Precise, repeatable valve control within an aircraft’s environmental control system is critical for regulating cabin temperature. Here’s why engineers typically select stepper motors for the job.
An aircraft’s ECS typically relies on stepper motor technology to regulate pressure and temperature in the cabin. Image: Portescap
An environmental control system (ECS) regulates an aircraft’s cabin pressure and temperature and includes an electronic expansion valve to manage air conditioning. This valve is integral to the system and requires precision control to efficiently regulate refrigerant flow, and subsequently, maintain crew and passenger comfort.
An electric motor actuates the electronic expansion valve via signals received from the ECS controller, which monitors cabin temperature. The motor drives precise valve regulation to control refrigerant flow into an evaporator. The evaporator receives air from outside the aircraft, which is heated by compression or through bleed air from the engine, and the blend of refrigerant balances air temperature within the cabin.
This simplified diagram shows how high-pressure and high-temperature liquid from the condenser flows through the electronic expansion valve and into the evaporator. The controller receives signals from the pressure and temperature sensors and sends a signal to the electronic expansion valve to adjust refrigerant flow. Image: Design World
The electric motor and controls provide variable modulation, which requires precision to refine refrigerant flow. Engineers typically choose stepper motors to open and close the expansion valve in small, controlled steps, with each step corresponding to a fixed angular movement. Depending on the motor’s resolution, these steps or increments are measured in fractions of a degree, achieving the needed precision and repeatable control. The stepper motor also generates sufficient holding torque to maintain the valve’s position without losing steps when under pressure from the refrigerant.
“Depending on how the motor is designed, you can have large or small steps. For such applications, the best solution is to have quite small steps. So roughly, we can say, the step size is 7° with an accuracy of ±0.5°. In most applications, this is good enough,” said Adrien Mettraux, industry manager for aerospace and defense at Portescap, a Regal Rexnord brand.
Although an aircraft’s ECS should include redundancy, protecting against motor failure is vital for minimizing maintenance costs and time. The stepper motor’s design is inherently durable, as it does not rely on mechanical brushes to achieve commutation, nor does it need a feedback device or a complex closed-loop controller. This simplicity also helps decrease procurement costs.
When selecting a motor for an electronic expansion valve, important considerations include torque, temperature, pressure, flow rate, reliability, and weight. Low weight and size improve an aircraft’s fuel efficiency and cargo-carrying capability. Stepper motors achieve high torque density for their low-speed operation requirements and do not require complex external electronics or feedback. This advantage reduces the total weight and size of the package.
“We are in an aerospace environment, so weight is an important criterion,” said Mettraux. “Most of the time, weight is not as important when you have, let’s say, a system in an industrial building. Ultimately, the systems work in the same way, but in aerospace, you also need to consider weight when designing the complete system. Even at motor level, this is something we consider.”
The motor must also resist corrosion and operate for an extremely long time under varying environmental and operating temperatures.
“Now, we are using more sustainable fluids inside of the system. And the new challenge it creates is that the working temperature of the system is higher than it was in the past. So, we have developed high-temperature stepper motors, mainly driven by the fact that the temperature of the refrigerant is higher in the system,” said Mettraux. “We simulate and calculate the performance of the motor at different working temperatures — not only at room and environmental temperature, but we also help our customers understand the behavior of the motor when the temperature is very low. You can start the plane at -40° C, so the system should work the same when the motor will be quite hot due to the high temperature of the fluid.”
Can stack technology achieves reasonable accuracy and moderate torque. Image: Portescap
Stepper motors are not the only solution, but they make sense for an ECS and operate as part of a closed-loop system. Engineers often use stepper motors in open-loop systems and can close the loop directly with an encoder that measures the motor’s position. However, in an ECS, sensors measure the fluid pressure and temperature. The ECS controller closes the loop by sending a signal to the motor to move the expansion valve and adjust the refrigerant flow. The feedback is not directly done on the motor but through its sensors that communicate to the ECS.
“We are not speaking about extremely high speed compared to other applications for which the motor needs to rotate at 15,000 or 100,000 rpm. So, the stepper motor, in this case, is a good solution because it’s open loop and the speed is not extremely high,” said Mettraux.
As an alternative to stepper motors, brushless direct current (BLDC) motors could enhance the speed and efficiency of control, while minimizing form factor and weight. While this design adds cost and needs external electronic controls, it could provide an advantage for aircraft that need more rapid changes in cabin temperature control. The more efficient operation of a BLDC motor can also add reliability and minimize the potential for overheating when under duress. However, engineers must also consider simplicity and cost-efficiency when selecting a motor for an aircraft’s ECS electronic expansion valve.
“The stepper motor is not extremely complex compared to a BLDC motor. It is simpler and more cost-effective for the application than a BLDC,” said Mettraux. “The stepper is a kind of BLDC because it doesn’t have a brush, but a traditional BLDC motor could be a solution. It has very good efficiency, the speed capability is higher, and one of the main advantages of the BLDC is that you can work way above its continuous operation range. With a stepper motor, you can go above the continuous operation range, but with a BLDC motor, you can go way beyond [it], meaning you can get … more torque. This is due to the way the BLDC motors are designed compared to steppers. In some cases, BLDC could provide advantages … but it also needs a more complex controller driving system, and most of the time this is not required for electronic expansion valves.”
In addition, stack stepper motors can also be advantageous for some electronic expansion valve applications. This permanent magnet stepper motor uses simple techniques and designs to create an effective solution with reasonable accuracy and moderate torque. Customization might be required as part of the motor specification, particularly to enhance design integration. Features such as customized mounting plates, output pinions, and modifications to the motor itself might be necessary.
Portescap
portescap.com
Regal Rexnord
regalrexnord.com
Filed Under: Aerospace + defense, Motors • stepper
