Hanna Instruments HI 720 Series - Manual (Page 21)

40 41
Example:
• Max. slope = 30 mS / 5 min = 6 mS/min
• Time delay = Tx = approx. 7 minutes
• Deviation = Tx * 6 = 42 mS
• Ti = Tx / 0.4 = 17.5 min
• Td = Tx * 0.4 = 2.8 min
Alarm Relay
During normal operation (no alarm condition) the alarm relay is energized, while
during an alarm condition or power failure the relay will be de-energized. As
long as a separate battery power system is used, an alarm will sound.
Example:
High alarm set at 1200 mS
Low alarm set at 400 mS
When the measurement is close to an
alarm value, the hysteresis will elimi-
nate the continuous relay energizing/
de-energizing sequence. The hyster-
esis amplitude is user-selectable.
Moreover, the alarm signal is generated only after the user selectable time pe-
riod (alarm mask) has elapsed since the controlled value has overtaken one
alarm threshold. This additional feature will avoid fake or temporary alarm con-
ditions.
Note If the power supply is interrupted, the relay is de-energized as in
alarm condition to alert the operator.
In addition to the customizing alarm relay, the meter is equipped with the Fail
Safe alarm feature.
The Fail Safe feature protects the process against critical errors arising from
power interruptions, surges and human errors. This sophisticated yet easy-to-
use system resolves blackout and line failure problems on both hardware and
software sides. The alarm function operates in a “Normally Closed” state and
hence alarm is triggered if the wires are tripped, or when the power is down.
This is a very important feature since with most meters the alarm terminals close
only when an anomaly occurs, and no alarm is generated upon line interrup-
tion, causing extensive damage. On the other hand, the software is employed to
set off the alarm in abnormal circumstances, such as dosing terminals closed for
too long. In both cases, the red LED will also provide a visual warning signal.
A simple and profitable procedure is described in this manual and can be used
in almost all applications.
The user can vary five different parameters, i.e. setpoint value (S1 or S2), devia-
tion (D1 or D2), reset time, rate time and proportional control mode period T
c
.
Note The user can disable the derivative and/or integrative action (for P
or PI controllers) by setting Td = 0 and/or Ti = MAX (Ti), respec-
tively, through the setup procedure.
Simple Tuning Procedure
The following procedure uses a graphical technique for analyzing a process
response curve to a step input.
Note This procedure allows only a rough setting of the PID parameters
and could not fit all processes. It is suggested that I and D param-
eters be set by technical personnel, because their inadequate val-
ues may cause undesired behaviors of the system.
Note Connect an external device (chart recorder or PC) to the controller
and the procedure will be easier, without requiring hand plotting of
the process variable.
1. Start from a solution with a conductivity value different from the dosed liquid
(the difference should be at least 15% of the full scale), and turn on the dos-
ing device at its maximum capacity without the controller in the loop (open
loop process). Note the starting time.
2. The conductivity value will vary and reach a maximum rate of change (slope).
Note the time at which this maximum slope occurs and the corresponding
conductivity value. Note the maximum slope per minute. Turn the system power
off.
3. On the chart, draw a tangent to the maximum slope point. Then read on the
time axis the system time delay (Tx), i.e. the time value corresponding to the
intersection between the drawn tangent and the starting conductivity value.
4. The deviation, Ti and Td, can be calculated as follows:
• Deviation = Tx * max. slope
• Ti = Tx / 0.4 (minutes)
• Td = Tx * 0.4 (minutes)
5. Set the above parameters, put the controller in the loop and restart the sys-
tem. If the response has too much overshoot or is oscillating, fine tune the
system by slightly increasing or decreasing the PID parameters one by one.