Understanding flap actuators and what drives them in automotive HVAC systems

This blog post was updated on April 16, 2019.

Whether it’s hot or cold outside, passengers inside a car are comfortable thanks to automotive heating and cooling systems. These heating, ventilation and air conditioning (HVAC) systems vary in complexity and level of automation depending on the vehicle class. While an economy car requires that a driver manually turn knobs to control the temperature, a higher-end vehicle uses sensors to automatically control not only the temperature but also the humidity and quality of air inside the cabin.

Moving the air

Regardless of vehicle class, an automotive HVAC system is all about moving the air from here to there. It conditions the air as well, modifying its temperature, humidity and quality while moving it.

Let’s look at simply moving the air for a second. The air could be drawn into the system from outside or inside the cabin. It could also be routed inside the HVAC system through the evaporator or heat exchanger for conditioning; that conditioned air is distributed throughout the cabin, whether it is to keep passengers’ feet warm or prevent the windshield from fogging.

The number of ways that the air can flow is remarkable: from outside to the evaporator to the windshield, or from inside to the heat exchanger to the vent in the footwell. So how does an HVAC system control which way the air goes?

Figure 1 is a side view of an HVAC system. Numbers mark the key components, while arrows show the direction of air distribution. Components 4 through 8 in Figure 1 show the flap actuators. The orange dotted lines represent the area where the flaps are moving, and the solid orange lines represent the flaps. The number of flap actuators in an HVAC system depends on the overall complexity of the system – if it has single or multiple-zone HVACs.

Figure 1: A car HVAC with eight components: 1 = blower, 2 = evaporator, 3 = heater, 4 = intake air flap, 5, 6 and 7 = air-distribution flap, 8 = air-mixing flap

Flap actuators

The air flows in an HVAC system through ducts or pipes; a flap is used to open or close, either fully or partially, certain portions of the duct in order to control which way the air goes. A flap actuator, also called a damper, is simply an electrical machine that moves the flap.

There are three types of flap actuators in an automotive HVAC system:

  • Intake air-flap actuator (component 4 in Figure 1): This flap actuator controls whether the source of air for conditioning will be outside air or recirculated cabin air. This flap actuator position is controllable by the driver using the recirculate button, or by the HVAC system using data from in-cabin air-quality sensors.
  • Air-mixing flap actuator (component 8 in Figure 1): This flap actuator mixes the warm (heat exchanger) and cool (evaporator) air in order to achieve the desired air temperature.
  • Air distribution flap actuator (components 5, 6 and 7 in Figure 1): These flap actuators, which could vary in number based on the vehicle class, distribute the air inside the cabin.

DC motor

What electrical machine moves the flap? Just as there are choices for controlling the flow of air, automakers have choices for the electrical machines that move the flaps. Potential choices include brushed DC motors with potentiometers to sense the position of the flap, three-phase brushless DC (BLDC) motors that use back electromotive force (back EMF) to measure positions or stepper motors that count the number of steps to measure the positions. These DC motors drive the flap through gears of varying sizes.

More choices

Having chosen the motor, HVAC systems engineers also have a choice of architectures for driving the motor. As I’ve mentioned, flap actuators can be controlled locally or remotely. For local control, the electronics to control the motor are located near the motor, which means that the motor control IC is integrated in the same housing where the motor itself is located (see flap actuator control in Figure 2). Communication protocols such as Local Interconnect Network (LIN) control the motor so that the flap is driven to a certain position. For remote control, the electronics to control the motor are located on the HVAC control unit (see Figure 3), which is away from the flap actuator. Communication between the motor driver and the microcontroller on an HVAC control unit could be realized with a serial peripheral interface (SPI) or even simpler with a parallel digital control interface. Texas Instruments’ DRV8912-Q1 is an example of a device that interfaces with the microcontroller via SPI and is used to can be used to drive the flap actuator remotely.

Figures 2 and 3 illustrate the two possible architectures. The architecture in Figure 2 is more complex than the one in Figure 3; however, the architecture in Figure 2 offers more design scalability and flexibility.

Figure 2: Remote control of flap actuator motor

Figure 3: Integrated motor driver for flap actuator
Even more choices
Let’s talk about the connection between the microcontroller and the motor driver control integrated circuit. HVAC system designers have choices regarding this connection as well. The microcontroller can connect to the motor driver using a digital communication interface such as SPI or can directly connect to the motor driver using control lines. Figures 4 and 5 illustrate these choices.
Figure 4: A microcontroller communicating with the motor driver using SPI

Figure 5: A microcontroller controlling the motor driver directly


Keeping it simple

The driver electronics for BLDC and stepper motors are more complex than the electronics needed to drive a brushed DC motor. If you choose to use brushed DC motors for moving the flaps, using a motor driver that directly drives the flap motor has a clear advantage – it is simpler both in hardware and software.

Additional resources to help you design automotive HVAC subsystems: