module resolution/scan times

can someone define resolution (briefly)from the standpoint of an analog module? Also, is resolution a function of the processor scan time?

Resolution for your PC monitor would be pixel density, pixels per square inch resulting in smoothness of image, the greater the resolution the more crisp the image. In an analog signal it would be how many parts make up the whole. An analog number viewed in 1000 parts would give you a much better resolution than the same number view in 10 parts.

Scan time is how many times you look at and up-date that analog signal in a given amount of time.

Roger

I will try.

The real world is analog, we use digital devices. DAC's ie digital analog converters are used to "translate" the signals from one to another.

Resolution applies to the numbers, the count, that can be used.
8 BIT = 256
10 BIT = 1024
12 BIT = 4096
13 BIT = 8192
14 BIT = 16384
16 BIT = 65536

The higher the resolution the higher the "count" available but this does not necessarily mean more accuracy. There are other issues to consider when accuracy is considered. A higher resolution does allow a more definitive range to be used in an application.

Example: When using an analog device (thermocouple etc) to measure temperature the "drift" range could be smaller..ie more defined using a higher resolution device.

For example in AB CtrlLogix analog output updates interval can set in unit properties conficurator. When set highest 1 ms inteval, the unit can use only part of channels, every 2'nd or every 4'th debending of unit.

In some manufacturers Analog I/O channels update interval can be 200 ms minimum and it could be too slow for some purposes. That case some have use digital outputs and D/A-transducers.

Same philosophy in Analog inputs and when need only one analog signal and not want bye Analog I/O-units.

Next homework question please.

And for Extra Credit

Do not confuse resolution with accuracy.

For example, an A-B Micrologix 1200 analog input card has an advertised resolution of 15 bits which translates to 1 part out of 32,768 (2^15 = 32,768) or ±0.003% (1 / 32,768 x 100).

The advertised accuracy of this same card is ±0.24% at 25°C/77°F. This includes offset, gain, non-linearity, and repeatability according to the A-B manual.

The accuracy for a typical good quality pressure transmitter - Foxboro - is ±0.20% including linearity, hysteresis, and repeatability.

So, just because you can get a lot of decimal places to display on your software it doesn't mean that all of them are actually conveying information. Just because one vendor's hardware has a higher resolution than another doesn't mean that your system will more accurately control the real world if you use that high resolution hardware!

Look up the total module count. I'll us Allen Bradley as an example
A 0-10 VDC signal would quite literally be looked at in .00003 VDC increments. PLC Scan Time has little to do with an analog card. Most cards update accurately in about 75MS I've seen many. Unless your scan time is longer than the settling time of the filtering on the front of the A/D Converter you will see nothing meaningful. You didn't bring up accuracy. The most importtant component is repeatability, and keeping the ambient temp stable. Again check out the spec. Usually you UNCERTAINTY is related to TEMP CHANGE. How much accuracy do you need?
If brute accuracy is nessecary then your better of buying an instrument that goes on devicenet or controlnet ,etc. Then you don't have to factor out the systems translation errors, The accuracy of the device is the accuracy of the device.

I hope I didn't take off in a direction that isn't meaningful.

If you have any questions

I'm at

[email protected]

[email protected]

Jim

More Extra Credit

Good point on response time, Jim. That same Foxboro pressure transmitter promises 1.0 second response to obtain 90% recovery from an 80% step change in input pressure. That doesn't take into account any filtering on the input hardware or A/D conversion that might be occurring.

The respononse to a step change isn't usually linear. That means that if my line pressure goes from 5% of range to 95% of range my signal after 1 second will be around 77% of range (0.05 + 0.80 x 0.90). That doesn't mean that after 1/2 second I'll be at 41% of range - I'll probably be higher than that but still not outputting a "true" pressure signal.

The net result is that the difference between 75 ms scan on the analog card and 200 ms on the analog card don't mean a thing in most process control applications.

(P.S. I don't know nuthin' about how these same considerations might affect motion control!)

Re: More Extra Credit

Tom Jenkins said:

(P.S. I don't know nuthin' about how these same considerations might affect motion control!)

Click to expand...

This is a good topic for another thread, but briefly think 1000 times faster and that is not counting the oversampling. Our sample rate is 122 micro seconds or 8192 times per second per channel.