Abstract:
A sensor for detecting the height above the soil of harvesting machinery such as a combine header includes an elongated, curved, flexible arm attached at one end to a rotation sensor mounted to the harvester. The sensor arm is concave in an upward direction and includes and outer sheath such as of plastic and an inner metal rod for increased strength to resist breakage such as during turns and when traversing irregular terrain. The arm is positioned forward of, or immediately adjacent to, the lowest part of the harvester which includes a cutter assembly for cutting and separating the crop. As the harvester encounters upraised portions of the soil, the arm&#39;s point of contact with the soil moves forward along the arm to provide an increasingly early warning of upraised soil to facilitate early height adjustment and the avoidance of soil contact and possible damage to the harvester.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to a sensor for detecting the height above the soil of a moving vehicle such as a harvesting machine, and is particularly directed to a ground height sensor having a curved ground engaging arm, wherein the ground engaging point on the arm moves forward as upraised portions of the soil are encountered to provide an increasingly early warning of ground impact as the soil-machine separation is reduced. 
   BACKGROUND OF THE INVENTION 
   A common approach to crop harvesting involves the use of a combine having a header on its forward portion for engaging and removing the crop from a field. The header is maintained a designated height above the soil as determined by the type of crop and various operating conditions. Operating with the header too high will result in failure to harvest all of the crop, while operating too close to the soil increases the possibility of damage to the header by impact with rocks and other obstructions in the soil. With the use of longer headers spanning wider tracts, the possibility of impact of the header with the soil and consequent damage to the header and/or combine has correspondingly increased. 
   Various types of height sensors are used to maintain the harvesting machine a designated height above the soil for optimum crop recovery. Most current height sensors employ a ground engaging arm suspended from the header and extending rearwardly relative to the direction of travel. A cutter assembly is located in a bottom portion of the header housing forward of the height sensor arm. Thus, the sensor arm provides information regarding vertical separation between the cutter assembly and the soil with respect to soil the cutter assembly has already passed over. The inability to sense and provide information regarding terrain in front of the header limits the accuracy of the height control signals provided by the height sensor. Moreover, as the header gets closer to the ground, current sensor arms engage the ground even further aft of the cutter assembly thus increasing the separation between the position of the cutter assembly and the location of the soil the height of which is actually being detected. 
   The height sensor typically includes a thin rod extending rearwardly and engaging the soil. These sensor arms are subjected to large forces. For example, a downward force is applied to the sensor arm to ensure that its distal end engages the soil. This downward force is of sufficient magnitude to allow the arm to penetrate plant residue in order to contact the soil. In addition, crop rows are frequently curvilinear to accommodate terrain contour. Harvesting curvilinear crop rows results in the application of large lateral forces on the sensor arm. The capability of combines, which incorporate rear steering, to rapidly turn and change direction increases the likelihood of sensor arm damage caused by the application of large lateral forces. In addition, field terracing wherein upraised strips of soil or elongated shallow depressions, or ditches, in the soil are formed in a spaced manner over a field are increasingly used to reduce erosion. Traversing these upraised strips of soil or spaced depressions also subjects the height sensor arm to large forces while placing greater demands on sensing and reacting to changes in soil elevation to avoid damage to harvesting machinery. Also, in an attempt to maximize crop recovery, harvesting headers are increasingly being employed at lower heights above the soil with increased force being applied to the height sensor arm. All of these factors tend to increase the likelihood of damage to the height control sensor resulting in harvester down time and production losses. 
   Finally, header height control sensors are generally not designed with the configuration of existing headers as a primary consideration. Thus, the typical header height sensor is not adapted for retrofitting on an existing header without header modification. For example, one current soil height sensor employs a pair of pivotally connected curved arms mounted to a lower portion of the header housing. In order to accommodate this multi-section height sensor arm, the lower surface of the header housing is provided with a recessed portion to receive the arm sections for storage and protection of the arms from damage when not in use. Not all harvester heads are provided with these height sensor arm storage recesses, thus, limiting the use of this type of sensor arm to headers having these recesses. 
   The present invention addresses the aforementioned limitations of the prior art by providing a height sensor arrangement particularly adapted for use in agricultural applications such as on a harvester which provides an increasingly early warning of upraised soil about to be traversed by the harvester as its height above the soil is reduced. This allows for more accurate height positioning of the harvester, thus reducing the possibility of impact with the soil and damage to the harvester. The height control sensor arrangement is particularly adapted for positioning in a forward, lower portion of the harvester to provide an even earlier warning of upraised soil to allow for harvester height adjustment. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a height sensor for an agricultural implement traversing a field which increases the time between detection and traversal of high points in the soil to facilitate implement height adjustment and the avoidance of impact with the soil. 
   It is another object of the present invention to provide a curved arm for a ground height sensor which is of high strength and rugged, is flexible allowing the sensor to be lowered to the ground without damaging or breaking the arm, and engages the ground at a point along its length which moves forward in the direction of travel as the height sensor is lowered to provide an earlier warning of contact with upraised portions of the ground. 
   Yet another object of the present invention is to provide a ground height sensor particularly adapted for agricultural applications, such as for use on a harvester of the combine header type, which can be easily mounted using conventional hardware at a location forward of or adjacent to the header&#39;s cutterbar. 
   A further object of the present invention is to provide a ground height sensor for use in a combine header which is easily installed on either end or on an inner portion of a header anywhere along its length without requiring modification of the header. 
   The present invention contemplates apparatus for use in an agricultural implement for measuring the height of the agricultural implement above the soil as the agricultural implement traverses a field. The apparatus comprises a shaft mounted to the agricultural implement; a rotation sensor coupled to the shaft and responsive to rotation of the shaft for providing an output signal representing rotational displacement of the shaft; and a curved flexible arm having a proximal end connected to the shaft and a distal end engaging the soil, wherein the arm is concave in an upward direction and rotates in a first direction when the arm contacts upraised soil and rotates in a second opposed direction when the arm contacts a depression in the soil, and wherein a point of contact of the arm with the soil moves forward in a direction of travel of the implement toward the proximal end of the arm as the height of the implement above the soil is reduced to provide an increasingly early indication of contact with the soil of the implement as its height above the soil decreases. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which: 
       FIG. 1  is a partial front perspective view of a combine with an attached header incorporating a height sensor in accordance with the principles of the present invention; 
       FIG. 2  is a partial perspective view of a lower portion of a combine header illustrating the mounting of a height sensor on an inner, lower portion of the header in accordance with another aspect of the present invention; 
       FIG. 3  is an exploded perspective view of one embodiment of a height sensor in accordance with the present invention; 
       FIG. 4  is a perspective view illustrating details of the manner in which the height control sensor shown in  FIG. 4  is attached to a combine header; 
       FIG. 5  is a partial perspective view shown partially in phantom illustrating additional details of the inventive height sensor; 
       FIGS. 5   a  and  5   b  are respectively perspective exploded and assembled views of the height sensor of  FIG. 5  which incorporates an adjustable feature for varying the downward, ground-engaging force exerted on the sensor arm; 
       FIG. 6  is a partial perspective view of the embodiment of the height sensor shown in  FIG. 2  illustrating additional details of the manner in which it is mounted in the header; 
       FIG. 7  is a perspective view of another embodiment of a height sensor in accordance with the principles of the present inventions; and 
       FIGS. 8-12  are side elevation views of another embodiment of the height sensor of the present invention illustrating the manner in which the point of contact of the sensor arm moves forward along the length of the arm in the direction of travel as the separation between the sensor and the soil is reduced. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a partial front perspective view of a height sensor arrangement  10  for use on a header  14  attached to a forward portion of a combine  12 . Combine  12  is conventional in design and operation and includes a chassis  30  disposed on and supported by four wheels, three of which are shown in the figure as elements  28   a ,  28   b  and  28   c . An operator is positioned within a cabin disposed in the chassis. Also disposed within the chassis are a means for propulsion and various grain processing stages as well as a storage bin for temporarily storing grain separated from plants ingested by the header. These portions of combine  12  are conventional and are not part of the present invention and are thus not shown in FIG.  1 . Header  14  is also conventional in design and operation and includes a crop gathering unit  36  (partially shown in the figure) used in the harvesting of various grains. Header  14  also includes an elongated housing oriented generally at 90° relative to the direction of travel of the combine during harvesting. 
   Header  14  includes first and second end panels  14   a  and  14   b . While only one header section is shown attached to a forward portion of combine  12  in  FIG. 1 , plural header sections attached by means of their respective end panels may be connected together and mounted to a forward portion of the combine to provide a wide harvesting path. Typically attached to an upper portion of header  14  is a crop engaging/gathering mechanism  36  for directing the severed plant residue into the combine for processing, with only a portion of this mechanism shown in  FIG. 1  because it does not form a part of the present invention. Shown respectively attached to the first and second header end panels  14   a ,  14   b  are first and second height sensor arrangements  10  and  11  which are similar in operation and configuration as they embody the principles of the present invention. Extending lengthwise along the header  14  is an auger  20  also oriented generally transverse to the direction of travel of the combine  12 . Auger  20  is rotationally displaced by means of a combination of a driven sprocket  24 , a drive chain  22  and a drive sprocket  26  which is rotationally displaced by the combine&#39;s engine (not shown). Auger  20  is provided with a pair of complementary spiral sections which direct grain and plant residue taken in by the header  14  toward the center of the header housing where it is directed aft into the combine&#39;s feederhouse (not shown) for processing within the combine. The bottom of the header  14  is provided with a skid plate  18  extending the length of the header housing which is adapted to engage and ride over upraised portions of uneven soil. Disposed adjacent a forward portion of the skid plate  18  is a cutterbar  16  which operates in a reciprocating manner to sever the upper portion of plants engaged by the header  14  as the combine  12  traverses a field. The thus severed plants, with grain attached thereto, are directed into the header&#39;s transverse auger  20  for processing as described above. 
   Because the first and second height sensor arrangements  10  and  11  are similar in operation and configuration, only the first height sensor arrangement will be described in detail for simplicity. The first height sensor arrangement  10  includes a curved, flexible arm  34  having a first proximal end and a second, opposed distal end. The height sensor arrangement  10  further includes a sensor mechanism  32  mounted to the header&#39;s first end panel  14   a  and attached to the proximal end of arm  34  for supporting the arm in a suspended manner. Sensor mechanism  32  is described in detail below. The distal end of arm  34  is attached to an aft portion of header  14  by means of a high strength connecting cable  38  which is preferably comprised of steel. Connecting cable  38  prevents damage to the sensor arm  34  in the event the combine  12  is reversed in direction. 
   Referring to  FIG. 2 , there is shown another embodiment of a height sensor arrangement  64  in accordance with the principles of the present invention. As in the previously described embodiment, height sensor arrangement  64  is attached to a header  50  having first and second end panels  52  and  54  as well as a crop gathering unit  58  attached to an upper portion of the header. A cutterbar assembly  56  is disposed in a lower, forward portion of the header  50  immediately forward of a skid plate  62  forming the bottom portion of the header. In the embodiment shown in  FIG. 2 , the height sensor arrangement  64  is attached to and suspended from the header&#39;s skid plate  62  by means of a mounting assembly  66  described in detail below. As in the previously described embodiment, the height sensor arrangement  64  includes a curved, flexible arm  68  and a connecting cable  69  coupling a distal end of the arm to an aft portion of the header  50 . A hydraulic cylinder  60  is connected between the header  50  and the combine for raising and lowering the header between a nonuse position and a use position and for changing the height of the header above the soil in accordance with output signals from the height sensor arrangement of the present invention. 
   Referring to  FIG. 3 , there is shown an exploded perspective view of a height sensor arrangement  70  in accordance with another embodiment of the present invention.  FIG. 4  is a perspective view of the height sensor arrangement  70  as installed on a header crop divider  120 , while  FIG. 5  is a perspective view of the height sensor arrangement in assembled form. The height sensor arrangement  70  shown in  FIG. 3  is adapted for attachment to a side panel of a header as shown for the case of height sensor arrangements  10  and  11  in FIG.  1 . Height sensor arrangement  70  includes an elongated, curved flexible arm  72  having an outer elastomeric sheath  73  shown in dotted line form in the figure and an inner high strength spring steel shaft  74  which is capable of flexing. Elastomeric sheath  73  protects arm  72  by absorbing high energy impact forces exerted on the arm such as when it engages an obstruction such as a rock or root in the field. Extending from a first end of the arm  72  and disposed within the outer elastomeric sheath  73  and connected to the spring steel shaft  74  such as by weldments is a metal reinforcing member  84  which provides the arm  72  with very high strength, particularly with respect to lateral forces. The combination of shaft  74  and reinforcing member  84  may also be formed by bending the shaft back upon itself and positioning the curved bent-back portion in closely spaced relation to the proximal end of the shaft as in the embodiment shown in FIG.  3 . Each of the elastomeric sheath  73 , shaft  74  and reinforcing member  84  is provided with the same radius of curvature along those portions of its respective length where it is in contact with one or more of the other two members, and none of these shaft members has a constant, fixed radius along its entire length. Each shaft member will assume its original curvature following removal of a force which changes its curvature. Arm  72  is flexible and has a curvilinear shape as shown in the figure for purposes which are discussed in detail below. One end of the spring steel shaft  74  is provided with an aperture  74   a  for installing the arm in the height sensor arrangement  70 . A first proximal end of arm  72  is provided with a first end aperture  72   a , while second opposed end of arm is provided with a second distal end aperture  72   b . The second end aperture  72   b  is adapted for receiving the combination of a threaded member  78   a  and a nut  78   b  for attaching one end of the connecting cable  76  to the second distal end of the arm  72 . The first end aperture  72   a  of the arm  72  is adapted to receive the combination of an elastomeric bushing  80  and an insert member  82 . The insert member  82  is inserted within the elastomeric bushing  80  and includes an aperture extending therethrough. The aperture in the insert member  82  is adapted to receive a threaded member  87  which is also inserted through the aperture  74   a  in the end of the spring steel shaft  74  for attaching the proximal end of the arm  72  to a bracket  86 . The proximal end of arm  72  is securely attached to bracket  86  by means of the combination of the threaded member  87  and a nut  88 . Also attached to bracket  86  by means of first and second threaded members  98   a  and  98   b  is a rotation sensor  94 . Rotation sensor  94  is electrically coupled to the combination of a header controller  114  and a controller interface  112  by means of the combination of an electrical connector  96  and one or more electrical leads  97 . 
   Bracket  86  includes a circular aperture through which is inserted a fixed shaft  92 . A first end of the fixed shaft  92  is attached to the rotation sensor  94 , while a second opposed end of rotating shaft is connected to a sensor dial  102 . Fixed shaft  92  is inserted in a cylindrically-shaped rotating shaft retainer  90 . Shaft retainer  90  is inserted in an aperture  100   a  of a sensor housing  100 . Disposed within sensor housing  100  is a torsion spring  106 , with the torsion spring disposed about and connected to the shaft retainer  90  as both of these components are disposed within the sensor housing  100 . Shaft retainer  90  extends through the sensor housing  100  and thus extends through aperture  100   a  as well as through a second aligned aperture in an opposing face of the sensor housing which is not shown in the figure for simplicity. A first combination of a bushing  108  and retaining ring  109   a  and a second combination of a bushing  110  and retaining ring  109   b  are disposed about the shaft retainer  90  in a spaced manner within the sensor housing  100  to maintain the shaft retainer within the housing while allowing the shaft retainer to freely rotate within the sensor housing. Retaining ring  109   a  is adapted for positioning within a first circumferential slot  90   a  within the rotating shaft retainer  90 , while retaining ring  109   b  is adapted for positioning in a second circumferential slot (not shown in  FIG. 3  for simplicity) for securely coupling the shaft retainer to housing  100  while allowing the shaft retainer to rotate. Also attached to the sensor housing  100  by means of the combination of a bolt  104   a  and a nut  104   b  is the aforementioned sensor dial  102 . Sensor dial  102  is in the form of a thin, elongated pin-like structure which is wrapped around bolt  104   a  and freely rotatable about the bolt. One end of the sensor dial  102  is inserted into a notched end portion  92   a  of fixed shaft  92 . In addition, one end of the torsion spring  106  is securely connected to the shaft retainer  90  within the sensor housing  100 . By engaging the notched end portion  92   a  of the fixed shaft  92 , sensor dial  102  securely maintains the fixed shaft in fixed position within the sensor housing  100  and establishes a zero elevation reference for the height sensor, which elevation reference point is adjustable. The elevation reference point may be easily changed by providing plural spaced apertures within housing  100 , with each aperture adapted to receive the combination of bolt  104   a  and its associated nut  104   b  for changing the position of sensor dial  102  and the orientation at which it engages the end  92   a  of the fixed shaft  92 . Shaft retainer  90  is freely rotatable on the fixed shaft  92  about which it is positioned. With the shaft retainer  90  attached to an end of the torsion spring  106 , the torsion spring urges the shaft retainer to a given rotational position within the sensor housing  100 . Rotational displacement of arm  72  which is attached to bracket  86  causes a corresponding rotational displacement of the combination of the shaft retainer  90 . In the arrangement shown in  FIG. 3 , an inner portion of rotation sensor  94  is maintained fixed by the fixed shaft  92 , while the sensor housing is allowed to rotate with the rotating shaft retainer  90  to provide an indication of the rotation of the sensor arm  72  about the axis of the shaft retainer within sensor housing  100 . Rotation of shaft  90  is detected by the rotation sensor  94  which provides a corresponding signal via electrical connector  96  and lead(s)  97  to the controller interface  112  which, in turn, provides a signal to header controller  114 . Header controller  114  is connected to the header for adjusting the height of the header in accordance with the rotation of sensor arm  72  as provided by the height sensor arrangement  70 . 
   Referring to  FIGS. 5   a  and  5   b , there are respectively shown exploded and assembled perspective views illustrating additional details of the height sensor arrangement  70  shown in  FIGS. 3 ,  4  and  5 . As shown in  FIGS. 5   a  and  5   b , torsion spring  106  is attached to the shaft retainer  90  by means of a threaded pin  89 , such as a bolt or screw, inserted through an inner portion  106   a  of the spring and into a threaded aperture  90   c  in a lateral surface of the shaft retainer. The outer end  106   b  of the torsion spring  106  is attached to the sensor housing  100  by means of a coupling bracket  95 . Coupling bracket  95  is attached to an inner surface of sensor housing  100  by conventional means such as a threaded coupling pin which is not shown in the figure for simplicity. Coupling bracket  95  is in the general shape of the letter “E” and includes first and second spaced recesses  95   a  and  95   b . Each of the first and second spaced recesses  95   a ,  95   b  is adapted to receive and securely engage the outer end  106   b  of torsion spring  106 . Coupling bracket  95  and the two recesses  95   a  and  95   b  disposed therein allow the outer end  106   b  of the torsion spring  106  to be positioned in accordance with the amount of tension to be applied to the torsion spring. For example, with torsion spring  106  applying a rotational force to the sensor arm  72  about it pivot axis aligned with rotating shaft  92 , this rotational force, and thus the downward force with which the distal end of the sensor arm  72  engages the ground or plant material disposed on the ground, may be adjusted, as desired. By positioning the outer end  106   b  of torsion spring  106  in the second, lower recess  95   b  within coupling bracket  95 , the torsion spring may be maintained under increased tension for urging the distal end of the sensor arm  72  downward with greater force. On the other hand, by positioning the outer end  106   b  of torsion spring  106  within the first, upper recess  95   a  of coupling bracket  95 , torsion spring  106  will be maintained under a reduced tension and will thus exert a reduced downward force on the distal end of sensor arm  72 . In this manner, the force with which the distal end of sensor arm  72  engages the ground or plant material disposed on the ground may be adjusted as desired to permit the sensor arm to penetrate a range of thicknesses of plant material disposed on the ground. While the figures show the coupling bracket  95  as having only two adjustment positions, coupling bracket  95  may be sized and configured to include a large number of tension adjustment positions to permit the torsion spring  106  to apply a wide range of the ground engaging force to sensor arm  72 . 
   As shown in  FIG. 4 , a sensor guard, or shielding plate,  124  is disposed adjacent the height sensor arrangement  70 . Shielding plate  124  is attached to the header crop divider  120  by means of one or more threaded mounting pins  126 . Shielding plate  124  is preferably comprised of a high strength, impact resistant material such as metal or plastic and protects the height sensor arrangement  70  from damage caused by impact with plant matter/crop residue as well as with obstructions in the field such as rocks. 
   Referring to  FIG. 6 , there is shown a partial perspective view of another embodiment of a height sensor arrangement  130  in accordance with the present invention. The height sensor arrangement  130  shown in  FIG. 6  is adapted for attachment to the skid plate  62  of a header  50  adjacent the header&#39;s cutter bar  149  such as illustrated in FIG.  2 . In the embodiment shown in  FIG. 6 , the height sensor arrangement  130  is attached to the header skid plate  62  adjacent to, and extends through, an aperture  132   a  within the plate. The height sensor arrangement  130  includes a sensor housing  142  attached to an upper surface of the header skid plate  132  by means of plural threaded connectors. Disposed within and extending through facing apertures in opposed surfaces of the sensor housing  142  is a shaft retainer  143 . Shaft retainer  143  is freely rotatable within the sensor housing  142  and is connected at one of its ends to a rotating shaft  144  as in the previously described embodiment. In the embodiment shown in  FIG. 6 , rotating shaft  144  is disposed within and extends through a torsion spring  146 . One end of the torsion spring  146  is securely attached to either sensor housing  142  or skid plate  132 , while a second end of the torsion spring is connected to the rotating shaft  144 . A rotation sensor is also connected to the rotating shaft  144 , although this is not shown in the figure for simplicity. Also attached to the rotating shaft  144  so as to rotate therewith is a coupling bracket  145 . Coupling bracket  145  includes a generally flat mounting plate  136  to which is attached one end of a sensor arm  134  by means of the combination of a threaded pin  138  and nut  140 . A combination of the coupling bracket  145  and mounting plate  136  extends through the aperture  132   a  within the header skid plate  132 . As sensor arm  134  is deflected and displaced upon impact with the soil in the direction of arrow  150 , the combination of the rotating shaft  144 , coupling bracket  145  and mounting plate  136  rotates about a generally horizontal axis passing through the rotating shaft. The rotation sensor (not shown) coupled to the rotating shaft  144  detects rotation of the sensor arm  134  and provides an appropriate signal for controlling the height of the header above the soil. A sensor guard  148  in the form of a generally flat, high strength plate such as of steel or plastic is attached by conventional means such as weldments or threaded connecting pins to a lower surface of the header skid plate  132  for protecting the height sensor arrangement  130  from damage caused by impact with the soil. 
   Referring to  FIG. 7 , there is shown another embodiment of a height sensor arrangement  152  in accordance with the principles of the present invention. As in the previously described embodiment, the height sensor arrangement  152  shown in  FIG. 7  is adapted for attachment to the lower, leading edge or surface of a header skid plate  154  adjacent the header&#39;s cutter bar  180 . In the arrangement of  FIG. 7 , the upper end of a sensor arm  166  is attached to a coupling bracket  164  by means of the combination of an elastomeric bushing  174 , an insert member  172 , and a threaded pin  168  and nut  170  combination. Coupling bracket  164  is also attached to a cylindrical shaft coupler  160  by conventional means such as weldments, which are not shown in the figure for simplicity. First and second ends of the shaft coupler  160  are securely attached to a lower surface of the header skid plate  154  by means of first and second mounting brackets  156   a  and  156   b , respectively. Shaft coupler  160  is rotatably attached to each of the first and second mounting brackets  156   a ,  156   b , allowing the combination of coupling bracket  164  and sensor arm  166  attached thereto to freely rotate with respect to the header. A first end of the shaft coupler  160  is attached to a torsion bar  158 , which is shown as having six sides, while a second opposed end of the shaft coupler is attached to a rotating shaft  162 . The other end of the torsion bar  158  is fixedly attached to the header in a conventional manner such that the attached end of the torsion bar is not free to rotate about its longitudinal axis. The other end of the rotating shaft  162  is attached to a rotation sensor  176  which measures the extent of rotation of the shaft and sensor arm  166  attached thereto as in the previously described embodiments. Rotating shaft  162  may be rigid or it may be in the form of a flexible steel cable to facilitate mounting of the rotation sensor  176  on the header. Torsion bar  158  maintains the sensor arm  166  at a given inclination relative to the header and exerts a rotational force on the sensor arm which must be overcome prior to rotation of the sensor as it contacts the soil. The force applied to the sensor arm  166  by the torsion bar  158  maintains a distal end of sensor arm  166  in contact with the soil. An elongated, curvilinear shield  178  is shown in dotted line form mounted to a forward portion of the header to protect the height sensor arrangement from damage caused by impact with the crop or with obstructions in the field. Height sensor arrangement  152  incorporating the rigid, elongated torsion bar  158  may also provide for varying the downward force applied to the sensor arm  156  as in the previously described embodiment. For example, torsion bar  158  may be in the form of a six sided shaft as shown in  FIG. 7  which is maintained in position by a mounting bracket (not shown) at least partially disposed about the torsion bar and attached to the header skid plate  154  by plural mounting pins (also not shown). Plural threaded apertures may be provided along the length of the mounting bracket, with each aperture adapted to receive a threaded pin which engages one of the lateral surfaces of the torsion bar  158 . With the lower end of the sensor arm  166  engaging the ground, torsion bar  158  may be rotationally displaced so that the desired amount of downward force is applied to the sensor arm. The rotational position of the torsion bar  158  may then be locked in position by tightening the threaded pins engaging lateral surfaces of the torsion bar  158  and preventing it from rotating for maintaining the desired downward force on the sensor arm  166 . Although this arrangement is not shown in the figures, it could easily be implemented by one skilled in the relevant arts. 
   Referring to  FIGS. 8-12 , the operation of the sensor arm  190  of the present invention will now be described. Sensor arm  192  includes first, second and third sections  192   a ,  192   b  and  192   c . The first and second sections  192   a ,  192   b  are securely connected together by plural connecting pins  206 , while the second and third sections of the sensor arm  192  are securely connected together by means of second plural connecting pins  208 . A lower distal end of the sensor arm  192  is provided with a bulbous portion  204  for engaging the soil  210 . Sensor arm  192  further includes a high strength plastic rod  200  and a metal reinforcing member  202  as in the previously described embodiments. Metal reinforcing member  202  is connected to and extends from a rotation sensor  194 . The sensor arm&#39;s first section  192   a  is connected to the metal reinforcing member by means of a threaded connecting pin  196 . The high strength plastic rod  200  is also connected to the metal reinforcing member  202  by conventional means and to the three sections of the sensor arm  192  by the first and second plural connecting pins  206  and  208  which draw adjacent portions of the arm together with the plastic rod between them in a clamping manner. Sensor arm  190  is first provided with a predetermined curvature as shown in the various figures. 
   In  FIG. 8 , the height sensor arrangement  190  is shown in an elevated position, where the distance between the rotation sensor  194  (and the header to which it is attached), is shown as h 1 . At this height, the bulbous portion  204  of the sensor arm  192  engages the soil  210  a distance x 1  aft of the rotation sensor  194 , where the combine is moving in a direction from right to left as viewed in  FIGS. 8-12 .  FIG. 9  is a side elevation view of the height sensor arrangement  190  at a medium height, where the distance between the rotation sensor  194  and the soil  210  is h 2 . At this lower height, an intermediate portion of the sensor arm  192  engages the soil a distance x 2  aft of the rotation sensor  194 , where x 2 &lt;x 1 .  FIG. 10  shows the height sensor arrangement  190  at a lower elevation relative to the soil  210 , where the distance between the rotation sensor  194  and the soil is h 3 . In the lower position of  FIG. 10 , the curved sensor arm  192  engages the soil adjacent the center of the sensor arm at a distance x 3  aft of the rotation sensor  194 , where x 3 &lt;x 2 &lt;x 1 .  FIG. 11  is a side elevation view of the height sensor arrangement  190  at an even lower position relative to the soil  210 . At this height, the sensor arm  192  engages the soil at a location close to the proximal end of the arm and in closely spaced relation from the rotation sensor  194 . At the reduced height of the rotation sensor  194  shown in  FIG. 11 , the point of contact of the sensor arm  192  with the soil is x 4  aft of the rotation sensor, where x 4 &lt;x 3 &lt;x 2 &lt;x 1 . From  FIGS. 8-11 , it can be seen that as the height of the rotation sensor  194  (and thus the height of the header) is reduced, the point of contact of the sensor arm  192  moves forward in the direction of travel of the combine to provide an earlier warning of upraised portions of the soil to facilitate raising the header and avoiding contact with the soil and reducing the possibility of damaging the header. 
     FIG. 12  is a side elevation view of the height sensor arrangement  190 , with the rotation sensor  194  in contact with the soil and the sensor arm in a substantially linear configuration. The header and height sensor arrangement would not be operated as shown in  FIG. 12 , but this figure illustrates the high strength and flexibility of the sensor arm  192  which allows for contact of a lower portion of the header with the soil so as to position the sensor arm in contact with the soil along a substantial portion of its length without damaging or breaking the sensor arm. In addition, the substantially flat configuration assumed by the sensor arm  192  when in substantially full contact with the soil without damage to the sensor arm eliminates the requirement for a recess in the lower surface of the header to receive the sensor arm when the header is in contact with the soil as in some current headers. 
   While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the relevant arts that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.