Re: [CAD_CAM_EDM_DRO] Re: I'm curious

High Alan;

PLC's can do process control, infact many "PLC's" offer alternative
programming languages. PLCs though excell at presenting ladder lgic
equivalents. When doing a lot of process control the clarity of the
PLC code can suffer a bit. Each brand of PLC is a bit different in
this respect to so research helps to find one that fits the task at hand.

At some point though I have to say that a PC or embedded controller
running C/C++ might be the easier path to follow for Process Control.
Depends on how much of the Job can take advantage of the PLC's
strengths. I suspect that your example though would map nicely to a
PLC. In any event the PLCs greatest strength is that the hardware is
optimized for an industrial environment.

Lots of luck with your project.

Dave



Alan Marconett wrote:

>Hi Dave,
>
>Thanks for the very informative tutorial on PLCs. I've not used or
>implemented one, I only remember ladder logic from the fire control
>(missile) systems on the FBM sub I was on. That was a long time ago!
>
>I liked the ladder logic diagrams we used, from what I remember, in
>troubleshooting you could walk right through them, looking for where the
>progress stopped, and determine what was missing from the "equation".
>
>Humm, in my new company, we're doing "process control" of signals coming in
>from an industrial machine (example, laundry), and controlling the
>dispensing of materials in response to those signals. Some of the
>"processing" involves logic that might lend it's self to a PLC language,
>rather then a complicated set of "rules" written by endless if-then
>statements. It sounds like it would make the process much easer to support
>and understand.
>
>I do see how some ladder logic or PLCs could aid in machine control (CNC).
>And the more ways you know how to control something...
>
>Perhaps I'll look up a simple example of a PLC language, and see how it
>could be compiled (interpreted?) on a PIC. Who knows!
>
>Thanks very much for the info!
>
>Alan KM6VV
>
>
>
>
>>-----Original Message-----
>>From: CAD_CAM_EDM_DRO@yahoogroups.com
>>[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of David A. Frantz
>>Sent: Thursday, July 21, 2005 10:20 AM
>>To: CAD_CAM_EDM_DRO@yahoogroups.com
>>Subject: Re: [CAD_CAM_EDM_DRO] Re: I'm curious
>>
>>Hi alan;
>>
>>Another dave here. PLC's are an abstraction of older machine
>>controls that where all relay based. In a sense the PLC environment
>>emulates a relay controller. To do this an interpeter is often
>>implemented on the target processor, though is also possible to
>>implement a compiler to do translation into another form. So to run
>>PLC code on a PIC you would need to implement an interpeter to execute
>>PLC "code" and the monitor software to allow communications.
>>
>>Communicaitons on a PLC is a key element in the technologies acceptance
>>in industry. This leads to the Ladder Logic displays on a laptop seen
>>in factories all over the country. This provides for an "easy" to
>>debug environment. In effect the PLC software does parse in a manner
>>similar to Gcode but is optimized to solve logic equations instead of
>>motion control commands. Many large CNC systems have a PLC function
>>integrated into their contoller. The CNC subsection handles motion
>>and the PLC handles the rest of the machines needs.
>>
>>As to your description of a PLC and its looping through equations that
>>is very very very close to being right on the button. What you have
>>to realize is that there are update operations that take place after
>>looping through the equations. That is the PLC handles things
>>roughly like this:
>>1. Read inputs and place them in a bit table for inputs.
>>2. Solve the logic equations.
>>3. Take the results setting in the output bit table and update the real
>>world outputs.
>>4. Handle house keeping such as timer updates.
>>5. Loop back to the top and start over.
>>
>>The logic equations are often solved via a stack mechnism. Generally
>>the elements of the equations look at the status of bits it the bit
>>tables for Inputs Outputs and Booleans to derive their Boolean value.
>>Timers and counter have to be supported also. At that point you have
>>a PLC, but with modern controllers much more is added to them to enhance
>>what can be done programmically.
>>
>>With the more advance controllers the enhanced functions allow for very
>>easy programming of state machines but often you end up loosing osme of
>>the advantages of a PLC. That would be the friendly debugging
>>environment and almost self documentation. Aften you can end up in a
>>situation where there is little advantage to a PLC over more
>>conventional languages when the advance functions are used. At least
>>from the programmers or technicians viewpoint. You still have the
>>PLC's biggest advantage on the plant floor which is the hardware and its
>>ability to snap in place various bits of hardened I/O.
>>
>>As to "ladder logic" there is actually an ISO standard for this.
>>Unfortunately I do not know of many complete implementations and it does
>>look like there was to much influence exerted by the big PLC
>>manufactures. Worth looking up though as a base point. Better yet
>>experience a PLC first hand by taking a training course at Allen Bradley.
>>
>>As far as implementing a PLC it doesn't take much, some PLC's didn't
>>even have a microprocessor in them to speak of. If any body here
>>remembers the 5TI and worked on them they can elaborate on just how
>>limited a usefull PLC can be. The size of the required microprocessor
>>depends upon what you wish to implement. A PIC could very well
>>implement a simple PLC that has the primary function of being an
>>enhancement to a PLC controller. It might not work to well as a
>>stand alone PLC competeing against PLCs with 32 bit embedded
>>hardware. I do not think that the PLC core on the PIC would be the
>>biggest part of the software project though, user interface and
>>communications would be a significant piece of work.
>>
>>In any event keep in touch with what you are doing. There is a long
>>way to go with respect ot low cost CNC hardware and software. PLC
>>support is probably wedged in there someplace as a future gota have.
>>Ideally all CNC I/O not axis specific would be handled on a PLC module.
>>
>>Thanks
>>dave
>>
>>Alan Marconett wrote:
>>
>>
>>
>>>Hi Dave,
>>>
>>>Do you actually implement ladder logic on the PICs?
>>>
>>>I have seen and worked with ladder logic hardware, (that was a long time
>>>ago) and I'm quite familiar with PIC ASM and C programming. But I don't
>>>
>>>
>>see
>>
>>
>>>any direct way to code ladder logic. Implement a state machine? Or is
>>>there a compiler for ladder logic? I can see that ladder logic could be
>>>expressed as Boolean equations, perhaps one just keeps looping through a
>>>
>>>
>>few
>>
>>
>>>equations? Just curious.
>>>
>>>Or maybe you meant do Gcode parsing in a PIC. This I can understand, I'm
>>>thinking of porting my CNC controller program STEP4 (the motion module)
>>>
>>>
>>to a
>>
>>
>>>PIC. OK, a BIG PIC!
>>>
>>>Alan KM6VV
>>>
>>>
>>>
>
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