Hanna Instruments HI 720 Instruction Manual page 20

If the "hold mode" is selected for the relay, then it is energized only when the
meter is in hold mode. In this case there is no time boundary for the relay ON
state.
Relays 3 and 4 can be configured to operate in three modes:
1. simple cleaning
2. advanced cleaning
3. hold mode
ON/OFF Control Mode
Once a relay is enabled (set 1, set 2), the setpoint can be configured to be
activated as a high ("OOHI") or low limit ("OOLO"). In both cases the following
values have to be defined through setup:
• setpoint value (conductivity value; setup item C.11 or C.21)
• hysteresis for setpoint (conductivity value; setup item C.12 or C.22)
A control device can be wired to the contact output: connect the device to the
COM and NO (Normally Open) or NC (Normally Closed) terminals of the re-
lay. The ON relay state occurs when the relay is energized (NO and COM
connected, NC and COM not connected), while the OFF state occurs when the
relay is de-energized (NO and COM not connected, NC and COM connected).
When measurement exceeds the setpoint threshold, a relay enabled as high
setpoint is energized until the reading falls below the setpoint minus hysteresis.
When the measured value is below the setpoint, a relay enabled as low setpoint
is energized until the reading goes above the setpoint plus hysteresis.
P .I.D. Control Mode
PID control is designed to eliminate the cycling associated with ON/OFF con-
trol in a rapid and steady way by combining the Proportional, Integral and De-
rivative control methods.
Using the proportional function, the activated control lasts for a time period
proportional to the error value (Duty Cycle Control Mode); as measurement
approaches the setpoint, the ON (relay energized) period diminishes.
During proportional control, the instrument calculates the relay activation time
at certain moments t , t +T , t +2T
0 0 c 0
t t +T
0 0
The ON interval (shaded areas) then depends on the error amplitude.
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etc.
c
t +2T
t +3T
c 0 c 0 c
Using the integral function (reset), the controller reaches a more stable output
around the setpoint providing a more accurate control than the ON/OFF or
proportional action only.
The derivative function (rate action) compensates for rapid changes in the sys-
tem, and reduces undershoot & overshoot of the conductivity value.
During PID control, the ON interval depends not only on the error amplitude
but even on the previous measurements.
Definitely, PID control provides more accurate and stable control than ON/OFF
controllers, and it is the ideal solution in system with a fast response, quickly
reacting to changes in the controlled solution.
P .I.D. Transfer Function
The transfer function of a PID control is given by the following relation:
Kp + Ki/s + s Kd = Kp(1 + 1/(s Ti) +s Td)
where Ti = Kp/Ki and Td = Kd/Kp,
The first term represents the proportional action, the second is the integrative
action and the third is the derivative action.
Proportional action can be set through the Proportional Band (PB), expressed as
input range percentage, and related to Kp:
Kp = 100/PB
The proportional action is set directly as "Deviation" (D) in conductivity units,
with the following relation:
D = Range * PB/100
Each setpoint has a selectable proportional band: PB1 for setpoint1 and PB2
for setpoint2. Two further parameters must be provided for both setpoints:
Ti = Kp/Ki, reset time, measured in minutes
Td = Kd/Kp, rate time, measured in minutes
Ti1 and Td1 will be the reset and rate times for setpoint1, while Ti2 and Td2 will
be the reset and rate times for setpoint2.
Tuning a P .I.D. Controller
The proportional, integrative, derivative terms must be tuned, i.e. adjusted to a
particular process. Since usually the process variables are not completely known,
a "trial & error" tuning procedure must be applied to get the best possible con-
trol for each process.
The target is to achieve a fast response time and a small overshoot. Many tun-
ing procedures are available and can be applied to HI 720.
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