MODEL 54eA | SECTION 15.0 |
| PID AND TPC CONTROL |
SECTION 15.0
PID AND TPC CONTROL
15.1 PID CONTROL (CODE -20)
PID Control
The Model 54eA current outputs can be programmed for PID control. PID control is used with a control device that is capable of varying its output from 0 to 100 percent in response to a changing signal in mil- liamps. Automated control valves or variable volume pumps are commonly used. These devices are referred to as modulating control devices because of their 0 to 100% adjustability. PID control is typically used where greater accuracy than is achievable with an on/off device is required, or where it is desirable to have the pump or valve "on" continuously, or where the existing or preferred pump or valve is of the modulating type.
Any process control system must manually or auto- matically hold the controlled variable (pH, concentra- tion, temperature) in a steady condition at selected set point values. For manual control, the operator looks at the value of the process variable, decides whether or not it is correct, and makes necessary adjustments. He decides the amount, direction, rate of change and duration of the adjustment. With automatic control, the controller does all of this. The operator only adjusts the set point of the controller to the selected value of the measured variable. Automatic process control such as PID is usually feedback control; it eliminates the devia- tion between measurement and set point based on continuous updates (feedback) from the process itself.
Measurement and Set Point (Feedback Control)
The Model 54eA controller is given two items of infor- mation: measurement and set point. The controller reacts to the difference in value of these two signals and produces an analog output signal to eliminate that difference. As long as the difference exists, the con- troller will try to eliminate it with the output signal. When measurement and set point are equal, the condition of the controller is static and its output is unchanged. Any deviation of measurement from set point will cause the controller to react by changing its output signal.
PID Control Mode Combinations
All PID controllers have several control modes which can be used in various combinations: proportional plus integral (reset), proportional plus derivative (rate) and a combination of proportional (P), integral (I) and deriva- tive (D). Each control mode produces a response to the deviation of measurement from set point that is the result of a specific characteristic of the deviation, and each control mode is separately adjustable. D, the derivative, or rate mode, is seldom used in water treat- ment and is beyond the scope of this manual.
Proportional Mode (Gain)
The simplest control is proportional. Proportional may also be referred to as sensitivity or gain. Although these terms may refer to a different version of proportional, the control function is still fundamentally the same - the error from set point is multiplied by this factor to pro- duce the output.
In the Model 54eA controller, proportional mode is referred to as proportional "band" which is config- urable from 0 to 299%. For good control of a specific process, the proportional band must be properly adjusted. The proportional band is the percent of the analog output span (the difference between the 4 (or 0) mA and 20 mA settings) through which the measured variable must move to change the output from mini- mum to maximum. The larger the proportional band, the less the controller reacts to changes in the measured variable. As the proportional band is made smaller, the reaction of the controller increases. At 0 proportional band, the proportional- only controller behaves like an on/off controller (an alarm set at 20 mA).
Most processes require that the measured variable be held at the set point. The proportional mode alone will not automatically do this. Proportional alone will only stabilize the measured variable at some offset to the actual control point. To control at an exact setpoint, proportional plus integral mode is used.
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