Abstract:
An automatic steering system for a work vehicle comprises a position sensor for detecting a position of the vehicle, a memory for storing information about a nominal path of the vehicle and a control unit coupled to the position sensor, to the memory and to a steering actuator for steering the vehicle. The control submits control signals to the steering actuator that depend upon a feedback gain and a lateral offset between the actual position and the nominal path. The feedback gain depends upon a sensed payload of the vehicle.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an automatic steering system and a corresponding method for a work vehicle. 
     BACKGROUND OF THE INVENTION 
     In the prior art, numerous automatic steering systems have been described that are operable to guide agricultural vehicles and implements automatically along a desired path over a field. Such automatic steering systems comprise a position sensor for detecting the actual position of the vehicle in the field, a memory storing information about a nominal, intended path of the vehicle over the field, a control unit coupled to the position sensor and to the memory and to a steering actuator that is suited to steer the vehicle. The steering actuator is controlled dependent upon the lateral offset between the actual position and the nominal path in a manner to keep the vehicle on the nominal path. A so-called feedback gain determines the relation or ratio between the lateral offset and the steering reaction of the steering actuator. 
     In the prior art, it was proposed to vary the feedback gain dependent on the propelling speed of the vehicle, wherein the feedback gain decreases with increasing propelling speed (U.S. Pat. Nos. 5,857,539 A, 3,946,825 A, 3,550,362 A, EP 0 381 073 A). Further, it was described to control the feedback gain dependent on the position of a moveable implement or upon a detected steering oscillation (U.S. Pat. No. 7,127,340 B), based upon a detected wheel slip rate (U.S. Pat. No. 6,901,319 B), and on vehicle kinematics (US 2004/0210357 A). It was also described to control the steering actuator based upon a detected lateral terrain slope (EP 1 688 027 A, U.S. Pat. No. 3,946,825 A). 
     The reaction of a moving vehicle to an action of the steering actuator depends among others on the mass or weight of the vehicle. For example, a combine with an empty grain tank follows a steering action of the steered rear wheels faster and easier than a combine with a full grain tank, since the grain mass in the grain tank corresponds to some 10% of the mass of the empty combine. This sensibility of the steering reaction to the vehicle mass is present in all work vehicles with variable payload, for example self-propelled and pulled sprayers, seeding machines, harvesting machines like combines and balers, and transport vehicles for any material. In the prior art, a variable mass or payload of a work vehicle has not been taken into account for determining the feedback gain of automatic steering systems. Dependent on the actual payload of the vehicle, the steering reaction can thus be too small or too large, leading to insufficient performance or stability of the automatic steering system. 
     Thus, there is a need for a simple and reliable system for controlling an automatic steering system taking the payload of the vehicle into account. 
     It is an object of the present invention to provide an improved automatic steering system responsive to the payload of the vehicle. 
     SUMMARY OF THE INVENTION 
     An automatic steering system for a work vehicle comprises a position sensor for detecting an actual position of the vehicle in a field, a memory for storing information about a nominal path of the vehicle over the field and a control unit coupled to the position sensor, to the memory and to a steering actuator that steers the vehicle or controls the direction into which the vehicle moves. The control unit is operable to submit control signals to the steering actuator that depend upon a feedback gain and a lateral offset between the actual position and the nominal path. The feedback gain depends upon a sensed payload of the vehicle. 
     It is an advantage of the invention that the actual payload of the vehicle is considered when the steering reaction to a deviation of the vehicle from the nominal path is determined. The performance and stability of the steering system is thus improved. The steering reaction will be larger for a vehicle carrying a large payload than for a vehicle without any payload or only a small payload. 
     The position sensor for detecting the actual position of the vehicle in the field can sense the absolute position of the vehicle and thus be a receiver of a satellite based positioning system, such as a Global Positioning System (GPS). In another embodiment, the position sensor for detecting the actual position of the vehicle can detect a relative position of the vehicle with respect to an object, for example to an edge between harvested and not harvested crop or a row of crop. The position sensor can then comprise at least one of a camera cooperating with an image processing device, a flight-time based scanning laser reflection sensor, and an ultrasonic or mechanical position sensor detecting the location of a crop edge or row. The type of information about the nominal path of the vehicle over the field stored in the memory depends on the type of the position sensor detecting the actual position of the vehicle. If the position sensor detects the absolute position, the nominal path information can be a map containing the desired path of the vehicle. If the position sensor detects a relative position, for example with respect to a crop edge, the nominal path information can just be a desired distance between the longitudinal center plane of the vehicle and the crop edge. It would also be possible to combine both types of position sensors and nominal path information. 
     The steering actuator is provided by the control unit with a steering signal that depends on the lateral offset between the vehicle position and the nominal path, and the payload-dependent steering gain. In one embodiment, the steering signal is obtained by multiplying the offset by the steering gain. 
     The invention can be used on any type of agricultural vehicles and corresponding implements. Examples are harvesting machines like combines and balers, and sprayers. The payload can be provided on the self propelled vehicle or on a trailer pulled by the vehicle. 
     The payload of the vehicle can be sensed directly, for example by weighing a payload container or by sensing the fill state of the payload container. It can also be sensed indirectly by detecting the flow into a payload container or out of it, and integrating the flow over time to obtain the payload. In another embodiment, the weight of the entire vehicle or of its chassis can be sensed. 
     These and other objects, features and advantages of the present invention will become apparent to one skilled in the art upon reading the following detailed description in view of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a vehicle with an automatic steering system; 
         FIG. 2  is a flow diagram according to which the control unit adjusts the steering actuator of the vehicle in  FIG. 1 ; and 
         FIG. 3  is a side view of another vehicle with an automatic steering system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , therein is shown an agricultural vehicle in the form of a combine  10  comprising a main frame  12  having driven front wheels  14  and steerable rear ground engaging wheels  16  supporting the main frame  12  for forward movement over a field of crop to be harvested. Although wheels  14  and  16  are shown, the ground engaging means of combine  10  could include or be composed of ground engaging tracks. Drive to the front wheels  14  is provided through a conventional hydrostatic transmission (not shown) by an internal combustion engine (not shown) mounted on the frame  12 . In the following, references to directions (like forward) are cited with respect to the forward direction of combine  10  that is directed to the left in  FIG. 1 . 
     An implement in the form of a header or harvesting platform  18  is used for harvesting a crop. The harvested crop is cleaned and separated, and the cleaned grain is directed to a grain tank  30 . The clean grain in the grain tank  30  can be unloaded into a grain cart or truck by unloading auger  32 . 
     The operation of the combine  10  is controlled from an operator&#39;s cab  36 . An automatic steering system of the combine  10  comprises a position sensor  38 , a control unit  40 , a steering actuator  42  operable to control the steering angle of the rear wheels  16 , and a memory  44 . The position sensor  38  comprises a camera mounted on the roof of the cab  36  and viewing into the forward direction. An image processing system provided within the housing of the position sensor  38  or included into control unit  40  processes the images from the camera and provides a signal representing the distance between a lateral edge of the harvesting platform  18  (or the longitudinal center plane of combine  10 ) and an edge of cut crop to control unit  40 . Memory  44  contains information about a desired distance between the cut edge and the lateral edge of the harvesting platform  18 , which usually corresponds to some cm (or to the longitudinal center plane of combine  10 , which corresponds to half of the cutting width of the harvesting platform  18 ). The control unit  40  is controlling the steering actuator  42  to steer the combine  10  on a desired path on which one edge of the harvesting platform  18  is guided along the crop edge. 
     In the grain tank  30 , a fill state sensor  46  is provided which senses the height of grain in the grain tank  30 , preferably with ultrasonic waves or optically. A grain flow sensor  48 , such as in the form of an impact plate is mounted at the outlet of the clean grain elevator  28 . However, it can be appreciated that other types of grain flow sensors can be used as well. 
     During automatic steering operation, the control unit  40  operates according to the flow diagram shown in  FIG. 2 . After start in step  100 , in step  102  the lateral offset of combine  10  from the intended path is determined, by subtracting the desired distance stored in memory  44  from the distance between the lateral edge of the harvesting platform  18  and the edge of cut crop measured by position sensor  38 . This lateral offset affects in which direction and with which angle the rear wheels  16  should be steered by steering actuator  42  to keep on the desired path. 
     In subsequent step  104 , the payload in grain tank  30  is calculated. This can be done using only the value from fill state sensor  46 , or by integrating the values from flow sensor  48  over time, or combining both values. If only the value from the flow sensor  48  is used for determining the payload, information about an empty grain tank  30  can be derived from a drive mechanism of unloading auger  32 , assuming that the grain tank  30  is empty once the unloading auger  32  has been running for a predetermined time. The payload can be calculated in volume units or in mass units, by multiplying the sensed volume in the grain tank with a density, which can be input to the control unit  40  by a user via a suitable interface or sensed with a suitable sensor. 
     In step  106 , a feedback gain is calculated in the control unit, using a stored table or formula. The feedback gain depends upon the payload and increases with the payload, preferably linearly. In a possible embodiment, the feedback gain is obtained by dividing the sum of the mass of the empty vehicle and the payload by the mass of the empty vehicle. Additional parameters, like the propelling speed of vehicle  10  can be taken into account for determining the feedback gain. The feedback gain can also be fine-tuned by an operator in the cab  36  via a suitable interface. 
     In step  108 , the steering signal is calculated by multiplying the offset with the feedback gain and sending the value to steering actuator  42 . Step  102  follows again. 
     A second embodiment of the invention is shown in  FIG. 3 . A vehicle  50  in the form of a tractor has steerable front wheels  52  and driven rear wheels  54  supporting a frame  56 . A three-point hitch  58  mounts an implement; in this case, a spraying implement  62  comprising a tank  60  and a boom  64  with nozzles. The automatic steering system comprises a position sensor  66  in the form of a GPS antenna, a control unit  68 , a memory  70  containing a map of the intended path of the vehicle  50 , and a steering actuator  72  operable to control the steering angle of the front wheels  52 . Two sensors  74 ,  76  mounted between the three-point hitch  58  and a frame  78  of the spraying implement  62  sense the weight of the spraying implement  62  including the payload within the tank  60 . In another embodiment, the weight of the spraying implement  62  can be derived by sensing the hydraulic pressure in hydraulic actuators of the three-point hitch  58 . It would also be possible to measure the fill state of tank  60  with a suitable sensor, preferably optically or mechanically or with ultrasonic waves, or to sense the flow of product from the tank  60  to the nozzles of the boom  64 , and to integrate the flow over time to determine how much product has been taken from tank  60 , thereby assuming the tank  60  has been full at the start of operation, or inputting or sensing the fill state. 
     During operation, the control unit  68  works also according to the flow diagram of  FIG. 2 , such that reference is made to the corresponding description above. The only significant difference is that in step  102 , the lateral offset is determined based upon an absolute position signal from the position sensor  66  and the map in memory  70 . The feedback gain is increasing with the sensed payload in tank  60 . 
     Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.