Stefano Mazza and Patrick Gall, Department of Mechanical Engineering,
University of Ottawa
First year
The following year
Description
This report outlines the design
steps and operation of the Auto-Bike bolt-on electronic gear
selecting package.
Reasons for
System
This system is meant to
create a more user friendly interface with the bicycle. There
are electronic group sets available for bicycles, however they
are very expensive and are only compatible with certain frames
types. The Auto-Bike bolt-on package will be able to mount to
almost any bicycle. Electronic gear selecting is not only more
accurate, but also puts less pressure on the cyclist to monitor
and maintain proper chain lines and gear settings.
Design Goals
The following points are design
features that were selected for this project. They outline a
specific application for this system.
·The system must remain
general enough to be able to bolt onto any bicycle.
·The system should display to
the user which gear the bicycle is currently in.
Using the System
The system is switched on simply by
plugging in the batteries located on the main mount, on the down
tube of the bicycle. The system will always initialize thinking
that it is in the highest gear, i.e. the gear with the fewest
teeth. For this reason it is important to shift the bicycle to
this gear before switching it off. In the case of the
demonstration bicycle, it is an 8 speed bicycle, and so the
system starts in 8th
gear, and should be returned to 8th
gear before shut down.
User input is done via three
buttons located on the handle bars. In the normal operating
mode, signified by the unlit warning LED, the ‘+’ and ‘-’
buttons will control the gear selecting. Once pressed, either
button will cause a gear change. Keystrokes during a change are
ignored. If a button is pressed and the warning LED flashes,
then the shift requested in not possible since there is no gear
in that direction. On the demonstration bike for instance, while
in 8th
gear, pressing ‘+’ will result in the warning LED flashing as
there is no 9th
gear. The same will occur in 1st
except when ‘-’ is pressed.
Set mode
is entered by pressing the ‘Set’ button on the interface. Set
mode is signified by the warning LED being constantly lit. In
this mode, the ‘+’ and ‘-’ buttons will simply move the step
motor in the direction specified for as long as they are held.
This can be used to fine tune a gear during riding, or to first
setup the system on the bicycle. Exiting set mode is simply done
by pressing the ‘Set’ button again.
Discussion
of Construction
The entire system has two main
output streams and one input stream. The input comes in the form
of 3 buttons actuated by the user to perform specific tasks. The
output streams go to the step motor and to the screen.
Figure
1:
The general outline of the gear selector.
Mechanical System
Lead Screw
Figure
2:
A model of the lead screw.
The lead screw transforms the
rotational input of the step motor to a linear translation of
the cable. The step motor’s shaft is retained into the lead
screw coupler by a single M4 set screw. The lead screw itself
uses a M6 threaded rod that rotates on ball bearings and is
threaded into a sliding block that holds the cable. The sliding
block fits into a square piece of aluminum tube which absorbs
the torque generated by the step motor while still allowing the
block to slide.
An approximation for the torque
required to pull the cable can be found using the following
equation:
Where, f
is the friction coefficient of the material, W is the load
carried (the cable tension in this case), is
the mean diameter of the thread, L is the Lead (which is equal
to the thread pitch in this case), is
the contact angle of the thread (which is 60 degrees as this is
an ISO thread). The above equation ignores the friction of the
ball bearings and only considers the friction from thread
contact; however the friction of the ball bearings is very low
to begin with, thus it can be neglected. Therefore the torque
can be found as follows:
It is important to note that the
cable tension (W) is a variable between different derailleurs.
It is therefore possible that, when implemented on some other
bicycles, the system would require a greater or lower torque to
accomplish the gear changing.
The Step Motor
The step motor used on the
demonstration bike was from a common printer. Very little
information could be found pertaining to its operating voltages
and maximum torque. Future versions of this system will utilize
specifically built step motors. Therefore, more accurate
predictions and calculations can be made.
Derailleur Modifications
The derailleur had to be modified
to allow the step motor to be increased in leverage over the
derailleur return spring. This was accomplished by increasing
the length of the lever arms used to actuate the derailleur.
Doing this would of course increase the cable travel required to
get to the next gear. The system was designed with a copious
amount of cable to travel. Under no circumstances, on any bike,
should the cable run out.
Figure
3:
The derailleur modifications to allow more leverage.
System Mounting
The system is comprised of two main
mountings.
The main mount holds the
microcontroller, motor driver, lead-screw, step motor, and
batteries. All these components mount onto an aluminum bracket
that bolts to the bottle cage bolts on the bicycle frame.
Figure
4:
The main mount of the system on the down tube of the bicycle
The Interface mount holds the
screen, the selecting buttons, and the warning LED. All these
components are on one small circuit board mounted somewhere on
the handle bars of the bicycle. Signal Cables run back from the
interface board to the main mount to communicate with the
Arduino.
Figure
5:
The user interface of the system
Electrical System
The main concern in designing the
electrical system was to not overload the Arduino
Microcontroller. The input and output ports of the Arduino have
very clear restraints on how much current can be sourced or
absorbed and at what voltages those ports can operate at.
Motor Driver
Due to the fact that the Arduino
cannot generate enough power by itself to operate the step
motor, a motor driver is required. The motor driver consists of
4 identical sections, 1 for each motor pole. Each section has
two circuits, the signaling circuit and the power circuit. The
signaling circuit takes the 5V output from the Arduino and uses
it to drive a transistor which switches the power circuit. The
power circuit simply has the battery and the step motor linked
to the transistor. The transistor used by the driver is an
ECG263, which is an NPN type transistor.
The signaling circuit also contains
a resistor and a diode. The resistor acts simply to limit the
current in the signaling circuit as to not exceed the limits of
the Arduino. The Arduino can only sustain an output of less than
0.040A at 5V. The size of resistor was chosen so that even in
the worst case scenario, a shorted circuit past the resistor,
the Arduino would still be okay. The resulting calculation is
therefore very simple:
Therefore resistors greater than
would
sufficiently limit the current being emitted from the Arduino to
below the max rated current.
The diode in the circuit protects
the Arduino from reverse currents that could be generated when
the pin goes from being high, to being low. There is a certain
back voltage generated by the motor windings, and the
transistor, that must be prevented from getting to the Arduino.
The magnitude of these back pulses could not be predicted with
current knowledge. Thus the diodes were installed in case they
were dangerous.
CIRCUIT
DIAGRAM:
Figure 6:
Motor driver circuit for 1 pole
User Interface
Screen Driver
The 7 segments common anode display
is driven by a 74LS47 screen decoder. It uses 4 Arduino outputs
which sent a BCD (binary coded decimal) to the decoder would
actuate the required screen LEDs. Summary diagrams of the chip
and screen can be seen below:
Figure 7:
The 74LS47 screen decoder and common anode display.
When supplying the chip with 5V and
a BCD across pins A, B, C, and D (where D is the least
significant bit) the corresponding outputs a, b, c, d, e, f, and
g will form the correct digit for the screen.
The Arduino output pins that go to
the screen decoder also needed to be current limited in the same
manner that the motor output pins had to be. The same base
calculation applied and so any resistor above would
suffice.
CIRCUIT
DIAGRAM
Figure 8:
Decoder Circuit for 1 pin
Both the
chip and the screen require separate power apart from the
signals to operate. This is done through the onboard Arduino
power supply. The 74LS47 chip used runs at the TTL standard,
which is signified by an operating voltage of 5V. Thankfully the
Arduino power supply can produce both 3.3V and 5V, so 5V is
supplied directly to the chip. The screen is simply a set of
LEDs with a common anode. The LEDs themselves operate at less
than 5V and so a resistor needs to be used reduce the voltage
(and therefore the current) going to the resistors. The
manufacturer of the screen states that the diodes should not be
exposed to more than 0.050A. For ease of construction, and to
save materials, it was decided that only the input
to the screen would be resisted. The alternative, resisting the
7 signal pins, would require 6 more resistors and much more room
on the circuit board. The negative side effect of the single
resistor is that the digit 1 would appear brighter than the
digit 8. This is caused by the current becoming more divided
when more screen elements are active, and so each element is not
as bright. Without more knowledge of how to predict the
resistance of LEDs, it was decided to build both configurations
and test them. The single resistor method was found to be
adequate; the difference in luminosity was not as considerable
as expected. To not exceed 0.050A when 1 lamp is lit would
require a resistor of size as
a minimum.
Buttons
The buttons located on the
interface are also resisted to limit the current that will flow
through the Arduino. The main difference in their implementation
is that the input pin on the Arduino is placed in parallel with
the resistor. The switch is outside of this section of the
parallel circuit. This means that when the switch is open the
pin is ‘pulled down’ by the resistor (which is connected to
ground). When the switch is closed, current is divided between
the resistor and the Arduino as to not overload the Arduino
input pin.
CIRCUIT
DIAGRAM
Figure 9:
Circuit for 1 Button
Arduino Program
The program operates in two main
modes, normal, and set mode. In normal mode the program runs the
loop() method and calls various other methods to update the
screen and advance the gears. The loop() method continually
checks if any of the buttons have been pressed, and then calls
the required methods to complete the task. As soon as the
methods have been called the program stops looking at the
buttons creating the behavior, noted above, that the program
ignores keystrokes when changing gears.
In set mode, the code is restricted
to a method called calibrate() by a ‘while’ loop that waits for
the set button to open again. In this calibrate() method, the
program looks at the input buttons and simply moves the step
motor in that direction 1 cycle (about 7.2 degrees) every time
it sees the button pressed. Pressing and holding the button
moves the step motor continuously.
Originally the program was to use
the two built-in interrupts on the Arduino, but there was an
issue with anti-bouncing. So the plan was scrapped in favor of
the current arrangement.
To implement the system onto
another bicycle, two constants in the program would have to be
changed. The first is a variable called ‘maxGear’ which
specifies the number of gears on the bicycle. The second is a
made-up index called ‘spacing’, which describes the number of
cycles the motor has to go through to get to the next gear.
Changing these two constants to the appropriate values would
allow the system to be implemented on almost any bicycle.
The entire code is attached.
Arduino
Code
//Screen Pins
const int D = 7;
const int C = 6;
const int B = 5;
const int A = 4;
//Motor Pins
const int MA = 0;
const int MB = 1;
const int MC = 2;
const int MD = 3;
//buttons
const int UP = 8;
const int DOWN = 9;
const int SET = 10;
//warning LED
const int OUTLED = 11;
//The gear spacing of the bicycle
const int spacing = 94;
//the gear that the bike arbitrarily starts in.
volatile int gear = 8;
void setup() {
//screen pins
pinMode(A,OUTPUT);
pinMode(B,OUTPUT);
pinMode(C,OUTPUT);
pinMode(D,OUTPUT);
digitalWrite(A,LOW);
digitalWrite(B,LOW);
digitalWrite(C,LOW);
digitalWrite(D,LOW);
//motor pins
pinMode(MA,OUTPUT);
pinMode(MB,OUTPUT);
pinMode(MC,OUTPUT);
pinMode(MD,OUTPUT);
pinMode(OUTLED, OUTPUT);
updateDisplay();
}
void loop() {
if(digitalRead(UP)==HIGH)
{
up();
}
if(digitalRead(DOWN)==HIGH)
{
down();
}
if(digitalRead(SET)==HIGH)
{
digitalWrite(OUTLED,HIGH);
calibrate();
}
if(digitalRead(SET)==LOW)
{
digitalWrite(OUTLED,LOW);
}
}
void updateDisplay()
{
if((gear/8)%2 > 0)
digitalWrite(A,HIGH);
else
digitalWrite(A,LOW);
if((gear/4)%2 > 0)
digitalWrite(B,HIGH);
else
digitalWrite(B,LOW);
if((gear/2)%2 > 0 )
digitalWrite(C,HIGH);
else
digitalWrite(C,LOW);
if((gear/1)%2 > 0)
digitalWrite(D,HIGH);
else
digitalWrite(D,LOW);
}
void up()
{
//check gear range
if(gear ==8)
{
flashThree();
}
else
{
//change gear number for screen
gear += 1;
//update display
updateDisplay();
//advance motor
for(int i = 0; i<spacing ;i++)
{
for(int j = 0; j<8; j++)
{
if(j==0||j==1||j==7)
digitalWrite(0,HIGH);
else
digitalWrite(0,LOW);
if(j==1||j==2||j==3)
digitalWrite(1,HIGH);
else
digitalWrite(1,LOW);
if(j==3||j==4||j==5)
digitalWrite(2,HIGH);
else
digitalWrite(2,LOW);
if(j>4)
digitalWrite(3,HIGH);
else
digitalWrite(3,LOW);
delay(4);
}
}
clearmotor();
}
}
void calibrate()
{
//allows the user to fine tune a gear for as long as the set
mode is active
