since the head makes additional stops or travels further, but are used because they are inexpensive, reduce the mass which must be carried on the head and one sensor can readily service multiple quills. 
     The essential purpose of a pick and place machine is to place an electronic component on a printed circuit board. Regardless of the type of sensor used to precisely locate certain features on the component, the host processor in the pick and place machine has an intended placement spot for the component. The location information from the sensor, vision camera or otherwise, is typically sent via a RS-422 port back to the host processor. Based on the sensor output and the desired placement position of the component, the host processor computes an orientation correction (x,y,θ) which includes an angular correction (θ) and a pick-up offset (x,y). Based on the orientation correction signal, the host processor instructs the motion control system in the pick and place machine to implement this correction movement, the motion control system responds, and then the host processor instructs the pick and place machine to place the component. 
     The electronic block diagram and general operational principles of a prior art shadow cast based sensor  2  and a prior art pick and place machine  12  is shown in FIGS. 1-2. A sensor system  8  consists of three parts; the sensor  2  (located on a sensor head, not shown), a controller module  4  and a two-wire coaxial cable  6  which connects sensor  2  to controller module  4 . Sensor  2  is located on the sensor head of pick and place machine  12 , while controller module  4  is located in a separate electronic enclosure, or alternatively, within machine  12 . Operation of the system  8  is controlled by a processor  10  in pick and place machine  12 , which generally controls the operation of machine  12  and issues commands to controller module  4  over communications bus  14 , typically in a serial communications protocol of RS-232 or RS-422. A motion control system  16  controls x,y,z,θ movement of the sensor head via motors  16 ,  18 ,  20  and  22 , respectively. Each motor has an associated encoder  24 ,  26 ,  28  and  30 , respectively, each encoder adapted to independently output a signal representative of the change in x,y,z and θ position of the sensor head in pick and place machine  12 . Pick and place machine  12  also includes an operator interface  32  which typically includes a CRT for viewing images of the component provided by the present invention, or optionally viewing summary results of operations of machine  12 . A power supply  34  is located in pick and place machine  12 , and supplies all the operating power required by sensor  2  over the bus  14 , through controller module  4  and over two wire coaxial cable  6  to sensor  2 . 
     The general operation of shadow cast sensor  2  is best explained in FIG. 2. A component  40  is secured via a vacuum nozzle  42  or the like to sensor head (not shown). A plurality of light rays  44  are cast onto component  40  from one of its sides and a linear image detector  46  on the other side of component  40  captures data representative of the shadow of component  40 . Sensor  2  is shown with a laser diode source  48  for light  44 , but any sort of high intensity light source may be used, regardless of whether the light is coherent or not. At several pre-selected angular positions of rotary motor  22 , the sensor  2  collects data from the detector representative of the shadow of component  40 . As appropriate for the type of component, the pre-selected angular positions do not have to be equally spaced over 360 degrees nor is component  40  required to spin at a constant angular velocity. For each preselected angular position, electronics in controller module  4  compute the width of the component&#39;s shadow on the detector and the location of shadow&#39;s center. For a rectangular component angularly rotated 180 degrees, a plot of shadow width v. angle will produce two minima; one corresponding to the width of the component and one corresponding to its length. Once the two minimum widths and their corresponding angles and shadow centers are known, the location of a feature on component  40  (with respect to an internal (x,y) reference in the pick and place machine) may be computed. The pick-up offset is calculated once the location of a feature and the internal (x,y) reference for a pick and place machine are known. This system does not provide any further information not already derived from the shadow edges, so that it is unable to identify misplaced or missing balls on BGAs or flip-chips, and cannot identify lead position errors for leaded components such as QFPs or TSOPs. This shadow casting sensor is shown in U.S. Pat. No. 5,278,634 to Skunes et al., assigned to CyberOptics® Corporation. Other applications of a shadow cast system include allowing the light to fall on the component while it is tilted, so as to be able to discern information about the leads on a component rather than exclusively the profile. The shadow cast approach takes advantage of the motion control system already installed on pick and place machine  12 , which has an extremely accurate rotary motor  22  and encoder  30  for reporting the angular position of nozzle  42 . 
     Since there is an electronics circuit board and a sensor associated with each of the effective data channels for placing components, it is cumbersome to change the electronics board whenever a difference type sensor is installed. Furthermore, the electronics board associated with each sensor in pick and place machine requires valuable board space and additional communications overhead is associated with each additional sensor/electronics board combination. Additionally, the amount of space consumed by the combination of the cabling between the board, the sensor and the host processor (which is not typically connected to the same card cage as the sensor electronics board) is excessive and a more space efficient solution is desired. In sum, a new interchangeable sensor which would allow various sorts of sensors to be installed in a pick and place machine without excessive downtime, which also provides for more efficient use of computations and communications capability within the pick and place machine and finally, takes up a reduced amount of space, is needed. 
     SUMMARY OF THE INVENTION 
     In the most basic form of the invention, a single housing sensor includes a detector in the housing receiving light from a light source. The detector provides an output representative of a profile of the component, as it is mounted on a nozzle of the pick and place machine. The sensor also includes a receiving circuit in the housing coupled to a motor encoder in the pick and place machine, the encoder providing an output representative of the rotational position of the nozzle to the sensor. Computing circuitry in the sensor computes a width of the profile of the component at a plurality of rotational positions of the nozzle, as a function of the output from the encoder and output from the detector, the circuitry providing a value of the output of the encoder at which the width of the component is minimized, sometime called the correction signal. In a preferred embodiment of the circuitry in the sensor, a UART (Universal Asynchronous Receiver Transceiver) is used to serially send the correction signal to the pick and place machine. 
     The sensor of the present invention is preferably used on-head, and also off-head, in a pick and place machine. The present invention may be used with any sort of light source of sufficient intensity to provide a detectable signal from the detector, including coherent light and incoherent light. When the housing is widened to allow for additional component, as in the case of a multi-nozzle sensor, the electronics in the sensor allows for as many “channels” as there are nozzles, one channel corresponding to one nozzle. The present invention is not limited to the specific sensors, controller circuits and pick and place machines as disclosed herein. For example, the present invention can be practiced with off-head sensors which detect a position of the component via smart cameras incorporating detector and vision processing circuitry in the same physical package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art block drawing showing a sensor head, a controller module and a pick and place machine; 
     FIG. 2 is a sketch of the method of operation of a LaserAlign® sensor, showing a component rotating in light and a shadow of the component shown on the detector; 
     FIG.3 is a block diagram of the control loop formed between the sensor of the present invention, the pick and place machine and the component; 
     FIG. 4 is sketch of a pick and place of the present invention; 
     FIG. 5 is a detailed block diagram of the electronics in the sensor of the present invention; 
     FIGS. 6A-D show various mechanical and electrical aspects of a 4-channel sensor of the present invention, FIG. 6A showing the physical layout of the sensor; FIG. 6B showing a side cross sectional view thereof, FIG. 6C showing general operation of the sensor for one of the four channels; and FIG. 6D showing an exploded drawing of the sensor. 
     For convenience, items in the figures having the same reference symbol are the same or serve the same or a similar function. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a preferred embodiment of the present invention, where a shadow cast sensor  100  is connected to a pick and place machine  102  via a cable  104  and a power supply cable  106 . Cable  104  carries communications and motor-encoder signals from a motion control system  108  in pick and place machine  102 , and in the other direction, transmits data from sensor  100  representative of the angular orientation and (x,y) position of a part  200  on nozzle  202 . Pick and place machine  102  includes a power supply  110 , an operator interface  120 , a processor  122  and motion control system  108 . A central bus  124  connects operator interface  120  to motion control system  108 , processor  122  and also to external sensor head  50 . Power supply  110  supplies power to sensor  100  over cabling  106 . 
     Motion control system  108  includes a set of motors  126 ,  128 ,  130  and  132  for moving a head  50  in pick and place machine  102  in the x, y, z and θ directions, respectively, including a set of encoders  134 ,  136 ,  138  and  140  attached to each of the respective motors to provide an output representative of the position of head  50  in the appropriate direction. As part of the data collection process for sensor  100 , motion control system  108  sends instructions to sensor head  50  to angularly rotate nozzle  202  while a light  52  is energized, so as to cast a shadow of the outline of component  200  into detector  54 . Light source  52  may be any type of light source as long as it has sufficient intensity to provide an adequately detectable shadow on detector  54 , although a preferred embodiment is a laser diode operated so that the light is substantially coherent. Sensor  100  is a slave-like device, in that it receives operating instructions from machine  102  via bus  104 , such as instructions to collect data, or specific instructions to report data about the width of the cast shadow and the associated angular position of nozzle  202 . A closed loop control loop is formed between sensor  100 , pick and place machine  102  and part  200 . 
     Sensor  100  is shown in pick and place machine  102  pictorially in FIG.  4 . Sensor head  50  includes vacuum nozzle  202  for picking up component  200 . Sensor  100  is shown configured to be able to shine light onto component  200  and to detect it within the same housing, but it will be understood that the present invention is not limited to the explicit configuration of sensor  100  as shown. For example, other configurations of the light source with respect to the detector electronics may be implemented, but will be understood to be within the scope of the present invention. A conveyor system (not shown) transports a printed circuit (PC) board (not shown) into and out of machine  102 . Sensor head  50  is linearly translatable in three directions (x, y, z) and is capable of angularly rotating component  200  via motion control system  108 . In pick and place machine  102 , each motor runs independently from the other motors by turning a threaded shaft which causes head  50  to slide along an appropriate gantry or slide, (x, y) gantries or slides shown respectively at reference designator  56  and  58 . The pick and place machine of the sort shown in FIG. 2 is typically called a lead screw pick and place machine, although other sorts of pick and place machines may be utilized in the method and apparatus of the present invention, such as those with valves or linear motors rather than lead screws. 
     FIG. 5 is a detailed electrical block diagram of sensor  100 . The main electronics board within sensor  100  has several connectors, although it is understood that the other embodiments may combine functionality within the electronics in a variety of ways. A power connector  204  is intended to connect to power supply  110  via cabling  106 . A voltage converter circuit  214  provides appropriate power to all of the circuits on electronics board  101 , in the form of converters from the plus 18 volt supply voltage to dual supply voltages of plus 5 Volts and plus 12 Volts. 
     A CCD connector  206  is intended to connect to CCD detector  54  within sensor  100 . Two channel analog processing circuits  216 ,  218  are shown, since detector  54  is of the sort which provides dual outputs, to provide faster image read-out from the CCD detector. A FIFO circuit  220  combines output from the dual circuits  216 , 218  and sends data representative of the intensity of the image collected in detector  54  to a digital signal processor (DSP)  236  over a common data bus  221 . 
     An encoder connector  208  connects to bidirectional bus  104 , which receives pulse width encoded positional information from encoder  140  and provides a digital signal via encoder counter circuit  226  through differential receiver  222 . Encoder counter  226  appropriately counts the number of pulses that have been received. A pair of circuits  222 , 226  is used for each channel. For example, in the embodiment shown in FIG. 6A, four pairs of differential receivers and encoders are required because there are four data channels in sensor  100 . Encoder  140 , which encodes the θ position of rotational motor  132 , has a frequency output as a function of angle and the phase of the two clocks coming from the encoder to indicate which direction motor  132  is turning nozzle  202 . A laser diode connector  210  receives power and provides a stand alone laser diode power supply for laser diode  52  within sensor  100 . 
     An RS-422 connector  212  receives commands and their associated data from host processor  122  over cabling  104 , except for power supply  34 , and allows data from sensor  100  to be transmitted to processor  122 . Any other standardized protocol may be implemented on cabling  104 , but in order to maintain the interchangeability of the present sensor, it is preferable that the connection be serial, since most parallel protocols requires some degree of specificity for a particular product. The desired placement position of component  200  is sent from host  122  to DSP  236  through connector  212 . 
     A timing control circuit CPLD  232  (complex programmable logic device) provides the clocking signals required for the operation of the CCD  206 , the analog processing units  216 , 218  and FIFO memory  220 , and also provides device decoding logic and control signals for connecting DSP  236  to the different devices such encoder counter  226 , FIFO  220 , UART  238  and EEPROM  234 . DSP  236  is responsible for all data acquisition and processing in sensor  100 , and includes a built-in storage of image data and other temporary information. The program required for operating DSP  236  is stored in non-volatile EEPROM  234 , which also stores calibration and configuration data. 
     A set of laser aligned functions which DSP  236  performs is shown below in Table 1. 
     TABLE 1 
     Perform alignment processing for one component 
     Read results of alignment processing 
     Load configuration parameters 
     Read operation and calibration parameters 
     Read raw image data 
     Read part width v. rotational angle data 
     Perform sensor diagnostic routines 
     Preset encoder-counters 
     The digitized image data from detector  54  is stored by the DSP in its internal memory and from such stored data, DSP  236  computes the orientation θ and offsets (x,y) of component  200  with respect to some standard reference point. DSP  236  operates on the desired position signal from machine  102 , and the orientation information computed by DSP  236 , to compute a correction signal representative of the amount of x,y and θ movement correction which must be accomplished in order to place component  200  in the desired placement spot. 
     A set of receiver/transmitter circuits  238 ,  240  send and receive data from and to connector  212 . The data is transmitted serially, according to a prescribed standard (e.g., RS-422) to provide fully interchangeable sensors regardless of manufacturer or type, as well as to minimize the number of connections between the sensor and the pick and place machine. 
     A preferred embodiment of shadow cast sensor system  100  is shown at FIG. 6A-E, where a single sensor housing  300  includes four separate channels, each channel adapted to sense the x,y,θ orientation of one of four components. Sensor housing  300  contains an electronics board  302  (not shown), and an opening  322  through housing  304 . Opening  304  is adapted for use with at least four components, FIG. 6A showing components  306 ,  308 ,  310 ,  312  secured to a nozzle  314 ,  316 ,  318 ,  320 , respectively. In general, the width of slot  304  is at least 80 millimeters long in order to accommodate four nozzles with 17 mm spacing, where any nozzle can handle a component which may be as large as 35 mm diagonally. Condenser optics  322  act to condense the shadow images of the components projected through slot  304  onto a CCD detector  324 , in order to allow use of a detector  324  which is narrower than the field of view. Detector  324  is typically a Sony ELX 510 CCD array detector which consists of a linear array of 5150 pixels, with a pixel pitch and height, respectively, of 7 microns. The width of the entire array  324  is approximately 36 millimeters. A laser diode or any other sort of light source, including incoherent light sources, is positioned at  326  in order to shine collimated light through a window slot  326  onto components  306 - 312 . The shadows formed by the components blocking the light from the light source  326  is focused through optics  322  and onto detector  324 . Sectional view FIG. 6B shows a cross sectional view of the interior of sensor  300 , including two optical windows  328 , 329  which prevent dust particles and the like from entering housing  300 . 
     The electronics shown in FIG. 5 apply to sensor  300 , except that encoder connector  208  receives four channels of rotating encoder output from motors driving the four nozzles  314 - 320  in FIG.  6 A. (Each of the four channels corresponds to one of the four components  306 - 312 .) DSP  236  provides image acquisition, data processing, communicating to the pick and place host machine and control for sensor  300 , including the basic command set shown in Table 1. Three useful parameters reported from sensor  300  are the rotational angle, θ, corresponding to the alignment of a major width of the component with the x or y axis of the pick and place machine, the center of the shadow cast from the component at a preselected value of θ and the width of the shadow cast from the component at a preselected value of θ. In particular, the center measurement is effective for use by machine  102  in determining the offset of the pickup point (the center of the nozzle when the component is secured to the nozzle) from the actual center of the component. In particular, if a shadow cast on detector  324  is between points x 2  and x 1 , the center is the average of the two points being: 
     
       
           c =( x   2   +x   1 )/2 −x   0   
       
     
     where x 0  is a constant derived from an installation process of the sensor in the beginning of operation and x 2  and x 1  are derived the actual shadow of the component on detector  324 . Similarly, the component width is equivalent to the difference between points x 2  and x 1 , and once reported to the pick and place machine, is useful in verifying the type of part of which is to be placed, whether the part which is being measured is the correct part, or whether the correct part has been improperly picked up on the wrong side. FIG. 6C shows a plot of the width of a shadow cast on detector  324  while component  310  is rotated. Shadow width data points  330 - 338  are collected at different angle θs, and the results may be viewed via operator interface  120  in this graphical form. The first minimum on the plot in FIG. 6C corresponds to the minor dimension, w 1  (width) of component  310  and the second minimum on the plot corresponds to a major dimension, w 2  (length) of the same component. Optionally, DSP may also compute and send to machine  102  a signal representative of the quality of the laser stripe, by differencing the intensity of the pixels on the detector in the presence and absence of a component so as to indicate that dirt or some other optical obstruction is present on detector windows  328 , 329 . Such information is helpful to schedule proper maintenance of sensor  300 . FIG. 6D shows an exploded drawing of sensor  300 , which shows slot  322 , which is long enough to allow for four components. The optical components within sensor  300  are located to the right of slot  322  in FIG. 6D, and the electronics and detector are located to the left of slot  322 . 
     The additional function performed by the DSP in the present invention is to compute a correction signal representative of the amount of x,y and θ movement correction required to place a component, as a function of the desired position signal from machine  102 , and the orientation information computed by DSP  236 . The present invention allows for direct communications between standalone sensor  300  and pick and place machine  102  using twisted wire pairs. The communications protocol between pick and place machine  102  and sensor  100  is similar to that used on presently available laser aligned sensors and can use RS-232 or  422  interface, as appropriate, so that one type of sensor  300  may be exchanged for another type without removing an electronics board from machine  102 . 
     The four channel system of the present invention, allows for a shadow cast sensor system with improved maintenance and interchangeability. With the interchangeable sensor of the present invention, unnecessary maintenance in the pick and place machine, and related downtime is obviated and multiple shadow cast sensors, regardless of manufacture, can be swapped in and out by merely changing the sensor and connecting cables from the sensor to connectors on the exterior of the pick and place machine. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For instance, insubstantial changes in the organization of the electronics, as well as nominal changes to the optical portions of the sensor or the pick and place machine should not be construed to change the essence of the invention. Finally, the present invention is not limited to use in the area of pick and place machines, but may be used in surface profiling, printed circuit inspection systems, solder paste inspection systems, component placement systems in general, vision assisted laser trimming and final assembly inspection systems which utilize shadow cast sensors.