[Firebug-cvs] firebug/doc/chassis chassis.tex,1.11,1.12
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[Firebug-cvs] firebug/doc/chassis chassis.tex,1.11,1.12
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From: Alex Do <use...@us...> - 2004-08-06 07:11:25
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Update of /cvsroot/firebug/firebug/doc/chassis In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv5025 Modified Files: chassis.tex Log Message: Developed background and need; added outline notes in DFA, mounting, and DFM sections Index: chassis.tex =================================================================== RCS file: /cvsroot/firebug/firebug/doc/chassis/chassis.tex,v retrieving revision 1.11 retrieving revision 1.12 diff -C2 -d -r1.11 -r1.12 *** chassis.tex 21 Jul 2004 18:56:00 -0000 1.11 --- chassis.tex 6 Aug 2004 07:11:16 -0000 1.12 *************** *** 1,255 **** ! \documentclass{article} ! ! \usepackage{chicago} ! \usepackage{graphicx} ! \usepackage{subfigure} ! %\input{comment} ! ! \setlength{\textheight}{8.5in} ! \setlength{\topmargin}{0in} ! \setlength{\oddsidemargin}{0in} ! \setlength{\textwidth}{6.5in} ! ! ! ! ! \begin{document} ! ! \title{Chassis design for motes} ! \author{Kevin Lee, Alex Do, David M. Doolin, Nicholas Sitar} ! \date{\today} ! \maketitle ! ! \section{Introduction} ! Wireless sensors networks (WSN) are currently emerging as solutions to monitor ! and collect data in situations where traditional ``wired'' sensing components ! are cumbersome or impractical. Though originally developed in the late 1990's ! for the military to monitor hostile combat situations~\cite{need_ref}, ! scientists, engineers and domain experts in many fields ! now see applications in monitoring structural conditions in large skyscrapers ! or bridges, industrial manufacturing operations, heating and cooling systems, ! and an endless addition of other applications. The technology, however, is ! extremely early in its lifecycle -- where commercial markets are minute and ! the bulk of the current customers are comprised of government, academic, and ! corporate researchers~\cite{appropriate_refs} ! ! These researchers are continually developing and optimizing WSN for use in ! untracked territory (often literally)~\shortcite{mainwaring:a2002}, ! and require hardware that can repeatedly be rapidly deployed, operated, and ! then taken down for constant revision. Because of the nascence of the ! technology, there are no commercially available housings or chassis units ! available that serve as the ``AT computer case'' of wireless sensor hardware. ! ! We are developing a mechanical chassis for wireless sensor network equipment ! distributed by Crossbow Inc. The chassis will allow for rapid assembly, ! deployment, and dissasembly of the Crossbow WSN hardware. ! ! This paper reviews the design process and technical issues for creating such a chassis. ! ! \section{Hardware Overview} ! -Components overview ! -sensing ! -mote board ! --radio antenna ! -power ! ! Discuss issues that stem from interconnections and assembly. ! ! \section{Design Objectives} ! Discuss needs for the following features: ! -Tool-less; requires only hands and possibly a coin to open, disassemble, reassemble (rapid and simple) ! -Separation of modular components: sensing, power, mote, antennas ! -Wiring harness to separate power connection ! -battery cage design to accommodate different battery candidates ! -Supports mounting by: screw, strap, zip-tie, magnet ! -Injection moldable for low-cost production ! ! ! \section{Design for Assembly} ! ! ! \section{Wiring harness} ! ! To create wiring harnesses for the motes, the issues that were encountered ! can be separated into roughly two sections: crimping the connectors ! and connecting the connectors to the battery cases. ! ! ! \subsection{Crimping the connectors} ! ! ! In order to utilize the Molex power connector on the mote, we ! purchased 50058-8100 Molex crimp terminals and 51021-0200 Molex ! crimp housings. To accommodate the size of these two parts, ! we found it best to use 26 AWG wire. In order to crimp these ! connectors, we were officially supposed to use a specific Molex ! crimping tool.However, the cost of this tool is roughly \$180, ! making it quite a hefty investment. Thus, we instead went with ! a \$7 crimping tool from a nearby electronics store, and did our ! best to make working connectors. But because of the size of ! these parts, crimping the terminals was very difficult, and even ! now there is no telling just how robust these connectors are. ! ! \subsection{Connecting the connectors to the battery cases} ! ! The battery cases provided with the motes caused a predicament ! when it came to connecting the connectors to an actual power ! source, which in this case meant batteries. For some of the ! battery cases that no longer had wires extending from them, we ! tried to solder wires to the cases, but to no avail. The cases ! just were not designed for soldering wires to them. And even ! for the cases that did still have wires connected to them, ! there were still issues to be addressed. ! ! To join the wires and form one piece that went from the battery case to ! the connector, solder was first applied on each pair of wire separately ! (the two red and two black wires) and then reinforced altogether with some ! plastic heat-shrinking material. This process, however, had some flaws. ! First, the diameter of the heat shrink (expanded diameter of .187) ! could not fit over the crimp housings, so the heat shrink had to be ! placed on the wires prior to soldering. It is important to note that ! since the heat-shrinking material only shrinks up to 50\% of its expanded ! diameter, a larger heat shrink would have been too loose to properly ! insulate the two sets of wires together. Thus, we could not use heat ! shrinks with a larger diameter to remedy this problem.As a result, ! since we had to place the heat shrink on the wires before soldering, ! the length of heat shrink that we were allowed to use was limited ! by where the joining of the wires would take place. This was rather ! disappointing, because we had hoped to cover up the two wires entirely ! with heat-shrinking material so that the two sets of wires would act as ! one. The second flaw we found was that the soldered wires could not ! just be wrapped in heat-shrinking material, because a short would then ! exist at the soldered areas between the red and black wires. To prevent ! a short from happening, the two options were to either first wrap each ! wire separately with heat-shrinking material and then wrap both of these ! with a heat shrink, or to use electrical tape and first insulate each of ! the wires before applying the heat shrink. The former could not be done, ! however, because we did not have heat shrink with a large enough diameter ! that would insulate the two heat-shrinked wires. The latter proved to ! be quite difficult too, because the heat shrink barely (and in some cases ! failed to) fit over the taped-and-soldered wires. ! ! \subsection{Suggestions} ! ! ! From these experiences with creating wire harnesses for the ! motes, we have a couple of suggestions to make. The first ! one is regarding the Molex power connector on the mote board. ! It would be nicer if Crossbow could use a larger (and possibly ! structurally different) connector so that we could manually make ! more robust connections. Secondly, it would be very helpful if ! Crossbow could provide better battery cases. Since there is a ! Molex power connector on the mote board, Crossbow should devise ! a way for people to utilize that connector instead of just the ! soldered connections. Using the current battery cases for this ! purpose is very difficult, largely because of the different sizes ! of the wires, and the difficulty of insulating them properly. ! It would be great if Crossbow could either provide battery cases ! that would allow us to connect our wires directly to them (instead ! of having to find a way to couple our wires with existing ones), ! or to maybe even provide a battery case that already has a molex ! connector on the end. ! ! ! \section{Batteries} ! ! ! When considering whether to use the Nickel-Cadmium (Ni-Cd) ! or Lithium Ion (Li-ion) batteries, we took into account ! several factors, focusing on its ease of use and whether ! there would be detrimental effects to the environment ! given the proposed usage of these batteries. Since ! firefighters will be placing these battery-powered ! devices on their helmets, one issue is to have the ! batteries be as light as possible. In this case, the ! Li-Ion battery chemistry is much lighter than the ! Nickel-Cadmium one, providing less of a burden on ! firefighters. Now the idea of using rechargeable ! batteries was so that firefighters would not have to ! frequently open up the device and replace the AA ! batteries. However, Ni-Cd batteries have what is ! known as ``the memory effect,'' where partial ! discharges will lead to a decrease in the capacity ! of the battery. Thus, to combat the memory effect, ! it is recommended that the Ni-Cd batteries be fully ! discharged before recharging. If we were to require ! firefighters to completely discharge the batteries ! first each time, then this idea of using rechargeable ! batteries would not be that much more convenient than ! having to replace the AAs. Moreover, if the batteries were to only be used for motes, then they would never be completely discharged. Luckily, Li-Ion batteries ! do not have this effect, and it is even recommended that ! only partial discharges be made before recharging them. ! Lastly, another concern was what effects the batteries ! could have on the environment if they were burned up. ! Again, the Li-Ion batteries prove to be the better choice, ! as Ni-Cd batteries are toxic and harmful to the environment. ! Li-Ion batteries do not even contain free lithium, thus ! making them much safer for this particular use. Given ! these considerations, Li-Ion batteries are much more ! suitable than Ni-Cd batteries for this project. ! ! \subsection{Combining the Battery with the Motes} ! ! ! Currently we are using 3.7V, 600mAh Li-Ion rechargeable batteries, manufactured by Ultralife Batteries, Inc. Unlike normal rechargeable batteries that are found in electronic devices, these OEM batteries do not have connectors on them already, only two wires with positive and negative polarities. What this means is that we also have to decide on the type of connector to use. Judging from the amount of difficulty we have had with the Molex 51021-0200 crimp housings, it would be nice if some other sort of connector could be used, especially a larger one, since the battery wires are larger than the ones used to attach the battery case to the Molex power connector on the board. ! (to be continued...) ! ! Ideally, it would be nice if we could use the one connector for the entire mote. This would leave us with three options: use the molex power connector, the adaptor jack, or a mini-USB jack. ! ! \section{Mounting} ! ! ! \section{Design for Injection Molding} ! ! ! \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} ! ! ! ! A prototyped ``candy bar'' chassis machined from acrylic ! is shown in Fig.~\ref{fig:exploded_view_2}. ! ! ! \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} ! ! An injection moldable chassis, as shown in ! Fig.~\ref{fig:cbim} was designed with the ??? software. ! ! \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{Conclusions} ! ! ! \bibliographystyle{chicago} ! \bibliography{tinyos,sensor} ! ! \end{document} ! --- 1,297 ---- ! \documentclass{article} ! ! \usepackage{chicago} ! \usepackage{graphicx} ! \usepackage{subfigure} ! %\input{comment} ! ! \setlength{\textheight}{8.5in} ! \setlength{\topmargin}{0in} ! \setlength{\oddsidemargin}{0in} ! \setlength{\textwidth}{6.5in} ! ! ! ! ! \begin{document} ! ! \title{Chassis design for motes} ! \author{Kevin Lee, Alex Do, David M. Doolin, Nicholas Sitar} ! \date{\today} ! \maketitle ! ! \section{Introduction} ! Wireless sensors networks (WSN) are currently emerging as solutions to monitor ! and collect data in situations where traditional ``wired'' sensing components ! are cumbersome or impractical. Though originally developed in the late 1990's ! for the military to monitor hostile combat situations~\cite{need_ref}, ! scientists, engineers and domain experts in many fields ! now see applications in monitoring structural conditions in large skyscrapers ! or bridges, industrial manufacturing operations, heating and cooling systems, ! and an endless addition of other applications. The technology, however, is ! extremely early in its lifecycle -- where commercial markets are minute and ! the bulk of the current customers are comprised of government, academic, and ! corporate researchers~\cite{appropriate_refs} ! ! These researchers are continually developing and optimizing WSN for use in ! untracked territory (often literally)~\shortcite{mainwaring:a2002}, ! and require hardware that can repeatedly be rapidly deployed, operated, and ! then taken down for constant revision. Because of the nascence of the ! technology, there are no commercially available housings or chassis units ! available that serve as the ``AT computer case'' of wireless sensor hardware. ! ! We are developing a mechanical chassis for wireless sensor network equipment ! distributed by Crossbow Inc. The chassis will allow for rapid assembly, ! deployment, and dissasembly of the Crossbow WSN hardware. ! ! This paper reviews the design process and technical issues for creating such a chassis. ! ! \section{Background} ! The architecture of a typical wireless sensor consists of the following layers: sensing, ! communications, and power. Because WSN sensors operate in mesh networks rather ! than through point-to-point communication, the communications boards (motes) are ! more than simple radio transmitters; they consist of an operating system that manages ! use of power, operates the sensors, and receives and transmits data packets. As this ! is a mechanical design study, we will not discuss software since it has no bearing ! on the mechanical package. ! ! >>need figure of mica mote ! ! The Crossbow Mica platform isolates the remaining layers into the following physical ! components: sensor boards, a mote board (along with an external radio antenna), and ! a battery clip that holds two AA battery cells. The sensor boards connect to the mote ! using a stackable 52-pin edge connector that allows multiple sensors to be used simultaneously. ! The battery clip is connected to the mote through two inch-long narrow-gauge stranded wires that ! soldered directly to the mote board (*footnote: earlier versions used solid wire that was not flexible). ! The mote board is secured to the battery clip with a small piece of foam adhesive. For the radio ! antennas, Mica users can choose from a solder-on wire antenna or a removable antenna ! that plugs into an MMCX connector on the mote board. ! ! While a Mica wireless sensor is functional "out-of-the-box," the default configuration is ! not rugged enough for practical use. Network testing and deployment requires the motes ! to be physically handled, mounted to walls and ceilings, disassembled and reassembled. The ! current interconnections between each of the components break or fail, ceasing operation of ! the sensor node until the unit is repaired. Here is a summary of common problems must be solved ! in the design of a chassis: ! -the edge connectors do not feature any locking engagement, so the sensor boards can become loosened from the mote or from each other ! -the wires from the battery clips are stressed very highly at the solder junctions, and break frequently even with light handling ! -the foam adhesive is not very strong, so the mote and sensors easily become loosened from the battery clip and often causing the fault above ! -the solder-on antenna becomes stressed from repeated repositioning and breaks off from the mote often ! ! In addition, the current mechanical package does not support for mounting. It is nearly impossible to ! secure a bare mote to a tree or a wooden post. Even in an indoor environment, the only common method ! of mounting a mote is by using Velcro (trademark) or other two-sided adhesive, and still many of the ! aforementioned problems occur. ! ! \section{Design Objectives} ! Early on we identified the following design objectives in a mounting chassis: ! -Tool-less; requires only hands and possibly a coin to open, disassemble, reassemble (rapid and simple) ! -Separation of modular components: sensing, power, mote, antennas ! -Wiring harness to separate power connection ! -battery cage design to accommodate different battery candidates ! -Supports mounting by: screw, strap, zip-tie, magnet ! -Injection moldable for low-cost high-volume production ! ! ! \section{Design for Assembly} ! >>Boothroyd references ! ! -mounting mote onto chassis; supporting edge connectors ! -using wiring harness to connect power to mote ! ! -pyramid assembly ! -using standoffs as thumbnuts ! -chamfers on the battery clip ! ! \section{Wiring harness} ! ! To create wiring harnesses for the motes, the issues that were encountered ! can be separated into roughly two sections: crimping the connectors ! and connecting the connectors to the battery cases. ! ! ! \subsection{Crimping the connectors} ! ! ! In order to utilize the Molex power connector on the mote, we ! purchased 50058-8100 Molex crimp terminals and 51021-0200 Molex ! crimp housings. To accommodate the size of these two parts, ! we found it best to use 26 AWG wire. In order to crimp these ! connectors, we were officially supposed to use a specific Molex ! crimping tool.However, the cost of this tool is roughly \$180, ! making it quite a hefty investment. Thus, we instead went with ! a \$7 crimping tool from a nearby electronics store, and did our ! best to make working connectors. But because of the size of ! these parts, crimping the terminals was very difficult, and even ! now there is no telling just how robust these connectors are. ! ! \subsection{Connecting the connectors to the battery cases} ! ! The battery cases provided with the motes caused a predicament ! when it came to connecting the connectors to an actual power ! source, which in this case meant batteries. For some of the ! battery cases that no longer had wires extending from them, we ! tried to solder wires to the cases, but to no avail. The cases ! just were not designed for soldering wires to them. And even ! for the cases that did still have wires connected to them, ! there were still issues to be addressed. ! ! To join the wires and form one piece that went from the battery case to ! the connector, solder was first applied on each pair of wire separately ! (the two red and two black wires) and then reinforced altogether with some ! plastic heat-shrinking material. This process, however, had some flaws. ! First, the diameter of the heat shrink (expanded diameter of .187) ! could not fit over the crimp housings, so the heat shrink had to be ! placed on the wires prior to soldering. It is important to note that ! since the heat-shrinking material only shrinks up to 50\% of its expanded ! diameter, a larger heat shrink would have been too loose to properly ! insulate the two sets of wires together. Thus, we could not use heat ! shrinks with a larger diameter to remedy this problem.As a result, ! since we had to place the heat shrink on the wires before soldering, ! the length of heat shrink that we were allowed to use was limited ! by where the joining of the wires would take place. This was rather ! disappointing, because we had hoped to cover up the two wires entirely ! with heat-shrinking material so that the two sets of wires would act as ! one. The second flaw we found was that the soldered wires could not ! just be wrapped in heat-shrinking material, because a short would then ! exist at the soldered areas between the red and black wires. To prevent ! a short from happening, the two options were to either first wrap each ! wire separately with heat-shrinking material and then wrap both of these ! with a heat shrink, or to use electrical tape and first insulate each of ! the wires before applying the heat shrink. The former could not be done, ! however, because we did not have heat shrink with a large enough diameter ! that would insulate the two heat-shrinked wires. The latter proved to ! be quite difficult too, because the heat shrink barely (and in some cases ! failed to) fit over the taped-and-soldered wires. ! ! \subsection{Suggestions} ! ! ! From these experiences with creating wire harnesses for the ! motes, we have a couple of suggestions to make. The first ! one is regarding the Molex power connector on the mote board. ! It would be nicer if Crossbow could use a larger (and possibly ! structurally different) connector so that we could manually make ! more robust connections. Secondly, it would be very helpful if ! Crossbow could provide better battery cases. Since there is a ! Molex power connector on the mote board, Crossbow should devise ! a way for people to utilize that connector instead of just the ! soldered connections. Using the current battery cases for this ! purpose is very difficult, largely because of the different sizes ! of the wires, and the difficulty of insulating them properly. ! It would be great if Crossbow could either provide battery cases ! that would allow us to connect our wires directly to them (instead ! of having to find a way to couple our wires with existing ones), ! or to maybe even provide a battery case that already has a molex ! connector on the end. ! ! ! \section{Batteries} ! ! ! When considering whether to use the Nickel-Cadmium (Ni-Cd) ! or Lithium Ion (Li-ion) batteries, we took into account ! several factors, focusing on its ease of use and whether ! there would be detrimental effects to the environment ! given the proposed usage of these batteries. Since ! firefighters will be placing these battery-powered ! devices on their helmets, one issue is to have the ! batteries be as light as possible. In this case, the ! Li-Ion battery chemistry is much lighter than the ! Nickel-Cadmium one, providing less of a burden on ! firefighters. Now the idea of using rechargeable ! batteries was so that firefighters would not have to ! frequently open up the device and replace the AA ! batteries. However, Ni-Cd batteries have what is ! known as ``the memory effect,'' where partial ! discharges will lead to a decrease in the capacity ! of the battery. Thus, to combat the memory effect, ! it is recommended that the Ni-Cd batteries be fully ! discharged before recharging. If we were to require ! firefighters to completely discharge the batteries ! first each time, then this idea of using rechargeable ! batteries would not be that much more convenient than ! having to replace the AAs. Moreover, if the batteries were to only be used for motes, then they would never be completely discharged. Luckily, Li-Ion batteries ! do not have this effect, and it is even recommended that ! only partial discharges be made before recharging them. ! Lastly, another concern was what effects the batteries ! could have on the environment if they were burned up. ! Again, the Li-Ion batteries prove to be the better choice, ! as Ni-Cd batteries are toxic and harmful to the environment. ! Li-Ion batteries do not even contain free lithium, thus ! making them much safer for this particular use. Given ! these considerations, Li-Ion batteries are much more ! suitable than Ni-Cd batteries for this project. ! ! \subsection{Combining the Battery with the Motes} ! ! ! Currently we are using 3.7V, 600mAh Li-Ion rechargeable batteries, manufactured by Ultralife Batteries, Inc. Unlike normal rechargeable batteries that are found in electronic devices, these OEM batteries do not have connectors on them already, only two wires with positive and negative polarities. What this means is that we also have to decide on the type of connector to use. Judging from the amount of difficulty we have had with the Molex 51021-0200 crimp housings, it would be nice if some other sort of connector could be used, especially a larger one, since the battery wires are larger than the ones used to attach the battery case to the Molex power connector on the board. ! (to be continued...) ! ! Ideally, it would be nice if we could use the one connector for the entire mote. This would leave us with three options: use the molex power connector, the adaptor jack, or a mini-USB jack. ! ! \section{Mounting} ! -strapping (velcro, zip-tie) ! -hooks ! -magnets ! ! \section{Design for Injection Molding} ! ! -straight-pull design ! -avoiding undercuts ! -constant wall thickness to avoid shrinkage ! -shelled design ! -strapping slots ribbed for rigidity ! ! \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} ! ! ! ! A prototyped ``candy bar'' chassis machined from acrylic ! is shown in Fig.~\ref{fig:exploded_view_2}. ! ! ! \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} ! ! An injection moldable chassis, as shown in ! Fig.~\ref{fig:cbim} was designed with the ??? software. ! ! \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{Conclusions} ! ! ! \bibliographystyle{chicago} ! \bibliography{tinyos,sensor} ! ! \end{document} ! |
