PLC ability to store data (not datalogging)

Hello,

I'm trying to find a controller, PLC or otherwise, to run a machine I'm helping develop for the wood products industry. My questions are:

1. Can a PLC store a series of 30 or so binary words? Each of the 16 bit words represent the L/W/H dimensions of one piece of lumber. The lumber is on a conveyor and there may be anywhere from 1 to 30 pieces of lumber between our tool and the machine providing the dimensions. If the answer to this question is no then don't trouble yourself to read any further.... but plaese post if this is a no.

2. This question is a little more difficult to ask and explain. Our tool weighs each piece of lumber and comes up with the Measured Wet Weight. Using the dimensions sent to our controller in the binary format mentioned in question 1 I need to calculate the percent moisture content of that piece of lumber. Basically:

MWWL = Measured Weight Wet Lumber (lbs per cubic foot)
KWDL = Known Weight Dry Lumber. ( " )

((MWWL-KWDL)/KWDL) * 100 = %MC

The KWDL value is a constant dry weight of one cubic foot of bone dry wood of a particular species. Depending on the species of wood being weighed each time, a different dry weight value needs to be substituted into the %MC calculation for KWDL.

Q 2: Can a PLC store a table of values like one of the following tables?)

White Cedar: 18.4
Red Cedar: 17.9
Oak: 45.2
RedwoodOG: 29.52
RedwoodNG: 28.3

or if I can't use characters:

1: 18.4
2: 17.9
3: 45.2
4: 29.52
5: 28.3

Normally this table would be one where the operator would select a species of wood at the beginning of the process and the same KWDL value is used in every calculation. However, I would like to have four bits coming into the controller that are appended to the binary string for the dimensions that signify which species of wood my tool is weighing and then use the appropriate dry weight value from a table stored in the PLC software.

I think it's now obvious to you after having read these questions that I know next to nothing about PLC's. If the answers to these questions are yeses then I'll be going to the local college to learn about PLC's. For now, all I need to know is if a PLC can operate like this...before I jump feet first into a class that would be useless to me. I have very limited experience with BASIC and C++ programming.

Anyway, thanks for the long read and thanks for your answers.

chris.

It can be done depending on what PLC you are using. You would have to use seperate memory locations for the ACII data and the numeric data. If thought is put into the memory map you can make the data so that it tracks nicely throughout the program and the different peices of data correlate well.

G'day mate!

I am far from an expert! I am working through a course on PLC's myself and will be interested in what the more experienced people around here have to say about this. From my limited knowledge I would say Q1 could be answered with sequencers and shift registers which can both handle up to 256 words each I think. I am learning on AB so this may differ!

Q2 should be easily handled by the high level maths these controllers can do. I am very much a learner myself but I would say go do the course because the power of the modern PLC just constantly amazes me.

No doubt this is a rather simplistic answer to your question and I am sure you will get better one's but thanks for asking this question and giving me a chance to learn aswell!

Question 1:- Yes . This would be a piece of cake for most of todays PLC's.


Question 2:- This could be done but you might consider using your message display to store the text that identifies each species of wood. When the species is selected on the message display, the appropriate constant (Weight per cubic foot of dry lumber)can be moved into a register to be used as part of the moisture content calculation. This sounds like a fun project. Good luck and let us know how it works out.

Just asking a question?

wborst said:

Question 1:- Yes . This would be a piece of cake for most of todays PLC's.


Question 2:- This could be done but you might consider using your message display to store the text that identifies each species of wood. When the species is selected on the message display, the appropriate constant (Weight per cubic foot of dry lumber)can be moved into a register to be used as part of the moisture content calculation. This sounds like a fun project. Good luck and let us know how it works out.

Click to expand...

Just wondering? This answer is accounting for there being something like a touch screen for the operator to use. Couldn't it be done with something as simple as a thumb wheel if neccessary using the PLC for all the calculations and data storage? Thanks for your time.

Go for it, it will be an adventure. As an example, we have a machine that identifies, via a barcode reader, the case coming in (out of a table of hundreds), then picks out the stored information for that case necessary for further processing. This is done for each case entering the machine. They can arrive in any order. This is done with a fairly simple PLC.

77blackhj45 said:

Just wondering? This answer is accounting for there being something like a touch screen for the operator to use. Couldn't it be done with something as simple as a thumb wheel if neccessary using the PLC for all the calculations and data storage? Thanks for your time.

Click to expand...

Since he was asking about the text, I made the assumption that you mention. You can certainly use labeled push buttons, thumbwheel or any other input device to determine what species is being measured. In any case, the calculations will take place within the PLC program.

I have a something similar that uses an 8 postition rotary switch, with a legend mounted next to the switch. A BCD thumbwheel would give you more selections.

This sounds like a nice project, that many PLC's could handle.

Chris B said:

Hello,

I'm trying to find a controller, PLC or otherwise, to run a machine I'm helping develop for the wood products industry. My questions are:

1. Can a PLC store a series of 30 or so binary words? Each of the 16 bit words represent the L/W/H dimensions of one piece of lumber. The lumber is on a conveyor and there may be anywhere from 1 to 30 pieces of lumber between our tool and the machine providing the dimensions. If the answer to this question is no then don't trouble yourself to read any further.... but plaese post if this is a no.

2. This question is a little more difficult to ask and explain. Our tool weighs each piece of lumber and comes up with the Measured Wet Weight. Using the dimensions sent to our controller in the binary format mentioned in question 1 I need to calculate the percent moisture content of that piece of lumber. Basically:

MWWL = Measured Weight Wet Lumber (lbs per cubic foot)
KWDL = Known Weight Dry Lumber. ( " )

((MWWL-KWDL)/KWDL) * 100 = %MC

The KWDL value is a constant dry weight of one cubic foot of bone dry wood of a particular species. Depending on the species of wood being weighed each time, a different dry weight value needs to be substituted into the %MC calculation for KWDL.

Q 2: Can a PLC store a table of values like one of the following tables?)

White Cedar: 18.4
Red Cedar: 17.9
Oak: 45.2
RedwoodOG: 29.52
RedwoodNG: 28.3

or if I can't use characters:

1: 18.4
2: 17.9
3: 45.2
4: 29.52
5: 28.3

Normally this table would be one where the operator would select a species of wood at the beginning of the process and the same KWDL value is used in every calculation. However, I would like to have four bits coming into the controller that are appended to the binary string for the dimensions that signify which species of wood my tool is weighing and then use the appropriate dry weight value from a table stored in the PLC software.

I think it's now obvious to you after having read these questions that I know next to nothing about PLC's. If the answers to these questions are yeses then I'll be going to the local college to learn about PLC's. For now, all I need to know is if a PLC can operate like this...before I jump feet first into a class that would be useless to me. I have very limited experience with BASIC and C++ programming.

Anyway, thanks for the long read and thanks for your answers.

chris.

Click to expand...

1. As already stated - Most plcs can do this, some being more powerful than others.
2. Same as above

Depends on your budget but id go down the HMI route and use this for 2 instead of the plc.

Although i havent used one yet, they come highly recommended on this forum is the Automation Direct Range of PLC's.

I tend to use Allen Bradley unless the customer specifies the PLC.

504bloke, (and eveyone else!)

Thanks for the replies, I'm looking at getting into a PLC programming class now.

I have to chuckle at your reply suggesting Autmation Direct. It's a good suggestion...that I will attempt again after I take the class or classes. I guess I didn't know enough of what I was talking about when I talked to Automation Direct the first time. I sent a fairly detailed description to the AD rep after he called me. (he called because I requested a catalouge several weeks before) After two weeks I still don't have much in the way of information from him so I asked my questions here. I'll try AD again when I have a better understanding of what I'm talking about.

Thanks again, chris.

if you are ok with fixed point integers instead of floating point every PLC will do it, just don't go for smart relay

If you choose a PLC which supports user defined data types (such as an AB ControlLogix or CompactLogix - the compactlogix will probably work just fine for your app) you can create a data structure which contains all of the information needed for a piece of wood.

For example one UDT would contain the following elements:
Length
Width
Height
MMWL
KWDL
PercentMC

Then you could create a tag array of that UDT type. The array could be 30 elements long or longer if needed.

Now the value Tag[10].Length would reference the lenght of the tenth piece of wood. Replace the "10" in the statement with a variable, such as Tag[index].Length and you can loop through each of pieces stored in the array.

You could visulaize it as a spreadsheet.

Likewise you could create a UDT containing a string and a real number. Then an array of that UDT would store information like White Cedar, 18.4. Again, the array could be as large as you need it to be.

As was stated, most modern PLCs can do what you describe, but life will be easier if you choose one that supports user defined data types or at least user defined files and file types.