1 July 2005
Total test traceability—TEDS
By Elizabeth Zybczynski
The plethora and cost effectiveness of PC-based instrumentation available today has increased the amount of data a person can routinely collect from a test system by several orders of magnitude.
As we find ourselves lost in this sea of data now available, among the many challenges that have arisen is traceability. Traceability is, in simplest sense, knowing definitively when, how, and on what equipment a particular device was tested. This information is imperative in order to provide on-going confidence so if calibration, test process, and/or system integrity are ever under question, someone can quantify and address the impact and scope. Good traceability can sometimes mean the difference between a recall and no action at all.
A virtual TEDS stores data in binary file that you can reference through the Virtual TEDS database.
The IEEE Standard 1451.4, commonly know as Transducer Electronic Data Sheet (TEDS), allows sensor information such as serial number, calibration constants, calibration date, etc. to go directly on the sensor itself. The data sheet is stored in Non-Volatile RAM (NVRAM) on a "1-wire" memory chip. The NVRAM can store either 256 or 4,096 bits of data and can exchange data and receive power over a single line.
The 1451.4 standard includes mixed mode interface (MMI), which allows for the sending, by the sensor, of its traditional analog signal as well as the sending and receiving of a digital signal containing the sensor information. The MMI allows traditional sensor data and TEDS information to transfer without additional connections.
Implementation of the MMI can occur two ways—Class I 2-wire or Class II multi-wire. Class I implementation includes a diode or analog switch, which allows the digital signal to multiplex the same line as the analog signal and sees use with constant-current powered sensors such as accelerometers and microphones. Class II implementation, the type used for most sensors, separates the wiring for the analog and digital signal but still maintains a signal sensor connection for easy and error-free hook-up. The Class II interface will allow TEDS capability with virtually any type of sensor and allow for relatively simple after-market upgrade of existing non-TEDS sensors.
TEDS in testing
In addition to the simplified troubleshooting and reduced test setup costs, TEDS technology can greatly improve test traceability in several ways. First, it eliminates the manually entered information during test setup needed to track critical items such as sensor identification and calibration information. This will increase confidence in and compliance with the traceability process. Second, it allows the system to actively confirm the proper sensor is present and in calibration. Additionally, for systems requiring end-to-end calibration or system environment revision control (considering calibration and self-test validity as specific to an exact combination of software, instrumentation, and sensors and explicitly tracking such combinations with all data), TEDS allows electronic automatic detection of sensor changes or recalibrations.
By incorporating TEDS technology in the test systems and calibration systems, you can fully automate calibration and calibration data handling with zero human handling of calibration data. During the calibration process, reference standard IDs, you can electronically query or calculate calibration date and calibration constants and then store them directly to the sensor memory. The data then "travels" with the sensor awaiting automatic retrieval by the test system. The test system automatically monitors upcoming calibration requirements.
As it is not feasible for most organizations to convert to all TEDS sensors at once or in the event that some sensors prove to be non-TEDS compatible, Virtual TEDS is available to bridge the transition smoothly. Instead of storing data sheet directly onto the sensor, virtual TEDS stores data in binary file, which you can then reference through the Virtual TEDS database, using the sensor serial number or other unique identifier.
The IEEE 1451.4 Standard and the commercially available sensors and signal conditioning equipment that supports it, will increase the confidence we have in our data as it ensures how correct and how current the calibration information is on a per sensor basis.
Behind the byline
Elizabeth Zybczynski is an engineering team leader at the Minneapolis office of VI Engineering, Inc. VI Engineering is a registered member of the Control and Information System Integrators Association (CSIA). Her e-mail is firstname.lastname@example.org.
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