[Firebug-cvs] firebug/doc/chassis chassis.tex,1.14,1.15

[David M. D. <do...@us...>]

Update of /cvsroot/firebug/firebug/doc/chassis
In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv20370

Modified Files:
	chassis.tex 
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Added alex material.

Index: chassis.tex
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RCS file: /cvsroot/firebug/firebug/doc/chassis/chassis.tex,v
retrieving revision 1.14
retrieving revision 1.15
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+ 
+ 
+ 
+ \section{Mica Mote Chassis}
+ 
+ Early on we identified the following design objectives in a mounting chassis:
+ \begin{itemize}
+ \item Tool-less; requires only hands and possibly a coin to 
+ open, disassemble, reassemble (rapid and simple)
+ \item Separation of modular components: sensing, power, mote, antennas
+ \begin{itemize}
+ \item Wiring harness to separate power connection
+ \item battery cage design to accommodate different battery candidates
+ \end{itemize}
+ \item Supports mounting by: screw, strap, zip-tie, velcro, magnet, hook, or similar 
+ technology.
+ \item Injection moldable for low-cost high-volume production
+ \end{itemize}
+ 
+ 
+ \subsection{Design for Assembly} 
+ 
+ Since the Mica Mote Chassis were to
+ be assembled and disassembled in the field, the most prominent choices
+ for mounting and securing were snap-fit joints and thumbscrews.
+ Snap-fitting joints however require custom injection molding and are
+ difficult to prototype, so early on it was a preference to target the
+ design aound tool-less screw types.  In addition, snap-fitting joints
+ would have only supported a fixed number of boards; one advantage of
+ the 51-pin edge connector employed by the Mica mote and sensor boards
+ is that many sensor boards are designed to be stackable such that the
+ pin-out leads can be passed through to additional boards.
+ 
+ The Mica mote and sensor boards support screw or pin mounting with two
+ 0.120" (just smaller than 1/8") diameter holes located at opposite
+ (diagonal) corners.  For such holes, it is advisable to use \#4 machine
+ screws.  A requirement in securing the mote and sensor boards was that
+ the additional boards needed to be fixed in the z-axis (normal to the
+ board), otherwise they would either loosen from the edge connector or
+ strain the connector in a way to possibly cause failure.  To provide a
+ 3/16" spacer between boards for support, threaded standoffs were used
+ so that they could fit between components in the screw assembly and
+ not become loose.  In this manner, a mote and several boards can be
+ secured to a plastic chassis with two screws and a number of 3/16"
+ standoffs.
+ 
+ To effectively use the screw holes on the mote, it was necessary to
+ remove the battery pack from the underside of the mote.  The chassis
+ was designed so that the mote-and-sensor assembly would be a stack of
+ components sitting alongside a power source, connected to the mote by
+ wire harness or the original battery pack wiring.  In this
+ configuration, the original battery pack could be peeled back from the
+ mote and secured to the chassis (still connected by wiring), or
+ another power source battery could be used.  The battery pack was
+ secured to the mote by using self-tapping screws, fastening through
+ two holes already present in the battery pack.  Battery clips or
+ sleeves, which could hold batteries of different form factors, could
+ then be screwed into that same mounting area.  In one case, a set of
+ battery sleeves which held lithium-ion batteries was fabricated to
+ screw into the battery mounting holes.  While the process of mounting
+ the battery clip or sleeve was not considered to be tool-less, it
+ could be done for a pre-configured chassis type before going to the
+ field.  Following Boothroyd's guidelines of design for manual assembly
+ (Boothroyd, 1994), the screw-holes on the chassis were chamfered so
+ that alignment of the very small screws could be done by feel.
+ 
+ One principal difficulty in using thumbscrews and standoffs in the
+ field is that they are very small, easy to lose, and difficult to
+ align.  To solve this problem and simplify the assembly method, a
+ "pyramid assembly" concept outlined by Boothroyd (Boothroyd 1994) was
+ applied to the design.  The primary concept of pyramid assembly is to
+ be able to stack multiple components onto a fixed guide axis.  In this
+ case, the implementation involved "flipping the logic" of trying to
+ insert a loose screw from above into a set of components with holes,
+ to fixing the screw from below as part of the chassis and individually
+ stacking the components onto it from below.  With this design, the
+ parts aligned themselves correctly to the component below them as they
+ were assembled.  This not only aided with the edge connectors, but
+ specifically made application of the standoffs a much simpler task.
+ The screws themselves were affixed to the chassis by two small holes;
+ the screws were inserted from the bottom and secured at the top with a
+ 3/32" hex nut.  While this process required the use of some tools, it
+ could be done before going into the field and once the screws were on
+ the chassis there was no reason to remove them.  The 3/32" nut also
+ provided clearance from the underside of the mote, eliminating any
+ interference from components to the chassis plane.  To finally secure
+ the components at the end of the screw, a 3/4" long threaded hexagonal
+ standoff was used.  The standoff's six edges allowed for a good grip
+ between fingers and a thumb while maintaining a small horizontal
+ profile which would not interfere with circuit board components,
+ unlike larger wingnuts.
+ 
+ Finally, for supporting chassis mounting to other surfaces, long slots
+ were designed which could support mounting by fabric strapping,
+ zip-ties, hard wires, or nails.  Keyhole type mounts were incorporated
+ to support nails, screws, hooks, and push-pins.  A magnet could also
+ be glued to the back of the chassis for mounting onto metal surfaces.
+ 
+ \subsection{Design for Injection Molding}
+ 
+ The first chassis designs were prototyped by machining 1/8" strips of
+ acrylic, shown in in Figure~\ref{fig:exploded_view_2}.  To make the
+ design injection-moldable, it was first created as a solid model using
+ the SolidWorks CAD package.  Three general design guidelines were
+ considered in adapting the prototype for moldability:
+ 
+ \begin{itemize}
+ \item Maintain constant wall thickness for best dimensional accuracy
+ \item Avoid undercuts so that a "straight-pull" tool can be used
+ \item "Shell" the design to use a reasonable wall-thickness for moldability and cost
+ \end{itemize}
+ 
+ The design was updated to use 0.08" wall thickness, reasonable for
+ polystyrene (PS), ABS, or polycarbonate (PC).  The shelled design
+ allowed features on the underside, such as the heads of the mounting
+ screws or the bosses of the battery clip screws, to be neatly nested
+ beneath the part as it lay flat on a table.  Structural stability with
+ a thinner design was aided by the perimeter wall but still a concern;
+ extending part wall around the slot features all the way to the bottom
+ effectively created a set of ribs which prevented extreme flexing.
+ Fig.~\ref{fig:cbim} shows early CAD renderings and early ABS
+ prototypes built from a fused deposition modeling (FDM) machine.
+ Later revisions of FDM prototypes durable enough for field testing.
+ Short-run injection molding through a service bureau such as Protomold
+ was determined to cost approximately \$2,500 for tooling and a fixed
+ lot of 25 parts, and between \$3 to \$10 per additional part (depending
+ on volume).
+ 
+ 
+ \begin{figure}
+ \begin{center}
+ \includegraphics[width=3in]{figs/exploded_view_2.eps}
+ \caption{An exploded view of the ``candy bar'' chassis 
+ used for the controlled burn test at East Bay Regional 
+ Parks Fire Department, Lake Chabot.}
+ \label{fig:exploded_view_2}
+ \end{center}
+ \end{figure}
+ 
+ 
+ 
+ 
+ 
+ 
+ \begin{figure}
+ \begin{center}
+ \subfigure[Front view.]{\label{subfig:candy_bar_injection_moldable_front}%
+ \includegraphics[width=2.5in]{figs/candy_bar_injection_moldable_front.eps}}
+ \subfigure[Back view.]{\label{subfig:candy_bar_injection_moldable_back}%
+ \includegraphics[width=2.5in]{figs/candy_bar_injection_moldable_back.eps}}
+ \subfigure[FDM front view.]{\label{subfig:fdm_front}%
+ \includegraphics[width=2.5in]{figs/fdm_front.eps}}
+ \subfigure[FDM back view.]{\label{subfig:fdm_back}%
+ \includegraphics[width=2.5in]{figs/fdm_back.eps}}
+ \caption{Injection mold design, and prototype using FDM technology.}
+ \label{fig:cbim}
+ \end{center}
+ \end{figure}
+ 
+ 
+ \begin{figure}
+ \begin{center}
+ \includegraphics[width=3in]{figs/fdm_assembled.eps}
+ \caption{Assembled mote using FDM prototype.}
+ \label{fig:fdm_assembled}
+ \end{center}
+ \end{figure}
+ 
+ 
+ 
  \section{Mica Mote Chassis}