Friday, July 03, 2009

Further improvements to the motor drive

By the end of last summer, I had built and installed a linear to angular motor drive that was vastly superior to the linear push/pull arrangement that I had been using previously.

Experience with it has led me to improve it further still in several ways. In this photo (click any photo to enlarge it) you will see the current motor drive. It has been operating for about two months now and this will be the motor drive detailed in the plans.

I kept the same basic arrangement: the gear motor (inside the right most box) turns a lead screw (the horizontal shaft) on which is mounted a circular brass nut that is able to ride up and down the channel in the drive arm (the one with the smaller box attached to it). The turning of the lead screw pushes or pulls the drive arm so that it rotates around the pivot at it's top. This pivot is in the same plane as the collector tubes and it's length is the same as the length of the arms that hang down from each reflector. In this way, the push rod which runs down the entire length of the array and couples all the reflectors, moves up and down along the same arc as the motor drive. The push rod now stays perfectly straight along the whole range of motion of the array, 50 degrees to either side of vertical.

The first improvement was in the construction of the backing board. Previously I had laminated a number of 2x6 boards of pressure treated wood together to get a suitably sized slab. While it worked well for a time and helped to show that the principle was correct, it wasn't long before the whole slab started to warp from the sunlight which shines mostly on the backside of this slab. The whole thing started to cup so that the drive arm began to bind against it's surface.

I made a new motor drive using the same technique as for the ribs. I cut the profile from good quality plywood and treated it with the same marine paints as I had used on the ribs to seal it. After two months now in some fairly direct hot sunny days, it is still perfectly straight.

The second improvement was to stiffen the stainless steel push rod by changing the 3/16" rod to a larger 1/4" rod. It had had a tendency to bend when pushing the array to the west (west is to the right in this picture). When moving to the east (in my arrangement at least) the rod is pulling and didn't have this problem. With the various changes made, the reflectors are quite as well balanced as they were intended and with 13 reflectors, there is enough force required that the rod needed to be stiffer.

Here is a closer view.

Another improvement I made was the use of standard blank plastic electrical boxes to contain the motor drive (the right box) the battery (the lower box) and the limit switches (the small box on the drive arm). I will write about the new limit switches in another update shortly. The new boxes look better than my handmade wooden ones of course, but they will also help in the weather sealing of the whole system.

The wiring was also completely replaced and run within plastic flexible conduit, another standard electrical product available at most home centers. The previous three year old wiring had become unreliable and needed replacing. The new wiring includes terminal blocks and crimped terminals throughout so should be much more reliable. I am trying to achieve a 5-10 year life for all components.

I currently run the system from a small (6 amp-hour) gel cell that I replace with a freshly charged cell about once a week. I intend to install solar panels to run this system as well as a DC operated pool pump this summer, so this battery should disappear shortly.

As a part of the general re-wiring, I changed the support and packaging of the sensor housing slightly. Previously, the Leviton "while in use" cover that I had removed the lettering from and polished (described in a previous article) had been bolted to the support platform (the white thing) and there wasn't quite enough room for the wiring and another improvement that I had wanted. I wanted to be able to swing the Red Rock sensor slightly on a pivot to adjust it's alignment.

I added a standard plastic electrical box under the sensor cover to give more space for the wiring and for the sensor swing mechanism. This will be described in another update shortly. The improvements have worked out very well. The wiring is now more easily accommodated and it is now very easy to swing the sensor slightly to change alignment and have it stay in place reliably.

Previously I had been bending the sensor on its wiring posts to change its position and this was not a good arrangement at all.

Here is a view of the new motor drive from the back.

One thing that was tricky was getting the pivot for the drive arm in the same plane as the collector rods. You can see the crossed wood struts at the back of the motor drive used to mount it to the post. These were used with wood clamps temporarily to get things into the right position with a bit of trial and error and then screws were driven in to hold everything permanently.

So far, everything has been working as planned.

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