Monitoring a hybrid drive system, function - GF08.30-P-5001FLH

ENGINE 276.952 in MODEL 212.095 up to model year 2014 

ENGINE 651.924 in MODEL 212.098/298 up to model year 2014 

Function requirements for monitoring a hybrid drive system, general points

• The hybrid system control units are activated by the wake-up function.

Monitoring a hybrid drive system - general points 

Monitors and limits the system monitoring as part of driving safety during acceleration the drive torque of the vehicle on the basis of the driver specification by means of the accelerator pedal position.

Function sequence for monitoring a hybrid drive system 

The whole monitoring concept is integrated into the CDI control unit (N3/9) (for a diesel engine) or the ME-SFI [ME] control unit (N3/10) (for a gasoline engine), in the power electronics control unit (N129/1) as well as in the battery management system control unit (N82/2). The monitoring concept for the CDI control unit or the ME-SFI [ME] control unit is subdivided into the following three levels:

• Function level (Level 1)
• First monitoring level (Level 2)
• Second monitoring level (Level 3)

While all implemented functions of the hybrid system such as torque coordination, energy management, sensor and actor actuation implemented in the CDI control unit or the ME-SFI [ME] control unit are in the function level, the further levels serve to monitor the system and thus achievement of functional reliability and plausible behavior of the vehicle.

Further safety systems such as deactivation of the hybrid system, temperature monitoring of components of the hybrid system and monitoring of regenerative braking systems are described in more detail in the following documents:

• Energy management for hybrid drive system, function 
• Deactivation of hybrid drive system, function 
• High voltage on-board electrical system cooling, function 
• Automatic engine stop, function 
• Automatic engine start, function 
• Emergency brake mode, function 

These two monitoring levels are described in more detail below:

• Function sequence for the interlock circuit 
• Function sequence for monitoring the control units functions 
• Function sequence for hardware-implemented monitoring of the control units monitoring 

Function sequence for the interlock circuit 

The interlock circuit is used as protection to protect people against inadvertent contact with contacts of the high voltage components of the high voltage on-board electrical system which are carrying a high voltage.

A 12 V/88Hz interlock signal is looped here through all high voltage on board electrical system components that are to be dismantled or opened. To do this there is a contact bridge in each removable high voltage plug connection which interrupt the interlock circuit during removal of the high voltage plug connection. The interlock circuit is also led switched in a series over the 12 V control units plug connection of the high-voltage components.

When detaching a control unit plug connection the interlock circuit is interrupted via the contacts Interlock input and output. Interruption of the interlock circuit causes opening of the contactor (A100s1) in the high-voltage battery module (A100) and powering down of the whole high voltage on-board electrical system, in particular shutoff of the power electronics control unit.

The Interlock signal is generated in the battery management system control unit and led in a series connection via the following components:

• Power electronics control unit
• High voltage distributor plate (contact bridge)
• Electrical machine (A791)
• Electrical refrigerant compressor (A9/5)

The battery management system control unit and power electronics control unit are equipped with an evaluation circuit for the interlock signal.

Fig 1: Battery Management System Interlock Circuit

Courtesy of MERCEDES-BENZ USA

Function sequence for monitoring the control units functions 

The first monitoring level (Level 2) contains the nominal-actual comparison from the driver's request torque (accelerator pedal position) and the overall drive torque (sum of all torques from the internal combustion engine and the electrical machine). Level 2 in the CDI control unit or the ME-SFI [ME] control unit checks whether the generated overall drive torque is greater than that requested by the driver via the accelerator pedal sensor (B37). The CDI control unit or the ME-SFI [ME] control unit assumes there is a fault if this is the case and the system changes over into the limp-home function of the internal combustion engine (rpm limitation 1500 rpm).

Level 2 in the power electronics control unit also checks whether the drive torque requested by the CDI control unit or the ME-SFI [ME] control unit via the hybrid CAN (CAN L) is always larger than the motor generated torque of the electrical machine calculated on the basis of the currents and the position angle of the rotor. The electrical machine is switched to passive by the power electronics control unit if this is the case.

Function sequence for hardware-implemented monitoring of the control units monitoring 

The second monitoring level (Level 3) is based on self-contained. hardware monitoring. It consists of a control monitor which checks the basic function of the first monitoring level in the CDI control unit or the ME-SFI [ME] control unit (checking the checker).