Electromagnetic proportional valves for manifold applications are known. In such a proportional valve, to achieve a correlation and preferably a linear dependency between an input variable and an output variable, the output variable is fed back to the input variable, so that for each input variable a corresponding equilibrium is established, resulting in a precisely defined relationship between the input variable and the output variable.
In the case where the proportional valves are used for motor vehicle brake systems for the sake of controlling a brake booster for automatic intervention into the brake system, for instance in the case of an electronic stability program or automatic adaptive cruise control, the input variable of the proportional valve is adjusted to the current supplied to the proportional magnet; the current is set by a control unit, for instance by voltage tracking or by pulse width modulation of a voltage. The output variable is the pressure level controlled in the brake booster. Since the maximum available potential for the negative-pressure brake boosters generally used is the difference between the ambient pressure and the pressure in the intake system of the engine, the maximum usable working pressure in Otto engines is only 0.8 bar (with the throttle valve closed). For the required high-precision metering of the braking action, a very precisely operating proportional valve is therefore necessary that adjusts the brake pressure with the least possible hysteresis.
In a known electromagnetic proportional valve of this generic type (European Patent Disclosure (EP 0 682 615 B1), the movable valve member and the sealing element are embodied integrally as a resilient rubber tube, which is reinforced on the inside with a metal ring in the region of the valve member. The free end of the tube, remote from the ring, is secured to the valve body in an airtight fashion. Such so-called rubber collars exhibit a temperature-dependent deviation in terms of their hysteresis, which has an adverse effect on the aforementioned purpose of high-precision metering of the braking action.
Proportional valves are also known in which the sealing element is embodied as a sliding seal, which seals off the valve member of the fixed valve from a chamber wall disposed between the two valve chambers. O-rings or lip seals are used as these sliding seals. Sliding seals of this kind have what is known as a stick-slip effect; that is, on running up against the valve member they have very strong adhesion, which decreases markedly when they slide. Once again, this makes fine metering of the pressure controlled in the brake booster impossible.