Control system

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Relays[edit | edit source]

A relay is one of the most basic and widely used method of control. A relay is an example of using electricity to control the flow of electricity. By breaking or making a connection a different connection is broken or made. 4-20mA control

0-10V Control and Sensor Reading[edit | edit source]

Less common these days but still available is the 0-10V scheme of sensor reading and control. For sensors a physical property is mapped linearly to a 0-10V output.

4-20mA A very common means of sensor reading and control of movement is via 4-20mA. A sensor will map a physical property to a current in a linear way. Example: A pressure sensor can read from 0-100PSI. If the sensor sees 0PSI it will output a 4mA signal. If it sees a 100PSI condition it will output a 20mA signal.

Basics of Motor Control[edit | edit source]

A motor can be controlled by 4 means. A contactor with external fusing, a combination motorstarter, a VFD, and a softstarter. A contactor is device similar to a relay but among many features rated for higher currents, a combination motorstarter is a contactor with circuit protection builtin, a VFD is a device that modulates AC frequency to control the speed of an AC motor, a softstarter is a type of combination motorstarter with the ability to limit current.

Schematics (naming convention)[edit | edit source]

Broadly speaking there are two ways to label a wire. Functionality or location. Labeling by functionality conveys to people what the wire is supposed to do. Labeling by location tells you where the wire is in the documentation and sometimes where it is in the real world.

Example of functionality labeling:

  • 24VDC
  • 1**s (i.e. wires labeled 100-199) for digital inputs.
  • 2**s (i.e. wires labeled 200-299) for digital outputs.
  • 3**s (i.e. wires labeled 300-399) for analog inputs.
  • 4**s (i.e. wires labeled 400-499) for analog outputs.
  • 5**s (i.e. wires labeled 500-599) for thermocouples.
  • CB**s (i.e. wires labeled CB01-CB99) for communication cabling

Cabinet Components (Panel Interface Connector)[edit | edit source]

A connector mounted on the cabinet. Usually with a cover that provides a NEMA rating. Typically used for laptop power and communication ports.

Cabinet Components (Panel Indicators)[edit | edit source]

Panels will often have light indicators on them. Standard convention is that Run state is colored Green, Stopped State is Red, and Alarm state is Red. However, the petrochemical industry often uses Red for Run, Green for Stopped, and Amber for Alarm. With the idea being that it is indicating to the operators if it is safe to approach the machine. Sometimes the light can be pushed down and it will automatically illuminate the light. This is a safety feature.

Cabinet Components (Discrete DC Power Supply)[edit | edit source]

DC power is used for many devices inside of a control system. By far the most common is 24volts DC. 12 volts DC is also used on occasion for older technology like VME.

Features a DC power supply may have:

  • dual-outputs
  • adjustable DC voltage output
  • relay output to show status. Changes state when powered on.
  • voltage range switch. The ability to operate on two voltage inputs. E.g. 400-500VAC and 110-230VAC
  • Dinrail mounting or panel mounting

Factors to consider for a power supply:

  • wattage
  • input voltage
  • phase: 1,2, or 3
  • Hazardous environmental rating
  • Inrush current
  • Connector types

PLC Components (Bus)[edit | edit source]

Some brands of PLCs do not have modules that clip together. Instead they have a rack where all modules sit. The rack or bus, can (depending on manufacturer) distribute power, provide mechanical support, and move data. Ones that do not move data instead have an access port on the side of the modules, effectively a secondary bus for data. We call those "passive backpanels" vs active ones that provide electrical connections.

Racks have a fixed amount of slots, almost always a power of 2. Some manufacturers provide filler modules that provide no function except to keep dust off the connectors. Another name for the Bus, besides rack, is backpanel. Often you can find 3rd party vendors who will make backpanels compatible with common PLC modules. When the rack is mounted vertical it is sometimes called a PLC chassis. Using a PLC rack is only an option for non-fixed PLCs. One advantage of using a 3rd party backpanel is the ability to provide additional environmental protection while maintaining smaller form factor.

Dampers[edit | edit source]

Dampers regulate the flow of air and other gases. The degree (modulation) they allow gases to pass is controlled by electricity. A discrete damper can open and close fully. An analog damper can open and close a variable amount. A discrete damper (depending on model) will respond to 24VAC or 24VDC or both. 24VAC is a legacy voltage from the days when AC-DC conversion was costly, but transformers were much cheaper. By lowering the voltage an electrician could step up the current which allowed the damper to better hold against gas pressure. A modulating damper uses 4-20mA signal/power to determine the amount to open and close. A careful consideration is needed to decide what a non-energized damper should do. If all power is removed should gases be blocked or allowed to pass? This covers the case where the PLC has become damaged or unpowered. For cooling applications it might be best to allow gases to pass to prevent too much heat. For moving flammable gas it might be best to stop the flow of gases to starve the combustion. Dampers rarely have feedback of any sort. If they do the overwhelming case are limit switches that allow the PLC to know that the damper is fully closed or fully open. In the rare case they have analog feedback it often 4-20mA. Smaller dampers can be usually powered by the control panel, if higher currents are needed consider a relay. Some dampers require an external source of compressed air/N2 to operate. For large gas systems sometimes two dampers are used. One large one to close the air flow most of the way, and a sealing damper to finish up.

See also[edit | edit source]

External links[edit | edit source]