Tension Load Cell – Common Issues..

Field service engineers require a variety of load cells spanning the numerous ranges necessary to calibrate their customers’ systems. They may also require the assortment to conduct a variety of force measurements for the testing application. The challenge begins when the engineer has to change the load cell that is attached to his instrument before he can continue. When the multi axis force sensor is attached to the instrument, the appropriate calibration factors need to be set up in the instrument.

Avoiding user-error is really a major challenge with manual data entry or with requiring the engineer to pick from a database of stored calibration parameters. Loading a bad parameters, or perhaps worse, corrupting the existing calibration data, can cause erroneous results and costly recalibration expenses. Instrumentation that automatically identifies the burden cell being connected to it and self-installing the proper calibration details are optimal.

What exactly is Transducer Electronic Datasheet? A Transducer Electronic Data Sheet (TEDS) stores transducer identification, calibration and correction data, and manufacturer-related information in a uniform manner. The IEEE Instrumentation and Measurement Society’s Sensor Technology Technical Committee developed the formats including common, network-independent communication interfaces for connecting transducers to microprocessors and instrumentation systems.

With TEDS technology, data could be stored inside of a memory chip that is installed within a TEDS-compliant load cell. The TEDS standard is complicated. It specifies a huge number of detailed electronic data templates with many degree of standardization. Even when using the data templates, it is far from guaranteed that different vendors of TEDS-compliant systems will interpret what data is put into the electronic templates in the same way. Moreover, it is not apparent that this calibration data that is needed within your application will likely be maintained by a certain vendor’s TEDS unit. You must also ensure that you have a means to write the TEDS data in to the TEDS-compatible load cell, through a TEDS-compatible instrument that has both TEDS-write and TEDS-read capabilities, or with the use of a few other, likely computer based, TEDS data writing system.

For precision applications, such as calibration systems, it also need to be noted that calibration data which is stored in the stress cell is the same regardless of what instrument is attached to it. Additional compensation for that instrument is not included. Matched systems in which a field service calibration group could be attaching different load cells to various instruments can present an issue.

Electro Standards Laboratories (ESL) has developed the TEDS-Tag auto identification system which retains the attractive feature of self identification found in the TEDS standard but may be implemented simply on any load cell and, when linked to the ESL Model 4215 smart meter or CellMite intelligent digital signal conditioner, becomes transparent towards the user. Multiple load-cell and multiple instrument matched pair calibrations are also supported. This is often a critical advantage in precision applications like field calibration services.

With the TEDS-Tag system, a tiny and inexpensive electronic identification chip is placed in the cable that extends through the load cell or it can be mounted within the cell housing. This chip includes a unique electronic serial number which can be read through the ESL Model 4215 or CellMite to distinguish the cell. The cell will be linked to the unit along with a standard calibration procedure is performed. The instrument automatically stores the calibration data inside the unit itself along with the load sensor identification number through the microchip. Whenever that cell is reconnected for the instrument, it automatically recognizes the cell and self-installs the appropriate calibration data. True plug-and-play operation is achieved. With this system the calibration data can automatically include compensation for the particular instrument so that high precision matched systems could be realized. Moreover, in the event the cell is transferred to another instrument, that instrument will recall the calibration data that it has stored internally for that load cell. The ESL instruments can store multiple load cell calibration entries. In this manner, multiple load cells can form a matched calibration set with multiple instruments.

Any load cell can easily be made right into a TEDS-Tag cell. The electronic identification chip, Dallas Semiconductor part number DS2401, is readily provided by distributors or from ESL. The chip is extremely small, which makes it simple to match a cable hood or cell housing.

Both ESL Model 4215 smart strain gauge indicator and the CellMite intelligent digital signal conditioner are connected to load cells via a DB9 connector with identical pin outs. The electronic identification chip does not affect the cell’s signals. Pin 3 of the DS2401 will not be used and can be cut off if desired. Simply connecting pins 1 and two from the DS2401 to pins 8 and 7, respectively, in the ESL DB9 connector will enable plug-and-play operation.

When you use off-the-shelf load cells, it is often convenient to locate the DS2401 in the hood in the cable. The cell comes with a permanently mounted cable that protrudes from the cell housing. After the cable, strip back the insulation from your individual wires and solder the wires in to the DB9 connector. The DS2401 is soldered across DB9 pins 7 and 8, and fits inside the connector’s hood. For a couple dollars in parts as well as a simple cable termination procedure, you have taken a standard load cell and transformed it into a TEDS-Tag plug-and-play unit.

For applications by which access to the load cell and cable is fixed, an in-line tag identification module may be simply constructed. A straight through in-line cable adapter can incorporate the DS2401 electronic tag chip. In this application, the cable adapter is actually positioned in series with the load cell cable before it is plugged into the ESL instrument. Additionally it is possible to use this technique in applications where different calibrations could be required on the same load cell. The ifegti may have a single load cell and instrument, but may change which calibration is auto-selected by simply changing the in-line cable adapter. Since each cable adapter features a different tag identification chip, the ESL instrument will associate an alternative calibration data set with every in-line adapter. This can be useful, as an example, when a precision 6-point linearization in the load cell is necessary in two different operating ranges the exact same load cell.

Now that the burden cell has been converted to a TEDS-Tag unit, it can be connected to the ESL Model 4215 smart strain gauge indicator or even a CellMite intelligent digital signal conditioner. The 1st time it is connected, a regular calibration procedure is conducted to initialize the cell’s calibration data within the instrument. The ESL instruments support a variety of industry standard calibrations including mV/V, shunt, 2-point, or multiple-point calibration. The instrument then automatically detects the presence of the force transducer and matches it with its calibration data. From this point forward, the system is entirely plug-and-play.

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