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Original Authors: Hans Thörnblom, Fingerprint Cards AB
Edited by Cyth Systems
The NI PXI hardware platform is used for the automated testing of fingerprint identification sensors.
Creating a fast and reliable system for both manual characterization and automatic regression testing to ensure that manufactured fingerprint identification sensors meet specification requirements.
Developing a PXI-based solution centered on a digital waveform generator/analyzer with PPMU for a flexible and easy-to-use system that can test and characterize all required parameters, resulting in full test coverage.
Fingerprint Cards AB (FPC) markets, develops, and produces biometric components and technologies that can verify a person’s identity by analyzing and matching an individual’s unique fingerprint. The technology uses biometric sensors, processors, algorithms, and modules separately or in combination with each other. FPC’s technology delivers competitive advantages including unique image quality, extreme robustness, low power consumption, and complete biometric systems. These advantages and the ability to achieve extremely low manufacturing costs make it possible to implement the technology in volume products such as smart cards and mobile phones, which face extremely rigorous demands. The company’s technology can also be used in IT and Internet security, access control, and more. One example of an implementation is in the Huawei Ascend Mate7 smart phone.
To ensure quality, FPC tests the functionality of the first samples of each version of a new sensor that come from our contractors before delivering them to customers. By design, we do not create dedicated prototypes. Once we design a new version, we send it to a manufacturer who produces first samples. If these samples are satisfactory, they are then the first in the new series. On top of the high-quality standards that FPC’s technology should have, the accelerated development rate in the last decade makes it important to also deliver the product fast. Thus, we use the test system for manual characterization testing and automatic regression testing to be sure that the first samples of a new sensor meet all the specifications necessary, and that mass production can continue as quickly as possible.
Left: The test system, adapter board, and user interface used in sensor validation, Right: A USB-connected PC sensor.
About the Test Application
When a new sensor goes into production, FPC gets 10–100 of the first produced samples from our contractor. Out of these, we test a handful in our lab to verify the correct functionality as well as to characterize power consumption, and to assure ourselves that we are using optimal settings for the highest possible image quality. We measure the quality of the AD converters; test all modes, parameters, and functionalities; and characterize the communication timing parameters. Since we use external contractors, we must quickly assure ourselves of proper functionality and assess the contractors’ production process as well as compare measurements to their production data.
An MSI Laptop featuring Touch Fingerprint Sensor.
We can do this quickly and reliably because of a solution based on NI PXI products we implemented. Our system consists of an NI PXIe-1073 chassis, connected to a stationary computer through an MXI-Express link. The chassis contains an NI PXIe-6556 digital waveform generator/analyzer with PPMU, an NI PXIe-6361 16-bit high-speed analog data acquisition device, and a PXI-5152 2-channel oscilloscope. The built-in signal conditioning of the PXI modules is a big benefit since it reduces the amount of custom design necessary. The only custom electronics we used is an adapter board that connects all the PXI instruments to the sensor and provides high-speed current sensing signal conditioning. This generic interface facilitates when we want to change between different types of sensors. The change happens in a manner of seconds, and the LabVIEW software is identical.
LabVIEW controls the entire application, which can perform several different tests automatically but also takes more investigative measurements. Common tests include input and output voltage thresholds, power consumption, leakage currents, continuity tests, and Iddq testing as well as characterization of the timing parameters of a serial communication bus, for example, the setup, hold, and valid times. The three modules complete each other very well. The NI PXIe-6556 digital waveform generator/analyzer with PPMU handles the high-speed digital communication and analysis and sourcing, and also accurately measures small currents. The PXI-5152 oscilloscope gives characteristics of the signals, such as amplitude, frequency, and rise time. The NI PXIe-6361 high-speed data acquisition card has excellent timing and synchronization and can measure fast power consumption transients well.
Flexible and Future-Proof Solution
Before moving to PXI, we used a mixture of USB-based data acquisition cards from NI and Cheetah, but the mixture of different USB units was a cumbersome solution and inefficient. The old instruments could not meet the performance requirements for testing coming versions of our sensors. After some investigation, we decided that LabVIEW and PXI provided an ideal platform for implementing our lab solution. Having all instruments in one system instead of multiple systems makes it easier to work with and saves desk space. It also offered advantages when we wanted to duplicate the system. We started off with a single system, but soon invested in two more identical systems. Thanks to the integrated hardware and software, duplicating the system was a very simple process. Measurement & Automation Explorer (MAX) gives a useful overview of the measurement hardware. To duplicate it we only needed to export the hardware settings from MAX to the duplicated systems and start using them. Different versions of the LabVIEW code can be easily maintained through Git version control system.
We can also create a custom user interface, and the intuitive graphical code of LabVIEW is also very convenient when communicating the test code to the sensor developers who also use the test systems for exploring new sensor solutions. The sensor developers often discuss their ideas for future functionality with the test developers who then create necessary test cases, which uses the competencies of each department, but the sensor developers can then make modifications in the programs as their work evolves to achieve fast development iterations.
Hans Thörnblom, Fingerprint Cards AB
Edited by Cyth Systems