October 2002Maintenance, Test & Measurement, Calibration
Instrotech's range of Revere stainless steel compression and beam load cells are now available with digital output. The RS485 I RS422 bus allows the scada or DCS to communicate directly with each load cell in the system, thus offering advantages in system set-up, control, corner correction, fault finding and load cell replacement.
Within the smart load cell is a multilayer printed circuit board with a power regulator, which supplies an excitation voltage to the Wheatstone bridge. The uncompensated output of the bridge is fed through an A/D converter and digital filter. A microprocessor controls the A/D converter, a temperature sensor and the digital output interface.
The cells offer an extensive diagnostics structure, which verifies the integrity and operation of the strain gauge circuit, power supply, EEprom, A/D converter and temperature sensor. Smart load cells produce a strong output signal, which is unaffected by electrical noise and temperature fluctuations on field cabling, thereby enabling the use of cable lengths up to 1200 m. A surge protection device is integrated to protect against overvoltages and lightning strikes.
These hermetically sealed IP 66/68 units are of stainless steel construction and are for accuracy classes C3 and C4. Digital output is via RS485 or RS422 interface and can be processed by a computer, PLC or a dedicated indicator. Multiple systems with up to 32 load cells may be connected to one control unit. Software drivers are available for the user to develop their own system software.
A rationalised output of 240 000 counts means that within a system, any load cell can be replaced without the need for recalibration. Low and medium accuracy systems can be automatically calibrated without the need for dead weights. Capacities for the beam load cell are 0,5 to 10 ton and 10 to 100 ton for the compression type.
The marginal extra cost of these load cells as opposed to standard units is offset by not requiring a load cell amplifier.
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