Debating PLC vs SoftPLC vs Embedded PC vs Motion Controller definitions

[Peter Nachtwey]

a motion controller must be deterministic. PLCs aren't deterministic. motion controllers have faster scan times and I/o with faster updates and I/O bandwidth. motion controllers have the motion commands/features built in.Motion controllers aren't meant to have a lot of Non motion related I/O.

 

[Archie]

This debate always interests me. It brings out some very strong opinions and sometimes even emotionally charged responses
especially when it comes to terms such as "PC Based PLC". This is often a result of one's own definition of "PC".
So what really is a "PC"? I think all of us would agree that a computer or laptop running Microsoft Windows is a PC.
What about an Apple Mac? Some would say it's a PC others would disagree and say it's a Mac.
If there were a piece of software to make a Mac operate like a PLC, would the debate then include "Mac-Based PLC"?

So what about a Raspberry Pi or Beaglebone? These are small microprocessor boards with an ARM processor that no where
resembles your conventional desktop computer, but yet they run Linux with a windows style OS, run user applications,
and are used with a keyboard and mouse.
If a PLC runtime software is installed on these devices, is it a PC based PLC? Now what if a real time embedded operating system
such as VxWorks is installed on the Pi and Allen Bradley's PLC software installed, what kind of controller is it now?

http://en.wikipedia.org/wiki/Raspberry_Pi

http://beagleboard.org/BLACK

Then we get to the Beckhoff hardware, such as the CX2020. By Beckhoff this is considered an embedded PC because it has
either Windows or Windows CE as it's operating system. Windows CE is Microsoft's version of a real time embedded operating system
that directly competes with VxWorks. Then there is the CX8090 which has an ARM processor with Windows CE operating system.
Beckhoff still considers this an embedded PC, but yet it is no different than an Allen Bradley Controllogix if you compare
the hardware and type of operating system.

http://www.beckhoff.com/english/embedded_pc/embedded_pc_series_cx2000.htm?id=97337418839693

http://www.beckhoff.com/english/embedded_pc/cx9020.htm?id=5563118742705

In my opinion, before we can express opinions like "PC based control will never be as reliable as a PLC",
we need to define what we consider to be a PC, or be more specific. So I think most of us could agree on this statement:

"A consumer grade desktop PC can never be as reliable as a PLC"

 

[RocketTester]

For hydraulic motion control, we have used both Delta Computer (post 16, Peter's company I think) and Beckhoff Automation. My memory is that Beckhoff is faster (0.1 ms cycle time, 0.05 ms possible), compared to 1 ms for the Delta, so I disagree w/ Peter on speed, though 1 ms was sufficient for our hydraulics. B&R's PLC's are also very fast. Delta's products are more purpose-built for motion control. The main unit has hard-wired PID controllers, whereas in the PLC one realizes PID via function blocks linked to more general I/O. In one project, we were unable to expand the Delta unit so had to kludge-in I/O from another vendor. With Delta, "programming" is mostly chaining pre-packaged motion functions, whereas in PLC's one programs w/ general IEC 61311-3 language. Delta does allow general coding via their own language and I recall has a plan for IEC 61311-3. In sum, it appears "motion controllers" are more purpose-built and thus easier to setup if all you are doing is motion control, but less flexible than a general PLC.

BTW, I have never popped open a Beckhoff CX PC, but I doubt it has any interconnecting wires on the "motherboard". Their early DIN-rail mounted PC's used the PC-104 bus for peripherals, which is a pin & socket connector. Most have no moving parts, using solid-state hard drives and fan-less cooling, but a few models do have a fan (w/ ball bearings) allowing them to speed the CPU clock. Beckhoff's "cabinet PC's" are probably more in-between "purely embedded" and "desktop" PC, but many people want that flexibility. The Beckhoff PC's I use have never hung up in 10 yrs of use. We have many Allen-Bradley PLC's around the plant and some have hung-up, in one case ruining a very expensive product, but hardly quantitative data and I don't know all the facts.

 

[gclshortt]

I find this topic very interesting. It really all boils down to this. We get the hardware/software to support the need.
We really have to look at the applications.
Definitions will be constantly changing as new technologies come out to satisfy more needs.

Garry
http://www.accautomation.ca