In direct digital control (DDC) a single computer is able to carry out PID control for multiple loops at the same time make that loop data accessible between loops for purposes of cascade control, feedforward control, ratio control or any other control strategy. The DDC controller is also accessible on networks like ethernet for operational and maintenance personnel to easily log on to it remotely.
An example of DDC controller is the Delta model DSC-1280. This controller does not have I/O cards but instead it is monolithic in design with inputs and outputs part of one large motherboard. This controller has 12 input channels and 8 output channels. It can network via BACnet protocol with personal computers.
Programming of DDC controllers ranges from text-based languages to function- block programming. DSC-1280 controller is programmed in text.
A common industrial application of direct digital control (DDC) is the use of programmable logic controllers (PLCs) to control multiple loops. Even though PLCs were initially developed for ON/OFF (discrete) process control functions, over the years, major improvements have been made in their speed and capability to handle analog PID control functions as well.
Related: Digital PID Controllers
The major advantage of employing PLCs for analog loop control is the ability to easily integrate discrete controls with analog controls; for instance, one can use a single PLC to coordinate the sequential start-up and shut-down functions needed for intermittent operation with analog PID controls required for continuous operation.
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