WO2002049896A1

WO2002049896A1 - Construction machine with traction control

Info

Publication number: WO2002049896A1
Application number: PCT/US2001/043311
Authority: WO
Inventors: Jeffrey E. Jensen
Original assignee: Caterpillar Inc.
Priority date: 2000-12-20
Filing date: 2001-11-21
Publication date: 2002-06-27
Also published as: DE10197080T5; JP2004525017A

Abstract

A construction machine (11) is provided with individual wheel brakes (36, 37, 38, 39), a pair of drive axles (17, 21) equiped with differential locking mechanisms, an electronic control module (90) connected in controlling relation to the brakes (36, 37, 38, 39) and differential locking mechanisms and wheel speed sensors (84, 85, 86, 87), and an articulation sensor (88) connected in input relation to the electronic control module (90). The machine operator may choose traction control by wheel braking or differential lock-up, or both. If both are selected, the control module (90) locks up the differential if the articulation angle is less than a predetermined number of degrees, and if the articulation angle is greater than a predetermined number of degrees the control module (90) unlocks the differential (33, 34) and individually brakes any overspeed wheel commensurate with the articulation angle.

Description

CONSTRUCTION MACHINE WITH TRACTION CONTROL

Technical Field

This invention relates to a construction machine in which wheel traction is governed by a wheel slip control and differential locking mechanisms.

Background Art

Traction control in construction machines improves their productivity. Although it has heretofore been suggested that vehicle wheel traction be controlled by braking drive wheels which are overspeeding, or by locking or controlling the slip of locking differentials, there is a need for a control for service braking, traction control, and differential locking for construction machines which operate in an automatic manner with safeguards against braking failure. The integrated traction, braking control and differential lock needs to be suitable for use in remote control and autonomous machine operations .

United States Patent Number Re. 36,152, reissued 16 March 1999 to J. Hosseini et al . for

Method and Apparatus for Controlling Differentially Driven Wheel-Slip for an Articulated Machine discloses use of a controller which receives wheel speed signals and an articulation signal, and automatically produces a braking control signal responsive to the received signals and energizes an electro-hydraulic brake valve to apply a braking force to an overspeeding wheel of an axle. In some operating conditions it is preferred to control traction by locking the differential of each drive axle, rather than by using the wheel brakes to control excessive wheel speed. United States Patent, Number 5,301,769 issued 12 April 1994 to H. Weiss, for Vehicle Power Distribution and Control System, discloses a vehicle with two drive axles, each having a fluid pressure operated clutch type differential lock. Individual wheel speed sensors and a steer angle sensor deliver signals to a controller which in turn controls the degree of locking of the differentials in response to the received signals. United States Patent Number 4,570,509, issued 18 February 1986 to L.L. Nighswonger for Differential Lock Control System Responsive to Steering and/or Braking Action to Unlock Differential, discloses a control system for a differential lock which unlocks the differential of an articulated vehicle upon the articulation angle reaching 10 degrees . Although the hereinbefore discussed patents reveal various traction control apparatus, there remains a need to provide a complete integrated braking and traction control with automatic operation in response to sensed conditions with safeguards against braking failure and with redundancy or back-up to ensure continued safe operation in the event of component failure or malfunction.

The present invention is directed to overcoming one or more of the problems set forth above .

Disclosure of the Invention

Each wheel of a construction machine is provided with a pressure fluid operated service brake which is actuated by pressure fluid delivered through an electrically controlled proportional service valve. Pressure fluid is preferably delivered to the service valves for the service brakes of the two wheels on each differential drive axle through an electrically controlled proportional safety valve connected in pressure fluid receiving relation to a source of pressure fluid. The service and safety valves are controlled by an electronic control module which includes signal lines extending to a manually operated signal generating control in an operator's cab. The service valves and the safety valve for each axle must all be shifted from their normally closed positions to open positions in order to operate the service brakes . This valve arrangement avoids unintentional application of the service brakes upon certain malfunctions or failures of components of the braking system.

Wheel speed sensors and a steer angle sensor are connected in signal delivery relation to the electronic control module, and traction is automatically controlled by the electronic control module through delivery of electricity to the appropriate safety valve to open it and by delivery of an appropriate amount of electrical current to the service valve for the brake of the overspeeding wheel to bring it to a non-slipping speed. The differentials are each provided with a fluid pressure controlled locking mechanism which may be a positive lock type mechanism or may be a slip type differential lock. An electrically controlled proportional lock valve controlled by the electronic control module is used to control delivery of pressure fluid to the locking mechanisms. A traction control switch and a differential lock switch are provided in the operator's cab and these switches are connected to the electronic control module by signal lines. When both the traction control and differential lock switches are activated the differentials are locked if the articulation angle is less than a predetermined amount, such as between 10 and 15 degrees, in which case the traction control by wheel braking is not operative. If the machine is articulated more than the predetermined degrees to either side of straight ahead position, the differential locking mechanisms are automatically deactivated by the electronic control module and the electronic control module will automatically control any overspeeding wheel by braking it. If the locking differentials are of the positive lock type, the electronic control module can be programmed to apply a modulated lockup force commensurate with the sensed travel speed of the machine; thus avoiding unnecessarily high applications of force to the lock-up mechanisms when the machine is traveling at moderate speeds.

The electronic control system for the traction control, the differential locking mechanism and the braking system includes the primary electronic control module and preferably, for safety purposes, a back-up electronic control module, both of which are preferably located on the machine outside the cab. Two sources of electric power and two sources of pressure fluid may be provided to ensure safe operation of the machine. By using this invention, a wire control operation of the construction machine from the cab is made possible. This invention is a brake by wire system with brake operating apparatus moved out of the cab and onto the chassis of the construction machine. In addition to facilitating remote and/or autonomous operation of the construction machine, the noise level in the cab is reduced and installation of the cab during manufacture of the machine is greatly simplified.

Brief Description of the Drawings

An embodiment of the invention is illustrated in the appended drawings, in which: Figure 1 is a side view of a wheel loader in which the present invention is advantageously utilized and

Figure 2 is a schematic illustration of a preferred embodiment traction control and braking systems utilized in the wheel loader shown in Figure 1.

Best Mode for Carrying Out the Invention Figure 1 illustrates a wheel loader 11 which has a chassis 12 made up of a front segment 13 and a rear segment 14 interconnected by a central articulation hitch 15 having a vertical axis 16 about which the wheel loader 11 is steered by a power steering arrangement, not shown. The front segment 13 is supported by a drive axle 17 having a pair of laterally spaced wheels 18, 19 and the rear segment is supported by a drive axle 21 having a pair of laterally spaced wheels 22, 23. The front segment 13 supports a boom with a bucket 26 at its forward end and the rear segment 14 supports an operator's cab 27. The rear segment 14 also supports an internal combustion engine or power unit 28 driving a transmission 29 which has an output shaft 31 connected in driving relation to the wheels 18, 19, 22, 23 through differentials 33, 34 of the axles 17, 21. A spring applied and pressure fluid released parking brake 35 is operatively associated with the output shaft 31. Referring to Figure 2, the wheels 18, 19, 22, 23 are driven through differentials 33, 34 of the drive axles 17, 21 and hydraulically applied and spring released service brakes 36, 37, 38, 39 are operatively associated with the wheels 18, 19, 22, 23, respectively, to control their rotation. A pair of electrically controlled proportional service valves 41, 42 are connected in fluid delivery relation to the service brakes 36, 37, respectively, by fluid conduits 43, 44 and a pair of electrically controlled proportional service valves 46, 47 are connected in pressure fluid delivery relation to the service brakes 38, 39 by fluid conduits 48, 49. The service valves 41, 42, 46, 47 have control elements 51, 52, 53, 54, respectively, having fluid delivery and closed positions of adjustment. Pressure fluid delivery to the service valves 41, 42 is by way of an electrically controlled proportional safety valve 56 and a fluid service conduit 57 having branches connected in parallel to the service valves 41, 42. In a similar manner an electrically controlled proportional safety valve 58 is connected by a fluid service conduit 59 to the service valves 46, 47. The safety valves 56, 58 have fluid flow control elements 60, 61, each of which have fluid delivery and closed positions of adjustment. The flow control elements 51, 52, 53, 54, 60, 61 of all the service and safety valves 41, 42, 46, 47, 56, 58 are normally in their closed positions of adjustment and are proportionally opened depending on the amount of electric current delivered to those valves .

The wheel loader 11 includes a source of pressure fluid including an engine driven pump 62 drawing fluid from a reservoir 63 and delivering pressurized fluid to two accumulators 64, 65 by way of a double check valve 66 and fluid conduits 67, 68. The accumulator 64 is connected in pressure fluid delivery relation to the safety valve 56 by a fluid conduit 69 and the accumulator 65 is connected in pressure fluid delivery relation to the safety valve 58 by a fluid conduit 70.

Each of the differentials 33, 34 have a fluid pressure released and spring applied lock-up mechanism. The lock up mechanisms of the differentials 33, 34 are connected in pressure fluid receiving relation to an electrically controlled proportional lock valve 71 by a fluid conduit 72 and branch conduits 73, 74 and the lock valve 71 is connected in pressure fluid receiving relation to the fluid conduit 70 by a fluid conduit 75. The differential lock-up mechanism may be a positive lock type or it may be a slip type differential lock wherein the amount of slip depends on the pressure of the fluid delivered to the lock-up mechanism. The fluid pressure delivered to the differential lock up mechanism can be varied because the lock valve 71 is a proportional valve. The wheel loader 11 is also provided with a fluid control for its spring applied and fluid pressure released parking brake 35. The parking brake 35 is connected in pressure fluid receiving relation to the accumulators 64, 65 via electrically controlled parking valves 76, 77 and a shuttle valve 78. The parking valves 76, 77 are connected in pressure fluid receiving relation to the fluid conduits 69, 70 by fluid conduits 79, 80, respectively. A pair of inlet ports of the shuttle valve 78 are connected in pressure fluid receiving relation to the parking valves 76, 77 by a pair of fluid conduits 81, 82 and a fluid conduit 83 connects an outlet port of the shuttle valve 78 to the parking brake 35. Individual wheel speed sensors 84, 85, 86,

87 are operatively associated with the wheels 18, 19, 22, 23, respectively, and a steer angle or articulation angle sensor 88 is operatively associated with the articulation hitch 15. These sensors are important components of the traction control and differential locking system which will herein after be described.

An electric control is provided for operating the parking valve 76, the service valves 41, 42, 46, 47, the safety valves 56, 68 and the differential lock valve 71, which includes a primary electronic control module 90 and a back-up electronic control module 100. The wheel speed sensors 84, 85, 86, 87 and the steer angle sensor 88 are connected in signal delivery relation to inputs of the primary electronic control 90. The service valves 41, 42, 46, 47, the safety valves 56, 58, the parking valve 76 and the differential lock valve 71 are individually connected to outputs of the electronic control module 90 by electric lines 91, 92, 93, 94, 96, 97, 98, respectively. The service valves 46, 47 and the safety valve 58 associated with the service brakes 38, 39 of the rear axle 21 and the parking valve 77 are individually connected to outputs of the back-up electronic control module 100 by electric lines 101, 102, 103, 104, respectively. As shown in Figure 1, the control modules 90 and 100 are mounted on the rear segment 14 of the wheel loader chassis 12 at a location outside of the cab 27. The service valves .41, 42, 46, 47, the safety valves 56, 58, the differential lock valve 71 and the parking valves 76, 77 are located in a console 105 beneath the operator's cab 27. The pressure fluid supply system is also carried by the chassis 12 outside of the cab 27.

The primary and back-up electronic control modules 90, 100 are connected to two power sources, namely, an engine driven generator 106 and a battery 107. The generator 106 and the battery 107 are jointly connected in power delivery relation to a pair of relays 108, 109 which are in turn connected in power delivery relation to the control modules 90, 100, respectively, by a pair of electric lines 111, 112. The relays 108, 109 are operated by a manually operated ignition type power switch 113 which is located in the cab 27 and connected to the relays 108, 109 by a pair of electric lines 114, 116. As illustrated, the sources of electric power 106, 107 and the relays 108, 109 are located outside of the operator's cab 27.

If the differentials 33, 34 are equipped with positive locking mechanisms, the control module 90 can be programmed to apply current to the proportional lock valve 71 commensurate with the sensed vehicle speed, thus automatically applying only the necessary force to engage the positive lock-up mechanisms. The greater the vehicle speed the greater the force required to engage the differential lock-up mechanism. The electronic control module 90 can be programmed to supply current to the proportional lock valve 71 in correspondence to the sensed slippage of a wheel, whereby a corresponding fluid pressure is delivered to the friction clutches in the differential locking mechanisms. This mode of traction control can serve as a back-up type traction control in the event the traction control using the service brakes should become inoperative.

In addition to the power switch 113, the various manually operated controls and warning devices located in the operator's cab 27 include a buzzer 131, a pair of warning lights 132, 133, a parking brake switch 136, a manually moveable service brake control 137, an electronic traction control switch 138 and a differential lock switch 139 which are connected by signal lines to inputs of the primary and back-up electronic control modules 90, 100 by way of a primary sheathing harness 161 and a back-up sheathing harness 162. It will be noted that all the operator controls, except the power switch 113, are connected by signal lines to the primary electronic control module 90; however, only the buzzer 131, the signal lights 133, 134, the parking brake switch 136 and the service brake control 137 are connected by signal lines to the back-up electronic control module 100. The service brake control 137 is a foot operated control with three pedal position sensors, not shown. The three sensors simultaneously sense the position of the brake control 137 and three positions sensing signals are transmitted to the primary electronic control module 90 and three signals are transmitted to the back-up electronic control module 100. The three lines actually employed to transmit the triple redundancy signals to each of the control modules 90, 100 are represented by single lines Figure 2. If one of the sensors fails, the active one of the control modules 90, 100 judges displacement of the brake control 137 based on two signals. Two correct signals override a wrong third signal. If only two position sensors were used, the active control module would not ascertain which signal was correct. The active one of the control modules 90, 100 delivers current to the service and safety valves it controls in proportion to the sensed position of the brake control 137 and those service and safety valves deliver pressure fluid to the service brakes with which they are associated to effect braking in proportion to the position of the brake control 137.

The primary and back-up electronic control modules 90, 100 are interconnected by a controlled area network (CAN) datalink 141 and a diagnostic signal wire 142 by which data is shared and by which the outputs of the back-up control module 100 are activated upon it sensing output failure of the primary control module 90.

The traction control and the differential locks are operated only through the primary electronic control module 90. When the traction control switch 138 is actuated, the primary control module 90 compares the sensed speeds of the wheels of an axle and applies appropriate braking force on any overspeeding wheel, with due adjustment for the steer angle of the machine as sensed by steer angle sensor 88. The preferred mode of traction control is the combined use of the individual wheel brakes 36, 37, 38, 39 and the differential locking mechanisms of the differentials 33, 34. When the traction control switch 138 and the differential lock switch 139 are both actuated the primary control module 90 maintains the differentials 133, 134 in a locked condition if the sensed articulation angle is less than a predetermined amount, such as 10 to 15 degrees, and the traction control by wheel braking remains inactive. Because of the different types of work performed and the variation in terrain or surface conditions, the control module 90 operating program can be adjusted to use a selected angle between 10 and 15 degrees. When the articulation angle exceeds the predetermined amount, the differential locking mechanisms are automatically disengaged and the traction control is automatically activated, and wheel overspeeding is controlled by the control module 90 through its applying the brake of an overspeeding wheel with due adjustment for the articulation angle sensed.

Industrial Applicability

The traction control system of this invention is particularly useful in construction machinery such as large wheel loaders. This invention provides a control by wire system with reduction of mechanical and hydraulic parts and linkages.

Elimination of mechanical and hydraulic parts and linkages from the operator's cab allows the cab to be substantially sealed from the outside environment, thereby greatly reducing the noise level to which the operator is subjected. This provides a healthier and more comfortable environment for the operator, which leads to higher productivity. Elimination of mechanical linkages and hoses between the cab 27 and the chassis 12 greatly simplifies installation of the cab during manufacture of the construction machine. Operating the machine by wire is a key segment of a complete drive by wire vehicle which may be operated by remote control . A drive by wire vehicle makes possible a complete or partial autonomous operation in construction, mining and landfill operations. The use of electrically controlled proportional service valves 41, 42, 46, 47 for controlling the flow of pressure fluid to the service brakes 36, 37, 38, 39 permits individual adjustment of the brake pressure at individual wheels. The use of an electrically controlled proportional safety valve 56 between the source of pressure fluid and the service valves 41, 42 for the front axle brakes 36, 37 and the use of the same type of safety valve 58 between the source of pressure fluid and the service valves 46, 47 for the rear axle brakes 38, 39 prevents the unintentional application of the service brakes 36, 37, 38, 39 in the event of a failure or malfunction of components. Construction machinery such as wheel loaders often operate on wet slippery footing. By using the differential locking mechanisms in combination with the wheel braking traction control, the friction wear of the brakes is avoided when operating the machine with the articulation steer angle below the predetermined amount, such as 15 degrees. By providing automatic operation of one or the other of these two types of traction control, depending on steer angle, the operator is free to concentrate on the work being performed by the machine .

An output failure of the primary electronic control module 90 is sensed by the back-up electronic control module 100 via the interconnecting datalink 141 and/or the signal wire and the outputs for electric lines 101, 102, 103 of the back-up electronic control module 100 are automatically activated to provide vehicle operator control for the parking brake 32 and the service brakes 38, 39 of the rear axle 21.

Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A traction control for a construction machine (11) with a chassis (12) having front and rear chassis segments (13,14) pivotally connected on a central vertical articulation axis (16) by an articulation hitch (15) and supported by front and rear axles (17,21), respectively, each of which have differentials (33,34) drivingly connected to a pair of laterally spaced wheels (18,19,22,23), a cab (27) supported on said chassis (12) and a power unit (28) connected in driving relation to said differentials (33,34), said traction control, comprising: a fluid pressure controlled service brake (36,37,38,39) operatively associated with each of said wheels (18,19,22,23); an electrically controlled proportional service valve (41,42,46,47) connected in pressure fluid delivery relation to each of said service brakes (36,37,38,39), said service valve (41,42,46,47) having fluid delivery and closed positions of adjustment; a fluid pressure operated differential locking mechanism operatively associated with each of said differentials (33,34); an electrically controlled lock valve (71) connected in pressure fluid delivery relation to said differential locking mechanisms, said lock valve (71) having fluid delivery and closed positions of adjustment; a source of pressure fluid (62,63,64,65,66) connected to said valves (41,42,46,47); a primary electronic control module (90) having outputs connected, respectively, in controlling relation to each of said service and lock valves (41,42,46,47,71) ; a wheel speed sensor (84,85,86,87) operatively associated with each of said wheels (18,19,22,23) and connected in speed signal input relation to said electronic control module (90) ; an articulation angle sensor (88) operatively associated with said articulation hitch (15) and connected in articulation angle signal delivery relation to said electronic control module (90) ; a traction control switch (138) connected in controlling relation to said electronic control module (90) and a differential lock switch (139) connected in controlling relation to said electronic control module (90) ; said electronic control module (90) being operative upon activation of said switches (138,139), and whenever the sensed articulation angle is at least a predetermined angle between 10 and 15 degrees, to compare the sensed wheel speeds and apply the service brake (36,37,38,39) of an overspeeding wheel (18,19,22,23) to slow it to a speed commensurate with the sensed articulation angle and said electronic control module (90) being operative to lock said differentials (33,34) when said sensed articulation angle is less than said predetermined angle.

2. The traction control as set forth on claim 1 wherein said predetermined angle is 10 degrees .

3. The traction control as set forth in claim 1 wherein said predetermined angle is 15 degrees .

4. The traction control as set forth in claim 1 wherein said switches (138,139) are positioned in said cab.

5. The traction control as set forth in claim 1 wherein said differentials (33,34) are fluid pressure controlled positive lock differentials and said differential lock valve (71) is an electrically controlled proportional valve, said electronic control module (90) controlling said differential lock valve (71) to apply a differential lock up force on said lock up mechanisms commensurate with the sensed wheel speed of said machine.

6. The traction control as set forth in claim 1, including a manually moveable service brake control (137) in said cab (27) connected to an input of said electronic control module (90) , said service brakes (36,37,38,39) being applied by fluid pressure in correspondence to the amount of movement of said service brake control (137) .

7. The traction control as set forth in claim 6, including a back-up electronic control module (100) interconnected by a data link (141) with said primary electronic control module (90) , said service brake control (137) being connected in signal delivery relation to said back-up electronic control module (100) and said back-up electronic control module (100) having normally inactive outputs connected in controlling relation to said service valves (41,42,46,47) , said outputs of said back-up control module (100) being automatically activated upon said backup control module (100) sensing failure of the outputs of said primary control module (90) .

8. The traction control as set forth in claim 6 having first and second sources of power (106,107) connected in power delivery relation to a pair of parallel connected relays (108,109) which are connected, respectively, in power delivery relation to said primary and back-up electronic control modules (90,100) .

9. The traction control as set forth in claim 1 wherein each of said differential locking mechanisms includes a fluid pressure controlled friction clutch, and wherein said lock valve (71) is a proportional valve.

10. A traction control for a construction machine (10) with a chassis (12) having first and second chassis segments (13,14) pivotally interconnected by an articulation hitch (15) on a vertical axis (16), said chassis segments (13,14) each being supported by a drive axle (17,21) having a differential (33,34) driving a pair of laterally spaced wheels (18,19,22,23) rotatably supported on opposite ends of said axle (17,21), a cab (27) on said chassis (12) and a power unit (28) connected in driving relation to a transmission (29) which has an output shaft (31) connected in driving relation to said differentials (33,34), said traction control comprising; a fluid pressure operated service brake (36,37,38,39) operatively associated with each of said wheels (18, 19,22,23) , an electrically controlled proportional service valve (41,42,46,47) connected in pressure fluid delivery relation to each of said service brakes (36,37,38,39), said service valves (41,42,46,47) having fluid delivery and closed positions of adjustment, a fluid pressure operated differential locking mechanism operatively associated with each of said differentials (33,34), an electrically controlled lock valve (71) connected in pressure fluid delivery relation to said differential locking mechanisms, said lock valve (71)- having fluid delivery and closed positions of adjustment, a spring applied and fluid pressure released parking brake (35) operatively associated with said output shaft (31) , a pair of electrically controlled parking valves (76,77) connected in pressure fluid delivery relation to said parking brake (35) ; a source of pressure fluid (62,63,64,65,66) connected to said valves (41,42,46,47,71,76); a primary electronic control module (90) having outputs individually connected in controlling relation to said service valves (41,42,46,47), said lock valve (71) and one of said parking valves (76) ; a wheel speed sensor (84,85,86,87) operatively associated with each of said wheels (18,19,22,23) and connected in speed signal input relation to said primary electronic control module (90) ; an articulation angle sensor (88) operatively associated with said articulation hitch (15) and connected in articulation angle signal delivery relation to said primary electronic control module (90) ; a manually moveable service brake control (137) in said cab (27) connected in controlling relation to said primary electronic control module (90); a traction control switch (138) , a differential lock switch (139) and a parking brake switch (136) mounted in said cab (27) and connected in controlling relation to said primary electronic control module (90) ; said differential lock switch (139) and said traction control switch (136) normally being activated when said construction machine is engaged in work at a job site, in which condition said primary electronic control module (90) causes said differentials (33,34) to be locked by said locking mechanisms whenever said machine is articulated less than 10 degrees, and whenever the articulation angle is at least 10 degrees, said primary electronic control module (90) causes said differential locking mechanisms (33,34) to unlock said differentials and applies the service brake (36,37,38,39) associated with any sensed overspeeding wheel with due regard to the sensed articulation angle, and a back-up electronic control module connected by a data link (141) to said primary electronic control module (90) , said back-up control module (100) having inputs connected, respectively, in signal receiving relation to said service brake control (137) and said parking brake switch (136) , said back-up control module (100) having normally inactive outputs connected to said service valves (46,47) and to one of said parking valves (77) , said normally inactive outlets being activated upon said back-up control module (100) sensing a primary control module (90) output failure.

11. The traction control as set forth in claim 9 wherein said control modules (90,100) and said valves (41,42,46,47,71,76,77) are mounted on said chassis (12) outside of said cab (27) .

12. The traction control as set forth in claim 9 including first and second sources of electric power (106,107) connected to first and second relays (108,109), said first and second relays (108,109) being connected in electric power delivery relation to said primary and back-up control modules (90,100), respectively.

13. The traction control as set forth in claim 11 including a power switch (113) in said cab (27) connected in controlling relation to said relays (108), 109) and wherein said sources of electric power (106,107) are supported on said chassis (12) outside of said cab (27) .

14. The traction control as set forth in claim 9 including a manually moveable service brake control (137) in said cab (27) , having at least three position sensors connected in signal delivery relation to said primary and back-up control modules (90,100) .

15. The traction control set forth in claim

13 including an electrically operated proportional safety valve (58) connected in pressure fluid receiving relation to said source of pressure fluid (62,63,64,65,66) and in pressure fluid delivery relation to said service valves (46,47) for said brakes (38,39) of one of said axles (21) and an electrically operated proportional safety valve (56) connected in pressure fluid receiving relation to said source of pressure fluid (62,63,64,65,66) and in pressure fluid delivery relation to said service valves (41,42) for said brakes (36,37) of the other of said axles (17), said safety valves (46,47) being connected in controlled relation to said primary control module (90) and said safety valve being connected in controlled relation to said back-up control module (100) .

16. A method of controlling traction of a construction machine (11) of the type having a chassis (12) with first and second segments (13,14) interconnected by an articulation hitch (15) on a vertical axis (16), said chassis segments (13,14) each being supported by a drive axle (17,21) having a pair of laterally spaced wheels (18,19,22,23), a cab (27) on said chassis (12) and a power unit (28) connected in driving relation to said wheels (18,19,22,23), said method comprising the steps of : providing a differential (33,34) with a fluid pressure operated locking mechanism in each axle (17,21), each of said differentials (33,34) being connected in driving relation to said wheels (18,19,22,23) of the associated axle (17,21) and in driven relation to said power unit (28) ; providing a fluid pressure operated service brake (36,37,38,39) for each wheel (18,19,22,23); providing a source of pressure fluid (62,63,64,65, 66) ; providing an electronic control module (90) ; providing individual wheel speed sensors

(84,85,86,87) connected in signal delivery relation to said control module (90) ; providing an articulation angle sensor (88) connected in signal delivery relation to said control module (90) ; providing electrically controlled proportional service valves (41,42,46,47) connected, respectively, in pressure fluid delivery relation to said service brakes (36,37,38,39) and in pressure fluid receiving relation to said source of pressure fluid (62,63,64,65,66); providing an electrically controlled lock valve (71) for each of said locking mechanisms of said differentials, said lock valve (71) being connected in pressure fluid receiving relation to said source of pressure fluid (62,63,64,65,66) and in pressure fluid delivery relation to said locking mechanisms; connecting said valves (41,42,46,47,71) in controlled relation to said control module (90) ; providing a source of electric power (106,107) for said control module (90); providing a traction control switch (138) , a differential lock switch (139) and a manually moveable service brake control (137) in said cab (27) and connecting said switches (138,139) and control (137) individually to said control module (90) , and programming said control module (90) to respond to actuation of said traction control switch (138) and said differential lock switch (139) to automatically lock said differentials (33,34) during operation of said construction machine (11) so long as the articulation angle is less than a predetermined number of degrees between 10 and 15 degrees and to unlock said differentials (33,34) when said articulation angle is more than said predetermined number of degrees and apply said service brake (36, 37,38,39) of any overspeeding wheel (18,19,22,23) to slow said overspeeding wheel to a speed commensurate with the sensed articulation angle.