PLC memory is very similar to personal computer memory. There is the operating system and firmware of the processor and connected modules. PLC programs and data that are used by the program are also stored in the memory.
We will now look at the basic understanding of memory in the PLC. Looking at two examples of PLC specifications. We will see how the program is stored and how long data memory will remain when the PLC is not powered up. Let’s get started. Keep on Reading!
PLC outputs are the next component of our PLC block diagram. The outputs of the PLC will be controlled by the PLC program. Controlling outputs is one way to get the inputs to change. We will be looking at digital and analog outputs that can be wired to the programmable logic controller.
PLC troubleshooting outputs will also be discussed. This will be done using a multimeter measuring voltage both at the PLC output and across the discrete output load. We will also measure and control an analog signal output. Let’s get started. Keep on Reading!
PLC inputs are one component of our PLC block diagram. The output actions of the PLC will be controlled based on the inputs. We will be looking at digital and analog inputs that can be wired to the programmable logic controller.
We will be looking at wiring of a normally open (NO) push button, normally closed (NC) push button, 3 wire PNP sensor, and an analog sensor to the PLC. These will all be sinking inputs. Let’s get started. Keep on Reading!
We will apply the five steps to PLC Program development to our next programming example of a process mixer.
The process mixer will be programmed using ladder logic. We will discuss each step of the PLC program development.
1 – Define the task:
How does the process mixer work?
A normally open start and normally closed stop pushbuttons are used to start and stop the process. When the start button is pressed, solenoid A energizes to start filling the tank. As the tank fills, the empty level sensor switch closes. When the tank is full, the full-level sensor switch closes. Solenoid A is de-energized. The mixer motor starts and runs for 3 minutes to mix the liquid. When the agitate motor stops, solenoid B is energized to empty the tank. When the tank is completely empty, the empty sensor switch opens to de-energize solenoid B. The start button is pressed to repeat the sequence.
2 – Define the Inputs and Outputs:
What sensors will be used in the PLC program?
3 – Develop a logical sequence of operation:
How the PLC example program is to solve the logic.
A flow chart or sequence table is used to fully understand the process. It will also prompt 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 is the step where you can save yourself a lot 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.
4 – Develop the PLC program
Writing the PLC sample ladder logic program for the process mixer.
Since we need to continue the sequence when the power goes off then memory retentive locations in the PLC must be used. In our example, we will use the ‘V Memory’ locations.
The first thing in our program is to control the start and stop functions. This is done through a latching circuit. From the sequence table, we know that to reset the sequence we need to have the timer done and the empty sensor off.
The filling of the tank is done through Solenoid A. It is turned on by the start signal and off by the full sensor switch. (Sequence Table) You will notice that we have a memory retentive output and the actual output to active the solenoid.
The memory retentive timer will start timing when we have the start sequence signal and when the empty and fill sensors are on. The timer will reset when the empty and fill sensors are off. Mixing motor will be on when the timer is timing and when the timer is not done.
5- Test the program
Simulate the PLC program of the Process Mixer
Test the program under many conditions. Check to see what happens when power is removed.
Using this five-step to program development technique will shorten your programming time. The result will be a better-defined logic and easier to understand the program, because it has within the documentation the logic flow chart or sequence table.
Watch on YouTube: PLC Programming Example – Process Mixer
Factory IO provides a 3D simulation of the process. Testing of the program is important and should be done in a variety of ways. Factory IO provides a straight forward method of seeing your program in action before you wire your application.
We will be using the BRX PLC Modbus TCP Server (Slave). Factory IO will be the Modbus TCP Client (Master). When the tank fills up we will start a dwell time instead of the mixer time for the simulation.
Here is the mapping of the inputs and outputs using Factory IO.
You can download the PLC program and Factory IO scene here.
Watch the following video to see this simulation in action.
Watch on YouTube: Process Mixer Test Simulation
If you have any questions or need further information please contact me.
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