P08427 – LED Lighting Technologies for a Sustainable Lighting Solution in Developing Nations

Product Improvement Recommendations & Future Direction for Work on the Project

16 May 2009

Rochester, NY

Dr. Stevens and Future Project Team Members:

The team members of P08427 would like to make recommendations for future work on this project. While our team was successful in establishing the base technology, there remain some critical points which should be addressed before prototype field testing is feasible. Additionally after discussing the matter with various professors it is possible that some of the specifications set for the product were not entirely reasonable. We would ask that future teams take our recommendations into consideration as we believe that they will lead to a more successful product. These recommendations, in addition to the documentation of the work already completed, should provide a clear point from which to start any future work. Thank you for your time and consideration – good luck with your work.

Sincerely,

The P08427 Team

Ian J. Frank – Team Lead (Mechanical Engineer)

Jesse Steiner – Electrical Engineer

Luke Spencer – Industrial & Systems Engineer

Matt Walter – Chief Engineer (Mechanical Engineer)

Mike Celentano – Electrical Engineer

Nick Balducci – Mechanical Engineer

SPECIFICATIONS:

Light Distribution Specification

After speaking with Dr. Borkholder and Dr. Stevens at the functional design review, the possibility of reconsidering the light distribution specification has come up as a possibility. The original spec states that 150 lux would be an ideal value for the distribution of the light, while 50 would be marginal. The original specifications were based on the rather high lighting standards of the United States. Given the fact that the kerosene lamps used in developing nations are capable of producing a maximum of 5 lux, the minimum 10 lux distribution seen at the edge of our lighting module’s lighted area provides twice as much light as is currently available.

Given this improvement as well as many other environmental, health, and financial benefits provided by the current system, it may be feasible to alter the light distribution specification. It would be best to collect end user feedback based on the current lighting levels to determine if the current light distribution is acceptable or if it should be increased. Suggestions for increasing the light distribution can be found in a later section of this document.

Operational Temperature Specification

Two questions arise with respect to the operational temperature specification. First, given the climate of the target nations, is the current specification (0-50°C) reasonable and second, do all of our components actually operate in the indicated range. Addressing the first question we must consider the fact that low temperatures in tropical climates are certainly never in the freezing range. To that end, it may be prudent to raise the minimum operational temperature to better fit the conditions of the intended regions for use. This revision will also help when considering the second question. It is not common to find batteries that will operate throughout this entire range of conditions. The high end of the range is generally fine, but the low end will likely cause one of two problems with the batteries. Either they will not function at all, or they will have a decreased lifespan. Such a decrease in lifespan would cause the current design not to meet the minimum battery lifetime spec of three years. So consideration should be given to the operational temperature specification to determine just how reasonable and necessary the current temperature range is to the design of product.

Drop Test Specification

The drop test specification was established when this project was in the DPM stage of its life. While some measure of durability is necessary, the proposed “drop test” is not necessarily the best way to proceed. Fifty drop cycles from a height of three meters provide a fairly extensivetest and would likely damage even the most robust prototype. Additionally, three meters is quite high as the prototype is not likely to be hung from a 3m (10’) ceiling. Similarly, fifty cycles is a fairly high degree of abuse and perhaps could also be decreased. The team would like to suggest that future teams consider a more reasonable specification with the help of their guide and customer contacts through our partner organizations (SOIL and H.O.P.E.).

LIGHT MODULE:

Power Switch

The current toggle switch design raises some durability questions. Given that the switch protrudes from the face of the light module, it would likely take the brunt of a frontal impact. Such an impact would damage the switch, perhaps to the point that it would no longer function. This problem could be averted by using a low profile push button switch (which would also be sealed and thus more water resistant) or by moving the location of the switch to a more protected location.

Increase Light Levels

  1. Look into different LED driver circuits, possibly ones that are capable of delivering higher currents.
  2. If the same driver is used (LT3474), consider placing the voltage divider that dims the LED from the REF pin to the Vadj pin, rather than from the Vin pin to the Vadj pin. This may require the use of two switches, one to turn the circuit completely off and one to switch between the two brightness levels.

BOM Changes

If the light distribution specification is reduced, it would no longer be necessary to use the same high power LED. A lower power LED (one rated for 0.8 watts) could be used instead. This change would help to reduce the overall cost of the light module. Additionally as the discharge duration spec is only 4 – 8 hours and the system is currently getting 12+, it would be possible to reduce the number of batteries used. If two batteries were removed from the design, the material cost of the unit would be reduced by $2.50 – a significant reduction.

Overall Design

Look into ways to make the lamp lighter. Perhaps only one central bolt is needed while smaller lower power LED go around it. This would also reduce cost and make machining in a third world factory much easier, cheaper, and faster.

Investigate LED that operate at the desired or lower power settings. Possibly use multiple lower power and cheaper LED versus our high power, high efficiency, and more expensive LED currently being used.

Make different sizes and shape lamps to accommodate different recycled cans. This would be extremely useful.

POWER MODULE:

Circuit Alteration

Currently there is one circuit board which has been populated and used. This is the board that we took to Washington, DC and used for all of our tests. Before we left, it appeared to be working. When we got to DC, the first guy who got on the bike pedaled as hard as he could and “let the smoke out” in short order. What I think happened here is that the voltage at the first stage input spiked, caused an internal short in the buck converter, which then raised the voltage at the second (constant current) regulator and caused it to push far more current than designed for. To fix this and prevent it from happening again, we added 3A fuses to prevent overcurrent and 38V zener diodes to prevent overvoltage. 38V was chosen because 40V is the max operating voltage of the buck converter. Two diodes are necessary since the motor can potentially produce 80W (just over 2A) at this current. In order to prevent the diodes from blowing up under high current conditions, they should be properly heatsinked, which they are not currently.

The first thing you should do is populate a fresh board with new components and test. If you need to order more, refer to the latest BOM for parts and suppliers. When populating a new PCB, there are a few alterations that need to be made:

  1. The diodes used in the buck converter circuit are not zeners, as their silkscreens suggest.
  2. The diode for the charger closest to the mounting holes has its silkscreen backwards. Installing a diode in the same direction will short the buck converter output to ground.
  3. The output capacitor of the buck converter shown on the schematic does not have a footprint on the PCB. It can be installed utilizing a via and a pad from another device.
  4. The fuses will need to be added using 20AWG wire. There is currently no mount for the fuse block.
  5. The input zener diodes do not have a footprint on the PCB. They need to be connected to the motor side of the fuses using 20AWG wire. There is currently no mount/heatsink for the zeners.

Once the board is populated, test each charger circuit individually. Check that the voltage is steady at the output of the buck converter and that the battery is charging with a constant current between 1.5A and 2A. Test under a variety of pedaling efforts. Test with and without a load (battery). If this circuit behaves the same as it does currently, the battery charge current will vary depending on pedaling speed. It will have two peaks and a valley. This is not correct, but it is the current state of the circuit that was being troubleshot. If the circuits all work, then the current problem involves one of the passive components or the PCB itself on the populated board.

If the freshly populated board is acting the same way with unstable charge currents, it is time to start thinking about new ways to charge the batteries. Here are some recommendations:

  1. Start at the battery and design backwards. Look at other NiMH charger chips, or consider not using one at all. A smart charger chip seems to be required to maintain battery lifetime and optimize charge time, but a useable and cheaper circuit may be possible without one. This will require a full understanding of NiMH chemistry cells. The basics available on most websites will likely not be enough, since they all focus on the “ideal” charging. You’ll find that the voltage of a NiMH cell drops slightly when it is fully charged. What happens if you don’t charge the cell fully, but instead use a voltage comparator to charge it to a specified voltage? This will obviously reduce capacity, but will it decrease battery lifetime as well? Is it a small enough decrease to be acceptable?
  2. Consider a new motor- look at the spec sheets and find one which will produce the desired power but possibly with a lower voltage at full speed. We picked our motor on price alone, and it came with no documentation. Characterization had to be done by us, which meant that we didn’t fully know how the motor would behave at full speed until the complete mechanical system was hooked up. At this point, the circuit was already designed using (now known to be false) assumptions.

Drive Components

  1. Motor mount should be connected in such a way that the two are always aligned. Currently the motor mount is fixed while the friction drive carrier is free to rotate. Rotation is necessary in order to accommodate the bike, so the motor mount should also rotate. The nut that currently holds the motor mount to the trained currently loosens and allows for easy misalignment of the motor and friction drive shafts.
  2. Use of a single coupling rather than the current two coupling and “spider” configuration to connect the shafts could provide several benefits to the drive system. Firstly, this change could reduce noise and parasitic losses in the drive system. Additionally, this connection would provide greater rigidity between the motor and the friction drive helping with the previously identified issue.
  3. Future project teams may consider the design of a custom support and drive system. This new part would accomplish the same tasks as the trainer but could be made specifically for this application. If designed correctly this could help accomplish the points set out in paragraphs one and two above in addition to saving a great deal of cost as the trainer is a fairly expensive component for the power module.

Overall Design

  1. Currently the box protecting the charging circuit is larger than the circuit within it. If after the power module circuit issues are addressed, this is still the case, then it might be reasonable to decrease the size of this box. This reduction would help provide a more reasonable profile for the power module and could also help to reduce costs.
  2. The use of a remote charging indicator introduces a fair amount of extra wiring to the overall power module setup. Future teams should evaluate the utility of mounting these indicators on the handle bars of the bike. If instead of being located on the handle bars, the indicators could be mounted on or near the rest of the power module the amount of wiring could be decrease and it would be easier to separate the bike and power module.
  3. Additional wiring issues exist on the power module. With all of the cable currently around the back of the bike, it is sometimes difficult to ensure that they will not get caught in the drive system. Additionally, when not in use, many of these wires sit on the ground. It would be safer and increase the lifetime of the cables if some sort of cable management were enacted. To this end, shorter cables and more cable ties might be considered.
  4. Look into other designs for a power-charging unit. Possibly, a treadle pedal design or a smaller power unit would be beneficiary. A unit that could be used by a household or community for charging multiple items, such as, cell phones, basic outlet for common appliances, and our lamps.

ADDITIONAL RECOMMENDATIONS/PROJECT DIRECTION:

Field Testing

Once a fully functional prototype system has been completed, field testing should be completed. Working through SOIL and H.O.P.E. the system should be distributed to the end user. Data should be collected to determine how well the individual subsystems (light module and power module) function and meet the needs of the end users. Special interest should be paid to the light distribution provided by the light module and the feasibility of someone operating a business based on charging lamps. Some questions to be answered would be:

  • Does the current light module design provide adequate light?
  • Are the light modules useable for a large range of activities?
  • Would people be willing to pay for the light module given the costs and benefits?
  • Could someone bike all day to charge modules?
  • Can they make enough money doing this to make the upfront costs worthwhile?

Do more field research! If you cannot get to where you are trying to design this for, find more resources and examine it extensively. This is one piece of advice that we learned was extremely valuable in our P3 competition interviews. This is the most important thing, understand the needs fully, otherwise the product could go nowhere.

Design Iterations

Based on the results and feedback from field testing, changes should be made to the design of both modules to better address the need of the customer and provide the best possible product. Further work should be done to reduce the cost while maintaining a high degree of quality so that it will be possible to provide a low-cost yet durable lighting solution.

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