Tag Archives: plc programming training

Create an Analog Voltage Input Tester for a PLC

We will create a simple and inexpensive analog voltage tester for a PLC using a potentiometer and a 9VDC battery. The potentiometer will be 5K ohms. This should be enough impedance for most analog inputs of the programmable logic controller. (PLC) Voltage impedance for analog voltage inputs are in the mega ohm range where current input is typically 250 ohms. Our tester will be for analog voltage inputs (0-10 VDC). Check your input specifications before wiring anything to your PLC. I have used this tester for other voltage inputs along with a meter to ensure that the voltage levels do not get out of range for the input signal.
Analog inputs to the PLC are continuous and can come in a variety of signals. These signals can come from temperature, flow rate, pressure, distance, etc. Continue Reading!

What Everybody Ought to Know About PLC Programming Languages

PLC programs are normally written in a special application on a personal computer, then downloaded to the PLC. This downloaded program is similar to compiled code to keep the program efficient. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory.

Albert Einstein said “The world as we have created it is a process of our thinking. It cannot be changed without changing our thinking” PLC programming languages have evolved to both adapt and change the way we program these units. We will look at all five programming languages as defined by the IEC 61131-3 Standard.

  • Structured Text (ST)
  • Function Block Diagram (FBD)
  • Sequential Function Chart (SFC)
  • Instruction List (IL)
  • Ladder Diagram (LD)

Not all of these programmable controller languages are available in every PLC. Ladder logic programming is by far the largest percentage of use in PLC’s today. Fundamental concepts of PLC programming are common to all manufacturers. Differences in I/O addressing, memory organization, and instruction sets mean that PLC programs are never interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible. This is true when looking at manufactures that private label other controllers.

Estimates are as high as 95% of installations use ladder logic programming in the programmable logic controller.

The PLC programming language that is used can be decided when you look at the following:

  • Maintenance and troubleshooting
  • Knowledge of language
  • Acceptance of the country, location, or individual plant
  • Application of the PLC
  • Ease of changing PLC program

The actual programming of the PLC is the second last step in the development of programs. The five steps to PLC program development is a good method to follow before picking what programming language to use. As mentioned before the languages supported by each PLC may differ. Please refer to the types of programming that are available for your model and version of PLC.

Let’s quickly review some of the different programming languages for the PLC.

Structured Text (ST) is a high level programming language that closely resembles Pascale programming. Statements are used to define what to execute.
ST MP50pro_st

Function Block Diagram (FBD) is a graphical representation of AND, NAND, OR, NOR gates, etc. that are drawn. It will describe the function between input and output variables.
FBD MP50pro_fbd

Sequential Function Chart (SFC) is like a flowchart of your program. It defines the steps through which your program moves.
SFC MP50pro_sfc

Instruction List (IL) can also be referred to as mnemonic code and statement list. It contains simple instructions for looking at your variables.
IL MP50pro_il

Ladder Diagram (LD) is the most popular programming language for the PLC. It was written to mimic the mechanical relays in the panel that the programmable logic controller replaced. It has two vertical rails and a series of horizontal rungs between them. Controllers will usually scan from left to right top to bottom. The output of one rung is available for the next rung.
LD MP50pro_ladder

Note: All pictures from PLCopen IEC 61131 Basics

PLC programming methods are evolving. PLC Open is an organization that is defining new methods to take advantage of the latest computer innovations. They have defined the IL method of programming to XML (Extended Markup Language) which is used for web development. This in my opinion keeps moving the ideal method, to a standard way to program PLCs.

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.

Five Steps to PLC Program Development

Programming a PLC can sometimes be a daunting task. The best method is to break the task into some smaller steps. These are the steps that I have used for years. We will apply them to a die stamping application.

1 – Define the task:

What has to happen?
Die Stamping

A master switch is used to start the process and to shut it down. Two sensors: an upper limit switch that indicates when the piston is fully retracted and a lower limit switch that indicates when the piston is fully extended. When the master switch is turned on the piston reciprocates between the extended and retracted positions. This is achieved with an up and down solenoid. When the master switch is turned off, the piston returns to the retracted position and all solenoids are off.

2 – Define the Inputs and Outputs:

Inputs:
Master Switch – On/Off
Upper Limit Switch – On/Off
Lower Limit Switch – On/Off

Outputs:
Down Solenoid – On/Off
Up Solenoid – On/Off

3 – Develop a logical sequence of operation:

This can be done with the use of a flow chart or sequence table. You can use anything to fully understand the logic of the operation before programming. Many people do not use this step and jump straight to programming.

Fully understanding the logic before starting to program can save you time and frustration.

Sequence Table: The following is a sequence table for our die stamping application. I usually review this sequence with the person with the most knowledge of the machine. This can be the designer and / or the machine operator.
Sequence Table

How to read the Sequence Table: Follow the steps from left to right, top to bottom. Inputs and outputs are labelled as 1 (ON), 0 (OFF) or X (Does not Matter). Step 1 indicates that it does not matter the upper and lower limit switch positions. The master switch is off, so the up and down solenoids are off. Steps 3 and 4 repeat themselves as long as the master switch is on.

Note: You will notice that at step 2, after the master switch turns on the up solenoid will be activated. So the piston always retracts when the master switch is first turned on.  This operation was picked up in development of our logical sequence.

4 – Develop the PLC program:

Look at the sequence table in respect to the following logic. I have used Set and Reset conditions so it is easily followed by the sequence table. When the master switch turns on the up solenoid is activated. Notice the first rung is a direct correlation. Follow the rest of the sequence table with this ladder logic.

PLC Program Die Stamping
Document, Document, Document This is a vital part of every program, which will save you time and money when you have to return to the program years later.

5 – Test the program:

Die_Stamping
Test the program with a simulator or actual machine. Make modifications as necessary. Check with the people most knowledgeable on the machine, to see if it is doing what they expect. Do they need something else? Follow up after a time frame to see if any problems arise that need to be addressed.

These five steps will help you in your PLC programming.

  1. Define the task
  2. Define the inputs and outputs
  3. Develop a logical sequence of operation
  4. Develop the PLC program
  5. Test the program

The five steps form the basis of all PLC development. You will notice that the actual programming does not occur until the second last step. Usually more time is spent on understanding the task and sequence of operation.

Watch on YouTube : Five Steps to PLC Program Development
If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.

How PLC Inputs Work

This post is a further follow up from my original ‘Here’s a Quick Way to Understand PLC Inputs and Outputs’. There are basically two different kinds of PLC inputs, Discrete and analog. Discrete inputs are either ‘ON’ or ‘OFF’; 1 or 0. You can think of them as a single switch. Analog inputs have a range to them. They are inputs that usually will sense pressure, temperature, height, weight, etc.  They usually have one of the following signals that are inputted into the PLC: 4-20mA, 0-10VDC, 1-5VDC.

Discrete Inputs
PLC Input

The above diagram has two inputs. A normally open (NO)  and a normally closed (NC). When we talk about normally open and close, think of the condition of the input if no one touches anything. A normally open contact will not turn on the input to the PLC card in its ‘normal’ state. The normally closed contact will turn on the input to the PLC card in its ‘normal’ state.

Normally Open Input
The NO contact when activated will complete a circuit and turn on the PLC input. Ladder logic will then turn on if you use a normally open (Examine On) input in your program. See above diagram.

Normally Closed Input
The NC contact when activated will break a circuit and turn off the PLC input. Ladder logic will then turn off if you use a normally open (Examine On) input in your program. See above diagram.

As you can see with the diagram above this can get tricky to determine the on/off condition of the input. PLC logic can convert any signal by using normally closed (Examine Off) inputs in the program. In the field, I usually look at the PLC input lights, and wiring diagrams to determine the current state of the input. This is before diving into the program to troubleshoot.

Here is a link to wiring up discrete 3-wire sensors in the field. Here’s a Quick Way to Wire NPN and PNP devices

Analog Inputs
BinMaster Analog Input

An analog input converts a voltage or current level into a digital value that can be stored and processed in the PLC. They use words to determine the signal coming from the device.
Example:
4 – 20 mA current Input – 8 bit resolution
4 mA = 00000000 base 2 = 00 base 16
20 mA = 11111111 base 2 = FF base 16
For a review of numbering systems, follow the link below:
What Everybody Ought to Know About PLC (Programmable Logic Controller) Numbering Systems

In the industrial environment noise from variable frequency drives, improper grounding, etc. can interfere with your analog input. The following post will show a quick method to reduce this noise.
The Secret Of Getting Rid Of Noise On Your Analog Signal

Here are some additional posts that you might find helpful.
How to make a Start / Stop / Jog circuit in a PLC
The Secret of Using Timers
The Secret of Using Counters

Watch on YouTube : How PLC Inputs Work

Watch on YouTube : Wiring (Testing) Analog PLC Input Click

Watch on YouTube : Wiring (Testing) Analog PLC Input Omron CP1H

Watch on YouTube : Wiring Contact (Discrete) PLC Inputs

Watch on YouTube : Wiring PNP Sensor to PLC

Watch on YouTube : Wiring NPN Sensor to PLC

If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.

Here’s a Quick Way to Wire NPN and PNP devices

Here’s a Quick Way to Wire NPN and PNP devices

I get asked often on how to wire NPN and PNP devices to the programmable logic controller. This can be confusing at first when looking at the wiring diagrams. I have managed to destroy a few sensors in the process….. so lets get started and I will share my experiences.

E2A_7069B_7l E2A_7002A_7l
NPN and PNP refer to the transistor in the output device.
NPN – Negative Positive Negative Switching. Sometimes referred to as ‘Sinking’ the load.  People have told me that when the NPN sensor blows it has a tendency to blow in an open state. (No Signal)
PNP – Positive Negative Positive Switching. Sometimes referred to as ‘Sourcing’ the load. People have told me that when the PNP sensor blows it has a tendency to blow in a closed state. (Signal On)

When the sensor blows, (malfunctions) it usually will also take out the power supply. (Fuse) It generally does not matter if you use NPN or PNP sensors provided they are all connected to the PLC using isolated commons.

You cannot mix PNP and NPN sensors on the same common point for inputs to the PLC. If you do mix the sensors, then the different common points on the PLC must be isolated from each other. This means that the commons are not connected internally to each other. Not ensuring this takes place will provide a short across the power supply and blow your sensors and supply. In general, machines tend to use all NPN or all PNP only.

Colour coding of the wires vary. Do not always rely on the colour code of the wires for connection. Refer to the wire diagrams in the documentation.

The following is a wire diagram of an open collector PNP sensor. You will notice that the load appears between the 0V (Blue)  and Switching wire (Black). When connecting to the PLC, the PLC input acts as the load. The 0V (Blue) will be attached to the common input and the Switching wire (Black) will be attached to the input number.PNP1

The following is a wire diagram of an open collector NPN sensor. You will notice that the load appears between the +V (Brown)  and Switching wire (Black). When connecting to the PLC, the PLC input acts as the load. The +V (Brown) will be attached to the common input and the Switching wire (Black) will be attached to the input number.NPN1

As you can see a direct short will be created if NPN and PNP sensors are wired into the PLC on the same common. The following shows an example of wiring of the 3 wire sensors into a PLC with isolated commons.

NPN_PNP_PLC

Watch on YouTube : Wiring NPN Sensor to PLC

Watch on YouTube : Wiring PNP Sensor to PLC

Watch on YouTube : Wiring Contact (Discrete) PLC Inputs

Wiring Interposing Relays
Watch on YouTube
: Wiring NPN and PNP Sensors into the PLC with an Interposing Relay
If you have any questions or need further information please contact me.
Thank you,
Garry



If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.

Building a PLC Program That You Can Be Proud Of – Part 1

What is the best way to program a PLC? 
My answer is simple. The best way is one in which someone can look at your program and understand it. I cannot stress enough the need for good documentation of your program. The best programs are ones that I can return to after several years and understand what it is doing within a few minutes. Programs should read like a book. This will aid in troubleshooting, modifying or teaching.

How do you approach a PLC program?
You must know everything about the logic or process before starting your program. Making a flow chart is one good method to learning the logic and process. The flow chart will bring out questions like the following:
What happens after a power outage? (In each condition of the outputs)
What happens if a sensor is not made? How long do you wait?
What are the critical items to monitor? (Ex. Air Pressure, Weight, Length, etc)
What happens…
Once you have written your program and are in the troubleshooting stage you can usually go back and add to your flow chart. Usually there is always something that needs to be added, changed or modified based upon the actual functioning of the program.
Consider each project a complete leaning opportunity.

Once you know what you want to do with the PLC and have a good understanding of the logic flow, then it is time to start coding. Remember that there is no write or wrong method to program the PLC, either the program will work or it will not work.

Let’s look at an example. We will start with something that we all know how it works.
Traffic Lights

We will look at three programming examples for the lights. Two different approaches to programming will be used, but the program function is the same. The last example will modify the logic for a car being sensed.

Logic:
First Example:
Traffic Light Program
Sample program for traffic light intersection with lights facing North /South and West /East.
Green is on for 5 seconds
Yellow is on for 2 seconds
Red has an overlap of 3 seconds
This program uses discrete bits and timers to accomplish this task.
The $FirstScan bit will reset the timers so if power is lost, the lights will start with Red / Red overlap before starting the sequence again.
The outputs are controlled by when the timers are on (Done) or off (Not Done)
North / South Traffic Lights
West / East Traffic Lights

You will notice that this program is fully documented and easy to understand.

This program is based upon time events. The base rate is one second. If we create a list of what the outputs look like after each second and then send them to the physical output channel we will have the second type of approach to this logic…

Logic:
Second Example:
Traffic Light Program

Sample program for traffic light intersection with lights facing North /South and West /East.
Green is on for 5 seconds
Yellow is on for 2 seconds
Red has an overlap of 3 seconds

This program uses indirect addressing to program
Lets look at the list of outputs we want based upon the following addresses: (Notice the Bit location)
Y0 – Red_Light_NS
Y1 – Yellow_Light_NS
Y2 – Green_Light NS
Y8 – Red_Light_WE
Y9 – Yellow_Light_WE
Y10 – Green_Light_WE

We have 20 steps to do in the sequence based upon 1 second increments. (V1000 to V1019)

Here is what the hex values translated to binary look like:
(Review of numbering systems from previous blog)

The $FirstScan bit will reset the pointers so if power is lost, the lights will start with Red / Red overlap before starting the sequence again.

Lets look at the program:

The $FirstScan bit will move  the number 1000 into V0. V0 will act as our pointer for the list of outputs. (V1000 to V1019)
Every 1 second ($1Second) V0 will increment by a value of 1. We will then compare the value to 1020 which indicates the end of the sequence. If the value is greater or equal to then our pointer is reset to the value of 1000. This is done by moving the number 1000 into V0.
The last step is moving our output word indirectly V0 to our output word V1. Indirectly means that the value in V0 will point to a memory location dictated by the number it contains.
ex: V0 has a value of 1000 so this means that V[V0] will move V1000 into our output word.

Set the outputs
Our physical outputs are set by casing our output word (V1) into bits. Depending on the programmable controller this can be done my moving to a word that can be addressed by bits or in our case we can cast the word into bits. [V1:#]

This program code is allot smaller than the first using discrete bits and timers. With documentation it is also easier to read.

One of the advantages of indirect addressing to program is that it makes modifications easier. Lets modify the last program…

The North will stay green until a car approaches from the West. It will remain green for 1 more second before turning yellow and completing the cycle. If the car is always there then the lights will always function.
X0 – Car West/East

Just a couple of contacts have been added to the logic on the line that increments the pointer. The setting of the outputs do not change.
If the value at V0 is equal to 1006 then stop incrementing V0. X0 (Car at intersection) comes then the pointer will increment. The cycle will complete and continue until X0 is not present. It will then stop when the pointer V0 equals 1006 again.

Watch on YouTube : Building a PLC Program That You Can Be Proud Of

In part 2 we will look at indirect addressing with a sequence that is event driven, not timed like the above.

Contact me for the above programs. I will be happy to email them to you.
If you have any questions or need further information please contact me.
Thank you,
Garry

You can download the software and simulator free at the following address. Also listed are helpful guides to walk you through your first program.
Do-more Designer Software

How to use video’s for Do-more Designer Software

One of the better PLC programming books is PLC Programming for Industrial Automation by Keven Collins. Here is the link to the free download.

http://staffweb.itsligo.ie/staff/kcollins/plc/plcprogramming.pdf




If you’re like most of my readers, you’re committed to learning about technology. Numbering systems used in PLC’s are not difficult to learn and understand. We will walk through the numbering systems used in PLCs. This includes Bits, Decimal, Hexadecimal, ASCII and Floating Point.

To get this free article, subscribe to my free email newsletter.


Use the information to inform other people how numbering systems work. Sign up now.

The ‘Robust Data Logging for Free’ eBook is also available as a free download. The link is included when you subscribe to ACC Automation.