Version 3.0

The big lesson from version 2.5 was stop making the positioning mechanism bear weight.  So the first task of version 3 was to find something to bear the weight of the mirror and enable good range of motion while requiring fairly constant (and minimal) power to position it.  At first I put something together with PVC and might go back to it but I want to try out just using a caster first.  It has a lazy susan-type bearing for azimuth movement and the axel provides the movement for altitude.  

The picture above is of this caster.  The wheel has been removed and in its place is a hinge strap like this one which can be fixed to the mirror.  For now I attached it to a piece of 3/4” plywood about the size and weight of the mirror to try it out.  When the wood is off-set a bit, the center of balance is at about a 45° angle and it doesn’t require much force to move it through a good range of motion (180° of azimuth and 90+° of altitude).

It’s mounted on four 1 foot lengths of 3/8” threaded rod which have been sunk into concrete in the cinderblock.  I still like the cinderblock base for being heavy, cheap, available and modular.  The holes were a bit small for the 3/8” rod so they had to be bored out.  For now I just want to test out the concept, if it’s worth developing I’ll source either a caster with larger holes or use smaller rod.  The four rods have nuts to level the caster/platform and fix it in place.  I would prefer metal for the platform but am using wood for now as it’s a easier for prototyping and was handy.

Next step is to fix something to the bearing which can be used to turn it for azimuth movement and can also bear the mechanism for altitude positioning.  For the azimuth movement, I’ll try to use a worm gear and fix the worm to the motor shaft which will be on the platform.  This will mean either sourcing or printing those gears.  Printing is preferable for
flexibility and cost however printing worm gears is a bit tricky for various reasons.  Reuttmeister has one on thingiverse and there is some discussion there about it.  I found two things that may help.  The first is  the SPGears Sketchup plugin.  It doesn’t do worm gears (he says he’d like to in a future release), but it can do helical gears with an angle and I think the simplest case of a worm (no enveloping) is a helical gear whose angle matches that of the worm.  Not sure but will try this out.  The second thing that may help is the Screw1_5.rb plugin – here’s a discussion thread that talks about doing screws in sketchup and has a link to the plugin.  That will be useful in making the worm itself.  Of course if you are using something other than Sketchup there are many options for modeling both worm gears and worms.

It would be great to use threaded rod with a nut as the worm – it has finer and stronger thread than I’ll ever do in plastic – but threaded rod is straight.  There is a discussion here about bending it but I’ve avoided it because it seems like it would require too much customization or manufacturing expertise to get it to work consistently.  I designed a little linear actuator that I was going to use for the altitude but am leaning against it at the moment because I can’t think of a simple way to have it track along the curve that the mirror will describe.  There is a solution where the mount for the actuator pivots that is used by Gabrial and others on his forum and maybe I’ll end up back there, but for now I’ll probably try to find a worm solution to the altitude positioning also.

One other thing to mention is rather than the Hall Sensor, I’m going to try out a rotary encoder for position sensing (cus I use a regular gear motor rather than a stepper to keep costs down).  I bought these at Jameco and will try them with the Arduino.

Time lapse of version 2.5

To be filed under the category 'declare victory and retreat', here's a time lapse video of v2.5 working.  This memorializes that it worked (barely) but that's all.  This design needs significant modifications, so I'm starting work on version 3.  The movie only captures about 30 minutes because then my phone overheated, but over a longer time (hour+) there was some drift of the reflection off target.  And, it turns out it only works for about 3-4 hours of the day anyway due to design flaws.

But, I've learned a lot about what doesn't work. The main problem with this design is that when the angle is anything significant, say >20 degrees off horizontal, the threaded rods which position it begin to bear too much weight which strains the rods and motors.  As a result, there's just not enough range of motion.  For version 3, I'm going to use bearings to take the weight off the positioning mechanisms.  The problematic linkages will be changed out.  But, the gear motors and hall switches will be kept.

Also, I just placed an order for a MendelMax kit.  3D printers are time sucks in that they take time to build, configure, tweak, etc.  But the design freedom they provide is just too great to pass up.  The MendelMax will give me a larger print bed (almost 8" cube up from a 4" cube on my current Makerbot Cupcake) which will be necessary for version 3 parts.

V 2.5. A bit more progress.

Too much chit chat in the video but it was easier than writing.
Should have said www.heliostats.org.

Hey Infinia! Use modular mirrors!

You gotta love Infinia’s PowerDish. Solar thermal generating instant residential spec A/C. It is a better-sized version of the SES attempt at Sandia. And as someone from Detroit, I love their interest in leveraging the manufacturing expertise there.

But they should consider modular mirrors. Check the specs on the PowerDish: 1900lbs with a 4.7m/15’ diameter mirror (call it 16m² or 175ft²). That’s big. I’m betting the installed cost of the dish sans motor is a lot more than $1600 ($100/ m²). Plus, if it breaks, it’s off line, whereas with 20 modular mirrors, if one breaks, it goes off line and you’ve only lost 5% of your light collection capacity.

Mirror curvature (and wind shear) could be addressed with modular heliostats. Ignoring a 1m² hole in the middle of your parabola, you’d have 8 1m² identical parabolic-slice mirrors in your first ring and 16 1m² mirrors in your second ring. You only need two mirror shapes. Injection molded curved plastic with PVD reflective coatings anyone? Include a few holes to reduce wind strain.

And let’s face it, that engine is designed for household use (1φ, 240V A/C). What you really want is for 1,000,000 homeowners to buy it and install it at their house. Is the hesitation releasing the IP into the wild before you’ve scaled?

Longer term, it just makes more sense to decouple the light collection from the power conversion. The power conversion component is where the value is added. Light collection should be a commodity. A monolithic design which ties these functions together reduces the chance that the power conversion part will be a success. And I hope Infinia is a resounding success.

V 2.5, catching up

The base: this new base sets the relations between the rods within the cinderblock and concrete instead of using angle irons that rest on top of the cinderblock. To make it, pre-cut holes in the pieces of wood above and below the cinderblock. Fit tubes into the holes for the shafts that the threaded rods move through, and wedge in the 1/2” rod which provides the main mirror support. Then, pour concrete around all of it and put the wood piece on top to help set positions as the concrete dries. The result is the two shafts are firmly held in position relative to the center support. This version used metal tubing for the shafts, but I think that tubing can be eliminated. PROs: cheap, sturdy, no custom parts; solid shaft path; wood creates a platform for motor mounting and simplifies prototyping. CON: the points may end up too close to each other and require too much force from itty bitty motors. If need be, there are larger CMU’s (concrete masonry blocks) but the one’s I’m using are extremely common and cheap.

Manufacturability: the new base is easier to make. There is a template which can be laid on to pieces of wood so you can pre-drill holes for the shafts and motors. Having a standard piece allows the rods to be fixed in place before concrete is poured and keeps them plumb and square. The wood provides a platform for mounting motors and other items. Later the wood platform could be plastic but for now wood makes it easier for others to play with the design if they wish.

Other: threaded rods are up-ticked to 3/8” to make the whole thing more sturdy. The way the motor interfaces to the platform and spur gear is simpler. I’m using store-bought clevis & pushrod pairs but this whole linkage (including the interface to the mirror) still needs improvement. My current thinking is to use a saddle joint for the weight bearing point which would allow movement in two axis but not rotation, but this is still a challenge.

Electronics: I’ll do a separate post when ready but briefly… Current approach is to put a motor controller at each ‘stat. Also, I’m driving the motors with 9V supply to make their work at the easier end of their power range so they are less stressed (motor failure is an area of worry) and also to combat voltage drop. Connections between the Arduino and each stat will be by Ethernet type cable because it’s fairly standard. Also, it’s mass produced and fairly cheap.

SketchUp: wrong tool for the job

I've been trying to improve the center mount so that it prevents rotation. The plan was to add a raised tab to the ball, then fit that tab into a slotted socket (constraining rotational movement down to one axis) then fit that slotted socket into another slotted socket (which would add one more axis of motion). The result was supposed to be movement in two axis without rotation.

This seemed easy enough to model in SketchUp for printing on the Makerbot, and if it worked, I'd send it off to shapeways. Well SketchUp is really the wrong tool. Whenever you make hemispheres with volume (5mm thick walls) and try to cut notches through it, you get messy messy results. I finally got something to work by downloading this person's work, hand editing out a notch, then making a copy that was 5mm larger all around, and sewing them together to create a closed volume. It works but is really lame. Results below.

So guess I'll try this in Autodesk Inventor which I've completely forgotten how to use. And we're moving in two weeks which should subtract lots of spare time. Oh well. Stiff upper lip and all that.

Hello hall sensor

Got the Hall Effect Sensor working. This is the code it's using. The other gotcha for me was wiring it up right -- took awhile to see the teeny tiny notations in the specs that tell you the wires are V, Ground and Signal read from left to right with the printing facing up. Feed signal wire into input 2 on the arduino (one of the two available interrupt pins). Also the Arduino sketch wasn't compiling until I put the pin information inside the (setup) section, but I think that's just me not understanding rules for declaring variables and such.

Here's what worked:

void setup()

{

pinMode(2, INPUT); //set the interrupt0 pin2 to input

digitalWrite(2, HIGH); // and enable it's internal pull-up resistor

Serial.begin(9600);

and so on.

This adds some useful options for position control on an otherwise simple gear motor.