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218 Tom E. Corley Building
Auburn University
Auburn, Alabama 36849
20 February 2015
Dr. Tim McDonald
Adviser, War Eagle Pullers
Biosystems Engineering Department
Tom E. Corley Building
Auburn University, Alabama 36849
Dear Dr. McDonald:
A preliminary design report for an actuated, non-linear control system for the Auburn University
War Eagle Puller’s continuously variable transmission is attached. This report contains our
design analysis and recommendation for the implementation of the control system. We believe
that this system will provide the War Eagle Pullers with a way to vary the gear ratio smoothly,
quickly, and independent of engine speed.
The purpose of this report is to provide a preliminary design recommendation for a control
system for a continuously variable transmission. Two design alternatives are presented, along
with necessary calculations and cost estimates. Based on our analysis, a recommendation is made
for proceeding with implementation of the control system.
On the behalf of Auburn University and the Biosystems Engineering Department, we would like
to thank you and the War Eagle Pullers for the opportunity to participate in this project. It has
served as an instructive, hands-on experience, and we appreciate any feedback or suggestions for
improvement.
Respectfully,
Conyers Coupland
Dustin Till
Sarah Ashworth
Enclosure
2
Non-linear, Actuated Control System for a CVT
Preliminary Design Report
Auburn University
February 12, 2015
CAT A TAC Engineering Team: Sarah Ashworth, Conyers Coupland, Dustin Till
Client: Auburn University War Eagle Pullers and Dr. Tim McDonald
3
Executive Summary
The American Society of Agricultural and Biological Engineers (ASABE) hosts an
annual Quarter-Scale Tractor Competition where university students design and build a quarter
scale tractors that compete in a variety of events. For the 2015 competition, the Auburn
University team, the War Eagle Pullers, have elected to use a continuously variable transmission
(CVT). The purpose of this report is to outline two designs for an actuated, non-linear control
system for a CVT that will be incorporated into the quarter-scale tractor of the Auburn
University War Eagle Pullers. An electric, linear actuator will apply lateral force the movable
sheave of the primary pulley. The actuator will mounted in parallel with the moveable sheave,
and a mechanical advantage will be utilized.
In the first design, the flyweights will left in the CVT, and an actuator will be used to
move the sheave of the primary pulley. The actuator will act parallel with the primary pulley, and
a 2:1 mechanical advantage will be utilized. A microcontroller will adjust the position of the
actuator according the user selected competition mode. In the second design, the flyweights will
be removed from the CVT. The mounting position of the actuator will remain the same as the
first design, and the same 2:1 mechanical advantage will be utilized. As with the first design, a
microcontroller will be used to control the position of the actuator. In order to evaluate each
design alternative, an economic breakdown and performance analysis of how well each design
adheres to the project constraints is provided. Since the control system and actuator for both
designs are the same, both designs have an expected cost of $610. Due to the 25% duty cycle of
the actuator selected, and the large pushing and pulling force required by the actuator to
overcome the flyweight force, CAT A TAC recommends proceeding with the actuator-only
design.
CAT A TAC will build the selected CVT control system, including the development and
programming of algorithms for the microcontroller and manufacture all mounting components.
CAT A TAC will also be responsible for testing the CVT and control system using a hydraulic
dynamometer. CAT A TAC will operate within a budget of $1,000 and remain in communication
with the War Eagle Pullers. The project will be completed by May 1st, 2015. This includes
assembly, programming, testing, and documentation.
4
Table of Contents
Introduction .................................................................................................................................................. 5
Overview ................................................................................................................................................... 5
Problem Statement ................................................................................................................................... 5
Design Objectives ...................................................................................................................................... 6
Background ................................................................................................................................................... 7
Competition Information .......................................................................................................................... 7
Technical Information ............................................................................................................................... 7
Design ............................................................................................................................................................ 9
Methodology and Approach ..................................................................................................................... 9
Specifications and Constraints .................................................................................................................. 9
Flyweight-Assisted Actuation .................................................................................................................. 10
Design Overview ................................................................................................................................. 11
Actuator-Only Control ............................................................................................................................. 15
Design Overview ................................................................................................................................. 15
Preliminary Analysis and Design Recommendations .................................................................................. 16
Economic Considerations ........................................................................................................................ 16
Safety and Durability Concerns ............................................................................................................... 16
Design Recommendation ........................................................................................................................ 17
Summary ..................................................................................................................................................... 17
Design Summary and Timeline ............................................................................................................... 17
Deliverables............................................................................................................................................. 18
REFERENCES ............................................................................................................................................... 20
Appendices .................................................................................................................................................. 21
5
INTRODUCTION
Overview
CAT A TAC was contracted by the Auburn University Quarter Scale Tractor team, the
War Eagle Pullers to design and implement an electro-mechanical control system for a
continuously variable transmission of a quarter scale tractor. The team plans to compete at the
2015 American Society of Agricultural and Biological Engineers (ASABE) Quarter-Scale
Tractor Competition. This is the first year that the team has competed in the since 2009. This
report outlines CAT A TAC’s approach to the project and includes two preliminary design
alternatives for the War Eagle Pullers. A recommendation and plan for proceeding with one of
the designs is made.
Problem Statement
The Auburn University Quarter Scale Tractor team, the War Eagle Pullers, is competing
in the ASABE Quarter-Scale Tractor competition in Peoria, Illinois on May 28th through the
31st. The teams will design and build a tractor that competes in a variety of events. The goal of
this project is to design and implement an actuated, non-linear control system for the Kawasaki
CVT that will be incorporated into the quarter-scale tractor of the Auburn University team, the
War Eagle Pullers. One of the events is a tractor pull competition that involves the use of a
progressive weight sled that increases the loading on the tractor with increased pulling distance.
One of the advantages of using a CVT in the competition is the ability to maintain a constant
pulling force on the sled without the need for shifting gears. CVTs are typically controlled by a
flyweight system as a function of engine speed. Replacing the flyweight control system with a
linear actuator, the system will be capable of varying the gear ratio between 3.5:1 and 0.98:1
smoothly, quickly, and independent of engine speed. The 12V actuator will be programmed to
operate in different modes, corresponding to optimal operating conditions for each competition
event. The programmable control system will be tested on a custom built hydraulic
dynamometer.
6
Design Objectives
Design and manufacture an actuated, non-linear control system for our Kawasaki
KAF620 CVT capable of varying the gear ratio between 3.5 and 0.98 smoothly, quickly,
and accurately.
o Design and machine a coupling method between the linear actuator and movable
sheave of the primary pulley.
o Design the system to be shock proof, ensuring operation continues over the
bumps on the durability course.
o Modify the hydraulic dynamometer to serve as a test bed for our 31-hp engine and
actuated CVT
Program a control system capable of controlling the actuator on the CVT, monitoring the
rotational speed of the engine speed and secondary pulley, and varying the gear ratio to
the optimal or desired setting in response.
o Allow for the operation in three distinct modes:
Sled pull mode – operate at peak power and torque, optimizing for the
greatest pull distance
Sound test mode – limit the stroke of the actuator to maintain speed at 4
+/- 2 mph at an engine speed of 3600 rpm
Durability/maneuverability test mode – operate similar to a normal, non-
actuated CVT, but maintain speed under 4 mph.
o Test the control system on the hydraulic dynamometer
Adhere to the ASABE 2015 Quarter Scale A-team Rules and Regulations.
o Ensure proper shielding (ASAE Standard S493).
o Ensure system returns to a safe condition in the event of a power loss.
7
BACKGROUND
Competition Information
Rules and regulations for the International Quarter Scale Tractor Competition are detailed
in the 2015 Handbook (ASABE, 2015a). The following section provides an overview of the
competition and applicable rules and regulations. The purpose of competition is to allow students
hands-on design experience. Each team has the same goal: “to develop a machine, which is
compact in size, relatively inexpensive, and can harness the power of a specific engine to achieve
maximum pulling performance for short periods of time” (ASABE, 2015a). Teams are required
to use the same Briggs and Stratton 31 hp engine and Titan 26 x 12.00 – 12 TRU POWER tires.
In addition to design, teams must consider the target market for their machine, whether it be
recreational or commercial, etc. A full design report is required, including an economic analysis.
Competition teams are comprised of university students. Students work throughout the
year to design and build a tractor that they present at competition. There are two classes of
competition: A-Team and X-Team. The A-Team class is composed of teams with a brand new
tractor that they are competing with for the first time. Teams that rework a previous year’s
tractor fall into the X-Team class. Since the War Eagle Pullers are building a new tractor, they
will be competing in the A-Team class.
During competition, the tractor must pass a technical inspection and weigh in. The tractor
is then evaluated by performance in certain events, including the maneuverability test, durability
test, sound test, and pull test. For the maneuverability test, drivers must negotiate an unknown
course. For the durability test, the tractor must tow a 1500 lb cart across bumps and loose sand.
Lastly, for the pull test, tractors must pull a progressive weight sled as far as possible. Of these
tests, the pull test is worth the most points (24.1% of the entire score as compared to 4.1% for the
maneuverability test and 8.3% for the durability test).
Technical Information
The CVT that is to be used in this design is a mechanical, rubber v-belt CVT. The basic
operating principle of this type of CVT is the use of a system of flyweights and springs to move
the sheaves of the pulleys based on engine speed. This system consists of two V-belt pulleys
8
comprised of a stationary and movable sheave separated perpendicular to their axes of rotation,
with a V-belt running between them. When one of the sheaves is forced inwards, the sheave of
the other pulley will be forced outwards, and vice versa. This lateral movement of the sheaves
changes the pitch diameter of the pulleys, forcing the V-belt to ride the pulleys at a different
radius, and thus, changes the speed ratio smoothly and continuously. Figure 1 illustrates this
operation.
Figure 1: CVT with 2 pulleys and V-belt (photo from: www.audizine.com)
One of the advantages of using a CVT in the quarter scale competition is the ability to
maintain a constant pulling force on the sled without the need for shifting gears. Standard belt
CVT’s are tuned for fuel efficient performance or high speeds, a non-ideal setup for a pulling
tractor. Replacing the flyweight control system with a linear actuator, the system will be capable
of varying the gear ratio between the full range of the CVT smoothly, quickly, and independent
of engine speed. The range of the Kawasaki KAF620 CVT is 3.5:1 (maximum) to 0.98:1
(minimum). The control system will allow for precise control of the actuator, forcing the CVT to
the optimum ratio for each event in the competition.
9
DESIGN
Methodology and Approach
Throughout the design process, CAT A TAC worked closely with the Auburn University
War Eagle Pullers. Communication was key, as any modifications to the tractor design could
potentially impact CAT A TAC’s design. Originally, CAT A TAC planned on purchasing a new
CVT with a calibration kit and creating a computerized model to optimize performance.
However, this was economically infeasible due to the War Eagle Pullers’ budget constraints.
After deliberation with the War Eagle Pullers and Dr. Tim McDonald, it was agreed upon that
the CVT already in the War Eagle Pullers’ possession, a Kawasaki KAF620, should be used. The
War Eagle Pullers requested that the CVT be controlled by a linear actuator and microcontroller.
To approach this project, CAT A TAC began by determining the system constraints and required
deliverables. CAT A TAC then began the process of researching and selecting the components
for the control system. Two designs were developed, focusing on the use of the flyweight spider
assembly.
Specifications and Constraints
The design of the control system and selection of components was largely driven by
budget. The War Eagle Pullers have a current balance of $2,800 in their bank account; this will
include all parts of the tractor and travel to and from competition. Thus, the War Eagle Pullers
and CAT A TAC determined that the control system should not exceed $1,000.
Competition rules also determined constraints for the system. The entire tractor must
weigh less than 800 lbs (ASABE, 2015b). The War Eagle Pullers are currently in the process of
weighing all components of the tractor to ensure adherence to this rule. CAT A TAC aimed to
have the control system as lightweight as possible, with a goal set at under 50 lbs. Competition
rules also specify shielding requirements. No-contact shielding is required for all rotating
components. Any belts and chains (including the belt of the CVT) must be peripherally shielded
with either mild steel or aluminum at a minimum thickness of 1/8 in or 1/4 in, respectively
(ASABEb, 2015). All other competition rules are contained in the ASABE International Quarter-
Scale Tractor Student Design Competition 2015 Handbook.
10
CAT A TAC also had to ensure that the control system would be compatible with the
12V power source the War Eagle Pullers are using for the tractor. Additionally, the components
of the control system must not draw more than 20 amps of current from the tractor’s battery due
to the 20 amp alternator of the Briggs and Stratton engine. Lastly, the control system must be
compact and fit within the physical bounds of the tractor without the need for excessive
modifications. According to competition rules, no part of the tractor may exceed a width of 72
inches (ASABEa, 2015).
Flyweight-Assisted Actuation
As aforementioned, two design alternatives were developed, focusing on the use of
flyweights with the CVT. In the first design, the actuator operates in parallel with the moveable
sheave of the primary pulley of the CVT, and 2:1 mechanical advantage is utilized via a lever
arm. Keeping the flyweights attached to CVT’s spider assembly, the rotational speed of the
engine will generate a centrifugal force pushing on the moveable sheave. The actuator will push
or pull the CVT depending on the desired gear ratio. Under normal, non-actuated operation, the
flyweights are the only force exerted on the CVT, meaning gear ratio and engine speed are
linearly related. In hybrid, flyweight assisted actuation, the flyweights will “help” in moving the
sheave inward when the desired gear ratio is less than the corresponding ratio under “normal”
operation. In the reverse case, the actuator must be able to pull on the movable sheave with
enough force to overcome the internal load of the flyweights when a higher gear ratio is desired.
A microcontroller adjusts the position of the actuator based on the competition mode.
Components include: competition mode input switches, microcontroller, H-bridge, and actuator.
The actuator is equipped with a potentiometer for positional feedback. Further details are given
below.
11
Design Overview
Due to the previously mentioned budget constraints, the KAF620 CVT was utilized in
both designs.
Figure 2: Kawasaki CVT used in our design modeled in Autocad Inventor.
A linear actuator will operate a lever arm that pushes the moveable sheave of the primary
pulley with a 2:1 mechanical advantage. Criteria for the actuator include the following:
positional feedback (potentiometer), compatible with our 12 volt battery, a stroke length of 4
inches, and an actuation speed of at least 0.5 inches per second. Additionally, the actuator must
be able to overcome the centrifugal force generated by the flyweights. As engine speed increases,
the moveable sheave of the primary pulley will move inward. The center of mass of the
flyweights follow an arc as they move outward, forcing the sheaves of the primary pulley closer
together.
It was necessary to find the travel path of the flyweights’ center of mass in order to
calculate the centrifugal force that is generated as well as how it increases as a function of
distance. This will give an accurate approximation of the amount of force that the actuator will
need to overcome in order to stop the inward sheave movement or pull it the opposite way. The
centrifugal force generated by the flyweights was calculated using the following equation:
𝐹 = 𝑚𝑟𝑉2, where m = mass of flyweights (g) (Equation 1)
r = radius of action (m)
V = rotational speed (radians/sec)
12
Note that since under normal, non-actuated conditions, engine speed is directly related to the
lateral movement of the primary sheave, this force could be calculated for any RPM of the
engine. For purposes of actuator selection, the highest speed of the engine was used in the force
calculation. This will set a limit on the amount of force that the actuator needs to produce. Table
1 shows the centrifugal force of the flyweights at the maximum speed of the engine (3600 RPM).
Thus, the actuator must be able to produce approximately 500 lb of force.
Table 1: Force generated by the flyweights as a function of CVT movement and gear ratio at 3600 RPM.
Based on these criteria, the Thompson Electrak 10 D24-10A5 actuator was selected
(Figure 3). This actuator has a rating of 500 lbs dynamic load at 0.8 in/s max load speed and is
equipped with a potentiometer for feedback. With the mechanical advantage, the force generated
on the actuator will be upwards of 1000 lbs. With a length of 10.3 inches and a weight of 13.7 lb,
this actuator is appropriately sized for the tractor.
Figure 3: Thomson Linear actuator D12-10A5-04, photo: www.thomsonlinear.com
The position of the actuator will be controlled by a microcontroller (Figure 4). Figure 5
represents the basic flow of the control system. User input will occur via three toggle switches
corresponding to the pull test, sound test, and durability test. When a new competition mode is
Gear ratio Closure of CVT (in) Flyweight Force (lb)
3.5 0 249.547
3.121 0.2 299.457
2.742 0.4 349.366
2.363 0.6 399.276
1.984 0.8 449.185
1.605 1 499.095
0.980 1.33 513.069
3600 RPM Engine Speed
13
selected, the microcontroller will assess the current position of the actuator via the potentiometer.
The microcontroller will then adjust the actuator to the new position. An H-bridge will be
employed between the microcontroller and the actuator. In order to work with the DC motor, it
was desirable that the microcontroller have an onboard Analog to Digital Converter (ADC) and
the capability for Pulse Width Modulation (PWM). The ADC will allow for the analog readings
from the potentiometer to be converted into a digital reading. PWM allows for the digital output
of the microcontroller to be converted into analog results. Two Hall Effect sensors act as
tachometers and measure the rotational speed of the primary and secondary pulleys. The
microcontroller would also have to be compatible with the actuator selected, including the input
of a 12V power supply. The H-bridge needed to be able to handle the 20 amp current of the
actuator. Lastly, the microcontroller needed to be user-friendly and well supported and
inexpensive.
Figure 4: Control system layout
Figure 5: Flow of data processing
14
After the actuator was selected, it was necessary to select an H-bridge that would be able
to handle a current of 20 amps. An H-bridge allows a voltage to be applied across a load in both
directions. In this application, it will allow the actuator to move forward or backwards. Due to
the high current rating, the MegaMoto Plus H-bridge shield for Arduino ($54.99 from Robot
Power) was selected (MegaMoto Plus Motor Control Shield for Arduino, 2015). The MegaMoto
is a shield, and therefore will easily stack on top of an Arduino microcontroller. The MegaMoto
is rated for a continuous current of 20 A. With a fan, this is increased to 25A. Logic levels of 3 V
- 5 V are supported. Supply voltage must be between 5 V and 28 V.
Since the MegaMoto H-bridge is an Arduino shield, Arduino was selected for the
microcontroller. The MegaMoto is specifically a shield for the Arduino Uno ($24.51 from
Mouser Electronics) (Arduino, 2015). The Uno has a recommended input voltage of 7-12 V and
a 5 V logic level. The board has 14 digital input / output pins, 6 of which have PWM capability.
Additionally, the board has 6 analog input pins. Figure 6 contains the Arduino Uno and the H-
bridge shield.
Figure 6: Microcontroller and H-bridge (photo: arduino.cc and www.robotpower.com)
For user input, 3 toggle switches ($2.94 from Sparkfun Electronics) were selected (Toggle
Switch and Cover, 2015). These will allow the driver to select the appropriate mode during
competition. To measure the rotational speed of the primary and secondary pulleys, Hall Effect
sensors will be used ($2.00 from Adafruit) (Adafruit, 2015). These will act as non-contact
tachometers and provide digital input to the microcontroller. Figure 7 contains these input
components.
15
Figure 7: Hall effect sensor and toggle switch (photos: www.adafruit.com and www.sparkfun.com)
Actuator-Only Control
In the second design, the actuator will utilize the same 2:1 mechanical advantage arm to
manipulate the moveable sheave of the primary pulley. A mechanical advantage allows for the
use of a smaller, less expensive actuator. The electronic components of this design are the same
as those in the first design. A microcontroller is employed to control the position of the actuator,
and a potentiometer allows for positional feedback. The distinction between the two designs is in
the use of the flyweights as an additional internal loading on the primary movable sheave.
Design Overview
As opposed to direct actuation, in this design all of the force on the pulley is generated by
the actuator. The force needed to manipulate the sheave will be much less than in the previous
design. While the actuator will need to “push” with more force on the CVT to move it in, the
force needed to backshift the CVT out to a higher ratio will be much less. This will reduce the
current drawn from the battery and increase the longevity of the actuator. The actuator will be
mounted to movable sheave buy using a thrust bearing coupled to the cover. This will allow the
manipulation on the rotating driven pulley by the static actuator.
16
PRELIMINARY ANALYSIS AND DESIGN RECOMMENDATIONS
Economic Considerations
The prices for each of the components can be found in Table 2. Since the control system
and actuator selected are the same in both the Flyweight Assisted Actuation design and the
Actuator-Only Control design, the cost is the same. The CVT the War Eagle Pullers currently
have already has flyweights. Thus, there is no cost to add them or remove them.
Table 2: Cost estimates for components
The budget allotted to CAT A TAC was $1,000. Based on the results in Table 2, both designs
cost approximately $600. This leaves $400 for shipping, contingencies, and any additional items
that arise. Once assembled, there will be no operational cost unless additional components are
desired.
Safety and Durability Concerns
Several electrical components are utilized in this design. Thus, it will be necessary to
ensure proper connections at all ports to minimize risk to the user. Additionally, moving
mechanical components will require shielding to prevent user injury. The system must be
compliable with ASAE Standard S493. Due to the rugged terrain of the competition courses, all
connections must be secure. Lastly, the control system must include a fail-safe mode to
disengage the clutch in the event of system failure.
Component Quanitity Price (USD)
Microcontroller 1 24.51
H-bridge Shield 1 54.99
Toggle switches 3 2.94
Hall effect sensors 2 2.00
Actuator 1 517.97
610.29
Control system:
Total:
17
Design Recommendation
Both designs had identical costs, so economics was not a major factor in deciding
between the two. Instead, the effect of the flyweights on the actuator force required drove CAT
A TAC’s decision. In the Flyweight Assisted Actuation design, the actuator will be required to
generate a large pulling force to overcome the opposing force of the flyweights, and due to the
25% duty cycle of the actuator, this would not be the best option. Therefore, CAT A TAC
recommends proceeding with the Actuator-Only Control design. Coupling the actuator to the
CVT assembly will be done using a thrust bearing and custom machined mount. Design of all
mounting components will be modeled using Autodesk Inventor and machined in-house.
SUMMARY
Design Summary and Timeline
CAT A TAC recommends proceeding with the Actuator Only design. The actuator will
drive the moveable sheave of the primary pulley. The position of the actuator will be set by a
microcontroller. Each competition mode will correspond to an optimal performance program.
CAT A TAC has carefully considered the next steps in development and has laid out a timeline
to ensure that the project is completed on time. Figure 8 illustrates the timeline CAT A TAC
plans to adhere to.
18
Figure 8: Timeline of planned work for project
All control components will be purchased by Monday the 23rd of February, so that the
control system can be constructed by March 16th, allowing for a three week delivery time. A
final design for coupling the linear actuator to the moveable sheave of the primary pulley will be
completed and manufactured by the same date. The hydraulic dynamometer will be operational
at this time so that it can be utilized for testing. Development of the algorithms for controlling
the actuator will begin prior to delivery of the components, ensuring we have programs to begin
testing with immediately upon delivery. The remaining time will be spent programing these
algorithms, testing the control system, and attempting to improve operation and computational
time. Once the tractor is fully developed, we will “drop-in” the CVT to continue testing at full
scale. Programming and testing will be done iteratively. Computational time will be monitored
and recorded. CAT A TAC will provide a final design report and presentation on May 1, 2015.
Deliverables
CAT A TAC will be responsible for implementing the chosen actuated non-linear control
system for the Kawasaki KAF620 CVT, including all electrical and mounting components.
Additionally, CAT A TAC will develop control algorithms and program the microcontroller for
the following operation modes: sled pull mode, sound test mode, and durability /
maneuverability test mode. Testing of the control system will be done in-house using a hydraulic
19
dynamometer at Auburn University. Such testing will allow CAT A TAC to verify the safety of
the control system and test the accuracy and computational time of the programmed algorithms.
Throughout the entire process, CAT A TAC will adhere to the ASABE 2015 Quarter Scale A-
team Rules and Regulations.
20
REFERENCES
Adafruit. (2015). Hall effect sensor. Retrieved from Adafruit: www.adafruit.com
American Society of Agricultural and Biological Engineers. (2015). 2015 Handbook International 1/4
Scale Tractor Competition. Retrieved February 16, 2015, from American Society of Agricultural
and Biological Engineers: http://www.asabe.org/membership/preprofessionalsstudents/14-scale-
comp.aspx
American Society of Agricultural and Biological Engineers. (2015). International 1/4 Scale Tractor
Competition A-Team Rules and Regulations. Retrieved February 18, 2015, from American
Society of Agricultural and Biological Engineers:
http://www.asabe.org/membership/preprofessionalsstudents/14-scale-comp.aspx
Arduino. (2015). Arduino Uno. Retrieved from Arduino: arduino.cc
MegaMoto Plus Motor Control Shield for Arduino. (2015). Retrieved from Robot Power:
www.robotpower.com/products/MegaMotoPlus_info.html
Toggle Switch and Cover. (2015, February). Retrieved from Sparkfun Electronics:
www.sparkfun.com/products/11310