Me 445 integrated manufacturing technologies experiment 2




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ME 445 INTEGRATED MANUFACTURING TECHNOLOGIES

EXPERIMENT 2

“PLC LADDER DIAGRAM PROGRAMMING"



OBJECTIVE



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In this laboratory work you will learn about a PLC Programming technique, namely the “Ladder Diagram Programming”. For this purpose, you will use a Festo Software Tool “Ladder diagram FST 100”, which is a basic ladder diagram editor to aid the designer to develop a PLC project for controlling several automation devices such as robot, conveyor, pneumatic piston, mechanical drill, rotary table, I/O buttons, switches and sensors. The system is a modular production station to demonstrate a typical drilling and pressing cycle. For the given equipment, you will design a typical sequence using the ladder diagram editor, download it to the PLC, debug and analyze the outcome.
Figure 1: General view of the system. Conveyor and Robot are not shown.

HARDWARE


The modular production system consists of 11 basic elements:

  1. Conveyor:

Basics:


The conveyor is an intercell positioning device having 14 cups (indexes) to provide the positioning of the workpiece. The workpiece is a typical Æ40, length of 25-mm bar made of plastic.

Control:


It starts to rotate by activating its output channel ON and stops when the output channel is OFF. It has a vision sensor mounted on it, which becomes ON when a part comes into the desired position.


  1. Robot

Basics:


The robot is the positioning device of the workpiece from the conveyor to the rotary table and vice versa. In this setup, it can perform two basic tasks:

  • Taking the part from conveyor and placing it in to the rotary table (Loading)

  • Taking the part from rotary table and placing it to the conveyor (Unloading)

Control:


The robot continuously traces the output channels and starts to operate when any of the output channels becomes non-zero. The output channel is a two digit binary number. For instance, when the channels are set as 10 which corresponds to 2 in decimal representation, it starts the task number two. Similarly, using its input channels it understands whether any task is active. You have no control on the input channels; the input channels are set by the program, running inside of the robot-controller. The setting of the output channels can be represented as:

00: No job

01: Start the job number 1 (take the part from the conveyor and place it into the rotary table)

10: Start the job number 2 (take the part from the rotary table and place it into the conveyor)

Similarly the input channels are identified as:

00: No active job is running

01: Job number 1 is running (robot is taking the part from the conveyor and placing it into the rotary table)

10: Job number 2 is running (robot is taking the part from the rotary table and is placing it into the conveyor)


Since the robot continuously traces its output channels and checks any non-zero assignment, when they are set, immediately must be reset (that is, set to 00) to avoid to repeat the same job for a second time.


  1. Rotary table

Basics:


The rotary table is a round table driven by an electric motor having 4 cups. It has a proximity switch, which becomes active (ON) when a single cup is rotated.

Control:


The rotary table starts to rotate when its output channel is activated (ON) and stops when the output channel becomes low (OFF). Its proximity switch is mounted such that when the table comes to the position (that is, when it rotates by one cup) the proximity switch becomes active (ON). Note that initially as the table is rotated by one and stopped when its switch becomes ON, the switch is still active.


  1. Drilling station with piston

Basics:


The drilling station consists of an electric drill and a pneumatic piston moving it upward and downward. The piston has two proximity switches on its up and down positions.

Control:


The drill starts to rotate when its output channel is activated (ON) and stops when the output channel becomes low (OFF). The control of this piston is accomplished by a 4/3 valve, such that to move it upwards and stay in that position, the upward channel must be triggered (that is set to ON and immediately set to OFF), and vice versa. When the piston is in the upward position the up switch is (ON) and the down switch is (OFF) and vice versa.


  1. Pressing station

Basics:


The pressing station consists of a pneumatic piston having two proximity switches on its up and down positions.

Control:


The control of the piston is accomplished by a 4/2 valve such that, the piston goes down when its output channel is activated (ON) and goes up when the channel is deactivated (OFF). Note that, the 4/2 valve is different from the 4/3 valve as used in the drilling station piston when its triggering mechanism is considered. The up sensor is (ON) and the down sensor is (OFF) when the piston is in upward position and vice versa.


  1. Emergency Stop Button

Basics:


The button to stop the system devices immediately.

Control:


When the button is pressed the system should be cut-off so that all active operations are stopped. The emergency stop button is specially designed as, if pressed it resumes its position until manually pulled out.


  1. Emergency Stop Pressed Button

Basics:


It is the button to inform the PLC that the emergency stop button is pressed.

Control:


As an external input, the button must be pressed (ON) when the emergency button is pressed and released. When it is pressed all flags and the necessary operations should be reset.



  1. Start Button

Basics:


The button to start the cycle.

Control:


As an external input, the button should be pressed to start the program downloaded to the PLC controller.


  1. Reset Button

Basics:


It is the button to reset all flags and active operations

Control:


When pressed (ON), all flags should be reset and all devices should return to their initial positions. It is similar to the Emergency Stop Pressed Button and can be used for miscellaneous purposes. For example, it can be used to cancel a certain operation at an intermediate state.


  1. Reset lamp

Basics:


Can be used to indicate whether the reset button is pressed .

Control:


It can be set (ON) when the reset button or the Emergency Stop Button is pressed. It must be reset when there is an active cycle.


  1. Operate lamp

Basics:


Can be used to indicate whether any cycle is active.

Control:


The lamps can be used for any purposes. For example when the PLC has started its operation it can be set (ON) and when the program is finished or if the Emergency Stop Button is pressed it is set (OFF) as an indication to the user.


  1. Programmable Logic Controller (PLC)

Basics:


The controller device for all the above mentioned items

Control:


Ladder Diagram Logic you have developed.

Hardware Allocation List


The allocation list denotes all input and output channels of the PLC as configured. If an item is an input (denoted by I), the programmer cannot set it externally, but can check its value, whether ON or OFF, at some point on the ladder diagram. Typically they are the buttons and sensors. If the item is an output (denoted by O), the user can set it, whether ON or OFF and it resumes its state until changed. The flags (denoted by F) are the variables, whether ON or OFF, which the programmer can use them to avoid unconditioned jumps (simultaneous uncontrolled operation of several devices).


Input

Description

Comment

I0.0

Start Button

One shot button. The operation should start when this button is pressed.

I0.1

Reset Button

One shot button. All flags should be reset and all devices should return to their initial positions

I0.2

Stop Switch

ON/OFF switch. Can be used to end operation

I0.3

Emergency Stop Pressed Button

All flags should be reset and all active operations should stop.

I1.0

Drilling Unit Position Up Sensor

ON, when the drill unit is in the upward position

I1.1

Drilling Unit Position Down Sensor

ON, when the drill unit is in the downward position

I1.2

Pressing Piston Position Up Sensor

ON, when the pressing piston is in the upward position

I1.3

Pressing Piston Position Down Sensor

ON, when the pressing piston is in the downward position

I1.6

Workpiece Received Sensor

ON, when there is a part in the received conveyor cup.

I1.7

Rotary Table Position Sensor

ON, when the table is at a quadrant perpendicular to its initial position.

I2.2

I2.3


Robot Input Bit1

Robot Input Bit2



These two inputs are controlled by the program inside the robot controller. “00” means there is no active jobs, “01” means that task no:1 is executing (robot is taking the part from conveyor and placing it into the rotary table), “10” means that the task no:2 is executing (robot is taking the part from rotary table and placing it into the conveyor)

Output

Description

Comment

O0.0

Drilling motor

When ON, the motor rotates.

O0.1

Rotary table motor

When ON, the table rotates.

O0.2

Reset lamp

Can be used to indicate whether the reset button is pressed

O0.3

Operate lamp

Can be used to indicate whether there is an active operation.

O0.4

Drilling Unit Go Up

When ON, the drilling unit goes up. After triggering, reset immediately.

O0.5

Drilling Unit Go Down

When ON, the drilling unit goes down. After triggering, reset immediately.

O0.6

Conveyor Operate

When ON, the conveyor rotates.

O1.0

Pressing piston down

When ON, the pressing piston goes down, when OFF the piston resumes its original upward position

O1.2

O1.3


Robot Output Bit1

Robot Output Bit2



These two outputs are controlled by the PLC. Sending a “00” means cancel all jobs, “01” means that execute task no: 1 (robot takes the part from conveyor and places to the rotary table), “10” means that execute the task no: 2 (robot takes the part from rotary table and places to the conveyor). When any task is set, must be reset to avoid successive executions.

SOFTWARE

To set up your programming software and design a ladder diagram, please follow these steps:


Step 1: Configuration

  • Visit the Integrated Manufacturing Systems Web Page and download the zipped file containing Festo Ladder Diagram FST 100 Editor.

  • Unzip the file into C:\ using Winzip. The zip file automatically creates a folder named “festo” and unzips the files in the archive into it. At the end you should have a “Festo” folder under C: driver with many files in it.

If you are using Win XP or an older Windows version:



  • Go to the directory C:\festo. Run “fst100.exe” which is the Ladder Diagram Editor running in MS-DOS mode. Press Alt-Enter and switch to full screen modee so that Ladder Diagram Editor works faster.

If you are using Windows Vista or Windows 7:

  • Graphics card drivers for Vista or Win7 do not support full screen mode for MS-DOS. Festo Ladder Diagram Editor still runs, however it runs very slowly. You can use one of the workarounds for this problem to run the editor faster:

  1. Right click My Computer, go to Device Manager. In the tree of devices, find the display adapter. Right click to your display adapter and disable it. This will probably cause your screen to go blank and you will have to restart your computer. After you restart your computer, Windows will use generic graphics drivers, which supports full screen MS-DOS. Go to the directory C:\festo. Run “fst100.exe” which is the Ladder Diagram Editor running in MS-DOS mode.

  2. The other solution is to install the MS-DOS emulator program DOSBox. Go to www.dosbox.com and download DOSBox for Windows. After you install DOS-BOx, run it and enter the command “mount C C:\festo”. This will mount the C:\festo folder as C folder in DOSBox. Now enter the command “C:”. Now you are in the folder where festo editor is. Finally enter the command “fst100.exe” to run the editor. You can switch to full screen mode by pressing Alt-ENTER.

  • Press Escape twice and you will see a screen like this:



Press F8 and then select “Save and Quit Editor” to exit the program.

Restart the program by running fst100.exe again. Program is now ready to use.

Create a new project by selecting “Project Management” and then “Create project”

Enter the project name as groupX and press F1 to confirm.

Select your ladder diagram from “Ladder diagram” and then “Ladder editor”.

Press F1 to confirm.

Try to add parallel branches, coils, timers and counters to become familiarized with the editor using the function keys.



Step 2: Creating a project

Before creating a project, first you should be familiarized with the ladder diagram logic. The diagram consists of several rungs and branches having multiple boxes, timers, counters, flags on it. There are two sides of the diagram. The left side is used to perform checks such as flags and inputs and the right side is used to activate or deactivate the outputs. When all the serial conditions on the left side are true, stated outputs are executed. Also the outputs can be used as inputs in the succeeding branches. Imagine the diagram as a simple If-then structure. So the diagram in the previous page can be formularized as:

If Input1.0 is true then Flag0.0 and Output1.1 and Timer2 and Counter2 is true.

If Flag0.0 is true and Timer2 is not true then Output1.2 is true.


Before proceeding further let us summarize the types of notations in the ladder diagram:

Notation

Name

Description

I1.1

Input 1.1

The value of input as given in the allocation table, whether ON or OFF

O0.2

Output 0.2

The output as given in the allocation table, can be set (ON) or reset (OFF)

F0.3

Flag 0.3

The flag (a variable), initialized by the programmer, can be set (ON) or reset (OFF)

F0.3

[ ]


Normally open contact with flag F0.3

The flow will pass to next if F0.3 is ON

I1.1

[/]


Normally closed contact with

I1.1


The flow will pass to next if I1.1 is OFF

F0.3 I1.1

[ ]--------[/]



Serial branch with F0.3 and Not I1.1

The flow will pass if F0.3 is ON and I1.1 is OFF

F0.3

---------[ ]--

I1.1

----[/]--



Parallel branch with F0.3 and Not I1.1

The flow will pass if F0.3 is ON or I1.1 is OFF

[ T2

2s

Timer ]



A two seconds timer named T2

The timer T2 becomes ON when initially activated (when time=now), becomes OFF after 2 seconds pass.(when time=now+2)

[ C1

3

Counter ]



A three units counter named C1

The counter C1 becomes ON when initially activated, becomes OFF when it becomes equal to 3 using (INC) and (DEC) contacts

C1

(INC)


Increase counter C1

Increase the value of the counter C1 by 1

C1

(DEC)


Decrease counter C1

Decrease the value of the counter C1 by 1

F1.1

(S)


Set

Make flag F1.1 ON

O1.1

(R)


Reset

Make output O1.1 OFF

SAMPLE LADDER DIAGRAM


Using the given table, create a ladder diagram which will execute the following steps:


  1. When operate button is pressed the conveyor starts running.

  2. When a workpiece is seen on the conveyor cup, the conveyor should stop and the robot will start the loading task

  3. After the loading is completed rotary table should rotate by 4 cups and than wait for 2 seconds.

  4. Robot should take the part and unload it to the conveyor

  5. Conveyor should start operating and wait for the next piece.

  • Cancel all flags and operations when the reset button is pressed.

Let us trace the solution:

R
ung1: Reset all the flags that have been used if the reset button is pressed (I0.1 is ON).


R
ung2: If the reset button is pressed (I0.1 is ON) then reset all the outputs corresponding to the devices that have been used in the ladder diagram (Note: O0.1 should also be reset)

R
ung3: If the start button is pressed (I0.0 is ON) then start the conveyor by setting the output O0.6. Use the convention of setting flags at the output of a rung and resetting it in the following rung.


R
ung4: If a workpiece is received on a conveyor cup (I1.6 is ON) then the conveyor should be stopped. Note that the previous flag is reset and a new one is opened.

R
ung5: Setting robot output 1.2 means that the task no:1, which takes a workpiece from the conveyor and places it to the rotary table, is started. As mentioned earlier, the robot controller continuously traces its output channels to see whether any of its outputs are on. If there are any, it immediately starts the previously downloaded program. That is, if the loading/unloading operation finishes and output channel (O1.2) of the robot is still ON, it re-executes the job no:1. To avoid this, after a small period of time (e.g. 2 seconds) the output channel should be reset. This is the reason why a timer is used here.

R
ung6: After 2s the robot program has started, reset the output to avoid re-execution.



R
ung7: If the robot has finished its operation (I2.2 is OFF) then start the rotary table (Set O0.1) and set a counter with 5 units. Note that a full rotation means 4 cups but a five unit counter is used here.

At the instant the table starts to rotate, the rotary table position sensor (I1.7) is ON which causes the value of the counter to increase by 1. Since its value is already 1 at the very beginning of the rotation, counter should make 4 additional counts for a full-rotation.

R
ung8: Increase the counter value by 1 if there is an ON signal from the rotary table position sensor.




R
ung9: A full rotation is completed. Stop the rotary table motor.


R
ung10: According to the given scenario wait 2 seconds before robot unloads the part.
R
ung11: Start unloading task of the robot (Set O1.3) if 2 seconds have passed.
R
ung12: Similar to Rung6. Reset the output channel of robot after 2s to avoid re-execution.
R
ung13: If the robot operation is finished, start the conveyor (Set O0.6), go back and re-initialize the first flag and re-execute the whole cycle.

In your program, in addition to these you will also use the drilling motor, drilling motor piston, pressing piston, their sensors, lamps and take a precaution for the emergency stop button. Try to optimize the timing and the sequence of operations. Note that, in ladder diagram logic, simultaneous operation of multiple devices is also possible.


INSTRUCTIONS FOR THE EXPERIMENT




Before the Experiment


Design the following sequence and construct the ladder diagram using the software.

  1. When operation is started the conveyor should run and the operate lamp should be ON.

  2. When a workpiece is seen on the conveyor cup, the conveyor should stop and the robot should start the loading task

  3. After the loading is completed, rotary table should position the workpiece under drilling station by rotating 2 cups.

  4. After the rotary table has stopped, the drilling unit should go down. Drilling motor should start operating at the up position. Drill for 3 seconds in the down position, go up and stop drilling motor.

  5. The rotary table should place the part under pressing station by rotating 1 cup and the press actuator has to make the pressing action 2 times. The pressing piston should stay in the down position for 2 seconds for each pressing action.

  6. Rotary table should position the part to the loading/unloading position by rotating 1 cup.

  7. Robot should take the part and unload it to the conveyor

  8. Conveyor should start operating and wait for the next piece.

If reset button is pressed all devices should return to their initial positions, which means:



  • Conveyor stop

  • Drilling Unit Up

  • Press Up

  • Drilling motor OFF

  • Rotary table positioned and stopped

  • Robot back to initial position

  • All flags are cleared

  • The reset lamp should be ON

  • Operate lamp should be OFF.

If at any instance the emergency button is pressed all the devices should stop unconditionally and immediately.


After developing the ladder diagram using the FST 100 software as described here, save the program and quit the ladder diagram editor. In “Project Management” menu there exists a submenu called “Backup/Restore”. Select backup drive as A: and backup the project to an empty diskette. Bring this diskette to CIMLAB at your lab hour. This diskette must not contain viruses. Infected diskettes and corrupted files will not be processed. After the experiment, the diskettes will be submitted back.

During the Experiment


  1. You will restore your project to the PC.

  2. Your ladder diagram project will be downloaded to the PLC controller.

  3. You will analyze and debug your project with the help of your assistant.

  4. Your PLC program will be demonstrated.

  5. Your individual contributions will be assessed and graded.




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