We will apply the five steps to PLC program development to a sorting station example. The program will use shift registers to track coloured parts down a conveyor and sort depending on colour into one of three locations.
Developing the PLC program is a process that can be clearly defined. In our series on the five steps to PLC program development we have done some similar practical examples.
Five Steps to PLC Program Development
– Process Mixer
– Shift Register (Conveyor Reject)
– Paint Spraying
– Delay Starting of 7 Motors
– Pick and Place
Define the task: (1)
Watch the sequence of operation video below.
Watch on YouTube : PLC Programming Example – Sorting Station Testing
A normally open start and normally closed stop pushbuttons are used to start and stop the process in automatic mode. Upon starting the entry, exit and sorter belt conveyors will turn on. This will bring parts to the vision sensor.
Each part will be tracked down the conveyor belt until it is in front of the appropriate exit ramp. (Three Ramps)
The sorter for the exit ramp will turn on allowing the part to slide down the exit ramp.
The number of part sorted are tracked by way of a counter that will display the number of parts that have been sorted for the exit ramp. Hitting the reset when the panel is in manual mode will allow the counters to reset to zero. (0)
To reset the tracking of the parts and clear the conveyors, hit the reset button when the machine is stopped.
If a part is stuck on the exit sensor to the ramp, this will block the sensor. A delay of 5 seconds takes place and then the machine will stop. The flashing reset button will indicate that a jam has occurred at the exit.
Define the Inputs and Outputs: (2)
Item at Exit – Sensor – On/Off – Normally Closed
Start Pushbutton – Normally Open – On/Off
Reset Pushbutton – Normally Open – On/Off
Stop Pushbutton – Normally Closed – On/Off
Emergency Stop – Normally Closed – On/Off
Auto / Manual – Switch – On/Off
Entry Conveyor – Contactor – On/Off
Stop Blade – (Not used)
Exit Conveyor – Contactor – On/Off
Sorter 1 Turn – On/Off
Sorter 1 Belt – Contactor – On/Off
Sorter 2 Turn – On/Off
Sorter 2 Belt – Contactor – On/Off
Sorter 3 Turn – On/Off
Sorter 3 Belt – Contactor – On/Off
Start Light – On/Off
Reset Light – On/Off
Stop Light – On/Off
Vision Sensor (Colour Sensor) – Register – 16 bits
Counter 1 exit ramp – Register – 16 bits
Counter 2 exit ramp – Register – 16 bits
Counter 3 exit ramp – Register – 16 bits
Develop a logical sequence of operation: (3)
A flow chart or sequence table is used to fully understand the process the needs to be controlled. It must also answer questions like the following:
What happens when electrical power and/or pneumatic air is lost? What happens when the input / output devices fail? Do we need redundancy?
This step is where you can save yourself allot of work by understanding everything about the operation. It will help prevent you from continuously re-writing the PLC logic. Knowing all of these answers upfront is vital in the development of the PLC program.
The main part of the program will be to track the parts down the conveyor belt. Using a shift register on each of the exit ramps is a good way to do this logic.
Here is a review of how shift registers work:
In our example we do not have an input to represent movement on the conveyor belt. This would be like a proximity sensor on a sprocket or an encoder on the conveyor shaft. We will accomplish this by using a system timed bit. (100ms interval) When the conveyor belt is running we will trigger our shift register every 100ms. This will act as the clock input to our instruction. Each 100ms pulse of the bit will indicate a movement of the conveyor belt.
Develop the PLC program: (4)
Writing the code for the PLC will be the next step in our program development. A quick review can be seen on our post: Buiding A PLC Program That You Can Be Proud Of
(This series takes you through using discrete inputs and outputs to control traffic lights and cylinders. As we progress we introduce additional methods to solve logic. We look at sequencers in a new way and learn how to write programs to allow users to teach the new sequence.)
The first thing in our program is to control the start and stop functions. This is done through a latching circuit.
Start / Stop Control Circuit
Start the conveyors in the system. This is done with the start light bit.
The three sorting ramps are tracked by their own shift register. The colour sensor on the input (MHR1) will determine the colour of the product. Based upon the colour the product will be sorted into one of three ramps.
The clock input to all three shift registers will be the start bit along with a 0.10 second system clock bit (100 msec). Normally this would be off of a conveyor pulse bit that would represent a distance travelled by the conveyor. Example: Gear turning triggers a proximity sensor.
Reset on the part tracking (shift registers) is done with the reset button when the system has stopped.
Sorter 1 tracking
If the colour sensor picks up a 0 or 4 this is a base unit. It will then put this as an input to the shift register. The output will trigger the sorter 1 turn mechanism.
Sorter 2 tracking
If the colour sensor input is greater or equal to 2 and less than or equal to 3 this is a blue part unit. It will then put this as an input to the shift register. The output will trigger the sorter 2 turn mechanism.
Sorter 3 tracking
If the colour sensor input is, greater or equal to 5 then this is a green part unit. It will then put this as an input to the shift register. The output will trigger the sorter 3 turn mechanism.
The actual output from each of the shift registers will have to be experimented with during the testing of the program. In our case Sort1 bit V0:8, Sort2 bit V21:15 and Sort3 bit V33:7 was used.
Upon the leading edge of the sorter turners, the output for each of the colour sorting ramps will increment by one (+1).
Will flash at 1 second intervals if product is jammed on the output conveyor
Light will be on when reset pressed.
Reset the counters back to zero. Move 0 into the associated word. MHR2 to MHR4
Product Jam at exit Timer. If the sensor is off for more than 5 seconds this will allow the timer to finish and produce an error.
This is the end of our program.
Test the program: (5)
We will be using Factory IO to test the program. This communicates Modbus TCP Client (Master) to our BRX Series PLC (Modbus TCP Server (Slave)).
We will now add the inputs and outputs to our program for communication to our scene in Factory IO. The output coils from Factory IO will set the actual input addresses in our PLC. See the following post on Understanding the PLC Program Scan. This will demonstrate how we can set the actual PLC inputs.
The inputs from Factory IO will be set by the actual outputs from our PLC.
The first step in testing out the program is to ensure that the inputs and outputs of the PLC are wired correctly. We would usually turn each input on manually and see the corresponding input of the PLC turn on. The same applies to the output devices. We would turn on each of the outputs to determine if they have been wired correctly.
If everything is working then we will start running our program in automatic mode.
Fine adjustments may be required for the output bits of the sequencers. In out example we are using the following bits within each of the sequencer instructions.
Test the program for things that can go wrong. Unplug a sensor or remove a box after the sensor has seen it. The program should be able to react.
Watch the video below to see an explanation and test of the program using Factory IO.
Watch on YouTube : PLC Programming Example – Sorting Station
If you have any questions or need further information please contact me.
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