P13415: Bike Charger II/public/
MSD IStatus as of the end of MSD I:
- Generator housing design will be fundamentally unchanged, but optimized to be cheaper and include a spring-driven normal force (internal to the housing) to reduce as much slip against the wheel as possible. Issues with slip and positioning were noted during a test run of the current model. Cost and maneuverability constraints are the driving reasoning behind using the existing model. Other designs simply add too much cost or complexity to be feasible.
- Blackberry support will be provided via a different generator housing "unit" including a higher output motor. The original idea to pursue two modular housings was abandoned due to cost of materials and complexity of the housing design, specifically in regards to frame attachment (see below).
- Breadboards will be used over PCB due to cost. However this includes an added risk of user tampering (and potentially safety issues and damage to the phones if used improperly). PCB is an alternative, at about $1 more in cost, but removes these risks and potential issues. Switching to PCB is at the customer's discretion.
- Breadboard will be kept in a separate housing from the generator and phone holder due to heat issues. The housing will be constructed from the same materials as the generator housing and contain three holes to provide air flow while minimizing exposure to the elements. This is the less ideal but safer (in regards to risk assessment) option to pursue as our main design choice, but thermal testing in MSD II will allow us to determine if the breadboard and generator housings couldn't be combined.
- Nokia supported power output does not need convection to cool the circuit design, but convection is required for Blackberry power output. The heat generation is due to the amount of power flowing through the transistor in the circuit. This transistor is necessary to ensure a low cost for the generator, i.e. the high voltage/higher current generator is cheaper than an equivalent lower voltage/higher current generator.
- There are two options for the phone holder. One is a purchased holder while the other is manufactured. The manufactured design (supplied by this team) is approximately $1.50 more expensive, but carries the benefit of supplying labor to shops in Haiti. The latter choice may end up being more desirable.
- Attachment between the generator housing and the frame of the bike will be done using two rubber-coated pipe routing clamps bolted together. This will drastically reduce the cost and number of components needed in the final assembly. In addition the routing clamps are flexible yet sturdy enough to provide some spring force against the wheel if rotated while constrained. There are concerns with the bolts becoming loosened during rough road conditions and/or continuous use, but re-tightening the fixture should be a minimal concern provided it happens infrequently. Fixture will not be permanently set so as to provide a means for the customer to remove the generator from their bike if they so choose. Zip ties will be provided for cable management and to ensure that the generator housing does not collide with the wheel if the fixture is loosened during use. Should the pipe routing clamps be insufficient, other backup options have been accounted for. This includes: other clamping mechanisms, adding an arm between the two clamping mechanisms for support, etc. The primary alternate method is to add a second clamp on the frame connected to a second point on the attachment arm. This will add stability and a second point of constraint for the housing, greatly reducing the moment on the bolt and placing most of the load on shear stress.
- Some simple drilling fixtures will be supplied to ensure that certain machining operations (drilling holes, etc) are done with reasonable precision. All manufactured parts are set to a low tolerance of +/- 0.1 inches to accommodate workshops in Haiti.
- Exposure is of minimal concern. The generator is positioned away from any splashes caused by the bicycle wheels and is high enough on the frame to avoid most contaminates from the road. Any holes in the structures will either be shielded or faced away from anticipated rain direction. Environment testing will still be done in MSD II to confirm these assumptions.