In most automotive heating, ventilation, and air conditioning (HVAC) systems, fresh air that continues to flow is conditioned and injected into the cabin. Often the driver can choose whether to interrupt the fresh air supply (recycle) or continue (ie keep fresh air flowing in). In recirculation mode, the high-end HVAC system monitors several cabin air parameters, recirculates air to the cabin through the air conditioner, and limits the fresh air intake to a minimum while practicing the parameters set by the driver or system specifications. Such recycling can reduce the fuel consumption of the HVAC system by up to 35%. Depending on climatic conditions and driving cycles, HVAC systems can consume up to 3 liters of fuel per 100 kilometers. This shows that large vehicles equipped with low-end HVAC systems benefit the most from the addition of automatic recirculation. However, small and medium-sized cars with advanced engines and low exhaust emissions can also benefit from smart air recirculation flaps because of their relatively high contribution to (saving) HVAC fuel consumption. Forecasts show that the percentage of cars equipped with semi-automatic or fully automatic HVAC systems will increase year by year. At the same time, the introduction of carbon dioxide (CO2) refrigerants has created potential requirements for additional sensors that are placed in the cabin. These trends indicate that small cars and/or cars with low-profile HVAC systems will increasingly reuse existing CO2 sensors and other fresh air sensor technologies. Although the problem with the automatic recirculation function sensor may have been solved, some problems concerning the motor drive flap (flapmotorization) have to be solved.
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Automatic air recirculation control system
The HVAC Electronic Control Unit (ECU) closes the fresh air conditioned control loop and operates the recirculation flap actuator (see Figure 1) to maintain the desired CO2 level in the cabin. The operating frequency of the circulating flap is a function of the maximum allowable number of passengers, the minimum amount of air inside the vehicle, and the maximum allowable offset (equal parameters) of the required CO2 level. Assuming that the number of passengers is 5 and the space inside the car is 3m3, it can be easily calculated that the CO2 concentration will increase by 100×10-6 within 30s.
The air recirculation control loop primarily requires low speed intervention to compensate for changes in pressure and airspeed in the "fresh air inlet" (see Figure 1). This phenomenon occurs frequently when driving speed changes, such as when driving in or near urban areas. The airflow also changes as the fan speed automatically adjusts, eliminating the effects of changes in sunlight (such as curved roads or intermittent shadows caused by buildings, trees or clouds).
The air recirculation flap actuator is a small motor valve that operates through the internal actuator of the ECU. As far as the steady state control algorithm is concerned, the position of the flap should be known at all times, so some type of position feedback is necessary. Since the control system frequently re-adjusts the actuator position, non-contact motor operation and sensorless position feedback are required.
Recirculation flap technology
There are several options for operating HVAC flaps (including recirculating flaps), which differ in the type of motor used in the flap actuator and the details and characteristics of the motor control. We will now discuss three common types of motors.
Brushed DC (BDC) motors are manufactured using proven and relatively inexpensive technology with only two wireleads from the drive to the motor terminals. BDC motor control is simple if bidirectional drive is provided through two transistor half bridges. In cases where position feedback is required, such as air recirculation flaps, position sensors need to be added. There are many sensors available, the most common being a potentiometer. The combination of this sensor with the associated ECU winding and the size of the electrical connector constitutes a significant portion of the system cost. It is also important to note that brushes and commutators are components of BDC motors that are the most prone to wear. Since air recirculation flaps require frequent operation, brush aging puts pressure on the long-term reliability of BDC motors equipped with recirculating valves.
The second type is a unipolar stepper motor with two windings per phase. These windings are electrically connected to the ECU and, like the BDC motor solution (with sensor position feedback), typically require 5 wires. The choice of a single-pole stepper motor in a motor valve is primarily determined by the availability of a low-cost driver integrated circuit (IC) or driver circuit (such as four low-end driver circuits). A disadvantage of the unipolar scheme is that only half of the windings store energy at any time (basically the unipolar stepper motor uses twice the amount of copper required to run the motor).
The third type is a bipolar stepper motor with one winding per phase. Compared to unipolar motors, this solution is advantageous in terms of size and weight because the amount of copper in the windings is only about half that of a single-pole motor with similar motor characteristics. The two windings are electrically connected to the ECU through only 4 wires (compared to a single pole motor or a BDC motor with sensors for 5 wires). Bipolar stepper motors are typically driven by a combination of dual full-bridge transistors, one for each winding. Compared to BDC and unipolar stepper motor architectures, the new bipolar stepper motor actuator technology provides a balanced solution: more system advantages (ie providing an optimized combination of features and quality) without a system The main reason for the loss in total cost is that the bipolar stepper motor essentially contains a "virtual" sensor, and it is also possible to infer the operating mode of the motor by monitoring the back electromotive force (BEMF) or BEMF signal (eg high speed operation, stall condition). Wait).
The advantages of virtual sensors
The BEMF signal based on the embedded stall detection algorithm enables the system to detect the end-stop of the flap very accurately. End stop is typically achieved during operation, such as when the flap is operating in a near-closed position. The closed-loop characteristic (or pseudo-closed loop only) involves a deliberate transition to a stalled state at regular intervals. The stall detection function then supports the precise marking of the new position from the fully closed flap position. By adopting this approach, even the smallest flap-opening can be accurately maintained and can be repeated to produce true proportional control. Obviously, this mode of operation is more advantageous than the traditional method of using open-loop absolute positioning based on step statistics. Since it is necessary to ensure that the end stop is reached in the referencing run, these methods require that the stepper motor be driven a few more steps after reaching the estimated end stop position. This causes the motor to run out of control, and the associated audible noise is aging with mechanical and magnetic components. In this way, the device that detects the end stop in one complete step can avoid noise and vibration problems in the stalled state. The stall detection in a single complete step also keeps the rotor and stator magnetic fields in sync. This avoids rotor demagnetization due to the AC magnetic field of the stator, which can cause any aging problems in the magnetic components and help maintain a stable actuator torque over the life cycle.
In the case where the flap needs to be closed as quickly as possible, such as when the external sensor detects the presence of externally contaminated air, the recirculation flap is closed, and high speed-critical positioning is critical. The BEMF signal makes it possible for stepper motors to operate at high speeds through dedicated adaptive speed motor drive algorithms. This allows the stepper motor to challenge one of the main advantages of a brushed DC motor actuator, namely the ability to rotate as quickly as possible with the supply voltage and load. Stepper motors run at the fastest possible speed, automatically modulating speed based on actuator and flap characteristics such as load. During this adaptive speed operation, sensorless stall detection works to ensure error-free positioning. These algorithms support speeds of up to 1000 full steps per second.
Summary of the blade actuator technology
Table 1 summarizes the "applicability" of the flap actuator technology we discussed. Both brushed DC motors and unipolar stepper motors have their advantages, but they are also weak. The new bipolar stepper motor technology appears to combine the strengths of the first two technologies and meets all of the requirements mentioned.
After summing up the actuator technology, it is the turn of the best HVAC system manufacturers to deploy the proper weighting of all of these features. Our observation is that the system-level costs of these three types of actuators are similar, but if only the procurement cost of the motor drive itself is taken into account, it may be the last time the automaker chooses a sub-optimal solution.
New recirculating flap driver IC
An integrated circuit that drives a bipolar stepper motor equipped with the above technology is now available. Figure 2 shows a typical block diagram of this type of IC. This IC is placed in the ECU and two full H-bridges drive the two phases of the bipolar stepper motor. The ECU's microcontroller (MCU) communicates with the IC via the SPI interface and a set of dedicated signals.
The current conversion table embedded in the driver applies the appropriate current to the windings. The microcontroller needs to be set only when the SPI register defines the winding current peak, the microstep mode, and the preset direction of operation. Thereafter, the microcontroller can step through the current conversion table by sending only the "next" signal to the IC. The motor driver then assumes full responsibility for generating the current waveform required for full-step, half-step or sinusoidal microstepping. The speed at which the "next" pulse is sent determines the speed at which the motor is running.
A simple and effective stall detection algorithm can be executed and activated via the SPI bus. The chip also supports adaptive speed control for closing the recirculation flap at maximum speed. The chip also performs the appropriate diagnostic functions to detect all relevant error conditions and prevent system and IC damage. This IC contains an interrupt output pin that is used to alert the microcontroller when an error occurs.
in conclusion
This paper discusses the existing recirculating flap actuator technology and analyzes the operational requirements of such recirculation valves. Brushed DC motor actuators and unipolar stepper motor actuators do not meet certain technical requirements. The bipolar stepper motor valve combined with the novel driver provides an optimal technical solution that meets the high quality operational requirements of future air recirculation valves.
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