Motor Control Sensing in Factory Automation?
Good day, my name is Harold Joseph, and we're going to be discussing motor control sensing and factory automation today. This is one of five webinars we're running in the factory automation area, each focusing on a different aspect.
Objectives
The objectives of this session will be to look at, first of all, our overall campaign, then get a better understanding of motor control applications in factory automation, and then finally understand how our solutions and products support those applications.
Overview
A little bit of market background first. The factory automation market is a more than $1 billion worth of analog content. So there's a lot of products spread out in many applications. The 16 largest suppliers address less than 50% of segment sales, so you've got a situation where many products are used in a wide range of areas and there's no particular dominant kind of product or focus in the industry. Our focus is going to be on some specific areas, and five areas we'll talk about where we focus on using our National components to address specific applications. So looking at key trends in the area, we're seeing expanding a competitive situation is causing an increase and a need for improved productivity, quality and throughput, and how that translates to our requirements is for the need for better or finer control. There's also a drive for more energy efficient systems. And in the motor control area, this is a major issue for us because the largest consumption of energy on the factory floor is related to motors and how they're used. There's a need for a high performance, high value AC and servo motor solutions, and there's also a drive for lower cost AC/servo motor control sensing solutions. And two of the areas that are used are Hall effect and SAR ADCs and optically isolated sigma deltas and ADCs. In our situation, we're focusing on the Hall effect and our SAR ADCs. That's what we will be presenting today to look at AC and servo motor applications.
Industrial Factory Automation Overview
The five areas that we'll be looking at in factory automation include a sensing webinar that was done on 06/25, motor controls, automatic controls webinar that was done on 07/09, this sensing motor control application today on 07/23, I/O modules which will be done at the end of next month, and then finally a machine vision presentation that is yet to be scheduled.
Focus Areas
The areas we'll focusing on today is, first of all, why is motor control sensing important, what needs to be measured. We'll look at some of the applications as to where it's used. Then we'll focus on particular areas like AC motors, servo motors, low side sensing, and then finally high speed motor synchronization.
Why Motor Control
So why do you control motors in the first place? As I mentioned, motors are one of the largest energy consumers on the factory floor. If you actually look at the lifetime cost of a motor, more than 99% of the cost of running that motor is due to the fuel it takes to run the motor. It's not the hardware. It's the energy consumption. Accurate motor control is the easiest way to minimize this energy consumption. If you don't have a way of controlling the motor, what you typically do is run the motor full out or near full out, and then you break it back down as you need to to run your system. If you can actually control the motor torque, now you can deliver the torque required for the application and you can minimize the energy consumption. In some areas, regions are actually charging factories for a tiered level of cost depending upon their energy usage. So, being able to cut down on motor control energy really helps in the long run in conserving their cost and making them more competitive. The applications that follow are going to focus on 12-bit ADCs for the primary sensor measurement, and this will be really looking at the current measurement that's used to control the motor. In general terms, the 12 bits provide a 0.024% signal accuracy over the specified temperature range of the device, and that's typically minus 40 to either 85 or 105C. So what you're talking about is the solution that gives a very good signal accuracy over a wide temperature range. The focus area that we're looking at is AC and servo motors. This is areas where discrete analog components are used in conjunction with processors, and it's that combined solution that is used to reduce energy consumption.
What Needs to be Measured?
So what needs to be measured? What I've shown here in this diagram is a rectifier, an inverter and AC motor and a controller with the various inputs that you might have to the controller. So highlighted in blue with the arrow is the current consumption from the high side of the inverter. This is actually two of the three phases going to the motor and controlling the motor. So knowing the current in each of the three phases allows the motor controller to adjust the motor torque, and then that provides the control you're looking for. The current in one of the phases can be calculated from the current in the other two phases. So actually what you really need to do is measure just two currents, and then you can calculate a third. And you can do that provided you know the phase relationship between the two currents you're measuring. So the easiest way to do that is to make a simultaneous measurement. So in this case, we're looking at two measurements that are made simultaneously as the primary control loop control input into the controller. So, the voltage on that right side of the inverter can be in the 600 volt or more range. And you'll see a line that says high voltage isolation barrier. And so what you're really looking at is making a measurement, but you have to be able to take that measurement from a high voltage side with more than 600 volts of difference back to the controller.
What Needs to be Measured? (cont.)
And the reason we -- one of the ways of doing that is with these current sensors, and they're called the Hall effect sensors. So those Hall effect sensors really make the measurement and they look at the magnetic field around each of those lines that's directly proportional to a current so we can do that without having to attach to this high voltage line. The other measurements that are made are to look at the bus voltage because in the equations of the control algorithm, the bus voltage actually shows up as a gain term. So what this means is that you have to have the bus voltage within a certain range to be able to ensure a proper control of the motor. So what you have to do then is to actually measure that and make sure the controller knows what that bus voltage is. A second input is the speed input to the controller. So that's coming externally, and that's driving the controller to move at a particular speed. And finally, a temperature input, and this is usually for general temperature monitoring around the system, if you've got a particular hot spot, to work as an alarm, and usually for environmental controls.
Applications / Industries
So the applications we'll be looking at are AC and servo motors. So AC motors look at areas like in the automotive area, you could have a robot, for example, building a car. You could have food and beverage applications, chemical applications, material handling and metals and semiconductor industries, conveying, pumping, process control, and as I mentioned, robotics in automotive and in other areas as well. So anywhere you've got a motor that needs to develop a fair amount of torque to these AC motors to drive and control your systems. A servo motor adds an encoder to the back of the AC motor. And so this is a way of getting finer control, so which are used in the same kinds of applications that you would find the AC motor in, but in areas where you'd have better or higher control. And we've added a couple of items to this list, areas like CNC machines, LCD manufacturing and surface mount manufacturing and material handling. But in essence, when you go to, and look at applications, you may find AC or servo motors used within the same kind of environment.
AC Motor Control Sensing
So in terms of the AC motor sensing, you've got a picture here that we looked at before where we looked at the control loops into the control, the inputs into the controller. So in this case, the Hall effect sensors are connected to simultaneous sampling ADCs. And as you recall, I mentioned that if you can make those measurements simultaneously, then you can calculate the current in the third of the three phases. The Hall effect sensors can usually interface directly with the ADC. Some earlier versions of Hall effect sensors have fairly low level output and require amplification. The Hall effect sensors today that are using these applications typically would input directly into an ADC. Now, I've talked about the common mode voltage on the high side of the rectifier, the 600 volt area. And, as I mentioned, the Hall effect sensors, because they are current sensors, effectively isolate the ADC from this high common mode voltage. They measure the magnetic field that's directly related to the current. They're not connected to this wire directly, so you don't have a high voltage path right back to the ADC, and you've got the isolation you need to protect the ADC and then the controller.
Recommended Products
These are a couple of the products that you would use in this application. I've listed here three different simultaneous sampling ADCs. The difference in these ADCs is in terms of sample rate. So the middle one that runs up to about 500 kilosamples per second is a good choice for most of these kinds of applications, but depending upon the requirements your customer or your requirement, may go to either lower or higher need. I've also listed some of the references you would use with the ADCs. And you would choose the reference depending upon the reference, the temperature range that you needed, the accuracy you needed, and the input voltage range that you had. So we have several choices that you would just tie, depending on your application, to that specific ADC.
AC Motor Control Sensing (cont.)
So the next area we'll take a look at is the bus voltage. Now as I mentioned before, you need to measure the bus voltage so that you can make sure your controller algorithm is within the proper range. This can typically be done with a 12-bit ADC. I mentioned in the beginning of the presentation that a 12-bit ADC has a 0.024% accuracy. So it's a very accurate measurement, and that's sufficient for your bus voltage applications. Depending upon the configuration, you may or may not need a reference with the ADC. The single-ended versions of the ADCs that we can look at actually take and work directly off the supply current. You stabilize the supply current before going into the ADC.
AC Motor Control Sensing (cont.)
So, in a way, it's similar to using a reference in those applications that you've got, basically both 12-bit ADCs to make that measurement. The speed input can vary. It can be a 12, 14, or 16-bit ADC, and it may or may not need an amplifier in front of the ADC. The speed inputs could be a voltage that could be, say, a five volt input. It might even be higher than five volts, so you might have to level shift it down to input into the ADC. The ADCs are typically five volt inputs. The temperature input then looks at environmental monitoring. So are you outside of your limits? Are you seeing something that exceeds a particular temperature area because something is -- particularly if you're in a system where you've got a lot of motion and motor control, you sometimes have issues where you might have some bearing wear, some other issues that caused some hotspots. So the temperature sensor gives you a way of measuring that, and we have both digital and analog temp sensors depending upon the particular application that's needed.
Recommended Products
So I've listed some of the op amps and differential input ADCs and references that are used in the diagrams. This is a full list of the op amps that would be used with any of the circuits that we talked about. And on the individual circuits, we've put the specific op amps that you would need in that particular application maybe as a good starting point. When you pull down this file, I think you'll get a better or clearer view of the individual components that are used. This is sort of a shorter list that will give you a little bit of benefit instead of going to, say, the large parametric tables that we have on our web site.
Recommended Products
So I've also listed here some single ended input ADCs, and those are labeled on the drawings as well, and a couple of different temperature sensors, both an analog and a digital part. In addition, we provided a list of power products. These would be typical products that you might use in those applications. So we haven't talked specifically about that today. Once we do the control loop, then the requirement of course is what kind of other power products do you need around the whole system. So while we won't be going into that today, this is a good starting point to look at what might be needed in your application.
Servo Motor Control Sensing
The next area to look at is servo motor control. Now in this case, you have either an encoder or resolver attached to the motor. And what this actually does is it looks at position and gives you another element or a finer control on the motor, and adding that function makes it a servo motor. So you have two choices. You have encoders, and you have resolvers. The encoder typically provides better performance than the resolver. It might be usually an optical encoder, sometimes a magnetic encoder, usually often though, an optical encoder. The resolver is made of windings. It's more of a transformer device, and it can be used over a wide temperature range. It's very tolerant of shocks and vibration. It's very rugged. So the two areas that you would look at is do you need the performance or do you need the temperature range and the ruggedness. Both solutions would require the same kind of analog interface to the controller. In this case, we've shown an ADC and an amplifier. And the ADC typically would look at an amplifier, because the output of the encoder might be, say, 300 millivolts. There are some that are higher voltage, but 300 millivolts would mean that you would have a 10x gain through the amplifier to the ADC. So we've listed that as the products you might take a look at.
Servo Motor Control Sensing (cont.)
And then, in addition, I wanted to make a couple of other points about the encoder. The encoder is very small, and so size is critical. You've got a picture of one here. So that means that one of the focuses that we need is to look at parts with a very small package size. Also, that encoder can run at a high sample rate, maybe 500 kilosamples per second or even one megasample per second. It typically doesn't run any faster than that. So you're looking at some ADCs that are running up in that range. We've listed those as well on the tables that follow. The encoder output varies. As I mentioned, it may need to have an amplifier to interface to an ADC or you may be able to go there directly.
Recommended Products
So these, again, are some of the products that would be used with those devices. We have op amps that we've listed before, and we've listed the particular ones that we would need with the encoder on the previous slide. We've also listed simultaneous sampling ADCs. These are in small packages, and you've got three sample rates here, the highest being one megasample per second that would then be used in the higher speed encoders. And finally, references that would be associated with those products.
Low Side Control
There's another approach that we wanted to talk about called low side control. Now the focus of this kind of control or where you would find this is more in applications like commercial washing machines, HVAC systems. They tend to be in applications where cost is critical. So there's some tradeoffs in going with this approach. So one of the positive tradeoffs in this approach is that you avoid the cost of the Hall effect sensors and the simultaneous sampling ADCs. So you're saving something on that end. On the other hand, this doesn't typically give you the kind of performance or accuracy that you do with the other applications or other configurations that we've talked about.
Low Side Control (cont.)
And the reason for that is that you actually don't get a full waveform when you look at measurements on this side. You actually have a couple of different measurements you're making for current and voltage. This approach is called the low side approach because you don't have that kind of high voltages you have on the other side of the inverter, so you don't have to worry about the isolation to the extent that you do on the high side. So the low side approach, though, takes and tries to -- it looks at pieces of the waveform. So the graph you have in the lower left shows a black line, and that actually represents what would be measured on the low side of the -- for this low side measurement. Then the red and blue lines would be the actual measured level of two phases of the three phase phased current. So those are actual measurements that are made on the motor. What the DSP would have to do in this case is go from the black lines to sort of fill in the gaps and try to extract what the actual currents on the high side would be doing. So here's where the tradeoff comes in. Because you don't have a full waveform to work with, you typically don't have the kind of performance out of this approach that you would do where you individually measure the currents on the high side going into the AC motor directly. Another comment I wanted to make is that this approach introduces some high speed harmonics in the measurement. So the blue line outlines the shunt that's used, and it calls out an LMP7711. So that's a very good amplifier. It's a fast amplifier, so it can deal with that kind of harmonics situation you have. The other measurement that's made with the other amplifier sometimes requires a high gain amplifier because the signal is very small. So you've got a couple choices there. It doesn't suffer from the kind of high speed harmonic issue that you would in the other shunt area, but you may have to worry about some of the gain issues, and we've called out an amplifier here, and there's a couple more we have in the table that you might use as well, but this is a good place to start.
Recommended Products
So again, we're listing the products that might be used in this area, single-ended input ADCs, op amps, and temperature sensors.
High Speed Motor Synchronization
And finally, wanted to look at high speed motor synchronization. So, up to now, we've looked at the individual measurements around an AC or servo motor or on the low side section of an AC motor. In this case, what we're looking at is applications where you would control a lot of motors simultaneously. This would be areas like pulp and paper manufacturing, some food and beverage packaging, semiconductor processing, areas where you've got a series of motors that have to be synchronized to maintain proper control and consistent production quality. So the key to this kind of control is having real time control. It means precision clocks. And that's an area where National has some real strength. So National's Ethernet PHYs have the industry's highest synchronization accuracy for distributed control, and they do this over a wide temperature range. So they're specifically designed for these types of applications. As part of the factory automation webinar, there is another presentation that covers the Ethernet PHY that goes into greater detail. For the purposes here, we just wanted to talk about this feature and how it might be used in motor control. So this would be looking at in synchronizing a lot of motors, while before we looked at the individual motor control.
Recommended Products
We've listed some of the products here for the interface parts. That's the full family of parts. There's some information about the temperature range, package and whether it supports fiber or not. As you can see, it also includes cable diagnostics, and that's a real benefit in some of the maintenance areas as well, as people are looking and gathering that data to make sure that they minimize the amount of downtime they have.
Summary
So in summary, motors are one of the largest energy consumers on the factory floor. As I've mentioned, almost all of the lifetime cost of a motor is based on the energy consumption of the motor, not the hardware itself. And accurate motor control is the easiest way to minimize this energy consumption. And we've talked about some of the National analog signal path products that can be used to provide the solution, and we've covered a number of what we call the higher performance applications today.
For Additional Information
For additional information, this webinar can be found at the Powerwise Design University, URL I've listed that. There's some additional information on other kinds of signal path solutions, sensing solutions on our web sites for Signal Path Solutions, Signal Path Designer, and Technology Edge. And we also have listed a WEBENCH Sensor Designer where if you want to look at particular kinds of sensors like, inputs into that. They will calculate the performance of the Signal Path based on those sensor requirements.
Up coming Factory Automation webinars
The two upcoming webinars we have are I/O Module and Machine Vision. Be sure to check them out.
Contact Information
And my contact information is listed here in case you have any questions about the presentation.
Thanks
And with that, hope you have a good day.
Digg
Del.icio.us
