Abstract: This paper presents a general-purpose automatic testing system for low-power servo systems with automatic adjustment capabilities, outlining the specific hardware circuitry and related software flow. Practical application demonstrates that this automatic testing system is not only fully functional and reliably stable, facilitating maintenance by ground personnel, but also features ease of use, intelligence, and high integration.
introduction
Nowadays, instruments based on low-power servo systems with automatic adjustment functions are widely used on aircraft. They use a servo system to realize signal conversion, calculation and long-distance transmission, which amplifies the signal energy. This not only improves the indication accuracy and load capacity of the instrument, but also has a more flexible output and display method [1]. The integrity of low-power servo system instruments directly affects the flight safety of aircraft. This paper designs a general automatic test system for low-power servo system instruments on aircraft.
1. Working principle of aircraft instrument servo system
The servo system used in aviation instruments is actually an electromechanical position feedback system, the principle of which is shown in Figure 1. It mainly consists of two parts: a sensor and an indicator. In the sensor, the sensitive element measures a certain physical parameter of the aircraft and converts it into an angular signal θ1 to control the movement of the servo system. Then, the angular signal θ1 is converted into a certain form of electrical signal e1 by a conversion device and output to the receiving device in the indicator. In the indicator, when the input signal e1 of the receiving device is inconsistent with the feedback signal e2 output by the motor assembly (i.e., Δe = e2 - e1 ≠ 0), an error voltage signal Δe is generated. This signal is demodulated and amplified by the servo device to control the operation of the DC motor assembly, thereby driving the pointer and counter to indicate, and at the same time providing the position feedback signal e2 to the receiving device (the dotted line in the indicator part of Figure 1 represents this mechanical position feedback relationship), causing Δe to tend to 0. We call this state the coordinated state of the servo system. When the corresponding aircraft parameters change, the servo system will repeat the above process to achieve a new coordination.
In an aeronautical instrument servo system , the conversion device and the receiving device are electrically integrated as a single unit, collectively referred to as the comparison conversion device. This device compares the main control signal with the feedback signal and converts it into an error electrical signal. The components of the comparison conversion device commonly include synchros or rotary transformers, both of which are characterized by high transmission accuracy and long service life.
1. Overall Design Concept
The general-purpose automatic test system designed in this paper uses an industrial control computer as a platform. Based on the test requirements of the device under test, a dedicated bus expansion card, such as a programmable excitation signal generator and a signal acquisition processor, based on GPIB bus data communication technology is developed. A dedicated interface adapter is made to connect the test system and the unit under test. Repeatedly occupied signal sources are allocated by relay switches and matrix switches, and the test is carried out under the control of a digital I/O card.
2 Hardware Design
The hardware system design block diagram is shown in Figure 2. The hardware platform mainly consists of seven parts: an industrial control computer, a digital I/O card, three synchronizer cards, test resources, an array interface, an interface adapter, and test cables.
(1) Industrial Control Computer: This is the control center and operating platform of the entire machine. In addition to the standard IPC configuration, it is internally expanded with four VXI bus expansion cards, digital I/O cards, and synchronizer cards. Through the software system and GPIB bus scheduling system, it automatically completes digital testing of the object under test and provides accurate test results.
(2) Synchronizer Card: This is a self-designed synchronizer/decomposer based on the ISA bus, which realizes the conversion between shaft angle signals and digital signals. On the one hand, it reads the digital angle data sent from the system bus, generates a voltage signal, and sends it to the instrument as an excitation; on the other hand, it can also receive the angle position signal output by the device under test and convert the angle position information into an angle value.
(3) Digital I/O card: In the test system, the digital I/O card can trigger the electromagnetic action of the relay module used and receive the switch signal sent from the discrete interface board[3].
(4) Testing resources: The system testing resources mainly consist of digital VXI module instruments and GPIB programmable instruments.
①VXI Test Resources: The selection and combination of VXI module resources take into account the testing requirements of the device under test (DUT) and the system's scalability. Considering the need for testing accuracy, a 16-channel 16-bit D/A converter JV53201, a 6.5-bit digital multimeter Agilent 34401B, a relay Agilent E1463A, and a matrix switch Agilent E1446A were selected. The discrete interface board is a custom-made board specifically designed for ATE to detect the discrete output voltage of the UUT. This printed circuit board also includes an additional resistor network, which uses electricity as its source and generates various voltage signal sources through a resistor network voltage divider.
②GPIB discrete instruments: mainly include DC flow control power supplies and AC flow control power supplies. Both DC and AC flow control power supplies have programmable adjustment capabilities and feature online monitoring, data readback, and self-testing functions.
(5) Array interface: It is the central hub for the transfer of input and output signals of the entire system.
(6) Interface adapter: The UUT (Object under Test) is not directly connected to the ICA (Array Interface Connector Frame), but is connected to a dedicated interface adapter. The interface adapter and UUT are used together during testing. All interface adapters have a unified structure for interfacing with the ICA, ITA (Interface Adapter). The sockets on the ITA and ICA are symmetrical [2], as shown in Figure 3.
The power supply for both the interface adapter and the unit under test (DUT) comes from the test system and is converted via the ICA and ITA. The interface adapter is a preprocessing device for the DUT's signals, connecting to the test system bus through a programmable interface to achieve effective automated testing. The ICA is equipped with a highly flexible control handle, allowing for easy and reliable connection and disconnection from the ITA, providing the most convenient way to interface with or disconnect the interface adapter chassis from the test system.
(7) Cable: Used to connect the device under test to the interface adapter.
3 Software Design Scheme
The system software adopts a modular structure, which is flexible and convenient, providing excellent conditions for hardware system expansion and software upgrades. NI's LabVIEW is selected as the software development platform. The main module program calls the various functional module programs to complete the corresponding test functions, and each functional module calls the underlying functions or sub-VIs to complete the corresponding operations. Data exchange between layers is carried out through common data files and real-time variables [4,5].
3.1 Overall Software Design of the Test System
The main control module of the test software system is responsible for system flow control, sub-module management, user management, help prompts, etc. The flowchart of the main module is shown in Figure 4.
The automatic testing system's testing process is as follows: First, the system power supply is automatically tested; the software platform is started, and system resource scanning and self-testing begin, entering test preparation. After the self-test is completed, the device under test is selected, and automatic testing is performed according to the detailed testing requirements of the device under test. During the test, it is determined whether the device under test has a fault. If a fault is found, the fault handling procedure is initiated to handle the corresponding fault. Simultaneously, fault signal information is recorded in real time to the detector's database through the storage module for future fault analysis and retrieval.
The functional modules include a data acquisition module, a data processing module, a signal output module, a data storage and historical data query module, a report module, and an error event handling module. It also adopts a hierarchical module design concept, with the main module calling each sub-module to realize the functions of data acquisition, processing, analysis, display, recording and printing, and complete the online testing of the low-power servo system.
3.2 Design of the User Graphical Interface
The operation panel of the automatic testing system is shown in Figure 5. The user interface and callback function names are created according to the overall design concept of the software. The operation panel is designed to be convenient and practical.
The general testing steps of the test system are as follows: (1) Connect the component under test (DUT) to the DUT adapter via the corresponding cable; (2) Connect the DUT to the test system via the standardized interface; (3) Connect the programmable DC power supply and AC power supply to power the test system; (4) Run the test program (TPS) of the DUT to enter the operation panel interface of the DUT. Select the test item to be performed in the operation panel, and press the "Start" button after selection. The computer will enter the test process. During the test, some operation prompts will be displayed. After the DUT is tested, click "Save Data" on the operation panel to record the test results.
4. Conclusion
This paper utilizes automated test system integration technology to design an automated test system for low-power servo system instruments on aircraft. Simulation verification proves that the design is correct and feasible, and can provide a basis for specific engineering implementation.
