Life is simpler with absolute positioning

April 27, 2023
Navigating the world is easier with absolute positioning than it is counting incremental steps. Posital Fraba's encoder product specialist Jim Stevens joins Control Design's editor in chief, Mike Bacidore, to talk all things encoder. What are they? What are the differences? Which encoder is best for which application? From kit encoders and Wiegand sensors to limit switches and homing cycles, component options have changed as a result of technology advances, scalability and price points. Absolute and incremental encoders are on equal footing.

Transcript

Mike Bacidore: Given your background, can we start with the basic overview of encoders? What are the basic types and differences for example?

Jim Stevens: Sure, absolutely. You know, was, we'll start with the very fundamental, you know, just just in case we have some newbies on the on the podcast here, you know, and encoders, a device, it's electromechanical device that converts mechanical motion either Rotary or linear linear motion. And there's some form of electrical signal such that control system controller or whatever can control a mechanical system in the real world. OK.

So now you know moving forward from that point, if you look at encoders. There's really three basic types.  You have standalone encoders, you know which are encoders that you can take, essentially have their own bearing, they're shaft and they're a complete unit. You can plug them in, fire them up, throw them on a table, turn the shaft and those in cards will give out signals. OK, those type incurs come with either solid shafts, a hollow blind board. So they go on to A to a shaft, or sometimes they even come with a complete through board. OK.

The next type of encoder that you'll typically see in control systems are linear encoders, and as the name implies you know they are able to transduce linear motions into electrical signals. OK, so they can be like last scales attached to machine. You read this glass scales as a tables moving on the machine gives you a linear displacement. You know so and also these sometimes can be seen as what they call, you know, you know a string encoder, essentially an encoder.

I Rotary encoder with a with a drum with a string on it or a cable on it that converts the as a string, pulls out of the product, converts the into linear Rotary motion and gives you signals.

And that and you know, the last type inquiry you come across in that these type of applications is a is a kit encoder.

And a kit encoder sent you though is is a bunch of parts, you know, postal could ship a kit encoder to a customer consisting of four main parts. You thought of parts on a table and they, the encoder is not functional. That type encoder is really requiring the customer or OEM or whoever is using the product to assemble it to shaft and bearing assembly. You know, in order to make it, you know, a functional device.

Mike Bacidore: Great explanation.

Uh, So what about incremental encoders and absolute encoders, for example. What what are the? What are the difference between those two types?

Jim Stevens: Sure, that's that's the fundamental, you know, types of types of encoders. Of course you know incremental encoder is a relative encoder.

OK. So an incremental encoder is is a device that as you turn the shaft or as it as it functions, it squirts out a a series of square waves.

OK, now those square waves have to be counted or or totalized to keep track of of position.

You know that you know a lot of times you you'll see the coming quarters as an optical encoder, you know may have 1000 lines on a disk. You know that. And as that that disk rotates when rotation you'll see 1000 squares coming out of the encoder.

It also has a second second channel which is all set by 1/4 of a cycle, which gives you the ability to discern direction of rotation with the incremental encoder. OK? And once again, you know incremental encoders are relative. So if you lose your account, OK, you have to go back to a known location, start over at zero and move forward.

OK, so so now now you know, conversely there are apps that encoders and absolute encoder essentially gives you a true position at any point in time. So basically it can be operating where you left off.

And and you, you know you are, you know, create the encoder. I'll give you a position. It gives you position. You turn power off you lose lose your your counts or whatever you fire back up the absolute encoder gives you actually true position at any given point in time. So basically it's exactly where you left off exactly right.

Mike Bacidore: Speaking of that, the Posital kit encoder product line, what would be saying appropriate application for that product line with its on access and how we're shaped absolute options compared to say a typical incremental kit encoder?

Jim Stevens: Sure, sure. So really when when it comes to kit encoders, you know incremental or absolute that the applications are very similar, OK, the application is that a kit assembles to a bearing and a shaft, OK, it can be incremental or absolute.

So the applications are are the same but but the the the the difference is is you know of course you have that relative versus versus you know absolute position.

So you know that you know many times both these both these anchors adding colors find themselves on the on the tail shaft or in bill of a motor as as a as a as a motor feedback device.

You know, and then we also have a a large hollow bore version of that kit encoder. And those type of encoders. You know, you usually find themselves on that, you know, larger, essentially larger diameter motors, you know, 30 or 50 millimeters shaft motors or they can find themselves on hollow bore motors. In other words, a motor that has a hollow shaft that goes completely through it, such as maybe if you think about it, a robotic arm.

I where you need to run cabling so robotic arm from one joint to the next internal to the arm. So you need an encoder that has a large board to go or you know to go hand in hand with a motor that has a hollow board such that you can run cabling or control lines or whatever to do through the encoder into the subsequent joins in a robotic arm.

Mike Bacidore: Yeah, a lot of robotic applications certainly springing up, especially lately with the influx of robotic I mean, even the robotics within different manufacturing cells that are that are coming out, that that's got to be a high growth industry I'm assuming.

Jim Stevens: Well, it is absolutely. You know the, you know, the way the world, you know today's dates, you know it's it's difficult to find labor you know. So you know that's driving automation our world of automation is a hot business right now. So robots are part of that automation.

So feedback device manufacturers such as Posital or are enjoying a a a great situation now in the in the world in the market.

Mike Bacidore: Right. Absolutely. Yeah. It was kind of like during the gold rush, it was not so much the the people panning for gold is the people selling the the shovels and the materials you need.

Jim Stevens: Correct, correct. That's a good. That's a good analogy. That's a good analogy.

Mike Bacidore: So let's talk. We can censor obviously a very important part to absolute encoders becoming battery free or not having mechanical turns counting gears. Can you share any insight about the sensor itself and and why it's so critical and and the impact that it's had?

Jim Stevens: Yeah, sure. The this this is actually a great point. Norwegian sensor, you know, so, so, so postal, you know, manufacturers a line of waking sensors and basically what awakened sensor does, in a nutshell, it's an energy harvesting device that can generate a pulse actually 2 pulses of of energy during a rotation of a magnet relative to that to that device. OK, so now if you think about, you know, traditional absolute encoders of the past.

You know how you know a lot of times I'm the old apps are encoders. You know, the very old ones have been in the business version, you know, 200 years. It feels like as you look back at the back in the old days, she would see these absolute encoders that were essentially a work of art. They had these very precise gear trains in them. They have a disk that rotates once per revolution. They give you a single turn resolution. Then they have another disk that's rotating on a gear train that gives you a multi turn count, right. And these gear trains were just.

They're actually work pretty cool, you know? But. But you know, so that that was an old version, you know, you have a gear trains, you know, running an absolute encoder. It gives you, it gives you absolute position. You know, you turn the power off with those encoders. You turn the shaft, you turn it back on. It knows exactly where it is. Even though the shaft turned during power off.

You know the then more recently you're seeing absolute encoders, you know, you know, sometimes mostly in the in the Asian market where they have, you know battery backup.

To you know, to maintain a turns count on on an absolute encoder, but how you know? But you know batteries, you know, batteries have a have a finite life. You know, at some point they need to be serviced or replaced and if the if it should happen to miss that cycle of replacing the battery.

You lose your count, you know, so you have to reset your system and start up and start over. OK. So now now you start thinking about well now the waking center that that Posital’s using.

So if you if you have a device and this is in Posital's magnetic encoders of course, because the wig and sensor the magnetic device. So, so if you if you think about that device it if you put that inside the encoder and you have our magnet rotating once for revolution giving you your single turn value resolution value such that you know when the when the the single turn rolls over from maximum count back to zero. If you're able to generate a pulse of energy at that point that's independent of shaft speed independent of external conditions, only dependent upon the magnet rotation and location in the encoder. If you could take that pulse of energy and wake up a controller and write a bit to a non volatile memory and go back to sleep, you can maintain your turns counts.

And since you, that's what the apostle was doing with there, the Wiggins sensors inside their magnetic encoders, you know, the wagon sensor allows you to essentially turn the encoder off remove it. Whatever you want to do. You know it's off. You turn the shaft, you know, clockwise counter clockwise so you're incrementing counts, decrementing counts, you'll multiturn.

And you can come back a day later, week later, a year later, and that that encoder when it wakes up powered up, you'll you'll provide you true physical location.

OK. So that's that. That's something that the the, the the wagon sensor has, you know, since you provided in terms of functionality for postal products, there's no batteries, no gear trains. And then there's horsing around to get an absolute position. This things a self-contained true absolute encoder. Sometimes I argue that that battery backed multiturn encoders are not really true absolute because if you lose battery, you're done. It's, yeah, it's a. It's a form of RAM, right? But but but but but with you know, but with the with the, with the waking sensor, it really, really provides a true absolute product exam.

Mike Bacidore: That is kind of important, I guess, to the to the encoder itself to be able to have that self-contained power and be able to provide the relative position, yes.

Jim Stevens: Yeah, yeah, exactly. So you powered up essentially and you query in code and it gives you it gives you true position. It's just it's just yeah, it makes it makes it transparent to the user.

Mike Bacidore: Right. Yeah, yeah, absolutely.

Let’s shift gears a little bit here and let's talk about the way technology has affected affected price points over the years.

Jim Stevens: Sure.

Mike Bacidore: Especially with absolute and incremental encoders, you know typically in the past those the price has been inhibitive.

Jim Stevens: Yeah, exactly that. That's a very good point.

Mike Bacidore: But how? How has the technology and the price point changed? Regarding absolute encoders.

Jim Stevens: Yeah, that's that's a good point. So. So I'm going to. I'm going to. I'm going to direct this towards a kit encoder, you know, incremental versus versus absolute magnetic kit encoders. You know, so.

So so, you know, traditionally, you know, if you if you were to let's say it was for example, let's say you, you, you.

They're buying out a stepper motor. And you wanted some kind of a a feedback device on the back of that step promoter, you know, a lot of times you would see a a nice little incremental kit encoder back to that separate motor. OK. So stepper motors are kind of a commodity item. My stepmother friends are probably cringing when I say that. But, but but. But you know, really you can buy a really nice stepper motor in low quantities, you know, for, you know, maybe 30 or 40 bucks, right. You can get a really nice little little stepper motor.

You know, so if you want to have a, you know, a position feedback and the back of that splinter field, if you want to control that stepper motor. If you wanted to do a, you know, like a servo motor is pretty like a, you know moving and verify types situation. You know you typically would see an incremental encoder back to that. And so in volumes you can see Nick incremental encoder on the back of a separate where maybe in that $30 range maybe $35 range. So you really have a.

You know a pricing that's par. You have a maybe a $3040 stepper motor and maybe you have a 35 to maybe $40 including the back of it, you know, so it's kind of a 1 to 1 pricing.

You know, but, but if you're. Wired you absolute feedback you know be before they the the the magnetic kit encoder. Absolutely Kit encoder was available.

You would have to throw a a a standalone encoder with a hub shape or a through shaft on the back of the of the stepper motor and that that ENCODER starts to look like you know 200 to $300.00 part on the back of a $35 was a stepper motor. It just doesn't make sense unless the application really needs the absolute feedback. People would just not go that direction you know.

But but now now with with the Posital. You know, kit encoder.

You know it, it's because it's becoming more, you know, closer in terms of pricing parity. You know, of course the, the, the, the absolute encoder from my postal kit is not a $35 part, but maybe in volume, it might be a $70 part, maybe it's a $75 part. It makes more sense now putting out maybe a $70 part on the back of a $35 separate motor, OK. It just makes more sense. And in reality, if you look at the world.

You know, in in terms of position in terms of control, the world is more absolute than it is incremental. You know relative you know you don't, you don't, you don't, you don't, you know drive down the road and you know and and start counting miles and stop and hope you have the right address. You know you look at the address location and say yeah I'm at the right place. Right. You know that's an absolute absolute given.

You know, and if you get lost, if you times you're you're really lost, right? If you're driving down the road trying to find a location. So the the the world is in in many cases just much more an absolute than it is incremental. It's just that the incremental guys have been around for a while and they've enjoyed it. Very nice price advantage against the magnetic or against the absolute guys.

Mike Bacidore: Sure that you know and that example you gave of stepper motors. Becoming. Commoditized, I mean that, that's a great example of pretty much anything within automation as well. As you know over time scale.

Increased technology, increased demand will bring price down and even things like we were talking previously about robotics and robot robot arms. I mean robots that have almost become commoditized. I mean, they're still a little more expensive than a $30 stepper motor, but. Umm, they've reached that point. Where? Programming uh integration. Price points applications have all just. Brought brought it to such a scale that yeah, I just need a a. Articulated arm for machine tending application and you know boom it's inserted it and and there you are.

Jim Stevens: Yeah, exactly. That's that's the trend of the world, right, that that's the way things earn. That's that's a normal normal progression products.

Mike Bacidore: Sure. And yeah, I mean even look at, you know things like low code, no code, you know, it's all about democratizing technology or democratizing automation so that. Pretty much I I won't say anyone, but anyone with an engineering some engineering know how can. And implement the technology.

Jim Stevens: Yeah, yeah, exactly, exactly. You know, you don't have to PhD to to get this application running, right? You know, it just, it just becomes a more of a day-to-day type of task in terms of manufacturing.

Mike Bacidore: Yeah, absolutely. So. So you've talked already about holding position. But let's talk a little bit about that in terms of. Like a TCO, a total cost of ownership consideration based on applications. So when a system shuts, shuts down and resets and you need to restart it. I mean, are there encoder options? That will hold position to avoid, you know scrapping a batch or interrupting critical applications such as what we're talking about robots. So like a medical robot, even.

Jim Stevens: Yeah. Yeah, exactly. Yeah, it's true. So. So indeed, on total cost of ownership, you know? So. So you're mentioning that the kit encoder will be applied to A to a motor, you know, getting closer to parity in terms of pricing, in terms of super low cost stepper motor.

You know that. The absolute encoders you know if you if you if you move to an absolute encoder control system, absolute position system. Really what happens is is that the need for for the homing cycle. OK, which is, which is time which is cost the need for maybe limit switches or some mechanical safeguards. You don't drive a a machine through a wall and factory. You know those type of things start start to disappear.

There's always a level of of fail safe safety you have to, you know, case something else happens. A sister or whatever you have to have a a way to maintain, you know, safety.

But you know when, when, when the system goes to an absolute encoder, you know, a lot of these old I should say like work arounds that people had to do to use a relative encoder. Where do you start to go away? That cost goes away. So those are bits and pieces on the machine. Bits and pieces on machine are added complexity. You've got cabling. You got all this stuff right? Maintenance or it it just it just life just becomes a little bit more simpler if if you start out with a with an absolute position system.

Also, like you say, the the worker, the work arounds just just kind of fade away which is cost you know fades away.

Mike Bacidore: So so is that. So, I mean, you really able to eliminate limit switches and homing cycles for positioning and in certain instances like that?

Jim Stevens: Yes, yeah, absolutely. You know, you know, suppose you have a suppose, you know, suppose you're you're processing a batch of wafers, you have some kind of wafer handling equipment.

And you know if if, if, if something goes on, if you if you happen to have a power glitch or something happens in the system and you lose your relative count.

It's really hard to reset that process without losing your your batch. You know, if you're you're making a batch of 300 millimeter wafers, that's a pilot, that's a pilot cost. You know, that could potentially be a scrap.

And and you know, you brought about medical robots, you know, you know, we're we're we're seeing applications with our products in medical industry and of course. Doing a home cycle or a reset during a procedure and the medical situation is is really a showstopper. Just say the least. Just say the least, you know, so.

There there are applications though. Yeah it it just it just makes sense. Right and and and absolute positioning systems you know enable that you know that that they they help that that process or that safety.

Mike Bacidore: Right. Yeah. That. Yeah, I yeah, I would call that critical. Yeah.

Jim Stevens: Yeah, it would be very critical, yeah.

Mike Bacidore: Yeah, right. Literally, yeah.

Jim Stevens: Sure.

Mike Bacidore: So. So what? And you've been very, very generous with your time. I have one more question. I I do want to ask you about so say on the back of a brushless DC motor for example, what are the tradeoffs between using an incremental encoder with a hall effect sensor versus the absolute absolute controller is. I mean is it more than just the commutation?

Jim Stevens: No, this is a good one. This is a great one. This is a great one. So this is one of my one of my soapboxes.

So. So so anyways, OK, so brushes DC motors. OK, so of course brushless DC motors. You have to have a way to switch currents, you know from the applier, through the motor, to make the motor to make the motor rotate, provide you know velocity or torque. OK, traditionally this is backing up a bunch of years. Once again, you're a couple 100 years back when I was a kid.

So you know you you see? Yeah. Brushes DC more as you see a brushless DC motors on the tail shaft. You would see a a magnetic drum.

And three hall effect sensors. OK, those hall effect sensors and drum would provide the commutation information for the for the drive such that it could switch properly, switch the phase current through the motor to control speed, direction or torque of that motor.

And and and and just as a side note, you know commutation. You know the commutation signals are usually A3 phase commutation. A backward brushes motors. Really, those commutation signals a lot of times are called UVW in the business. Are really an absolute signal. OK, they're an absolute signal over a small portion of rotation of the of the motor. OK, because it needs absolute position to switch the face currency through the motor or when the face switch phase current to the motor so.

Now going further ahead, hearing open the old traditional design of you know motor, you got the you got your your commutation stuff on the back of the motor. Then you'd see it on incremental encoder on the back of the motor to provide your position information for the control system. OK. So you're moving forward from that point. You started to see as encoders improved and we're able to provide more information. You might be able to see any commander that would have the A, Quad B with index position, traditionally incremental signals plus the three phase. Permutations they're taking now. That's a functionality of the magnetic drum and three hall sensors, you know, essentially putting that functionality inside the encoder. So. So then you know, remove that portion from the motor. So you'd have a relative encoder with commutation signals. OK.

Well, now that that that that you have a a kit encoder that's absolute. That's magnetic that's that's you know. Yeah. Comparable in price to maybe an incremental encoder or or even more comparable to A to a hall incremental encoder.

You can place that on a backroom motor. OK, you can use the single turn absolute position information to provide your commutation points to your to your drive. OK, so you can control your drive.Uh, so it can switch the the the phase current motor so you can control direction rotation. I mean rotation velocity and torque of the motor. But you can also use the absolute information for your position feedback.

So it becomes an absolute control system. And using the absolute Control Center, you're absolutely no functionality to single turn. You can control your phase currents. You know we have, we have quite a few applications go in that direction. You know, with our with our kit encoders. Or speak.

Mike Bacidore: Great information. Wow. I think in corners are probably one of the more. Well, probably more. One of the more misunderstood and.

Jim Stevens: Correct.

Mike Bacidore: Definitely one of the more utilized components with any within any type of motion control system for sure that you have, you know, absolutely critical in. So many applications. Uh, and I think you've brought a lot of insight into. The latest technology and the way that's affected, price and applications within industrial settings of nothing else.

Jim Stevens: Yeah, yeah. Excellent.

Mike Bacidore: So thanks for joining us today. Jim, really appreciate your insights.

Jim Stevens: Oh my you know, my my pleasure is is it's always fun to talk shop right? So this this is a great opportunity.

Mike Bacidore: Yes. Yeah. Looking forward to doing this again.

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