DCS or PLC or PAC or RTU?

One often hears the question what is the difference between a PLC and DCS?, or question what is the difference between a PLC and RTU? This article the differences between these devices are clarified.

PLC vs DCS

PLC vs DCS

Programmable Logic Controllers (PLC)

The PLC born to replace relay logic which were used as control systems for industrial processes sequential or discrete. Discrete processes are those that have dynamic or can be modeled with discrete event systems, these processes usually have equipment such as motors, valves, switches, lights, buttons, equipment with a small number of discrete states (open or closed, run or stop, pressed or release and normal or in failure). In this sense the first instructions of the PLC were open and closed contacts, coils, timers and flip-flop and had only discrete inputs and outputs.

Currently, the PLC has the ability to handle analog inputs and outputs, also have a fairly wide set of instructions for handling them, and can also be connected to a variety of industrial networks. One advantage that still retain the PLC against the PAC and DCS is the price, the PLC are cheaper.

Programmable Automation Controllers (PAC)

PAC is newer devices. We can see the PAC as a control device which has the functions of a PLC and additional functions such as:

  1. Servo control (motion control),
  2. Advanced processing for analog signals,
  3. Advanced communications, as gateways between networks and
  4. Set of instructions for handling drives

Distributed Control Systems (DCS)

DCS born to replace analog signal controllers such as PID controllers, initially these teams only had analog inputs and outputs and functions to handle them. At present the DCS have a large number of digital signals; they connect to a variety of networks and sometimes have built-in HMI.

Distributed control system also refers to those control systems where the control elements are distributed throughout the process, in contrast to the centralized control, in which the decision is taken in a single controller. These distributed systems usually have more than one controller and these can be a mixture of PLC, PAC, DCS or RTU.

Remote terminal unit (RTU)

RTU is an electronic device designed to get or set data from one process, through inputs and outputs, analog and / or digital, and send them to a controller.

Often RTU are installed in remote locations and therefore need to have some features such as: ability to communicate over long distances (radio modem or GSM / SMS etc.), low power consumption, data log capacity, operation in a temperature range wide, DC power.

Conclusions

In the past distinguish between the use of a PLC and a DCS was easy, but now these devices have many features in common and can solve the same problems. These differences are still less when DCS and PAC are compared.

Unlike the PLC the main function of the RTU is the data acquisition and not the control. Some PLC can operate as an RTU.

An example of each device with links is shown in the list located below

RTU: AutoLog

PLC: Micrologix 1400

PAC: Controllogix

DCS: DeltaV

1 Comment

  1. martin

    informative

    Reply

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