Monthly Archives: March 2012

Motion Control (Part 2)

I wrote about this little USB tilt sensor I found back in October.

I am not recommending this unit for purposes of motion control. The unit firmly affixed to a desk shows a +/- 5 degree variance which would A) require some kind of algorithm to smooth the input and B) make it off by 5 degrees at any given time. The effect I am seeing in my computer models is an oscillation due to latency  and the existing error.

I am very pleased with the D/A controller I got from Phidgets and stumbled upon the fact that they also sell a tilt sensor that is marketed more as an accelerometer.

I have one on the way and plan to play with the code on the next rainy day.

 

Moving right along, I attached a new picture of the platform with the plywood sheathing completed.

I have since put fiberglass batting in the cavities and installed the 1/2 plywood floor where Ferro’s feet go. Next step is painting/weatherizing the parts exposed outside. Finally I have to drill the holes for the universal joints. Then I need to find 2 or 3 people to help me carry it and lift it into position!

Plywood Sheathing

Reposted from apawood.org:

An engineer recently asked me about the Exterior and Exposure 1 classifications of plywood. Specifically, he wanted to know if APA Rated Sheathing Exposure 1 plywood panels could be used outdoors.

Wood structural panels may be produced in three bond classifications: Exterior, Exposure 1 and Interior. The bond classification relates to adhesive bond, and thus to the structural integrity of the panel. By far, the predominant bond classifications are Exposure 1 and Exterior.

Exterior Plywood Panels are bonded with fully waterproof glue and are suitable for repeated wetting and redrying or long-term exposure to weather or other conditions of similar severity.

Exposure 1 Plywood Panels are suitable for uses not permanently exposed to weather. Panels classified as Exposure 1 are intended to resist the effects of moisture due to construction delays, or other conditions of similar severity. These panels are manufactured with the same fully waterproof glues as used in Exterior plywood; however, because of other compositional factors that may affect bond performance, only Exterior panels should be used for long-term exposure to weather. Exposure 1 panels may, however, be used where exposure to the outdoors is on the underside only, such as at roof overhangs.

APA Rated Sheathing Exposure 1 plywood, commonly called CDX, is oftentimes confused with an Exterior plywood panel and misused in applications. It is important to understand the difference between these classifications in order to select the best panel for your project.

On the APA trademarks shown here, the bond classifications are described as Exposure 1 (left) and Exterior (right).

Floor Frame Continued

I spoke with a decking expert named Chris at our local Home Depot today over lunch.

Chris had a couple of statements:

Joist hangers are stronger than nailing from the other side of the board

The joist hangers are more for anchoring the joist in a fixed position as explained here.

Gluing plywood between two sistered beams makes it a lot stronger

I researched this one a little because it made some sense to me and wouldn’t be hard to do but this article really puts it in perspective (the main point is that only some of the plies are oriented in the correct direction making a lot of it potentially wasted).

Nails have more sheer force than screws

Follow this link for an interesting debate. Apparently, as long as the design load isn’t exceeded, the withdrawl force needed to pull a screw out is greater. However, screws are more brittle so if they are “overdriven” or subjected to a lot of repeated stress (wood warping, foot traffic) they will break whereas nails will bend and bend.