SYSTEM AND METHOD FOR DIFFERENTIAL LOCK CONTROL
Technical field
The present invention relates to a system and a method for differential lock control.
Background
Heavy vehicles, e.g. trucks, often have between two powered wheels, a differential gear which has a differential lock by means of which the powered wheels can be mutually locked. When the differential lock is engaged, the wheels are mutually locked, making it easier for them to cope with bad road surfaces, since both wheels run at the same speed even if one of them loses contact with the road.
At a transition to good road surface, the driver usually wishes to disengage the differential lock so that the mutual locking between the powered wheels ceases. However, the differential lock, which is usually an axial gearwheel combination, may be difficult to disengage because the gearwheels in the differential lock often have an angle of engagement which tends to "pull together" the differential gear as soon as it comes under load. The disengaging movement, normally effected by compressed air or return spring force, is not strong enough to pull the gearwheels apart if the torque across the gearwheels is too great. For it to be possible to disengage the differential lock, this pulling-together effect has to be overcome, preferably by the differential lock being relieved of load so that it becomes entirely torque-free, which in practice only happens either when the vehicle is travelling forwards in an exactly straight line on a smooth level road with powered wheels which are exactly alike in size (i.e. exactly equally worn) or during the brief time when the torque between the left and right powered wheels changes direction. To make sure that the differential lock really is disengaged, the driver has at present to pivot the control to and fro about the neutral position when disengaging the differential lock. At present, in the worst case, the differential lock indicator may indicate "differential lock disengaged" even if the differential lock control has been operated to disengage the differential lock but the
differential lock itself has not been disengaged. The differential lock has sometimes begun its disengaging movement, but this may be halted in a half-disengaged position by torque across the differential lock gearwheels, thereby making it impossible to rely on the differential lock indicator.
An example of a system and a method for differential lock control is referred to in SE 514 011 C2, which refers to a vehicle with differential lock whereby the forces to which the differential lock is subjected are reduced by using components in the vehicle's braking system before the differential lock is engaged.
A problem with differential locks of the abovementioned kind is that they may possibly not disengage despite the driver's thinking that they have disengaged.
The consequences of driving on in the mistaken belief that the differential lock is disengaged can be very serious. In the first place, the vehicle severely understeers with the differential lock engaged as compared with disengaged, which may take the driver by surprise at the next bend. Secondly, the differential lock combination may become overloaded and fail, resulting in immobilisation and expensive repairs.
Brief description of the invention
The problem of the differential lock possibly not disengaging despite the driver's thinking that it has disengaged is solved according to the invention by arranging a system and a method for differential lock control in a vehicle which comprises a differential gear between left and right powered wheels and a differential lock which, when engaged, locks together the left and right powered wheels, with a control unit arranged to effect alternate braking of left and right powered wheels when a differential lock control is operated to disengage the differential lock, resulting in the differential lock being relieved of load so that it becomes torque-free, thereby ensuring that the differential lock becomes fully disengaged.
A system for differential lock control and a method for differential lock control which have the characteristics of claim 1 and claim 10 respectively afford the advantages that the differential lock becomes fully disengaged so that the driver is not liable to be surprised by the vehicle severely understeering at a bend due to the differential lock being engaged despite the driver's thinking that it is disengaged, and that overloading and failure of the differential lock are prevented, thereby making it possible to avoid immobilisation and expensive repairs. Moreover, relieving the differential lock of load is performed automatically, so the driver can forget about relieving the load on the differential lock when disengaging it.
Brief description of the drawings
The invention is explained in more detail below with reference to the attached drawings, in which:
Fig. 1 depicts schematically a view of a vehicle, and
Fig. 2 depicts schematically a block diagram of a system for differential lock control according to an embodiment of the invention.
Description of preferred embodiments
The same characteristics are provided with the same reference notations in the various drawings.
Figure 1 depicts schematically a view of a vehicle 2 in which a power unit 4 in the form of a combustion engine situated forward in the vehicle is connected rearwards to a propeller shaft 6 for transmitting driving power to powered wheels 8,10 via rear driveshafts 12,14. The propeller shaft 6 is connected in a conventional manner to a rear axle gear 16 provided with a differential gear 18 which is itself connected via two driveshafts 12,14 to the powered wheels 8,10 of the vehicle 2.
The vehicle's system for differential lock control comprises a differential gear 18 between left and right powered wheels which is provided with a conventional differential lock 26 which, when engaged, locks together, preferably mechanically, the driveshafts 12,14 and hence also the left and right powered wheels 8,10. The driving location of the vehicle 2 is provided with a differential lock control 28 connected to a control unit 30 which controls both a brake regulator 36 for regulation of braking force to the powered wheels 8,10, and an operating unit 31 for engaging and disengaging the differential lock 26. The operating unit 31 for engaging and disengaging the differential lock 26 is normally a compressed air cylinder. The driving location is also provided with an indicator 32 which shows whether the differential lock 26 is engaged or disengaged.
The differential lock 26 is intended to be engaged when the vehicle 2 is travelling on slippery road surfaces. When the differential lock 26 is engaged, the ordinary function of the differential gear 18 is disengaged and the two powered wheels 8,10 are instead both powered at the same speed.
At a transition to good road surface, the driver usually wishes to disengage the differential lock 26 so that the mutual locking between the powered wheels 8,10 ceases.
Figure 2 depicts schematically a block diagram of a system for differential lock control according to an embodiment of the invention. The operation of the differential lock control is coordinated with brake operation by the fact that the control unit 30 which controls both a brake regulator 36 for regulation of braking force to the powered wheels 8,10, and an operating unit 31 for engaging and disengaging the differential lock 26, is arranged to effect, preferably repeatedly, alternate braking of left and right powered wheels 8,10, one at a time, when the differential lock control 28 is operated to disengage the differential lock 26, thus relieving the load on the differential lock 26 so that it become torque-free and hence ensuring that the differential lock 26 is fully disengaged. Upon a disconnecting signal from the differential lock control 28, the control unit 30 activates simultaneously both the operating unit 31 for the differential
lock 26 and the brake regulator 36. The control unit 30 may thus on the basis of information from the differential control 28 deliver control signals to a brake regulator 36 which itself controls brake devices 38,40 at the powered wheels 8,10. The brake regulator 36 and the brake devices 38,40 preferably form part of the ordinary braking system of the vehicle 2 and enable, in conjunction with the control unit 30, braking as necessary of the two powered wheels 8,10. The control unit 30 may also preferably form part of the ordinary braking system of the vehicle 2.
A method for differential lock control in a vehicle comprising a differential gear 18 between left and right powered wheels 8,10, and a differential lock 26 which, when engaged, locks together the left and right powered wheels 8,10, comprises the following steps:
- operating a differential lock control 28 to disengage the differential lock 26,
- sending this disengagement information from the differential lock control 28 to a control unit 30,
- delivering control signals from the control unit 30 both to the brake regulator 36 which itself controls brake units 38,40 at the powered wheels 8,10, and to an operating unit 31 for engaging and disengaging the differential lock 26, and
- alternately braking, preferably repeatedly, the left and right powered wheels 8,10 at the same time as the differential lock 26 is being disengaged.
The number of alternate brakings may either be predetermined as one or more or be controlled by their cessation when the operating unit receives a disengagement completion signal from the differential lock.
The control unit 30 is preferably separate from the differential lock control 28 but they may alternatively also be arranged as a single unit.