DDC or Direct Digital Controller in BMS System
What is DDC ?
To understand the DDC, we need to know a little bit of history about what was the things before the DDC invention and why it was invented? So that we can have a broader view of the primary purpose of DDCs.
The Programmable Logic Controller or PLC used to control and monitor the Process mainly in the industry like automobile and other manufacturing factories.
Richard Morley invented PLC in 1968 to fulfil the primary needs of control and protect the production capacity of machines and manufacturing lines in the industry, and this PLC used initially was in the area of transfer lines in automotive plants.
Due to these PLC or Programmable logic controllers were designed and invented mainly for controlling and monitoring or automating the productions in the industry.
But when it comes to buildings, this PLC cannot fulfil the exact needs in terms of tenants comfort, environmentally green or can say effective management system for buildings. And still, we can use PLC for Building automation whereas it will be an excessive investment and different performance.
So here DDC or direct digital controller invented in
order the process and automated the building equipment needs almost which PLC
can do with minimal investment from installation to engineering.
What is the Main
Difference between PLC and DDC?
What is DDC or
Direct Digital Controller?
In a nutshell, DDC is a controller which use the analogue
or digital signals from various devices of a field sensor and actuators and
then process and control the system based on the programme written inside the
controllers and has the capability to sends the information to another
controller or DDC.
Basic Features of
DDC
·
DDC or Direct digital controller usually has
the followings features
·
The Analogue Inputs is to monitors the fields
sensors values.
·
Digital Inputs to monitors the on/off status
from switches/contactors.
·
The analogue output is to control the field
actuators devices.
Digital Output is to control relay or provide low voltages.
·
DDC must have internal ROM/RAM to store
control logic and sensor values.
·
It must have networking protocols inbuilt to
transfer the data between the devices.
·
Modern DDC controller should have the
capability to implement BACnet protocols for communication.
Note that there are various DDC controllers available in
the market from the different manufacturer and those DDCs are available with a
variety of function and features based on the specific needs like controller
has all inputs/outputs like Analog inputs, Digital input, analogue output and
digital output and some controller has only digital/analogue inputs.
Let us see below DDC
Controller
·
Eleven 10-bit universal inputs whereas we
connect either analogue input or digital input using a jumper select, eight
binary outputs, and eight analogue outputs.
·
Terminal 23,24 used to connect other DDC
controller to communicate between devices through BACnet over MSTP.
·
It has non-volatile memory used to store
program and work independently.
·
It has the 24vdc used to give power for
field devices.
Now Let us see how
DDC used to control the BMS System,
Consider the followings scenario which we need to control
and monitor through above DDC.
Let us say in Building, we need to control Pump control
and control filling sequence through DDC whereas we have 2 Booster pumpS,
one is for filling the water tank and other is to pump the water to buildings
purpose to tenants like toilet etc.
This two-pump motor is controlled through the pump
control panel by manually and it should work automatically based on the
following sequence
·
Pump-1 should run if the water level below
the high level and stops once above the high level.
·
Pump-2 should run if the pressure on the
supply line lesser than the defines let us say 2.5bar.
· Pump-2 Should not run if water lesser than the lower level switch even pressure lesser than defined.
So based on the above sequence we will have following
parameters to monitor and control
·
Booster pump-1 Run status from control
panel-Binary Input
·
Booster pump-1 Run command from control
panel-Binary output
·
Booster pump-2 Run status from control
panel-Binary Input
·
Booster pump-2 Run command from control
panel-Binary output
·
Water Low-Level status-Binary Input
·
Water High-Level status-Binary input
·
Liquid pressure on supply line-Analog Input
Let us connect the above points in DDC Controllers as follows
BP-1 Run sts- IN-1
BP-2 Run sts- IN-2
Low-Level Sts- IN-3
High-Level Sts-IN-4
Liquid Pressure-IN-5
BP-1 Run Command-BO-0
Bp-2 Run Command-BO-1
Logic will be as follows to execute the above sequence
If IN4==1 ##(means lesser than
high-level status)
then
BP1=1
##(On Pump)
else
##(means above than
high-level status)
BP1=0
##(Off-Pump)
endif
If (IN5<2.5 and IN4==1) ##( if pressure
lesser than 2.5bar and water above the low-level sts)
then BP2=1 ##(on Pump)
else
BP2=0
##(Off-Pump)
endif
Note that this program may change for each vendor
controllers.
Not only this small sequence but also DDC can execute
complex and critical sequence in BMS System for HVAC.