Requirements Specification
1
Overview:
Our project team will design and build a portable generator powered by the
rotary action of a bicycle wheel, capable of generating environmentally clean
energy which may be used for supplying low power applications (up to 100 W).
The energy storage unit may either be simply charged exclusively while pedaling,
or some of the power may be routed to an attached headlight, lowering the
charge rate on the energy storage unit by a small amount (not more than 20 W).
Once charged, the energy storage unit may be detached and later used as a
standalone power supply.
Problem Statement:
In a world where more people in our society are exhibiting environmental
conscientiousness, bicycles are rapidly becoming the most economically and
ecologically viable alternative transportation method. This can be attributed to
the bicycle’s lack of emissions and gasoline expenses. Consumers who have
already acknowledged the benefits of this form of transportation would
undoubtedly be attracted to a way to increase the bicycle’s positive
environmental impact even more.
Through P.E.E.V.S., we aim to reach out to these potential consumers. By
designing a portable energy storage unit which uses the bicycle as a renewable
mechanical source of energy in order to supply low power loads (cell phones,
laptops, etc.), we will add yet another benefit to the list of the bicycle’s
environmental contributions.
Operation:
Operation of the device will be simplistic and intuitive, such that the bicycle
is not any more difficult or unwieldy to ride (based on O2 consumption) than any
other conventional bicycle. While in use, the movement of the bicycle’s wheel
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automatically stores energy to the unit through a generator. The user may flip a
switch attached to the handlebars if he/she wishes to instead power a mounted
headlight.
Once charged, the user may detach the energy storage unit from its
housing on the bicycle, which may then be used to power other electrical loads
once an on/off switch allowing electrical discharge has been flipped to the ‘on’
position. The user will be notified once the supply has been depleted of energy
and has cut off discharge, so that the user may then reattach it to the bicycle in
order to charge it again.
Deliverables:
 Mobile bicycle with gear shifting capabilities and power supply
mount
 Detachable energy storage unit with LED indicators
 Generator
 LED headlight with on/off switch
 User’s manual
 Circuit diagrams/schematics including analysis and simulations
 CAD drawings and analysis
 Final report including test results, list of materials, and final design
Draft User’s Manual:
Setup:
 Flip power supply discharge switch to the ‘off’ position
 Mount power supply
- Fasten and tighten housing clamps
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- Plug in LED headlight (ensure headlight switch is in the
‘off’ position)
- Connect energy storage unit to generator system
Operation:
 Ride bicycle in order to charge power supply
 For night riding, user should flip both the power supply switch
and the headlight switch to the ‘on’ position to power the
headlight
 Once the power supply has been fully charged as indicated by
an LED on the power supply, the supply may be detached after
ensuring that both switches are in the ‘off’ position
 At this point, one or two low-power loads (not to exceed
100W) may be plugged into the energy storage unit
 Charging of the low power device will begin once the user has
flipped the discharge switch to the ‘on’ position
 The energy storage unit will no longer discharge once the
voltage level drops to a predetermined threshold, as indicated
by a low power LED
 Repeat setup instructions for further use
Maintenance:
 Store bicycle in a dry location protected from the weather
 Ensure that the discharge switch is set to the ‘off’ position
when not in use
 Normal bicycle maintenance applies (chain tension and
lubrication, tire pressure, etc.)
User Interface:
 On/Off switch for headlight
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 On/Off switch for energy storage unit
 LEDs indicating maximum charge and low power
Physical Capabilities:
 Easily rideable (such that the supply may be fully charged in
2.5-3 hours of pedaling at a rate of approximately 50 rpm with
the rider’s oxygen consumption ranging from 1-1.5 L/min.)
 12-14 W headlight operated by an On/Off switch with a 15-20o
beam dispersion, able to illuminate a distance in front of the
rider up to at least 30 meters with maximum intensity of at
least 20 candela.
 Headlight will run continuously (with constant luminosity,
beam dispersion, etc.) as long as the power supply discharge
switch is in the ‘on’ position and the charge level in the supply
is above the cutoff threshold
 Will be able to support two low-power electrical loads (up to
100 W each for two hours)
 Generator able to generate 125-150 watts
 Energy storage unit maximum capacity of at least 400 watthours
 Waterproof power supply casing
 Automatic cutoff when the charge drops below 5% of its
maximum capacity
System Testing:
Supply testing: The energy storage unit will be tested over 20 complete
charge/discharge cycles at a supply rate of no less than 40 W*hr to confirm that
the system performs as well as originally specified. The cutoff system must be
shown to occur within a 20% tolerance of the 5% charge level cutoff threshold.
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Peripheral testing: Through night-time testing it must be demonstrated that
the headlight is capable of meeting all of the above listed physical qualities.
Mechanical testing: Due to the addition of external parts on the bike,
testing will need to be done to ensure that the structural integrity of the bicycle is
not compromised. We will measure and record the initial tire pressure, tread
height, bolt tightness, etc. of the bike. We will then ride it at a steady rate of 4050 pedal rpm for a total of 25 hours and record the new measurements, which
will then be compared quantitatively. All measured values should be within 10%
of the original values.
Ease of ride testing: In order to verify the comparative ease of riding the
bicycle with existing industry standards, the oxygen consumption for a batch of
three students will be measured while riding for five minutes at an average rate of
50 pedal RPM. The average required oxygen consumption must be less than 1.5
L/min.
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Mechanical Accomplishments
7
Crankshaft and Power Supply Mount
Once we decided on using a crankshaft/piston mechanism, we had to
decide how to implement it. It was decided to use a chain to drive a separate
crank wheel, much like the way the bicycle chain drives the rear wheel of the
bike. To do this we had to think of where we would mount the crank wheel on the
bicycle. Because we knew that we would already be building a shelf on which the
power supply would sit, we decided to fabricate it in a way that would facilitate
the use of the crankshaft and piston assembly.
Power Supply Rack
The crank wheel is bolted through a slotted hole in the right side support of
the power supply rack (see figure 4.1) and a secondary bike chain is driven by the
rear gear set and in turn drives the crank wheel which has a single bicycle gear
mounted on the back of it. The power supply rack had to be made differently than
other similar bike racks because it had to account for the motion of the chain
around the right side support. Because of this, the right side support consists of a
single upright steel bar which tapers downwards and features an inward bend.
This bend is necessary because the chain, which is driven by a gear inside the
frame of the bike where the rack support is fastened, needs to then drive the gear
on the crank wheel, which is mounted to the outside of the support. The support
on the other side of the rack is made of two narrow steel bars which are welded
together at the bottom and create a triangular shape with the rack platform. This
shape is designed to provide extra stability to the rack which is lacking on the
right side. The rack as a whole was made by shaping bars of mild steel and
welding them together. The two side supports fasten to the bike with screws in
the pre-existing holes in the bike frame near the rear axle. The platform attaches
to the bicycle by way of an adjustable bar and a clamp which tightens around the
seat post of the bike.
8
Slotted hole through
which the crank wheel is
mounted
Figure 4.1
Power Supply Mount (Side View)
Figure 4.2
Power Supply Mount (Top View)
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Chain
To drive the crankshaft we purchased a standard bicycle chain and adjusted
it so that it is the correct length to have the necessary tension between the rear
gear-set and the crank wheel. A slotted hole in the power supply rack support also
helps to adjust the tension in the chain by allowing the crank wheel to be raised
or lowered depending on the length of the chain and the amount of tension
needed.
Crank Wheel
The crank wheel itself (see figures 4.3 and 4.4) is a 7.5 inch (19.05 cm) disc
made of ¼ inch (0.635 cm) thick aluminum with a ball bearing press fit into the
center. This bearing allows the wheel to rotate around the large bolt that holds
the crank wheel to the power supply rack. Another smaller bearing is press fit into
a hole which is placed 3 inches (7.62 cm) from the center of the crank wheel. This
hole acts as the pivot position between the crank wheel and the connecting rod. A
22 tooth chain-ring and another smaller aluminum disc (which provides extra
stability to the chain-ring) are bolted to the large aluminum disc using spacers
which provide clearance between the chain and the disc.
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7.5” Aluminum Disc
Bearing
22 tooth gear
Hole for connecting
rod
Figure 4.3
Crank Wheel (Back View)
Figure 4.4
Crank Wheel (Front View)
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Connecting Rod
The connecting rod (see figure 4.5) is made out of a 15 inch (38.1 cm) long
bar of ¼ inch (0.635 cm) aluminum. Much like the crank wheel it has two bearings
press fit into it, one on each end. One end of the connecting rod attaches to the
crank wheel at the small bearing and the other end attaches to the piston. The
connecting rod was originally designed to be a straight bar, but upon further
consideration, it was decided that putting a bend in the bar to close the gap which
would normally exist between the end of the piston and the connecting rod
would significantly decrease the moment that inevitably occurs due to the
distance between these two parts without weakening the aluminum bar
noticeably.
Bearings
Figure 4.5
Connecting Rod
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Gear Ratio
One issue that arose in the operation of the crankshaft was that the gear
ratio between the pedals and the crankshaft needed to be high enough to
produce the needed voltage, but low enough to not cause problems in the
crankshaft or the piston. The secondary chain was adjusted and both chains
arranged on the rear gear-set to allow for a 1.6:1 gear ratio. This means that for
every rotation of the pedals the crankshaft rotates 1.6 times.
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Faraday Apparatus
As stated in our design from last semester, our goal was to make a
mechanism which can generate electricity by way of Faraday’s principle. The
complete system includes the magnetic piston, aluminum cylinder, magnet wire
solenoid, wheel crank and connecting bar.
Crank Wheel
Piston
Crankshaft
Figure 4.6
Faraday Apparatus
The figure above shows the Faraday apparatus which we made. The wheel
crank, which has radius 3” (76.2 mm), is driven by the rotation of the rear gear
connecting to the driving gear. The connecting bar, which connects the piston
and wheel crank, then transfers the angular motion of the wheel crank into
translational oscillation. The piston, which is 8” in length (203.2 mm) and 1” (25.4
mm) in diameter, moves inside of the aluminum cylinder, and the aluminum
cylinder is covered by the outer plastic cylinder. The plastic cylinder is fixed to the
power supply mount with various nuts and bolts.
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The wheel crank, the connecting bar, the piston and the plastic outside
casing were made with exactly the same dimensions as ones we selected in the
last semester. Only the aluminum cylinder causes contributes to the piston
catching is at oscillates, an issue which we will discuss shortly.
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Aluminum Cylinder
The aluminum cylinder was originally 6.5” (165.1 mm) in length, 1.875”
(47.6 mm) in diameter at the thickest point and 1.1” (27.9 mm) at the thinnest
point. After testing the mechanical system with all the mechanical components
mounted, we found that the piston would stick at its furthest extension away
from the rider. This created a jolting effect when the bicycle was being ridden,
manifesting itself most noticeably for the rider at the pedals. To fix this problem,
we made an extension for the cylinder to be attached to one of its sides. In this
way, when the piston comes out of the cylinder, a much longer portion of the
piston will still be in the cylinder, effectively eliminating the issue with the piston
catching as it rotates.
Connecting Joint
Additional
Aluminum Cylinder
Original Aluminum
Cylinder
Figure 4.7
Aluminum Cylinder w/ Length Extension
Figure 4.7 shows the complete aluminum cylinder with the new addition.
The length of the extension is 2.5” (63.5 mm) and the diameter is 1.875”
(47.6mm), just like the main cylinder. Because both pieces have the same
diameter, they meshed together perfectly, held in place with high tensile glue.
The total length of the cylinder with the extension is 9” (228.6 mm).
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Fixture of the Cylinder
In order to fix the aluminum cylinder to the outside plastic casing, screw
threads were made through the aluminum cylinder, as shown in Figure 4.8.
Screw
Threading
Figure 4.8
Aluminum Cylinder Inside Plastic Casing
To fix the plastic and aluminum cylinders together, we welded two steel
plates onto the supports for the power supply mount, allowing us to fix the
cylinder in place both vertically and horizontally. Bolts and nuts were then used to
attach the cylinder to the supports. By having been fixed so securely, the cylinder
is able to withstand the forces inherent in high speed operation of the piston. A
figure is shown below illustrating how the combined plastic and aluminum
cylinders are mounted to the bicycle below the power supply mount.
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Power Supply Mount
Connecting
Bolts and Nuts
Horizontal
Fixture
Plates
Phenolic Piston
Figure 4.9
Mounted Piston and Casings
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Aluminum Cylinder
Drive System
As mentioned before, the purpose of the mechanical system is to convert
rotational motion of the wheel crank into linear motion of the piston within the
aluminum cylinder. The drive system allowing this to take place includes two
chains: one chain connecting the pedal to the real wheel gear cassette, and one
chain running from the rear wheel to the wheel crank. As the figure below shows,
the two chains are wound around different gear settings and thus rotate and thus
have a variable speed ratio. The current gear ratio between drive gear and the
driven gear is 1:1.375, which means the piston goes into the cylinder and comes
out to the start position 1.375 times within one full rotation of the pedals.
Primary Drive
Chain
Secondary Drive
Chain
Figure 4.10
Chain System (Side View)
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Mechanical Efficiency
Mechanical efficiency is a necessary quality to evaluate concerning the
mechanism, and is defined as the ratio of work in to work out. Specifically, we
can calculate how much energy the rider must use to ride the bicycle, but it is
hard to determine how much of that energy is transferred to the piston because
the piston is moving during the measurement. For this reason, we chose to
measure the forces exerted on the piston and pedal statically using fish scales.
Since we use force transfer efficiency to determine the mechanical efficiency, the
mechanical efficiency is the ratio of these two forces.
In the experiment shown below to calculate the force transfer efficiency,
the angular reference is set to be the position of the wheel crank when the piston
is at its fullest extension out of the aluminum cylinder, and is defined as 0°.
The “Original Efficiency” column on the table below indicates the efficiency
before the extension was made to the cylinder, while the “Current Efficiency”
column shows the effect of adding that extension on the overall efficiency of the
bicycle. All the data in the table below is an average of four separate
measurements.
Angle of
pedal (0)
10
25
40
Force on
pedal (N)
6.78
8.8
11.3
Force on
piston (N)
2.95
3.89
6.21
Current
Efficiency
0.435
0.442
0.550
Original
Efficiency
0.255
0.346
0.546
Figure 4.11
Mechanical Efficiency at Various Angular Positions of the Crank
It is readily apparent that current efficiency is much higher than the original
efficiency, especially near the “problem area” close to that 0° mark at the furthest
extension of the piston. Because the addition of the extra length to the cylinder
was the only mechanical change to the system since original efficiency was
recorded, it can be inferred that the extra length is the factor that increased the
efficiency to its current value.
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Power Supply Case
The purpose of the power supply case is used to protect all the electrical
components from the environment. The dimensions of the power supply case are
12.9” (327.7mm) long by 7.65” (194.3 mm) wide by 7.75” (196.9 mm) high,
exactly as we designed. The power supply case contains the battery, the power
inverter, and the PCB and associated electrical parts (relays, inductors, etc.). The
case lid rotates on hinges and is locked shut with a latch. The figure below shows
the power supply with only the lead acid battery.
Hinges
Latch
PCB Location
Figure 4.12
Power Supply Case w/ Battery
Glue is used to connect each plate of the power supply case. The whole
case is attached to the power supply mount with bolts and nuts. The below figure
shows the construction of the fixture. To make the power supply case be
perfectly horizontal, two holes were drilled in the bottom of the case to leave a
space for the bolts which are used to connect the mount and the plastic cylinder.
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Holes for the
Connecting Bolts
Fixture of
Bolt and Nut
Fixture of
Bolt and Nut
Fixture of
Bolt and Nut
Fixture of
Bolt and Nut
Figure 4.13
Power Supply Case – Top View (at top), Front Bottom View (at left), Rear
Bottom View (at right)
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Electrical Accomplishments
23
Cutoff System Changes
Since the midterm progress report, multiple changes have been made to
the overcharge protection and discharge cutoff systems. Both circuits have had
their PNP transistors removed, which were found to be unnecessary. The error in
our former wiring scheme for both systems stemmed from a misunderstanding of
the functionality of the MAX8211 voltage detector, which we have learned
actually sinks current until its threshold is exceeded. Thus, the output of the
voltage detector itself may be used as a switch, with a pull-up resistor to supply.
Resistances in both systems were also adjusted slightly to put the thresholds
closer to our intended experimental values of 12.9 V and 11.8 V, respectively. As
will be discussed in the “System Testing” section, both systems have been shown
to function appropriately once soldered into our professionally made circuit
board. Multisim schematics of both circuits are shown below.
Figure 4.14
Multisim Schematic of Overcharge Protection System
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Figure 4.15
Multisim Schematic of Discharge Cutoff System
The series logic inverters in both circuits are simply logical buffers
implemented to ensure consistent signals. Also, because six inverter gates are
used, and the 7404 hex inverter contains six gates, no additional ICs will need to
added to the design in order to implement the logical buffers.
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Circuit Board Design
Since our last progress report was submitted, several changes have been
made to the PCB schematic on PCB Artist: namely, corrections to the design such
as fixing incorrect lead paths, spreading out the design to maximize the available
surface area, and adding a diode immediately before the battery to prevent backcurrent. The final design at the time of ordering is shown below.
Figure 4.16
PCB Artist Schematic Layout
Note that there are several mistakes with the board, which were not noted
until the board had already arrived. Firstly, because there are not 8-pin DIP
component templates in the PCB Artist library, we attempted to place each pin
individually at the correct spacing. The results can be seen above in the circled
regions of Figure 4.16. As you can see, the pins for the MAX8211 voltage
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detectors are thus spaced much too close together on the horizontal (compare to
the 7404 hex inverter, located in between the two voltage detectors). As a result,
it became necessary to determine a workaround, as a new board could not be
ordered. To solve the problem, we cut off all the pins on the right sides of both
voltage detectors, inserted the pins on the left sides normally into the board, and
then ran wires from the board to the ICs for the two pins on the right side that are
used in each chip (supply and ground). The result is not very aesthetic, but
continuity tests determined that the patch fix had worked correctly.
Another complication arose with the connectivity of the leads where the
MAX1745 DC/DC converter was to be soldered onto the board. Because of
unfamiliarity with the PCB Artist software and the processes involved in the
professional manufacturing of a printed circuit board, the design was sent in such
that those leads would be insulated by solder mask and silkscreen. The solution
in that situation was simply the careful scratching away of the solder mask
covering those pads so that connectivity with the DC/DC converter could be
achieved.
Finally, the geometry of the board proved to be less than ideal. The 330 µF
capacitors implemented in the filtering subsystem were much larger than
anticipated, and as such only four of the six total capacitors to be used there were
able to be fitted onto the board. At this point, with the level of energy generation
we are observing, four capacitors will certainly be ample, but for the purposes of
completing the circuit board the capacitors will simply be staggered vertically to
fit all of them in. Spacing issues also arose with the toroidal inductor, which was
significantly larger than predicted. To correct this, we will simply tape the
inductor to the side wall of the power supply case and run wires from the two
ends of the inductor to their respective through holes on the board. This not only
solves the geometry issue for the inductor, but prevents the electromagnetic field
generated by the inductor from interfering with the rest of the circuitry on the
circuit board.
Aside from these things, there were no other major issues with the board,
and none of the issues mentioned were irreparable.
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Circuit Board Construction
All soldering on the circuit board has been completed. The circuit board
may be seen below without input and output wires, as those wires were included
only after the circuit board was installed in the power supply case.
Figure 4.17
Circuit Board w/ Soldered Components (Top View)
Figure 4.18
DC/DC Converter Lead Solders
Note from the top view that a PFET to the right of the converter is not yet
placed. This PFET was, unbeknownst to us, not ordered because of a confusion in
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Harding purchasing, and has just recently been ordered. Its function in the circuit
is to create the duty cycle by which the converter steps down the input voltage by
rapidly switching on and off, and as such is absolutely necessary to show charging
of the battery (all current flowing from the generator must go through that PFET
to get to the battery).
Figure 4.19
DC/DC Converter Schematic w/ Peripheral Circuitry
Also, if one observes the top view of the circuit as seen in Figure 4.19, it is
apparent that two resistors are missing to the left and down from where the
DC/DC converter is to be placed on the board. These resistors comprise a voltage
divider running from the OUT pin to the FEEDBACK pin of the MAX1745 which
sets the duty cycle of the output signal. Effectively, these resistors determine the
ratio of the output voltage to the input voltage, allowing us to set how much the
input is stepped down. Although we are generating roughly 35 V RMS, we plan to
step the voltage down by dropping the drive chain to a lower gear setting. Until
we know what voltage this new gear setting will generate at our specified riding
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rate of 50 pedal RPMs, we cannot know how to set these resistors. Those values
will be determined and implemented by the time that the next rough draft is
submitted.
30
Solenoid Construction
The first winding of the solenoid contained approximately 850 ft. (259 m) of
34 AWG magnet wire. The wire was wound onto the aluminum cylinder by fixing
the cylinder snugly onto a ratchet bit for a power drill using electrical tape. The
spool of magnet wire was slid vertically onto a metal rod held in place by a shop
clamp, and the first portion of the wire from the spool was taped to the cylinder.
The power drill could then be used to rotate the aluminum cylinder and
mechanically “feed” the wire onto the cylinder, winding it much more quickly and
evenly than could have been done by hand. Once the winding was complete, the
thin, brittle magnet wire was wrapped around and soldered to thicker lab wire,
with heat shrink covering the connection. Electrical tape was then wound around
the coil to keep the windings in place, and the positive and negative lead wires of
the coil were taped such that no tension would be placed on the magnet wire.
However, as will be discussed in the testing section, the observed voltage
produced by passing the magnet through that coil was much lower than
expected, and it was decided to unwind the aluminum cylinder so that the
mechanical alterations elaborated upon in the “Mechanical Accomplishments”
section could be made. Once those adjustments were complete, the cylinder was
rewound with the remaining 1100 ft. (335 m) of magnet wire. The hope was that
the combination of the reduction of the cylinder diameter and the addition of
more windings to the coil. As will be discussed in the “System Testing” section,
those results were determined much more precisely after surmounting some user
error issues with regard to the oscilloscope. The image of the completed coil is
shown below.
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Figure 4.20
Wrapped and Wired Aluminum Cylinder
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System Testing
33
Holding Force Testing
Because initial readings, both on a voltmeter and an oscilloscope, had
shown the waveform generated by the Faraday apparatus to be peaking at only
about 4 V (and then dropped even more as testing proceeded), there was concern
that the magnet contained within the phenolic piston might have been partially
demagnetized since it was received in the fall. In order to determine if such a
thing had occurred, the magnet was taken to the GAC to see how much weight it
could pick up. The magnet was able to pick up 17.5 lbs. (a 10 lb., 5 lb., and 2.5 lb.
weight), but not 20 lbs. Considering that the magnet was rated for 70 lbs. of
holding force at the time of its purchase, it is fair to conclude that the magnet has
been demagnetized significantly.
Although the actual cause of this may not be determined for certain using
experimental methods (at least not without further worsening the condition of
the magnet), culpability most likely rests on eddy currents circulating within the
magnet. These eddy currents could be induced by the reactionary magnetic field
which is in turn induced by the current with the solenoid. Called the Lenz effect,
this phenomenon manifests itself quite strongly in our prototype, to the point
that the magnet cannot be forced through coil by hand due to the extremely
strong oppositional force generated in response.
The demagnetization of the magnet does not spell the end for the project,
as we are still able to demonstrate a voltage being generated. More importantly,
because the magnet has been reduced to approximately 20% of its original
holding force, it is not unreasonable to assume that it would be able to generate
roughly five times as much voltage at that holding force. This is because the rate
of change of magnetic flux, and thus the induced electromotive force, must
increase proportionally to any increase in the strength of the magnetic field lines
coming out of the magnet. If we multiply our current RMS voltage of ~2.8 V by 5,
we get 14 V. With a little tweaking of the solenoid (changing the magnet wire
diameter and number of coils experimentally to find an optimum value), it could
be very feasible to generate enough voltage to charge the battery. Because of
drops across the rectifier diodes and relays, this “target voltage” is approximately
17 V.
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Energy Generation Testing
When the first winding of the aluminum cylinder was complete, testing
showed a peak generated voltage of roughly 2.5 V (measured via voltmeter and
one of the older lab oscilloscopes). Approximating the energy waveform as
sinusoidal, we can find the RMS voltage by dividing by the square root of two,
with a result of 1.77 V. These low generation figures prompted us to make all of
the changes outlined previously, reducing the cylinder diameter and rewinding
with more magnet wire. Using the same voltmeter and oscilloscope, a new peak
voltage of approximately 4 V (2.8 V RMS) was observed: still unacceptable for
purposes of meeting our requirements specification.
Regardless, because a waveform was being shown at all, it was important
to record it for submission in our final design report. To do so, we used one of the
new oscilloscopes with a USB hook-up and the included software package to
capture waveforms being generated by the Faraday apparatus. The resulting
waveform is shown below
Figure 4.21
Energy Generation Characteristic of the Faraday Apparatus
The rotational speed of the pedals when that waveform was capture was
approximately 60 RPM. As was mentioned in the “Holding Force Testing” section,
these numbers certainly show proof of concept, which is a step in the right
direction. It goes without saying that the Faraday apparatus is critical both for
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meeting most of the requirements, and because it links the mechanical and
electrical systems of the prototype together.
Observe how the waveform is not perfectly sinusoidal, as might be
expected. This is because of the constant acceleration and deceleration of the
piston as it oscillates linearly within the aluminum cylinder. Also notice that
approximately two complete periods of the waveform occur in one second, the
time it takes for the pedals to undergo a full rotation at 60 pedal RPM. This may
be attributed to the fact that, at the time of these tests, the gear ratio for the
mechanical system was set to slightly less than 2:1.
Finally, compare this waveform to the one shown below (Figure 4.22). This
waveform shows the output characteristic with a 5 Ω resistor attached in series
with the generator.
Figure 4.22
Energy Generation w/ Resistive Load
Inspection reveals a peak voltage of about 0.5 V, or 0.35 V RMS. Using
Ohm’s Law and dividing this voltage by the sum of the resistances of the coil and
the resistor yields current as follows:
𝐼=
𝑉
𝑅
=
0.35 𝑉
314 Ω+5 Ω
= 1.1 𝑚𝐴
In the same manner, power delivered to the load can be calculated by:
𝑃 = 𝐼 2 𝑅 = (0.0011 𝐴)2 ∗ 5 Ω = 6.05 µ𝑊
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Obviously, with values such as these, it was not possible to meet our
requirements specification defining how long it would take to charge the battery
(2.5-3 hours). Knowing that the electrical system restricts the battery to a voltage
range between 11.8 V and 12.8 V, and is rated at 40 A*h, the total energy
necessary to charge the battery is (12.8 V-11.8 V) * 40 A*h = 40 W*h. Assuming
that the magnet was strong enough to generate adequate voltage to charge the
batter, and that charging takes place at a rate of 6.05 µW, the time required to
charge the battery from empty to fully charged would be as follows:
𝑡=
40 𝐴 ∗ ℎ
= 36,364 = 1,515 days = 4.15 years
0.011 𝐴
Although these numbers do not seem to indicate any level of practicality,
they do provide theoretical proof that the bicycle could effectively function as a
source of energy for the lead acid battery. Making dimensional adjustments to
the solenoid, obtaining a higher gear ratio and purchasing a more powerful
magnet are all alterations that would increase the power supplied to the load. If
the prototype were to be designed again, these are the first changes to the design
of the Faraday apparatus that we would have made.
37
Oxygen Consumption Testing
Despite our recent successes in generating energy, no amount of energy
generation will make the prototype marketable if the device is too difficult to
ride. To ensure that that is not the case, one of our testing requirements is to
measure the oxygen consumption of three different subjects each riding the
bicycle for a period of five minutes at 60 pedal RPMs. The pace was set by a
metronome, and two sets of data, one with the Faraday apparatus and one
without, were to be taken.
That testing was performed with the assistance of Dr. Ken Turley in the
kinesiology department, using a specialized piece of equipment which measured
the amount of air inhaled and exhaled. Using that information, the computer
connected to the device calculated key pieces of information such as percent
oxygen metabolized, carbon dioxide exhaled, etc. The printed results of those
tests may be found in Appendix C.
Most important to us, however was the VO2 data, which is the oxygen
consumed by the rider in L/min. In the appendix, this data is the second column
in each experiment. Below you can see that data consolidated into a single table,
along with mean values for each experiment. To avoid outliers caused by the
body’s initial adjustments to the physical exertion of riding the bicycle, Dr. Turley
suggested that the first few points in each experiment not be used for
calculations. Note that data samples were taken in 20 second intervals.
38
Data Points
Average Values
Oxygen Consumption Rate Test Results (L/min)
With Piston Connected
With Piston Disconnected
Subject Subject Subject Subject Subject Subject
1
2
3
1
2
3
0.72
0.63
0.7
0.67
0.67
0.82
0.64
0.69
0.76
0.87
0.74
0.6
0.62
0.67
0.74
0.76
0.74
0.97
0.85
0.65
0.78
0.65
0.69
0.82
0.62
0.63
0.6
0.65
0.66
0.72
0.77
0.75
0.49
0.54
0.46
0.57
0.65
0.67
0.42
0.64
0.6
0.56
0.74
0.72
0.62
0.74
0.72
0.62
0.62
0.7
0.62
0.62
0.72
0.7
0.67
0.7
0.64
0.54
0.71
0.66
0.69
0.62
0.69
0.7
0.6
0.52
0.57
0.56
0.44
0.6
0.58
0.57
0.86
0.73
0.49
0.46
0.63
0.63
0.73
0.7
0.58
0.88
0.77
0.64
0.5
0.69
0.8
0.7
0.61
0.57
0.57
0.7
0.37
0.65
0.63
0.52
0.72
0.64
0.44
0.43
0.64
0.54
0.69
0.63
0.604
0.66
0.658
0.603
0.679
0.673
Figure 4.23
Oxygen Consumption Rates for Three Subjects
A paired t-test of the above data was conducted with the help of Dr. Olree,
with a resulting p value of 0.2136. From that result, one may reasonably and
statistically conclude that there is no additional increase in riding difficulty for the
operator due to the Faraday apparatus. The fact that the averages for the tests
with the piston connected are actually lower is almost certainly due to
experimental error, such as inconstant riding rates, metabolic processes as the
body adjusts to aerobic activity, etc.
39
Updated Budget
Already purchased:
Item
Bicycle
Headlight
Headlight Replacement
Copper wire
Magnets
LEDs
Phenolic, Aluminum, Plastic
Solid-State Relays
Pencil Sharpener (mock-up)
Power Inverter
Lead Acid Battery
12 x 330 µF Capacitors
4 x Rectifier Diodes
Chain
Bearings
22-Tooth Gear
34 AWG Magnet Wire
2 x Voltage Detectors
2 x DC/DC Convertors
DC/DC Converter
Components
Glue
Circuit Board
Power Supply Case
PFET
Total Spent:
Vendor
Wal-Mart
B2Cshop24.com
B2Cshop24.com
Smallparts.com
Bunting Magnetics
jameco.com
jameco.com
Staples
Wal-Mart
BMF Batteries
Pololu Robotics
Allied Electronics
Sport’s Basement
Grainger
Sport’s Basement
Powerwerx
Maxim
Maxim
Jameco.com
Cost
$214.92
$7.34
$7.34
$26.59
$40.60
$8.08
$56.00
$49.90
$10.79
$19.59
$152.04
$11.15
$9.68
$8.74
$25.96
$7.65
$13.21
$0.00 (Samples)
$0.00 (Samples)
$20.20
Lowe’s
Advanced Circuitry
Lowe’s
Jameco
$10.00
$49.58
$10.00
$9.88
$769.24
Possible Vendor
Cost
$80.76
$80.76
To be purchased:
Item
Contingencies
Total Remaining:
40
Budget Analysis
At the beginning of the fall semester, the estimation was that nearly $300
of the allotted $850 for the project would remain simply as contingency funds.
Unlike other projects this year, no microprocessor and relatively few electrical
components were necessary for our construction. However, purchases could
have been planned more carefully, such that more orders were placed in bulk.
What we quickly learned was that shipping was a major issue in managing the
budget, and one we had certainly not anticipated. Many of our parts orders,
especially from Jameco, were done individually, and there were several cases
where shipping and handling cost more than the part itself.
However, the project is still over $80 under its expense limit, leaving us
ample room to potentially buy a transformer to step up the generator voltage or a
new magnet, if time had permitted and the necessity arisen. Had our project
been more complicated (i.e. contained a microprocessor) our expenditures would
have brought us very close to going over budget. Future students of this class
should take note to take such factors into consideration when formulating a
budget for their project, and always assume that something will cost more than
the price tag value.
41
Appendices
42