EME 812
Utility Solar Power and Concentration

3.5. Engineered devices for solar tracking


3.5. Engineered devices for solar tracking

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Image credit: Afloresm via Flickr

The main elements of a tracking system include [Rockwell Automation, 2011]:

  • Sun tracking algorithm: This algorithm calculates the solar azimuth and zenith angles of the sun. These angles are then used to position the solar panel or reflector to point toward the sun. Some algorithms are purely mathematical based on astronomical references, while others utilize real-time light-intensity readings.
  • Control unit: The control unit executes the sun tracking algorithm and coordinates the movement of the positioning system.
  • Positioning system: The positioning system moves the panel or reflector to face the sun at the optimum angles. Some positioning systems are electrical and some are hydraulic. Electrical systems utilize encoders and variable frequency drives or linear actuators to monitor the current position of the panel and move to desired positions.
  • Drive mechanism/transmission: The drive mechanisms include linear actuators, linear drives, hydraulic cylinders, swivel drives, worm gears, planetary gears, and threaded spindles.
  • Sensing devices: For trackers that use light intensity in the tracking algorithm, pyranometers are needed to read the light intensity. Ambient condition monitoring for pressure, temperature, and humidity may also be needed to optimize efficiency and power output.
  • Limit switches are used to control speed and prevent over travels. The mechanical over travel limits are used to prevent tracker damage.
  • Elevation feedback is accomplished by either 1) a combination of limit switches and motor encoder counts, or 2) an inclinometer (a sensor that provides the tilt angle).
  • An anemometer is used to measure wind speed. If the wind conditions are too strong, the panels are usually driven to a safe horizontal position and remain in the safety position until the wind speed falls below the set point.

Three classes of tracker drive types to operate the moving receiver:

  1. Passive trackers use the sun's heat to expand the compressed gas, which is used to move the panel. Selective heating of some cylinders versus others create more expansion on one side of the panel and make it tilt. These systems are relatively simple and low-cost, although may lack due precision necessary for the solar conversion systems using concentrated sunlight.
  2. Active trackers use hydraulic or electric and an actuator to move the panel based on sensor response. Light sensors are positioned on the tracker at different locations for higher precision. These systems work best with the direct sunlight and are less efficient with cloudy skies.
  3. Open-loop trackers use pre-recorded data on the sun position for a particular site. Simple timed trackers move the panel at discrete intervals to follow the sun position, but do not take into account the seasonal variations of the sun's altitude. The altitude/azimuth trackers employ astronomical data to determine the position of the sun for any given time and location.


Linear actuators are common technical tools that proved to be effective solution for moving the solar receivers. An electric linear actuator is a device that converts the rotational motion of an electric motor into linear motion. With linear actuators you can lift, slide, adjust, tilt, push or pull objects of various mass, and they are easy to implement in many different applications. Mechanically, linear actuators are quite simple devices that have been extensively deployed in 2-axis and 1-axis trackers due to their precision and service reliabilty.

The following video provides a rather detailed overview of the design, principle of operation, and specifications of electric linear actuators:

Video: Linear Actuators 101 (19:43)

Credit: Robert Cowan. "Linear Actuators 101." YouTube. June 24, 2018.
Click here for a transcript of the Linear Actuators 101 video.

Hello, everyone. In this video, I'm going to be talking about linear actuators. Linear actuators are really cool. And in this video, I want to explain what they are, how to use them, how to drive them, and how to pick them based on the various specs for your application. So, let's get started.

So, first off, what is a linear actuator? Well, really, linear actuators are just motors, but instead of moving around in a rotational direction, they move in and out along a linear plane. So, a normal motor would spin around like this. It just kind of spins, and it spins indefinitely. If you apply voltage one way, it goes this way.

If you apply it the other way, it goes the other way. Whereas linear actuators, when you apply voltage to the motor, the shaft here either goes out this way, or if you reverse it, it goes back the other way. Ultimately, you drive them in the same way that you would a standard DC motor. If you go positive to positive, negative to negative, it will go usually out. And if you flip the polarity of those around, it will go the other way.

I will be taking one of these apart later so that you can kind of see how it works. But as you can see, each one of these basically has this big motor on the end of it. Right there, right there and right there. And that motor spins around, essentially a lead screw, which pushes that lead screw out or pushes it back in. Now, with standard rotational motion, there are no end stops.

It kind of just keeps spinning indefinitely. But when you have something like this, there is an end stop to where it can hit the end on the extended travel, or it can come all the way back in. And there's a couple of little mechanisms that stop that from basically causing damage to the actuator. But that's really all there is to it. You can use these in any application where you want linear motion rather than rotational motion, like lifting up forks on something.

They're used a lot of times in combat robots for lifting up, clamping down. Or I used it in my little wheelchair snowblower thing for actually lifting the blade up and down. They are really good for any kind of linear motion. Just like with standard DC motors, there's a lot of various sizes and configuration to linear actuators, but ultimately, they all pretty much do the same thing. They provide linear movement.

There are a couple different variations that you need to look out for when purchasing a linear actuator for your application. If we look at the end of this service city one, we see that there are five wires coming off the end of it. And if we look at these two that I salvaged from the wheelchairs, there are two wires and two wires. Now, the difference is all of these can be driven just like a standard motor. You apply voltage, it goes out, you switch the voltage, it goes back in.

However, this one actually has a feedback mechanism inside of it. What I was saying earlier about the shaft reaching the end of travel, on either extreme, all of these have some sort of protection mechanism or some sort of electronics inside that stop them from overextending themselves. It's usually in the form of just a diode, so that when it goes out to the far end of its reach, the diode will stop current from flowing in that direction, and then you can only reverse it back the other way. Almost every linear actuator has that. There's probably some exceptions.

I'm sure there's some really cheap ones out there that don't have that, but generally speaking, that's a feature of a linear actuator. So, what does the servo city one have that is different? And this isn't unique to servo city, by the way. It's just the one that I have here that has the feedback mechanism inside of the base of this. When I open up, you'll see it in better detail.

There is essentially just a potentiometer that is connected to this output shaft, and it is, I guess, wired proportionately so that when this shaft is all the way in, that potentiometer is at one end of its range. And then when this shaft is all the way out, it is at the other end of the range. And this happens to be a ten k potentiometer. So here it's going to read zero, and all the way out there it's going to read ten kilo ohms. So this is very useful when hooking up to a motor controller or something else to determine pretty much where this is in the travel.

Now, with both of these, they were for a wheelchair, I think this was like the power lift for the seat, and this was like a tilt for the seat, something like that. You're less concerned with where that is. And in some applications, you might not really care where this shaft is. You just kind of want to move it forward and backward, much like a standard motor. You just want to move it at a certain speed in a certain direction.

So that is something to look out for, is some of them do have some kind of feedback mechanism, and others do not. You might not need it, or you might need it. The other thing to look out for is some of these are actually linear servos. So, they take it one step further, and they actually take the motor controller and put it directly inside. So, you control it much the same way you would with a servo.

You just send it a pulse command, and then it travels wherever you dictate. So if you say, I'm going to give it this pulse, then it would go halfway, or you can go like 80%, 20%, whatever it is. And so linear servos are very different. Servo city does have a very good video that shows you the difference between a standard linear actuator, one with feedback, and a linear servo. So that's worth checking out.

And that is linked below. But that is something that you might want to look at, because not all of these have the exact same features inside of them. Driving linear actuators is pretty straightforward because they are just motors. So, you drive them the same way you would a standard motor. I have these leads connected up to my benchtop power supply over there, and this has 12 volts on it.

So if I connect one lead down here and then the other one up there, this will just start moving. There we go. So, it's moving in. And if I switch these around, this one up here, it will move out once I clip it in. Right?

And now it moves out the other way. As you can see, linear actuators are not the fastest thing in the world. There's a lot of power. There is absolutely no way I could stop this from moving, but they are relatively slow, and I will get into that a little bit later. But the speed at which these move is one of the things that you should factor in when you are picking these out.

So if we look over at the Servo city, we have all these five wires coming out. We can just ignore these three for now, and then we can connect it and control it the exact same way. I'm just going to go red to red and black to black. This one is a little bit zippier, and then we can reverse it and it will go the opposite direction. So, it's really that simple.

And some of these actually do come with a little toggle switch that is pre wired for these. But if you don't have that, or if you want to make your own, you can use any dual pole, dual throw switch. Basically one that has six connections like this. You wire a power supply into the middle. One side is one polarity, and then the other side is a different polarity.

So let's say this is red from the supply, this is black from the supply. You would just go red and black directly to the motor, but in this side, you would actually flip them to the other side. And I will have a link to a wiring description down below if that's confusing. But you're going to want a motor that is not latching like this one. You're going to want kind of a toggle that goes like a normal toggle switch.

This isn't the right one to use, and I didn't have one on me, so I won't show that. In addition to using just a simple switch, which will just go full speed one direction or full speed the other direction, you can use a motor controller. The main difference between using just a switch directly up to a power supply and a motor controller is with a motor controller like this, Roboclaw is, you can actually vary not only the speed, but the direction on the fly. This you can connect to a microcontroller. You can connect it to all sorts of other things.

And this is what I'll be using for my application. But the real big difference is controlling the speed and other parameters. This can actually also accept the encoder feedback, and we can do all sorts of fun things. But for basic control, you really don't need anything, really, beyond a switch and a power supply. So they're pretty easy to drive.

Now it's time to disassemble this, dissect it, and show you what's going on inside the linear actuator. I'm using the servo city one only because it's actually the most well laid out and the easiest to get into. So let's just start taking it apart. There's three screws at the bottom of this, so I'm just going to go ahead and take those out.

You can see we've got some plastic gears around here, and at the very bottom, you can see that is our potentiometer. Now, don't fear, these plastic gears actually have nothing to do with the driving of the actuator. This is just for the feedback side. So I'm going to take this off, take this shell off, and then we can actually see the gears that are driven by the motor directly. But this is, I want to say, maybe probably a five turn or ten turn potentiometer.

And it spins directly with the output shaft, and that gives you the feedback that ends up going on these main wires. So, it's a pretty simple little mechanism that as this main output shaft turns, it also turns the potentiometer. And then you just read the potentiometer. So, nice and simple. All it.

So, now that we've got this open, there's yet another gasket here. And then you can see all the output gears. Let me see if I can get a little bit better shot of that. So you've got the motor right there, and that is a plastic pinion gear. And then it moves into all the metal gears right here.

So this particular motor comes in a couple of different configurations. And ultimately, the difference between the stroke or how far this travels out is really just going to be the gear reduction here, because the pitch of the linear rail inside here, all of that good stuff. But this is just the gear train that slows down the motor and drives the linear actuator inside this shell. And you can see you've got a lot of decent gears in here. There's a lot of nice oil.

So awesome. That is pretty cool. So that's really all there is to it inside, it's just basically a really simple gearbox that drives a linear shaft over here. So, let's take this outer shell off, and then you can see what's inside of here. It.

So there you go. There's a little gasket that goes all the way at the bottom down here. Let's get that out of the way. And then you can see we've got two little micro switches. One here and then one here.

And then we have a couple of diodes in place. Now, this one does not appear to have adjustable end of travel. So, basically, as this comes down, it will hit against there. And as it goes out to this side, it will hit against there. So, it's just this little nub or this little piece right there that travels up and down and is hitting your end stop.

So, pretty cool. Some of the higher end ones actually do have adjustments. So, where you can kind of slide these along and you can have different end stops. However, this one does not have that. So you would need to rely on software to do that.

And if you do use something like the roboclaw, which I misplaced, if you use something like this roboclaw, you can actually put that into the software in here and configure that separately. And then right here, you can see we just have a simple lead screw right there, and it's all nice and greased up. And then this just simply slides along the lead screw. So when the motor turns, it turns these gears, it turns this and that, either presses this out or brings it back in. It is really that simple.

There's not a whole lot to it. And then this is just a cover to protect the whole thing. So, yeah, that's all there is to a linear actuator. Let's put this back together. Okay, so we've talked a little bit about what a linear actuator is, how it functions.

Taken a look inside and I've kind of covered the very basics on how to control them, either directly with a power supply or just wiring them up to a switch. If you want to take this further, I will be doing another video as a supplement to this that shows you some of the more, I guess, advanced parts of controlling these with something like the roboclaw or another type of motor controller. That's a little bit beyond the scope of this video, but I will be coming out with that separately because I am working on my own project using a couple of these linear actuators. So, the next thing that we need to talk about is how to spec them for your project. As I discussed earlier, linear actuators are essentially just motors, and you can treat them the same way in terms of specking them out.

For your project, you're going to need to pay attention to the voltage that the motor runs at and the current requirements behind that. And really you can run these at different voltages and that's a larger discussion. But essentially the amount of voltage that you put into a DC motor is going to proportionately relate to how fast it spins. However, if you over voltage, a DC motor apply more voltage than it is speced at, you will lessen the lifespan of that motor. So keep that in mind.

And a lot of these have a duty cycle rating that is only like 25%. I. E. This is not meant to run 100% of the time. It is spec to run 25% of the time.

Now, the big difference in driving these motors with just a power supply directly and driving them with a motor controller like, let's say the roboclaw is going to be being able to vary the speed. If you put 12 volts into this motor, it's always going to run at that exact same speed, either forward or backwards. It's always going to be the same. If you use a motor controller, that gives you the opportunity to control it at different speeds. And that's really the only reason why you would want to use a motor controller, is to vary that speed of how fast the shaft moves.

There are a couple specifications for linear actuators that are unique to them, and you won't find them on a normal DC motor like the voltage and the current. You're going to want to pay attention to the stroke. The stroke is probably the most important part of the linear actuator. And it basically means how far the shaft can travel. It's a total travel, not necessarily the total size of it, but just how much it can travel.

This one is a twelve inch, meaning when this is all the way in to all the way extended, it moves twelve inches. These are somewhere around like six inches. So definitely pay attention to the stroke. The other spec that you're going to want to pay attention to with a linear actuator is going to be the load rating. There's going to be two load ratings.

Usually there's going to be the dynamic load and the static load. The static load is quite simply how much force can you put against this before it will fail statically? So let's say it was just sitting there like that and it had a load on top of this. This one's rate, I think like a 500 pound static load. It means it can just sit there with 500 pounds resting on it without failing.

The dynamic load is how much force it can actually exert on the thing that you're trying to use it with. I want to say this one's rated like 100 and 5175 pounds. So that means it can actually press or exert a force of 175 pounds. So that is the difference between a static and a dynamic load. The last thing that you want to look at is the speed at which these things move.

Usually it's some kind of like inches per minute or something like that, and it's going to be how fast it can move. That's something that you really want to pay attention to because this has a twelve inch stroke. Let's say it was one inch per hour. That means it's going to take 12 hours to fully extend from fully non extended. So that is something you really want to pay attention to.

Now, when you look at these linear actuators, you will see that there's usually multiple variations of the same thing. For Instance, with this model, there are, I think, three different versions of it, and they're all the exact same price. One of them has like a 50-pound loan rating. One's like 100 and 5175, and the other is like 500. Let's say.

Why are they all the same price? Why wouldn't I go with the one that's like rated 500 pounds? Well, there's no free lunch here. These all have a fixed amount of power that they contain, and power is equal to work done over time. So guess what?

That one that has the really high load rating is also going to be incredibly slow because it is the amount of work done over time. You're doing it in a lot longer time, so then the work can be a lot higher. And likewise, on the other side of the spectrum, a really fast one just can't do as much work because it's doing it in a much shorter period of time. So that's something you want to pay attention to when you're specking. These is not only the stroke, which is how much they can move, but how fast they can move.

And then of course, you want to pay attention to the static load rating because if you're trying to lift something really heavy, but something's pushing against it, you want to make sure that it can handle those static forces as well as the dynamic ones. So I think that's about all I wanted to talk about with linear actuators. I think the last point to talk about is where to find them. These two were salvaged from an electric wheelchair. I do a lot of electric wheelchair salvaging because there's a lot of good parts in there.

Look for some that have a dead battery, dead charger, dead controller, and you can usually find a linear actuator in there. If they have some kind of tilting seat mechanism, usually the fancier ones do. Just keep in mind the voltage is almost always going to be 24 volts and they are going to be a weird form factor that might be really difficult to use. And typically, they have a very short stroke and they are very, very slow. So there's a lot of caveats to getting them free out of a wheelchair or really inexpensive out of a wheelchair.

But you can find them on eBay. There's a lot of sellers that have them on eBay, and there's a lot on Amazon. Just keep in mind, the ones on Amazon and eBay tend to not have any feedback. They're just two wires. Some of them do have feedback, but they tend to start getting kind of pricey.

And sometimes the load ratings are not really exactly what you're looking for and they have limited strokes, stuff like that. Also, you might want to check out Servo City. Servo City has a really nice selection of them, and I'm not just simply plugging them. They really do have a nice selection of linear actuators in all shapes and sizes. So it's at least worth a look to see what's available for your project.

As always, thanks for watching this very long and informative video on linear actuators. You can check out my Facebook page for all my little project updates and such. And down below there's a lot of links that you can check out. There's an Amazon link down below that. If you use and shop with Amazon using that link, you can give a little bit of a kickback to my channel to help support my projects and my videos.

As always, thanks for watching. See you next time.

The technical details of all the components of tracking systems would be beyond the scope of this course. It is important to understand though that additional components and more complexity, while improving efficiency of the solar panels and reflectors, add to the cost of the whole system and consume additional energy.

This following video (4:25) demonstrates some technical features of a single-axis tracking system:

Video: Renewable Energy: Single-Axis Tracker (4:24)

Credit: Snmsuaces. "Renewable Energy: Single-Axis Tracker." YouTube. February 14, 2012.
Click here for a transcript of the Renewable Energy: Single-Axis Tracker video.

THOMAS JENKINS: Here we have an example of something that might be at more of a commercial application of photovoltaic systems. We have several photovoltaic panels. We have two sets, we have one type on this structure, we have another type on the structure behind it. Both are mounted on what's called one-axis trackers, in that there are electric motors which turn these panels such that they track the sun as it goes from the east to the west.

With this type of tracker, it's fairly sophisticated in that it's a computer-controlled tracker. There is a little computer in here that runs some very sort of mid-complexity algorithm that knows the latitude of your location, where you are-- Las Cruces, New Mexico, 32 degrees latitude-- and it knows the day of the year-- for example, January 28, day 28-- and it knows the time of the day-- 2:00.

With that information, it can predict exactly the angle of the sun, relative to east and west, and it knows to turn the tracker exactly that many degrees every day to point directly to the sun. This increases the efficiency or the amount of electricity that comes from the solar panels, but you have some additional complexity in your system. These are what's called active trackers, in that it requires electrical motors, it requires some mechanical components, some electrical components, and it tracks the sun, but you get more electricity from this type of system.

This system is a German design, and it's being tested here at SWTDI. Right next to it, we have a good bit of data collection that is brand new, very sophisticated, and it's connected via cell phone and land lines such that all the data-- which is being collected real time-- can be accessed through a cell phone from anywhere in the world. For example, at the headquarters of the German company, who's looking at the system design and the system components and seeing how they're interacting.

You can see on this structure here that we have a couple of instruments that are being used to characterize the sun's energy. These are called pyranometers. They are reading the amount of sunlight so we know how much energy is striking the surface. And, from that, we can see how much energy the panels are delivering to us and determine the efficiency of the panels. So a lot of instrumentation is going in this because this is an evaluation system, but this might be a system that might go into, for example, a desert environment over several acres that might be a large scale electrical production.

Tracking the sun, in some cases, is very important, especially on systems that use a new type of system with lenses, called Fresnel lenses that are used to concentrate the sun onto a smaller section of photovoltaic, so the total amount of photovoltaics that you need is smaller, but they produce the same amount of energy because you're concentrating the sunlight onto the cell. So we're looking at two different types of modules on the same structures, under the same ambient conditions, the same location, the same amount of sunlight, and we're comparing those, seeing how efficient they are relative to one another and relative to traditional solar photovoltaic systems.

PRESENTER: The preceding was a production of New Mexico State University. The views and opinions in this program are those of the author and do not necessarily represent the views and opinions of the NMSU Board of Regents.

Additional Reading

Journal paper: Mousazadeh, H. et al., A review of principle and sun-tracking methods for maximizing solar systems output, Renewable and Sustainable Energy Reviews 13 (2009) 1800–1818.