Abstract:
Motorized slides are inserted between the end of a robot arm and a robot tool/sensor arrangement to provide additional positioning ability. A control unit of the slides cooperates with the control unit of the sensor to maintain the tool correctly positioned over a feature while the robot arm moves following a programmed path. The control unit of the sensor has look-ahead and additional buffers from which corrected information is determined to compensate for robot teaching inaccuracies, calibration and robot arm response errors. A sensor with two distinct probing zones is used to get information about the position of the tool tip and of the feature to assist in calibrating the sensor/tool relation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to robot feature tracking devices and methods, and more particularly to an assembly, a system and a method for providing additional positioning ability to a tool at an end of a robot arm, and improving the positioning accuracy of a robot tool over a feature to be processed. The invention applies for example to laser processing, such as laser welding, and to arc welding. It also applies to other types of processing that involve the guidance of a tool over a joint or feature to be processed. 
     BACKGROUND 
     It is well known that process robot tasks are often programmed using the method of play back of a taught path. If the work piece to be processed by the robot is not accurately positioned and oriented to correspond with this taught path, the robot will not position its tool accurately over the work piece and flaws will result. 
     The current solution to this problem is to install a sensor in front of the robot tool and to link this sensor with the robot through a special interface. In a welding operation, for example, the sensor measures the position and orientation of the joint, and communicates this information to the robot to correct its trajectory and tool orientation at the right time and place. 
     One problem is that many robots are not equipped with this type of interface. They cannot be linked with a sensor for joint or feature tracking. 
     Another problem is the calibration that is required in order to define the physical relation between the tool center point and the sensor. This relation must be well defined to allow the control unit of the sensor to accurately control the position and orientation of the tool while getting position information about the joint some distance in front of the tool. This calibration is usually performed by accurately positioning the tool center point over a reference object. If the operator does not accurately position the tool center point during this operation, the calibration will not be accurate. The robots usually have a very good positioning repeatability but poor absolute positioning accuracy. This means that the tool center point can be repeatedly sent back to the same position with a good accuracy, but the coordinates of this position in space will not be known accurately. The robot also makes an error when it informs the sensor about its current position during joint or feature tracking because of the response time of the robot arm and because of its mechanical elasticity. In the case of arc welding with a filler wire, the calibration problem is further complicated by the fact that the filler wire is not always straight when it gets out of the tool tip. It often gets out with a variable curve so that the tip of the wire does not correspond to the position of the tool center point. In the case of laser welding, the focal point of the laser beam moves relative to the theoretical position of the tool center point because of imperfections in the optical path. 
     U.S. Pat. No. 4,952,772 (Zana), U.S. Pat. No. 4,954,762 (Miyake et al.), U.S. Pat. No. 4,969,108 (Webb et al.), U.S. Pat. No. 5,006,999 (Kuno et al.), U.S. Pat. No. 5,014,183 (Carpenter et al.), U.S. Pat. No. 5,015,821 (Sartorio et al.), U.S. Pat. No. 5,066,847 (Kishi et al.), U.S. Pat. No. 5,463,201 (Hedengren et al.), U.S. Pat. No. 5,465,037 (Huissoon et al.), U.S. Pat. No. 5,582,750 (Hamura et al.) and U.S. Pat. No. 5,624,588 (Terawaki et al.) provide examples of welding control systems and methods of the prior art, some of which including error correction algorithms. Yet, none of them provides easy robot path correction for joint and feature tracking by an industrial process robot, which would be applied even at very high speed and without directly intruding into the robot control itself. Likewise, none of them satisfactorily solves the problem of accurate computing of the sensor to robot tool center point geometric relation, in static and dynamic operating modes, which is so critical to high speed joint tracking due to the use of the delayed shift method usually applied when a laser vision system is used in front of the robot tool. 
     SUMMARY 
     An object of the invention is to provide easy robot path correction for joint and feature tracking by an industrial process robot, which can be applied even at very high speed and without directly intruding into the robot control itself. 
     Another object of the invention is to provide additional positioning ability to a tool at an end of a robot arm. 
     A subsidiary object of the invention is to allow a robot to perform joint and feature tracking even if the robot is not equipped with the proper interface. 
     Another object of the invention is to provide a solution to the problem of accurate computing of the sensor to robot tool center point geometric relation, in static and dynamic operating modes, which is so critical to high speed joint tracking due to the use of the delayed shift method usually applied when a laser vision system is used in front of the robot tool. 
     According to the present invention, there is provided a motorized slide assembly for providing additional positioning ability to a tool at an end of a robot arm. The assembly comprises a slide arrangement having a base and a sliding element movable along a predetermined course relative to the base. A motor is mounted onto the slide arrangement. A drive device is connected to the motor for moving the sliding element along the course upon operation of the motor. Fasteners are provided for fastening the base of the slide arrangement to the end of the robot arm, and for fastening the tool onto the sliding element. 
     According to the present invention, there is also provided a motorized slide system for providing additional positioning ability to a tool at an end of a robot arm. The system comprises a motorized slide assembly including a slide arrangement having a base and at least one sliding element movable along a predetermined course relative to the base. A motor is mounted onto the slide arrangement. A drive device is connected to the motor for moving the sliding element along the course upon operation of the motor. Fasteners are provided for fastening the base of the slide arrangement to the end of the robot arm and for fastening the tool onto the sliding element. An encoder is operatively coupled to the motor to provide motor positional information. A control unit is provided for the motorized slide assembly. The control unit includes a communication interface for receiving sensor related data, a I/O interface for receiving and transmitting synchronization signals, a CPU for controlling positions of the sliding element, a memory, a servo-amplifier circuit for powering the motor, a slides control for controlling the servo-amplifier circuit in response to the CPU and the motor positional information provided by the encoder, and a bus circuit interconnecting the communication interface, the I/O interface, the CPU, the memory and the slides control together. 
     According to the present invention, there is provided a compensation method for compensating errors made by a control unit of a robot sensor when evaluating a relation between a position of a robot guided tool behind the sensor and a position of a feature to be followed by the guided tool. The method comprises the steps of recording position data generated by the sensor during a dry pass of the guided tool over the feature, the position data representing consecutive positions of the feature detected by the sensor, and subtracting the recorded position data from joint position errors computed by the control unit during a feature tracking operation where the guided tool is operated to process the feature. 
     According to the present invention, there is provided a control unit for a robot sensor tracking a feature to be processed with a robot tool positioned behind the robot sensor. The control unit comprises a sensor interface having a sensor control output and a video input. A memory is connected to the sensor interface. A CPU is connected to the sensor interface and the memory. A communication interface is connected to the CPU, the memory and the sensor interface, and has a communication port. The memory includes a look-ahead buffer that stores a number of successive feature position data computed by the CPU from signals received at the video input, as a function of tracked successive positions reached by the robot sensor during displacement over the feature. An additional buffer is connected to the look-ahead buffer, and stores a number of the successive feature position data as a function of tracked successive positions reached by the robot tool. The CPU has an operating mode causing a computation of a corrected position value required to maintain the robot tool correctly positioned over the feature by subtracting a current position of the robot tool and one of the position data stored in the additional buffer related to the current position of the robot tool from one of the position data stored in the look-ahead buffer related to the current position of the robot tool, and a transmission of the corrected position value through the communication port of the communication interface. 
     According to the present invention, there is also provided a robot sensor assembly for simultaneously detecting a position of a feature at a given look-ahead distance in front of a tool and a position of a tip of the tool. The robot sensor assembly comprises a sensor body, a bracket for side attachment of the sensor body to the tool, a first probe device attached to the sensor body and directed toward the feature in front of the tool, for providing surface range data along the feature whereby the position of the feature at the look-ahead distance in front of the tool is determinable, and a second probe device attached to the sensor body and directed toward a target region including the tip of the tool and the feature under the tip of the tool, for providing an image of the target region whereby the position of the tip of the tool is determinable. 
     According to the present invention, there is also provided a sensor control unit for a robot sensor assembly as hereinabove described. The sensor control unit comprises a range processing circuit having an input for receiving a video signal produced by the robot sensor, and an output for producing surface range data extracted from the video signal. A frame grabber has an input for receiving the video signal produced by the robot sensor, and an output for providing image frames stored in the frame grabber. A main CPU has an input connected to the output of the range processing circuit, and a communication port. A secondary CPU has an input connected to the output of the frame grabber, and a communication port. A communication link interconnects the communication ports of the main and the secondary CPUs. A communication interface is connected to the communication link. The secondary CPU has an operating mode causing a processing of the image frames stored in the frame grabber, a determination of the position of the tip of the tool from the image frames, and a transmission of the position of the tip of the tool to the main CPU via the communication link. The main CPU has a sensor-tool calibration mode causing a storage of the position of the tip of the tool received from the secondary CPU as calibration data, and a subsequent processing mode causing a comparison of the position of the tip of the tool received from the secondary CPU with a corresponding position in the calibration data, a computation of tool positioning correction values, and a transmission of the correction values through the communication interface. 
     To sum up, the addition of motorized slides at the end of a robot arm and the installation of the tool and the sensor on the motorized slides allow for joint and feature tracking to be performed even if the robot is not equipped with the proper interface, and thereby provide additional positioning ability to the tool at the end of the robot arm as the orientation of the slides can be set as needed and desired. 
     In one preferred embodiment of the invention, motorized slides are added at the end of a robot arm in order to enable real-time seam tracking while the control unit of the robot is not necessarily equipped with a sensor interface. A tool and a sensor are installed on the motorized slides so that a control unit, provided with a vision system to process the data from the sensor and a slides driver to control the position of each slide, maintains the tool correctly positioned over a joint or feature of an object by moving the motorized slides according to the position information computed by the vision system, while the robot arm moves along the joint or feature by following a programmed path. 
     The compensation method compensates for robot teaching inaccuracies, for calibration errors in the robot arm and for errors caused by the response time of the robot arm. 
     In another preferred embodiment of the invention, the compensation method, based on data recorded while the robot follows a programmed path, is used to modify the position correction information computed by the control unit of the sensor. This method compensates for errors made by the control unit of the sensor when it evaluates the relation between the position of the tool and the position of the joint or feature to be tracked, these errors being caused by incorrect programming of the robot path or by inaccuracies in the robot. 
     Accuracy can be improved also with the use of a sensor that gets information from the joint or feature in front of the tool and from the real position of the tip of the tool. 
     In another preferred embodiment of the present invention, a sensor with two distinct vision zones is used to get information about the position of the tip of the tool, as well as the position of the joint or feature some look-ahead distance in front of the tool, in order to help in calibrating the sensor/tool relation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of preferred embodiments will be given herein below with reference to the following drawings, in which like numbers refer to like elements: 
     FIG. 1 is a perspective view of a motorized slide assembly according to the present invention, installed at an end of a robot arm. 
     FIG. 2 is an enlarged view of the motorized slide assembly shown in FIG. 1, with a tool and a sensor. 
     FIG. 3 is a schematic block diagram representing control units for the sensor, the motorized slides, and the robot, according to the invention. 
     FIG. 4 is a schematic diagram representing a control system for the motorized slide assembly according to the invention, and the I/O interface with the control unit of the robot. 
     FIGS. 5A,  5 B and  5 C are complementary flow charts representing the method for the feature tracking with a motorized slides assembly according to the invention. 
     FIG. 6 is a schematic diagram representing a processing made by a CUS for the trajectory control of a tool, including a look-ahead buffer. 
     FIGS. 7A and 7B are schematic diagrams illustrating an error that a sensor makes during a feature tracking if the operator did not correctly position it while teaching the path of the robot. 
     FIGS. 8A and 8B are schematic diagrams illustrating an error that a sensor makes during a feature tracking if the taught path is not straight but follows a deviation of the feature to be tracked. 
     FIGS. 9A,  9 B and  9 C are complementary flow charts representing the compensation method used to compensate for the errors illustrated in FIGS. 7 and 8, according to the invention. 
     FIG. 10 is a schematic diagram representing a processing made by a slides control unit for the trajectory control with compensation according to the invention, including a look-ahead buffer and an additional buffer. 
     FIG. 11 is a side elevation view of a sensor that integrates two distinct vision zones, according to the invention. 
     FIG. 12 is a cross section view of a possible optical arrangement for the sensor shown in FIG.  8 . 
     FIG. 13 is a block diagram illustrating the data acquisition and processing system for a sensor with two distinct vision zones, according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, there is shown a motorized slide assembly for providing additional positioning ability to a tool  2  at an end of a robot arm  4 , according to the invention. 
     In the illustrated embodiment, the slide assembly combines two motorized slide arrangements  6 ,  8  assembled in a block  12 . Each slide assembly  6 ,  8  has a base  14 ,  16  and a sliding element  16 ,  18  (the sliding element  16  of the slide assembly  6  being provided by the back of the base  16 ) movable along a predetermined course relative to the base  14 ,  16 . The assembly is installed at the wrist of the robot arm  4 . For this purpose, any kind of suitable fastener and fastening arrangement can be used, like a mounting bracket  10  which fastens the base  14  of the slide arrangement  6  to the end of the robot arm  4  as best shown in FIG.  2 . The base  14  provides a mounting surface adapted to receive the end of the robot arm  4 . 
     The processing tool  2  is mounted on the motorized slide assembly, with the tool center point  20  being preferably as close as possible to the position where it used to be without the motorized slide assembly, so that the robot can be programmed to weld a piece  22  as usual. For this purpose, any suitable fastener and fastening arrangement can be used, like a clamp  24  projecting from the sliding element  18  opposite the base  16  thereof. 
     A sensor  26  can be affixed to the tool  2  or the motorized slide assembly to detect the joint feature  28  to be tracked in front of the tool  2  as best shown in FIG.  1 . For this purpose, the clamp  24  has preferably a mounting surface opposite the sliding element  18 , adapted to receive the sensor  26 . 
     Only one motorized slide arrangement  6  or  8  can be installed if the trajectory corrections must be made in only one direction, for example laterally or vertically, perpendicularly to the programmed trajectory. A second motorized slide arrangement  6 ,  8  can be added perpendicularly on the first one if the trajectory corrections must be applied both laterally and vertically. If necessary, other motorized slide arrangements including linear and rotational slide arrangements can be used to support more degrees of freedom for the movement of the tool  2 . 
     Referring in particular to FIG. 2, the sliding element  18  of the slide arrangement  8  is in the form of a plate and the base  16  has spaced apart, opposite lateral surfaces  44  slideably receiving the plate. In the case of the slide assembly  6 , the equivalent of the plate is provided simply by the back of the base  16  of the slide arrangement  8 . Each base  14 ,  16  may take the form of an elongated frame having spaced apart, opposite end faces  38 ,  40 , extending between the lateral surfaces  42 ,  44 . 
     Each slide arrangement  6 ,  8  has a motor  30 ,  32  mounted onto the slide arrangement  6 ,  8  and preferably one of the end faces  42 ,  44 . A worm screw  34 ,  36  extends between the end faces  42 ,  44  and is coupled to the motor  30 ,  32 . A toothed member (hidden by the base  16  and the sliding element  18 ) projects from the plate  18  or the back of the base  16  and is meshed with the worm screw  34 ,  36 . The worm screw  34 ,  36  and the toothed member form a drive mechanism for moving the sliding element  16 ,  18  along the corresponding course upon operation of the motor  30 ,  32 . Any other suitable drive configurations can be used. The toothed member can be made for example by a nut screwed about the worm screw  34 ,  36 , which has the advantage of holding the sliding element  16 ,  18  against the base  14 ,  16  without requiring additional guiding members. 
     The motors  30 ,  32  are preferably provided with encoders  46 ,  48  for control of the motors&#39; positions. 
     Referring to FIGS. 1 and 3, in use, the robot can be first programmed off-line or by a “teach and play back” method as usual. During this robot teaching phase, the motorized slides  6 ,  8  are maintained in their central reference position, in order to provide the maximum trajectory correction range on either side of the programmed trajectory. The relation in the 3D space between the tool center point  20  and a given reference position in the sensing range of the sensor  26  must also be determined. This relation is used to calibrate the position of the tool center point  20  in the coordinate system of the field of view of the sensor  26 . This calibration data is programmed in the control unit  50  of the sensor (CUS). This allows the control unit  50  of the sensor (CUS) to calculate the position of the tool center point  20  relative to the position of the joint  28 , knowing the position of the joint  28  in the sensing range of the sensor  26 . 
     Referring to FIGS. 3 and 4, the CUS  50  is interfaced through a communication link  52  with the control unit  54  that drives the motorized slides  6 ,  8  (CUMS). The CUMS  54  is interfaced with the control unit  56  of the robot (CUR) through a I/O line  58  for synchronization. The I/O signals can be sent through electrical wires  60 ,  62 ,  64 ,  66 ,  68 ,  70  and can consist of voltage variations, a high voltage representing the activated state and a low voltage representing the deactivated state. The six signals required for the synchronization between the CUMS  54  and the CUR  56  are illustrated in FIG.  3 . This synchronization can also be accomplished by sending messages through a communication device, such as a serial communication link or a parallel bus. The CUS  50  has a sensor communication interface  51  for communicating with the sensor  26  through a communication link  53  over which control and video signals are transmitted. A bus  55  interconnects the sensor interface  51  with a memory  57 , a CPU  59  and a communication interface  61  forming a processing circuitry of the CUS  50 . The CUMS has a communication interface  63  for receiving sensor related data from the CUS  50  through the communication link  52 . 
     Referring to FIGS. 5A-5C, there is shown a flowchart illustrating the steps that can be carried out by the system for feature tracking with motorized slides installed on a robot, according to the invention. As depicted by block  72 , the CUR  56  activates a home signal to inform the CUMS  54 , through the I/O link  58 , that it is time to bring the motorized slides to their central position, operation which is depicted by block  78 . 
     Referring to FIG. 3, the CUMS  54  is interfaced with the motorized slides  6 ,  8  through a slides controller  74  and servo amplifiers  76 . The motors  30 ,  32  of the slide arrangements  6 ,  8  are powered by the servo amplifiers  76  and the slides controller  74  senses their position through the position encoders  46 ,  48  that are coupled to the motors  30 ,  32 . By sending the successive positions to be reached to the slides controller  74 , the CPU  80  of the CUMS  54  controls the position of the motorized slides  6 ,  8 . A memory  65 , an I/O interface  67  and an interconnecting bus  69  complete the processing circuit of the CUMS  54 . 
     Referring to FIG. 5A, once the central or home position of the motorized slides is reached, the CUMS  54  activates a signal to inform the CUR  56 , through the I/O link  58 , that the home position of the motorized axes is reached and that the process can start, as depicted by block  82 . 
     Once the home position is reached, the tool  2  is brought to the beginning of the path, where the sensor  26  will start looking for the joint or feature  28  to be tracked, as depicted by block  84 . The CUMS  54  waits for a search start signal from the CUR  56 . When this signal comes as depicted by block  86 , the tool  2  starts moving forward along the programmed path and the CUS  50  starts looking for the feature  28 . 
     Referring to FIG. 6, there is shown computations related to the trajectory control and a look-ahead buffer  88  implemented in the CUS  50 . Once the CUS  50  has found the YZ v  coordinate of the feature  28  (in the reference system of the tool  2 ) at the current X s  sensing position along the feature  28 , it adds this coordinate YZ v  to the current YZ TC  coordinate of the tool to get the YZ F  coordinate of the feature  28 . The CUS  50  stores this YZ F  value in the look-ahead buffer  88 , associated with the current X s  position of the sensor  26 . Since the motorized slides  6 ,  8  are still at their home position, the YZ TC  coordinate is considered to be (0,0) at this moment. The look-ahead buffer  88  is a circular buffer that contains the data sampled along the feature  28  between the observation zone  90  of the sensor  26  and the position of the tool center point  20  as shown in FIG.  1 . The CUS  50  carries on this process until the tool  2  reaches the X position where the feature  28  was first found, as depicted by blocks  92 ,  94  in FIG.  5 A. 
     When the tool  2  reaches the X position where the feature  28  was first found, the CUS  50  extracts from the look-ahead buffer  88  the YZ F  coordinate of the feature  28  at the current X T  position of the tool  2  along the feature  28 . It computes the position correction YZCORR by subtracting the YZ TC  coordinate from the YZ F  coordinate. The new YZ TC  coordinate of the tool  2  after this movement is computed by adding the YZ CORR  correction to the previous YZ TC  coordinate. The CUS  50  informs the CUMS  54 , through the communication link  52 , that the start position is reached as depicted by block  96  in FIG. 5B, and sends the YZ CORR  position correction required by the CUMS  54  to move the motorized slides  6 ,  8  to bring the tool  2  above the feature  28  as depicted by block  98 . Once this operation is achieved, the CUMS  54  activates a start position signal to inform the CUR  56 , through the I/O link  58 , that the tool  2  reached the start position, as depicted by block  100 . When the CUR  56  receives this signal, it stops the movement of the tool  2 . It then starts the welding operation as depicted by block  102 , starts moving the robot arm  4  along the programmed path and activates a signal to inform the CUMS  54  that the tracking operation can start as depicted by block  104 . 
     The CUMS  54  informs the CUS  50  that the tracking operation started. The CUS  50  computes a new YZ v  feature coordinate in the tool reference system, adds this coordinate to the current YZ TC  coordinate of the tool  2  and stores the resulting information YZ F  in the look-ahead buffer  88 , associated with the current X s  position of the sensor  26 . The CUS  50  extracts from the look-ahead buffer  88  the YZ F  position data corresponding to the current X T  position of the tool  2 . It subtracts the current YZ TC  position of the tool  2  from the YZ F  position to obtain the YZ CORR  position correction required to maintain the tool center point  20  correctly positioned over the feature  28 . It sends this correction to the CUMS  54  that moves the motorized slides  6 ,  8  to apply the correction. This tracking cycle continues until the tool  2  reaches the end of the feature  28 , as depicted by blocks  106 ,  108 . The CUS  50  recognizes that the end of the feature  28  is reached when the look-ahead buffer  88  does not contain valid position information at X positions that are beyond the current X position of the tool  2 . The CUS  50  informs the CUMS  54  that the end position is reached, as depicted by block  109  in FIG.  5 C. The CUMS activates an end of feature signal to inform the CUR  56 , through the I/O link  58 , that the tool.  2  reached the end of the feature  28 , as depicted by block  110 . The CUR  50  stops the movement of the robot arm  4  and terminates the welding process, as depicted by block  112 . 
     This joint or feature tracking process assumes that the path of the tool  2  was perfectly programmed in the CUR  56 . However, because the operator cannot maintain the sensor  26  at a constant position over the joint or feature  28  during the robot teaching phase, the sensor  26  will detect that the feature  28  moves while the robot executes its program, even if the tool center point  20  maintains its position over the feature  28 , as illustrated in FIGS. 7A and 7B. FIG. 7A shows an example of the possible path  114  of the sensor  26  and the path  116  of the tool  2  and the position of the feature  28  during a robot teaching phase, with the sensor  26  and tool  2  moving in the direction of the arrow  118 . FIG. 7B shows the position  120  of the feature  28  detected by the sensor  26  for the case of FIG.  7 A. The same problem happens if the robot is programmed to follow a deviation in the path of the feature  28 , as illustrated in FIGS. 8A and 8B. In these cases, the CUS  50  will try to correct the error that the sensor  26  detects and will bring the tool center point  20  out of the joint  28 . To eliminate this error, a compensation method is added according to the invention, to record the error during a dry pass over the joint  28  after the robot teaching phase. 
     Referring to FIGS. 9A-C, a dry pass is added for the memorization of the position of the joint or feature  28  while the tool  2  moves along the programmed path, according to the invention. During this dry pass, the same general sequence is followed (as hereinabove described and illustrated in FIGS. 5A-C) and the same signals are activated through the I/O link  58  between the CUMS  54  and the CUR  56 . However, the CUMS  54  does not move the motorized slides  6 ,  8  after being informed by the CUS  50  that the start position is reached, and it does not move the motorized slides  6 ,  8  to track the joint or feature  28 . 
     Referring also to FIG. 10, the CUS  50  memorizes the consecutive positions of the feature  28  associated with the X TM  position of the tool  2  in a second or additional buffer  122 , where X TM  means X position of the tool  2  during the memorization pass, as depicted by blocks  124 ,  126 ,  128 ,  130 , with the CUR  56  setting the robot in motion as depicted by blocks  132 ,  134 . 
     During a normal processing pass, when the CUS  50  computes the corrections that are sent to the CUMS  54 , it extracts from the look-ahead buffer  88  the YZ F  position data corresponding to the current X T  position of the tool  2 . It also extracts from the second buffer  122  the YZ FB  position data corresponding to the current X TT  position of the tool  2 , where X TT  means X position of the tool  2  during the tracking. It first compensates the YZ F  position data extracted from the look-ahead buffer  88  by subtracting the YZ FB  position data extracted from the second buffer  122 . Knowing the current YZ TC  position of the tool  2 , it computes the YZ CORR  correction required to maintain the tool center point  20  correctly positioned over the joint or feature  28 . Because the position data extracted from the look-ahead buffer  88  is compensated for the teaching errors, the CUS  50  will compute corrections that will not track the errors illustrated in the FIGS. 7A-B and  8 A-B. This compensation method applies to the feature tracking performed with the motorized slides  6 ,  8  installed on the robot arm  4  as well as to the feature tracking performed directly by the robot without motorized slides. 
     When the feature tracking is performed on a robot without additional slides, this compensation method is used to compensate for the calibration errors of the robot arm  4  that cause its absolute position inaccuracy and for the dynamic errors that are caused by its response time and its mechanical elasticity. To compensate for these errors, the CUR  56  is programmed to maintain the tool center point  20  correctly positioned over the feature while moving at the desired production speed. A dry pass is then performed while the compensation process in the CUS  50  records, at consecutive X positions of the sensor  26 , the feature position data and the tool center point position information received from the CUR  56 . During the processing operation, the compensation process subtracts the recorded data from the position error calculated by the CUS  50 , at a given tool center point position received by the robot, to compensate for the positioning errors of the robot and to send the tool center point  20  accurately over the real position of the feature  28 . 
     Referring to FIG. 11, the problem of sensor/tool calibration can also be solved by using a special sensor  136  that simultaneously detects the position of the joint  28  in front of the tool  2  and the position of the tip of the tool  20 . This special sensor  136  can be a vision sensor that contains two detectors or probes, a first one looking at the joint  28 ′ at a look-ahead distance in front of the tool  2  for providing surface range data along the feature  28  such that the position of the feature  28  at the look-ahead distance is determinable, and a second one looking at the tip  20  of the tool  2  for providing an image of a target region including the tool tip  20  and the feature  28  under the tool tip  20  such that the position of the tip  20  of the tool  2  is determinable. In the illustrated embodiment, the sensor  136  has a body  138  and a bracket  140  for side attachment of the sensor body  138  to the tool  2 . 
     Referring to FIG. 12, the special sensor  136  can also be embodied by a vision sensor that contains only one detector  142 , a section of this detector  142  receiving the signal from the feature  28 ′ in front of the tool  2  and another section receiving the signal from the area  144  of the tool center point  20 , by using a special arrangement of optical components. It must be understood that other optical arrangements are also possible as long as the simultaneous recording of the joint or feature  28  and of the tool center point  20  is made possible. The area  144  of the tool center point  20  preferably includes the tool tip and the joint under the tool tip. A laser source  146  produces a laser beam which is focused and expanded by a line generator  148  in a crosswise direction relative to the general direction of the feature  28 . The expanded laser beam is reflected by a double mirror arrangement  150  and projected on the work piece  152 . The light line derived from the line generator  148  is thus directed at a tilt angle relative to a plane in which the feature  28  extends and substantially crosswise to the feature  28  in the measuring field in front of the tool  2 . The scattering of the laser beam from each intersection point between the spread laser and the surface of the work piece  152  is collected by an imaging lens  154  and focused on a CCD array sensor  156 . The CCD sensor  156  is properly positioned and oriented so that every point within the measuring field is preferably in exact focus. A diaphragm  158  with two apertures separates two optical input channels. The left aperture of the diaphragm  158  faces a filter  160  and limits the collection of radiance from the scattering of the laser beam. The filter  160  lets only the laser light pass and blocks the background lighting, which is considered noisy light for profile measurement. 
     One part of the sensitive area of the CCD  156  is reserved for profile data acquisition and the other part is used for passive  2 D imaging. The range measurement profile data acquisition of the sensor  136  is based on an active optical triangulation principle. The position of the peak of a focused point on one horizontal CCD scan line is related to the range information of the corresponding point on the surface of the work piece  152 . The second vision module integrated in the same sensor  136  is a passive  2 D imaging module. The passive imaging module has an orientation-adjustable mirror  162  directed toward the target region  144 , a fixed mirror  164  facing the mirror  162 , a group of optical filters mounted on an adjustable disk  166 , a wedge prism  168 , the right side aperture of the diaphragm  154  and a second part of the CCD sensor  156 . The mirror  162  is oriented to capture the desired observation scene. The observation scene is then reflected by the mirror  164 . The light rays are filtered by one of the optical filters mounted on the filter disk  166 . One of these filters with a particular spectral transmission is selected to emphasize the observation of a specific process. The selection of one filter is realized by simply turning the filter disk  166 . The wedge prism  168  deviates the incident light rays from the mirror  164  into the right side aperture of the diaphragm  158 . This wedge prism  168  physically separates two optical channels so that the mirror  164  can be used without blocking the incident beam of the first vision module. The light rays from the right side aperture of the diaphragm  158  is focused by the imaging lens  154  on the sensitive area of the second part of the CCD sensor  156 . 
     Referring to FIG. 13, in order to support the added function of the special sensor  136 , a frame grabber  170  and its associated CPU  172  are added in the CUS  50 . The video signal from the sensor  136  is transmitted to the range processing hardware  174  and to the frame grabber  170 . The range processing hardware  174  extracts from the video signal the range information that is used by the main CPU  59  of the CUS  50  to compute the position of the feature  28 . The frame grabber  170  stores consecutive image frames from the sensor  136 . Its associated CPU  172  processes the stored image frames and extracts the position of the tool tip  20  and sends this information to the main CPU  59 . 
     During a sensor/tool calibration procedure, the position of the tool tip  20  is detected by the secondary CPU  172  and sent to the main CPU  59  to be recorded in the calibration data (e.g. in the memory  57  as shown in FIG.  3 ). During the subsequent processing operations, the main CPU  59  compares the position of the tool tip  20 , received from the secondary CPU  172 , to its position recorded in the sensor/tool calibration data. If there is a difference, it is included in the computation of the corrections sent through the communication link  52  by the CUS  50  to the CUMS  54  or the CUR  56  when motorized slides are not used. 
     While embodiments of this invention have been illustrated in the accompanying drawings and described above, it will be evident to those skilled in the art that changes and modifications may be made therein without departing from the essence of this invention. All such modifications or variations are believed to be within the scope of the invention as defined by the claims appended hereto.