Tagging Load Cells

Aug. 10, 2006
Automatic identification of load cells and other sensors reduces human errors while speeding the task of swapping in new hardware.

The key to creating a TEDSTagcompliant load cell is this DS2401 electronic ID chip from Dallas Semiconductor. Its small size lets it sit inside a load-cell housing or within the connector hood.

The electronic identification chip is installed within the connector hood of a standard S-type load cell. The chip automatically identifies the sensor to the associated electronics by assigning the device a unique serial number.

When it's impossible to mount the identification chip within the sensor itself, a special adapter is created to insert the device in-line. The sensor connects to one end while the other plugs into the instrument.

The front panel of a dual-channel ESL Model 4215 Intelligent Indicator signals the presence of two TEDS-Tag load cells attached to it. The numerical data shown with the Auto ID label is a cell serial number that a user entered during initial calibration. The 4215 can hold the calibration data for up to 25 load cells.

Dr. Raymond Sepe Jr.
Electro Standards Laboratories
Cranston, R.I.

Suppose you're out on the road and carrying a variety of load cells that span different ranges to calibrate a system in the field. Or maybe you must measure a wide range of forces for the application at hand. In either situation, you must change the load cell connected to your instrument before you can continue. Then you'll have to enter new calibration factors for the instrument.

Manually entering this data, or just making the operator select from stored calibration parameters, introduces the possibility of user error. Entering the wrong parameters for a load cell, or worse, corrupting the existing calibration data, can bring bad results and costly recalibration.

It would be better, of course, to let the instrument automatically identify the load cell attached to it and self-install the proper calibration data.

Help along those lines is available through TEDS, the Transducer Electronic Datasheet. With TEDS, calibration and other data reside inside a memory chip installed within a TEDS-compliant load cell. At first glance the TEDS standard seems complicated. For flexibility, the TEDS standard specifies a huge numberof detailed electronic data templates with some degree of standardization. But the TEDS working group traded simplicity for flexibility. Even with the data templates, there is no guarantee different vendors of TEDS-compliant systems will interpret the data in the electronic templates the same way.

More importantly, a particular vendor's TEDS unit may not support the calibration data your application needs. The TEDS user must have a way to write the data into a TEDS-compatible load cell in the proper format. This takes place either through a TEDS-compatible instrument that has both write and read capabilities, or through some other TEDS-datawriting system, typically using a personal computer.

It should be noted that the calibration data stored in the load cell is the same regardless of the instrument connected to it. In other words, the load cell can't compensate for the eccentricities of specific instruments. Precision applications that include matched load cells and multiple instrument systems can present problems. For example, a fieldservice calibration group might attach different load cells to different instruments during the course of the day. And calibration is, of course, by definition highly precise.

A new auto-identification system developed by Electro Standards Laboratories (ESL) addresses these limitations. Known as the TEDS-Tag, it retains the self-identification feature found in the TEDS standard but works on any load cell. Operation of the TEDS-Tag becomes transparent to the user when the cell connects to either the Model 4215 Smart Indicator or Model 4325 CellMite embedded digital signal conditioner. The Tags even support multiple load cell/ instrument pairings.

The TEDS-Tag system places a small identification chip in the cable that extends from the load cell or within the load-cell housing. Each chip contains a unique electronic serial number the Model 4215 or CellMite reads to identify the load cell. The load cell connects to the device under test and a standard calibration procedure takes place. The instrument automatically stores the calibration data along with the unique load-cell identification number from the microchip.

Whenever that load cell reconnects to the instrument, it is automatically recognized and self-installs the appropriate calibration data. The result is true plug-andplay operation.

The calibration data automatically includes compensation for the particular instrument to realize high-precision matched systems. If the load cell moves to another instrument, that instrument recalls the appropriate calibration data stored internally. With the ability to store multiple calibration entries, load cells can form a matched calibration set for multiple instruments.

The key to creating a TEDSTag-compliant load cell is an electronic identification chip made by Dallas Semiconductor, the DS2401. The chip is readily available from many distributors and typically costs under $2.00. Housed in a TO92 transistor case, the chip easily fits into a cable hood or load-cell housing.

Both the Model 4215 and 4325 CellMite connect to the load cell via a DB9 connector with identical pin outs. The electronic identification chip does not interfere with the load-cell signals. Direct connections from the DS2401 to the DB9 connector enables plugand-play operation.

When using off-the-shelf load cells, it is often convenient to locate the DS2401 in the hood of the cable. Thus any standard load cell is transformed into a TEDSTag plug-and-play load cell once the chip is installed.

Applications with restricted access to the load cell and cable can use an in-line tag identification module. A straight-through in-line cable adapter can incorporate the DS2401 electronic tag chip. Here, the cable adapter actually goes in series with the load-cell cable before it is plugged into the instrument. This technique also works for applications that demand different calibrations on the same load cell. The user may have a single load cell and instrument but can change which calibration is autoselected by simply changing the in-line cable adapter.

Because each cable adapter has a different tag identification chip, the instrument associates a different calibration data set with each inline adapter. For example, suppose a load cell needs a precision six-point linearization in two different operating ranges. Once converted, the now-TEDSTagcompliant load cell connects to the ESL Model 4215 or CellMite instrument. A standard calibration procedure-takes place to initialize the calibration data in the instrument the first time the cell connects.

The ESL instrument supports a variety of industry-standard calibrations including millivolt/volt, shunt, two-point, or multipoint calibration. The instrument automatically detects the presence of the TEDS-Tag and matches it with its calibration data. From this point forward the system is completely plug-and-play. A typical startup procedure now consists of plugging the load cell into the ESL instrument, turning on the power, and taking a reading.

After initial calibration, a cell change simply involves plugging the new cell into the instrument and turning on the power.

Electro Standards Laboratories,
(877) 943-1164, electrostandards.com

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