Abstract:
Method and device for delimbing a tree trunk ( 60 ) in a compliant gripping state defined by a set of limbing knives ( 18, 24, 26, 28 ) enclosing the trunk. The limbing knives are repeatedly adjusted through a control unit ( 46 ) for adaption of the gripping state to a varying thickness of the trunk when the trunk is advanced between the knives for the delimbing of the trunk. The invention comprises entering a desired compliance (D 1 ) of the gripping state in the control unit ( 46 ), detecting an actual compliance (d) of the gripping state, and performing each adjustment of the limbing knives via the control unit until the actual compliance (d) corresponds to the desired compliance (D 1 ).

Description:
This application is a national phase of International Application No. PCT/SE2009/050797 filed Jun. 23, 2009 and published in the English language. 
     TECHNICAL FIELD 
     This invention is relates to a tree delimbing method and device for delimbing a tree trunk in a compliant gripping state defined by a set of limbing knives enclosing the trunk, comprising repeated adjustment of the limbing knives through a control unit for adaption of the gripping state to a varying thickness of the trunk when the trunk is advanced between the knives for the delimbing of the trunk. 
     BACKGROUND 
     When delimbing tree trunks using a tree processing assembly, it is desirable to cut the branches as close as possible to the trunk. During a delimbing operation it is therefore necessary to adapt the grip of the surrounding knives to the varying thickness of the trunk section that is currently in the grip. 
     In a prior art delimbing device of the above mentioned type and disclosed in U.S. Pat. No. 4,898,218, the compliance of the gripping state is predetermined by changing a closing extent of a gripping means provided with the limbing knives each time a top face of the trunk has been moved a predetermined distance in a direction of movement for a resiliently compliant limbing knife. In the example disclosed in the above patent, there is provided a switch which is actuated by the compliant limbing knife so as to when the switch is not actuated by the limbing knife, the closing extent of the gripping means increases to lift the tree trunk in the delimbing device until the compliant limbing knife actuates the switch that then interrupts the closing of the gripping means. When the tree trunk advances further in the delimbing device in the direction to its top end, the switch again becomes non-actuated by the returning resiliently compliant limbing knife such that the closing extent of the gripping means once more increases. This procedure is repeated until the whole tree trunk has been advanced through the delimbing device. 
     A disadvantage of the prior art device is that the compliance of the gripping state, i.e. the remaining free stroke of travel of the resilient limbing knife, is invariable after each increase of the extent of closing of the grip of the trunk. However, in order to obtain a good delimbing operation, it may be necessary to cut the branches of different types of tree trunks with mutually different compliance or remaining free stroke of travel of the resilient limbing knife. Birch trees, for example, can have coarse branches that may excessively load the processing assembly when attempting to cut the branches too close to the trunk. The gripping state should then have a relatively large compliance, capable of allowing the tree trunk to move away from the limbing knives, or allowing the limbing knives to move away from the tree trunk a further distance, when one or more such coarse branches are coming into knife engagement. The compliance, play or remaining free stroke of travel of the gripping state should then be about 30 to 40 mm. On the other hand, spruce trees, for example, may have a linearly tapering trunk with relatively thin branches. The gripping state should then have a relatively small compliance so as to cut the branches close to the trunk. If the compliance then is too large, the elastic branches may resiliently bend toward the trunk and slide along the limbing knives without being cut. The compliance, play or free stroke of travel of the gripping state may then be about 5 mm. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is to further develop a method and a device of the type defined above so that it is capable of more effectively delimbing tree trunks of mutually differing branch qualities. 
     Another object may be considered as to adapt the delimbing operation to the requirements of the actual tree trunk. 
     These objects are obtained by the features of the appended claims. 
     In one aspect of the invention, a method according to the invention comprises 
     setting a desired compliance of the gripping state in the control unit; 
     detecting an actual compliance of the gripping state; and 
     performing each adjustment of the limbing knives through the control unit until the actual compliance corresponds to the desired compliance. 
     Thereby the compliance of the gripping state can be adapted to the quality of the actual tree trunk. The desired compliance may empirically be determined by an operator of the device based on a judgment of the tree trunk and its branches and be set in an input unit as a category such as “Spruce”, “Birch”, etc. but also as a direct distance value, for example 40 mm. 
     While the compliance of the gripping state may be included by all limbing knives, according to one embodiment of the invention, the compliance comprises a remaining free stroke of one limbing knife resiliently engaging the trunk. 
     Other features and advantages of the invention may be apparent from the claims and the following detailed description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a front view of a vertically oriented tree processing assembly that may be incorporated with a device according to the invention; 
         FIG. 2  is a diagrammatic side view with parts broken away of a tree trunk received in a horizontally oriented tree processing assembly; 
         FIG. 3  is a view corresponding to  FIG. 2  illustrating an increasing distance between the tree trunk and a bottom face of the assembly when the trunk is advanced therethrough; 
         FIG. 4  is a diagrammatic front view of a horizontally oriented tree processing assembly gripping a tree trunk; 
         FIG. 5  is a view corresponding to  FIG. 4  where the assembly is gripping a thinner section of the trunk; and 
         FIG. 6  is a graph showing a characteristic of a proximity sensor that may be incorporated with a device according to the invention 
     
    
    
     Throughout the drawing, components having similar function have identical reference numbers. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a front face of a vertically oriented tree processing assembly  10  of the single-grip type. Assembly  10  has a main body  12  supporting inter alia a pair of opposite feed wheels  14 , a lower forward limbing knife  18 , a pair of upper forward limbing knives  24 ,  26  and an upper rearward limbing knife  28 . 
     The two feed wheels  14  are pivotally connected to the main body  12  to clamp and longitudinally feed a tree trunk  60  ( FIGS. 2-5 ) felled by the assembly  10 . The likewise pivotally connected limbing knives  18 ,  24 ,  26 ,  28 , likewise pivotally connected to the main body  12 , enclose the tree trunk to cut off branches therefrom when the trunk is advanced through the assembly. 
     Each feed wheel  14  and the limbing knives  18 ,  24 ,  26  are adjusted to their positions enclosing the trunk by means of actuators. In the examples shown, the feed wheels  14  are adjusted by respective hydraulic cylinders  16 , and the lower limbing knife  18  as well as the upper forward limbing knives  24 ,  26  are also adjusted by respective hydraulic cylinders  20  and  56 ,  56  ( FIG. 4 ). The upper rearward limbing knife  28  is resiliently forced to the tree trunk  60  by means of a spring  30  ( FIGS. 4, 5 ). 
     The diagrammatic representation of  FIG. 2-5  shows a horizontally oriented tree processing assembly  10  gripping a tree trunk  60  by the upper forward limbing knives  24 ,  26  and by the upper rearward limbing knife  28 . Accordingly, the upper forward limbing knives  24 ,  26  as well as the lower forward limbing knife  18  (indicated only in  FIG. 2 ) support the weight of the horizontally oriented tree trunk  60  received in the processing assembly  10 . 
     To facilitate the description, the function of the adjustable lower forward limbing knife  18  is omitted. If the assembly  10  is provided with such a limbing knife  18 , in the following description, limbing knife  18  is understood to be controlled in a manner corresponding to the control of the adjustable upper forward limbing knives  24 ,  26 . 
     When the horizontally oriented, thus gripped tree trunk  60  is advanced to the left through the assembly in the manner that is diagrammatically shown in  FIGS. 2 and 3 , i.e. from the butt end to the top end, the actual distance d from the trunk to a bottom face of the main body  12  of the assembly  10  will increase. The spring-loaded limbing knife  28  will then follow the movement downwards of the top face of the trunk  30  and—to a limited extent—also upwards. Distance d may then be considered as a measurement of the compliance of the enclosing engagement of the limbing knives to the vertical movements and variations of thickness during the feed of the trunk. The bottom face of the main body may represent an upper end position, formed by an upper stop (not shown) for the spring-loaded limbing knife  28 , the lower end position of which may be defined by a lower stop  32  ( FIGS. 4, 5 ) of the assembly  10 . 
     The distance d is critical for a correct delimbing operation. Distance d may be regarded as representing the play or remaining stroke of travel of the spring-loaded limbing knife  28 —and thereby also the play in the vertical direction of the horizontally oriented tree trunk  30 . 
     If, on the one hand, the play is too small, the trunk may get stuck in the assembly  10  or subject the limbing knives to a far too high load by cutting into far too massive wood sections of the branches or the trunk  60 . If, on the other hand, the play is too large, the branches of the trunk  60  may bend to the trunk and slide under the the knives whereby they will not be cut off during the delimbing operation. 
     As is apparent from  FIGS. 4 and 5 , the actual distance d is detected by a proximity sensor  42 , such as an analog inductive or magnetic proximity sensor. Proximity sensor  42  is continuously signaling the actual distance d, for example as an electric current i, via a signal connection  44  to an electronic control unit  46  of a control system  40 . In one embodiment, a processor (not shown) in the control unit  40  can be programmed to emit a control signal via a signal connection  48  to a magnet valve  50  when the electric current indicates that the distance d has risen to a critical value D 1  ( FIG. 6 ) that may be stored in a memory (not shown) in the control unit  46 . Valve  50  then opens a path between a hydraulic pump  52  and the respective hydraulic cylinders  56 . The limbing knives  24 ,  26  will then further tighten the grip of the tree trunk  60 , whereby the distance d decreases to an adjoining value D 2  ( FIG. 6 ) when the tree trunk  60  is lifted further up in the assembly  10 . During the continuing feed of the trunk  60  in the assembly  10 , the distance d may further increase until it again reaches the critical value D 1 , whereupon the above process is repeated. The number of repetitions of this process depends on the magnitude of the interval D 1 -D 2  and the length of the tree trunk. The magnitude of the interval D 1 -D 2  may vary depending on the accuracy, for example, depending on inertia and delay in the hydraulic system and influence of hysteresis, of the control system  40  and the sensor  42 . 
     As indicated in  FIG. 6 , in certain circumstances, D 1  and D 2  may have relatively large values that typically amount to between 30 and 40 mm. This means that the limbing knife  28  has a relatively large play above the tree trunk  60 . Such a case may be suitable for trunks of birch, the branches of which may be relatively massive close to the trunk. The large play of the limbing knife  28  will then allow the knife to be forced further out from the trunk before it cuts the branch. The spring-loaded limbing knife  28  and also the other limbing knives can have such a cutting angle that they are guided away from the trunk to a certain amount during the cutting operation. The process may then also allow that the whole trunk  60  is forced away from the adjustable limbing knives  24 ,  26  when these knives encounter massive branches. The result, however, is that a larger play d will generally allow coarse branches to be cut off at a larger distance from the trunk  60  than will a smaller play. The spring-loaded limbing knife  28  and the weight of the trunk  60  will, however, provide for that the thinner branches still are cut close to the trunk. 
     A relatively small play where D 1  and D 2  have relatively small values of about 5 mm may be suitable when delimbing trunks of spruce, for example, where generally the trunk surface tapers relatively linearly and the branches are relatively thin and therefore are quite easy to cut off. The branches are then cut close to the trunk  60  and are thereby prevented from bending against the trunk and sliding along and past the limbing knives  18 ,  24 ,  26 ,  28 . 
     In order to take account for varying quality of tree trunks where at least certain branches need to be cut off at different distances from the trunk to obtain a good delimbing result for a certain processing assembly, the desired play or the above-mentioned critical value D 1  can be set in the control unit  46  prior to each delimbing operation. To this end, the control unit  46  is provided with an input unit  47 , having one or more push buttons or keys  49 , for example, in the vehicle (not shown) that supports the assembly  10 . In one embodiment of the invention, the operator (not shown) then makes a visual examination of the tree to be felled, and, after an empirical conclusion, makes a decision about the quality of the trunk, for example in the form of a category, that is entered into the unit  47 . Apart from “Birch” and “Spruce”, in a set of categories to be entered, also other tree species, sub-categories such as “Slender Birch”, as well as varying qualities such as decay or rotten wood, may be included. An experienced operator may also enter the value of D 1  directly, if the control unit  46  so allows. The set of keys  49  of the control unit  47  may be located accessible for the operator in the proximity of a joy stick (not shown), for example, provided for controlling another function of the tree processing assembly. 
     To relieve the operator, in the scope of the appended claims, it is at least imaginable to perform the setting of tree trunk category automatically by using a camera and an image processing system (not shown) having the capability of identifying the different categories of tree trunks. 
     The foregoing detailed description is given primarily for clearness of understanding and no unnecessary limitations are to be understood therefrom. Modifications will become obvious to those skilled in the art upon reading this disclosure and may be made without departing from the spirit of the invention or the scope of the appended claims.