Automated Tire Pressure Control System for Multi-Purpose Wheeled Vehicles

The cross-country capability of multi-purpose wheeled vehicles is one of the most important operational properties of these technical objects. In many ways, it is determined by their traction characteristics. There are a number of ways to improve traction and coupling properties of multipurpose wheeled vehicles, the main ones are the use of various kinds of traction control systems, blocking of interaxle and interwheel differentials, the use of ballast and several others. Recently, one of the ways to improve the traction properties and cross-country ability of vehicles on soils with weak load-bearing capacity is a regulation of air pressure in the tires of the driving wheels of multi-purpose wheeled vehicles. The paper describes the process of interaction of the wheel mover with the ground surface when the air pressure in the tire changes. The influence of air pressure on the traction properties of wheeled vehicles is established. The system of automatic control of air pressure in tires of mobile cars depending on road conditions is offered. The use of the proposed regulation principle will significantly increase the cross-country ability of multi-purpose wheeled vehicles in heavy traffic conditions, eliminating the subjective factor in the person of the vehicle operator.


Introduction
It is known that the air pressure in the tires of multi-purpose wheeled vehicles (MWV) determines the shape and size of the contact fingerprint and affects their traction and coupling qualities and patency on the supporting surface. This process is accompanied by slipping and a change in the strength of resistance to movement due to deformation of the soil. With a decrease in pressure, the supporting surface of the tire increases and the pressure of the propulsion on the ground surface decreases, which leads to an improvement in the traction and coupling properties and throughput of the MWV. This is especially evident when driving on soils with weak bearing capacity. On the contrary, the increase in pressure when MWV moves along artificial roads such as concrete, asphalt, stone or rolled dirt roads when a track is not formed improves the efficiency of the machine. Tire air pressure regulation is currently carried out manually by the driver when the car is moving in different road conditions. However, when the MWV moves on soils with weak bearing capacity, the driver does not always have time to catch the moment of decrease in the tangential traction force Fк with an increase in skidding ере, which leads to loss of patency or due to loss of adhesion of the mover to the supporting surface or landing on the bottom due to the increase in the track depth. Based on the results of an analysis of modern studies on soil mechanics when a dynamic load is applied to them, a set of dependencies is formed that determine the interaction of driven sprockets with the ground surface. The main ones are soil resistance to compression and shear when applying dynamic loads. As a method, a theoretical study of the traction and coupling properties and patency of wheel propulsions when driving on dirt surfaces has been adopted. The result of theoretical studies is the creation of an electronic device that monitors the moment of decrease in traction force or complete slipping and gives a signal to reduce air pressure in the tires, and, conversely, to increase pressure when the machine enters a solid supporting surface. Creation of a system for automatic regulation of air pressure in MWV tires depends on road conditions.

The process of interaction of the propulsion multi-purpose wheeled vehicles with a soil surface
To determine the favorable moment of turning on or off the air flow valve in the tire, it is necessary to determine the adhesion properties of the propulsion, slipping and track depth. These properties are determined, on the one hand, by the system-forming parameters of the MWV, such as: machine weight, engine power, propulsion structure, etc. On the other hand, they depend on the physical and mechanical properties of the soil surface, such as: structure and mechanical composition, humidity, soil resistance to compression and shear [1][2][3].
The most appropriate real conditions are the dependencies proposed by V. V. Katsygin [4,5], namely: -normal soil compression stress  is determined by the formula where  0 -bearing capacity of the soil, N/m 2 ; kcoefficient of volumetric crushing of the soil, N/m 3 ; h -immersion depth of the stamp, m; -shear stress  arising from the deformation of the soil are determined by the expression   Fig. 1 shows that there are three sections of this dependence: the first section reflects elastic deformation; the second is plastic; the third is the flow of soil.
A graphical representation of the dependence of shear stresses arising from soil deformation is shown in Fig. 2. Figure 2 shows that the shear stresses reach a maximum at some strain  0 , and then decrease. This phenomenon is explained by the fact that in section I the soil is compacted (static friction), and in section II it is shifted (sliding friction).
The reduced radius of the wheel can be determined by the expression In this expression, the value of tire deformation under load can be determined by the Heideckel formula where р sh -tire pressure, Pa; r c -radius of the tire section, which can be equated to half the width of the tire, m, those r c  b/2. The tangential traction force (driving force) is determined according to the formula τ 0 where L pr -reduced length of the supporting part of the wheel, m; q x -mover pressure on the ground, N/m 2 ; f p , f sk -coefficients of friction of rest and sliding;  x -slipping of the wheel at the point of contact with the x coordinate; k  -strain coefficient. The reduced length of the wheel support is calculated as: where h -rut depth, m.
A typical dependence obtained when calculating the expressions (1)-(6) of the tangential traction force F k from slipping  of the drive wheels is shown in Fig. 3. Fig. 3. The dependence of the tangential force on slipping when driving wheel on a dirt surface Fig. 3 shows that the tangential traction force F k increases depending on slipping  to a certain value  opt , and then begins to decrease. This is due to the fact that when a tire with lugs interacts with the ground surface, the latter move the soil in the direction opposite to the movement of the machine and in the section from 0 to  opt , the driving force is proportional to the shear forces T sd .
Upon reaching slipping  opt , the lugs cut off the soil "bricks" and an "earth" wheel is formed, i. e. shear friction T sd is replaced by sliding friction T sk . It is known that T sd  T sk [9][10][11].
Thus, when slipping a wheel, there are two modes of slipping (Fig. 3): -traction increases with increasing slip; -the traction force drops and tends to a constant value, due to the friction forces of the "earthen" wheel with a dirt surface. Fig. 4 shows one of the options for the proposed system of automatic regulation of air pressure in the tires of mobile cars.