Control System Fundamentals

This post evolved to support a string of posts I will start next to support an exercise in one of the classes I am involved with at the Pacific Energy Center.  In those posts, we will look at how you can use Eikon for Educators or WindLGC to develop and verify control logic.

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Applying these packages as a design tool is very useful since they both allow you to simulate your logic before it goes to the field.  Having that ability to allows you to find and fix any bugs that might exist.    And designer or not, using the tools will help you learn about logic and how control systems work. 

If you are involved with building systems in any way, be it as a designer or an operator or a commissioning provider or a field technician, it is very helpful to have a basic understanding of control system fundamentals and the building blocks used to develop control logic. My hope is that the slides modules provided with this post will facilitate that a bit by providing you with the slides I generally use when I do a control system basics class. 

These slides are not narrated but I think the graphics and links to resources in them can facilitate understanding the basic idea behind the slides.  They certainly are not a comprehensive presentation of the control system topic.   But they do provide information on most of the fundamental principles you might need to design or understand a control system and the logic and programming behind it.

The post is divided into three main parts.  Module Organization provides an overview of the content in each module.  Using the Modules explains how you go about downloading and running the modules if you are interested.  Taking Notes on Slides explains how you can use Microsoft One Note if you want to add  your own notes to the slides as you work through them. 

Module Organization

The slides are divided into seven modules, as listed below.  If you click on the link, it will take you to a description of the module and there will be a link from there to the actual module on my Google Drive.  The “Back to Contents” link and the end of the description will bring you back here.

Introduction and Overview

This module starts out by defining the basic, functional goal of a control system.

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It then looks at what we have been able to do with control systems historically, from fairly early in the industrial age to something representing the epitome of technology and compares that to where we seem to be in terms of applying control systems in the HVAC industry currently, which many (including myself) would say is not particularly good.

The module then proceeds to explore why (in my opinion) the current process may be dysfunctional and proposes a process (that I and others I know have used) to address what I believe might be the causes of the dysfunction.  We should and can do better than we are on most projects these days.  A narrative that complements the information in the slides (which is basically what I talk about as I go through them) is included in the second chapter of the Control Design Guide.

Next, the module looks at some of the National Electric Code requirements that impact the installation of a current technology control system, including a case study that illustrates how these requirements might affect an installation in a tight equipment room.  Current technology control systems are basically electrical systems.  Thus we need to make sure we install then in accordance with the requirements of the codes that govern electrical systems.

From there, the module looks at some basic functional testing requirements that will help ensure that your design goals are realized.  As the Designer or Owner of a control system, you should have a fairly good sense of what you expect it or need it to do.   It turns out that if you know that, and test for it, you can find out if the system you have is delivering or not, even if are not up to speed on the details, networking, and theory behind a modern DDC system.

The module concludes by introducing the basic building blocs for most control systems;

  • Inputs
  • Outputs
  • Control Processes
  • Set Points

The modules that follow explore these topics in more detail.

Click here if you want to download the Introduction module from my Google Drive.

Click here to go back to contents.

Inputs

This module focuses on the “Inputs” building block and starts out by looking at some of the basic concepts behind sensors and transmitters.  All though we tend to use those terms interchangeably, at a technical level, there actually is a difference and understanding that is important from an application stand point.

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The module then looks at how the dynamics that are going on in an operating system might impact what a sensor and transmitter “see”.  It turns out that for accurate results, you need more than an accurate sensor and transmitter.  It needs to be installed correctly and arranged so that the data it picks up is truly representative of what is going on in the system.  The accuracy can also be impacted by application considerations for the sensor and transmitter itself, including position effects and the effect of mass.

Calibration is an important component of accuracy, but there is often more to it that simply verifying the sensor at one operating condition.  For one thing, the accuracy of many sensors will vary over their range.  And even if that is not the case, a single point calibration may not deliver accuracy over the entire range.  These topics as well as others are focused on in the Calibration and Accuracy section of this module.

The bottom line is that there are a lot of things between what we see at the work station and what is actually going on out in the field.  All of them can make a difference and of particular importance is the sampling frequency.  The Nyquist Theorem allows you to predict and appropriate sampling frequency and if you don’t pay attention to it, then you may be misled by your data.  The final section of this module explores this topic, including examples of what can go wrong.

Click here if you want to download the Inputs module from my Google Drive.

Click here to go back to contents.

Outputs

Outputs are another important building block in our control systems and this post starts out by looking at the types of actuators and the signals that are typically used to cause them to move, both pneumatically and electrically.

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Just because an actuator has a linear signal applied to it does not mean that the response in the system it is controlling will be linear.   In fact, most of the time it won’t due to they way system pressures and flows interact with the valves and dampers, the characteristic curves for the valves, dampers, and other final control elements in our systems, and the linkages that connect the actuators with the final control elements.  These topics, including examples, are explored as a part of this module.

Click here if you want to download the Outputs module from my Google Drive.

Click here to go back to contents.

Control Processes

Control processes manage our outputs based on our inputs and the set point for the process.  So they are a pretty important part of the control system.  This module starts by looking at where we came from in the HVAC industry, starting with the analog days, as illustrated below in the photo, which is an analog, pneumatic PID (Proportional plus Integral plus Derivative) controller and moving forward through the evolution to DDC.

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The content in this module includes a section that takes a detailed look at how pneumatic one pipe and two pipe controllers work and their pro’s and con’s relative to each other, the pro’s and con’s of pneumatic vs. electric actuators (pneumatic control is not the same as pneumatic actuation) and pneumatic control resources before moving on the DDC systems.

The module then explores digital and analog control processes, including floating control, Proportional control (P-only), and Proportional plus Integral plus Derivative (PID) control.  This part of the module includes examples illustrating what happens when you narrow down the throttling range on a P-only control process and what happens when you take a P-only process and add integral gain and derivative gain to it,including the units of measure associated with both types of gain.

The open and closed loop tuning methods are also explored including illustrating what the response of a well tuned control process should look like.  It turns out that lags in the control process can have a huge impact on your ability to tune it tightly and get it to settle out in a reasonable period of time.  The module illustrates this along with showing how the open loop tuning method can be used to identify the lags in your system, which will provide a lot of insight into where you can expect to end up in terms of response and settling time once you have tuned your process.

The module concludes by providing you with some resources that will give you a deeper understanding of PID control and all of its variations as well as some general rules to apply as you go about tuning control loops and starting up control systems.  The rules are generally the same as the rules you would use in any functional testing process, probably because the control system plays such an important role in virtually all of our HVAC systems.

Click here if you want to download the Control Processes module from my Google Drive.

Click here to go back to contents.

Control Logic

This module supports the previous module by taking a closer look at the logic behind control processes, including how that logic is developed for a current technology system.

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Having a sense of the logic required to implement and improvement to a system is very helpful if you are involved with commissioning and operations, new construction, existing building, or otherwise, which is one of the points that is made early-on in this module via an EBCx (Existing Building Commissioning) case study.

The module then looks at relay logic, which is how digital logic was accomplished in the “olden days”, and which is the foundation for modern digital logic.   This includes an example that compares a narrative sequence with the relay logic required to accomplish it.  It then contrasts the relay logic with the DDC logic that would be used to accomplish it.

The module concludes by comparing the two primary ways logic is implemented in current technology systems along with guidelines and resources you can use to help you develop the logic for the systems you are working with.

Click here if you want to download the Control Logic module from my Google Drive.

Click here to go back to contents.

Set Points

Set points are critical in terms of coordinating a logic sequence that might apply to a given system type in any location with the specifics of a given location, the loads the system serves, and the nuances of the equipment serving the system.

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For example, the perfect set point to return an economizer process to minimum outdoor air on a hot and humid day in San Francisco, California could be an energy efficiency disaster if you used it on an a similar air handling system serving a surgery in St. Louis, Missouri.

This module uses a number of case studies to illustrate why the set point you select for a given control process needs to consider the operating environment, the nature of the load, and the characteristics of the equipment serving the system if your goal is optimal performance and efficiency.

Click here if you want to download the Set Points module from my Google Drive.

Click here to go back to contents.

An Exercise

This module is the exercise that I will be working with in the blog posts that follow.

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The intent of the module is to let you work with Automated Logic’s Eikon for Educators or Idec’s WindLGC to build and test some basic logic and then modify it to achieve energy savings.  Both of these tools are free resources and are fully functional versions of their respective manufacturer’s programming software, including the ability to simulate and test your logic once it is complete.  They only thing you can’t do is download the logic to an actual controller.

The exercise starts by having you put together the logic for a control loop that will modulate the steam valve on the heat exchanger serving the hot water system in the system diagram above so that a constant discharge temperature is maintained.  Once you have that working, you modify the logic to add two energy saving features.

The point of the exercise is to help you become familiar with logic by working with it.  While the building blocks you will be working with are specific to the manufacturer’s software you will be using, I think you will find that once you understand how to develop logic for one vendor, the knowledge is easily transferable to other vendors, at least that is how it was for me when I first started going control system programming.

The other point of the exercise is to illustrate how having the ability to understand, work with, and communicate logic can open the doors to some very low-cost/no-cost savings opportunities.   I plan to illustrate this by developing the cost/benefit metrics for the two improvements you will make to the logic.  So, not only will you learn about logic, you will learn a bit about energy savings calculations and cost estimating by the time you are done with the exercise and related blog posts.

Click here to download the Exercise Module from my Google Drive.

I have provided the system diagram that goes with the exercise as both a PowerPoint file and a .pdf file.   The reason is that if you have PowerPoint and are interested in developing system diagrams, having the system diagram in the PowerPoint format will give you a start on some symbols since you can copy and paste them from my diagram.  If you don’t have PowerPoint, the .pdf version of the diagram is really all you need to do the exercise (along with the programming software mentioned above).

Click here to  download the system diagram that goes with the exercise as a PowerPoint file.  Click here to download the system diagram that goes with the exercise as a .pdf file.

Click here to go back to contents.

Using the Modules

The modules are PowerPoint slides that have been converted to html and flash files to allow you to view them even if you do not have PowerPoint.  I believe they should work on PCs and IOS devices but currently, I don’t have a way to verify that so let me know if you have problems with running them on a MAC.

The various modules are saved on my Google Drive (the link takes you to the location) as zip files.   I created the files with Winzip, but they typically will open with the standard Windows file compression utility.   If you have problems getting that to work, you can download the free trail copy of Winzip for a PC, Linux, or a Mac off the internet.

Downloading the Modules

In the paragraphs that follow, I am going to show the download process on a Windows PC using Internet Explorer.  Other operating systems and web browsers should look similar to what I show.  I should mention that I have run into an occasional problem working with Google Drive using Internet Explorer, but I was always able to solve them by using Google Chrome instead.  So if you run into a problem try doing the steps in Google Chrome.

To use the modules, you should down load them and then extract the files to the location of your choice, making sure to preserve the file structure.    Specifically, you should navigate to the Google Drive location where the module of interest is located by clicking on the Click here to download the …  link at the end of each section describing a module. which should give you a window that looks something like this.

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To download the zip file containing the files listed in the window, you click on the download icon at the upper right side of the Google Drive window.  When you select the download option, you should be asked if you want to open or save the file. To get them on your computer, you need to save them and if you are doing that, you are given the option of where to save it, at least that is how it works for a Windows PC.   Here is what that looks like on my system.

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If you select “Save” on most systems, the file will end up in your default “Download” folder.   If you pick “Save As”, then you an select any location on your system’s hard drive (or drives).  Here is what that looked like on my system when I created a folder on my Desktop named “Download Example” and then saved the Introduction and Overview module there.

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The next step is to extract the files from the compressed folder that you downloaded.   To do that, you highlight the folder of interest, which in this case, since the folder is a compressed file folder, will open up a set of “Compressed File Tools”. 

I’m sure that what this looks like and the specific steps involved with vary from PC to PC and operating system to operating system and may even vary with the file compression tool set as the default tool on your system.  But here is what that looks like on my system, which is running Windows 8.1 Pro.

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To extract (unzip) the files in the folder, select the “Extract” menu and then the “Extract All” option. 

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When you do that, you will be given the option of selecting a location for the extracted files.  The default is typically a normal (non-compressed) folder of the same name in the same folder (location) as the zipped file, but you can put them anyplace you want by using the “Browse” button.

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When you select “Extract”, you typically will get a little window showing the progress of the operation …

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… and then a window with the folder that you extracted the files to open, showing the files you extracted.

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For the “Play” function to work, it is important that the files be in the structure shown above.  For most systems, that will happen automatically if  you extract the files using the process I illustrated, but I mention it just in case somehow, in the process of doing this, you don’t end up with something that looks like the preceding.

Click here to go back to contents.

Playing the Modules

If you double click or otherwise open or run the file with the name that starts with Run This …, it should launch your html viewer (Internet Explorer, Firefox, Google Chrome, etc.) and allow you to play the slide show.

Note that, depending on your security settings, you may get a message asking you if you want to allow blocked content to execute.  To get the slides to run, you will need to click the “Allow Blocked Content” button.  Ultimately, you should end up with a window that looks like this.

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You can display the slides “Full Screen” by clicking the little square button with the diagonal arrows as shown below.

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You can advance the slides in a number of ways including pressing the “Play” button, pressing the “Previous” or “Next” buttons, pressing the “Page Up” or “Page Down” keys on your keyboard, or pressing the “Space Bar” on your keyboard.

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Note that there is a subtle but significant difference between using the “Play” button or “Space Bar” to advance the slides vs. the “Previous” or “Next” buttons, or pressing the “Page Up” or “Page Down” keys.  Specifically, the “Previous” or “Next” buttons and the “Page Up” or “Page Down” keys will literally advance to the next slide even if the animation associated with the current slide has not run.

In contrast, using the “Play” button or “Space Bar” will step through each animation (if they are not automatically triggered by the previous one) and only advance to the next slide after you have seen all of the content on the current slide.   So, I would recommend using the “Play” button or “Space Bar” to move through the slide deck.

To facilitate things when I teach using these slides, I have hyperlinks set up to jump between the various slide decks and back to the table of contents.   So far, I have not been able to figure out how to make those links work in the html versions of the slides.  Meaning that to open the next module, you will literally need to go to the location that the Run This to Play the …. Module.html file is located in and launch the module that way.   To return to a previous module, you will need to manually navigate to it if it is still open or simply launch it again.

Click here to go back to contents.

Taking Notes on Slides

Frequently, when you are working through a presentation or listening to a webinar, you might find it handy to be able to take notes on a particular slide or anything that you have showing on your monitor for that matter.  OneNote makes this pretty easy (the link takes you to web page the describes some of the features) so I thought I would close this post by illustrating how you go about using OneNote to make notes on the slides if you want to do that.

What follows is specific to OneNote, which is a free Microsoft utility. I use it a lot for taking notes in meetings, keeping a phone journal, taking notes as I work through projects, taking notes when I am listening to presentations or webinars, etc.  OneNote will not run natively on a MAC, but you could run it in a Windows Shell.  But I believe there are MAC specific programs that have the same basic functionality as OneNote.

Lets say I was working through the Inputs module and got to this slide and wanted to make a note reminding myself that one of the points of the image was to illustrate that while the two devices appeared to be similar a closer examination of the nameplate reveals that they actually have different ranges.

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To do that, select the window you want to capture so that it is on top on your monitor.  Then switch to OneNote window and select “Insert” and then “Screen Clipping”.

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When you do that, OneNote is hidden and your entire screen is covered by an opaque white layer.

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You can now use your mouse to drag a window on what-ever portion of the screen you would like to capture …

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… and when you release the left mouse key, OneNote will re-appear, but now, the portion of the screen you selected will be included on the OneNote page and you can add your notes to it by simply clicking anyplace on the page and typing.

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Pretty cool and really fast once you know how to do it.  

Incidentally, when I say “drag a window with your mouse” what I mean is that you use your mouse to put the little cross-hairs that appear with the opaque screen over one corner of the area you want to capture.   Then, while holding down the left mouse key and moving the mouse to the other corner, you will discover that you will “make a hole” in the opaque screen defined by your use of the mouse.

Click here to go back to contents.

In closing, I should mention that this is the first time I have tried doing this (putting slide modules up on line in a format that should allow you to view them).   Everything seems to work on the machines I have tried it on, but please let me know if you have are having problems getting the slides to open up via a comment on the blog and I will try to figure it out.

David-Signature1_thumb1_thumb

David Sellers
Senior Engineer – Facility Dynamics Engineering
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This entry was posted in Controls, HVAC Fundamentals, Operations and Maintenance, Pneumatic Controls. Bookmark the permalink.

2 Responses to Control System Fundamentals

  1. Shane Park says:

    Happy New Year Mr. Sellers. I really enjoy reading your blogs. You make everything so insiteful and I want to learn more about Eikon logic programming. I would like to know if you have any futher reading material for programming control logic with Eikon? Thank you

    • Hi Shane,

      Thanks for your kind words. I am behind on my next post on developing the logic for the HHW system using Eikon, but soon it will be up, I hope. That said, here are a couple of resources that will give you something to look at in the meantime.

      Jay Santos (one of the two founding principals of FDE) wrote an article for Consulting Specifying Engineer a while back that is basically a text book for how to develop a control sequence and/or logic diagram, Eikon or other wise. I have a copy of that on my Google Drive at this link:

      http://tinyurl.com/JaySantosHowToWriteSequences

      About the same time, I wrote and article for HPAC which is kind of the “I’d like to teach the world to sing in perfect harmony … ” Coke commercial version of Jay’s article titled The Joy of Writing Control Sequences, which is also on my Google Drive at this link:

      http://tinyurl.com/ControlSequenceJoy

      In addition, if you download the Resource List I link to from the Commissioning Resources links on the right side of the home page and go to the appendix, you will find a string of articles Jay wrote and one I wrote for the Control Freaks column in HPAC a long time ago now. But they are still a pretty good guideline for developing control system designs since the physics and definition of what needs to happen for of all of that has not changed much. Here is the link to the current version of the resource list.

      http://tinyurl.com/EBCxResourceList

      Let me know if you have trouble with the links but they seem to work from my machine currently.

      Hope you are having a Happy New Year too.

      David

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