Controlled power distribution mechanisms in the transmission of an integral tractor

. The key features of the integral tractor concept are considered. The roles of controlled inter-axle and inter-wheel power distribution mechanisms are analyzed. Variants of the transmission structure are proposed. An example of simplified kinematic diagrams of interaxle and interwheel power distribution mechanisms is considered. Approaches to determining the external parameters of such mechanisms, their control technology, and the technological base for their design and manufacture are proposed.


Introduction
The features of integral tractors, regardless of the traction class, are mainly the combination of the maximum number of technological agricultural operations in one pass.
The consequence is the presence of two sets of attachments, the expansion of the field of view from the cab, the potential need for all-wheel drive and transmission reverse, and other design features.
An integral tractor can be designed as a new product, or it can be created on the basis of the principle of maximum use of commercially available components.The energy source can be a heat engine, a hybrid power plant, an onboard energy storage device.The latest trend is "trendy", but there is reason to believe that the hybrid power plant is currently the most effective technical solution.
The machine is integrated into the machine-tractor unit with the involvement of a line of agricultural implements, mastered either by the enterprise that produces the tractor, or common on the market.
Depending on the traction class of the tractor, kinematic steering control, the principle of turning the tractor sections in plan, a combination of the latter method with a side turn can be used.
Finally, an integral tractor can be inhabited and uninhabited.Moreover, based on the principle of increasing versatility and reliability, as well as maximizing the use of commercially available components, it may be rational to base the project on a commercially available habitable tractor, adding the modules necessary for autonomous operation.
However, it is the combination of technological operations and the presence of a front and rear technological module, coupled with the need to ensure an efficient transport mode, that determines the need to control the power distribution between the front and rear axles, depending on the specifics of the operating mode of the machine-tractor unit.An additional option -combining the turn of the tractor sections in the plan with the redistribution of traction forces along the sides, can be implemented by using controlled gear interwheel power distribution mechanisms.
The question of the rational implementation of the complete reverse of the transmission is beyond the scope of this study.The aim of the work is to propose a methodology for designing the transmission of an integral (primarily knee) tractor.
Tasks to be solved:  choice of transmission structure;  selection of the principle for determining the external parameters of the main transmission units;  proposal of approaches to the control of units, principles of their design and technological base for their manufacture.At present, the practical application of controlled interwheel power distribution mechanisms are typical for passenger car transmissions.Such mechanisms work as part of dynamic stabilization systems and solve the problem of torque redistribution between the wheels of the axle to improve the controllability and stability of the car [1][2][3][4][5][6][7][8][9][10].In the Russian Federation, based on the analysis of foreign publications, the study of the operating experience of mechanisms ZF Vector Drive, AYC, etc., the authors of this article proposed the theoretical models necessary to create controlled cross-wheel power distribution mechanisms and practical results protected by patents of the Russian Federation, the feasibility of using cross-wheel controlled differentials on tractors and transporttechnological wheeled and tracked vehicles.Based on these results, the concept of an interaxle controlled power distribution mechanism was proposed [11].In this paper, the issue of using such technologies in the transmission of an integral tractor is considered.

Methods and materials
If there is more than one drive axle, the competing solutions will be the concept of a parallel hybrid, the principle of using electromechanical drive axle modules, and a fairly traditional fully differential transmission in the automotive industry (see Figure 1).
The work is based on methods and technologies for the synthesis of planetary mechanisms with two and three degrees of freedom; the theory of movement of tracked and wheeled vehicles; design of transmissions of transport vehicles, theoretical mechanics and machine science, theory of automatic control systems.

Results and discussion
Integral wheeled tractors of a small traction class are characterized by the use of the principle of steering control by turning the steered wheels.A variant of a simplified block diagram of the transmission for such a tractor is shown in Figure 1.
The chassis of integral wheeled tractors of higher traction classes seems to be built according to the "breaking frame" scheme -the turning is controlled by turning two sections of the chassis in the plane of the plan.The scheme according to Figure 1 can be adapted to this option: it will not be necessary to use equal angular velocities on the front axle, the mechanical connection between the sections will be complicated (or a transition to the parallel hybrid principle will be implemented, when the front axle is driven by a heat engine, and the rear axle is driven by a traction electric motor.
The concept allows the use of the principle of a series hybrid, and fully "electric" traction for the tractor [15].
The scheme under consideration allows you to fully control the value of the torque on each wheel, due to which traction properties, maneuverability, controllability, environmental safety, and product efficiency are increased.
Controlled power distribution mechanisms (CPDM) are key units in such a transmission.
Interaxal CPDM allows you to change the distribution of torques between axles, depending on the specifics of the tasks being solved.
Cross-wheel CPDM allow you to solve issues of controllability, stability, maneuverability, cross-country ability.
Shown in Figure 1 scheme allows you to provide full control over the distribution of power on the driving wheels of the mobile platform.The scheme can be implemented using an internal combustion engine, a hybrid power plant or an electromechanical drive as an energy source.
Due to the specifics described above, a controlled interaxle PDM must combine the capabilities of symmetrical and asymmetric differentials typical for transport engineering.It is also desirable to provide for the possibility of disabling one of the drive axles.Shown in Figure 2, the scheme is based on an asymmetric differential, made in the form of a simple planetary mechanism with a kinematic parameter equal to -2, and providing a 1:2 distribution of moments between the axles.For the mechanism under consideration, this mode of operation is implemented when the blocking clutch C 2 is turned on.An operating mode equivalent to a symmetrical differential (planetary mechanism with a kinematic parameter equal to -1) is realized when the brake T 1 is turned on.
In both cases, the mechanism initially has three degrees of freedom.When the control is turned on, the number of degrees of freedom is reduced to two, thus, in both cases, the differential connection between the bridges is preserved.
The use of traditional methods of kinematic and force analysis [13] confirms that the distribution of angular velocities and torques between the bridges corresponds to the indicated analogies.
Disabling the rear drive axle is implemented when the TA brake is turned on.The power flow is directed to the front axle, the gear ratio increases.movement speed is reduced.
Shown in Figure 2, the simplified kinematic scheme belongs to a family of similar ones; it is not difficult to offer analogues by varying the kinematic parameters of the original mechanisms.
Using the principle of controlled slipping of the disc friction control element [12], it is possible to control the redistribution of torque between the axles.Such a solution may be of interest when creating a transmission of a special transport and technological machine, but for the case of an integral tractor, it can be considered as optional.The control over the operation of the mechanism in this case must be transferred to a closed digital control system with feedback on the angular velocity of movement and on the angular velocity of the wheels.
The brakes and blocking clutch shown in Figure 2 can be replaced by gear nonsynchronized clutches in order to simplify and reduce the cost of the design, if the technology of using the machine implies a mandatory stop when switching the operating modes of the interaxle power distribution mechanism.The control drive in this case can be completely mechanical (for example, through the use of a tear mechanism).
Cross-wheel controlled power distribution mechanisms in terms of functionality are close to the known solutions of ZF Vector Drive and AYC [10,9].Such mechanisms can be called symmetrical: the inclusion of a control element (brake) causes a redistribution of torque between the sides, compared to the case of a symmetrical differential.But one control is responsible for turning right, and the other is responsible for turning left.
Effective operation of such mechanisms requires the use of a closed digital control system with feedback on the angular velocity of movement.The mechanism can perform the function of maintaining rectilinear motion, the function of accelerating the exit to the trajectory, etc.It is advisable to use such mechanisms as part of the dynamic stabilization system for the motion of a mobile platform.
For any mechanism, external parameters can be proposed: such that characterize its functionality, but do not depend on the kinematic scheme.For the power distribution mechanism, the external parameters are the gear ratios for typical operating modes.
For an interwheel power distribution mechanism in rectilinear motion, the equality of the angular velocities of the sides during rectilinear motion requires the fulfillment of the condition (therefore, it is advisable to build a kinematic scheme of such a mechanism based on a simple differential, or functionally similar to a simple planetary mechanism with a kinematic parameter equal to (+2)).
Integral tractors of light classes carry out the turn by controlling the position of the steered wheels.In this case, it is advisable to agree on the values of the minimum "kinematic" and "power" turning radii.The steering mechanism and steering gear are responsible for the kinematic control, and the interwheel power distribution mechanism is responsible for the power control.
In this case, you can use the dependencies proposed in [14]: In these expressions, B is the track width.
For most transport and technological machines, the value   3,4  remains relevant.
Using the principle of controlled slipping of a disk pack [12], it is possible to obtain large intermediate stable turning radii.
A variant of a simplified kinematic diagram of the mechanism of this functional group is shown in Figure 3.The mechanism makes it possible to realize the value of the minimum relative turning radius ρ = 2.78, which corresponds to the value of the inter-board gear ratio u(ρ) = 1.44.These values correspond to the use of simple planetary mechanisms with an internal gear ratio k 3 = 2.05 and k 4 = 2.37.
The synthesis of kinematic schemes of a planetary interwheel controlled power distribution mechanism based on known gear ratios is a trivial task (see, for example, publications [13]).The design methods of units and manufacturing technology are considered in [12,13].

Conclusion
In conclusion, the following main conclusions can be drawn.
1.A fully differential transmission based on controlled power distribution mechanisms can compete with a parallel hybrid and find application in all-wheel drive wheeled vehicles (primarily trucks, skidders, forwarders, harvesters, special off-road chassis).2. The proposed approach makes it possible to create a family of transmissions with a high degree of unification of parts and assemblies for use on machines for various purposes.3.In Russia, there is a theoretical base, a technological reserve and production experience that make it possible to implement the necessary components and assemblies with the maximum degree of localization.Such technologies are concentrated primarily in the field of tank building.

Fig. 1 .
Fig. 1.The structure of the distribution part of the transmission of an all-wheel drive two-axle wheel integral tractor of a small traction class: 1 -power supply; 2 -interaxial PDM; 3 -cardan gears; 4main gear and MRM of the rear axle; 5 -axle shafts; 6 -onboard or wheel gear; 7 -power outlet to the drive wheels; 8 -main gear and PDM of the front axle; 9 -half shafts with CV joints.

Fig. 2 .
Fig. 2. Simplified kinematic diagram of an interaxle MRM with friction control: 0, X and Y -driving and driven links; A and 1 -connecting and brake links; T1, A and C2 -controls; k1,2 -kinematic parameters of planetary mechanisms.