Control System Choices for OEMs

Doug Johnson - Embedded Hardware/Firmware Engineer
Originally Published:

Foreword - Often in the business of electronic  control system design, while the tools of the trade change and shift to  meet an ever-accelerating technological landscape, the methodology  behind the designs tend to permeate well into the future. Such is the  case with semi-custom control system design, which in fact is more  effective now than ever before, with the growing prevalence of  community-based support and higher focus on powerful development tools  by hardware developers. Because the concepts detailed in this paper are  as relevant now as they were when this paper was written 20 years ago,  it is being republished with only aesthetic revisions as of January  2021.


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Control System Choices for OEMs

Introduction - OEM's that use control systems in their equipment have four choices of control solutions:


  • PC's

  • PLC’s

  • full custom

  • and semi-custom.


The  PC and PLC categories seem well defined. They are off the shelf,  relatively easy to implement, and are targeted at industrial  applications. Full custom seems well defined also - write a  specification for the requirement, and design a custom control to meet  the spec. With this choice you get exactly what you need, no more, no  less. The semi-custom category, however, is less well defined, and is  challenging not only to describe, but also to understand.

In  just about every issue of the professional trade journals I read, there  are advertisements for off-the-shelf controls that can be tailored to  any set of customer control requirements (i.e. semi-custom). Here is a  partial list of some of the offerings:


  • "SoC" (system on chip)

  • "SBC" (single board computer)

  • "Modular Single Board Computer"

  • "Embedded PC", "PC/104"; "PC based Open Control"

  • "PC on a Chip"

  • "Embedded Controller"

  • "C programmable Controller"

  • "Stand Alone Controller" (used mostly with motion controllers)

  • "Industrial Computer"

  • "Embedded Development Kit"

  • "Platform Based Design".


The  software offerings for semi-custom are equally daunting, but are, for  the most part, the same as those used for full custom... for example:


  • "Windows" (of all varieties)

  • "DOS"

  • "Linux"

  • "Real Time Operating Systems"

  • "C"

  • "C++"

  • "Java"

  • "Assembly Languages (of all varieties)"

  • "Object Oriented Programming"


and so on.



An engineer that needs a control for an application should be asking:  "What are the tradeoffs between PC's, PLC's, full custom, and  semi-custom, and under what circumstances does one make more sense than  the other?" To answer, not only do you need a good understanding of the  hardware/software issues, but here is also a partial list of some other  important issues to consider:

1. Production Volume - The lower your production volume, the less likely you will be able to  justify the custom or semi-custom approach. The NRE of a custom  (semi-custom) control wants to be paid back through the recurring cost  savings. If you are only making 50 units per year, a $200 recurring  savings returns only $10K in the first year.

2. "Look and Feel" - If you care less about "look & feel" and more that you can just  buy the hardware off the shelf, the PC and PLC options are appealing. If  you are more interested in "look & feel" and want to customize the  control to suite your application, making your own using semi-custom or  full custom control allows more flexibility.

3. Internal Development Staff vs. Technical Outsourcing -  The PC's & PLC's are easier, and the custom more difficult, whether  internally done or outsourced. The more "off-the-shelf" the solution,  the easier it is to outsource. PC's & PLC's are commodities and  there are many companies proficient in their use. The custom &  semi-custom are more difficult, and there are fewer capable sources.

4. Recurring Cost - Feature for feature, the more "custom" the control is, the less the  recurring cost should be. That is because with a custom control you get  exactly what you need, no more.

5. Non-recurring Cost - The more "custom" the control is, the more the non-recurring costs will be.

6. Features / Benefits - The more "custom" the control is, the more you can tailor the control  to the application. This usually results in a control with features  better suited to the application, which increases usability and "look  & feel". One area where custom controls are at a disadvantage to  their PC counterparts has to do with Graphical User Interfaces. If you  need a "Windows" type user interface, PC's of any type are the control  systems to use.

7. Technical Risk and Technical Difficulty – These, in fact, are two different concepts. Technical risk refers to  the probability that your plan is fatally flawed in a manner that is yet  unknown. Technical difficulty can be judged independently of technical  risk. For example, using an 8051 based microcontroller to implement a  PID temperature control is a low technical risk project, but for a  beginner, it is likely to be technically difficult. The more "custom"  the control, the more these two concepts can become issues.

8. Part Obsolescence Risk - This is a tough characteristic to categorize because all controls  carry this risk to some extent. With PC's & PLC's it is more a  function of the product life cycle of the specific control you are  purchasing. With custom and semi-custom, it is more a function of the  schematic, if there are single-sourced parts, and how popular those  parts are. My feeling is that the more you are in control of the  schematic, the better you can control this risk.

9. Business Cycle Assessment i.e. Where in the Business Cycle is the Product?
(Each of these could dictate a different control implementation)


Market Study(trying to determine if there is a market for your product)


This  product cycle wants to have the lowest up-front cost. It does not make  sense to spend a lot of money developing a product if you do not know  what the potential market is.


Market Penetration (trying to capture market share before your competitors)


This  product cycle wants to have the quickest time to market. If you have  identified a market that is yearning for a solution, you need to react  quickly to make a lasting impression. If you react slowly, someone else  may notice your idea and may outspend you to buy market share.


Market Maturation (trying to maximize efficiencies in your on-going production)


This  product cycle wants to have the lowest recurring cost, and the fewest  on-going production issues. If you have good production volume, your  up-front costs to get to this point are less of an issue.


Conclusion - Make  an uninformed choice among these difficult technical/business issues,  and the ramifications can be disastrous. Stories about failures abound,  and the scars are long remembered. I suspect this is the biggest reason  the full custom solution is viewed with such trepidation... it is  probably the most difficult, has the highest development cost, and  carries the highest technical risk. The semi-custom solutions want to  claim the same benefits of the full custom solutions, only at a lower  development cost and with lower technical risk, and I suspect that many  of them do... but which ones? Will a semi-custom solution that worked  well for one project, work well for a significantly different project?  How are you going to implement a still technically challenging  development... with internal staff or sub-contractors? (Semi-custom is  still no piece of cake) This remains a dicey proposition, and again, is  probably why the PC/PLC, which is the least complicated lowest up front  cost solution, is frequently chosen, even in applications where it's  probably not the best choice.

What would be ideal is a  semi-custom solution that provides low implementation complexity, low  up-front cost, low recurring cost, high performance and a high level of  flexibility. This is exactly what WRD's "ProductMaker" was designed to  do. By mitigating many of the disadvantages of custom controls, and  retaining most of the advantages, this solution has been providing low  cost, low risk, high quality "custom controls" to WRD's customers for  many years.

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