This article was printed in CONTROL's May 2009 edition.
Any multiple-load-cell system requires a summing junction box to sum and trim the outputs. To achieve the speed necessary for many applications, load cell users traditionally had to use a single-channel, analog-to-digital converter (ADC) and an analog summing box. In doing so, the ability to diagnose individual load cell performance was lost. Some digital summing boxes offer beneficial cell diagnostic data, but as the number of modules in a platform increase, a system will lose speed. In addition, their proprietary protocol requires using a specific indicator, such as a display, panel meter or other readout device.
The process control industry has been waiting for an affordable, dynamic solution that can bring weight-based processes into the 21st century. Rice Lake Weighing Systems says it has that solution with the release of iQUBE2 junction box, which can update up to 200 times per second on multi-channels. The iQUBE2 offers simultaneous weight and diagnostics capabilities, isolated communications, transient protection, multiple scales and sums, emulation and and system scalability up to 16 load cells.
For more information, call 800/472-6703, email firstname.lastname@example.org or browse to www.ricelake.com/exclusive.
A digital junction box should do much more than just sum and trim, says Rice Lake. “Our junction box can simultaneously monitor load cell function and provide incredible speed,” claims Tony Boehm, senior electrical engineer. “What’s more, advanced diagnostic features also alert the user the instant a problem is detected, and the system can compensate for that failure immediately to keep the scale operational.”
Boehm says a theoretical calibration option uses each load cell’s physical characteristics to calibrate the scale with a single command. Additional trimming and calibration options include section-match for trimming based on paired cells, corner-match for individual cell trimming and scale-span for when trimming isn’t required. “If a load cell is damaged, the system can emulate that cell’s output based on the output of its neighboring cells,” adds Boehm. “A damaged board can be replaced simply by uploading its parameters to the replacement, and the scale typically will retain its calibration to 0.02% of full scale output.”
Each iQUBE2 circuit board is designed with four high-speed, 24-bit Sigma-Delta ADC converters, but the daisy-chainable system can be expanded to four boards and accommodate 16 load cells. Boehm says ADC channel sample rates range 2.5–500 Hz with noise performance ~3 nV @ 15 Hz and 17 nV @ 500 Hz.
Each ADC channel has cell-level diagnostics that check for cell connection faults, under- and overloaded cells and excessive noise. The health of each load cell is indicated by a bi-colored LED, but can also be communicated serially and/or configured for remote monitoring using the four I/O channels. In addition, each circuit board continuously monitors the input power supply voltage and excitation voltage. Scale specific diagnostics check for cell linearity, failure of a scale deck to return to zero and cell drift under load.
When configured as an input, the digital I/O can perform common scale tasks such as Tare, Zero, Units, etc. As an active low output, an I/O channel can sink 25 mA and can be controlled via host commands or be assigned to a scale as a free-running set-point, e.g., Trip Higher, Lower, In-Band, etc.
“The iQUBE2 was designed with a simple ASCII-based open protocol allowing any HMI to retrieve usable weight data,” says Boehm. “Six data formats can be customized for serially streaming or polling in nearly any process controller format.”
The serial communication port is isolated to prevent ground loops and uses a multi-protocol system that accommodates RS-232 and RS-422/485 (full or half-duplex). Available option cards include fiber optic, Ethernet, wireless Ethernet, USB and multi-protocol RS-232/422/485.