Category Archives: analog

Click PLC Numbering System and Addressing

Continuing our series, we will now look at the numbering systems and addressing used in the Click PLC. Previously we have discussed:
Click PLC System Hardware
Click PLC Installing the Software
Click PLC Establish Communication
The programming software and manuals can be downloaded from the Automation Direct website free of charge. These are being used exclusively in our Click PLC series.  Keep on Reading!

Click PLC System Hardware

Many people ask me what I do when looking at a new PLC model or system. My approach is very straight forward and we will view this in action with this Click PLC series. This series will go from examining the hardware to programming and communicating to the PLC in several ways. If you have questions along the way, please let me know. Keep on Reading!

AdvancedHMI to Solo Process Temperature Controller

Modbus RTU will be the serial (RS485) method in which we will communicate between the AdvancedHMI Screen and the Automation Direct Solo Process Temperature Controller.
We can address up to 247 (Solo 1 to 247) devices on this master – slave protocol. A maximum of 32 devices (Nodes) on the network can communicate to the master. A review of the Modbus RTU protocol can be seen at the following URL.
http://www.rtaautomation.com/technologies/modbus-rtu/

AdvancedHMI is a free HMI programming package the runs on Microsoft Visual Studio. It can be downloaded at the following URL.
https://sourceforge.net/projects/advancedhmi/

Connections:
We will be running the AdvancedHMI software on the computer. One of the USB ports will have an USB to RS485 adapter and communicate RS485 to the Solo process temperature controller.

See the following post to install the USB to RS485 adapter.
http://accautomation.ca/usb-to-rs485-pc-adapter-installation/

Solo Controller Settings:
In the Initial Setting Mode we will change the on line configuration to on and make the changes to the Modbus settings as follows: 9600 Baud, Even, 7 Data Bits, 1 Stop Bit, Modbus ASCII Format. We will leave the default unit number as 1. See the following post to set the controller:
http://accautomation.ca/solo-process-temperature-controller/

Modbus RTU (Addresses)
The following address will be used in our project:


AdvancedHMI will use the Modbus Decimal value in the PLCAddressValue to determine the information that you want to get. For a list of all Modbus addresses that can be used in your project, refer to the Solo Manual located a the following URL:
https://www.automationdirect.com/adc/Manuals/Catalog/Process_Control_-a-_Measurement/Temperature_-z-_Process_Controllers

Screen Display: (AdvancedHMI)
Here is what our screen will look like:

We have mimicked the look of the solo process temperature controller. Our PV and SV values are DigitalPanelMeters from the AdvancedHMI toolbar.  The eight output indicators are just labels.

Our ModbusRTUCom1 settings are as follows:

Settings: 9600, 8, Even, One StopBit and Station 1 should all match the settings in the Solo process temperature controller that we did previously.
PollRateOverride will allow us to determine how often the communication will take place to the controller. (250msec)
The PortName will be the same port number that the computer will communicate out of. This will be set when you install the USB to RS485 adapter. It may change if a different USB port is used.

The DataSubsciber1 will be used to determine the status of the controller.

PLCAddressValue = 44139

We read the value of the eight status bits and convert this into a string so we can determine the status of each of the individual bits. Here is the code that is used to do this. It is the only code required for this application.

Private Sub DataSubscriber1_DataChanged(sender As Object, e AsDrivers.Common.PlcComEventArgs) Handles DataSubscriber1.DataChanged
        Dim i As Integer = DataSubscriber1.Value
        Dim Status As String
        Status = Convert.ToString(i, 2).PadLeft(8, "0") '8 bits
        'There are 8 bits that we need to check and account for on our screen. 
        'Modbus Decimal - 44139
        'Bit 0 - ALM3 - Alarm 3
        'Bit 1 - ALM2 - Alarm 2
        'Bit 2 - C degrees
        'Bit 3 - F degrees
        'Bit 4 - ALM1 - Alarm 1
        'Bit 5 - OUT 2 
        'Bit 6 - OUT 1
        'Bit 7 - AT - Auto Tuning

The complete AdvancedHMI code for this application can be downloaded at the end of the post.

The PV and SV indicators are DigitalPanelMeters as mentioned above.


They both have four digits with a decimal position of 1. This will give us a value between 000.0 and 999.9.
The SV includes a keypad to change the set value. KeypadScaleFactor is set to 0.1 to allow for the decimal place.

Included in our display is a BasicTrendChart from the AdvancedHMI toolbar.


You want to make sure that the YMaximum and YMinimum settings are set so the values will not go past these settings. If they do then the graph line will disappear from the chart at that point and time.
This will show a running trend for the last 5 minutes.
Polling rate is 250msec x 1200 points in the chart = 300 000msec
300 000msec / 1000 = 300 seconds
300 seconds / 60 (seconds in minute) = 5 minutes

Notes: Displaying Extended ASCII Symbols in Visual Studio (VB.NET)
You can display any symbol in the visual studio environment by holding the ‘Alt’ key down and typing the decimal number of the symbol that you want. In our example the degrees symbol is Alt 248.
Here are the extended ASCII symbols:

http://www.asciitable.com/

Running the Application:

You will notice that the response rate is very quick. (250msec) As the PV, SV or indication values change, the screen will get updated.

The trend chart will show the last 5 minutes of the PV value. 

As you can see, programming the AdvancedHMI to communicate to the Solo process temperature controller is very easily done.

Download the AdvancedHMI code for this project here.

Watch on YouTube : AdvancedHMI to Solo Process Temperature Controller
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.

Solo Process Temperature Controller

The SOLO Temperature Controller is a single loop dual output process temperature controller that can control both heating and cooling simultaneously. It is available in 1/32, 1/16, 1/8 and 1/4 DIN panel sizes and is UL, CUL and CE approved.

There are four types of control modes:
PID (Auto Tuning (AT) function)
P stands for proportional and accounts for present values of the error – It reacts to the amount of error which is the difference between the SP (Set Point) and the PV (Present Value)
I stands for integral and accounts for past values of the error – It uses math to basically find the approximation of area under the curve.
D stands for derivative and accounts for possible future values of the error, based upon rate of change – It uses math to determine the slop of the error over time and multiply this by the derivative gain.
AT – Auto Tuning as the name implies will automatically cycle your control system through two cycles and set the PID parameters.

ON / OFF
On/Off control is the simplest form of control. In the case of temperature the output will be on when the temperature is below set point. When the temperature gets above the set point of the controller the output will be off. When this cycling occurs frequently, you can add a hysteresis to the output. This will limit the time the output goes on and off by a number of degrees.

Manual
Manual mode is when you need to control the output directly. By entering manual mode the operator can adjust the values of the output(s).

Ramp / Soak control
The Ramp / Soak control mode is used to control the outputs according to the pre-programmed SP patterns with the PID control method. The Solo can have eight programs with eight steps each. Note: This can be increased with the additional use of hardware and software via HMI or PLC.

The available outputs include relay, voltage pulse, current, and linear voltage. There are up to three alarm outputs available to allow seventeen alarm types in the initial setting mode. SOLO can accept various types of thermocouple, RTD, or analog input. This means that cascade control is possible with these controllers.

Cascade (Application)
When you use the output of one of the Solo process controllers as the input to another, this would be cascade control.

It has a built in RS-485 interface using Modbus slave (ASCII or RTU) communication protocol.
The Solo Process Temperature Controller can be configured through the buttons on the front of the unit or by the configuration software. Monitoring of up to four controllers at once can be done thought the same configuration software.

The following is the wiring of the Modbus Serial communication. We will use a USB to RS485 converter from Automation Direct. (USB-485M)
Installation and operation instructions can be found at the following link: USB to RS485 PC Adapter Installation

The solo process temperature controller needs to be setup before we can communicate to it. The default setting is ‘Off’ for the On-Line Configuration. Here is the way to change into the different modes in the Solo.

In the Initial Setting Mode we will change the on line configuration to on and make the changes to the Modbus settings as follows: 9600 Baud, Even, 7 Data Bits, 1 Stop Bit, Modbus ASCII Format. We will leave the default unit number as 1.

Our controller is now set to communicate.

Download the documentation and/or configuration and monitoring software at the following URL link:
http://support.automationdirect.com/products/solo.html

The configuration and monitoring software does not have to be installed. You just need to download the file “slsoft.zip”.
Once downloaded right click on the file and select “Extract All…”. The file ADC1105.exe can now be run.

The recommended screen resolution is 1024 x 768. If you do not have this resolution then there is another program that will allow us to create a shortcut to this program and change our default resolution. After we exit the program, our screen resolution will return to its original state.

Reso is a free application that works well. It can be downloaded at the following URL link:
http://www.bcheck.net/apps/reso.htm

Download the exe file (reso.exe) into the same extracted folder that you have the ADC1105.exe file.

Click on the reso.exe file in the folder to run the application.
Click the Browse… button and select the ADC1105.exe solo configuration software.
Under the Graphics Mode: Resolution: select 1024×768
We can leave the rest to the defaults as shown below. Now Click Create Shortcut.

Put the shortcut in the same directory as the software that was downloaded.

We will receive notification that the shortcut was created. Click OK.

Our folder will now look like this.

Click on our shortcut (ADC1105 (at 1024×768)) to start the Automation Direct Solo Series Configuration Software.

There are six icons on the main menu. Follow these in order from left to right to setup or troubleshoot your system.

Under the Com port setup we configure the serial port of the computer to communicate to the Solo. In our case we will use COM5, 9600, Even, 7 Data Bits, 1 Stop Bit, Modbus ASCII Format.

In the Configuration menu we will select the address of the controller that we will be communicating. This will be the default of the Solo which is 1.

Selecting Connect will then communicate to the Solo process temperature controller and you will see on the screen a picture of the controller with the PV, SV and indicator lights active.

This screen will now allow you to set up all of the parameters in the Solo. Once you enter a value it will be red on the screen. After hitting enter on the keyboard and the value will then be sent to the controller and be displayed in black again.

The recorder screen is used to monitor the temperature over time. You can monitor up to ten Solo Controllers at once.

Command Test is used to send individual Modbus command out. This is done in Hexadecimal.

The software for the Solo Process Temperature Controllers is very functional. Using the Reso software, you will be able to put the Solo Software in the 1024 x 768 that it was created for without manually changing the screen every time.

Watch on YouTube : Solo Process Temperature Controller

Watch on YouTube : Analog Input to a Solo Process Temperature Controller
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.

PLC Bits Numbers and Position

People often ask “What is a PLC?” and “PLC Meaning”. A programmable logic controller (PLC) is a piece of hardware that isolates inputs from outputs. Programs are written to look at the inputs solve logic and set the outputs to perform work. Today we are going to look at the basic fundamental way we program. Every PLC company will do this…

Everything in the programmable logic controller actually boils down to bits in the memory.

It is these bits that we manipulate in order to accomplish the work that we need done by the PLC. The instruction set is the method we use to do this. In general, there are several ways to view the bits. Discrete input and output, Numbers and Position of bits will be covered. Understanding the different ways in which we can view these bits will help in developing programs.
Bits are part of the memory of all PLC systems. The memory can be retentive or non-memory retentive. Memory retentive means that if power is lost to the PLC, the status of the bit remains the same when power is restored. If the bit is non-memory retentive, and power is lost the bit returns to the off state. Addressing refers to how the controller understands what memory location to look at. When we address memory in the PLC we can do this in two different ways:
Direct Addressing: Specify a location of the memory location
Indirect Addressing: Specify a location that contains a value to point to the memory location required.

Refer to the manual of the specific PLC that you are using for the way in which memory is addressed and if it is memory retentive or not.

Discrete bits are the basic building blocks in the PLC. When we talk of digital I/O this is referring to the individual bits that you can wire switches, pushbuttons, proximity sensors, or any other device that is either on or off. (1 or 0) They can be usually wired to the PLC as a normally open or normally closed contact. The ladder logic is written in a way that you examine the bit as either on or off.
HOW PLC INPUTS WORK

HOW PLC OUTPUTS WORK
We also must look at the frequency (rate of change from off to on) of the input bits or output in some cases. The maximum frequency that we can read an input to the PLC will be determined by the scan of the PLC.
Example:
A 2 ms Scan (0.002 second) means that we can read the inputs and solve the logic in 2 ms. In order to ensure that the input is read in both states (on / off) we have to ensure that the input is off or on for at least 2 ms. The maximum frequency (Switching / Second) that the input could switch would be 2 ms = 1/.002 times per second = 500hz

Numbers in the PLC are all based on binary. Analog inputs and outputs are based upon the number of bits put together in order to display the range for the input. (12 bit or 16 bit) The values from the analog 12bit input will go from 000 to FFF base 16 (Hex). Hexadecimal is used to display the binary bits in the word or register. Some of the more common numbering systems in the PLC are binary, hexadecimal, BCD (binary coded decimal) and octal (based on 8 bits)

Additional Information on understanding numbering systems in the PLC:
What Everybody Ought to Know about PLC (Programmable Logic Controller) Numbering Systems

Position of the bits within the word, stack or accumulator can be very useful. Usually we can use this to track items. The typical example of this is to track items on a conveyor belt. The belt movement is usually a pulse input from an encoder. A sensor indicates the item on the conveyor.
PLC PROGRAMMING EXAMPLE – SHIFT REGISTER (CONVEYOR REJECT)

Bits are the basic building blocks that we use to program programmable logic controllers. The three ways to view bits (Discrete, Number and Position) will help use to understand the different ways to program.
Here are some additional links that you may find helpful:
Five Steps to PLC Program Development
PLC Programming Example – Process Mixer
PLC Programming Example – Shift Register (Conveyor Reject)
PLC Programming Example – Paint Spraying

The Secret of Using Counters
The Secret of Using Timers

Watch on YouTube : PLC Bits Numbers and Position
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.

Create a PLC with HMI Training and Learning Environment Free

Learn PLC programming and use a powerful HMI (Human Machine Interface) easily and free. We will use the Automation Direct Do-More programming software tied into the Advanced HMI package via Modbus TCP.

Our application will show a HMI screen with a panel meter and a reset button. The panel meter value may be changed by clicking it. This will bring up a input screen to put in a number. When the reset button is selected the input value entered will show on the panel meter.

Since we will be communicating via Modbus TCP, the following table shows the Coil/Register Numbers and the associated Do-More PLC Addresses.

Coil/Register Numbers Data Addresses Type Do-More PLC Table Name
00001-09999 0000 to 270E Read-Write MC1 to MC1023 Discrete Output Coils
10001-19999 0000 to 270E Read-Only MI1 to MI1023 Discrete Input Contacts
30001-39999 0000 to 270E Read-Only MIR1 to MIR2047 Analog Input Registers
40001-49999 0000 to 270E Read-Write MHR1 to MHR2047 Analog Output Holding Registers

Note: The Do More PLC uses the Modbus area to communicate. This is because having direct access to the digital I/O can be dangerous when connected via Ethernet to the internet. Data must move in and out of this area via the PLC program.

We will first start with the PLC.
Automation Direct has a powerful simulator with their Do-More PLC. It is the Do-More Designer Software. This software simulator includes the entire instruction set (Not Just Bit Logic) as well as communication protocols. It can be downloaded and installed for free from the above link.
Our PLC program will have the following addresses:
Digital Panel Meter Present Value (PV) – MHR1 – Modbus 40001
Digital Panel Meter Set Value (SV) – MHR2 – Modbus 40002
Reset Button – MC1 – Modbus 00001

The first rung of the ladder will use the 1 second pulse bit and increment the PV value of our digital panel meter. This will also compare the current value to 4000 and if greater or equal, move the value of zero into the PV value.

The second rung of the ladder will move WX0 analog value from our simulator into the PV value of our digital panel meter.

The last rung of ladder will move the SV value into the PV value of our Digital Panel Meter. This happens when the reset is hit.

The simulator is showing X0 on and we can then use the WX0 slider to change the PV value of the Panel Meter.

Advanced HMI is a powerful HMI/SCADA (Supervisory Control and Data Acquisition) development package that takes advantage of Visual Studio. There is no coding required and you can simply drag and drop items onto the page. The best thing is that the software is free.

Communications drivers include the following and are accessible via VB or C# code:

  • Allen Bradley DF1 RS232 Driver
  • Allen Bradley Ethernet/IP Driver for SLC,MicroLogix, ControlLogix, and CompactLogix
  • Beckhof TwinCAT Driver
  • ModbusTCP Driver
  • ModbusRTU Driver
  • Omron Ethernet FINS Driver – Ethernet for newer controllers such as CP1H with Ethernet module
  • Omron Serial FINS Driver – Serial (RS232 / RS485) for newer controller such as CP1H
  • Omron Serial HostLink Driver – Serial (RS232 / RS485) for controllers such as CQM1, C200H, K-Series (C28K), C200, etc

The power of Advanced HMI is that it works within Visual Studio. This is a program integrated development environment (IDE) that you can take advantage of to modify or create new features including data logging applications.

Advanced HMI runs on Visual Studio 2008 or higher and will need to be installed on your PC. Visual Studio Community Edition 2015 is the latest version of the software. If you do not have it installed, please download and install from the following link.

https://www.visualstudio.com/en-us/products/visual-studio-community-vs

We will now need the Advanced HMI project. Here is the link to download the zip file.

http://sourceforge.net/projects/advancedhmi/

After downloading ‘AdvancedHMIBetaV399a.zip’ extract the files from the zip file. (Right Click.. Select Extract All)
Note: Your version might be different than the one above.

Open the solution file (AdvancedHMIv35.sln) from the extracted files in the root directory.

Our initial screen looks like the following. The project will now need to be compiled in order to add the components to the Toolbox.
Select Build | Build Solution from the menu
The next thing to do is add the communication to the form. On the left hand side of the screen you will see the ‘Toolbox’. Click on it and under AdvancedHMIDrivers Components we will select ModbusTCPCom. To actually add a component to our form you need to drag it. Select the component and as you hold the mouse button down move to the form.

After adding the ModbusTCPCom component, it will appear at the bottom, beneath our form.
Click on the ModbusTCPCom1 at the bottom of our form. On the right hand side you will notice the properties for this communication driver. Under Communication Settings | IP Address, enter the value  of the IP Address for the PLC. (192.168.1.3) Ensure that the port number is 502. This is the default port number for Modbus TCP.

We can now add the digital panel meter. From the toolbox select and drag the DigitalPanelMeter to our form.

Resize the panel meter on the form by dragging a corner of the component.
While the panel meter is clicked, set the Properties | PLC Properties of the component:
PLCAddressValue – 40001 – MHR1 – Value to display on the meter.
PLCAddressKeypad – 40002 – MHR2 – This is the location of the stored number when the operator selects the meter and enters a number in the keypad.

Add a MomentaryButton to our form by selecting and dragging it from the toolbox.

After re-sizing the component, we can change the colour to blue under Properties | Misc. Also change the text on the button to ‘RESET’
Set the PLCAddressClick value to 00001. This is address MC1 in the Do-More PLC.

Run the application by selecting the ‘Start’ form the top menu. This also can be started by hitting ‘F5’. The form will then show in a separate window and the panel meter will be incrementing the value. Hitting the reset button will reset the value to the one entered when you click the panel meter.

When you hit the panel meter on the display a keypad will then pop up on your screen. Enter the new value and then select ‘Enter’. The new value will appear in MHR2 in the Do-More PLC.

Watch on YouTube : Create a PLC with HMI Training and Learning Environment Free
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.

Now You Can Have Data Logging Free

A data logger is also known as a data recorder or  data acquisition. It is a method to record data over a period of time and/or events.

The recorded information can come from sensors in the field. They can be digital or analog. With analog (voltage or current) we can measure temperature, pressure, sound, weight, length, etc. Digital data can be used for counts, times, events (motor overload), etc.

Data collecting can be time or event driven. Time based would be like collecting data every minute, shift, day etc. An event based collection would be from an error in the field such as an overload of a motor or a fault with a temperature controller.

Data mining / analysis is the most important part of the data logging.

Data mining / analysis is the way in which we look at the data and determine  what to do. Clustering is a method to look at the data in similar groups for comparison. An example of this would be the amount of material made on individual shifts in the plant.  Setting up the data logging in a way to examine the output over time is very helpful in determining methods to increase productivity in the manufacturing environment.

Time studies or observations are vital in the lean manufacturing world. Data logging can be useful in assisting with these studies. However, unlike the usual manual approach, this time study can be continuous.

Doing Time Observations

Data logging does not have to be expensive. It is also not as intimidating as it may sound.

The ‘Robust Data Logging for Free’ eBook is available in a free download. Just subscribe to ACC Automation to get the link for the free download.
 
This eBook will walk you though step by step on getting information into a database so you can start analysing the data. With traditional loggers, software will read the memory of the PLC and store in a local computer. If the network stops or the PLC communication fails then the logging will stop.
Creating a robust PLC data logger allows the communication to be stopped for a period of time without losing any of the data for collection. This is accomplished by storing the data locally on the PLC until communication is restored. All of the data is then read without loss. The amount of time that the connection can be lost will be dependent on the memory size of the PLC and the frequency of the data collected.
This series will walk you through the steps to create and implement a robust PLC data logger using the following equipment and hardware.
  • Automation Direct – Do-More – H2-DM1E PLC (Ethernet Modbus TCP)
  • Do-more Designer 1.3 (Simulator instead of PLC mentioned above)
  • Windows based computer running IIS
  • Visual Basic 6

Additional information on Omron Host Link Protocol and Indirect Addressing can be found in the eBook.

The ‘Robust Data Logging for Free’ eBook is available for a free download. Just subscribe to ACC Automation on the left side menu of the website to get the link for the free download.

Watch on YouTube : Now You Can Have Robust Data Logging For Free
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.

Who Else Wants to Learn About On Off and PID Control?

Process control can be a bit intimidating. We will try and break down both On/Off and PID control in a fun way. This is a simple analogy without any math.

On/Off control can be used effectively with temperature control. Everyone’s house usually has a temperature controller that uses an On/Off control. When the temperature is below the set value (SV) the output switches on. The output will remain on heating the house until the present value (PV) is above the set value. At this point the output will then go off. The house will constantly be doing this in a cyclic way. This means that the temperature of the house will vary a few degrees.

We can plot this out like the sign wave above. The set point is at the middle. By the time the output is turned off the thermo mass continues to heat the house, before starting to cool down. The same is true when the output is turned back on. It will cool down a little more then start to heat up again. This is called hunting. We can not get exactly on the set point value and stay there.

Lets look at another way to explain:

You are in a car and can only use full gas or full brake. Racing toward the stop sign at full gas, you use full brake at the stop sign line. Naturally you go passed the stop sign and eventually come to a full stop. Putting the car in reverse, you again use full gas back toward the stop sign line. When you hit the line you apply full brake. Missing the mark again.  This is like On/Off control action.

If we wanted to control the method a little closer then we could program in a hysteresis. (Dead band) This is just a range in which nothing would happen. It would take into consideration the amount that we went over the line in both directions.

If we need to hit the stop sign target a little more accurately then we can now introduce another control method.

PID is a time based control logic. It will look at a control period (CP) and determine what to do for the next. In a temperature control application the control period would be 20 seconds. In a servo valve application, it can be 1 second. Lets look at each of the control methods in the PID with respect to our car analogy.

Proportional Control (P) – This will increase in amount based upon the error. The closer we get to the set point, the control period will be on for a longer period of time. (Reference to the output percentage  of control period time.)

In our example the car can be seen applying the brakes proportionally longer and longer times before the stop sign line is reached. If it goes over the stop sign line the car will apply the brakes even longer depending on the amount over the line. This is proportional control.

Integral (I) – Using just proportional control would always leave us below the set point. We need a method to reset us to the actual set point. This is where integration comes into play. It is interesting to note that PI control is one the most commonly used in the industry.

The car above is travelling along the road, following the dashed lines. If we used just proportional control we would find ourselves riding in the ditch. The integral control will move us into our lane and keep us close to the dash line.

Derivative (D) – This mode of control will look at rate of change and adapt our control to get us back to set point. Remember that everything is based upon a control period which is time. PI rely on the fact that everything remains constant in your control. D will take into account the differences over time.

In our car analogy the derivative function of the control will continually adjust as we move up the hill and down the other side. It will not do much as we drive along the straight road way.

We have looked at a very basic analogy of control logic without all of the details of math. This can aid in understanding what your process is doing and methods to correct. Further information can be obtained by the following references:

PID without a PhD By Tim Wescott
Understanding PID in 4 minutes
PID Control – A brief introduction
PID Controllers Explained

Nice project using PID:
Desktop Line Following Robot

Watch on YouTube : Who Else Wants to Learn About On / Off and PID Control?
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 Outputs 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 outputs, Discrete and analog. Discrete outputs are either ‘ON’ or ‘OFF’; 1 or 0. You can think of them as a single light bulb. Analog outputs have a range to them. They are outputs that usually will control proportional valves, drive speeds, etc. They usually have one of the following signals that are outputted from the PLC: 4-20mA, 0-10VDC, 1-5VDC.

Discrete Outputs

The above diagram has three outputs. A coil, light and motor. The Ladder outputs Y0, Y1 and Y2 control the outputs respectfully. You will notice that when the Ladder output turns on, the corresponding output card bit LED turns on. This then will energise the output hardwired to the device.

The outputs are turned on or off at the end of every PLC Scan. The PLC logic is solved left to right, top to bottom in most PLCs. Physical outputs are not set / reset until an I/O refresh is performed at the end of every scan. This means that if I have a scan of 1msec, then the maximum time it will take to turn on/off the output is 1msec.

PLCs will sometimes have the ability to update the I/O in the middle of a scan. Please refer to your PLC manufacturers manual for this instruction. This can be used for updating the I/O quickly or controlling stepper drives for motors by giving them a pulse train output from the discrete PLC output. A pulse train is just a quick series of on/off states of the output.

Analog Outputs

An analog output converts a digital value to a voltage or current level that can be used to control (vary) physical outputs. In the example above we are controlling the speed of the motor. Words in the PLC will control the analog value.
Example:
4 – 20 mA current Output – 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

Previous Post:
How PLC Inputs Work

Watch on YouTube : How PLC Outputs Work

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

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

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.