A client asked NI Gold Alliance Partner Circuit Check Inc to design a solution to parallel test industrial inkjet print heads in dramatically less time. As an added challenge, the tester needed to be compliant with a class 10,000 clean room on the production floor. The test system also needed a high level of flexibility to deliver testing capabilities for future products.
The Platform and Instrumentation
We designed a unit and built it into a 19 in. cabinet. We also built a custom fixture to fit six print heads simultaneously and provide the connection interface to the PXI modules. We built the core of the system on the PXI platform with an NI PXIe-5160 2-channel, 2.5 GS/s oscilloscope in an NI PXIe-1062Q chassis. We connected a PC running LabVIEW software and TestStand software to the chassis through MXI link. The PXI system also contains a number of high-speed switching cards to deliver the required channel count to the print head test fixture.
The modular approach of PXI means we can adapt the system if the test requirements change. This helped satisfy the customer’s desire for a flexible test system capable of meeting future requirements.
The print heads were driven with a special digital pattern. We developed a custom state machine to control the print head driven by an Ethernet interface. This was a crucial component in making a fast solution. We could deliver test patterns in milliseconds instead of seconds, leading to spectacular reductions in test time.
LabVIEW was the perfect choice for the heavy lifting of the analysis this because of its easy to understand graphical design method and build in multi-threaded support.
Using TestStand and LabVIEW
We chose TestStand as the test executive because it delivered a comprehensive framework beyond that of a simple test sequencer. Key features within TestStand include:
- A test sequencer provides control of the test execution order (with loop s and branches)
- Parallel execution makes it possible to execute a single sequence concurrently for each print head using the batch process model
- A custom user interface shows the test execution in real time and reports the status of the test (for example, pass or fail)
- An administrative structure delivers operator traceability with server login and access
- Database connectivity manages test limits and results storage
We implemented individual tests with LabVIEW VIs that used the PXI digitiser and a programmable power supply to take voltage and current readings. TestStand brought these individual tests together into a sequence to meet the test specification. This structure inherently encouraged code reuse and a disciplined approach to test specification implementation.
Due to the complexity of the test system, we used a simple LabVIEW user interface for TestStand, which eased system use for the operator. We expended considerable effort to ensure that hardware errors caused by the device under test were recovered gracefully and fully logged to file. TestStand delivered the ideal framework for a solid foundation whilst maintaining the flexibility to customise the environment when necessary. For this application, we customised the recovery, user interfaces, and custom steps, as well as elements of the data handling to meet the specific user requirements.
We developed a maintenance application to manage basic tester functions and provide engineers with a debugging interface to use when looking for electrical or mechanical faults with the print heads.
To achieve the desired test throughput, the customer needed the ability to test six print heads concurrently. We can configure TestStand to run with as many separate sockets as needed through a simple change within the sequence settings. When running parallel tests, TestStand can manage hardware availability between individual instruments to achieve the highest system throughput.
We used a batch model so that, when possible, tasks occur in parallel on the six print heads. Using this testing method reduced overall cycle time by approximately 14 percent.
A key feature built into TestStand is that it offers logging for the test data. We created a log each time a value is checked against limits. We reported this data at the end of the sequence, and there are a number of formatting options available. The XML format is easy to view, but with a typical count of 1,500 distinct tests, is inconvenient for examining all but the most basic of failures. The built-in database connectivity works as a more appropriate solution for storing these large data sets. For this project, we decided to use a MySQL database, so we could link data in TestStand to a data source within the database. Only the hard disk of the server limits the database size.
To assist production and maintenance, we developed a LabVIEW application to interrogate the data history and display a number of useful summary views, forming a very useful debugging tool. We developed the user interface to include a left hand panel that shows the nest, serial number, and colour-coded pass/fail status.
Upon delivery of the system, the customer instantly saw benefits in three key areas: data analysis, reliability, and yield. The quality and detail of data analysis offers the opportunity to go beyond simple pass/fail results, instead potentially helping to determine strength of bond and predict early life failures. The machine’s ongoing reliability is popular with both the manufacturing teams and equipment support engineers, and gives the customer confidence in manufacturing stability and capability. The yield has increased by 60 percent, which is a significant improvement over the previous tester. The ease with which users can load the product means that, together with the speed at which the test is conducted, significant throughput improvements on original testing methods have been realised.
Conclusion—The Deployed System
The tester could test parts from two stages of the process, automatically detecting the state of the device under test and changing the sequence, thanks to the flexibility of the PXI cards used in the system. The tests generate better quality measurements than before, due to the use of the more accurate digitiser and better signal management. This means we can refine the tests in the future to give even more diagnostic information on faults. The fixturing is more accessible for support staff, leading to faster turnaround for preventative maintenance. The system is free-standing and easily moved, which suits a dynamic customer that relocates often.
The primary objective of this test system was to reduce test times. The new tester reduced a 6–7 minute test to just over 1 minute, and more speed improvements are possible with the 64-bit upgrade now available in TestStand. This is a revolution to the manufacturing process, setting the standard for future testers.
Circuit Check Inc
Circuit Check Inc, Unit C9, Endeavor Business Park, Penner Rd
Havant PO9 1QN