Understanding damper actuators and what drives them in automotive HVAC systems

Whether it's hot in the summer or cold in the winter, passengers can always enjoy a comfortable interior environment through the car's heating and cooling systems. The complexity and degree of automation of these heating, ventilation and air conditioning (HVAC) systems vary in different vehicle categories. 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.

Air flow

Regardless of the class of vehicle, automotive HVAC systems need to exchange air and, in the process, change its temperature, humidity, and quality.

Let's look at how air flows. Air can be drawn into the system from outside or inside the cabin. It can also enter the HVAC system through an evaporator or heat exchanger to be conditioned; the conditioned air is distributed throughout the cabin to keep passengers' feet warm or prevent the windshield from fogging up.

There are many ways that air can flow: from the outside to the evaporator and then to the windshield, or from the inside to the heat exchanger and then to the vents in the floor of the cabin. So how does the HVAC system control the way air moves?

Figure 1 shows a side view of an HVAC system. Key components are numbered and arrows indicate the direction of air flow. Components 4 through 8 in Figure 1 are shown as damper actuators. The orange dashed lines indicate the zones where the dampers move, while the solid orange lines represent the dampers. The number of damper actuators in an HVAC system depends on the overall complexity of the system - whether it is single-zone or multi-zone HVAC.

Figure 1: Automotive HVAC consists of eight components: 1 = blower, 2 = evaporator, 3 = heater, 4 = air intake damper, 5, 6 and 7 = air distribution dampers, 8 = air mixing damper

Damper Actuator

Air moves through HVAC systems through ducts; dampers control how the air moves by fully or partially opening or closing sections of the ducts. Damper actuators (also called dampers) are electrical devices that move dampers.

There are three types of damper actuators in automotive HVAC systems:

Inlet flap actuator (item 4 in Figure 1): This flap actuator controls the conditioned air source - outside air or recirculated air from the vehicle. The flap actuator position can be controlled by the driver using the recirculation button or by the HVAC system based on data from the air quality sensor in the vehicle.

Air Mixing Flap Actuator (item 8 in Figure 1): This flap actuator mixes warm air (heat exchanger) and cool air (evaporator) to achieve the set air temperature.

Air distribution flap actuators (components 5, 6 and 7 in Figure 1): These flap actuators, which vary in number depending on the vehicle type, are used to distribute the air in the vehicle cabin.

DC Motor

What electrical devices are responsible for driving the damper? Just as there are many ways to control air flow, automakers have many choices in the electrical devices that drive the damper. These include brushed DC motors with a potentiometer to sense the position of the damper; 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 damper through gears of varying sizes.

More options

After selecting the motor, the HVAC system engineer can also select the architecture that drives the motor. As mentioned earlier, the damper actuator can be controlled locally or remotely. In local control, the electronics that control the motor are located near the motor, that is, the motor control IC is integrated into 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, which is 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. The DRV8912-Q1 from Texas Instruments (TI) is an example of a device that interfaces with a microcontroller via SPI to remotely drive a damper actuator.

Two possible architectures are illustrated in Figures 2 and 3. 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

More options

Let’s look at the connection between the microcontroller and the motor drive control integrated circuit. HVAC system designers also have several choices for this connection. The microcontroller can connect to the motor driver using a digital communication interface such as SPI or it can connect directly to the motor driver using control lines. Figures 4 and 5 illustrate these options.

Figure 4: Microcontroller communicating with motor driver using SPI

Figure 5: Microcontroller directly controlling the motor driver

Simplify components

The driver electronics for BLDC and stepper motors are more complex than those required to drive a brushed DC motor. If you choose to use a brushed DC motor to move the damper, there is a clear advantage to using a motor driver that drives the damper motor directly—it is simpler in both hardware and software.