3A2: Distributed and Logic control

What is a DCS?

• distributed control system

• A system in which measurement and control components of the system are distributed at locations throughout the process area and perform 2 functions:

  • measurement of analog/digital inputs which are transmitted to controllers and to the OIU

  • Generation of output signals to actuators that can change and control process conditions

What is a data highway?

• a shared cable arrangement that’s used for communications

• Minimizes quantity of wiring while allowing for unlimited reconfiguration flexibility

What are the components of a DCS?

• transmitters

• final control elements

• Input/output cards

• Controllers

• Operator console

• And a Network (data highway)

What kind of displays are used for tracking process?

• graphic displays

• Detail displays

• Trend displays

• Alarm displays

What’s the function of a graphic display?

• allow the operator to view a process in the form of a picture or animation

What’s the function of a detail display?

• specific to a single loop (single controller) and often appears as a single loop digital controller faceplate

• Allows control of setpoint, auto/manual transfer and controller output

What’s the function of trend displays?

• DCS equivalent of chart recorders

• Allows operator to select many process parameters and plot the points on a graph

What’s the function of an alarm display?

• once alarm conditions arise they’re brought to the attention of the operator visually or audibly

What are the components of a PLC and how is it programmed?

• components are input/output cards, processor card and operator interface

• The PLC is programmed by a programmer that connects to it so that an operator on instrumentation tech can enter ladder logic into it

What are the various inputs and outputs of input/output cards?

• DI - discrete inputs, only 2 positions (on/off)

• AI - analog inputs, ever changing signal (temperature)

• DO - discrete outputs, control signal for 2 position devices

• AO - analog outputs, ever changing signals that control process

What is a processor card?

• an integrated circuit that fits onto a single chip

• Contains arithmetic unit, control circuitry and memory registers

What is ladder logic?

• is what controls the sequence of events:

  • basically various outputs will energize if certain conditions are proven to be true

  • For example, if pilot flame valve is proven open, the flame will energize and so on

What is a SCADA system?

• supervisory control and data acquisition

• Allows continuous process monitoring and simple loop control

What are communication methods used in SCADA?

• on dedicated wires that run directly from the remote terminal unit (RTU) to the central computer

• Through the telephone system using modems at the RTU and the central computer

• Using radio, microwave signals or satellite links