Multiple-circuit brake system

A multiple-circuit brake system for motor vehicles in which a multiple-circuit braking value transducer is embodied partially electrically and partially as a pneumatic control valve and is intended for triggering pressure control valves, the wheel brake cylinders being disposed following these pressure control valves. One pressure-medium reservoir is associated with each pressure control valve, and the controlling energy for each of these pressure control valves is drawn from the pressure-medium reservoir associated with that valve. In the pneumatic brake circuit (I), a further pressure-medium reservoir is provided preceding the braking value transducer, as a result of which it is possible, with the aid of two-way check valves, to actuate the wheel brake cylinders with the pneumatic brake circuit (I) whenever the electrical brake circuits (II, III) have failed.

BACKGROUND OF THE INVENTION 
The invention is based on a multiple-circuit brake system for motor 
vehicles such as a tractor-trailer. A brake system of this type is known 
from German Offenlegungsschrift 29 37 657. In this known brake system, the 
multiple-circuit braking value transducer is a two-circuit compressed-air 
brake valve, which is combined with an electric potentiometer in order to 
actuate a third brake. As a result, however, this known brake system has 
many parts and is expensive. 
OBJECT AND SUMMARY OF THE INVENTION 
The multiple-circuit brake system has the advantage over the prior art in 
that it represents a simple structure for a multiple-circuit brake system, 
and the reliability of braking is assured in a particular manner. The 
electrical system provides that the main brake circuit is fast-acting, and 
the pressure medium supplies an auxiliary brake circuit which if the 
electrical system fails puts the brakes into operation upon the actuation 
of the braking value transducer. 
A further advantage is that identical valves are used at several locations 
in the system. As a result, the equipment is very economical. 
The invention will be better understood and further objects and advantages 
thereof will be better understood from the ensuing detailed description of 
preferred embodiments taken in conjunction with the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A multiple-circuit brake system such as one disposed on a tractor, 
including a compressor 1 with a pressure regulator and a four-circuit 
protective valve 2, to which three operating brake circuits I, II and III 
are connected. The brake circuit I is a pneumatic brake circuit. The other 
two operating brake circuits II and III include a supply line 4, to which 
two supply tanks 5 and 6 are connected, which act as reservoirs for the 
pressure medium. A two-circuit braking value transducer 7 is also disposed 
on the tractor and has an electrical current transducer 8 and a fluid 
valve member 9, both of which are actuatable simultaneously by a driver of 
the vehicle via a pedal 10. 
Preceding the two-circuit braking value transducer 7, in a supply line 3, 
is a further reservoir 11 for a fluid pressure medium, and a two-way check 
valve 13 is connected in fluid pressure line 12 which monitors the fluid 
access to two brake cylinders 14 and 15 of a front axle 16. The check 
valve 13 is disposed following the braking valve transducer 7 in a 
pressure-medium brake line 12 belonging to the pneumatic brake circuit I. 
The other side of the two-way check valve 13 is connected via a pressure 
control valve 17 to the supply tank 5 through a short line. Parallel to 
the pressure control valve 17 is a pressure control valve 18 of the same 
type, which is indented for supplying two rear-axle brake cylinders 19 and 
20 in the brake circuit III. 
For their actuation, the two pressure control valves 17 and 18 each have an 
electromagnet 21 and 22 respectively, which are connected via respective 
electrical lines 23 and 24 to a load-dependent switching member 25 which 
is electronically controlled by a load sensor 25'. The switching member 25 
is triggered by an electronic control unit 26, which is connected via an 
electrical line 27 to the electric current transducer 8. 
A magnetic valve 28 is disposed following control unit 26 with which a 
control valve 29 for the brakes on a trailer can be electrically 
triggered. 
MODE OF OPERATION 
Upon the actuation of the braking value tranducer 7, both the variable 
electric current transducer 8 and the fluid valve member 9 are switched 
over. The current reaches the electronic control unit 26 via the line 27 
and is converted there; then a braking value is imposed by the 
load-dependent switching member 25, and the product is carried via the 
lines 23 and 24 to the electromagnets 21 and 22. These elements can be 
switched through in accordance with the prevailing times and 
electric-current intensities. The electromagnets 21 and 22 switch the 
pressure control valves 17 and 18 over, and stored air is directed in a 
controlled manner from the supply tanks 5 and 6 on a short path to the 
brake cylinders 14, 15 and 19, 20. Braking is effected with appropriately 
metered force. In the case of the front-axle brakes, a second position of 
the two-way check valve 13 is thereby blocked off. 
In a slower manner, as compared with the electric-current actuation, 
compressed air passes from the reservoir 11 via the valve member 9 into 
the brake line 12. However, since the two-way check valve 13 has already 
been switched over, this imposition of pressure has no effect. 
However, should the electrical system fail, then as a result of the 
pressure increase in the brake line 12, the two-way check valve 13 is 
switched over into its other terminal position, and air for braking 
cylinders 14 and 15 flows into the brake cylinders 14 and 15 of the front 
axle 16 of the vehicle, so that it remains possible to bring the vehicle 
to a stop. 
FIG. 2 shows a further development of the design according to FIG. 1; the 
same reference numerals as in FIG. 1 are used for corresponding elements. 
An electronic switching member 30 of an anti-skid apparatus is disposed 
following the switching member 25 and receives signals from wheel rpm 
sensors 30'. Two pressure control valves 17' and 18' are embodied in the 
dual-channel manner; that is, they each have two separate outputs 31, 32 
and 33, 34, respectively, and one two-way check valve 13 and 13' is 
provided for each of the two outputs 31 and 32 of the front-axle brake. 
For anti-skid monitoring, a 3/2-way valve 35 is also inserted into the 
pressure-medium brake line 12; this valve 35 is actuatable by a 
electromagnet 36, which is connected to the electronic switching member 
30. Finally, a pressure reduction valve 37 is also inserted into the line 
12, in order that on the one hand, the pressure of the pneumatic circuit 
will be restrained in the two-way check valves 13 and 13' in comparison 
with that of the electric circuit, while on the other, it will 
nevertheless still be possible to supply the full pressure to the trailer 
control valve 29. 
By using the 3/2-way valve 35, a complete anti-skid control can be 
established, with reduction, maintenance, or increase of a given pressure 
level. This control functions in the dual-channel manner for both the 
front and rear axles; however, for the rear axle it functions only with an 
electric brake circuit. 
If operation is intended with both an electric and a pneumatic brake 
circuit at the rear axle as well, than a branch line 38 of the 
pressure-medium brake line 12 must be provided leading to the rear-axle 
brake cylinders 19 and 20, such as is shown in FIG. 3, and one two-way 
check valve 40 and 41 must be disposed preceding each rear-axle brake 
cylinder 19 and 20. 
With such a design, it is then possible to actuate both the front-axle and 
the rear-axle brakes via the pneumatic circuit on an auxiliary basis, if 
the electrical system should fail. The anti-skid apparatus can be imposed 
in this case as well. 
The foregoing relates to preferred exemplary embodiments of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.