Hanna Instruments HI520 - Page 95

95 Control Modes & Algorithms
The relation between D and PB is:
D = Range * PB/100
Ti = Kp/Ki, Reset time
Td = Kd/Kp, Rate time
AO
PWM
DB
DBG
SP
PV
I
P
error
D
K
PM
K
IM
K
DM
-
+
+
+
+
Figure 39: Controller Structure Representation
SP – Set Point DBG – Dead Band Gain
PV – Process Value K
PM
– Maximum proportional term representation
P – PID proportional term K
IM
– Maximum integrative term representation
I – PID integrative term K
DM
– Maximum derivative term representation
D – PID derivative term AO – Analog Out
DB – Dead Band PWM – Output driving relays
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.