In hydraulic vehicle braking systems which have anti-slip regulation, the brake circuits are usually connected to a brake master cylinder via separating valves and are separated hydraulically from the brake master cylinder by closing the separating valves during an anti-slip regulation operation. As a result, feedback effects of the anti-slip regulation on the brake master cylinder are avoided, in particular a pulsing brake pedal on account of a pulsing delivery of brake fluid into each brake circuit by means of a hydraulic pump which is usually configured as a piston pump. In the reverse case, a disruption of a build-up of brake pressure and brake pressure regulation by the brake master cylinder are also avoided during pressure regulation by closing the separating valves. Anti-slip regulation means are known as anti-lock braking system, acceleration-slip regulation and vehicle-dynamics regulation or electronic stability program; the abbreviations ABS, ASR, FDR and ESP are customary. The list is not exhaustive.
The separation of the brake master cylinder from the brake circuits of the vehicle braking system by closing of the separating valves has the consequence that a brake fluid volume is shut in the brake master cylinder and as good as no pedal travel or, in more general terms, no actuating travel is possible at the brake master cylinder on account of the incompressibility of the brake fluid. The brake pedal which feels “hard” and virtually immovable unsettles a vehicle driver and makes the metering of a pedal force or, in general, an actuating force difficult, that is to say a muscular force which is exerted by the vehicle driver for brake actuation. To make matters worse, there is the additional situation that the “hardened” brake pedal regularly occurs in a driving situation which is critical anyway, when the anti-slip regulation intervenes.
Pedal travel simulators are therefore known. These are usually spring-loaded hydraulic accumulators which are connected to the brake master cylinder and into which the brake master cylinder displaces brake fluid when it is actuated. The pedal travel simulator makes an actuating travel possible at the brake master cylinder when the latter is separated hydraulically from the brake circuits by closing of the separating valves. A spring element of the hydraulic accumulator generates a pedal force; an at least approximately usual pedal feel is possible, that is to say the usual dependence of an actuating force on the actuating travel as in a brake actuation without anti-slip regulation. The spring loading can also take place, for example, by gas pressure, and the hydraulic accumulator can therefore be a gas pressure accumulator.
In order that no brake fluid is displaced out of the brake master cylinder into the pedal travel simulator in the event of a normal brake actuation without anti-slip regulation, a simulator valve can be provided between the pedal travel simulator and brake master cylinder, which simulator valve separates the pedal travel simulator from the brake master cylinder and, when the separating valves are closed, connects it to the brake master cylinder. Solenoid valves are used as simulator valves.
A further application for a pedal travel simulator is electrohydraulic vehicle braking systems. These are power-assisted vehicle braking systems, in which the brake pressure is generated by power assistance from a hydraulic pump and the brake master cylinder is likewise separated hydraulically from the vehicle braking system by closing a separating valve in each brake circuit. In the case of a disruption, auxiliary braking by way of the brake master cylinder is possible by all the separating valves remaining open.