MPXM2102AS datasheet(560/670 Pages) MOTOROLA

560 / 670 page

AN4007

3–414

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 2. Braking Phase

Fp + Fb

Once the brake pedal is activated (force Fp), the vacuum

valve is closed and the air intake valve is open proportionally

to the displacement of the push rod (Figure 2). The working

chamber is progressively open to atmospheric pressure,

which creates a differential between the vacuum chamber and

the working chamber. This differential pressure applied to the

surface (S) of the piston results in a force

Fb = (Pw – Pv) x S.

The forces Fb + Fp 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.

VACUUM GENERATION

On most passenger cars, vacuum is generated by the

engine itself. When the engine throttle valve is closed, the

displacement of the pistons produces vacuum in the intake

manifold. Thanks to a tube or hose connected between the

engine intake manifold and the brake booster, vacuum can be

applied to the chambers. A backslash valve inserted between

the intake manifold and the booster maintains the vacuum in

the booster when the engine throttle valve is open.

This principle has some limitations, however. For example,

it can be only used on engines that have the ability to generate

enough vacuum. On diesel engines, which have no throttle

valve, it is necessary to use an auxiliary pump to generate

vacuum. This will also be the case on the Gasoline Direct

Injection (GDI) engine, where in some driving conditions (idle,

lean burn) the electrically assisted throttle valve will be

maintained slightly open.

In this situation, the vacuum

available on the intake manifold is not sufficient to provide an

efficient braking.

Figure 3. Vacuum Pump Monitoring

PUMP CONTROL

CIRCUIT

ELECTRICAL

VACUUM

PUMP

VACUUM FEEDBACK

VACUUM GENERATION

PRESSURE

SENSOR

BUS INTERFACE

Therefore, it is necessary and desirable to use an electrical

pump that will generate the vacuum for the brake booster. The

use of an auxiliary electrical pump (Figure 3) provides several

advantages over the “intake manifold” vacuum.

• Vacuum generation is no longer related to the engine

running condition. Vacuum is only generated and con-

trolled by the pump thanks to a vacuum pressure sensor

that provides an accurate reading to the pump electrical

control circuit.

• The electrical pump can be switched on and off based on

the required vacuum. To compensate atmospheric pres-

sure variation in order to maintain a constant booster

effect, the pump also can be switched on independently

from the atmospheric pressure. Various algorithms for

driving the pump can be implemented depending on the

required braking conditions.

• Pressure variations during braking can be measured,

and the pump can be activated to generated additional

vacuum if required to increase the braking force.

• Leakage can be detected by the pressure sensors and

the pump can be switched on to compensate them. The

driver can be informed of any type of failure thanks to the

bus interface. Vacuum level, and thus available braking

force can be communicated through the bus to other

braking systems such as, for example, ABS or ESP.

Motorola,

worldwide

leader

automotive

semiconductors, has introduced a new integrate pressure

sensor dedicated to 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 component within the brake

booster, thus providing flexibility, system integration and

reduced system cost.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 560 Page - Motorola, Inc

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