Presentation for management..

Pete Smith

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P
Hi guys,

Great website, very very helpful.

I'm a 3rd year instrument apprentice, and as part of my apprenticeship, each year, I have to produce and deliver a presentation regarding a chosen subject of my choice. I have chosen PLC's, have written a report, and generated a presentation.

All is good so far.

At the end of the presentation, the managment then torture you and fire questions at you (lol).

I should be ok, but I was wondering if you guys could help me with a few of the questions I'm expecting to be asked.

One of which is analouge to Digital conversion and vice versa. I know if you have an analouge value, it is divided by the number of bits designated (ie 2^14)..with some bits ued for polarity etc) to get the resolution for the output. But what about when you get to the output? digital to analouge conversion? What device is used? Triac outputs?

Another, which I really find hard to answer is, Why use PLCS? I have looked at cost issues, but whats a few relays from RS? Pence? Was thinking reliability, troubleshooting, physical space, ability to reprogram easily (although at my work they are normally programmed, and then left for years until a slight modification or improvement is required..) any other ideas?

Any other questions you can think I may be asked (we are talking quite basic, as the PLC expert isn't attending the presentation..!!).

Thanks for your time,

Pete
 
In a way, you already answered your second question. The Reliability factor. Consistency - product quality goes up with a more consistent product. Not to say it replaces humans, but many times, humans have atendency to 'daydream' and miss things. Safety. Human safety is a big one. By automating hazardous processes, you protect human life. Reliability - more uptime and more product produced. THat affects the bottom line. THose guys should love that. THesee aree in addition to your items already mention.


A D/A converter takes the Digital Pulse Signal and generates a current(in most cases) based on that signal. I would think it is an operational amplifier that is used to generate an accurate signal. ALmost certain, but I have been out of the electronics side of things for a while.

David
 
Yes! Never thought of anything related to safety and quality. Like you say management always love that kind of stuff, I'll add that to my answer script!

Thanks mate, I'll also look at a op amps on google, one guy at worked seemed to mention Triacs alot, so I'm wondering if they are used at all in the D/A conversion.

I shall investigate futher!
 
Getting ready for the inquest!

TRIAC's are not used in DC circuitry at all. They are AC switching devices and used in the output of solid state devices, like the discrete output modules for PLCs. As David pointed out, the digital to analog and analog to digital conversion is done onboard PLC modules by D to A and A to D converters. I am sure that there are electronic circuits such as OP amps employed in the D to A process. That would allow for the output to be either current, or voltage. A Google search is a good idea to turn up more info on the process.

 
Pete Smith said:
I have looked at cost issues, but whats a few relays from RS? Pence?

A few relay and a timer or two will gennerally cost MORE then a small PLC like the AD-DL05 ($119). Let's say you have a circuit that will reqiire 3 KH relays and 2 KH socketed timers:

3 relays w/sockets: ($15 x 3) = $45.00
2 timers w/sockets: ($35 x 2) = $70.00
Total for hardwire circuit = $115.00
+ the cost of manual wiring of same.... Maybe $50.00

So the small hardwire circuit costs $165.00 & the PLC costs $119.00..

It's very diffucult to beat the cost, reliability & flexabilty of a PLC vs hardwire components.

Engineering Econ 101.
 
Going back to my electronics days, I will say this about the D/A conversion.


Essentially what happens is this:

THe digital signal is processed by a specifically design IC to take the digital signal in. THe following is a breif simple explanation from the website http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/dac.html

Four-Bit D/A Converter

One way to achieve D/A conversion is to use a summing amplifier.

dac.gif
This approach is not satisfactory for a large number of bits because it requires too much precision in the summing resistors. This problem is overcome in the R-2R DAC.

Goto the website and check it out. It is more detail than you will ever need, but it might help you to understand.

OP Amp (operational amplifiers) are used because of the precision that can be had by using them. Understand that the binary representation is converted in this basic method.

The reverse is a little different in that it follows a comparison methodology:

this is from the same website above:

Digital Ramp ADC

adc.gif
Conversion from analog to digital form inherently involves comparator action where the value of the analog voltage at some point in time is compared with some standard. A common way to do that is to apply the analog voltage to one terminal of a comparator and trigger a binary counter which drives a DAC. The output of the DAC is applied to the other terminal of the comparator. Since the output of the DAC is increasing with the counter, it will trigger the comparator at some point when its voltage exceeds the analog input. The transition of the comparator stops the binary counter, which at that point holds the digital value corresponding to the analog voltage.

This is a little harder to follow, but with some reading you will understand. If I really took some time to think about it now, I could explain it, but I have to take my son to Soccer.

David
 
Woohoo, I won the presentation competition out of the other guys..

Only a few questions I didn't know, one was whether fieldbus instruments could be used with PLCs.. and I cant quite remember the other.

Really pleased though, thanks alot.
 

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