Sunday, January 24, 2010

Mirror Mount

I'm convinced that the specifics of the mirror mount are important. A simple, efficient mount which bears almost all the weight of the mirror and includes a counterbalance system to minimize the force required to move (tilt) the mirror is the key. If well done, the motors which move and hold the mirror in place can be minimized.

Motors are one of the key cost variables on the project. My target motor is a cheap low torque stepper. Cost goes up fast when more torque is needed. I'm also hoping to integrate a worm gear to trade speed for torque. Also worm gears have a certain holding force even at rest. I hope to use that to hold the mirror in place under ordinary conditions, so power is used only when it's moved (once every few minutes). Since power generation is the ultimate goal, the system should be miserly with power.

I'm investigating two mount directions at this point. First is the ball and socket, second is the "gyroscopic drink holder", both pictured below. I like the "gyroscopic drink holder" better. It was literally sold as a device you could attach to your dashboard with double stick tape then put a hot cup of coffee on your dashboard while you drove to work. It would keep it level so it didn't spill while you drove around corners and over hills. Comedy. My friend found it at a garage sale.

It's not a gyroscope but nested curves with ball bearings in between. You can see in the picture plastic ribs on the lower bowl/curve create six pie wedge sections, one for each ball bearing. I'm pretty sure there's a also a rib ring that keeps the bearings from rolling down to the bottom. So the top bowl moves smoothly over the bottom bowl. Then, the red screw is attached to a weight which hangs below the second bowl. The weight is the counterbalance so the top bowl always wants to stay level. To make it, I'm trying to find some nesting bowls, put some ball bearings in between and hang a counter balance. The mirror would go on top facing directly up, but could tilt in any direction, the counterweight pulling it back to level.

The ball and socket is easier to get parts for. I bought some ball thrust bearings (the kind used for lazy susans, not the radial motion type) from VXB. First pic is how the bearing would look on top of a sphere, next pic is how the mirror frame would sit on top of the bearing on a sphere. It's easy enough to buy the spheres, and this bearing type would work on them. This is a second choice at the moment because even with a counterweight, once the mirror moves past rest point, the mount itself makes it want to keep going. Whereas with the bowl style mount, when out of rest position the dynamics of the mount itself make it want to go back to rest. More potential energy at the rest point of this mount as my high school physics teacher would say.

Anyway, just posting as an update. Need to make something and post pics and how to's. Meanwhile, working in the background on sourcing motors.

Should also note that for v2, the configuration is an array of mirrors reflecting light up to a target that is raised but in the center of the array -- more like Ausra in the layout. Whereas v1 was more for an array all to one side of a target, reflecting back toward the sun -- more like a classic power tower. The difference is because how I guess retail solar (especially lightly concentrated PV) might play out. So v2 has a rest position for the mirror at something close to the horizontal, where the rest position for v1 was something close to vertical.

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