LTH MOD53, Operation Manual

Page: 51 / 110

8 Set Points
50
4. When the trigger is set to “Latch” the relay will become activated when the
source input becomes lower than the “Band Low” level, and will remain energised
until it becomes higher than “Band High” level. It will then remain de-energised
until the sensor input falls below the “Band Low” level again.
SET POINT MODE
The relays can operate in one of three modes.
On/Off Mode
The On/Off mode is the default mode of operation for the relays. The relay
energises when the set point is activated and is de-energised when the set point is
de-activated.
“Delay”: In order to prevent, short duration changes at the input affecting the relay
operation a delay can be set before the relay is energised. If the input is still the
same after the delay, then the relay will be energised.
“Hysteresis”: A facility to apply hysteresis to the set point level allows the user to
avoid relay “Chatter” when the sensor input level approaches the set point level.
“Chatter” is caused when the sensor input is sufficiently close to the set point value
that noise on the signal repeatedly crosses the set point level, thus causing the
relay to switch on and off rapidly.
The hysteresis level should therefore be set to be a little larger than the input noise
level.
Time Proportional Mode
The MOD53 provides two forms of pseudo proportional control, which can be used
to control the levels to a defined value when used in conjunction with a pump or
valve. When the reading deviates from the programmed set point level the relay
pulses at a rate proportional to that deviation. It is possible to control any on/off
device such as a solenoid valve or dosing pump using the time proportional mode.
The proportional band is displayed as a percentage of the full range value. For
example, a proportional band of 20% on the dissolved oxygen range of 0-30 ppm
would give a band of 6 ppm. If the set point trigger was selected as LOW and the
set point value was 10 ppm, the band would cover 4 to 10 ppm. When the reading
falls below 4 ppm the relay would be energised. As the input rises and approaches
the set point the output relay starts to cycle on and off with the on time reducing
and the off time increasing, respectively. The cycle time is adjustable and is the
sum of the on and off times.
Pulse Proportional Mode
The Pulse Proportional (or PP) mode is intended to drive solenoid type dosing
pumps which have the facility to accept an external pulse input. The proportional
band operates in the same way as the Time Proportional mode.