EUT 1040 Lecture 10: Programmable Logic Controllers
PLC‟s Are ... • Similar to a Microcontroller: – Microprocessor Based – Onboard Memory for Storing Programs – Special Programming Language: Ladder Logic – Input/Output Ports
PLC‟s Are... • Dissimilar to Microcontrollers: – Intended for Industrial Applications – I/O Designed to interface with Control Relays – Emphasis on Maximum Reliability
PLC‟s • Widely Applied in Every Industry • Were Developed to Simplify the Implementation of Control Automation Systems in Plants and Assembly Lines • Designed to Minimize the Number of Control Relays in a Process and Maximize the Ways Relays can be Used • First Applied to Automobile Ind...
I/O Modules • Input Modules: Input Signals can be AC or DC, Analog or Digital • Output Modules: Outputs are either AC or DC Analog Signals (Although it is possible to „Construct‟ Digital Outputs) • Modern PLC‟s have Expansion Ports to Increase the Number of Available Inputs and Outp...
Examples of I/O Signals • Inputs: – Pushbutton (Energizing or Grounding an Input) – Relay Contact Output – DC Voltage Level – Digital Logic Signal (+5V or 0 V, etc) • Outputs: – 24 V ac – 120 V ac – 120 Vdc – etcetera
PLC‟s Use Ladder Logic • Ladder Logic Diagrams Provide a Method to Symbolically Show How Relay Control Schemes are Implemented • Relay Contacts and Coils, Inputs and Outputs lie on “Rungs” Between the Positive and Ground Rails
Example of Ladder Diagram
Relays • In General, Relays Transform a Control Signal into a Control Action • Relays Provide: – Isolation Between Input and Output – Leverage (Small Signal Can Control Large Action) – Automation (Minimize Human Interaction with a Control Process)
Basic Relay Symbols
Relay Applications • Relays can be Designed to Perform Many Functions – Detect Out of Limit Conditions on Voltages and Currents – Start Motors – Prevent Motors from Over Heating – Control Assembly Lines – Adjust Lighting
PLC Timers and Motor Protection
Industrial Communications • RS-422 (EIA 422): Asynchronous Serial Communications , similar in many respects to RS-232 • Faster (up to 100 Kbps) than RS-232 • Better Noise Immunity – Differential (Balanced signal) Protocol – Makes use of Twisted Pair lines - 1 pair for transmit, one pair...
EIA-422 Basics • Can be 1 Master Transmitter feeding up to 10 Slave Receivers • Can be Peer-to-Peer, like RS-232 • Data is sent and received via Differential Ports - Common Mode Rejection (Noise common to both inputs is attenuated) • Twisted Pair also reduces EMI at low cost
Common Mode Rejection
EIA 485 (RS-485) • More Modern, Faster and Flexible (supports TCP/IP) • Since it uses a differential balanced line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). • In contrast to EIA-422, which has a single driver circuit...
IEEE 802.3 (Ethernet) • Star Topology (Hub and spokes) • Standard for computer networks since the 1990‟s • Becoming more and more popular in Industrial settings • Uses twisted pair data cables terminated in 8P8C (sometimes incorrectly called RJ45) modular plugs, wired according to TIA/...
Twisted Pair Cables • Twisting a pair of wires that act as a communication channel will: – Minimize the loop area between the pair (minimize the self-inductance and capacitance) – Which in turn tends to cancel out much of the electromagnetic interference from external sources and crosstalk ...
PLC Special Features • Time Delay Relays • Counter Relays • Special Functions • User Defined Functions • Special Bits
Time Delay Relays • When TD Relay Pick-Up Coil is Energized, a Delay is Initiated • Normally Open Contacts Wait to Close until Delay is Completed • Normally Closed Contacts Wait to Open until Delay is Completed • Very Useful for Creating a Sequence of Control Events
Time Delay Symbol • Can be Constructed With or Without a Reset Input
Making Use of Delays • Delay Motor Start While Alarm Sounds for Safety
Counters • Counter Relays must “Count” a pre-determined number of events before changing contact status • Can Count Up (UpCounter) or Count Down (DownCounter) • e.g. An UpCounter is set to 8 and is programmed to detect every occurrence of a 5 Volt pulse. When it has detected 8 such oc...
Special Functions • Modern PLCs can perform many Math and Logic Functions without additional Ladder Logic Programming – Differentiation, Integration – +, -, *, / – Boolean Logic Functions (AND, NOT, OR) – Master Control Functions (Reset, etc)
Motor Protection • Essential Part of Motor Control • Protect against: – Under Voltage – Under Frequency (AC Machines Only) – Over Current – Over Heating – Over Speed – Over Load
Motor Protection Schemes • Incorporated Directly in Ladder Logic Control Schemes