Hanna Instruments HI 21 Series - Manual (Page 10)

19 18
P.I.D. CONTROL MODE (HI 21523 ONLY)
PID control is designed to eliminate the cycling associated
with ON/OFF control in a rapid and steady way by means of
the combination of the proportional, integral and derivative
control methods.
With the proportional function, the duration of the activated
control is proportional to the error value (Duty Cycle Control
Mode): as the measurement approaches setpoint, the ON
period diminishes.
The following graph describes the pH process controller be-
havior. Similar graph may apply to the mV controller.
During proportional control the process controller calculates
the relay activation time at certain moments t
0
, t
0
+T
c
, t
0
+2T
c
etc. The ON interval (the shaded areas) is then dependent to
the error amplitude.
With the integral function (reset), the controller will reach a
more stable output around the setpoint providing a more
accurate control than with the ON/OFF or proportional ac-
tion only.
The derivative function (rate action) compensates for rapid
changes in the system reducing undershoot and overshoot of
the pH value.
During PID control, the ON interval is dependent not only to
the error amplitude but even to the previous measurements.
Definitely PID control provides more accurate and stable con-
trol than ON/OFF controllers and it is best suitable in system
with a fast response, quickly reacting to changes due to ad-
dition of acid or base solution.
ON
OFF
14 Setpoint +
Hysteresis Setpoint
t
0
t
0
+T
c
t
0
+2T
c
t
0
+3T
c
dosage, if available).
An upper boundary is imposed for acid/base dosage time
when relays are energized continuously, i.e. when relay works
in ON/OFF mode or in PID mode but in the latter case only
if the relay is always ON. This parameter can be set through
setup procedure. When the maximum boundary is reached,
an alarm is generated; device stays in alarm condition until
relay is de-energized.
ON/OFF CONTROL MODE
Either for mode 1 or 2 (base or acid dosage) the user has to
define the following values through setup:
• relay setpoint (pH/mV value);
• relay hysteresis (pH/mV value).
Connect your device to the COM and
NO (Normally Open) or NC (Normally
Closed) terminals.
The ON relay state occurs when relay is energized (NO and
COM connected, NC and COM disconnected).
The OFF relay state occurs when relay is de-energized (NO
and COM disconnected, NC and COM connected).
The following graphs show relay states along with pH mea-
sured value (similar graph can be derived for mV control).
As shown below, a high setpoint relay is activated when the
measured pH exceeds the setpoint and is deactivated when it
is below the setpoint value minus hysteresis.
Such a behavior is suitable to control an acid dosing pump.
A low setpoint relay as can be seen from the following graphs
is energized when the pH value is below the setpoint and is
de-energized when the pH value is above the sum of setpoint
and the hysteresis. The low setpoint relay may be used to
control an alkaline dosing pump.
ON
OFF
Setpoint 14 Setpoint –
Hysteresis