What is DDC?
DDC is an automatic control of conditions or processes by digital equipment (computers).
DDC has the following properties:
Part of the computer which is the central part of the control system, is a replacement system that includes input and output modules. If there is a failure from the computer on the control side, as soon as possible other computers that are in a standby state will automatically take over so that there are no problems during the operation.
All DDC logic was originally made with maintenance tools for IDOL that have been developed by MHI. By applying IDOL, we can easily edit, modify or monitor DDC logic in a very short time without specific knowledge for programming.
A microprocessor performs the following tasks:
It has the program for control loop
It read process variable from the different transmitter through multiplexer and ADC
It determines errors for each control loop and executes control strategy for each loop
Its outputs the correction value to field device through DAC.
Components of DDC:
The internal components of the Direct Digital Controller (DDC) are as follows:
System Transfer Unit (STU):
This unit has two main functions, namely to change to the backup control system and for the alarm system in the control system. After detecting abnormal signal status from the computer, power supply, fans and cable connection, certain signals from computer A and computer B give a signal. Then the system can change automatically according to the logic transfer system and each alarm indicates the front panel of the unit. The selection of the control side of the computer can be done by pressing the “push” button on the front panel at STU. Each computer has an alternative button “CONTROL” and “STAND BY”.
CPU cards and PI / O cards are arranged in this chassis. Data for DDC (loop data) is given in the battery back up memory, so that after switching off the power source from the loop data from the maintenance device does not need to be transferred.
RS232C Change Unit
This unit functions to change two RS232C (Master and Slave) connection ports with remote operation. Both IDOL maintenance devices and DDC terminals can be connected to the computer board A computer and computer B through this unit.
Signal Conditioners (MS900 series)
The I / V converter (converts the voltage after the signal from the pressure transmitter is isolated, etc.) and the V / I converter (converts to current after the voltage signal from the computer is isolated) each arranged in the Signal conditioner rack. And the mV / V converter is also arranged on this shelf.
Digital Input Signal
The operating signal from the block control desk and the contact signal is transmitted to RELAY through prefabricated cable and connectors. The signal from the isolated RELAY photocoupler is divided into computer A and computer B, and transmitted to DI card via flat cable. Contact signals from the computer plant (the regulator to increase or decrease the value) are transmitted to RELAY via prefabricated cables and connectors in this panel.
Analog Input Signal
Signals from local transmitters and thermocouples are received at the signal conditioner and converted to voltage signals. This signal is input to the AI card from computer A and computer B via a flat cable from the connector other than the signal conditioner.
Digital Output Signal
The ON and OFF values shown by computer A and computer B are transmitted to the relay module from the DO card itself. In the relay module, the signal selector transmitted from STU is also transmitted to the computer side A or B that has been selected on dry contact.
Analog Signal Output
The voltage value shown by computer A and computer B is transmitted to the signal output modules from the AO cards. In this module, the selected transmission signal from the STU is also transmitted to the computer side A or B as the output voltage. The output signal for the recorder on is isolated and transmitted to the current signal.
Communication Signal Line
Computer A and computer B are provided with interfaces for communication of maintenance devices and CRT monitors. Cables from IDOL maintenance devices and CPU terminals can be used to connect communication line connectors for computers A and B through RS232C change units. Control data is transmitted periodically from the operating side computer to another computer with a stand-by status via a DMA card.