Abstract: To meet the requirements for normal operation under harsh environmental conditions, the performance of an electromechanical servo system was verified by testing at a low temperature of -45°C. Based on the test results, process improvements were made. First, the impact of low temperature on the servo control drive section was studied; second, the operating status of the linear displacement sensor and motor rotary transformer under low temperature conditions was tested; finally, the assembly quality of the transmission mechanical structure was quantitatively analyzed. Through this research, the main factors restricting the operation of the electromechanical servo system in harsh environments were identified. Process improvements enhanced its low-temperature performance, increased production efficiency, and ensured product delivery. The research results have broad practical value.
Keywords: electromechanical servo system; low-temperature performance; process improvement
1 Introduction
The servo system is the actuator of the aircraft control system. Its main function is to drive and control the aerodynamic load according to the control command, quickly respond to the position command, complete the attitude control of the aircraft, and achieve the purpose of stabilizing its flight.
Electromechanical servo systems typically operate in ambient temperatures of 20-30°C. However, actual aerospace applications face more complex and variable conditions, such as harsh outdoor environments with low temperatures and low pressures (-45°C) at high altitudes, placing higher demands on electromechanical servo systems.
To expand its application scope and meet the needs of complex conditions and wide temperature ranges such as land, sea, and air, this project focuses on process improvement research on the low-temperature environmental adaptability and batch production capacity of electromechanical servo systems, so as to improve its low-temperature working capability.
2 Low Temperature Test Conditions
A certain type of servo system consists of a servo control drive part and an actuator part. The actuator part includes components such as a motor, a rotary transformer, and a linear displacement sensor. The connection of its components is shown in Figure 1-1.
Figure 1-1 Simplified diagram of the servo mechanism's components and connections
The servo system must be able to operate reliably in a low-temperature environment of -45℃. Before delivery, it must undergo a -45℃ low-temperature operation test. When the servo mechanism is to be operated at -45℃, the entire servo mechanism (1 servo control driver and 2 electromechanical actuators) must be placed in a low-temperature chamber and kept at that temperature for 4 hours. Then, the servo mechanism is started up and operated under the -45℃ low-temperature conditions to conduct functional performance tests. The low-temperature test conditions are shown in Table 1-1.
Table 1-1 Low Temperature Test Conditions
Currently, this type of servo mechanism is about to enter the mass production stage, but the pass rate of the -45℃ low temperature test of the servo mechanism is only 90.1%. Each batch of servo mechanisms exhibits jitter when starting up in a -45℃ low temperature environment. The typical command and feedback curves of the abnormal phenomenon are shown in Figure 1-2a, where the black curve jdB_sf(2) is the command curve and the red curve wyB_sf(2) is the displacement feedback curve. As can be seen from the curves, the red feedback curve has abnormal spikes, and the servo mechanism cannot accurately follow the command, causing abnormal jitter. After returning to normal temperature, the jitter phenomenon disappears, and the command and feedback curves of the normal servo mechanism are shown in Figure 1-2b.
a) Abnormal feedback curve
b) Normal feedback curve
Figure 1-2 Schematic diagram of low-temperature vibration problem of servo mechanism
The occurrence of low-temperature jitter issues will cause delivery delays and product scrapping, seriously affecting the research and development and mass production progress. Against this background, with the goal of improving the first-pass yield rate of the servo mechanism's low-temperature operation test, we further analyzed the causes of the servo mechanism's low-temperature jitter, investigated and analyzed the components of the servo mechanism, namely the single-machine servo control driver and electromechanical actuator, and used experimental verification methods to further locate the location of the servo mechanism's low-temperature operation jitter problem.
3 Servo Control Driver Verification Test
Two actuators are connected by a servo control driver via a cable. The servo control driver is placed in a low-temperature chamber, and the actuators are placed outside the low-temperature chamber. The chamber is kept at -45℃ for 4 hours. Then the servo mechanism is started to conduct a 30° position characteristic test. The test curve is shown in Figure 2-1.
Figure 2-1 Displacement feedback diagram of the servo mechanism (photograph of the curve displayed on the testing instrument)
Two actuators are connected by a servo control driver via a cable. The actuators are placed in a low-temperature chamber, while the servo control driver is placed outside the low-temperature chamber. The chamber is kept at -45°C for 4 hours. Then, the servo mechanism is started to perform a 30° position characteristic test. The test curve is shown in Figure 2-2.
Figure 2-2 Displacement feedback diagram of the servo mechanism (photograph of the curve displayed on the testing instrument)
The above comparative tests show that under the low temperature of -45℃, the servo control driver is not sensitive to low temperature. When the electromechanical actuator works alone in the low temperature environment, abnormal jitter occurs, which is manifested as the position feedback curve not tracking the command curve normally and generating a large number of glitch. Therefore, the abnormal jitter of the electromechanical actuator under the low temperature of -45℃ is the key reason for the low first-pass yield of the servo mechanism in the low temperature operation test.
4. Cause Analysis
An analysis of the abnormal vibration problem of electromechanical actuators under -45℃ low temperature conditions identified the following possible causes:
1. Damage to the resistor plate of the linear displacement sensor in the electromechanical actuator causes intermittent non-conduction between the brush of the linear displacement sensor and the resistor plate.
2. Excess material exists between the resistive plate and the brush of the linear displacement sensor of the electromechanical actuator, causing intermittent non-conduction between the brush and the resistive plate.
3. During the assembly of the electromechanical actuator, there is an axial clearance. Due to the low temperature stress, the clearance is amplified, resulting in a large axial clearance in the electromechanical actuator.
4. The servo motor rotary transformer malfunctions under low temperature conditions, causing deviations in angle and speed measurements.
To address the problem of abnormal vibration of electromechanical actuators in a low-temperature environment of -45℃, brainstorming was used, and cause-effect diagrams were employed for causal analysis, as shown in Figure 3-1.
Figure 3-1 Cause-and-effect diagram for analyzing abnormal vibration at low temperatures in electromechanical actuators
5. Verification Test
Linear displacement is one of the important position feedback sensors in electromechanical actuators. Abnormal damage to the linear displacement component will prevent the entire servo mechanism from completing closed-loop control, causing the actuator to fail to respond accurately to commands or even suffer open-loop damage. Therefore, targeting the pattern of localized damage to the resistive film of the linear displacement plate during operation, the linear displacement cover and linear displacement plate of the abnormal electromechanical actuator were disassembled. Observation of the resistive film on the linear displacement plate revealed dark, viscous excess material, which was lubricating grease, at the locations where vibration occurred during low-temperature operation testing, as shown in Figure 4-1. After cleaning the linear displacement plate and brushes, as shown in Figure 4-2, both sides of the linear displacement plate were relatively smooth, and the brush filaments were also in good condition, with no damage. After cleaning the surface of the linear displacement plate and brushes with alcohol, the surface of the linear displacement resistive film was relatively smooth, with only slight brush marks, and no obvious damage such as scratches or pits was found. It is believed that localized damage to the linear displacement plate has a small impact on the low-temperature vibration of the electromechanical actuator.
Figure 4-1 Original state of the linear displacement plate
Figure 4-2 shows the state of the linear displacement plate after cleaning.
Localized grease buildup on the resistive film of the linear displacement plate forms excess material. At low temperatures, its fluidity (viscosity) and conductivity decrease, causing intermittent non-conduction between the linear displacement brush and the conductive film. This problem leads to vibration or even open-loop damage of the electromechanical actuator. After disassembling the faulty electromechanical actuator, localized grease buildup on the resistive film of the linear displacement plate is visible. The ball screw assembly of the electromechanical actuator is coated with low-temperature wide-temperature-range grease 7014. If too much grease is applied or unevenly, it will be splashed out during operation. If it splashes onto the resistive film of the linear displacement plate, it forms localized buildup, becoming excess material. At low temperatures, the grease viscosity increases and conductivity decreases. During operation, the linear displacement brush is unable to push away this excess material, causing intermittent non-conduction between the linear displacement brush and the conductive film, thus causing vibration at low temperatures. When the temperature rises, the grease viscosity decreases and conductivity increases, the linear displacement brush and the conductive film regain conductivity, and the vibration disappears. To verify that the low-temperature vibration problem of the electromechanical actuator was caused by grease splashing onto the linear displacement plate and forming excess material, the linear displacement plate that came with the electromechanical actuator was cleaned with alcohol and reassembled into the actuator. Three more low-temperature tests at -45°C were then conducted. The test results are shown in Figure 4-3. In each test, the electromechanical actuator operated normally without any low-temperature vibration. The test results indicate that when the grease on the surface of the linear displacement plate was wiped off before the low-temperature test, the low-temperature vibration problem in the electromechanical actuator disappeared. Therefore, it can be confirmed that the presence of excess material (grease) on the surface of the resistive film of the linear displacement plate is the main factor causing the low-temperature vibration of the electromechanical actuator.
Figure 4-3 shows the test data of the linear displacement plate after cleaning (photograph of the curve displayed by the testing instrument).
An electromechanical actuator is a precision transmission mechanism. Its main principle is that a motor drives a reducer to rotate a ball screw pair, converting the motor's rotational motion into the screw's linear motion. During assembly, the transmission clearance of the electromechanical actuator is a crucial factor affecting control accuracy and stability. Large transmission clearance can also cause actuator vibration, which is more pronounced at low temperatures. Therefore, this study decomposes and inspects the actuator for unacceptable axial clearance during the closing process. A simplified structural diagram of the ball screw assembled with the electromechanical actuator is shown in Figure 4-4. Axial clearance control is one of the key factors determining the actuator's transmission accuracy. When adjusting the clearance, appropriate adjusting shims must be selected to ensure an interference fit of 0.04-0.08 mm. If the selected shim size does not meet these requirements, axial clearance may occur in the electromechanical actuator, leading to vibration at low temperatures. Forty-one electromechanical actuators with normal low-temperature performance and seven electromechanical actuators exhibiting low-temperature vibration were randomly selected from the same batch. The thickness of their adjusting shims and their interference fit values were summarized and compared. The distribution is detailed in Figure 4-5. The blue line segment represents the interference fit of qualified products at low temperatures, and the red line segment represents the interference fit of unqualified products at low temperatures. As shown in the figure, the interference fit of all products is within the allowable range. Comparative analysis revealed that the axial interference fit of both normally operating and low-temperature vibration products is distributed within the range of 0.04mm to 0.07mm, suggesting that the non-compliance of axial clearance during actuator closing has a relatively small impact on the low-temperature vibration of the electromechanical actuator.
Figure 4-4 Assembly drawing of ball screw assembly
Figure 4-5 Distribution of Interference Points
The resolver is a crucial sensor for detecting motor speed, angle, and position. It is also one of the key sensors for servo mechanisms to achieve three-loop control of current, speed, and position. Under low-temperature conditions, the normal operation of the resolver is essential for ensuring smooth motor operation and stable output. Therefore, this study decomposes and inspects the patterns of deviations in measuring motor speed and angle using the resolver. The servo motors associated with the faulty electromechanical actuators were removed, and the servo control driver software was modified to control the motor movement using a resolver closed-loop method. Two servo motors were connected via cables using the servo control driver. The servo motors were placed in a low-temperature chamber, while the servo control driver was placed outside the chamber. The chamber was kept at -45°C for 4 hours. Then, the servo was started, and a 30° position characteristic test was conducted using the resolver closed-loop method. The test results are shown in Figure 4-6. It can be seen that the position curves of the two motors completely overlapped, and the current was stable without any abnormalities. This indicates that the motor can operate normally at -45°C, and the low temperature did not affect the normal operation of the resolver.
Figure 4-6 Servo motor position feedback curve (photograph of the curve displayed on the tester)
6 Process Improvements
Through the above verification tests, the main cause of abnormal vibration in the electromechanical actuator was identified as: excess material on the surface of the resistive film of the linear displacement plate. This excess material was 7014 grease. While this grease does not affect the linear displacement feedback function at room temperature, it causes abnormal burrs in the linear displacement feedback at -45℃. Therefore, the grease application process for the ball screw assembly was improved by changing the cleaning procedure. Instead of soaking in alcohol and then refilling with grease, the process was changed to no longer requiring grease replacement. During cleaning, the screw surface was wiped with a silk cloth soaked in alcohol. This avoids excessive grease usage and uneven lubrication caused by manual secondary grease replacement, ensuring that grease is not splashed onto the linear displacement plate during high-speed rotation, thus fundamentally solving the low-temperature vibration problem.
Verification tests were conducted on the process improvement scheme: Two new ball screw assemblies were taken. One was cleaned and greased according to the old process flow, while the other was wiped according to the new process flow. The two ball screw assemblies were then assembled onto two electromechanical actuators, and position and speed characteristic tests were performed. After the tests, the linear displacement of the electromechanical actuators was disassembled, and the grease splashing on the linear displacement of the two products using the old and new processes was compared. The test results are shown in Figures 5-1 and 5-2 below.
Figure 5-1 shows the linear displacement plate配套 with the ball screw in the old process flow.
Figure 5-2 shows the linear displacement plate配套 with the ball screw in the new process flow.
Through testing, the new process flow has been verified to effectively prevent grease splashing and solve the problem of low-temperature vibration in electromechanical actuators.
The first batch of 20 products after the process improvement were inspected, and all passed the low-temperature operation test. Randomly selected three electromechanical actuators were disassembled for linear displacement testing, and the results showed that the linear displacement resistive film was clean and free of excess grease. The pass rate reached 100%. The process documents for this type of servo system were revised, and the relevant assembly and testing specifications were revised and published.
7 Conclusions
Through verification experiments and analysis improvements, an improved process for ball screw assembly was summarized. Based on this achievement, we will continue to carry out technical research on improving the low-temperature performance of electromechanical actuators, add a sealing structure to the ball screw assembly to avoid the risk of grease overflow from the structural design, and conduct in-depth research on other high-performance low-temperature greases to further improve the environmental adaptability of electromechanical actuators.
References
[1] Tao Yonghua, Yin Yixin, Ge Lusheng. Novel PID Control and Its Applications, Machinery Industry Press, 1998.9
[2] Li Liansheng. Radar Servo System [M]. National Defense Industry Press, 1980: 127-149
[3] Gong Zhenbang. Servo Transmission Device (II) [M]. National Defense Industry Press, 1980, 8.
[4] Hu Youde, Ma Dongsheng. Servo System Principles and Design [M]. Beijing Institute of Technology Press, 1999, 7
[5] Yan Hao, Li Changchun, Chen Ce. Comprehensive load simulation test system for servo mechanism [J]. Acta Ordnanceologica Sinica, 2012, 33(5):588-593
[6] Zhang Jie, Sun Ningsheng. Transfer function between load simulation and no-load characteristics of launch vehicle servo mechanism [J]. Aerospace Measurement & Control, 2002, 18(2):12-16
[7] Hao Jingjia, Zhao Keding, Xu Hongguang. Study on the influence of stiffness and inertia load on the performance of electro-hydraulic load simulation platform [J]. Chinese Mechanical Engineering, 2002, 13(6):458-460
