1756-ENBTA Allen- Bradley ControlLogix EnterNet

The 1756-ENBTA has a hassle-free tuning feature, so during the tuning process, peaks are created by changing the controller gain or derivative settings.

It is due to the collisions in the controller output caused by changing the tuning settings that it is good practice to temporarily place the controller in manual mode when making tuning changes. Since most controllers have unobstructed transmissions, the collision can be eliminated when switching the controller back to auto after a manual mode tuning. However, this becomes a problem if the tuning settings are changed, as in the case of gain scheduling.

Trouble-free tuning can be achieved without placing the controller in manual mode by calculating how much the controller output will jump due to the new scale and derivative settings, and subtracting the equivalent amount from the integral term so that the sum of the three terms (controller outputs) remains constant. Similar to faultless transfer, faultless tuning can also be achieved by using a velocity algorithm

The 1756-ENBTA means that as the controller tries to close the valve and reduce the pressure, the output of the PIP-2 will decrease. When the output drops below 30%, it has no effect on the valve because the high select logic selects the PIP-1 output and the controller has full control of the valve.

Everything is fine, except that the PIP-2 output continues to drop to 0%. If, for any reason, the suction pressure of the compressor drops below the set point than PIP-2, the control output must now go from 0% to 30% before the valve can be opened to increase the pressure.

At this point, the pressure could easily drop further and the compressor could be triggered at a low suction pressure.