Abstract:
A drive assembly for an agricultural machine. A power take off (PTO) output shaft is connected to a driving motor for driving an attachment. A drive transmission is positioned between and interconnects the driving motor and the support wheels of the agricultural machine. An input device is connected to an electronic control unit. A sensor senses the torque transferred between an attachment and the PTO output shaft, and an actuator is configured to change the transmission ratio of the drive transmission, thereby setting the propulsion speed of the agricultural machine such that the torque transferred between the PTO output shaft and an attachment does not exceed a pre-determined value. The electronic control unit further receives data from the input device and calculates a pre-determined value based on the data received, and thereafter controls the actuator accordingly.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to a drive assembly of an agricultural machine with a driving engine, which is connected to a power take off (PTO) output shaft for driving an attachment, wherein the torque taken off at the PTO output shaft is limited to a predefined desired value. 
   2. Background of the Invention 
   In the prior art, agricultural tractors are equipped with a driving engine, which in operation drives the wheels to advance the tractor, and a power take-off shaft, which is used to drive attachments, such as agricultural implements or balers. A power take-off gearbox is switched between the engine and the PTO output shaft. Such power take-off gearboxes normally have several transmission stages, which may be selected by an operator, and result in the desired speeds of the PTO shaft. The transmission ratio of a drive transmission inserted into the drive train between the driving engine and the wheels can be charged stepwise or continuously to change the driving speed of the tractor. 
   In a prior tractor and a baler combination, by measuring the torque at the power take-off shaft of the tractor, information is obtained on the flow rate, which is proportional to the torque, of the material picked up by the baler, and is compared to a desired flow rate. If the desired flow rate is above or below a certain value, the propulsion speed of the tractor is changed by automatically regulating the transmission ratio of the drive transmission. The speed of the driving engine, and consequently, of the power take-off shaft, remains constant. A storage medium (ROM or CD-ROM), found on board the tractor equipped with a device for reading the storage medium, is used to store the flow rates as a function of the transmission stages of the drive transmission and different swath densities. The current flow rate is determined using the measured torque. The train is driven using a comparison between the current flow rate derived from the torque at the power take-off shaft and a preset value for an optimal flow rate. The transmission stage of the drive transmission to be chosen is selected using the ratio, stored in the storage medium, between the flow rate and the transmission stage. Here, it is considered a disadvantage that the described automated control functions only when there is a suitable storage medium used each time for the attachment. If the tractor and the attachment are supplied by different manufacturers, a combined effect in the described manner is accordingly almost impossible. This control is likewise not suitable for other attachments for which there are no measurable flow rates, such as for tilling machines. 
   Another prior tractor is equipped with a measuring device for recording the torque taken off on a PTO output shaft. The maximum value of the torque at the PTO output shaft is stored by the operator. This may be done by manually entering the values or by selecting the corresponding attachment (cutter bar, root harvester, manufacturer) from a list of attachments stored in the central computer of the tractor, where the appropriate limiting values are stored. To prevent damaging the drive train of the tractor or of the attachment in the event of overloading, a comparison between the torque at the PTO output shaft and the specified maximum value is made. If the maximum value is exceeded, the speed of the internal combustion engine is automatically reduced. One disadvantage is that, in the event of exceeding the allowable torque, the power take-off shaft as well as the traveling gear is driven with less speed. Eliminating a possible congestion in the attachment is also not possible here because, for instance, less material than before is picked up by the intake elements of a baler. Since the speed of the power take-off shaft remains constant in relation to the driving speed, the quantity of material picked up per distance covered remains the same. 
   Therefore, it is desired to have a drive assembly with an internal combustion driving engine, a PTO output shaft, and a drive transmission, which enables an operator to smoothly set, irrespective of the type and manufacturer of the attachment, a desired or maximum torque at the power take-off shaft and to reduce the traveling speed when the torque is exceeded. 
   SUMMARY OF THE INVENTION 
   A self-propelled agricultural machine includes a driving engine, usually an internal combustion engine, such as a diesel engine, which drives a PTO output shaft. The driving engine also drives, via a drive transmission, drive means such as wheels or tracked driving gear to move the machine over a surface to be cultivated by an attachment that is driven by the PTO output shaft. An electronic control unit is connected to an input device, to a sensor for determining the torque at the PTO output shaft, and to an actuator, which is used for stepped or step-less changing of the transmission ratio of the drive transmission. In operation, the torque at the power take-off shaft is measured by the sensor and compared with a predetermined target torque value. If the target torque value is exceeded, the control unit causes the actuator to change the transmission ratio of the drive transmission to reduce the speed, thereby reducing the load on the attachment, and therefore, the torque at the PTO output shaft. A device enabling data input by an operator provides the data for the control unit to derive the target torque value. 
   Unlike the prior art, the target torque value is not pre-set, but may be entered by the operator. In this manner, the torque at the PTO output shaft can be limited for attachments of any kind and any manufacturer. 
   In one embodiment of the invention, not only is the torque at the power take-off shaft limited, but also controlled. The propulsion speed of the machine is regulated by the control unit such that the target torque value at the PTO output shaft is maintained, at least approximately. To prevent the operator on board the machine from having to make constant changes in speed, a change in the transmission ratio of the drive transmission takes place only when the torque measured at the PTO output shaft departs from a boundary range extending above and below the target torque value. This boundary range may be pre-set or entered by the operator, since the fluctuations of the torque taken off at the PTO output shaft may vary, depending on the type of attachment. Thus, for balers in general, smaller torque variations appear than for a self-loading forage wagon, so that in the former, a smaller boundary range would be selected than for the latter. It would also be conceivable for the control unit to automatically derive the boundary range from information about the type of attachment. 
   In a simple, but universally applicable variation, the input device allows quantitative input of the target torque value, for example via a keyboard or a rotatable knob or slide button that is connected to a potentiometer or to a rotary encoder disc working in combination with a photo sensor. The operator can then derive the target torque value for any attachment from an appropriate specification sheet. Another possibility is to enter information on the attachment, such as the name of the manufacturer and the type name or type number of the attachment. Using stored information, or information downloaded from a database at another site, such as via a radio data circuit and the Internet, the control unit can then determine the target torque value. Instead of entering this data, it is also possible to equip the attachment with a storage device that communicates with the control unit via an existing bus system and transmits to the control unit the information on the target torque value or manufacturer and type of attachment. In another variant the machine is started, and as soon as the desired target torque value is reached, the operator makes the input, which causes the current torque value to be taken as the target torque value. 
   Advantageously, the speed of the driving engine remains at least approximately constant, while the control unit varies the propulsion speed. As a result, the PTO output shaft turns at a constant speed, so that the attachment is also always driven at its optimal speed. In the event of overloading the attachment, such as high quantities of material picked up for a baler or a self loading forage wagon, or for compressed soil areas in soil cultivation, less new material is supplied to the attachment due to the reduced speed, so that the overloading can be reduced. 
   The sensor for determining the torque at the PTO output shaft is preferably on board the work vehicle. The advantage of this is that there is no additional link to be established between the control unit and a sensor arranged on the attachment, and the sensor may be used for all attachments. A measurement of the torque may be made such that, for example, the torque is determined by sensors at the input and output of a power take-off shaft clutch, where the clutch slip is determined from the torques in front of and behind the clutch, by continuously setting and maintaining the clutch slip at a constant value and determining the torque transmitted by the clutch from the constant slip value and a value corresponding to the clutch pressure. Such a torque sensor is disclosed in U.S. Pat. No. 6,729,459, that is assigned to the assignee of the present application and whose contents are incorporated by reference in the present application. Suitable torque measuring shafts may be obtained from the company GKN Walterscheid, Lohmar, Germany, under reference DLC 250. 
   The control unit may vary the propulsion speed of the machine only at specified limits, or by means of limits to be entered by the operator. This avoids falling short of and exceeding the limits that are reasonable in field work. The upper limit may be set by means of the elements normally used for setting the speed, such as a gas pedal or a hand throttle lever or the service brake. In particular, the speed of the machine when automatic control was activated is used as the upper speed limit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a lateral view of an agricultural machine in the form of a tractor with a trailing attachment, in the form of a round baler; 
       FIG. 2  is a schematic representation of the drive assembly of the machine; and 
       FIG. 3  is a flow chart of a program under which the control unit operates when regulating the torques. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a lateral view of an agricultural machine  10  in the form of a tractor and an attachment  12  in the form of a round baler coupled to a drawbar clutch (not shown) of the machine  10  by means of a shaft  14 . Instead of the round baler, or in addition thereto, any other pulled or attached or suspended attachments, such as field choppers or driven agricultural implements, such as rotary harrows or ordered combinations, can be attached to the machine  10 . The machine  10  is constructed on a supporting frame  18 , which is supported on steerable front wheels  20  and movable rear wheels  22 , and supports a cabin  24  where the operator&#39;s workstation  26  is located. 
   The operator&#39;s workstation  26  includes a seat  28 , a steering wheel  30 , a gas pedal  16 , and another pedal for the brake and clutch (not shown), and input elements arranged in the operator&#39;s workstation  26  within reach of the operator for setting the selectable functions of the machine  10 . The latter includes a selection device  32  for the transmission gear of a power take-off shaft gearbox  46 , a hand throttle lever  70 , a power take-off switch  68 , an input device  74  for setting a target torque value at the PTO output shaft  56 , and a virtual terminal  72  of a bus system that operates according to ISO 11783 with a keyboard  90  and a display device  94 . The selection device  32 , the input device  74 , and/or the power take-off switch  68  could also be implemented as menu items on the terminal  72 . Instead of the terminal  72 , any other input and display devices may be used. The gas pedal  16  is provided with a sensor that transmits electrical signals to an operator interface logic unit  64 , the electrical signals containing information on the particular position of the gas pedal  16 . 
     FIG. 2  schematically illustrates the drive assembly of the machine  10  for driving the rear wheels  22  and a power take-off shaft  34  used for driving movable elements of the attachment  12 . A driving engine  36 , usually a diesel engine, drives a shaft  38 , which, via a gearwheel  40 , is used to drive the rear wheels  22  and preferably also the front wheels  20 , and, if necessary, other movable devices of the machine  10 , such as an air conditioning compressor and a power generator. The rear wheels  22 , and the front wheels  20 , are driven by the gearwheel  40  via a clutch  88  and a drive transmission with continuously variable transmission ratios, or with stepwise selectable ratios, but constant within the individual transmission stages, and a differential  78 . 
   The drive transmission can, for example, be a purely hydraulic transmission with a hydraulic pump driven by the clutch  88  and a hydromotor or hydrostatic transmission moving the wheels, wherein the transmission ratio of the gear can be changed by adjusting the swash plates of the pump and/or of the engine. In another embodiment, it is a multistage mechanical gearbox with a torque converter connected in series, such as used in passenger cars, or a mechanical gearbox with a sufficient number of transmission stages and an automatically switched clutch or planet gears and friction clutches. Also conceivable is the use of a drive transmission with step-less adjustable gearing, which for instance, works with V-belts that rotate around pulleys with adjustable diameters, or with a mechanical and a hydraulic (or electrical) drive path, wherein a planetary transmission comprises an element driven mechanically with a fixed or stepwise variable speed and one driven hydraulically or electrically with variable speed, and the third element is used for output. A combination of several of the mentioned transmissions would also be conceivable. 
   In the illustrated embodiment, the drive transmission comprises a power shift transmission  92 , which contains planetary gear sets with clutches and brakes that facilitate the changeover of speeds under load, and a secondary, manually shifted, or synchromesh, transmission  76 . The power shift transmission  92  and the synchromesh transmission  76  are switched off each time with actuators,  104  and  106  respectively, for selecting the transmission stage. Since both actuators  104  and  106  are controllable independently of one another, a high number of different transmission stages are available, derived by multiplying the number of transmission stages of the power shift transmission  92  with the number of transmission stages of the synchromesh transmission  76 . Further inserted in the drive train between the power shift transmission  92  and the synchromesh transmission  76  is a clutch  88 , which is movable through a clutch actuator  102  between a closed and open position. 
   The shaft  38  is also connected to a hydraulically operable clutch  42 , which is connected to the output with an input shaft  44  of a power take-off gearbox  46 . The clutch  42  is actuated by means of a valve assembly  48 , which is also connected to a brake  50  positioned on the input shaft  44 . 
   The power take-off gearbox  46  has three different, selectable transmission stages and therefore comprises three meshing gearwheel pairs. The transmission stage is selected by means of an electro-hydraulic (or electrical) actuator  52  operated by external force. Movable clutch elements determine which of the three gearwheels arranged on a driven shaft  54  of the power take-off gearbox  46  is in positive torque connection with the driven shaft  54 . Alternatively, the actuator  52  could establish which of the three gearwheels arranged on the input shaft  44  is in positive torque connection with the input shaft  44 . The actuator  52  enables the selection of three transmission stages. In the illustrated embodiment, a first transmission stage provides, at a nominal driving engine speed of 2100 rpm, a driven shaft speed of 1000 rpm. A second transmission stage, at a lowered driving engine speed of 1800 rpm, the driven shaft speed is 540 rpm. A third transmission stage, at a nominal driving engine speed of 2100 rpm, the driven shaft speed is 540 rpm. Moreover, it is possible to not couple any of the gearwheels of the power take-off gearbox  46  with the input shaft  44  and the driven shaft  54 , thereby placing the power take-off gearbox  46  into a neutral position, wherein the driven shaft  54  is freely rotatable, even when the brake  50  is activated. 
   The driven shaft  54  is connected to a power take-off output shaft  56  in the form of a power take-off stub shaft. The power take-off shaft  34  of the attachment  12 , equipped with a sleeve shaft end piece  58 , is slid onto the power take-off stub shaft. The power take-off shaft  34  is normally designed as a universal-joint propeller shaft. 
   An electronic control unit  60  is connected to the valve assembly  48  and to the actuator  52 . Moreover, the electronic control unit  60  is connected to an operator interface logic unit  64  by means of a device bus  62  (e.g., CAN bus), which is connected to the selection device  32 , the input device  74 , the gas pedal  16 , the hand throttle lever  70 , and the power take-off switch  68 . The control unit  60  is moreover connected by the bus  62  to the virtual terminal  72 , the actuators  104  and  106 , and the clutch actuator  102  of the clutch  88 . The selection device  32  comprises four pushbuttons, which are respectively assigned to the previously mentioned three transmission stages and to the neutral position of the power take-off gearbox  46 . 
   The selection device  32  makes it easy for the operator, through manual operation of the four pushbuttons, to select one each of the three transmission stages of the power take-off gearbox  46  or the neutral position. The user interface logic unit  64  transmits information on the operator&#39;s setting via the bus  62  to the control unit  60 , which controls the actuator  52  in accordance with the information received. Each transmission stage selected may be displayed to the operator by illuminating the assigned pushbutton of the transmission stage selection device  32 . 
   If the operator puts the power take-off switch  68  into the operating position, the operator interface logic unit  64  transmits the corresponding information, via the bus  62 , to the control unit  60 , which prompts the valve assembly  48  to release the brake  50  and to engage the clutch  42 . If the operator puts the power take-off switch  68  into the non-operating position, the operator interface logic unit  64  analogously transmits corresponding information, via the bus  62 , to the control unit  60 , which prompts the valve assembly  48  to disengage the clutch  42  and to activate the brake  50 . 
   The control unit  60  is connected, via the bus  62 , to an engine-actuated control  80 , which in turn controls an injection assembly  82  of the driving engine  36  and to which information about the respective speed of the shaft  38  is fed by a speed sensor  84 . A sensor  86  connected to the control unit  60  determines the torque transmitted by the driven shaft  54 . The speed sensor  84  may optically (through perforated rotary encoder discs connected to the shaft  38 ) or magnetically (through magnets connected to the shaft  38 , which work together with induction coils, reed relays or hall sensors) interact with the associated shaft  38  to determine the speed. The speed may also be determined by other suitable means. The sensor  86  comprises strain gauges positioned on the driven shaft  54 , whose terminals are connected to the control unit  60  by means of sliding contacts, in order to determine the elastic torsion of the driven shaft  54  caused by the transmitted torque. Any other embodiments of the sensor  86  may be used, for instance, laser sensors for determining the torsion of the driven shaft  54 . 
   The third input device  74  comprises three keys  96 ,  98  and  100 . The first key  96  is used to switch on an automatic mode of operation and the other keys  98 ,  100  to reduce (key  98 ) or enlarge (key  100 ) the target torque value at the driven shaft  54 . 
   In operation, the control unit  60  instructs the engine-actuated control  80  to let the driving engine  36  run at a speed that corresponds to the current input of the selection device  32 , i.e., depending on the pressed key of the selection device  32 , with a fixed speed of 2100 rpm or 1800 rpm. Alternatively, the speed is entered by the operator via a torque input device (not shown) or via the virtual terminal  72 . If the neutral position is selected, the torque of the driving engine  36  will depend on the position of the gas pedal  16  or of the hand throttle lever  70 , or is entered by the user, or is pre-set. 
   In operation, the propulsion speed of the machine  10  is initially specified by the gas pedal  16  or the hand throttle lever  70 . The control unit  60  then controls the actuators  104  and  106  and, before and after operating the actuator  106 , the clutch actuator  102  for disengaging and engaging the clutch  88 , so that the drive transmission drives the wheels  22  at the desired speed. The torque at the driven shaft  54 , measured by the sensor  86 , is displayed on the display device  94  of the virtual terminal  72 . 
   As soon as the displayed torque corresponds to a target torque value that is optimal and appropriate for the attachment  12 , the operator can operate the key  96  of the input device  74 . The control unit  60  then ignores the inputs from the gas pedal  16  or the hand throttle lever  70 , and by operating the actuators  104  and  106 , adjusts the propulsion speed of the machine  10  such that the torque measured by the sensor  86  at least approximately corresponds to the target torque value. Key  98  facilitates a step-by-step decrease of the target torque value and key  100  facilitates a step-by-step increase of the target torque value, e.g., in increments of 10 Nm. Alternatively, or in addition, the desired target torque value may be entered via the keyboard  90  of the virtual terminal  72  as a numerical value. Furthermore, the manufacturer and type of attachment  12  may be entered via the keyboard  90  and the control unit  90  derives the target torque value therefrom using a database. The target torque value can also be stored in memory, connected to the bus  62 , on board the attachment  12 , and the control unit  60  obtains it from the memory  108  as needed. 
   The routine that runs in the control unit  60  is illustrated in the program flow chart of  FIG. 3 . The power take-off shaft torque is recorded in Step  200  via the bus  62 , and is smoothed out in Step  202  with a sliding average formation for 0.1 sec, i.e., 10 values at a 100 Hz sampling frequency. After computing the deviation in Step  204 , the computation of the change in speed takes place in accordance with a three-step controller with secondary proportional control and an acceleration as well as velocity limiter in Steps  206  to  226 . 
   If the mean torque taken in Step  206  moves within a tolerance range of 75 Nm above or below the target torque, the driving speed remains constant in Step  208 . The value of 75 Nm represents a compromise among the various tasks. For greater torque deviations in self loading forage wagons, 100 Nm would be better, whereas 50 Nm will suffice for mowing or baling straw. It is conceivable to provide the driver with an opportunity to input the tolerance range. 
   If the amount of deviation is greater than the tolerance of 75 Nm, but less than the so-called adjustment tolerance of 275 Nm (Step  210 ,  214 ), a change in speed takes place (Step  212 ,  216 ). If the torque is too high, the range of acceleration in the region of 0 to 0.9 m/s 2  is computed proportional to the torque deviation; on the other hand, if the target torque is not reached, acceleration is done at 0 to 0.45 m/s 2 . The mentioned maximum values of acceleration of −0.9 m/s 2  when braking and 0.45 m/s 2  when accelerating were chosen during field tests in accordance with subjective sensitivity for a comfortable automatic controller action. Even the value of 275 Nm above and below the tolerance limit for the acceleration range coupled to the torque deviation has been empirically determined in practice, and can remain set constant for all tasks. For even greater deviations, the speed is changed by an established value (Step  218 ). If necessary, the brake of the machine  10  could also be activated through the control unit  60 . 
   After Step  220 , wherein the new theoretical value for the propulsion speed is calculated, there is no increase in the target speed if the difference from the actual speed is greater than a determined value (here, 1 km/h, for example) (Step  222 ,  224 ). Otherwise, (Step  226 ), the speed is increased. The maximum speed accordingly corresponds to the propulsion speed driven when the control algorithm is switched on. Consequently, it is possible at anytime for the driver to limit or reduce the speed manually by pulling back the hand throttle lever  70  or stepping on the brake pedal without increasing the target speed without limit. At the start of a swath, actuating the brake for a short time will suffice, for instance, to bring too high a driving speed in the headland to a reasonable level for travel over the field. 
   As will be appreciated by one skilled in the art, it is anticipated that the functions of the operator interface logic unit  64 , the engine control unit  60  and the engine control  80  may be realized in an alternative manner, for example through a single unit and or hardware. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described.