Distributed Control

  What is distributed control ?
          An electrical, electronic or electromechanical system where a number of devices are “controlled” in some way based on the requirement to perform a pre-specified set of operations while also , possibly, taking account of sensor readings, can be described loosely as a control system. In any control system there is a controller which acts as the “brain” behind the process. In a typical PLC control system the PLC controller is a single entity with lots of inputs and outputs suited to a wide variety of devices and sensors.   


    Once the PLC has been programmed with the particular requirements of the application and it has been connected to all its inputs and outputs , it is ready to control. The PLC would typically repeat the following loop continuously as it performs its tasks….

1. Read all Inputs
2. Determine New Outputs based on current state and current inputs
3. Set the New Outputs

    All processing needed to be done in carrying out this loop would be done by the PLC’s own local processor. All wiring to and from the sensors and controlled devices would also be routed to somewhere near the PLC’s processor. Typically this would be in a cabinet fitted with DIN rails holding the PLC module and additional modules for connection to the “devices”.
     In contrast to this is the idea of Distributed Control. In this case there is not just one processor dealing with all of the system. The overall system is divided up into local processing modules which can read and set their own attached devices/sensors. Each different module can specialise in a particular type of device: for example, motor control, digital input/output, analogue measurement, relay switching etc… etc… However, each module must have some general guiding control which coordinates the operation of all of the modules in a given system. This means that there must be communication between them. This can take many forms but some of the simplest and most reliable are those using serial multi-drop protocols such as RS485. This has the convenience of requiring just two wires to link together all modules in a multi-module system. It should be noted that the ability to have separate modules linked only be a pair of wires can greatly reduce the overall wiring effort in a control system since the modules can be placed near to their own devices.
   The “guiding control” can be provided by a “Master Controller” which has the ability to communicate with all of the modules in such a system. Although it may seem that taking this approach to distributed control has just added an extra layer of complexity to a simple control system, it is the separation of the “housekeeping “ processing to each of the individual module processors that makes it very efficient and generally easier to program and maintain.



    For example: If the PLC processor needed to make a measurement of a temperature using one of its attached sensors it would need to initiate the analogue to digital conversion of the “raw” input signal before using the result in its main job of deciding what to do at “such and such” a temperature. When doing the same thing using a dedicated analogue input module the “Master Controller” can simply request the current temperature from the module using the communications channel. The analogue module would carry out the A/D conversion automatically in the background. Similarly, a stepper motor requires a special sequence of pulses to each of its phase coils to make it move in a particular direction at a particular speed. With a separate stepper motor controller it would simply require the “master” to send a command telling it to take “so many steps” at “such and such a speed” leaving the modules local processor to take care of the correct pulse sequence generation.
    Another advantage in distributed control is the ability to have the modules separated by quite large distances without having to worry about cable lengths and associated power distribution problems. For example: To operate a large motor requiring large currents at a long distance from the controller (eg 1Km) would require expensive cable to prevent voltage drops between the supply and the motor. The local controller option means that the motor power supply can be situated exactly where it is needed; close to the motor and controller.


Control Master Distributed Control System


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