Abstract : This paper analyzes the working process of a hybrid stepper motor driving the ram air turbine of a refueling pod in an aerial refueling aircraft. The structure and usage characteristics of the stepper motor are analyzed, and methods to improve its performance under the influence of multiple factors are summarized. The impact of acceleration/deceleration curves and microstepping on motor performance is analyzed in detail.
Keywords : aerial refueling tanker; refueling pod; ram air turbine; hybrid stepper motor; performance
0 Introduction
The electric motor studied in this paper is a special high-temperature resistant stepper microstepping control motor used in the refueling pod control system of a certain type of aerial refueling aircraft. It is a key actuator in the refueling pod control system, as shown in Figure 1. After receiving command signals from the pod control system, the motor drives the push rod, changing the blade angle of the ram air turbine, thereby adjusting the fuel pressure and flow rate output by the refueling pump in the refueling pod. With the development of microelectronics and computer technology, stepper motors are widely used in various automated control systems.
Figure 1 shows the position of the drive motor in the refueling pod.
Although stepper motors are widely used, they cannot be used in the conventional way like ordinary DC or AC motors. They require a control system composed of dual-ring pulse signals, power drive circuits, etc., as shown in Figure 2. Therefore, using stepper motors effectively is by no means easy; it involves a great deal of expertise in mechanics, electrical engineering, electronics, and computers. The stepper motor in the ram air turbine of the aerial refueling tanker pod requires small size, high torque, and high control precision.
1. Theoretical Analysis of Stepper Motor Driven by Ram Air Turbine
The main types of stepper motors are variable reluctance stepper motors (VR), permanent magnet stepper motors (PM), and hybrid stepper motors (HB).
Figure 2 Block diagram of stepper motor system
Hybrid stepper motors, also known as permanent magnet inductor stepper motors, were first seen in the US patent and are called SLO-SNY motors [1]. Hybrid stepper motors are usually available in two-phase, three-phase, four-phase and five-phase configurations. Figure 3 shows the structure of a typical two-phase hybrid stepper motor. The stator core has several large pole teeth, and each large pole tooth is designed with several small teeth. The windings are placed in the slots of adjacent large pole teeth of the stator. The rotor consists of a rotor core, permanent magnet material and shaft. The rotor core of this type of motor is divided into two parts, and the two parts are assembled with a difference of half a tooth pitch. The tooth pitch of the small teeth of the stator and rotor is usually the same. The permanent magnet material is an axially magnetized ring located in the middle of the rotor. From the fact that it contains permanent magnets in its magnetic circuit, it should be said that it is also a permanent magnet stepper motor. The role of the magnets in the operation of the motor reflects the characteristics of the permanent magnet motor. On the other hand, like the reactive motor, the stator and rotor surfaces have slots to make the step angle very small. It is a combination of reactive and permanent magnet stepper motors, which can make small stepping like reactive motors, and also has the advantage of low control power of permanent magnet motors, thus meeting the various working performance requirements of the ram air turbine of the aerial refueling tanker.
The main characteristics of the hybrid stepper motor in the ram air turbine are:
(1) The step angle can vary over a wide range, from a few minutes to tens of degrees. With a small step angle, it can achieve high torque and stable operation at ultra-low speeds and can drive the load directly without a reducer.
(2) It has a certain self-locking capability when the power supply is stopped.
(3) The angular displacement output corresponds to the number of input pulse signals. The step error does not accumulate over a long period of time and is almost unaffected by external conditions. When the accuracy is high, it forms a closed-loop control system.
(4) It has good controllability and responsiveness, and is easy to start, stop, reverse, and change speed. The speed is synchronized with the frequency of the control pulse and can be smoothly adjusted over a fairly wide range.
Figure 3. Typical structure of a hybrid stepper motor
However, this stepper motor has the following disadvantages:
(l) The load of the stepper motor must be properly matched with the parameters of the motor and the transmission device in order to obtain good stepping performance.
(2) Due to the existence of step loss and resonance, mechanical dampers are required. Therefore, the acceleration and deceleration methods of stepper motors are complicated depending on the utilization state.
2. Working process and principle of stepper motor driven by ram air turbine
The fuel pressure output by the refueling pump in the refueling pod of an aerial refueling tanker is achieved by adjusting the blade angle of the ram air turbine.
The blade angle is adjusted using a stepper motor. As the stepper motor rotates through the cylindrical gears, the ball screw assembly converts the rotational motion into linear motion of the lead screw. The stepper motor has three operating modes: power-off state, forward/reverse operation state, and phase-locked state.
When the stepper motor is powered off, the blades are in the initial position, which is called feathering. Let the blade angle α be zero at this time.
Figure 4 is a schematic diagram of the blade mounting base, with the blade rotating around its axis α. As the blade angle α increases, the turbine speed increases. There are five torques acting on the blade mounting base to cause the blade to rotate:
M is the rotational torque generated when the aerodynamic center of the blade deviates from the axis of rotation, and its direction is the direction that increases the blade angle α.
M is the rotational torque generated when the centrifugal force center of the blade deviates from the axis of rotation, and its direction is the direction that increases the blade angle α.
F-drive—The push rod acts on pin A with the rotational force from the stepper motor, and the torque it generates increases the blade angle α;
M-counterweight—The torque generated by the centrifugal force of the counterweight deviating from the axis of rotation, its direction is to reduce the blade angle α;
F_spring—the feathering spring force acting on pin A, which generates a torque that reduces the blade angle α.
Figure 4 Schematic diagram of ram air turbine blade mounting base
When the driving force F of the stepper motor is present, the torque that increases the blade angle must be greater than the torque that decreases it. When F is absent, the torque that increases the blade angle must be less than the torque that increases it. Therefore, when the stepper motor is running in the forward direction, the blade angle α increases, while when running in the reverse direction, the push rod advances backward, F = 0, and the blade angle α decreases, thus achieving the purpose of adjusting the blade angle.
When the stepper motor is in phase-locked mode, the push rod is locked, so the blade angle α does not change. Once the stepper motor is de-energized, the feathering spring overcomes the frictional forces of the air, clutch, and ball joint, causing the stepper motor to reverse and quickly return to the feathering position (α=0).
3. Factors affecting the performance of ram air turbine driven stepper motors and their improvement
The overall performance of the stepper motor driving the ram air turbine of an aerial refueling pod is influenced by many factors. Not only are the motor's own performance parameters crucial, but its related phase resistance and phase voltage are also vital. As the driving element of the stepper motor, the driver can be selected according to actual needs. For example, a higher voltage model can be chosen if high speed and high torque are required, while the application of microstepping can effectively solve many practical problems. Furthermore, the writing of the control program, the design of the acceleration, and the interrelationships between steps all affect the actual performance.
3.1 Selection of Stepper Motor Driver for Ram Air Turbine Drive
Two-phase hybrid stepper motor drivers can be categorized by motor drive technology into single-stage drive and bipolar drive; by control core technology into traditional analog control and the rapidly developing digital PID control; and by drive voltage into DC drivers and AC drivers.
The same motor paired with different drivers can sometimes produce significant differences. The ram air turbine-driven stepper motor studied in this paper requires the following performance characteristics: positioning accuracy in a single pulse with no cumulative error; high torque and high-speed operation (5000 rpm); rapid start-up (reaching 3000 rpm in 30 ms); and frequent start-stop operations (nearly 30 start-stop actions per second). To achieve these stringent performance requirements, a high-drive-voltage digital PID-controlled driver is employed. Compared to analog-controlled drivers, this type of driver has the following advantages:
(1) Lower quiescent current noise;
(2) Flexible and adjustable starting current;
(3) Low calorific value;
(4) Automatic matching of motor performance;
(5) Online debugging can be performed via a host computer.
3.2 The role of the acceleration and deceleration curve of the stepper motor driven by the ram air turbine
Figure 5 is a working curve diagram of a stepper motor driven by a ram air turbine. As can be seen from the figure, when the motor is running in the shaded area, the motor cannot start directly. It must first start in the starting area and then accelerate to reach the working area. Similarly, when the motor decelerates, if it stops directly at high speed, the stepper motor will lose steps and cannot achieve accurate positioning. It must decelerate to reach the starting area and then brake.
Figure 5. Acceleration/deceleration working curves
As can be seen from the figure above, the higher the starting frequency, the lower the starting torque. When the stepper motor is unloaded, it can start at a higher speed (such as 10 rpm). However, when the motor is under load, the range of the starting frequency needs to be determined according to the size of the load. It is generally recommended that the starting speed be between 2 rpm and 4 rpm.
Simply put, microstepping of a stepper motor is to divide an action that was originally completed in one step into several steps.
A two-phase stepper motor has a step angle of 0.9°. When 400 step pulses are input, the motor will rotate 360°, which is the most basic full-step control method. With the development of control technology, the control of stepper drivers has evolved from half-step (400ppr) control to the current 125 microsteps (25000ppr). The application of microstepping can avoid the resonance zone of the mechanism, improve the vibration of the motor at low speeds, improve the accuracy of each pulse, and enhance the output performance of the motor.
4. Conclusion
Stepper motors, as actuators, are widely used in automatic control systems in the aerospace field and are one of the key products in mechatronics. This paper studies a ram-air turbine driven stepper motor, focusing on solving the problems of high temperature resistance, small size, light weight, low power consumption, high torque, high control precision, and fast response speed of the new type of motor. The structure, working principle, advantages, and disadvantages of hybrid stepper motors are summarized, and several factors affecting the overall performance of hybrid stepper motors are analyzed.
About the author : Zhou Xueping, born in 1982, male, Zhuang nationality, from Ningming, Guangxi, assistant engineer, master's student at the Air Force-oriented Electronic Science and Technology University, specializing in power electronics and electric drives.
Contact number: 13548137542
Email:[email protected]
Mailing Address: Room C1405, School of Mechanical and Electronic Engineering, Qingshuihe Campus, University of Electronic Science and Technology of China, No. 2006 Xiyuan Avenue, High-tech West Zone, Chengdu, Sichuan Province, China. Postcode: 611731
Project background: The author has long worked on the front line of aircraft maintenance, focusing on the performance of this type of equipment.
