Emerson E2 operation manual Other PID Features, Changing the Output at Setpoint

THROTTLING	OUTPUT AT
RANGE	SETPOINT
	(shown here as 50%)

0% 100%

CONTROL INPUT

26512028

Figure D-4- Centered Setpoint PID Control (Output @ Setpoint = 50%)

Therefore, as the input value gradually drops below the setpoint, the Proportional Mode of PID will gradually bring the output percentage down. When the input drops below the low end of the throttling range, the output will be zero.

The opposite happens with the input value begins to rise above the setpoint. The output percentage is gradually brought up from 50% until, when the input is at or above the throttling range, Proportional Mode will bring the out- put to 100%.

Output at Setpoint for Condenser/HVAC PID Control

For Condenser and HVAC PID Control, the Output at Setpoint defaults to 0%. This places the 0% end of the Throttling Range at the setpoint value, and it generally means Proportional Mode will strive to pull the input back down below the setpoint and attain a 0% output percent- age. As the input climbs from the setpoint to the top of the Throttling Range, Proportional Mode likewise increases the output percentage to 100%.

Figure D-5- End Setpoint PID Control (Output @ Setpoint = 0%)

The differences between the two PIDs can be seen by observing the throttling ranges. For non-Condenser Con- trol applications, PID reacts to error that exists on either side of the setpoint. If the input goes lower than the set-

point, the output percentage is dropped from 50% (it is assumed this will result in the input rising back to the set- point). Likewise, when the input is higher than the set- point, the output percentage is raised. For Condenser Control PID, no reaction is made to an input that is lower than the setpoint, since the output percentage is already 0%.

Changing the Output at Setpoint

Advanced users may wish to change the Output at Set- point in order to change the stabilization characteristics of their system(s). In most cases, the default values of the Output at Setpoint (0% for Condenser Control and HVAC, 50% for all others) are appropriate and recommended. Changing the value of the Output at Setpoint may have some beneficial results, but at the same time it may also hamper the system’s ability to react to changing condi- tions, and it may also result in decreased energy efficiency.

Other PID Features

The following section lists other PID-related constants or control structures that are a part of some PID control algorithms. Many of these features are advanced features that are accessible only by service personnel or advanced users. As a general rule, it is best to contact Retail Solu- tions before changing any of these values.

Output at Minimum / Output at Max- imum

The Output at Minimum and Output at Maximum val- ues are used to change the range of possible output per- centages. The default values (0% for minimum, 100% for maximum) allow the output to vary from fully off to fully on; changing these values, therefore, would only be neces- sary if you desire a limited output range.

Two things occur when the Output at Minimum or Output at Maximum values are changed:

1.The throttling range is shortened as dictated by the new values. For example, if you specified a 20% minimum, the portion of the throttling range that would usually represent the range of outputs from 0% to 20% is cut off. Any input value that falls within the previous 0% to 20% portion of the throttling range will cause the Proportional Mode to move the output towards 20%.

2.The Output at Minimum or Output at Maximum value replaces any calculated adjustment that would normally call for the output to be outside of the specified range. In other words, if the Out- put at Minimum and Output at Maximum values

Other PID Features Appendix D: PID Control • D-5