Isolated A-D loadcell transformer
November 2005
Maintenance, Test & Measurement, Calibration
The SNAP-AILC module provides one channel of transformer and optically isolated analog-to-digital conversion for loadcell inputs. This module supports 6- or 4-wire loadcells. (4-wire is at reduced performance.) It provides isolated 10 V d.c. bridge excitation for up to four parallel 2 mV/V or 3 mV/V 415/350 Ω loadcells as a loadcell front end. 24 V d.c. is required at the separate connector on top of the module for the isolated 10 V d.c. bridge excitation.
The bridge and 10 V d.c. bridge power is isolated from the 24 V d.c. bridge source and the 5 V d.c. source used by the mounting rack. The 24 V d.c. input is reverse polarity protected, and it may be used for multiple modules. The 10 V d.c. bridge source is current limited so that an external fault on the loadcell is isolated and will not affect another loadcell module.
There are two channels of data: channel 1 is unfiltered, and channel 2 is filtered. Filtered data has a fast settling feature with a programmable fast settle level. The module snaps into Opto 22 SNAP B- & M-series mounting racks. The field connections are made by a pluggable terminal strip located on top of the module.
Features
* Works with 2 mV/V and 3 mV/V loadcells.
* Provides isolated 10 V d.c. bridge excitation for up to four 350 Ω loadcells.
* Powered by a single 5 V d.c. power source supplied by rack.
* Convenient pluggable wiring terminals, including terminals for external 24 V d.c. power supply, which converts to isolated 10 V d.c. for bridge excitation.
* Rugged packaging.
There are two channels of data on the SNAP-AILC module. The first channel provides raw Analogue-to-Digital (ADC) data, while the second channel provides filtered data. Filtering is configured in ioControl software using two commands: Set Analogue Load Cell Filter Weight and Set Analogue Load Cell Fast Settle Level. The Set Analogue Load Cell Filter Weight command filters noise on the filtered channel of a loadcell module to a range of 0 to 255 (1 byte), at the expense of settling time. The filtered reading is calculated from the ADC reading after every ADC conversion. For maximum resolution, the filter weight is set to less than half the peak-to-peak system noise as seen while trending the data in ioDisplay.
The Set Analogue Load Cell Fast Settle Level command sets the fast settling trigger level (FSTL). The filtered weight is reduced when the difference between the ADC data and the filtered data is greater than the FSTL. This feature is useful to decrease settling time when there are large step changes in the loadcell output and a large filter weight.
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