Control System Choices
Doug Johnson WRD 7/31/02
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 pretty 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's 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
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, here's a partial list of some other
important issues to consider:
The lower your production volume, the less likely you'll 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're only making 50 units per year, a $200 recurring
savings returns only $10K in the first year.
"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're 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.
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
Feature for feature, the more "custom" the control is,
the less the recurring cost should be. That's because with a custom
control you get exactly what you need, no more.
The more "custom" the control is, the more the non-recurring
costs will be.
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
Technical Risk and Technical Difficulty
These are actually 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 a 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.
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's more
a function of the product life cycle of the specific control you're
purchasing. With custom and semi-custom it's 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.
Business Cycle Assessment i.e. Where In The Business Cycle Is
(Each of these could dictate a different control implementation)
(trying to determine if there is a market for your product)
(trying to capture market share before your competitors)
This product cycle wants to have the quickest
time to market. If you've 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.
(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.
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's 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.