sispro 3p - ladder diagram 2011

Ladder DiagramProgrammable Logic Controller

Production SystemPart 2/3 (ASA) – Session 3

PLC (1)

PLC (2)

PLC (3)

PLC (4)

Definitions

A ladder diagram is a symbolic and schematic way of describing the sequence of events of a discrete-state control system.A discrete state process is one for which the process variables can take an only two states, and a discrete state control system is one that causes the process to pass through a sequence of events. Each event is described by a unique specification of the process variables. Industrial control normally involves a series of events.

Basics

It is called a ladder diagram because various circuit devices are connected in parallel across the line that looks like a ladder, with each parallel connection called a "rung".Each rung of the ladder is composed of a number of conditions or input states and a single command output.The nature of the input states determines if the output is to be energized or not energized.

Elements of Ladder Diagram

Relays (Control Relays)A relay coil is represented by a circle identified as CR for control relay and an associated identifying number. The contacts for that relay will be normally open (NO) or normally closed (NC) and can be identified by the same number.


Time delay relaysIt is available a time-delay relay as one for which the contacts do not activate until a specified time delay has occurred. The coil is indicated by a circle, with the designation of TR to indicated timer relay. These have an arrow to indicate NO-to-close after delay or NC-to-open after delay. This is called an on-delay timer relay. When the coil is energized, the contacts are not energized until the time delay has lapsed.


Time delay relays (cont'd)There is also available an off-delay timer relay. In this case, the contacts engage when the coil is energized. When the coil is de-energized, however, there is a time delay before the contacts go to the de-energized state.

Mechanical Relays


MotorsThe symbol for a motor is a circle with a designation of M followed by a number. The control system uses this symbol to represent the fact of a motor, even though other operations may be necessary in the actual hardware to start the motor.


SolenoidsThe solenoid is designated by SOL and a number. The symbol itself tells nothing of what function the solenoid plays in the process. For example, it may be a solenoid to open a flow valve or move material off of a conveyor or a host of other possibilities.


LightsA light symbol is used to give operators information about the state of the system. The colour of the light is indicated by a capital letter in the circle, for example, R stands for red, G for green, A for amber, and B for blue, etc.


SwitchesOne of the primary input elements in a discrete-state control system is a switch.The switch may be normally open (NO) or normally closed (NC) and may be activated from many sources. Different symbols are used to distinguish between different types of switches.


Switches (cont'd)There are push-button switches. These switches are typically used for operator input such as to stop and/or to start an operation. Also available, NO and NC limit switches. These devices are used to detect physical motion limits within the process.


Switches (cont'd)There are also pressure switches, both NO and NC. Thermally activated switches, such as for ovens or overheating protection of a motor are available. Other types are level switches. In all of these switches, the NO is closed by rising pressure, temperature, or level.

An example (1)

DescriptionA ladder diagram for an elevator control system is discussed. The objective is that when the UP button is pushed, the platform carries something to the UP position, and when the DOWN button is pushed, the platform carries something to the DOWN position.

An example (2)

Inputs (6 elements)

LS1 = NC limit switch to indicate UPposition

LS2 = NC limit switch to indicate DOWN position

START = NO push-button for STARTSTOP = NO push-button for STOPUP = NO push-button for UP commandDOWN = NO push-button for DOWN

command

An example (3)

Outputs (2 elements)

M1 = Motor to drive the platform UP

M2 = Motor to drive the platform DOWN

An example (4)The sequence of eventsWhen the START button is pushed, the platform is driven to the DOWN position.When the STOP button is pushed, the platform is halted at whatever position it reaches at that time.When the UP button is pushed, the platform, if it is not in DOWN motion, is driven to the UP position.When the DOWN button is pushed, the platform, if it is not in UP motion, is driven to the DOWN position.

An Example (5)

Ladder Diagram Software

An Application: Olive Oil Plant

An Application (2)

An Application (3)

References

Several websites (online material)

http://industrial.omron.co.uk/http://metalpass.com/http://www.automation-drive.com/http://www.kollewin.com/http://www.tekmix.co.uk/

sispro 3p - ladder diagram 2011

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