Direct Digital Controls ( ddc panel ) in BMS System , are controls operated by one, or more, small computer processors inside encluser . The computer processor uses a software program of instructions to make decisions based on the available input information. The processor operates only with digital signals and has a variety of built-in interface components so that it can receive information and output control signals.
There are many instances where the types of controls are mixed. For example a DDC system could have all electric “sensors,” the units that measure temperature, humidity, pressure or other variable properties. This same system may also have pneumatic actuators on all the valves, since pneumatics provide considerable power and control at low cost. A “transducer” creates the interface between the electrical output of the DDC system and the valve. The transducer takes in the DDC signal, say a voltage between zero and ten volts, and converts it to an output of 3psi to 15psi. Thus, at zero volts the output will be 3psi, rising to 15psi at ten volts.
The Controller in the DDC panel :
- Micro-processor based
- Contains a program (“logic”) Script , Leader ,FBD..
- Reads Inputs à Controls Outputs
- Makes decisions The controller controls the system
These are the four main types of input and output in a control system. Lets consider each one briefly in terms of a DDC system.:
Analog Inputs :
- Measuring Devices (Continuous)
- Measure physical things/conditions and convert to electrical signal
- What can we measure?
- Temperature (resistance ; Kohms)
- Humidity (voltage ; 0-10VDC)
- Pressure (current ; 4-20 mA) –Light level
- Flow –Energy
Analog Outputs :
- Speed Control (VFD)
- Modulating Dampers
- Modulate a valve –to regulate flow of water in a pipe
- Regulate 0 to 100%
Digital Inputs :
- Volt Free Contacts
- Dry Contacts
- Potential Free Contacts
- Limit Switch
- Pressure Switch
- Normally Open
- Normally Closed
Digital Outputs :
- Control Relays, Starters, Lights
- Dampers
- open/close a valve to regulate flow of water in a pipe
- turn things ON and OFF