Tech Briefs
Integrated pressure sensor for brake-booster monitoring

Figure 1. Simplified schematic of a vacuum brake booster. Figure 2. Motorola's MPXV4115V integrated pressure sensor is for vacuum-pump and brake-booster monitoring and other applications employing a microcontroller with A/D inputs.

Vehicular braking systems are either activated by the driver's effort on a hand lever or pedal, which is directly transmitted via a hydraulic link to the brake pad, or are partially or completely power assisted. Power-assisted brakes are found on most passenger cars and some light-duty trucks, with the driver's effort amplified by a brake booster to increase the force applied to the brake pedal.

A vacuum brake-booster system uses the difference between atmospheric pressure and a lower pressure (vacuum) source to assist braking operation. The brake booster is located between the brake pedal and the master cylinder (Figure 1). When no pressure is applied on the brake pedal, the air intake valve is closed and the vacuum valve open. Thus both the vacuum and working chambers are at the same pressure, typically around 70 kPa (10 psi) below atmosphere.

On most passenger cars vacuum is generated within the engine itself, either by the intake manifold or an auxiliary vacuum pump. When the engine throttle valve is closed, the displacement of the pistons produces vacuum in the intake manifold. With a tube or hose connected between the engine intake manifold and the brake booster, vacuum can be applied to the chambers. A valve inserted between the intake manifold and the booster maintains the vacuum in the booster when the engine throttle valve is open.

Once the brake pedal is activated, the vacuum valve is closed and the air intake valve is opened in proportion to the displacement of the push rod. The working chamber is open progressively to atmospheric pressure, which creates a differential pressure between the vacuum chamber and the working chamber. The pressure applied to the surface of the piston results in forces that are then applied to the brake pads through the master cylinder and hydraulic links. When the brake pedal is released, the spring moves the piston back, closing the air intake valve and opening the vacuum valve to rebalance the pressure between the two chambers.

However, this principle can only be used on engines that have the ability to generate enough vacuum. Diesel engines, which have no throttle valve, need an auxiliary pump to generate vacuum. This is also the case for gasoline-direct-injection (GDI) engines, in which the electrically assisted throttle valve is maintained slightly open during idle or lean-burn driving conditions, limiting the amount of vacuum generated by the displacement of the piston. On other gasoline engines, the vacuum for the brake booster is generated by the intake manifold. But the amount of the vacuum in the brake booster is not known by the braking system, thus the amount of amplification is not known. Also, there is no possibility for the brake system to interact with the intake manifold if additional amplification is required for severe braking.

Motorola has introduced a new integrated pressure sensor, the MPXV4115V, for vacuum measurements in applications such as brake-booster monitoring. The single-chip vacuum sensor may be placed directly onto the pump electronic control unit or integrated as a component within the brake booster. The advantage of having the vacuum generated by an auxiliary pump is that the brake system then manages the amount of vacuum, thus amplification, with the braking system operation becoming independent of the engine's. Pressure variations during braking are measured, and the pump activated to generate additional vacuum if required to increase braking force.

Motorola's pressure sensors are based on technology that consists of a silicon micromachined diaphragm and a diffused piezoresistive strain gauge. When vacuum or pressure is applied on the die, the diaphragm is deformed and stressed. An excitation current then passes through the gauge and a voltage proportional to the applied pressure and excitation current appears between the voltage taps. The MPXV4115V is a monolithic, amplified, and signal-conditioned sensor that uses thin-film memorization and bipolar semiconductor processing to provide a high-level analog output signal proportional to the applied pressure.

Jean L. Broge