Actuators and Sensors
Actuators and sensors
The following lines are connected to the a/c control panel (control unit) to supply the necessary inputs for individual functions:
- Terminal 31
- Terminal 30
- Signal ground
- Reference voltage 5.0 V
- CAN interface
The remainder of the air conditioning system breaks down into actuators and sensors. The software of the control panel (control unit) monitors all of the system's measurement and control operations.
The sensor system comprises the following components:
- Interior temperature sensor (integrated into the control panel)
- Temperature sensor for fresh air intake (in the fresh air intake port)
- Temperature sensor for footwell air outlet
- Sun sensor
- Check-back signals from the drive motors (see section "Air and flap controls")
- Check-back signal - actual fan voltage
- Check-back signals - a/c compressor status (compressor voltage)
- Pressure sensor
Interior temperature sensor
The interior temperature sensor which is integrated in the middle of the a/c control panel (behind the slotted cover) ensures that the automatic air conditioning control system responds to temperature changes with a substantially greater degree of precision. In order to avoid measuring errors resulting from heating-up of the control panel area, the interior air is extracted over the sensor by means of a micro-fan.
Temperature sensor for fresh air intake
The fresh air intake temperature serves as a reference variable to control the compressor.
The outside temperature also serves as a control variable. This value is supplied from the front-end control unit via the CAN bus. The lower of the two measurements from the fresh air intake sensor or the outside air sensor (behind the ribs on the right-hand opening at the front end) is employed to assess the system (system outside temperature).
In the case of a falling temperature, assessment of the system outside temperature is subject to a time constant of t = 1 second. When the system
outside temperature rises, a vehicle speed-dependent filter is employed. At speeds under 15 km/h, the evaluation of temperature changes is "frozen". This prevents stop-and-go traffic from leading to incorrect system outside temperature values (incorrect measurements due to heat emitted from the engine, for example).
Sun sensor
This sensor is located in the middle of the dashboard.
The sensor is supplied with a reference voltage of 5.0 V. The measured values are subject to 2 different time constants so as to rule out any disruption in the system if the sun intensity changes briefly (e.g. driving through a tunnel, on tree-lined streets, etc.). This means that the system only responds to changes in sun intensity after this time has elapsed. A different mean time constant applies for increasing and decreasing sun intensity respectively.
The evaluation of these changes in sun intensity is employed to control the drive motor of the temperature mix flap and to correct the air flow rate (fan adjustment).
Pressure sensor
The pressure sensor is an essential instrument in assessing the air conditioning system. It detects the current pressure in the system and supplies a PWM signal (Pulse Width Modulation) to the a/c control unit. The a/c control unit relays this signal to the CAN bus for further use.
This PWM signal is employed for the purposes of
- low-pressure shut-down (p < 2 bar)
- high-pressure shut-down (p > 28 bar)
- condenser cooling (see also section "Condenser fan")
The following components serve as actuators:
- Drive motor for temperature mix flap
- Drive motor for central distributor flap
- Drive motor for defrost/footwell distributor flap
- Drive motor for fresh air/recirc. air distributor flap
- PWM - fan motor voltage
Main state variable (Energy Requirement Level)
When the air conditioning system is in operation, all control functions for the system are dependent on this main state variable, ERL.
The main state variable to determine the thermal energy requirement level in the vehicle, in the form of the ERL signal, is derived from the key influencing variables:
- Set interior temperature
- Actual interior temperature
- Outside temperature
- Solar irradiation energy
- A/c compressor on/off
... the main state variable for determining the thermal energy requirement in the vehicle - the ERL signal - is derived.
The ERL state variable varies as a digital number in the range from 0 to 255 (8 bit).
This corresponds to the extreme cases of "0" for maximum cooling requirement and �255" for maximum heating requirement.
The influencing variables 'coolant temperature' and 'engine running time' serve as additional reference values to adapt the control functions in special cases, e.g. start-up of the air conditioning, cold start, hot start of the vehicle.
Temperature mix flap
The temperature mix flap is presented here as an example of flap control based on the main state variables and also serves to illustrate how all the other flaps function.
Cooling mode applies below state variable ERL < 85.
The command variable for the mix flap position is directly dependent on the result of the flap position calculation (check-back from potentiometer of the drive motor).
Heating mode applies above ERL 150.
In this case, the position of the mix flap is determined solely by a subordinate footwell air outlet vent temperature sensor.
At ERL > 85, the mix flap control system is overridden by the initiated footwell air outlet vent temperature sensor.
Diagram - Temperature mix flap position
(without footwell air outlet vent temperature sensor)
Air flow control (fan)
The air flow rate is set by means of control programmes according to the ERL state signal by means of voltage control of the fresh air fan. Corresponding characteristics are defined both for automatic mode and for manually selected air distribution.
NOTE: The air throughput is influenced by the back pressure which arises in the area of the fresh air inlet according to the vehicle speed. In order to compensate for this dependency, the distributor flap for fresh air/recirculated air (back pressure flap) is moved in the direction of "closed" (recirculated air) according to the vehicle speed (from 90 km/h). The flap setting is also dependent on the fan voltage.
Condenser fan (cooling water fan)
The required speed for the cooler fan is calculated in the DME engine control unit according to the coolant pressure (supplied via CAN from the a/c control panel) and the cooling water temperature sensor (see also section "DME"). The DME control unit sends this speed data via CAN Drive to the gateway and via CAN Comfort to the front-end control unit. The front-end control unit controls the cooling water fans (right and left) together via an output module. To this end, the front-end controller transmits a pulse width-modulated (PWM) signal to this output module. The output module evaluates and processes the PWM signal and relays it to the fans.
For the purposes of the air conditioning system, the fans are controlled in linear mode according to the recorded value from the coolant pressure sensor.