Article |

PCs offer strengths in other areas

The following letter was received in response to a two-part series by David Newcorn, Special Projects Editor, on PC vs PLC controls, published in September and December 1999 (see packworld.com/go/pcctrl).
FILED IN:  Controls  > Strategy
The letter was written by James Ingraham director of software engineering for an industrial robotics firm Sage Automation Inc. and has been edited for brevity.

We have been using PC-based controls for more than four years. While your article in the December Packaging World touched on a few of the reasons why we use PCs it missed several important ones.

First of all the “determinism” factor of PCs is not an issue. We handle motion control with a DSP-based motion controller or with intelligent servos. All of the loops are closed independently of the PC. So at worst the system waits a few hundred milliseconds before getting its next move. Since our robots spend a lot of time waiting for something to do anyway it’s not an issue.

ADVERTISEMENT
Second integration is more than just hardware. The fact that I only need one language [for] control data HMI and communications is a blessing. On a [PLC] I need one language for control (ladder logic) one for the HMI and one for motion commands. There is no real data handling capability i.e. a real relation database with logging and reporting capability. No HMI has the power of a full-blown database even though [some suppliers] claim to have logging and some other functions.

PLC communications schemes are limited; it is difficult to have a heterogenous network of Ethernet DH+ Modbus+ Profibus etc. And on top of everything else the PC is an integrated runtime and development platform. [In contrast] I have to pull a laptop out to the machine to even look at information on a PLC much less change the ladder [logic]. A PC can use one language for all tasks and allow a developer to simply walk up to the machine and make changes without having to lug a laptop out there.

Third ladder logic is an [inappropriate] language for some tasks. The way I usually describe it is this: What PLCs are good at they are very very good at. For everything else they are [very difficult]. Procedural machine control is a perfect example of this especially once a complicated data structure is thrown in. Don’t believe me? Every major PLC manufacturer has recognized the shortcomings of ladder [logic] and provided work-arounds. Siemens and Modicon both add Sequential Function Charts and Function Block Diagrams as part of their IEC-1131-3 implementations. Siemens provides a C compiler with its PLC as the target system. Even Rockwell Automation created a PC-based soft PLC as well as planning IEC-1131-3 full compliance to their ControlLogix platform.

Make no mistake traditional PLCs are [limited] outside their niche. You won’t survive as a controls manufacturer unless you can provide an integrated control solution that goes beyond straight ladder logic.

Author David Newcorn responds: Your point about the need to mix logic HMI and motion control in one programming environment is one that resonates with several users. Indeed this very point was brought up at the OMAC meeting at Pack Expo Las Vegas in October ’99 by Procter & Gamble (see packworld.com/go/omac). Although still not entirely bullish on PC control P&G’s Robert Aleksa corporate controls section head identified the need for a single programming environment: “[Mixing] logic and motion easily in the same environment is one of our key needs.”

At the end of the day many end users and machinery builders still feel PLCs are appropriate for machine control. Others are moving into PCs. Packaging World will continue to track the issue including publishing the results of an exclusive survey that documents the current packaging end-user perspective on PC vs PLC control this summer.

Comments(0)

Add new comment

E-BOOK SPECIAL REPORT
42 Best Package Designs: 2014 edition
Sign up to receive timely updates from our editors and download this e-book consisting of our editors' picks of most notable package designs. Updated for 2014!
x

 

Newsletters
Don’t miss intelligence crucial to your job and business!
Click on any newsletter to view a sample. Enter your email address below to sign up!
GENERAL INTEREST

New Issue Alert

Packaging World Magazine

eClip

Breaking packaging news

Packaging Insights

Pertinent packaging issues

PACKAGE DESIGN/
DEVELOPMENT

Greener Package

Sustainable packaging

Shelf Impact

Package design strategies

Each newsletter ranges in frequency from once per month to a few times per month at most.