PLC Analog Outputs

First an introduction since I'm new here. I'm involved with design of ICs that are used in PLC analog output modules. I rarely talk with the guys who actually use PLCs though so this seems like al goo d place.


Second - hopefully this isn't a repeat but when I tried the first time the thread didn't post.


Now the real question. It seems like the trend is for higher resolutions (16-bit) and fast settling time (<1ms, sometimes 20us). Typically a furnace temperature control system doesn't need anything like 1ms settling time. A module with 1ms will work fine but so would something that settles far slower.

So anyone have any examples of systems that need fast settling or 16-bit resolution?

Thanks, Dave

15 years ago, researchers optimizing LED crystal growth production needed 16 bit AO resolution for something or other. I handled a PID controller with 16 bit output resolution and they bought several. The process was a very slow thermal process, but they insisted they needed every bit of resolution available.

I'm can't recall that I've run into it since.

I had a student a couple of years ago who was engineering a steel casting process ... he was convinced (and I believed him) that the rate at which the steel was dispensed had a significant effect on the final product (its crystalline structure, grain, and so on) ... he was specifically trying to move a valve-type arrangement to regulate the flow of the molten steel ... the speed of the signal transfer to and from the process were a particular sticking point ...

the following write-up that I did might be of interest to you – and hopefully will help to answer your question ...

http://www.plctalk.net/qanda/showthread.php?p=391975&postcount=1

good luck with your project ... and welcome to the forum ...

I hope this encourages a trend with all manufacturers - asking 'what is actually needed' in PLC designs.

Not with outputs but with analog inputs I have often run into a need for speed and high resolution. Of the top of my head, aside from motion controllers, I can't think of a time where I needed more than 12 bit resolution or very high speed for an analog output.

If the PLC analog output is an analog motion card, then certainly speed and resolution could be critical.

MaximDave, it is great to see a person working in that field visiting this site, and I am sure you did come to the right place.

Thanks for the replies. Some of the message I'm getting is that the faster PLC outputs are used for machine control.

Since I'm a semiconductor guy, my customers design PLC IO modules. Their marketing folks give them the specs. They get them from .........

As you can see I'm quite away from the actual useage of the PLC IOs themselves. So forgive some really simple questions.

Anyone have any examples of using PLC analog I/O for machine control?

what are the commonly applications that commonly use analog IO?

Common uses for analog IO is giving drive a speed reference to tell a motor how fast to run, temperature control to tell something how hot to get, or opening/closing valves to a certain position.

Normally when something requires extreme speed or precision, a digital communication is used for speed and accuracy.

Thanks Helliana. It still doesn't explain why the PLC manufacturers are asking for high resolution & fast settling. They obviously have to provide for markets where they need fast settling or high resolution, so they make modules that support both.

If we use motor speed control, then I'm after an example of when the PLC is regulating the speed by taking either a tacho feedback or something from somewhere else. Is there an example where the PLC needs to respond in a few ms rather than several 100s of ms. Clearly something with a small time constant.

Any ideas anyone?

MaximDave,

I'm not in your business, so forgive me if my assumptions are incorrect. I'd always assumed that the reason most recent analog I/O modules were coming with 16-bit resolution was that the A/D and D/A chips for 16-bit were not costing much more than older 12-bit versions or that the 12-bit versions were actually more expensive due to older manufacturing techniques. If I can get 16-bit analog I/O cards without paying much of a premium, then I see no need to produce a 12-bit version.

That being said, the settling time for analog outputs is not usually an issue. Analog outputs controlling physical devices like control valves or even variable speed drives just aren't asked to move that fast. As Helliana said, we have other means to handle signals that require extreme performance.

That doesn't mean your marketing department doesn't want to post better specs than your competitors. They just need to know where it begins to impact the cost. I've got no problem with having better performance than I need as long as it isn't costing me a significant premium.

MaximDave said:

Thanks Helliana. It still doesn't explain why the PLC manufacturers are asking for high resolution & fast settling. They obviously have to provide for markets where they need fast settling or high resolution, so they make modules that support both.

If we use motor speed control, then I'm after an example of when the PLC is regulating the speed by taking either a tacho feedback or something from somewhere else. Is there an example where the PLC needs to respond in a few ms rather than several 100s of ms. Clearly something with a small time constant.

Any ideas anyone?

Click to expand...

Most likely a Motion application; Motion positoning loops' response is measured in μs.

This may be a marketing driven request, not an engineering driven one. They may be looking for an "advantage" they can include in advertising. In point of fact, outside of motion control, very few transmitters are as accurate as 16 bit resolution implies, and very few transmitters can respond as fast as your spec suggests. This is from process control perspective.

Is perception reality? Not really, but if it can help sell product then reality doesn't matter.

Hi Mellis, typically a 16-bit part will cost more than a 12-bit part. However, some PLC manufacturers will use the DAC to adjust the output range too. This saves money overall since they don't have to use analog gain control which is expensive when you consider the accuracy requirements. i.e 16-bits for a -10V to +10V output. Set the output to 0V to 10V & you only use half of the codes so get 15-bit. Now use 0V to 5V & you get 14-bit.

Secondly, you use a digital multiplier in the module to calibrate gain. So, the code that hits the DAC isn't the exact code that's written to the module. The multiplier causes 2LSB steps or 0LSB steps in the output at certain points in the characteristic. If you have a 12-bit DAC, then you will get LSB steps at 12-bit. If you have a 16-bit DAC, then the steps are at 16-bit and much smaller.

The question I'm trying to get a handle on is why people need the speed. Is it perhaps that the PLCs are getting faster. If we have a loop that's sampled every 5ms, an output that slews at 10ms could screw up the algorithm development. If the output was where the algorithm told it to go when it's next sampled, then the algorith is easier to develop. I would have assumed that a PID algorithm could take care of a slow slewing output even if it slewed slower than the sampling speed.

You guys develop the PID algorithms. Any comments?

Check with Peter Nachtwey at Delta Computer for motion-control products that need fast outputs.

http://www.deltamotion.com/

Hydraulic servo systems generally require very fast controls, most PLC outputs are not fast enough.

Another app that I can think of off the top of my head that needs fast analog outputs is automatic weighing systems for powders for small component manufacturing - 1ms can be the difference between a part that is good and a reject. The pharmaceutical industry probably has similar high speed weighing needs.

>Anyone have any examples of using PLC analog I/O for machine control?

>What are the commonly applications that commonly use analog IO?

Good questions when related to machine control, and I can't answer because I'm in the process world.

But Banner Engineering is in the discrete manufacturing world and they provide a weird wiring diagram for the AO's on their wireless nodes and gateways:


3 wires for an analog output? Huh?

The 3rd connection might be intended as a shield connection, maybe, but who calls a shield a ground?

My customers can't figure this one out. Neither can I, because to me an AO is (+) and (-). Simple.

Maybe the discrete world does AO differently than the process world?