Still more than one way to 'plug-and play’

Dr. Raymond B. Sepe Jr. -- Control Engineering, 10/1/2006

The basic TEDS architecture supports different data storage and retrieval methods.

Goal: take a sensor, connect it to electronic instrumentation, and collect data, with automatic calibration of the sensor with the instrumentation. The problem: every sensor and every instrument differs.

A sensor is a transducer that changes a mechanical or environmental parameter into an electrical signal. That signal is passed to conditioning electronics to scale the signal into the desired unit of measure. To work properly, the gain introduced by the sensor and the electronic instrumentation must be determined with a calibration procedure.

Virtual TEDS ID system shows PC used to retrieve database information and act as a host for data acquisition electronics.

For accuracy, the sensor and instrument are paired, and the calibration of the effective gain of the system is conducted in its entirety. However, the electronics instrumentation and the sensor are often calibrated separately. The sensor manufacturer provides the sensor gain factor in a calibration certificate, while the instrumentation manufacturer has already compensated for the gain of the electronics within the instrument. (The sensor gain factors are entered into the instrumentation through a front panel or computerized interface.) To automate this process, an instrument needs to be able to gather the sensor gain factor automatically whenever the sensor is attached to the instrument.

The TEDS (transducer electronic data sheet) IEEE 1451 specification provides an industry-standard transducer electronic data sheet containing the sensor gain factor in a memory location that can be read automatically by an instrument whenever a new sensor is attached. The 430-page TEDS 1451 document contains complicated template formats, interpretive language specifications, and protocols that are burdensome on embedded instrumentation.

Variants of the TEDS standard differ on how data are stored, updated, and retrieved. In a basic TEDS architecture, data are stored in the sensor via an electronic chip added within the sensor housing. This chip contains an electronic identification tag and programmable memory. A TEDS-compatible instrument or computer writes the sensor’s calibration data and related information into the memory chip in the electronic format specified by TEDS 1451.

When this sensor is connected to TEDS-compatible instrumentation, the sensor calibration data are automatically read from the memory and installed into the instrumentation. For proper operation, the vendor writing the data into the memory chip and the instrument manufacturer need to interpret the TEDS standard identically. When the sensor is moved to other instruments, the procedure is automatically repeated. Note that the sensor and the instrument are calibrated separately.

TEDS identification system includes a tiny electronic chip with only electronic tag incorporated in housing. Sensor data are stored in computer database.

TEDS tag identifier and electrical interface are housed within the sensor. Sensor calibration data are stored directly within the instrument.

Computer-based instrumentation and PC cards that support the TEDS standard are available. Not all TEDS-based instruments write calibration data into the sensor. Some only read the prerecorded data. Others support basic TEDS, containing serial number and manufacturer, not calibration data. PC-based data acquisition systems use the computer to format, interpret, and translate TEDS data. The electronic chip cannot fit in some sensor housings because of space constraints or demanding environments, or because the legacy sensors are not outfitted for TEDS. In these cases, the TEDS concept can be supported with a virtual TEDS system.

With virtual TEDS, the electronically stored sensor data are saved in a computer database, accessed via the Web, and downloaded into a local machine. Although automatic sensor identification is not supported, the calibration data are available electronically. In this system, a PC is used to retrieve the database information and act as a host for the data acquisition electronics. This architecture is not well-suited to embedded instrumentation applications.

The virtual TEDS system uses a computer database to store sensor data supplied to the user electronically, but it does not have the desired plug-and-play operation. With the TEDS identification system, a tiny electronic chip with only the electronic tag is incorporated into the sensor housing, while the sensor data are stored in a computer database. This allows for plug-and-play operation, but requires a PC to download the sensor data. This architecture is not well-suited for embedded instrumentation or stand-alone instruments that do not use computers.

With the TEDS identification system, the tag identifier and electrical interface are housed within the sensor, while sensor calibration data are stored in the instrument. A PC is not required. The user enters the calibration data into the unit once, then the unit recognizes the sensor and recalls the necessary sensor calibration data from internal memory. No computer, database, or additional connectivity is required, and plug-and-play operation is achieved. The sensor may be paired with a particular instrument and precisely calibrated as a system.

Because each instrument can store its own calibration data for a particular sensor, slight differences in gains between different systems that include the sensor, cabling, and instrument are automatically compensated for during the calibration process, allowing for matched calibration sets. The number of sensors identified on a particular unit is limited by the unit’s internal memory, but with products that can store 25 sets of sensor data, there is no practical limitation. If the electronic tag cannot be incorporated directly into the sensor, it can be enclosed within the sensor’s cable hood or in a small inline adapter. System is suited to embedded instruments and stand-alone instrumentation.

Dr. Raymond B. Sepe Jr., Electro Standards Laboratories, www.ElectroStandards.com; www.ieee.org