Active trailer with variable base for the timber industry

. For forest machines (skidders, forwarders), due to the peculiarities of work cycles and a wide range of changes in resistance to movement, a power plant with a heat engine most of the time operates in a non-optimal mode. It is proposed to overcome this disadvantage by using an active trailer link. The issues of the features of the use of active trailers as part of road trains of forest machines are considered. A trailer with a variable base was chosen as the basis, as the most popular and widespread in the forestry complex. The structure of an electromechanical transmission based on the principle of using a central electric motor and a controlled interwheel power distribution mechanism is proposed. The principles of determining the power of thermal and electric motors are considered. A method for calculating the external parameters characterizing the power distribution mechanism based on the principle of matching the radii of the kinematic and power turn is proposed, and a design scheme for turning the road train is given. The main assumptions are formulated to justify the possibility of the proposed calculated dependencies. The way of synthesis of kinematic schemes of such mechanisms, approaches to design and production are indicated. A technology for controlling such mechanisms is proposed. An example is considered, a variant of the kinematic scheme of the power distribution mechanism is given. The ways of project realization are shown.


Introduction
Industry statistics show that forestry vehicles are characterized by a wide range of changes in resistance to movement, about 40% of the time is idling (movement without a payload (see, for example, the data given in the monograph [1] and other publications of its authors).
From the point of view of reducing energy consumption during timber removal, skidding methods and the use of skidders should, if possible, be preferred to the use of forwarders of various designs. From an economic point of view, in many cases it is expedient to operate a truck in this capacity as part of a road train with a dissolution trailer. The dissolution trailer is characterized by the presence of one or two driven axles, and the design often allows you to change the length of the trailer. An overview of modern technical solutions on the issue of timber transportation is given, for example, in the book [2].
A significant disadvantage of the trailer-dissolution is the underutilization of the grip weight of the loaded road train, since trailers of this type are traditionally passive.
In the USSR, road trains with an active trailer link and various options for driving the trailer wheels were created and tested. The mechanical transmission turned out to be unnecessarily complex and expensive, while the electromechanical transmission turned out to be heavy, bulky and unreliable. There is also the option of using a volumetric hydromechanical transmission to drive the wheels of the trailer, but it should be considered as too expensive. Active trailers were initially considered as an object intended primarily for the defense industry. Therefore, when the military gave preference to all-wheel drive multiaxle monohull vehicles, interest in active trailers decreased and they did not become widespread in the forestry industry. An active trailer with a mechanical transmission is used as a link in the DT series articulated caterpillar transporter, which is commercially produced in Russia.

Formulation of the problem
The modern level of electronics and electrical engineering allows us to return to the issue of introducing an active trailer into a road train in order to improve operational characteristics (handling, driving stability, maneuverability, cross-country ability, fuel efficiency).
Actual tasks are to determine the structure of an electromechanical transmission, the external parameters of its constituent units, and the principles of their control. As a basic model, it is proposed to consider an active trailer with one axle.
The work is based on the analysis of statistics on the operating modes of forest machines and promising approaches to managing power distribution in the transmissions of transport and transport-technological platforms. The main provisions of the work are based on the approaches of the theory of machines and mechanisms, the theory of the movement of transport vehicles, the principles of using modern computing technologies.

Results of theoretical studies
Regardless of the design solutions for organizing the drive of the wheels of the active trailer, it is fundamentally important to ensure the safety of the road train. A feature of a road train with an active trailer is the requirement to prevent the trailer from running into the tractor, which can cause loss of stability and folding of the road train. The traction force on the tractor coupling device in all operating modes must be positive (theoretically, it can be zero).
To ensure a minimum positive traction force on the tractor coupling device, the driving wheels of the active trailer must operate in a mode close to neutral (the case when the force applied to the wheel axle is compensated by the torque applied to the wheel, see, for example, publication [3,4]). Direct measurement of the longitudinal force on the coupling device of a serially produced tractor presents some technical difficulty and leads to an increase in the cost of the road train. But at present, it is possible, using information about the value of the normal reaction for the tractor and the load of its engine, at a known actual speed, to calculate the required traction force on the wheels of the active trailer by calculation. Work in this direction is being successfully carried out at the Higher School of Transport at Peter the Great St. Petersburg Polytechnic University. Modern internal combustion engines provide the greatest economy while realizing maximum torque. The use of the principle of an electromechanical drive of the driving wheels of the trailer will make it possible to store energy in on-board electric storage devices when the chassis is moving without a payload, and to release this energy in the active trailer mode. This implies an increase in the power of the generator installed on the tractor, and the need to install an on-board energy storage device on the trailer.
When the machine is self-propelled, the excess power of the power plant stored in the onboard energy storage device can be estimated as a first approximation by the dependence where M N is the engine power at maximum torque, N  is the power of resistance to movement (excluding aerodynamics), w N is the power of the aerodynamic component.
Additional information for evaluating the power balance of the mobile chassis is given, for example, in publications [3,4].
A review and analysis of the prospects for using various strategies for organizing the electromechanical drive of the driving wheels of mobile platforms is carried out in the article [5]. The use of motor-wheels is accompanied by an increase in the total mass of electric motors and the necessary intermediate gearboxes (range boxes) in comparison with the principle of using a central electric motor. For high-speed chassis, the use of motor-wheels negatively affects the increase in unsprung masses. But in a number of special cases, the concept of motor-wheels may be in demand. For transport and technological chassis, this issue is considered in detail, for example, in the publication [6]. At present, the "motorwheel" scheme is preferred to the "motor-half shaft" option, when the TEM is placed on a frame or load-bearing body and is connected to the wheel by a semi-axle by means of constant velocity joints. With an increase in the cost of construction, the advantages are a decrease in unsprung masses, a decrease in the requirements for the compactness of the TEM and the gearbox working with it, and an improvement in the cooling conditions of the TEM.
Fundamentally common for works [5] and [6] is the thesis: to reduce the installed power of the electric motor, it is necessary to use at least a two-mode mechanical range box connected in series with it, in which the first gear is responsible for implementing the maximum required traction force, and the second gear is responsible for achieving the maximum customer-specified speed.
The gear ratios of the modes are determined by the dependencies [5]: Here: ψ -coefficient of resistance to movement; G -coupling weight; tr u and tr gear ratio and transmission efficiency in the considered branch; Р  -efficiency of the gearbox.
The principle of using a central electric motor with a sequentially installed range box and controlled interaxal power distribution mechanisms has the greatest prospects in terms of reducing energy consumption for movement (see work [5] and others).
In this case, two basic schemes of the mechanism seem to be competitive: a symmetrical lockable differential and a controlled power distribution mechanism.
In the first case, the differential principle of uniform distribution of torque allows you to equalize the torque between the axles, and if necessary, the use of blocking will effectively overcome a straight section of the route with extreme conditions of resistance to movement. This principle is widely used on a high cross-country chassis. Questions of the theory and design of symmetric differentials are described in sufficient volume, for example, in [7]. In the domestic transport engineering industry, self-locking differentials have not received distribution, but symmetrical differentials with forced locking are being introduced into the series, for example, at the St. Petersburg Tractor Plant.
In the second case, the interaxle power distribution mechanism works like a simple (symmetrical) differential until there is a need for forced redistribution of torques between the sides. For example, when entering a turn with a small radius, the threat of skidding or a significant mismatch in the bearing capacity of the soil under the wheels of the sides. A close analogue of such a system is the dynamic stabilization system, which is actively used abroad on passenger cars. As part of such a system, commercially available controlled cross-axle differentials ZF Vector Drive, AYC, etc. work [8,9,10].
In publications [11,12,13,14], the question of choosing the external parameters of such a power distribution mechanism is considered. It is based on the need to coordinate the kinematic turning radius of the tractor and the power turning radius for the trailer (Fig. 1). A feature of the scheme under consideration is an additional degree of freedom, which allows the bridge of the trailer-dissolution to turn around relative to the platform with the load. For a passive trailer, the turning angle will be determined by the tractive force and external resistances. For an active trailer, the turning angle can be controlled kinematically or forcefully. This opens up the possibility of increasing maneuverability at low speeds and rollover stability at relatively high speeds. Fig. 1 illustrates simple relationships relating the main parameters of trailer movement: The actual radii and speeds differ from the theoretical ones due to the influence of slipping and skidding, but in this case this is not important. The necessary corrections in calculations can be introduced using traditional methods of the theory of motion of wheeled and tracked vehicles (see, for example, [3]).
Using Fig. 1, it is convenient to enter the trailer's theoretical turning radius:   21 2 R R R  (4) and the relative theoretical turning radius of the trailer: RB  (5) Using these concepts, it is possible to apply the dependencies characteristic of [15]: Therefore, knowing the range of relative turning radii that is appropriate from the point of view of operating conditions, it is possible to estimate the required kinematic gear ratio and proceed to the problem of synthesizing possible kinematic schemes of interwheel power distribution mechanisms. A similar question was considered, for example, in [16], and the approaches outlined in this article can be effectively used in the case under consideration.
For transport and transport-technological machines, the value of the minimum relative turning radius in the range of 3 ... 4 is typical (see, for example, the source [17]).
For example, the value ρ=2.78 corresponds to the inter-board gear ratio u=1.44. On Fig. 2 shows an example of a kinematic diagram of a controlled interwheel power distribution mechanism that implements a given inter-board gear ratio. This technical solution is protected by the RF patent [18].
The mechanism with the kinematic parameter k0=2 is functionally similar to a simple (symmetrical) differential.
A mechanism with kinematic parameter * 0 k performs the function of a step-up gearbox and is introduced into the circuit to reduce the radial dimension of the device.
Mechanisms with kinematic parameters k3 and k4 are used to control the distribution of power between the sides of the trailer.
The controls are disc brakes and an electromechanical or hydromechanical drive. Service brakes and wheel reduction gears in fig. 2 are conventionally not shown.
The operating modes of the GSM are given in Table. 1. Specific values k3=2.05 and k4=2.37 are proposed for the minimum relative turning radius of the platform ρ=2.78.  To ensure a smooth change in the turning radius ranging from infinity (rectilinear movement) to the minimum, due to the kinematics of the power distribution mechanism, it is advisable to apply the principle of controlled slipping of the disc packs of friction control elements (see, for example, articles [19, 20,21,22,23 ,24]).
Implementation of the principles discussed above in real time is difficult without a proven technological base. However, the technologies for calculating and designing planetary gearboxes operating with disk friction control elements with a hydraulic and hydromechanical drive have been successfully tested in domestic engineering (see, for example, publications [19,25,26]).

Practical Implications and Perspectives
Implementation of the principles discussed above in real time is difficult without a proven technological base. However, the technologies for calculating and designing planetary gearboxes operating with disk friction control elements with a hydraulic and hydromechanical drive have been successfully tested in domestic engineering (see, for example, publications [19,25,26]).
The equipment and technologies for testing such mechanisms have been tested [27].
Of further interest is the use of the proposed mechanism in the design of the chassis of new tracked and wheeled vehicles and the modernization of existing models of ground vehicles for various purposes [28,29,30].

Acknowledgments
The work was carried out within the framework of the Strategic Academic Leadership Program of the Kazan (Volga Region) Federal University ("PRIORITET-2030").

Conclusion
1. The greatest prospects in terms of increasing the cross-country ability and improving the maneuverability of a timber carrier with a trailer-dissolution have a design with an electromechanical transmission and a controlled power distribution mechanism. 2. Based on the considered methodology, a family of electromechanical transmission schemes can be built.
3. In the domestic transport engineering industry, methods for designing and manufacturing the necessary parts and assemblies have been tested, and it seems possible to establish the manufacture of the considered transmissions with a large percentage of component production localized within the Russian Federation.