Data Acquisition in Factory Automation Harold Joseph and Jason Seitz
HAROLD JOSEPH: Good day, we're going to be talking about data acquisition in factory automation.
Panelists
My name's Herald Joseph and this is Jason Seitz and together we are going to cover a number of different applications.
Objectives
The objectives of this session are to briefly review our overall factory automation solutions. Then we're going to look at data acquisition requirements in factory automation and then our goal is to gain a better understanding of those solutions and how they apply to data acquisition applications.
Industrial Factory Automation Overview
This focus as I mentioned of this presentation will be data acquisition and we will be talking about I/O modules. I/O modules are the area where on a factory floor you collect a lot of the inputs. We really focus on that as sort of general data acquisition. So far we've given a number of webinars. There's been a webinar in June on sensing. There were webinars in July on motor control and control networks and in November we'll do the final webinar in the series on machine vision.
Data Acquisition - IO Module Inputs
Within the factory floor we can look at the variety of different signal inputs and sensing measurements. So common signals that we'll be talking about today inputs are millivolt input. You could have voltage inputs either from less than a volt up to more than 10 volts, even up to plus or minus15 volts and 4-20 milliamp as well. And that would be primarily used in process areas where you're sending signals long distance. The types of sensor inputs that go into these systems are areas like pressure, load, force, temperature, optical devices for position, ranging and distance, chemical sensors, pH flow level. These are the most common ones we see. We covered a lot of these in earlier seminars. We're also seeing requirements within a system for system health monitoring and we will look at that today a little bit as well. Looking at systems that measure temperature as well as voltages within a box.
Focus Areas
The areas that we'll focus on today are 4-20 milliamp inputs and voltage inputs, then data acquisition systems, looking at I/O module inputs for single or multi-channel and also the outputs to those I/O modules and then finally system health monitoring.
4-20mA and Voltage Inputs
Jason you're going to talk to us today about 4-20 milliamp and voltage inputs.
4-20mA Input
JASON SEITZ: Yes I will, so we go to the next slide. Here we have a typical 4-20 milliamp application. This is often seen in process control where you have items like gas, chemical, pulp and paper. One of the benefits of this application is it can be carried for long distances which may be necessary in typical factory automation up to a thousand feet without needing a repeater basically. And overall the entire configuration is generally going to be cheaper than a voltage interface. Before I go into the receive side right here we have a picture of the entire signal path transmitter and receive and your goal here is really to increase the loop length. And you could do that several ways but your end goal is to reduce the voltage drop across the loop. So you could either have a very small voltage requirement on the transceive side, you could increase the voltage available on the receive side or you could decrease the drop around the loop by actually decreasing the resistance through the loop. However there are some drawbacks. You have to increase the diameter of the wire that increases the cost of your system.
4-20mA Input Configuration
Focusing on the data acquisition receive side here we have the current sense amplifier. It's basically a differential amplifier and it's going to take that 4-20 milliamp current source, pass it through a sense resistor and amplify it by our current sense amplifier. Here we point to the LMP8601. A couple of nice features for this amplifier is it has a very high common mode input voltage range that could be necessary on a high voltage system. Also it has a fixed gain of 20 which allows you to minimize the sense resistor. My minimizing the sense resistor you could decrease the voltage drop across that resistor and decrease the overall voltage loop requirements.
HAROLD JOSEPH: Now you were telling me before the session that in some of customer applications they require a certain kind of filtering if they have a lot of data acquisition loops, where they want to look at a particular band and you mentioned that this amp is a good selection for that. Could you go into that a little bit?
JASON SEITZ: Yes this particular amplifier current sense amp has a unique feature where if you look closely at the schematic brought out at pins three and four you're able to attach a capacitor which will act as a low pass filter. So depending on what noise is involved in your system, maybe the 60 Hz or 50 Hz noise from the mains, you could pick appreciate capacitor to basically implement a low pass filter very simply with this amplifier. Other important features are gain, gain accuracy and gain drift as well as reducing the offset voltage and the input bias current. You want to have a precision amplifier based on how precise your measurement needs to be. Here we're suggesting the LMP8601. Also you could couple that with the ADC121S021, control 12-bit 200 kilosample per second ADC and the LP2951 to make sure that the reference on that ADC is stable and have a highly accurate system.
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Some amplifiers that can be used in the signal path, again we're going for current sense amplifiers here. This is our portfolio. You see the voltage gains typically are around 20 and we have one at 14. Again you want to minimize the input bias current. You also want to have precision so the offset voltage drift wants to be minimized as well.
HAROLD JOSEPH: And on the ADCs I see there's a section for the signal accuracy as well as INL and ENOB. That's something we don't typically carry in our datasheets but both, I guess we've been introducing that in our seminars and we did it first in the sensing seminar. So the signal accuracy in this case, because a lot of the applications are very low sample rate applications near DC, INL is the governing factor in terms of performance, so we're actually calculating signal accuracy for that.
Voltage Input
JASON SIETZ: Great, moving on to different voltage inputs and the different voltage input configurations for these factory automation systems. Listed above are some typical voltage ranges, 1-5 volts, 0-10 volts plus or minus10, plus or minus15 etcetera. Historically high voltage has been used and basically the reason for this is to increase noise immunity. Higher voltage allows you to have higher signals. If you keep the noise constant then that'll help improve your signal to noise ratio. However as systems become more localized we have distributed control. That allows us to not be as concerned with noise and allows us to actually decrease the system supply to 5 volts. Also with this you're able to get better value, lower cost ACDs due to lower voltage processes and you're able to minimize the level shifting circuitry necessary for taking a signal from plus or minus15 volts down to the 5 volt full scale of an ADC.
Voltage Input Configuration
Here we go to some amplifiers suggested for these different voltage ranges. From the 0-5 volts we have our precision parts which are available. Moving on to plus or minus10, plus or minus15 we also have high voltage parts which are available in this range. The circuit to the top on your right is a level shifting circuit in a sense where if you're using a plus or minus15 volt system and you need to shrink that voltage to a full scale range of an ACD you could use this differential amplifier configuration here. Basically what you're going to want to have is a lower feedback resistor when compared to the gain setting resistor on the front end there.
HAROLD JOSEPH: Are there any concerns about noise when you level shift?
JASON SEITZ: Possibly yes and looking at the capacitors there that'll help minimize some of that noise and also the capacitors there are to increase stability. Potentially depending on what amplifier you select you may have stability concerns because you're actually using at a gain less than one in this particular configuration. Some parts that we suggest as starting points for the 5 volt and under range, the LMP7715 is nice. Its got considerable precision and speed. If you want to bump it up to the 10 volt and above bracket then the LM7341 is a good choice. The higher voltage amplifiers that we have listed here are also promoted as unlimited or near unlimited capacitance driving capability.
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We have amplifiers broken up into two categories, our high voltage categories. What's nice about these amplifiers are they're both (inaudible) around input and output and you could see they could go up to 32 volts, supply voltage no problem. The lower half of the slide we'll see our precision amplifiers starting with one of the most precise precision amplifiers is the LMP2021/22. And if you want a little bit more speed lower noise you could go to that other starting point that we pointed you out to, the LMP7715. You'll also see that many of these come in single dual varieties and if you're looking at the LMP7701/02/04 you have a quad available there as well.
HAROLD JOSEPH: So the precision amps provide very good performance but you were saying to me earlier that in a lot of these applications if you need a 12-bit A to Ds. and the kind of accuracies you listed on the other slides up to say a 1/10 of a percent then any of these amplifiers will work, is that correct?
JASON SEITZ: Correct, when basically selecting an amplifier it's all about the precision requirements and if it's an 8-bit system, a 10-bit system, a 12-bit system etcetera you want to pick an amplifier accordingly. And not only the amplifier, you want to also have the passive within a certain tolerance to achieve that accuracy.
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Again our ADCs, we have them broken up into the first three or SPI interface ACDs. Bottom three are I2C and we have 8, 10 and 12-bit buckets there. And again we're giving you the signal accuracy to help get a better understanding of what exactly can you expect in a typical factory automation environment.
Data Acquisition IO Module Input
HAROLD JOSEPH: The next area we're going to talk about is data acquisition and I/O modules. And as I mentioned earlier I/O modules are places on the factory floor where you pull in a lot of different signals. But you might also have data acquisition systems where you're looking at only a couple of dedicated inputs or you've got say a portable unit like a data logger that has maybe a dozen of different inputs. These kinds of applications vary quite a bit over the factory floor. We're looking at some general kinds of cases for these applications.
Single and Multi Channel Inputs
The two diagrams you see here or the two inputs you see here are a single channel input and then a multi-channel input. We're going to talk about a 16-bit single channel input and then a 12-bit multi-channel input and going to go through each of those briefly.
Single Channel Input
In the case of a single channel input we see this use a lot because of the systems where you're pulling signals in from long distances. And when you do that you sometimes have different ground potentials on the factory floor. And so you could have one to even 400 volts of different ground potential between where you're collecting the data and where you're actually making the measurement. And so when you do that you want to isolate the analog electronics because if you try to put that much voltage through these analog electronics it's not a pretty situation. The typical way of doing that is putting digital isolation behind the ADC. And that also means that if you do that you typically, because you've got different inputs coming in you actually want to have individual channels because two channels next to each other might vary quite a bit in terms of your different input. So if you look at each individual single channel versus a mux channel you have the protection you need. And right now with today's technology it's quite a bit more expensive and not as practical to do the analog isolation on the front end so it's not really practical to do this before a mux device. In this case we recommended a 16-bit single channel product, an LMP7716 which is the precision amplifier that Jason talked about earlier and if you wanted to go to high voltage then you would go to an LMP say something like a 7702. Not quite the performance but still gives you very good performance throughout from the kind of different sensors we're measuring.
HAROLD JOSEPH: So Harold I noticed you picked the 16-bit. Is that allows necessary for this type of bridge sensor application?
HAROLD JOSEPH: Okay well as you pointed out in your earlier areas 12-bit is fine for a lot of the requirements that we have. The reason we picked 16-bits is that if we look at I/O modules today about half the loops go to high resolution ADCs, 16-bit ADCs for SARs or even 24-bit sigma delta ADCs. The choice is really made because some companies like to make what they call universal devices, they don't know what's going to be plugged into the I/O modules. So they go ahead and choose the higher resolution ADC because it provides a little bit more flexibility in their systems. One of the other things to note and you had mentioned in your earlier applications you're taking signals a long distance. Here we've shown a bridge directly connected to the ADC. But you might take a voltage input from a very remote location like Jason was talking about earlier on a 4-20 milliamp loop or a voltage loop that could come into a factory floor. In this case visually you'd have that sensor fairly far away from the ADC.
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In terms of the product selection we've in this case identified a 12, 14 and 16-bit A to Ds and given you the signal accuracy for each. That signal accuracy is still focused around the INL of the device because a lot of the signals as we mentioned are close to DC are fairly slow. So INL is the governing factor in this case and that's been used as the basis to determine the signal accuracy for this table. Then we listed some references for the ADC based on different performance requirements.
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And for the opamps we've listed the opamps that Jason has talked about before because these really meet the same requirements that Jason has already discussed. So depending on what you'd need you'd either look for high voltage or high performance. And as you mentioned before you look at the performance you look at the components around it, the passives to make sure you have the right performance for the application.
Multi Channel Inputs
The multi-channel application that we've identified here, we've actually identified an 8 channel 12-bit device for this where the different signals are coming directly into the ADC and there is no isolation. And this is actually from an example from one of our HVAC customers that actually had a series of 8-bit ADCs in their system and in that case everything was close together. They're monitoring or their measurement points were very close to where the ADC was located. They didn't care about or they didn't have concerns about ground potential issues so the least expensive alternative for them is to use a mux device.
JASON SEITZ: So because no ground potential issues that eliminates the need for that isolation, is that a true statement?
HAROLD JOSEPH: Well you might have isolation but it actually might not be around the ADC. For example let's say that this is part of a system and you're connecting it to somewhere else on the factory floor for looking at some other kind of control system. So you may have isolation issues if the ground potential here is different from what you're connecting it to someplace else. But from the sensor back to the micro controller at this point in this particular application we're talking about there were no isolation issues, so you could just go directly from the measurements into the micro controller and we've identified as I mentioned an 8-bit, 12-bit or an 8 channel 12 bit-device. Also in this example shown a couple of different alternatives for input either a temperature input directly. We picked one of our LM35s, it's a very good temperature sensor that would be a good starting point. There are others depending upon the requirements if the measurement was very local or say it was part of a, or how remote it was. You might have a thermocouple input which would be more of a remote application and also the 4-20 milliamp loop input into the ADC here as another alternative might be from a standalone device. And you mentioned Jason this could be up to a thousand feet but sometimes you could have these devices very close to the input at the A to D.
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There are a number of different products we've listed here, a 12-bit 8 channel for a channel and two channel devices. We've also listed a differential two channel device. In some bridge applications you might want to have a differential input from a bridge. In a lot of applications on the factory floor they're single ended so in most cases we see the requirements being the single ended ADCs from 2-8 channels. We've listed the performance again to give people an idea of what that signal accuracy will be and we've taken this up to 8 channels. One of the things we were talking about earlier is that while you might have more channels as you go up in channels you add a lot of pins to the package. It's actually more economical and requires fewer board space to try to keep your channels in a manageable range. In this case the HVAC application we talked about, they use three 8 channel parts. For the temperature applications we've listed a couple. The LM35 is the remote temperature input. The LM94022 was actually used for cold junction compensation on the thermocouple and that's something that you and I covered earlier in the sensing webinar. That is the reference that the thermocouple is measured against and so in that case we would have a device in a very small package. The LM35 supports a much wider voltage range and much tighter accuracy.
JASON SEITZ: Yes so you see the LM94022 has a little bit looser accuracy. And you had that opportunity to do that with the thermocouple measurement because again this is being used specifically for cold junction compensation. It's not measuring the temperature of your measured environment. It's basically allowing you to utilize a thermocouple in a very extreme environment. Thermocouples have a relative accuracy of about 1%, 2% so this pairs off nicely with the thermocouple.
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HAROLD JOSEPH: Okay and on the opamps again the same kind of considerations that we talked about before whether you're looking for high voltage or whether you're looking for precision.
Data Acquisition IO Module Output - Output modules - Single Channel Output
If you look at a lot of those input measurements there are clearly cases where then you need to if you're measuring say flow or pressure or temperature you need to take some action and drive an output and that is where you would typically use a DAC. And these DACs and the I/O modules have the same kind of isolation concerns that you might have with the input side. It really depends on where you're making in this case the drive function, what you're driving, is it at the same potential as your micro controller. In this case we've shown a single channel DAC. It could be single, dual, quad or 8 channel. We talked about the output buffer and LMP7701. A reference may be needed for the DAC and we've included a negative bias generator. And Jason you were telling me before our session about the negative bias generator and why you would use this to get to exactly the 0-5 volts. Could you go into that a little bit for us?
JASON SEITZ: Sure so you see the output of the amplifier here has the 0-5 volts. And people familiar with working with amplifiers know that even though they claim to be rail to rail output, they really only go hundreds of millivolts to maybe if you're luck tens of millivolts to the actual rail. What the LM7705 enables you to do is put actually a slight negative bias on the negative supply of the amplifier and for this particular negative bias generator it's negative .23 volts. That allows you to get a true 0 volt output on your amplifier and again the goal here is to maximize the full scale potential of the amplifier.
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HAROLD JOSEPH: In the DAC products we've listed a couple of different alternatives here either some single channel or multi-channel products. The first one is a 121C, that's an I2C device. The others are SPI devices. The S stands for SPI. And then the opamps that we discussed in the drawing, the LMP7701/2/4 depending upon the number of channels required that would be matched up with the DAC.
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And the negative bias generator here that Jason discussed the need for as well as some references that might be required depending upon the DAC you choose.
System Health Monitoring
The next area we're going to be talking about is system health monitoring.
We've talked about I/O modules and taking inputs in from a variety of different either local or remote locations. What we also see in the industrial space is that sometimes you were making measurements within a system or a box so everything is self contained. And in this case you often have a need to do a system health measurement meaning you're looking at the different voltages within the box. You may also want to measure temperature. If you have a processor it's possible to overheat those so you want to be able to look at that. You might want to be able to look at temperature and other areas like in a battery or in memory locations. A lot really depends upon how small the box is and sort of how confined things are. In this kind of situation you could actually buy almost or get almost a system on a chip that would address a lot of these requirements. And the ones we're talking about here can accept has a local temperature measurement which means it measures its own temperature onchip. It also can have an external measurement like an LM94022 hooked into it so it can measure a remote location or a variety of different other voltage inputs. And it also has outputs that could be used to control a fan, either turn on a fan like you would hear in your notebook would be an example. Obviously not industrial but when you hear the fan turn on that's actually a system that's making a decision to say, okay things are getting a little hot. I need to cool it off, I'm going to turn on a fan. Or even clocking the processor, adjusting the actual speed of the processor so that it can cool down and doesn't overheat.
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Some of the systems that we've identified here, there's a family depending upon the need. You have a temperature monitor which could be either a single local input or you could have multiple remotes like a diode coming into it or voltage inputs. And those voltage inputs could be another temperature input or it could be other voltages on the board, other sensors, power monitoring. And then a variety of outputs either for fan (inaudible) monitoring, different kind of outputs either DACs to drive something or PWM outputs. And then the kind of fan control, you've got different approaches that are needed depending upon the end piece of equipment and then some information on the supply. This is a nice little family of system on a chip system monitors that we see used in the industrial space.
Summary
In summary we have a wide range of data acquisition and I/O module applications or we see them in factory automation and those applications requirements vary by area. And important issues are the actual application and measurement, the input type and signal level that Jason discussed whether you're doing a local or distributed control system. These things all influence the type of analog solutions required by the customer. Also along with our other factory automation webinars what we've got here is a nice little family of webinars that really addresses as we've discussed sensing, motor control, control networks, machine vision in the future. But it gives you the ability to look at solutions from the sensing input right all the way through the system to the final control network.
HAROLD JOSEPH: We've also added some helpful web links. We've added a link to our industrial solutions page and that will cover a lot of the material that we've discussed here and the other webinars. And then on the individual webinars we've added links in to our factory automation overview, our sensing solutions in factory automation motor control, and industrial applications for our precision fighters. That's for the control networks. And finally as I've mentioned before our machine vision webinar is scheduled for 11/5 and that'll be the final in the series. This is our contact information, mine and Jason's. If you have a particular question please let us know, we'd be happy to help you out. And in the appendix we've added one slide to help you with the amplifier selection. Jason has talked about the LMP7715 as being one of our important precision opamps. This is a slide that gives you a way of saying, here's a good starting point and then as you need to optimize your performance for the particular application requirement it gives you some typical choices for bandwidth, power, voltage, performance near DC, noise and drift. And this is put together by our application folks and marketing folks on the amplifier side. And that's about it so thank you for your time. Anything to add Jason? JASON SEITZ: Again like Harold said, we're here to help you out so give us a ring. HAROLD JOSEPH: Okay thanks very much.
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