Abstract: To address the challenges of complex operation and difficult maintenance under real loads in the production and development of a certain type of servo mechanism, a modular simulated load platform is proposed. Through the establishment of a load mathematical model, dynamic characteristic testing, and resonant frequency analysis and identification, the mechanical structure, loading control, inertial load module, friction load module, elastic load module, and overall scheme of the simulated load platform are described. Targeted design of swing angle measurement and force measurement modules is also presented. Through research and practice, a simulated load platform was successfully produced and replaced the real load, exhibiting high dynamic performance and testing efficiency. It meets the requirements for dynamic characteristic testing of servo systems in ground-based semi-physical experiments and possesses significant engineering application value.
0 Introduction
Simulated load testing is an essential step in the hardware-in-the-loop (HIL) simulation process during aircraft development. Real loads typically present challenges such as compact assembly structures, inability to maintain stability over long periods, and difficulties in maintenance and calibration. Therefore, research on simulated load testing has always been a hot topic in the aerospace field both domestically and internationally. Servo mechanisms are the actuators of an aircraft's control system. Based on command signals from the control system, they achieve thrust vector control or aerodynamic control of the aircraft's engines or control surfaces to stabilize attitude and control direction.
The servo mechanism simulated load platform is a specialized torque loading device for studying and testing the characteristics of servo systems. It simulates the inertial and frictional loads experienced by aircraft during actual operation. Taking the production and development of a certain type of servo mechanism as an example, and to address the problems of complex operation and difficult maintenance under real loads, as well as to improve equipment utilization and save space and costs, a modular simulated load platform is proposed. This allows for the analysis of the servo mechanism's load performance under laboratory conditions. The platform has the following advantages: stable system characteristics, simple maintenance, adjustable and easily measurable load performance, and modular structural design.
1. Load Characteristic Analysis
This servo mechanism controls the rudder load through the linear extension and retraction of an internal piston rod and an external connecting rod. Considering actual flight conditions, the load is decomposed into three parts: inertial load, frictional load, and resonant load. According to technical requirements, the load torque is 400 Nm and the moment of inertia is 0.303 kg·m². The control block diagram of the servo mechanism under load is shown in Figure 1, and the simplified mathematical model is shown in Figure 2.
Figure 1. Control block diagram of the servo mechanism
Fig.1Servomechanismcontrolblockdiagram
Figure 2 Simplified mathematical model of the servo mechanism
Fig.2Briefmathematicmodeloftheservomechanism
The load characteristics of the servo system are measured using an angle sensor coaxial with the rudder. Simultaneously, a displacement sensor is installed parallel to the piston rod within the servo mechanism to measure the linear displacement of the servo mechanism and its closed-loop negative feedback. By intermittently inputting sinusoidal signals of varying frequencies, amplitudes, and periods to the servo mechanism, and subtracting the measured angle and displacement in logarithmic coordinates, the load characteristics of the servo mechanism can be obtained, as shown in Figure 3.
Figure 3 Load characteristic curve
Fig. 3 Load characteristic curves
2 Overall Layout
The simulated load platform has an open, single-channel, linear layout, mainly consisting of a fixed platform, a spindle module, an inertia adjustment module, an angle measurement module, and a stiffness adjustment module, as shown in Figure 4. Each module is relatively independent and can be partially assembled according to usage requirements. It features a simple structure, high overall strength, long-term stability, easy replacement, and is equivalent to a real installation structure.
Figure 4. Composition diagram of the simulated load platform
Fig.4The composition of the simulated load platform
3 Mechanical Structure
The mechanical structure of the simulated load platform is shown in Figure 5. The fixed platform is welded from steel plates, employing a frame structure with locally reinforced diagonal supports to improve overall rigidity. It is connected to the test site's ground rails via anchor bolts. The angle measurement module achieves a gapless connection with the output shaft through a coupling, ensuring measurement accuracy and compensating for installation errors. The stiffness adjustment module changes the extension length of the spring steel plate by adjusting the clamping position of the tail pressure plate. The pressure plate is deformed by the thrust of the servo mechanism, simulating the load characteristics of the servo mechanism and adjusting the load platform's stiffness. The inertia adjustment module ensures coaxiality with the spindle by adding or removing shims, preventing additional torque. The rotational inertia is adjusted by adding or removing half-plate inertia disks. The spindle module uses high-precision ball bearings for support, ensuring fit accuracy and stability, and facilitating control of frictional torque. Mechanical limit devices are installed at both ends of the crank, acting as limiters when the rotation angle exceeds the set value, thus protecting structural components, as shown in Figure 6.
Figure 5. Simulated load platform structure diagram
Fig.5The structure of the simulated load platform
a) Fixed platform structure diagram
b) Angle Measurement Module Structure Diagram
c) Stiffness adjustment module structure diagram
d) Structural diagram of rotational inertia adjustment module
e) Spindle module structure diagram
Figure 6 Module Structure Diagram
Fig. 6 The structure of each module
4. Load characteristic design
4.1 Stiffness Design
Stiffness is the ability of a load platform to resist deformation caused by external forces that vary over time. Its significance lies in its frequency response to the amount of deformation generated under external forces (such as a servo mechanism driving a load for high-frequency characteristic testing). The dynamic stiffness of the load platform is related to its stability against forced vibrations (including vibrations transmitted from the ground; vibrations caused by imbalances in rotational motion within the structure; vibrations caused by varying frequency points during testing) and self-excited vibrations.
To ensure the long-term reliable use of the load platform and reduce the impact of nonlinear factors such as deformation and clearances in transmission and connections on product characteristics, the rigidity design of the load platform is crucial: the support of the swing shaft assembly is machined as a single piece to improve structural strength; the main mounting hole is machined in one go to ensure coaxiality; the bearings inside the swing shaft assembly are radially and axially adjustable thrust bearings to prevent axial movement of the swing blocks, reduce rotational clearance, improve dynamic balance, and adjust the rotational friction load by adjusting the bearing clearance to meet phase angle correction requirements; the base adopts a heavy-duty welded frame structure and a locally reinforced diagonal support structure, with a compact layout of the swing shaft assembly on the platform to improve the static rigidity of the base. Tight fixing to the ground rail with screws reduces the foundation vibration source while increasing damping and attenuating vibration energy.
4.2 Dynamic Characteristic Correction Design
When the servo mechanism is tested for frequency characteristics at different frequency points with a rated amplitude signal under a real load, the amplitude and phase angle will differ due to the servo mechanism's response capability and the output displacement changes at different frequency points caused by the deformation of the load transmission mechanism. For the load platform, the base is rigidly connected to the ground to ensure sufficient rigidity, and the rigidity and moment of inertia of the elastic steel plate are adjusted to approximate the real load characteristics.
The dynamic correction design of the load platform mainly includes frequency test amplitude simulation and correction design, and resonance point simulation and correction design. Due to the limitations of the servo mechanism installation structure, the elastic steel plate assembly is not of uniform cross section. The maximum adjustable stiffness of the elastic steel plate is greater than the stiffness of the system resonance frequency. The simulation calculation results through ANSYS are shown in Figure 7.
Figure 7. Maximum stiffness of the elastic steel plate (ANSYS simulation)
Fig.7ANSYSsimulationoftheelasticsteelplate'smaxrigidity
When calculating stiffness in simulation, the input force value is based on the maximum output force of the servo mechanism. The stiffness of the servo mechanism is calculated according to the technical requirements, using the formula:
(1)
In the formula, f is the maximum output force of 2.32 x 10⁴ N; y is the displacement under the input force of 0.658 mm.
We can obtain:
k = 3.859 x 10⁷ N/m (2)
From the formula:
(3)
In the formula, R is the lever arm length of 70mm; I is the moment of inertia of the rudder.
For the inherent frequency of the transmission system
We can obtain:
k = 1.76 x 10⁷ N/m (4)
Furthermore, ANSYS simulation yielded a deformation of 1.484 mm, which is basically consistent with the theoretical value of 1.43 mm for the simplified rectangular model, as shown in Figure 8. With a design value of L = 240~360 mm for the steel plate length adjustment range, the stiffness adjustment range is k = 0.855x10⁷~3.859x10⁷ N/m. This proves that by finely adjusting the steel plate length, the cumulative error of machining and assembly can be covered, achieving the goal of equivalently simulating a real load.
Figure 8. ANSYS simulation of the true stiffness of the elastic steel plate.
Fig.8ANSYSsimulationoftheelasticsteelplate'srealrigidity
4.3 Periodic verification
To avoid the impact of wear and tear on the simulated load platform on the test results, calibration must be performed every six months. The calibration includes:
Visual inspection: Key parts should be inspected for any obvious mechanical damage; the servo mechanism and the transmission mechanism of the angular displacement sensor should be free from damage and cracks, and should be securely fastened without any looseness.
Dimensional check: Use a micrometer to measure the interface dimensions of the servo mechanism and the angular displacement sensor to ensure that the mating clearance is within the range of 0~0.006mm.
Load characteristic check: The stiffness of the simulated load platform is adjusted using a process servo mechanism to ensure that the load characteristics are consistent with the real load; the loading device is adjusted so that the amplitude of the resonant point of the simulated load platform is consistent with the real load.
5. Experimental Verification
In the frequency sweep test (amplitude 1°) of the servo mechanism under real load and simulated load conditions, the stiffness adjustment module and inertia adjustment module were adjusted according to the difference in test results to achieve basically consistent load characteristics, which can be used as the basis for the production test of the servo mechanism, as shown in Figure 9.
Simulated load test data
Real load test data
Figure 9 Comparison curves of frequency sweep test
Fig.9Comparisoncurvesofthespectrumscanning
The application of the simulated load platform has solved the problems of complex operation and difficult maintenance in real load scenarios. To date, the simulated load platform has been used to accept 500 servo mechanisms, improving production efficiency, as shown in Figure 10.
Each module of the simulated load platform can be installed independently, making operation simple and shortening installation and calibration time;
The simulated load platform has a long service life and stable load characteristics. With proper maintenance, its lifespan can be guaranteed.
Figure 10. Physical diagram of the simulated load platform
Fig.10Photooftherealsimulatedloadplatform
6 Conclusions
This paper, taking the production and development of a certain type of servo mechanism as its background, proposes a modular simulated load platform for the servo mechanism. It elaborates on the mechanical structure, modular design, load characteristic correction, and overall scheme, and specifies the periodic verification of the simulated load platform using real loads. Through research and practice, the simulated load platform has been successfully used in production to replace the real load, solving the problems of complex operation and difficult maintenance of real loads, improving production efficiency. The product has successfully undergone hundreds of test tasks, demonstrating significant engineering value and broad application prospects.
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Author Biography
Tang Li, male, born in May 1987, title: Engineer, degree: Master's degree, major and research direction: overall design of servo systems, mailing address: P.O. Box 77, No. 1, Nanda Hongmen Road, Fengtai District, Beijing (Department 1, Institute 18, China Academy of Launch Vehicle Technology), Postcode: 100076, email: [email protected], telephone: 13810866130, fax: 010-68382904
