Hi!

I'm really interested in your project! I'm a engineer in computer science and robotics, and in parallel, I'm going to run a workshop on building solar ovens at a recycling center. I'd love to base the workshop on your project and learn more about it.

Would it be possible to get your contact information so we can communicate further if you're open to it?

Looking forward to your reply!

Best regards, Julien (email in bio)

Cool work! I do research in nonimaging optics, the optics of achieving high concentration ratios (or wide tolerances to errors) in solar concentrators.

I like that you are implementing closed-loop control. This is all the rage also in large-scale heliostat fields. Most traditional heliostats are controlled using open loop, which places very strict requirements on both the mechanical structure, the actuators, and on the kinematic model, leading to expensive and very stiff heliostats. People are therefore moving towards cheaper heliostats where the tracking precision is achieved through closed-loop control. Implementing closed-loop control is a little bit more tricky when you have overlapping focal spots from thousands of mirrors, but there are approaches that are being developed, e.g. having cameras around the target looking back out over the heliostat field (developed by Heliogen among others).

You mention the challenge of light only being focused for a few hours per day. This is also a problem with large helisotat fields, and is also a field of active research. There's a group at University of Arizona with Professor Roger Angel developing heliostats that actively deform through the day to keep the perfect shape, and there's also an Australian company (Heliosystems) building heliostats that passively deform from gravity to keep as correct shape as possible.

When you are only using a single heliostat, as in your project, you could also consider building it as a Scheffler reflector - placing it on a single-axis polar-aligned tracking axis that passes through your target. Then it only requires single-axis tracking through the day, with some (possibly manual) seasonal adjusting.

I am very happy to see that you are highlighting the inherent risks in concentrated sunlight. There are lots of stories about people accidentally settings stuff on fire if the tracking doen't track correctly.

Thank you very much for your kind and detailed reply!

Indeed, the closed-loop control was the initial idea which convinced me that it would be possible to build the mechanical parts by hand with common tools. In other words, the software "smartness" compensates for the mechanical "ugliness".

Another initial idea was to do multi-panels (several orientable panels) with a single camera looking at the target. Indeed, it's not easy, so I finally went back and decided to finish and release something with a single panel.

Nevertheless, I have some ideas to do multi-panels with a few more cameras. I would like to work on them in the near future.

Thank you for all the references, I will spend time to explore them.

There is also a company that uses vacuum to adjust the mirror shape, I'll try to find it and post it here.

I wanted to emphasize the inherent risk because my project is not a finished product, but a work-in-progress/proof-of-concept.

FYI, this is the "vacuum adjustable focus mirror" I mentioned:

https://lm.solar/order/4-square-rigid-aluminum-composite-mir...

Yeah exactly!

I like your cable-drive concept by the way. Did you describe it in more details anywhere? Heliogen were also developing a cable-drive system for their commercial heliostats, but I don't know if they are still working on it.

How are you getting the right mirror orientation for each mirror (aka canting)? Custom spacers for each mirror?

One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver. When they look back at the mirrors, they will see the circumsolar radiation (how the sky gets brighter as you get closer to the sun). By comparing the brightness of the sky at different cameras, you can estimate which cameras is "closest" to seeing the real sun, and get an estimate for the real position of the sun.

> I like your cable-drive concept by the way. Did you describe it in more details anywhere?

I didn't take the time to describe the cable-bot concept in detail.

I modeled almost all the mechanical parts with OpenSCAD, but I struggled to model the cable itself.

In the "mechanics" README [0] you can click on any image to view it in an online 3D viewer.

The following note in the same README tries to explain how the cable is used :

    Each cable is actually wrapped around the motor axis, then passed through the pulley and tied to a fixed ring in the corner of the panel.

English is not my first language, is this sentence clear enough ?

You can see these elements in the main 3D viewer [1]

> How are you getting the right mirror orientation for each mirror (aka canting)?

I use one bolt that pulls the mirror holder in the center and 3 bolts that push it in the corners.

By screwing or unscrewing the corner bolts you can precisely orient each mirror independently.

The "panel_board_exploded" view tries to show this [2]

> One trick for closed-loop control with many heliostats/panels is to have a few cameras surrounding the receiver.

Super clever, thanks for the explanation!

I think it might be tricky to calibrate.

[0] https://github.com/remipch/solar_concentrator/blob/master/me...

[1] https://remipch.github.io/solar_concentrator/view_3d.html?mo...

[2] https://remipch.github.io/solar_concentrator/view_3d.html?mo...

> Each cable is actually wrapped around the motor axis, then passed through the pulley and tied to a fixed ring in the corner of the panel.

Thanks, when seeing the video again now it makes sense! I didn't catch the counterweight the first time I saw it. Nice! In the Heliogen concept I mentioned previously they got around having to use a counterweight by attaching the other side of the cable to another part of the panel, such that the cable length stays approximately constant. Then they used a spring to compensate for the small changes in cable length that are inherent to the geometry.

> I use one bolt that pulls the mirror holder in the center and 3 bolts that push it in the corners. By screwing or unscrewing the corner bolts you can precisely orient each mirror independently.

Nice! Even in large heliostat fields it is often done in a similar way. It becomes quite labor intensive when you have thousands of heliostats in a field, with 10+ segments each, so there are ongoing efforts to find ways to do it automatically or to get around the need for doing it in the first place.

I wonder if you could set up 3 multi panels with a known pattern of red green or blue gels on each, then filter the camera image by color to identify which of the panels is pointed where so you can adjust them individually?

With the low-cost camera used here, the small red/green/blue pattern would be indistinguishable because the whole spot is saturated.

It might be possible with an expensive camera.

Speaking of risks and fire, is there a known limit to the temperature achievable by concentration? I was wondering if I could melt a piece of tungsten with this method.

Yes, and it is super-interesting!

The fundamental limit is given by the 2nd law of thermodynamics - you can never reach higher temperatures than the surface of the sun, or around 5800 K. We have the atmosphere that absorbs and scatters some of the light, so on the surface of the earth it is a bit lower, but not by a huge amount.

This means that there is a fundamental limit to how small and intense you can make the focal spot in a solar concentrator. The limit is around ~45 MW/m² or 45000 "suns" (which is plenty high, but far from infinite).

Concentrators used for eletricity generation use much lower concentration than this, on the order of 25 suns to 1000 suns depending on the type. There are also solar furnaces designed for reaching much higher concentration by using a different type of optics. The most impressive one is the huge Odeillo solar furnace [1]. I would guess that they could melt tungsten, but I have not actually run the numbers.

I did a talk last week about a concept we are developing for reaching furnace-level concentration ratios with conventional heliostats [2].

[1] https://en.wikipedia.org/wiki/Odeillo_solar_furnace

[2] https://folk.ntnu.no/haakonjj/talks/2024-08-19-nonimaging-fr...

The thing that I can't wrap my head around is that if the concentrator "pumps" power into an object, and say you can somehow insulate it to stop the losses, how is this limit not unbounded? Where does the energy go once we reach the cap?

Does the black body radiation send the energy back out?

> Does the black body radiation send the energy back out?

Exactly, this is the issue. If an object is able to absorb sunlight, it is also able to emit blackbody radition back towards the sun. When the temperature limit is reached, these two exactly cancel each other. The object will emit blackbody radiation with the same brightness as the surface of the sun.

Another way to look at it is to imagine yourself standing at the center of the concentrated sunlight and looking out towards the concentrator. The concentrator makes the sun look "bigger" from your perspective, and this is what makes the sunlight concentrated. The limit to this effect is if the sun fills all directions in the whole hemisphere above you. Now it will be as if you are standing on the surface of the sun, and all you can see in any direction is sunlight. Normally, the solar disc fills 1/45000th of the hemisphere above you here on earth, thus the limit of 45000 suns concentration.

Thank you so much. It's the first time I do understand the _why_ of that fact.

But I could build up a lot of solar panels and use the electricity to heat up an oven more than the surface of the sun, right? Is that "cheating" in terms of thermo dynamics?

> I could build up a lot of solar panels and use the electricity to heat up an oven more than the surface of the sun, right?

Yes, this would be like using a hydroelectric dam to power a fountain that sprays higher than the initial reservoir. Machines can convert a large amount of low-quality energy into a small amount of high-quality energy, even when passive components (e.g. mirrors or pipes) cannot.

So what is a "passive component"? I guess, a water wheel that drives a pump that pupms water way above the surface level would count as active?

What about a material that absorbs photons and emitts them at a higher energy level (emitting one after absorbing two)?

Great question, and this shows why we could never get a 100% efficient solar panel. Otherwise your scheme would brak thermodynamics.

The most efficient possible way to convert sunlight to electricity is ~86% and is related to the second law of thermodynamics. So we use the heat flow from a hot reservoir (sun) to a cold reservoir (earth) and are able to convert some of that heat into work (electricity) which can then be used to heat something else to a higher temperature without breaking the second law.

That's a very insightful way to look at it, thank you!

ps. It's like making a VPN tunnel to the surface of the sun :-)

> ps. It's like making a VPN tunnel to the surface of the sun :-)

Wow, I love that! Great analogy!

Nice explanation, thanks

Yes...in the situation where somehow the oven became hotter than the surface of the sun, then the oven would start heating up the sun.

Wikipedia says a max temperature of 3500C, which is above Tungsten's melting point. Graphite is the only thing I know of with a melting point above that at 1atm, but I'm not a chemist so I'm sure there are other things.

Check out: https://what-if.xkcd.com/145/

I think it depends on the mirror area. The more mirrors, the more power. The more power, the higher the temperature will be.

Anyway, tungsten melts at 3422°C, I don't know if it's feasible.

Imagine we built a giant concentrator in space so we can melt tungsten without having to worry about containment!

Platinum was first melted using concentrated solar iirc.

> with some (possibly manual) seasonal adjusting.

Any pointers for floating PV seasonal mechanism?

With a 5m high prism, and panels along ONE face, how do you get seasonal adjustment of the panels tilt angle.

An adjustable/inflatable ballast seems the simplest?

Awesome project OP! Especially the power comparison. Who would have thought that you can achieve 1kW of energy from 1m2.

On a side note and in a similar direction. Would it be feasible to make a solar concentrator that heats a molten-salt reactor that powers a turbine engine? On a small-ish scale though, such that it'd be achievable as a back-yard reactor?

So the description I used above was my memory-driven understanding of it. But here is what I actually meant: https://en.wikipedia.org/wiki/Solar_power_tower

Edit. I went down a little rabbit-hole, HN. This is what I eventually found about small-scale energy generation using solar-concentration.

https://en.wikipedia.org/wiki/Solar-powered_Stirling_engine

https://en.wikipedia.org/wiki/File:Dish-stirling-at-odeillo....

Could be a semi-viable alternative to solar, perhaps? Though cost-wise, it's probably quite high now that solar-panels and their auxillary hardware have been commoditized so much.

1kw/m^2 is the "standard" rule of thumb for heat energy from solar irradiation. I'm unsure whether OP has done the calculation (in which case, credit to a well built system) or simply cited the standard rule.

There are some large Stirling engines out there that operate on hot oil. With a large diameter piston, quite a bit of torque can be generated with even small ∆T. Oil can be heated as with traditional solar water heaters (i.e., with no concentration), though concentration doesn't hurt.

I have developed a simple simulator [0] to estimate the theoretical power received by the target for a given hardware configuration:

- the global position on the planet

- the date and time

- the size and position of some background elements

- the number, size and position of the panels in the grid

The solar power estimation uses :

- the Python code provided in this article [1] to estimate position of the sun (thank you John Clark Craig)

- the simplified formula [2] to estimate the direct insolation from the sun position

- a custom light projection implemented using Panda3D game engine [3]

[0] https://github.com/remipch/solar_concentrator/blob/master/so...

[1] https://levelup.gitconnected.com/python-sun-position-for-sol...

[2] https://en.wikipedia.org/wiki/Direct_insolation#Simplified_f...

[3] https://www.panda3d.org/

Yes, there are several industrial applications that use a solar concentrator to drive a turbine engine or a Stirling engine.

I'm not sure it's a viable way to produce electricity on a small scale because:

- high thermodynamic efficiency requires high temperature difference

- photovoltaic panels are mass-produced and increasingly efficient

Personally, I think small scale concentrated solar power is most useful for applications that require direct heat (cooking, desalination, foundries).

In these cases, photovoltaics have a lower efficiency and a shorter lifetime.

My undergrad senior year thesis project was exactly this, back in 2011 - to use a solar concentrator to generate electricity. When we started the project, we thought the economics were cheaper than solar PV. At the end of the year, solar PV had already halved in price (this was when Germany and China were doing subsidy wars on solar).

Solar concentrator electricity must be super expensive compared to PV by now.

Sterling engines might be a bit too much maintainance to make it worthwhile compared to solar panels?

A engine wont run for years without part changes.

Solar panels are really cheap. Like, comparable to a mirror of the same size cheap.

A 15cm x 15cm mirror used in this project cost 1€.

That's 48€/m2, I couldn't find a photovoltaic panel at that price.

Add to that:

- photovoltaic efficiency is about 20%, while such mirrors reflect 90% of the energy

- photovoltaic panels have an average lifespan of 20 years, while mirrors do not wear out.

Anyway, we're comparing apples and oranges, because we have to add the mechanical installations, which are very different depending on the specific application.

I'm not against photovoltaic in general, I just think that for some applications there are some interesting alternatives.

> That's 48€/m2, I couldn't find a photovoltaic panel at that price.

Still apples to oranges but, $68 for > 2sqm at the factory, probably closer to $100 at retail:

https://www.alibaba.com/product-detail/144cells-Jinko-Solar-...

And in Europe: https://venturama-solar.de/produkt/ja-solar-jam54s30-425w-lr...

Oh mirrors do wear out...

Oh yes, they do. I can be a little naive sometimes :-)

I still naively think that we could make mirrors completely encased in glass to limit their degradation (pure speculation here).

Which would make the cost of each mirror higher, right? Additional processing will naturally increase the cost per unit.

By the way, super cool project and thank you for sharing. My experiments with concentrating sun power when I was a child were directly related to the spontaneous combustion of insects. Still making amends for the number of ant hills my brother and I cooked with the sun.

Concentrated sunlight is deceptively powerful. We probably all have played with small handheld magnifying glasses to focus the sun to a small spot, burning paper or small wood blocks.

When I was a kid I had a Fresnel lens, probably 2' in diameter, out of an old projector or some similar thing. It would set asphalt on fire. Almost instantly. You could probably weld steel with sunlight, though not very conveniently.

There was a project a few years back to demo 3d printing with solar sintering of desert sand: https://www.sciencedirect.com/science/article/abs/pii/S09596... - very neat idea.

Very interesting, do you know if there are any actual applications today?

I've not heard of it since, no. It's got some steep hills to climb as a concept before it's a better option for building materials than just pouring more concrete.

Not sure what those hills are, but it’s always surprised me that this type of hobby project hasn’t been ever been scaled up industrially.

Setting aside applications for moon base 1 and other sci-fi, there’s a lot of desert on earth. If the construction method is automatic or even semi automatic, and costs almost no energy, then who cares if it’s slow? A legion of robots that can’t even make other robots but can make glass bricks from sand seems like it could be paving the Empty Quarter one decade, finishing the glittering glass towers in the next.

I don't think we want to encourage more people to live in the desert.

See https://www.youtube.com/shorts/M0LUdqFJEPI

If you're going to start slinging youtubes at people, isn't it common courtesy around here that it should be cool machine-shops or tokamak reactor videos or something kinda educational instead of this awful attempt at comedy/memes? Get out of here with that.

You can also sinter sand into 3d structures.

https://www.youtube.com/watch?v=ptUj8JRAYu8

In theory if you could manufacture lenses and optical fibers, you could 3d print active solar powered structures anywhere where there is enough sun. Imagine a 3d printed little bottlegarden, with a water collector using a heated silicagel cycle printed in situ on mars. Still low atmospheric pressure, but warm and wetter.

Here's a huge solar concentrator that melts thick steel in seconds:

https://youtu.be/8tt7RG3UR4c

Action around 1:25

I can imagine this would be very useful for water desalination.

Excellent!

Do you know the mirror area used here?

Indeed; I've played around with various concentration methods (along with sun imaging using lenses). My US letter-sized Fresnel lens can easily start a tinder/kindling fire in a few seconds, and I've managed to melt a lot of things accidentally jsut by pointing the lens at the sun for a few seconds when the solar filter was not installed.

Lots of interesting experiments with solar collection here: https://www.youtube.com/@sergiyyurko8668/videos

Whaoo, super interesting, thanks for the link.

Some of their projects follow the same idea of a grid of small square mirrors.

However, they choose to put the mirrors on the ground (which is simpler) and move the target at the focal point (which is not simpler).

Good source of inspiration, I will watch their videos.

If you make a large (1+ meter diameter) curved lump out of wet sand, you can use that to lay a fiberglass parabola which could then be chrome plated, painted, or otherwise finished on the inside of the parabola after it’s cured.

This would allow you to further concentrate the solar power beyond the current 48x limitation.

Good idea, thank you!

However a 20cm x 20cm square spot can be better for some applications.

If you want to cook something it's best to spread the heat over the entire baking sheet to ensure even cooking.

For the specific use case of cooking, I think the problem you will run into is that solar power is inconsistent which is problematic for cooking via direct heat as you tend to need consistent temperatures.

So instead I would look at indirect heat. By super-concentrating the full parabolic area into a single point, you can heat cooking stones which will radiate heat more consistently even when clouds momentarily block the sun.

Using a simple store-bought pizza stone suspended at a 45-degree angle above your food, the homemade solar "laser" (said in Dr. Evil's voice) could be targeted on the underside, directly above the food.

Placing firebrick or other insulating stone directly on the opposite side of the pizza stone would help ensure that minimal energy is lost through the rear.

They've got active/closed-loop control of the mirrors.

Right now, it looks like it's just using the camera feed to aim the reflected bright spot into the centre of the fiducial markers to align it with the oven window. If you wanted consistency temps, you could over provision enough mirror, then use a temp sensor in the oven to intentionally aim some of the bright spot outside the oven window - "wasting" that solar energy, but it was free anyway. (Maybe put a solar hot water system collector right next to the oven window, so any heat not needed for cooking could be used to make hot water for cleaning up after cooking?)

Yes, this is exactly how I planned to control the temperature : add a temperature sensor in the oven and implement a simple PID in the supervisor to do so.

Decades ago, my brother used the inside of a large umbrella covered in aluminum foil. A small grill attached to the handle was used to grill hotdogs. Not sure how he had it mounted in the correct orientation though. In this case, imperfect surface shape may have been a good thing.

Old school satellite dishes too.

Yes I've seen a solar dish warmer with a satellite dish (manually adjusted).

It was clever.

This is so cool, thanks for sharing! If I had a yard/space to build one of these, I would totally try rigging one up to drive a little heat engine.

Thanks for your nice comment.

Yes it's definitely fun to build one and use that free energy from the sun.

I was particularly happy to eat my first solar gratin :-)

Hmmm. A solar espresso machine, with the sun heating the brew water and also a boiler to drive both the steam wand and the steam engine that runs the espresso pump!

(I'm mostly looking for an excuse here to stay in bed on bad weather days...)

Very cool. Curious if you have looked into non-imaging (anidolic) solar collectors? My understanding is that they are actually more efficient than mirror or lens based collectors as they do not require precise aiming and are able to collect indirect sunlight as well.

I wasn't aware of such systems.

If I understand correctly, their main feature is that they do not focus on a focal point, but instead diffuse the light.

So I'm not sure if it would be applicable to my project.

I need to dig deeper to fully understand how it works exactly, thanks for the hint.

Am I understanding correctly that the angle between each mirror segment and the backboard is fixed once during construction and then not dynamic?

Exactly

This has the advantage of requiring fewer motors (only two motors for the entire panel) instead of two motors per mirror.

The disadvantage is that the light is only focused for a few hours a day.

It is possible to make a mechanical angle bisector. Think of a compass for drawing circles but with a 3rd arm bisecting the angle. If you attach the bisector normal to a mirror, you can point one leg at the sun and the other at a target. An array of these would require linkages to aim all inputs parallel toward the sun and all outputs fixed toward the target - the outputs don't have to be parallel just have each pointing at the target. 2-axis actuation to move all mirrors as needed. Not sure if this has ever been tried, it's just an idea I had some time ago.

This is a really cool concept! The term used for connecting mirrors to rotate together is a ganged heliostat. Most papers about ganged heliostats don't do anything fancy like what you describe, but there are some patents that show a nice way of connecting the rods to give the correct angles using a slightly different concept than what you are describing [1]. I previously made a visualization of how that concept works (click and drag the sun) [2].

I've not seen exactly what you describe published anywhere, but it sounds very smular to something I thought about as well. See this 2d illustration [2]. Is this the same as what you are describing?

I had a masters student try to make it mechanically. It turns out that though it's an elegant concept, you still end up with quite a few moving parts so it's a bit tricky.

[1] https://patents.google.com/patent/US20060060188

[2] https://folk.ntnu.no/haakonjj/ganged_heliostat_illustration....

[3] https://folk.ntnu.no/haakonjj/ganged_heliostat_gear/

Nice interactive visualizations !

Yes, it might be complicated to build.

Another disadvantage being that your distance between the mirror system and the target must be fixed the same each time you setup.

This isn’t an inherently bad thing. As a marketable product this would imply a one-time manual alignment for all 48 mirrors but cheaper lifetime maintenance costs.

If you were going to sell a kit, having 48 sets of 3d printed mirror mounts of the right length for each position wouldn't be out of the question. Just a screw-on clip with the right length standoff for each mirror corner would be all you'd need. You could conceivably have different sets with different focal lengths.

I don't think it would be too fiddly done that way but it might take some work to make fine adjustments straightforward.

I remember something similar back in ~2000 - made out of a directv satellite dish.

It was dead simple. Mirror the parabolic surface of a satellite dish, put a hole in the dish, and use an optical sensor to track the beam of light that shone through that hole on the rooftop. Use a couple of motors and an IC to move the dish according to the suns' position.

Hopefully this isn't a stupid question as I know very little about solar, but could the risk/danger be reduced with some kind of diffusion layer behind the target to "de-concentrate" the light in the event of a failure?

The risk includes targeting the wrong thing, or stepping into the beam in front of the target. A non-flammable backstop for the target is kind of a base level safety measure.

The thermal 'inertia' of concentrated solar power may be cost competitive with PV solar and batteries.

You can use the mass of the concentrator target as a thermal battery effectively, as the conversation of sunlight to electrical energy is not instantaneous like solar PV.

Almost anything you can do with concentrated PV, I can do better with photovoltaic and a resistor immersed in the same thermal storage medium. It is wildly more versatile in the face of clouds and piping + insulation costs.

What do you mean by "better"? There are several aspects to consider.

Indeed the advantage of your photovoltaic solution is versatility in the face of clouds and lower mechanical costs.

But the drawback is the larger panel area, higher panel cost and probably shorter panel lifetime.

Photovoltaic efficiency is about 20%, while even low-cost mirrors reflect 90% of the energy.

It really depends on the specific application and the answer does not seem so obvious to me.

More cost reduction is to use mylar sheet.

Magnifique, I would like to try baking a pie with such an oven!

With regards to the efficiency of the motion system, it occurs to me that your system only requires an occasional movement rather than a rapid movement for tracking purposes and therefore could conceivably build up the charge required for such motion over time. Such a build-up is inherently suited to renewable (solar, wind or thermal-recovery based) energy harvesting rather than the approach of powering the system from a separate electrical supply.

Secondly, the motion system appears to be single-ended and based upon a rope and a stepper motor. It would perhaps be useful to consider conversion to a more rigid system. I would suggest removing the line entirely, though if you want to keep it one possible option is that of steel wire, which can be had in a range of gauges and metallurgies. These are relatively very strong and inexpensive. Furthermore conversion to a closed loop would be desirable, ie. ability to turn in each direction. However, I would recommend a geared motor and a rotary track cut or milled in to a heavy base plate as a simpler solution with less parts and a higher probable efficiency. Ideally you could add a line-based system in case the motor has no power or breaks or something so people could point the thing manually in a worst-case scenario.

The use of a video/optical sensor path is inefficient from a processing perspective. While this is a great way to prototype an initial version, you are going to see issues around the presumption of a flat surface, the need for establishing a visual datum based upon peripheral codepoints, and distance estimates. Toward a more reliable, elegant and configurable solution one might suggest that a system of curves be established. Each row could become a single dimensional array with its own curve controlled by a single actuator (eg. a line) and then balanced off with springs, graduated mounting points, or some other counter-force, in order to effect inward curve at the elements closed to the end points. A major array would then separately angle all rows along the alternate axis. Done carefully this may be adequate for close actuation scenarios, though there is surely tradeoff between rigidity, precision, mechanical and control complexity, and efficiency. Assuming short distances, I suspect a simpler-is-best approach would be adequate. This could be combined with the critical element of a laser TOF ranging sensor to determine the subject range, which would inform the required algorithmic adjustment of the array geometry. In short, if you have knowledge of your own geometry, have known orientation relative that fixed point, and can before activation determine the TOF distance to your target, and have a GPS fix thus solar inclination at that moment is calculable, then it should be possible to track the target without the need for visual feedback.

In terms of temperature sensing, you can obtain remote temperatures using infra-red linear systems which should be effective.

In terms of safety, laser TOF ensuring no change in distance would ensure the subject is still within the programmed range and an intermediate object hasn't been introduced for combustion. Also, adding an IMS would be cheap and effective to detect events like being knocked over/moved and misalignment.

In terms of portability, it may be useful to use a collapsible carbon fiber structure derived from those seen in modern tents. Make the mirrors interchangeable so the collapsed form is tighter, they can clip on and also be readily replaced. If a concern is that the structure becomes too flexible, then consider adding more triangular structural elements, and potentially some form of self-tensioning system with a sprung tie-down.

In terms of gathering interest, not sure where you are based in France but I could suggest structuring the system as a public sculpture and holding events including eating food cooked by the system which could involve the additional sponsorship of local wineries, cider, etc. to garner social support.

As next steps first I would recommend mechanical prototypes for a single line operated adjustable curve system. Then I would recommend an array of those with the perpendicular axis on a stand. If necessary this could be prototyped as a fixed curve initially. Next the revised and compact stand (line-free), incorporating perhaps solar recovery for slow-tracking actuation. Finally an integrated control system with GPS, laser TOF distancing, IR thermal, IMS and the novel safety features.

Smiles from Sydney. I applaud your work toward the application of technology to social and environmental concern, we need more of this.

Thanks for your kind and detailed reply.

> it occurs to me that your system only requires an occasional movement rather than a rapid movement for tracking purposes

Right, actually the system wakes up every 10 seconds to check if the angle needs to be adjusted. It often does not and waits for the next 10 seconds.

There is an opportunity to implement true "hibernation" while the system is waiting to save some more power, but it's not done yet.

> Secondly, the motion system appears to be single-ended and based upon a rope and a stepper motor. It would perhaps be useful to consider conversion to a more rigid system.

Actually there are two geared motors, allowing to control both angles.

Yes, I use simple ropes, the system is stable enough thanks to the counterweight (all ropes are always under tension, making the whole system stable).

> In short, if you have knowledge of your own geometry, have known orientation relative that fixed point, and can before activation determine the TOF distance to your target, and have a GPS fix thus solar inclination at that moment is calculable, then it should be possible to track the target without the need for visual feedback.

My project takes the opposite approach: using a low-cost camera board to avoid having to measure all the geometric aspects precisely.

> In terms of temperature sensing, you can obtain remote temperatures using infra-red linear systems which should be effective.

Good idea, I haven't implemented temperature sensing yet, but I'll look into infrared sensors.

> In terms of safety, laser TOF ensuring no change in distance would ensure the subject is still within the programmed range and an intermediate object hasn't been introduced for combustion.

It may be too late, the intermediate object/person is already heating up by the time we detect it.

> In terms of gathering interest, not sure where you are based in France but I could suggest structuring the system as a public sculpture and holding events including eating food cooked by the system which could involve the additional sponsorship of local wineries, cider, etc. to garner social support.

Yes I'm in France, it would be nice to present the system at such events. For the moment I'd much rather get to work on the technical side.

> Smiles from Sydney. I applaud your work toward the application of technology to social and environmental concern, we need more of this.

Thank you

Taking a computer out of the loop is the best way to guarantee speed and reliability.

Any safety response is going to rely on speed of detection and speed of response. Almost no sensor system will be faster than laser TOF and IMS. They are both very fast and very low power, you can afford to sense at high frequencies and this costs almost no power. Some models may include programmable interrupt lines to further reduce aggregate power utilization by avoiding the need for polling.

In terms of speed of response, anything within 1-2 seconds should be safe. Perhaps having an emergency actuation function in which the array is inverted to prevent convergence (and attract attention as a side effect!) may be safest. You could also cheaply and easily add a siren or audio announcement.

Bon travail et bonne chance!

> Taking a computer out of the loop is the best way to guarantee speed and reliability.

As suggested by pjc50 in another comment [0], a passive safety would be good for this application.

The main drawback being the space required to protect the entire danger zone.

> Bon travail et bonne chance!

Merci beaucoup

[0] https://news.ycombinator.com/item?id=41390704

Well short of an enclosure, the standard for power tools is to project some form of laser mark. Only when the operator verifies the mark is in the correct position should they activate the system. This would be a viable approach for a CNC-shaped array of curves and would not require a complete enclosure, which has various downsides (windage, size, weight, potential for damage, suitability for rough terrain, etc.)

> not sure where you are based in France

I answered your question too quickly, I'm near Clermont-Ferrand, we have several local brewery here.