“Whether it’s hot summer or cold winter, passengers can always enjoy a comfortable interior environment with the car’s heating and cooling system. The complexity and degree of automation of these heating, ventilation and air conditioning (HVAC) systems vary across different classes of vehicles. Economy cars may require the driver to manually turn a knob to control the temperature, while in high-end vehicles, sensors can automatically control the temperature inside the car as well as the humidity and quality of the air at the same time.
Whether it’s hot summer or cold winter, passengers can always enjoy a comfortable interior environment with the car’s heating and cooling system. The complexity and degree of automation of these heating, ventilation and air conditioning (HVAC) systems vary across different classes of vehicles. Economy cars may require the driver to manually turn a knob to control the temperature, while in high-end vehicles, sensors can automatically control the temperature inside the car as well as the humidity and quality of the air at the same time.
Regardless of the class of vehicle, automotive HVAC systems exchange air, changing its temperature, humidity, and quality in the process.
Let’s take a look at the principle of air flow. Air can be drawn into the system from outside or inside the cabin. It can also be regulated by entering the HVAC system through the evaporator or heat exchanger; conditioned air is distributed throughout the cabin, keeping passengers feet warm and preventing windshield fogging.
Air flows in many ways: from the outside to the evaporator to the windshield, or from the inside to the heat exchanger to the vents in the bottom of the cabin. So how does an HVAC system control the way air flows?
Figure 1 is a side view of an HVAC system. Key components are numbered and arrows indicate the direction of air flow. Components 4 to 8 in Figure 1 are shown as damper actuators. The orange dotted line represents the area where the damper moves, while the orange solid line represents the damper. The number of damper actuators in an HVAC system depends on the overall complexity of the system – whether it is a single-zone or multi-zone HVAC.
Figure 1: Automotive HVAC consisting of eight components: 1 = blower, 2 = evaporator, 3 = heater, 4 = intake damper, 5, 6 and 7 = air distribution damper, 8 = air mixing damper
Air flows in an HVAC system through ducts; dampers control how air flows by fully or partially opening or closing segments of the duct. A damper actuator (also called a damper) is an electrical device that moves the damper.
There are three types of damper actuators in automotive HVAC systems:
Intake damper actuator (part 4 in Figure 1): This damper actuator controls the source of conditioned air – outside air or recirculated air inside the vehicle. This damper actuator position can be controlled by the driver using the recirculation button or by the HVAC system based on data from the in-vehicle air quality sensor.
Air Mix Damper Actuator (Item 8 in Figure 1): This damper actuator mixes warm air (heat exchanger) and cold air (evaporator) to achieve a set air temperature.
Air distribution damper actuators (parts 5, 6 and 7 in Figure 1): The number of these damper actuators varies according to the vehicle type and is used to distribute the air in the cabin.
Which electrical device is responsible for driving the damper? Just as there are multiple ways to control air flow, automakers have multiple options for the electrical equipment that drives the dampers. Includes brushed DC motors with potentiometers for sensing damper position; three-phase brushless DC (BLDC) motors that use back electromotive force (back EMF) to measure position or stepper motors that measure position by counting steps. These DC motors drive the dampers through gears of different sizes.
After selecting the motor, the HVAC system engineer can also choose the architecture that drives the motor. As mentioned earlier, damper actuators can be controlled locally or remotely. In local control, the electronics that control the motor are located near the motor, i.e. the motor control IC is integrated in the same housing as the motor (see damper actuator control in Figure 2). A communication protocol such as the Local Interconnect Network (LIN) controls the motor to drive the damper to a specific position. In remote control, the electronics that control the motor are located in the HVAC control unit remote from the damper actuator (see Figure 3). Communication between the motor driver and the microcontroller on the HVAC control unit can be achieved through a serial peripheral interface (SPI) or even a parallel digital control interface.
Figures 2 and 3 illustrate two possible architectures. The architecture in Figure 2 is more complex than the architecture in Figure 3; however, the architecture in Figure 2 provides greater design scalability and flexibility.
Figure 2: Remote control of the damper actuator motor
Figure 3: Integrated motor driver for damper actuator
Let’s look at the connections between the microcontroller and the motor driver control IC. HVAC system designers also have several options for this connection. The microcontroller can be connected to the motor driver using a digital communication interface such as SPI, or it can be directly connected to the motor driver using control lines. Figures 4 and 5 illustrate these options.
Figure 4: Microcontroller using SPI to communicate with motor driver
Figure 5: Microcontroller that directly controls the motor driver
The driver electronics for BLDC and stepper motors are more complex than those required to drive brushed DC motors. If you choose to use a brushed DC motor to move the damper, there are clear advantages to using a motor driver that drives the damper motor directly – simpler in both hardware and software.