A small PLC (Allen Bradley MicroLogix 1000) (Courtesy of Allen-Bradley).
Definition of PLC
PLC is a digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes.
Modern PLC can be viewed as specialized computer
Specialized in logic control
Handle harsh and noisy environment
Analog valves and actuators
DC and AC motor drives
Bar code, encoder
Special purpose I/O
PC based programming software
Functions of PLC
Timer and counter
Others, such as sequencers
Counter timing diagram (the count value is 5)
Timer timing diagram (the timing value is 5)
Control actions are taken by making decisions depending on the values associated with various inputs or variables and the control logic in the program.
Decisions by attributes (go-nogo decision)
Is the machine turned on?
Is the gate to the work cell open?
Has the AGV arrived at the docking station?
Decision by variables
How deep is the hole?
What is the cutting speed?
What is the required surface finish?
How many parts are to be made?
Attribute decision can be represented by a logic variable (true or false) and its value can be determined by a logic expression
Boolean algebra is used for logic expression
Logic network diagram and ladder diagram
A robot is to be used to unload finished parts from a machine onto an AGV and to load raw parts from the AGV to the machine. Assume that there are sensors at the AGV's docking station to indicate the arrival of a vehicle and onboard sensors indicating whether the vehicle has brought a raw part to be processed as well as whether the AGV has space to carry away a finished part. Also assume there are sensors on the machine to indicate whether the machine is loaded with a part and also to signal completion of part processing. The robot is required to unload a processed part from the machine onto the AGV, pick up a new part for processing from the AGV, and load it onto the machine. The AGV is to be dispatched after completion of the cycle. Construct a ladder logic diagram for this task.
01 AGV has arrived
02 AGV is carrying a new part to be processed
03 AGV has space to store a processed part
04 Machine has a finished part to be unloaded
20 Unload old part from machine onto the AGV
21 Pick new part from the AGV and load onto the machine
22 Dispatch the AGV
The Ladder Diagram
Features of Ladder Diagram
A ladder logic diagram is made up of inputs and outputs connected according to the appropriate logic.
Each rung in the ladder represents a set of logical relationships between the inputs that leads to a particular output.
The output from one rung of the ladder could be used as an input in another rung of the same ladder.
Except when special provisions are made, it is considered that all rungs in a given ladder logic diagram are executed simultaneously, so the order of the rungs on the ladder in general does not matter.
During the powder metallurgy process, a punch is used to press blended metal powder into a compact inside a die. A pushbutton is used to start the process. When the start button is pressed, the die is filled with powder. The punch is then advanced and it applies pressure to the powder for a duration of 10 s, after which it is retracted. The pressed compact is then ejected from the die and the cycle repeats. The cycle can be interrupted by pressing a stop button. If the stop button is pressed, the punch is required to retract (if it had been advanced) before the process is stopped. We are required to construct a ladder logic diagram for this task.
This example illustrates the use of both the timing and counting functions. Consider a production line in which parts requiring processing are brought to a machine by a conveyor. A robot is used to load parts from the conveyor onto the machine and, after the part has been processed, unload the part from the machine and place it on a pallet. The cycle time for processing each part is 10 min. The robot is to palletize the parts by placing 125 parts on each pallet. The parts are to be arranged on the pallet in five layers of 25 parts each. Once a pallet is complete, it is dispatched and a new pallet is started. An allowance of 30 s is to be made for pallet dispatching and presentation of a new pallet. Construct a ladder logic diagram to effect the required control.
10 Machine has a part to be unloaded
30 Load machine with a new part
40 Unload a process part from the machine
45 The current pallet layer is full
48 Increment layer being palletized by 1
50 The pallet is full-dispatch
60 Present a new pallet
C100 Up counter with limit set to 25
C200 Up counter with limit set to 5
T250 Timer with limit set to 600 s
T300 Timer with limit set to 30 s
Design methods for ladder diagram
Ladder diagram for Example 10.2
Rules of State Diagram
State diagram for Example 10.4
What is industrial robot?
Classification of robot
Basic components in a robot
Robot in Manufacturing
Selection of Robot
Brief introduction to robot kinematics
Basics of robot kinematics
Economy justification of robot
1. What is robot?
In a broad sense, any autonomous machine can be called a robot.
Industrial robot is a general-purpose computer-controlled manipulator consisting of several rigid links.