If the wheels on one axle are stationary and those on the other axle are turning, the system detects that a test is taking place and
switches to the corresponding mode. If the wheels on the front axle are being driven by the roller, the parking brake is engaged. If
the wheels on the rear axle are being driven, the parking brake remains disengaged. This allows the service brakes to be tested as
usual. The electromechanical parking brake test corresponds to the emergency braking procedure.
Integration of hydraulic brake system in vehicle’s recuperation system
A braking procedure initiated by the driver can be achieved 100 % hydraulically, 100 % electrically or using a mix of the vehicle’s
hydraulic and electric braking power. The main conditions for recuperation are the ability of the battery to take in electric energy, a
dynamic driving state suitable for recuperation and a suitable vehicle speed. As a result, the system predominantly generates hy-
draulic brake pressure in corners as it is more likely that stabilising brake inputs (ESC regulation) will be needed. Regulation on
specific wheels can only be performed by the hydraulic brake system. Even if each wheel were driven by a separate electric motor,
the regulation of the electric motors would not be fast enough. Hydraulic brake pressure is also generated under braking at low
vehicle speeds in addition to recuperation. This ensures that the vehicle can be brought to a standstill safely. Autonomous braking
procedures (e.g. by the adaptive cruise assist, stop and go) are also supported by the ESC in conjunction with the electromechanical
brake servo. If braking procedures are required by/for assist systems where there is a certain level of danger potential (e.g. in
emergency braking procedures), any recuperation currently in progress is switched to a fully hydraulic braking procedure.
To ensure that the battery has a good level of charge and by extension a large range, the system recuperates energy as often as
possible. This also means that braking procedures with low to medium vehicle decelerations (up to approx. 3 m/s2) are usually per-
formed electrically. The driver will not be able to tell whether the braking procedure which he/she has initiated is being implemen-
ted electrically, hydraulically or as a mix of the two.
The decision as to which system contributes to what extent to providing the necessary braking torque is complex. Running gear
control unit J775, ABS control unit J104 and motor control unit J623 are primarily involved. The motor control unit permanently
transmits information on the current maximum recuperation capacity/braking power to the ABS control unit. If the driver presses
the brake pedal or an assist system (e.g. adaptive cruise assist) requests braking deceleration, the ABS control unit determines
whether the braking procedure required can be performed fully electrically via recuperation or whether additional hydraulic braking
power needs to be provided. It transmits the specified generator torque to the motor control unit. This is the amount of braking
power required from recuperation. At the same time, it sends a message to the running gear control unit with the calculated “ide-
al” distribution of the braking torque on the front and rear axle. As the running gear control unit is continuously determining the
current dynamic driving state, it coordinates the transition between acceleration and overrun recuperation phases and recuperation
distribution. It sends a specification for the torque distribution to the motor control unit, which then activates the power electron-
ics of the electric drive motors accordingly. The electromechanical brake servo works together with the ABS control unit and the
ESC hydraulic unit if hydraulic brake pressure also needs to be generated. If brake regulation procedures are required during an
active recuperation phase, the ABS control unit can, in the same way, initiate a reduction in recuperation or end it completely. At
the same time, the system generates hydraulic brake pressure which can be used (if hydraulic brake pressure has not already been
generated) to initiate regulation procedures for individual wheels.
As the proportion of hydraulic braking procedures performed by the service brakes is significantly lower in normal usage than on
vehicles with conventional drive systems, the hydraulic brake system is activated in certain situations even though an electric brak-
ing procedure using recuperation would also have been possible. If the vehicle has been stationary for a longer period (> 6 hours),
the first braking procedures are made hydraulically by the service brakes.
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