Emerson E2 operation manual How Condenser Control Hvac PID Differs From Others, Output at Setpoint

sate for error.

To reduce this lag time, Derivative Mode is used. Derivative Mode constantly analyzes the rate of change of the error, makes a prediction about what the future error will be, and makes an adjustment to the output in an attempt to reduce the rate of change in the error.

In layman’s terms, Derivative Mode causes PID con- trol to “overshoot” the amount of output percentage to compensate for the slow reaction times of the P and I Modes. As a result, Derivative Mode slows the rate error change down to a level the P and I Modes can handle.

The “D” Mode Calculation

To determine the “D” Mode adjustment for each update, PID performs the following calculation:

“D” mode adjustment = Kd * (E – (2E-1/Δt-1)+(E-2/Δt-2))

Kd = derivative gain E = current error

E-1=error from the previous update

Δt-1=the amount of time elapsed since the previous exe- cution

E-2=error from the update before the previous update Δt-2=the amount of time elapsed between 2 executions

ago and the previous execution

The factors E-1/Δt-1and E-2/Δt-2are the rates of change of the error (in units per minute). The rate of change for the previous error (E-1) weighs twice as much in the

Derivative Mode calculation as the 2nd previous error (E-

2), since E-1is closer to the current rate of change than E-2.

The derivative gain Kd is a multiplier that changes the total size of the Derivative Mode adjustment. If Derivative Mode is causing PID control to react too quickly or too slowly, the derivative gain may be adjusted to correct the problem. Higher values of Kd result in quicker reactions; lower values result in slower reactions.

How Condenser Control and

HVAC PID Differs From The

Others

The RMCC approaches condenser control and HVAC control from a different angle than other PID-controlled systems such as Pressure Control and Case Control. PID control for Pressure Control and Case Control seeks to maintain a constant equality between the input and the set- point. Specifically, in Pressure Control, the RMCC tries to keep the suction pressure or temperature equal to the suc- tion setpoint, and in Case Control, the RMCC tries to keep

the case temperature equal to the temperature setpoint.

Condenser Control and HVAC Control seek only to keep pressure or temperature values below or above their setpoints. Thus, the system is only concerned when the input value is on the wrong side of the setpoint (e.g., above the setpoint in Condenser Control and Cooling Control, or below the setpoint in Heating Control). Any value on the other side of the setpoint is considered an acceptable value for the purposes of controlling, and therefore the output will be at or near 0%.

Condenser PID and HVAC Cooling Control only react to pressure or temperature levels that climb above the set- point. Likewise, in HVAC Heating Control, the tempera- ture level must be below the heating setpoint in order to begin heating. The 0-100% output percentage is then determined based on the distance between the input and setpoint, and the rate of change.

Output at Setpoint

Mathematically, the only difference between PID for Condenser and HVAC Control and PID for other systems is the Output at Setpoint value.

The Output at Setpoint value is simply the percentage the output will be when the input value is stabilized at the setpoint. In other words, when the PID input equals the PID setpoint, the PID output percentage will be fixed at the Output at Setpoint value.

Output at Setpoint is the value that determines where the throttling range is placed. As mentioned in “Throttling Range” on page 1, the Throttling Range is the range of input values across which Proportional Mode will gradu- ally move the output percentage from 0% to 100% (excluding effects by the Integral and Derivative Modes). The Output at Setpoint value basically tells the RMCC where to place the Throttling Range in relation to the set- point (this is explained in further detail below).

Output at Setpoint for Non-Condenser/

HVAC PID

For all non-condenser and non-HVAC PID control, the Output at Setpoint is fixed at 50% (except for Analog Out- put Modules, which may be programmed with any value from 0-100%). As mentioned before, this means that PID control will constantly strive to achieve a stable system where the input is equal to the setpoint and the output is 50%.

The throttling range in a PID Control application with a 50% Output at Setpoint is placed in such a way as to put the setpoint right in the middle of the throttling range, as shown in Figure D-3.

D-4 • E2 RX/BX/CX I&O Manual 026-1614 Rev 4 5-JAN-2013