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CONTROL MODES
PID control of a batch pH process using a pump as external dosing device
As with On/Off and Proportional control, a dosing solution can be an acid or base depending on the
desired results; and the control mode can be set High or Low.
With PID control enabled in Setup, the dosing time depends on the Deviation, Control period, Reset
time, Rate time, as well as how far the measurement is from the Set point.
Once enabled, a controller in proportional/integral mode (PI mode) works in a fashion similar to a
controller in proportional mode, but also integrates the error over time to reduce the variance error to zero.
A controller in PID mode incorporates the three control functions into a single control scheme. The
addition of derivative function to the PI mode results in the capacity to attenuate overshoots to some
extent, but adds the risk of instability if the process is noisy.
Proportional Function
With the proportional function, control output is proportional to the variance value.
Figure 37
illustrates the process controller behavior with a pH probe. Similar graph may apply for mV
measurements.
SP
Relay out
Time
Time
Tc
pH
Error
Process value
Figure 37:
Proportional Function with pH Probe Connected
When a relay is assigned to proportional control, the controller calculates the relay activation time at
certain moments e.g. t0, t0+Tc, t0+2Tc (Tc=Control period). The On interval (the shaded areas)
is dependent on the error value.
Integral Function
With the integral function (Reset time), the controller will reach a more stable output around the Set
point, providing a more accurate control than with the On/Off or proportional action only. The integral
function uses feedback.
Derivative Function
The derivative function (Rate time) compensates for rapid changes in the system reducing undershoot
and overshoot of the pH value. The derivative function utilizes predictive behavior.
During PID control, the On interval is dependent not only on the variance value but on previous
measurements too.
