Whenever we need to determine whether a tension input problem is a load cell, or internal to the Z4 controller itself, it is a simple matter of measuring the voltage at the input terminals. If these voltages are correct and repeatable with no force applied to the load cell(s), the problem is internal. On the other hand, if these voltages vary over time, or temperature, the issue may be the load cell(s) themselves.
The excitation voltages should be a nominal +2.5V and -2.5V. Note that these can vary slightly from the nominal 2.5V magnitude. It is of utmost importance that they remain consistent during the operation of the load cell within the system. If these voltages are inconsistent, the tension readings will also be inconsistent.
Positive Load Cell Input
As force is applied to the load cell, this voltage should swing in the positive direction. When force is removed from the load cell, it should provide a “reasonably consistent” reading over time.
Negative Load Cell Input (If Present)
This input is only present when using dual half-bridge load cells, or, a full-bridge load cell. As force is applied to the load cell, this voltage should swing in the negative direction. When force is removed from the load cell, it should provide a “reasonably consistent” reading over time.
Due to stiction and other mechanical effects, load cells may not return to their “zero” output levels; however, they should be “reasonably” close over multiple load cycles.
During full-scale calibration, the voltage at the Positive Load Cell Input (as well as the Negative Load Cell Input if used) should be “reasonably” repeatable. Variations in this voltage could be due to quite simple things like differences in the calibration weight as well as using a slightly different wrap path when hanging the weight. If all things are equal, these voltages should be as well.
In some load cells, there is a temperature dependency where either the gauge resistance or gauge factor, can vary with temperature. Keep this in mind when calibrating load cells and try to maintain a consistent calibration temperature. Although this is typically a subtle difference, it is still a possible source of variation.
When using full-bridge load cells, it is the difference between the Positive Load Cell Input and the Negative Load Cell Input that is being monitored for changes in tension. Voltages relative to ground can vary slightly; as long as they vary in common, the measurement will be correct. This is part of the advantage of full-bridge over half-bridge load cells.
There is a phenomenon in many load cell gauges called “gauge creep” caused by the relaxation of the adhesive bonding the gauge to the beam. If a constant force is applied to a load cell over very long durations, the strain applied to the gauge “relaxes” and is slightly reduced. Once the force is removed, it may take a short period of time for the bond between the gauge and beam to return to its “normal” distribution.