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BACKGROUND 
     The present invention relates to roadway maintenance and crack repair, and more particularly to systems for cleaning and filling such cracks that typically develop over time in roadways having coatings of concrete or asphalt including bitumen and aggregate and the like. 
     In roadway maintenance, crack repair traditionally involves manually cleaning the cracks by removing material therefrom, and filling with a material that may contain aggregate. In some cases, a stream of the thermoplastic material is manually directed onto the roadway generally into the crack, with some of the thermoplastic material (such as tar) remaining on the roadway surface on opposite sides of the crack. In other cases, a crack sealant is used to fill cracks that are first enlarged by sawing or routing during the cleaning. U.S. Pat. No. 5,215,071 to Mertes et al. discloses a riding pavement saw having a hydraulically powered blade that rotates on a generally horizontal axis, the blade being supported approximately midway between a pair of front wheels, the machine being steered by a single rear wheel that is operator-controlled. 
     U.S. Pat. No. 4,407,605 to Wirtgen discloses a powered vehicle having a pavement heater and milling device, modified to provide a rotating chisel cutter for removing a relatively wide portion of the pavement to an adjustable depth in the repair of longitudinal cracks. The milling device is mounted for laterally adjustable positioning relative to the vehicle, whereby initial positioning of the vehicle can be simply parallel to the crack, the vehicle being further maneuvered to follow the crack after the lateral positioning of the milling device is completed. The cutter rotates on a transverse horizontal axis, with chisel elements being spaced along the axis and located within a container that is fed with aggregates and/or liquid to be mixed with pavement that is removed by the milling device, the augmented aggregate mix being reapplied from the container onto the roadway. 
     U.S. Pat. No. 4,954,010 to Montgomery et al. discloses a maintenance truck equipped with a transverse-mounted slurry sealant box assembly for filling transverse roadway cracks and blending stepped pavement segments 
     These and other devices for roadway crack cleaning and repair have a number of disadvantages including, for example, one or more of the following: 
     1. They are excessively large and consequently difficult to maneuver, in many cases also requiring excessive lane width to be closed to regular traffic; 
     2. They are difficult to use in that the operator is required to manually steer the vehicle to accurately follow the crack, being particularly difficult when the driver&#39;s position does not afford a clear view of the crack and/or when lighting conditions are poor; 
     3. They have limited capability for following random cracks in that the cutters rotate on horizontal or generally horizontal axes; 
     4. They have limited effectiveness in that optional secondary operations that are indicated for portions of certain cracks (based on visual inspection, for example) are not permitted prior to filling; and 
     5. They are expensive to provide and operate in that they require heating and/or removal of excessive amounts of roadway material. 
     Thus there is a need for a roadway crack cleaning and repair system that overcomes the disadvantages of the prior art. 
     SUMMARY 
     The present invention meets this need by providing a roadway repair system including an appliance platform that is transversely driven relative to a vehicle in response to sensors thereby to automatically follow the crack as the vehicle is driven generally over the crack. In one aspect of the invention, the repair system includes a steerable vehicle for navigating the roadway; a platform assembly supported relative to the vehicle, at least a portion of the platform assembly being spaced directly above the roadway and being laterally movable relative to the vehicle; a sensor unit mounted on the movable portion of the platform and directed to the roadway below the platform assembly for detecting local variations in a sensed parameter during movement of the vehicle, a sensed continuous local deviation of the parameter being indicative of a roadway crack; a controller responsive to the sensor unit for generating an actuator drive signal that reflects a sensed lateral position of the crack relative to the sensor; an actuator for laterally positioning the platform in response to the actuator drive signal; and an appliance supported on the movable portion of the platform for performing a maintenance operation on the crack as the appliance follows the crack. As used herein, the terms laterally and longitudinally are orthogonal orientations relative to the vehicle being generally correspondingly aligned with the crack. In the case of cracks running generally longitudinally in the roadway, lateral and longitudinal have direct correspondence with a vehicle frame of reference. However, it is also contemplated than when cracks run generally across the roadway, laterally means forwardly and rearwardly with respect to the vehicle, there being a cross-slide carriage on the vehicle for supporting the platform. In that case, the vehicle is maneuvered generally perpendicular to the crack with the sensor unit positioned over the crack, and the cross-slide carriage is operated for traversing the crack, the appliance following the crack as indicated above. 
     The sensed parameter can be a spacing of the roadway below the platform assembly, a sensed continuously locally increased spacing being indicative of the crack. The sensor unit can include an array of sensor elements, each sensor element being directed to a respective portion of the roadway and producing a corresponding sensor signal. Preferably each sensor includes a source of radiation for projecting onto the roadway, the sensor signals being responsive to reflected portions of the radiation whereby the sensor unit is effective in the absence of ambient lighting. Preferably the sensor unit includes at least three of the sensor elements for enhanced crack detection, the elements being in a single laterally extending row. In cases wherein the width of the sensor elements is greater than a desired resolution of the sensor unit, the sensor unit preferably has the sensor elements in two rows, there being at least five of the sensor elements with an even number of the elements being in a first laterally extending row, an odd number of the elements being in a second laterally extending row that is longitudinally adjacent the first row, the odd number being one different from the even number, the sensors of the different rows being laterally interleaved for avoiding detection gaps between adjacent sensors. As used herein, the term longitudinally means orthogonally to the direction of lateral movement of the platform assembly, being generally in the direction of the crack. The first row can have four of the sensors, the second row having three of the sensors. Preferably the second row of sensor elements is displaced rearwardly of the first row for locating a central one of the sensors more closely to a following appliance. As used herein, rearwardly means perpendicular to the lateral movement of the platform assembly and opposite to the direction of movement of the sensor unit in tracking the crack. 
     The movable portion of the platform assembly can be supported by at least one glide shaft being connected to the vehicle, the actuator including a gear rack supported in parallel relation to the glide shaft, and a control motor supported on the platform assembly and having a pinion engaging the gear rack. 
     The appliance can be a router unit including a spindle assembly having a spindle shaft rotatably mounted in a spindle stator; a resilient mount for yieldingly supporting the spindle stator vertically oriented and laterally aligned with the sensor unit rearwardly thereof; means for rigidly mounting a routing cutter to the spindle shaft with the cutter extending below a nominal roadway height; and means for powering the spindle, whereby when the vehicle is maneuvered to generally follow the crack, the router unit is positioned on the movable portion of the platform to follow the crack with sufficient accuracy that the router cutter is guided by the crack, the resilient mount deflecting as required to limit side loading of the cutter, the cutter widening the crack by removing material from at least one side of the crack, unless the crack is wider than the cutter. The cutter will also function to break up dirt, rocks or weeds that may be present in the crack. The maneuvering of the vehicle to generally follow the crack is defined to include positioning the vehicle with the sensor unit approximately over a crack that runs generally crosswise to the path of the vehicle, and transporting the platform on a cross-slide to generally follow the crack, the sensor unit signaling the controller to laterally drive the platform in a manner more closely following the crack. 
     Preferably the system further includes a vacuum system having an inlet fixture supported on the platform behind the router unit for transporting loose material from the roadway, the transported material including material dislodged by the router unit. The inlet fixture can be a router vacuum inlet, the system further including an advance vacuum inlet supported on the platform ahead of the sensor unit for removing foreign material from the roadway to provide unobstructed indication of the roadway crack. Preferably the system further includes a scarifier appliance mounted on the platform ahead of the advance vacuum inlet for dislodging the foreign material. The scarifier appliance can include a laterally spaced array of scarifier wires and a laterally extending anchor bar for cantilevered support of the wires with root portions of the wires projecting from the anchor bar, a free end portion of each wire projecting downwardly and forwardly below the anchor bar, forward extremities of the wires being displaced rearwardly of the root portions. 
     Preferably the appliance includes a sealer outlet mounted to the movable portion of the platform assembly laterally aligned with the sensor unit rearwardly thereof for feeding a sealant medium into the crack. The system can further include a pressure roller mounted in depending relation to the platform rearwardly of the sealer outlet for compacting the sealant and the roadway proximate the crack; a sand outlet mounted to the platform rearwardly of the sealer outlet and ahead of the pressure roller for feeding a particulate medium onto the roadway ahead of the pressure roller; and a vacuum system having an inlet fixture supported on the platform behind the pressure roller for transporting loose material from the roadway, the transported material including portions of the particulate material not bondingly joined to the sealant and/or the roadway by the pressure roller. 
     The vehicle can be a first vehicle, the sensor unit being a first sensor unit, the controller being a first controller, and the actuator being a first actuator in a cleaning module wherein the appliance is a cleaning appliance selected from the group consisting of a scarifier, a router, and a vacuum system having a vacuum inlet for removing loose material from the roadway in and proximate the crack, the system further having a sealing module for filling the crack subsequent to operation of the cleaning module including a steerable second vehicle for navigating the roadway; a second platform assembly supported relative to the second vehicle, at least a portion of the second platform assembly being spaced directly above the roadway and being laterally movable relative to the second vehicle; a second sensor unit mounted on the movable portion of the second platform and directed to the roadway below the second platform assembly for detecting local variations in a sensed parameter during movement of the second vehicle, a sensed continuous local deviation of the parameter being indicative of the roadway crack; a second controller responsive to the second sensor unit for generating an actuator drive signal, the drive signal reflecting a sensed lateral position of the crack relative to the second sensor; a second actuator for laterally positioning the second platform in response to the actuator drive signal of the second controller; and a sealing appliance supported by the movable portion of the second platform for filling the crack as the sealing appliance follows the crack. The sealing appliance can include sealer outlet mounted to the movable portion of the platform carriage laterally aligned with the sensor unit rearwardly thereof for feeding a sealant medium into the crack. 
     In another aspect of the invention, a method for repairing roadway cracks includes: 
     (a) providing a crack cleaning module including a steerable first vehicle for navigating the roadway and having a first platform assembly laterally movable relative to the first vehicle in response to a first control signal, a first sensor unit being mounted on the first platform for detecting local variations in a sensed parameter during movement of the first vehicle, a sensed continuous local deviation of the parameter being indicative of the roadway crack; a first controller responsive to the first sensor unit for generating the first control signal, the control signal reflecting a sensed lateral position of the crack relative to the first sensor; a first actuator for laterally moving the first platform assembly in response to the first control signal; and a router unit mounted on the first platform behind the sensor unit and laterally aligned therewith; 
     (b) providing a crack sealing module including a steerable second vehicle for navigating the roadway and having a second platform assembly laterally movable relative to the second vehicle in response to a second control signal, a second sensor unit being mounted on the second platform for detecting local variations in a sensed parameter during movement of the second vehicle, a sensed continuous local deviation of the parameter being indicative of the roadway crack; a second controller responsive to the second sensor unit for generating the second control signal, the control signal reflecting a sensed lateral position of the crack relative to the second sensor; a second actuator for laterally moving the second platform assembly in response to the second control signal; and a sealer outlet mounted on the second platform behind the sensor unit and laterally aligned therewith; 
     (c) driving the first vehicle to generally follow the crack; 
     (d) activating the first controller for driving the first actuator thereby to track the crack; 
     (e) activating the router unit for machining opposite sides of the crack as the first platform is positioned for more closely following the crack in response to the first sensor unit; 
     (f) removing loosened material from the crack and from the roadway proximate the crack; 
     (g) driving the second vehicle to generally follow the crack after the machining by the router unit; 
     (h) activating the second controller for driving the second actuator thereby to track the crack; and 
     (i) feeding a sealant medium through the sealer outlet as the second platform is positioned for more closely following the crack in response to the second sensor unit, at least a portion of the sealant medium flowing into the crack, thereby repairing the crack. 
     Preferably the method further includes: 
     (a) providing a scarifier appliance on the first platform ahead of the first sensor unit for loosening foreign material on the roadway proximate the crack; and 
     (b) vacuuming loosened material from the roadway between the scarifier appliance and the first sensor unit. 
     Preferably the method further includes: 
     (a) providing a pressure roller on the second platform behind the sealer outlet for compacting the sealant medium and the roadway proximate the crack; 
     (b) feeding a particulate material onto the roadway between the sealer outlet and the pressure roller for preventing adhesion of sealant medium onto the pressure roller; and 
     (c) compacting portions of the particulate material into the sealant within the crack by the pressure roller. 
     The crack can include a singular segment and a branched segment, the method preferably further including: 
     (a) activating a bias input of one of the controllers for selecting one branch of the branched segment to be followed by a corresponding one of the sensor units; and 
     (b) biasing the control signal for driving the platform toward the one branch in response to the bias input when the one sensor unit detects a laterally spaced pair of continuous local deviations of the sensed parameter. 
    
    
     DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where: 
     FIG. 1 is a perspective view of a roadway having cracks being repaired by a roadway crack repair system according to the present invention; 
     FIG. 2 is a side view of a crack cleaning module of the system of FIG. 1; 
     FIG. 3 is a perspective view showing platform carriage guide and drive assemblies of the cleaning module of FIG. 2; 
     FIG. 4 is a sectional detail view of a router unit of the cleaning module of FIG. 2; 
     FIG. 5 is a sectional view of the router unit of FIG. 4; 
     FIG. 6 is a side view of a crack sealing module of the system of FIG. 1; 
     FIG. 7 is a perspective view showing platform carriage guide and drive assemblies of the sealing module of FIG. 3; 
     FIGS.  8 A- 8 G are plan diagram views of alternative crack sensor arrays of the modules of FIGS. 2 and 3; 
     FIG. 9 is a block diagram of a position control system for the carriage drive assemblies of FIGS. 3 and 7; and 
     FIG. 10 is a flow chart for a microprocessor control program for the modules of FIGS.  2  and  6 . 
    
    
     DESCRIPTION 
     The present invention is directed to a modular system for roadway crack cleaning and repair that is particularly effective in tracking and following typical roadway cracks. With reference to FIGS.  1 - 10  of the drawings, a crack repair system  10  includes cleaning module  12  and a sealing module  14  for use on a roadway  15 , the cleaning and sealing modules  12  and  14  each including a maneuverable vehicle  16  (the vehicles being individually designated cleaner vehicle  16 A and sealer vehicle  16 B) that is equipped with a transversely movable carriage  18  having appliances  20  thereon as shown in FIG.  1 . According to the present invention, each of the carriages  18  has a sensor unit  21  supported thereon for signaling irregularities of the roadway  15  to a controller  22  (See FIG. 9) that is carried by the vehicle  16 , the platform carriage  18  also having an actuator  24  for positioning the carriage in response to the controller  22  as described below for following a roadway crack  25  that may have a variety of shapes. The platform carriage  18  is supported as described below by a frame  26  that extends forwardly from the vehicle  16 . The carriage  18  supports the appliances  20  generally in longitudinal alignment with the sensor unit  21  so that each appliance is approximately centered over the crack  25  while the vehicle is maneuvered generally along the crack and the actuator  24  is maintaining the sensor unit  21  more or less accurately centered over the crack  25 . In an exemplary and preferred implementation of the present invention, a pair of supportive swivel casters  28  are fastened under front corner extremities of the frame  26 . A guide assembly  30  for the carriage  18  engages a circularly cylindrical glide shaft  32  that is transversely mounted to the frame  26 , there being a parallel-spaced pair of the guide assemblies as shown in FIG.  3 . Thus the carriage  18  is movable in parallel relation to the frame  26 , as the vehicle  16  traverses the roadway  15 , the front of the frame  26  being supported at a predetermined distance from the roadway by the casters  28 , the rear of the frame  26  being supported by the vehicle  16 . 
     As shown in FIG. 1, the platform carriage  18  of the cleaning module  12  supports appliances  20  including a scarifier  34 , a scarifier vacuum inlet  36 , a router unit  38 , and a router vacuum inlet  40 . The cleaning module  12  also has a vacuum unit  42  supported on the cleaner vehicle  16 A (or on a trailer towed by the vehicle), respective flexible vacuum conduits  44  and  46  fluid-connecting the vacuum inlets  36  and  38  to the vacuum unit  42 . The purpose of the scarifier  34  is to dislodge material that may have accumulated in the crack  25 , the dislodged material being drawn into the scarifier vacuum inlet  36  and carried into the vacuum unit  42  for providing a clear view of the crack  25  to the sensor unit  21 . Accordingly, the scarifier  34  is located at the front of the carriage  18 , the scarifier vacuum inlet  40  being located between the scarifier  34  and the sensor unit  21 . As shown in FIG. 3, the scarifier is formed by a transversely spaced plurality of scarifier wires  48 , an upper end of each wire  48  being rigidly supported on the carriage  18  by a comb bar  49 , a lower end of each wire projecting downwardly and forwardly to a location below and slightly behind the point of attachment to the comb bar  49  for allowing the lower ends of the wires  48  to deflect upwardly and rearwardly when encountering rigidly fixed irregularities of the roadway  15 . The scarifier  34  is not required to completely clean the crack  25 , but rather to remove material that would otherwise obscure the location of the crack from the sensor unit  21 . 
     The router unit  38  is located immediately aft of the sensor unit  21  for minimizing the effects of skewed orientation of the crack  25  relative to the platform carriage  18 , the router vacuum inlet  40  being located behind the router unit  38  and ahead of the guide assembly  30 . Also, a router shield  39  is interposed between the sensor unit  21  and the router unit as shown in FIG. 2 for blocking forward movement of routed material that might otherwise interfere with operation of the sensor unit  21 . As shown in FIGS. 4 and 5, the router unit  38  includes a spindle assembly  50  mounted on a base plate  51  of the platform carriage  18 , the spindle assembly  50  having a spindle shaft  52  rotatably supported by antifriction bearings  53  in a stator structure  54 . In the drawings, the stator structure  54  is shown as a pair of circular plates, the bearings  53  having a flanged configuration, and the spindle shaft  52  being stepped for defining a spacing between the plates of the stator structure  54 . The shaft  52  rotates on a spindle axis  55 , having conventional means for rigidly holding a router cutter  56  that is used to more thoroughly clean the crack  25  in preparation for sealing by the sealing module  14 . Typically, the router cutter  56  has a diameter of approximately 0.5 inch. 
     In order to allow for variations in tracking of the crack  25 , and for some skew in the orientation of the crack relative to the platform carriage  18 , the spindle assembly  50  is resiliently supported on the carriage  18  by a flexible spindle support  57  that allows the spindle axis  55  to be displaced laterally and/or angularly as indicated at  55 ′ in FIG.  4 . As best shown in FIG. 5, the spindle support  57  includes a circularly spaced plurality of support springs  58 , opposite ends of each spring  58  being fastened by suitable spring anchor fasteners  59  to the stator structure  54  and the base plate  51 , the plate  51  being rigidly connected to the platform carriage  18  by any suitable means. Thus the router cutter  56  can deflect laterally, and the spindle axis  55  can deflect angularly from being vertically oriented, in response to side forces encountered by the spindle assembly  50  as the cutter  56  rotates in the crack  25 . The spindle assembly  50  may be rotatably powered using any suitable means, exemplary means being a belt drive  60  having a spindle sheave  62  that is mounted on the spindle shaft  52 , the belt drive  60  being powered from a suitable driven shaft (not shown) of the cleaner vehicle  16 A. 
     As best shown in FIGS. 3 and 7, an exemplary configuration of the actuator  24  includes a control motor  64  having an output pinion  66  that engages a gear rack  68 , the rack  68  being mounted to the frame  26  near one of the glide shafts  32  and parallel thereto by a pair of brackets  69 . (Only one of the guide assemblies  30  and glide shaft  32  is shown in FIG. 7.) Alternatively, the motor  64  and pinion  66  can be mounted to the frame  26 , the rack  68  being mounted to the carriage  18  for simplified wiring of the motor  64 ; however, it is preferred to have the rack  68  stationary as described above for permitting extended lateral travel of the carriage  18  without the rack  68  having to project beyond the width of the vehicle  16 . 
     As shown in FIG. 1, the platform carriage  18  of the sealing module  14  supports at least one appliance  20  which can be a crack sealer outlet  70  that is fluid connected by a sealer conduit  71  to a sealer feeder  72  being carried by the sealer vehicle  16 B. The sealer outlet  70  is located closely behind the sensor unit  21 , being longitudinally aligned therewith for following the crack  25  and filling same with a suitable sealant medium  73  when the controller  22  and the actuator  24  drives the platform carriage  18  for tracking the crack  25  as described above and in more detail below. The sealant medium can be a material that adheres to both sides of the crack  25  and, particularly in cases of wider cracks, the medium can contain a filler such as sand. 
     Optionally, the sealing module  14  is provided with a pressure roller  74  for leveling the roadway surface immediately adjacent to the crack  25 . The roller  74  is sufficiently wide to contact the roadway  15  on both sides of the crack  25 , and being sufficiently large in diameter for making rolling contact with the irregular matter as may be present following the cleaning and filling of the crack as described above, a suitable roller diameter being from approximately  5  inches to approximately  24  inches. Also, the roller  74  is effective for compacting the sealant medium being applied to the crack  25  and pressing the medium down into the crack. When the pressure roller  74  is employed, a quantity of sand  75  is preferably applied in the vicinity of the crack  25  upstream of the roller for avoiding adhesion and collection of the sealant medium on the roller  76 . Accordingly, a sand hopper  76  is mounted on the platform carriage  18  and having a sand outlet  77  between the sealer outlet  70  and the roller  76  for distributing the sand in a sufficiently wide pattern to encompass the width of the pressure roller or at least to cover all of the applied sealant medium  73 . A portion of the sand  75  is expected to become imbedded in the sealant, advantageously forming a composite having greater structural integrity than the sealant alone. Further, a sand vacuum inlet  78  is preferably located behind the pressure roller  74  for collecting sand  75  that does not adhere to the sealant medium  73 , the vacuum inlet  78  being fluid connected by a sand vacuum conduit  79  to a counterpart of the vacuum unit  42  being transported by the sealer vehicle  16 B. 
     With particular reference to FIGS.  8 A- 8 G, exemplary configurations of the sensor unit  21  include a transversely oriented sensor array  80  of sensors  82 , the sensors  82  individually signaling variations of the roadway  15  to the controller  22  by suitable means such as by parallel or multiplexed outputs. The sensors  82  can be distance-sensitive proximity sensors for signaling as cracks local depressions in the roadway  15 , such sensors being commercially available as the Cutler-Hammer Perfect Prox diffuse reflective sensor No. 13104A6517 from Kaman Industrial Technologies Corp. of Rancho Cucamonga, Calif. The above-identified sensor is configured for a 2-inch range, other such sensors being configured for 4-inch and 6-inch ranges, the version having the 2-inch range being preferred based on its particularly sharp cut-off at the threshold range. These sensors each have a width of approximately 0.47 inch, with an active width of approximately 0.25 inch. It is contemplated that the sensors  82  will be mounted with the cut-off range being slightly (by approximately 0.25 inch) below the nominal level of the roadway  15 , irregularities extending more than approximately 0.5 inch below the nominal roadway level being reliably signaled to the controller  22 . 
     In FIG. 8A, there are four of the sensors  82 , five of the sensors being shown in FIG. 8B, and seven of the sensors  82  being shown in FIG. 8C, the sensors  82  being arranged in a single row in each case and having a center-to-center spacing S. With the above-identified sensors  82  closely spaced in a single row, coverage is not continuous in that the active width, identified as ω in FIG. 8A, is less than the spacing S. Consequently, a narrow crack that is centered between adjacent sensors  82  may go undetected, the controller  22  being unable to distinguish conditions of tracking from no tracking except by inference from past history. Thus it is preferred to have an odd number of the sensors  82  as shown in FIGS. 8B and 8C so that perfect tracking would produce an active output from the center sensor as long as the crack  25  is sufficiently wide to be detected. Also, conditions wherein the crack extends to or beyond an outside one of the sensors  82  are less precisely distinguished because the width of the crack is variable. Thus it is preferred that the array  80  be sufficiently wide to encompass commonly encountered crack widths; however, it is also preferred to have the sensors closely spaced for avoiding cases of narrow cracks being undetected by being located between the active widths of adjacent sensors. Thus it is further preferred to have a relatively large number of sensors that are closely spaced. 
     In each of FIGS.  8 D- 8 G, there are odd numbers of the sensors  82  in two staggered and closely spaced rows, there being five of the sensors  82  in FIGS. 8D and 8E, and seven of the sensors in FIGS. 8F and 8G. In each case one of the rows has an even number of the sensors  82 , there being an odd number that is one different than the even number in the other row. Also, the row having the odd number of sensors is located forwardly of the other row in FIGS. 8D and 8G, the relative positions being reversed in FIGS. 8E and 8F, the forward movement of the sensor array  80  being indicated by arrows in each of FIGS.  8 D- 8 G. It is preferred that the row having the odd number of sensors  82  be to the rear, in closer proximity to the following appliance  20  (the router unit  38  of the cleaner module  12  and the sealer outlet  70  of the sealer module  14 ) for enhanced tracking accuracy when the sensor array  80  signals perfect tracking in the cases of the crack  25  being skewed. Accordingly, the configuration of FIG. 8F is most preferred among those depicted in FIGS.  8 D- 8 G, the configurations of FIGS. 8B and 8C being meritorious for providing wider coverage, the configurations of FIGS. 8B and 8C also advantageously having an odd number of the sensors  82  but the configuration of FIG. 8B having narrower coverage than that of FIG.  8 C. It will be understood that arrangements of greater numbers of sensors  82  are possible, such as nine sensors in two rows, although with commonly practiced microprocessor technology implementation of the controller  22  is facilitated by having not more than eight of the sensors  82 . It is contemplated that suitable sufficiently narrow sensor elements will become available such that the spacing S can match the sensed width ω, in which case the single-row configuration of FIG. 8C would be the most preferred among those depicted. 
     As shown in FIG. 9, the controller  22  includes a microprocessor  90  and a driver  92 , an input interface of the microprocessor receiving signals from the sensor array  80  of the sensor unit  21 . The driver  92  has inputs connected to an output interface of the microprocessor  90 , outputs of the driver being connected to the control motor  64  for lateral movement of the platform carriage as described above. It will be understood that interface connections of the microprocessor may be defined as input or output by program instructions that are executed during a software or firmware initialization sequence. Other connections to the microprocessor include a bias input  94  and other appropriate operator controls (not shown). The bias input  94  is used by the operator for urging the controller  22  to drive the platform  18  selectively toward one branch component of the crack  25  when the sensor unit  21  passes from a singular crack segment as indicated at  25 A in FIG. 1 to a branched segment as indicated at  25 B and  25 C in FIG.  1 . The bias input  94  can be implemented by a simple SPST toggle switch that signals the microprocessor  90  on a single line. A preferred alternative is to use a SPDT switch having a center-off position and separately signaling right- and left-bias inputs to the microprocessor. In that case, there can also be momentary push-button switches or equivalent means for temporarily augmenting or overriding the setting of the SPDT switch. Operation of the controller  22  in response to the bias input  94  is further described below. 
     As shown in FIG. 10, an exemplary control program  100  for the microprocessor  90  is configured for analyzing relevant combinations of signals from the sensor array  80  in order to generate suitable drive signals for the control motor  64 . The program  100  includes an interrupt routine  102  that is periodically initiated by a timer module of the microprocessor  90 , and a main routine  104 . The Main routine  104  includes an initialization sequence  106  wherein variables are reset and the clock timer is activated, followed by an endless loop  108  that continues until an interrupt is encountered. The interrupt routine  102  includes a clock service sequence  110  that decrements a timer variable and returns directly to the main routine  102 , except that when the timer variable underflows, a sensor subroutine  112  is executed for reading and analyzing the sensor array  80 , followed by an output routine  114  for correspondingly signaling the driver  92  to move the platform carriage  18  to move a designated distance for tracking the crack  25 . 
     In the sensor subroutine  112 , the sensor array  80  is read and tested for any change relative to a last previous reading. When there is no change, the subroutine is exited immediately, control being transferred to the output routine  114  for maintaining a previously signaled activation of the driver  92 . It will be understood that although various implementations of the control motor  64  are contemplated, one such implementation is a stepper motor wherein a plurality of motor phase windings are driven in a progressive sequence to advance the motor, the motor holding its position when the sequence is halted. When the designated distance is more than a single step, the output routine  114  can produce steps following the first one in subsequent executions of the output routine. Also, some known stepper motor implementations involve dynamic damping of motor oscillations, which can also be implemented in repeated executions of the output routine. In the present invention, the control motor  64  can be implemented as a stepper motor that is responsive to both “half-step” and “full-step” activations in a manner known to those having skill in the art. In this case, the output routine  114  advances an output phase of the driver  92  by the indicated half-step or full-step distance, merely holding that phase in subsequent entries of the output routine  114  unless the sensor subroutine  112  calls for a new position of the platform carriage  18 . In a preferred variation that is potentially more accurate, the control motor is driven at a rate proportional to the positional error, known in the control system art as a type- 1  servo. 
     When a change of the signals from the sensor array  80  is encountered in the sensor subroutine  112 , program control advances to an analysis sequence  116  that proceeds by a process of pattern matching to derive an “error signal” to be used by the output routine  114 . In the exemplary case of the control motor  64  being a stepper motor having “half-step” and “full-step” responses, the error signal may have only five possible values, namely −1, −½, 0, ½, and +1. Also, with the array  80  having seven of the (digital) sensors  82 ,  128  output states of the sensors are possible. While a pure table look-up using a memory of 128 addresses of 3-bits each is possible, an exemplary and preferred implementation of the analysis sequence  116  uses a decision tree having series of masks and comparisons of sensor outputs to generate the error signal. Table 1 lists the possible output states of the sensors  82  grouped by cases considered to have like significance, with indications of the resulting error signal. The table lists an “address” being the decimal equivalent of the sensor output states; the states of the individual sensors  82  (from the leftmost, L 3 , L 2 , L 1 , center, R 1 , R 2 , progressively to the rightmost, R 3 ); “move” (a preliminary form of the error signal); “mask”; and “match”. In cases 1 and 2, all seven outputs are the same, and control is transferred to an acquire sequence  118  that is described below, it being assumed that the crack  25  is not in view, or the height of the sensor array  21  is improperly adjusted. (Another possibility is that the crack  25  extends beyond opposite sides of the array  80 .) 
     In cases 3-17, the sensor output states are interpreted as detecting a single crack  25 , the crack being centered under the array  80  in cases 3, 6, and 11, with the resulting error signal being zero. Cases 4 and 9 produce an error signal of −½, cases 7, 12, 14 and 15 produce an error signal of −1, while cases 5 and 10 produce an error signal of +½ and cases 8, 13, 16, and 17 produce an error signal of +1. In an exemplary case, case 4, the center and L 1  sensors signal a depression, the sensors L 2  and R 1  on either side signaling no depression. Accordingly, the crack  25  is deemed to be centered halfway between the center and L 1  sensors, the sensors L 3 , R 2  and R 3  being ignored. Thus the analysis sequence includes instructions for each of the cases, masking off the ignored bits of the address in case 4 by the hexadecimal value of 3C, which has zeroes in the ignored bit locations. (This masking portion of the analysis is common to cases 4, 7, 17, and 20.) Following the masking, the result is compared with a match quantity of 18 hexadecimal of case 4, a match confirming that case and the error signal being set to −½ with control being returned to the calling portion of the interrupt routine  108 , at the output routine  114 . Absent a match, other possible cases are similarly tested, a positive result being eventually assured in that the table encompasses all possible output combinations of the sensor array  80 . 
     In cases 18-24, the sensor output states are deemed to indicate the presence of two cracks in the roadway  15 . In one common possibility, a single crack branches to form separate cracks, it being necessary to elect which of the cracks to follow. Consequently, an operator of the vehicle  16  can use the bias input  94  for making the election, the error signal being set according to the bias input  94 . Alternatively, the analysis sequence  116  for the cases 18-24 can have separate tabulations of the error signal for each state of the bias input  94  (two in the case of the SPST switch described above, three in the case of the SPDT switch). Also, the analysis sequence  116  can be responsive to the pair of momentary push-button switches described above in each of the cases 3-24 for producing a manual offset in the controlled position of the platform carriage  18 . 
     In the acquire sequence  118 , the error signal can be set according to the bias input  94  as in the cases 18-24, the operator manipulating the bias input for moving the sensor unit  21  over a visually observed crack  25  in the roadway  15 . Alternatively, the acquire sequence  118  can produce repetitively alternating error signals in a predetermined pattern for producing a scanning lateral movement of the platform carriage  18 . In either alternative, the acquire sequence  118  is automatically bypassed in succeeding cycles of the interrupt routine  102  in response to the analysis sequence  116  processing sensor output states other than cases 1 and 2. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, the sensor elements  82  can provide a multiple bit or analog indication of the sensed variable instead of a single-bit digital signal, for enhanced effectiveness in tracking the crack  25 . The scarifier  34  can be configured as a rotating wire brush, ground-contacting portions of the brush preferably moving in a direction for sweeping debris away from the path of the sensor unit  21  such that use of the vacuum inlet  36  is not required. Also, the router unit  38  can be supported on an elevator mechanism for adjusting the depth of routing, and for raising the router cutter  56  to clear the roadway  15  when the router unit is not being used. Further, the vacuum unit, the sealer feeder, and the sand hopper can each be optionally carried on the vehicle  16 , the frame  26 , or on the platform carriage  18 . Moreover, when the roadway  15  has the cracks  15  running generally crosswise, the platform carriage  18  can be oriented orthogonally to the vehicle  16  relative to the orientation shown in the drawings, being supported on a cross-slide carriage. The vehicle is driven along the roadway generally perpendicular to the crack and stopped or nearly stopped with the sensor unit positioned over the crack, and the cross-slide carriage is operated for traversing the crack, the controller  22  operating to cause the sensor unit  21  to track the crack. Although it is preferred to have separate vehicles for the cleaning and sealing modules  12  and  14 , it is also contemplated that the complete system can be provided on a single vehicle. Therefore, the spirit and scope of the appended claims should not necessarily be limited to the description of the preferred versions contained herein.

Summary:
A roadway crack repair system includes a crack cleaning module and a crack sealing module. Each module includes a steerable vehicle for navigating the roadway, a platform being laterally movable on the vehicle in response to a control signal, a sensor being mounted on the platform for detecting local variations in the roadway during movement of the vehicle, a sensed continuous local deviation indicating a roadway crack. A controller is responsive to the sensor for generating the control signal as a sensed lateral position of the crack relative to the sensor, causing the sensor to accurately follow the crack when the vehicle is generally driven along the crack. A router is mounted on the platform behind the sensor of the crack cleaning module for cleaning and widening the crack, and a vacuum system removes loose material from the roadway behind the router. A front-mounted scarifier dislodges foreign material that might otherwise interfere with operation of the sensor, another vacuum inlet being located between the scarifier and the sensor. A sealer outlet is mounted on the platform behind the sensor of the crack sealing module for feeding a sealant medium into the crack. A pressure roller behind the sealer outlet compacts and levels the sealant medium and the roadway, sand being dispensed ahead of the roller for preventing adhesion of sealant on the roller, loose sand being removed by vacuum behind the roller. Also disclosed is a method for repairing roadway cracks.