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Section Valve Production Test System Using LabVIEW RT and TestStand
Products Used:
LabVIEW Real Time 6.1
TestStand 2.0
PCI-8330
PXI-8330
PXI-6025
PXI-6602
PXI-6508
PXI-8461
PXI-7030/6040E
SCXI-1102B
SCXI-1124
PXI-1010
The Challenge:
Providing a high volume production test system that is both reliable on a 24/7 basis and flexible for future expansion.
The Solution:
Build the system using National Instruments LabVIEW RT, TestStand, and PXI devices. Dedicated LabVIEW RT control code running on PXI ensures the system reliability. PXI, with its modular approach, and TestStand, with its tight integration of LabVIEW, make it possible for both hardware and software functions to be expanded without affecting the existing system.
Test System Requirements:
The test system is for the production floor to test section valves used for off-highway mobile equipment. Section valves are individual hydraulic valves that can be mounted together as a valve stack to perform a particular task on the vehicle. Different section valves have their own functions and require different tests. When tested, each valve stack is a Unit Under Test (UUT) and all the valves in the stack need to be tested.
The major requirements for the system are:
- Keep pressures constant during a test to ensure accurate test results. This is especially critical in dynamic tests.
- Allow new types of section valves to be tested with minimum down time for system upgrade.
- Provide a simple and intuitive user interface to configure and run the test sequences.
System Architecture:
PXI-SCXI modules are used in the system for data acquisition and control. To meet the IT department requirements, a separate desktop PC is used instead of a PXI controller. The communication between the PC and PXI is through MXI-3. Because of their modular architecture, both PXI and SCXI allow easy system expansion for the future.
The user interface and the test software are written in LabVIEW. TestStand is used for test sequence configuration and execution control. This greatly reduces the software development effort and takes full advantage of each product's best capabilities.
Real Time PID Control:
To ensure the deterministic and reliable closed-loop PID control of the input pressures, PXI 7030/6040E real time card is used. PID control code written in LabVIEW RT is downloaded to the real time card at the system boot-up. The pressures are continuously monitored and adjusted during the tests. The user interface and the tests code is running on the desktop PC. The set points and process values are exchanged between the PID RT code and the LabVIEW PC code through shared memory.
The benefits of this approach are:
- Reliability. The real time card provides deterministic and reliable control of the input pressures, which is critical to the accuracy of test results.
- Simplicity. The control code running on the RT card and the user interface code running on the PC are almost independent of each other. This greatly reduces the software complexity and simplifies the maintenance of the software.
- Expandability. It also makes it possible to add new test code without affecting the other parts of the test software, especially the complicated PID control code.
- MAX usability. Only the pressure control part is run on the real time card, the rest of the inputs/outputs are still run by the LabVIEW Windows software. This allows Measurement and Automation Explorer (MAX) to be used to test and debug the hardware channels before running the whole test system. This is especially helpful during system installation.
Test Sequence Execution:
There are multiple kinds of valve sections, but these sections can be stacked together in more than 50 ways to become a different product. Each valve stack needs to have its own test sequence. The following TestStand features make it the best choice for this application.
- Drag and drop capability. TestStand provides an advance sequence editor utility. From here, with a few mouse clicks, the tests for different section valves can be configured the same way the valves are physically stacked. This provides the most intuitive way to configure a test sequence.
- One limits file. The ability of TestStand to load test limits from a single file helps reduce the effort needed to maintain the sequence files. Because all the test sequences share the same tests, one limits file ensures all the test sequences use the same limits and any change in the limits is guaranteed to be updated for every sequence run.
- Tight integration with LabVIEW. Because this test system is running on the production floor, simple and intuitive user interface is important to maintain high production yield. LabVIEW has proven itself to be one of the best environments to develop advanced user interface, with its close integration with TestStand, the combination of the two becomes the obvious choice for this system. Figure2 shows the LabVIEW user interface.
Conclusion:
The independent, yet tightly integrated NI products allow the test system to be developed in a modular fashion. Each NI product is used for what it is best for. This makes it possible to develop a robust, flexible, and easy to maintain system in a short period of time.
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