TRANSISTOR- BASED LINE FOLLOWING ROBOT WITH FRONT ... · The circuit was a combination of a line...
Transcript of TRANSISTOR- BASED LINE FOLLOWING ROBOT WITH FRONT ... · The circuit was a combination of a line...
ATENEO DE DAVAO UNIVERSITY
SCHOOL OF ENGINEERING AND ARCHITECTURE
TRANSISTOR- BASED LINE FOLLOWING ROBOT WITH FRONT COLLISION
SENSOR
Submitted by
Kurt Irving S. Barcelona
Lilibeth Diane L. Yu
In Partial Fulfillment of the Requirements in ECE 320:
Electronic Devices and Circuits 1
Engr. Raymond Pidor
July 13, 2015
PRINCIPLES ON A LINE FOLLOWING ROBOT
Differential Drive Mechanism
The robot heavily relied on the differential-drive mechanism of the motors. It
was an expensive approach because of the use of two motors, but it removed the
complication of the design and manufacturing of the steering and motor driving gears.
Two motors were independently placed on the sides of the robot, and the
difference of their revolutions per minute (RPM) steered the robot in a given direction
to follow a line on a flat surface. A diagram is placed below on how this operates.
Fig. 1.1. Line Following Mechanism. Retrieved from http://www.emicro.com/blog
Fig. 1.2. Differential Drive Steering. Retrieved from http://www.emicro.com/blog
Approach and Principles behind the Robot
In order for the robot to follow a line using a differential drive steering and
control of the robot, a mechanism was made to detect or differentiate a line from a
clean surface.
In compliance with this specification, a simple approach was used, which was
the utilization of the properties of light. It was to observe contrast between the line
and a clean surface. It was an easy method to distinguish the line from a planar
surface and vice versa. A bright surface indicated that a greater intensity of light was
reflected from a surface. A darker surface indicated that light was more absorbed by
that surface. This principle of contrasting shades was simple yet effective. It was also
assumed here that the surface was opaque and was also matte in texture so to diffuse
or scatter light rather than reflecting it at a specific direction.
It was already decided that the robot would have been electronically and
mechanically based, which would possess a device that will detect contrasting
surfaces by exhibiting a certain response. This certain device responds on the change
of light intensity by changing its electrical resistance. This device or sensor senses or
reacts to the difference between a surface that reflects light and a surface that absorbs
light. This component is called the Light Dependent resistor.
Connecting it in certain ways yielded various outcomes. Its electrical
resistance decreased as the intensity of light increased, and its electrical resistance
decreased as the intensity of light increased.
With this method, the robot didn't require complex calculations and logic for it
to fulfill its specifications, and it only used simple transistors.
PLANNING AND PARTS DESCRIPTION
Parts: Power Supply
Unlike in the original schematic diagram of the basis circuits, whose power
supply was shared between the motors and other components, the power supply of the
circuit were separated independent power supplies because the power needed by the
motors used in this circuit was quite large, and it may affect the current distribution to
other components if it were a singular power supply due to the large power dissipated
in the internal resistance of a battery. In order for the circuit's logic and other
components to function properly and not to be disrupted in case the motors required a
large current, an independent power for the motors and the logic circuits was
implemented.
Also, a 6 VDC given by four AA Carbon-Zinc batteries was the power supply
used for the logic circuits because of the 6VDC requirement of the relay, and Carbon-
Zinc provided enough current draw for the low power transistors.
The power for the motors was two AA Carbon-Zinc batteries, each having
1.5VDC. Connecting them in series yielded 3.00 VDC. According to the motors'
specification sheet, 3VDC was enough to drive the geared DC motors in a slow RPM,
which was necessary since the LDR's response was not as fast contrary to the robot's
expected speed at which the motor will run if given a higher RPM. Carbon-Zinc
batteries provided a low current rating, but this was enough for the motors and also
for the safety of the other components if in case the circuit had a short between the
supply and the transistors that controlled the motors, which was quite common
whenever a motor got stuck in operation.
It was also evident that the use of heavier AA batteries were apparent in the
design. It was known that AA batteries hold up much more charge compared to the
9VDC battery. The AA batteries also gave out more current when needed compared
to a 9VDC battery, and large amount of current was needed in powering motors. Also,
the motors and relays were rated only for 6VDC, and applying 9V would have been
destructive to the parts. Another problem was that 9VDC batteries easily heated up,
and their internal resistance hindered a large amount of current to be pulled away
from, and power would have just been wasted in heat during power dissipation.
Lastly, when a 9VDC battery's output was voltage divided, it’s was a waste of power
because the power loss of the circuit would have been dissipated into the resistors of
the voltage divider circuit. Compensating for these problems, it was good enough to
sacrifice weight with performance of AA battery. Thus, AA batteries were used in the
robot rather than 9VDC.
Parts: Transistor
The transistors used here were from the original basis circuits. The 2N3904
transistors were used to switch on and switch off the motors. Unlike in the original
design, five transistors were used to divide the current among the five transistors to
reduce the heat generated in each transistor. The amount of heat dissipated would
have been minimized in case a larger power consuming motor were to be used.
In a single 2N3904, it had about a maximum of 300mA collector current, or
the current that flowed through the collector down to the emitter of the transistor. It
was the maximum rating of the transistor if it permitted current through it from the
collector then to the emitter. If the collector current would have been near the
collector current value, heat would have been dissipated due to resistance. By using
five transistors, one had increased the operational range from 300mA to 1500mA, and
also aided the heat dissipation by dividing the current through each transistor, it also
provided a larger surface area for the heat to dissipate.
Parts: Motors
The motors were not ordinary 6VDC motors. Motors required of the design
were ready made geared DC motors. The one used in the design was a 48:1 gear
reduction ratio 3-6VDC motor. These motors used simply converted the speed into
more torque. Non-geared DC motors were high speed but low in torque. Their torque
wasn't enough to drive the robot's large and heavy wheels, much more its weight. To
reduce complication, it was recommended that buying the readymade motors would
have been beneficial than designing a gear reduction mechanism and then
implementing it in the design.
Parts: Wheels
The robot had three wheels for its design. The one at the front was a pivot
wheel, rotating in any direction where the robot moved. There would be two wheels at
the back directed only frontward. These wheels were made to be in free-wheeling
configuration, meaning that these wheels were not directly driven by the motor. The
motor had its own smaller wheels that rim-drove the larger wheels at the back so to
increase torque and reduce speed. Speed reduction was important because once again,
the LDRs' response was slow, and would not instantaneously detect a change in light
intensity.
Parts: Body
The robot's body was a plastic lunchbox container to minimize cost and also to
protect the circuitry. It was also chosen so to make it aesthetically appealing. It was of
hard plastic, slightly flexible and slightly brittle to give extra flexibility while
providing strength. Holes were easily made by melting the enclosure with a soldering
iron.
DESCRIPTION OF SCHEMATIC DIAGRAM
Background of the Schematic Diagram
The circuit was a combination of a line following circuit, motor driver circuit
and an obstacle detection circuit. It was the brain of the robot. The whole circuit was a
drastically modified form of the basis circuits to suit a more modular approach in the
design of the robot, which meant that the robot parts could be easily replaced or
improved over time with new circuits and processes.
Line Sensor Circuit Setup
The Line following circuit had two LDRs as sensors- one for the left and one
for the right of the robot, as shown in the bottom left and bottom right of the
schematic diagram. These controlled the motors.
As the light intensity increased for one LDR, cascading BC547s were
triggered to let charges flow from the voltage source (6VDC- 4x AA batteries) to the
base of the 2N3904 transistors.
The rush of charges to the base of the 2N3904s let current from the 3VDC
2xAA power supply enter the motor coils and then down to the five transistors'
collector to emitter, and then went go to the ground.
This mechanism didn't vary the speed of the motor. It simply turned on the
motor in full swing or turned it off. Unlike in the original schematic diagram of the
basis circuit, the use of the 'switch on and off' for the motor rather than a linear
amplifier method used in the original circuit tightened the turn radius of the designed
robot without sacrificing a lot of time and space.
Motor Control
The motors were controlled by the switching mechanism of the LDR and
transistor circuit. When the LDR detected a high intensity of light (clean surface,
usually a bright surface), the five 2N3904s turned on, and charges flowed to the
motors, able to run at full speed, and when the LDRs detected a low intensity of light
(dark surface, usually a dark line over a surface), the motor turned off. In series with
the base of the 2N3904 transistors was a 10K Ohms trimmer resistor. It was placed
there in case the motors didn't have the same calibrated RPM due to a factory defect
or wiring problem. In most cases, this was only used to reduce the current going to the
base of the transistor, thus current control was only its purpose,
This caused problems in testing in testing. When the trimmer was set to a
point that only minimal current was passing through the base of the 2N3904s, the
motors didn't start turning and they only remained at their initial position unless acted
upon by an external tangential force. Worse was when the motors created a short
whenever the motors didn't move, simulating that there was no apparent load, so most
of the current went through the 2N3904 transistors, which dissipated power as heat
and might had even destroyed the circuit especially when the power supply provided a
very large current, which didn't in this case due to the use of Carbon-Zinc, a low
current draw battery.
Obstacle Detection and Countermeasure
In the obstacle detection circuit, an LDR-relay switch was used to make or
break the connection of the motor to its power source. A relay was used here rather
than a power transistor so that in case a larger power source or a larger motor was
inserted, the circuit would have been flexible enough not to break the other
components, and it was simpler than designing a set of cascading transistors just for
the similar purpose. It was part of a modular-flexible design of the circuit.
A LDR was used to determine the distance of the robot to an object in front.
As the light intensity increased, the reflected amount of light from the object to the
sensor was strong, which meant that the distance from the robot to the object was
near. Whenever there was a weak intensity of light, it meant that the object was far
away from the robot.
The power rail of the motor's power supply was connected to the pole of the
relay, and the motors were connected to the normally-open pin of the relay.
Whenever the LDR on the top left of the schematic diagram didn't detect a
near object, the light intensity was weak, and charges flowed from the power supply
down to the base of the first cascaded BC547 transistor. That transistor then supplied
enough current to the last of the cascaded transistors, in an amplifier configuration to
the inductor of the relay, thus the pole connector moved from the normally closed pin
to the normally open pin of the relay. Basically, the circuit turned on the relays, thus
power was fed from the motor's power supply (3VDC 2xAA batteries) to the motors,
but when the LDR detected a high intensity of light, lesser current entered the two
cascading BC547 transistors and cut the flow of charges from the collector of the last
cascaded BC547 to ground, thus charges didn't flow from the power supply down to
the coil of the inductor of the relay and to ground. Basically, the relay was switched
off, and the pole connector of the relay will go back to the normally closed pin, which
was designed to be an open terminal.
It was noted that this configuration of the relay and LDR was set so that the
normally open switch was connected to the motors and the pole was connected to the
power source. It was be questioned, why not configure the circuit in a way that the
power was connected to the pole, and the motors connected to the normally closed pin
of the relay? Only when the LDR detected a high intensity of light it switched on the
relay to save power by reducing power dissipation on the relay's coil. Actually, this
mentioned method wasn't part of the circuit design also to save power, and also to
reduce complication so to easily modify the circuit when needed. Whenever the
circuit was not powered by any sort, which all parts didn't have charges flowing
through them, the relay's pole was connected to the normally closed pin. In the robot
design, the normally closed pin connection was open, thus power from the motor's
battery won't dissipate to the motors themselves that were connected in the normally
open pin of the relay. Only when the circuit was activated and the LDR didn't detect a
high intensity of light it allowed the connection of the motor's power to the motors
themselves. The design was basically a switch that won't permit the connection of the
motor's power and the motors themselves whenever the whole circuit was turned off,
and only when the circuit was turned on, together with the LDRs detecting a high
intensity of light it allowed the motors to be connected to their power source.
Weakness
The circuit was not intended to be a perfect design. It had drawbacks due to its
design philosophy and parts availability.
Transistors weren't designed to be easily replaced because of the lack of
available parts which houses a sturdy modular a fault finding system was not
installed. When a transistor went out, it wasn't easily detected among the other
parallel-connected transistors unless all transistors of a motor burnt out. The circuit's
LDRs were not quick in detecting a dark surface from a bright surface, so the speed
was only limited to the detection time of the LDR, and also the reaction of the motors,
which were factored by the gear ratio quality, charging time of the motors and the
internal forces acting upon the gears. Another weakness was the heavy weight of the
circuit. A lot of nickel nuts and bolts were used, including the fully metal pivot wheel
in front of the robot. The heavy weight contributed to the large force needed to move
the robot, and it was difficult to slow down since no braking mechanism was made in
the design to reduce complication and provide more space for more important parts in
the robot. Another was the inaccuracy of LDRs in detecting a line due to lack of light
or too much light entering the sensor, or the surface was not completely planar.
Another would be that the motors' speed was not easily controlled by the lights, but
only trimmer resistors to balance the RPM for each motor. Adding to this, when the
trimmer resistors were set which there was minimal current through the base of the
2N3904 transistors, the motor didn't immediately turn unless an external tangential
force was applied to the motor's axle or wheel. This was due to a lack of current that
flowed through it, and this in turn did create a short between the Power supply and the
collector’s pins of the transistors, even if the voltage was held constant across the
motors. The lack of current on the base of the transistors reduced the current supplied
to the motors, thus they didn't start turning. The short between the supply and the
transistors dissipated heat and might have destroyed the transistors. Worse was when
the supply was able to provide a large amount of current through that short, but that
didn't happen since a low current draw power source since a Carbon-Zinc battery was
used.
Basis Circuits
The Schematic diagram was based on the Transistor-Based Line Following
Robot of http://www.ermicro.com/blog/?p=1097 and the LDR switch from
http://www.buildcircuit.com/how-to-use-a-relay/. The line following robot schematic
incorporated the two mentioned circuits in a modified and a more modular circuit that
is suited to the mechanical design and the philosophy of the proponents, which is, the
design will be based on the available parts. The original authors of the line following
robot schematic had parts unavailable to the proponents, and changes were made to
suit this. One was the modification of amplifier-type LDR sensor and motor
triggering.
SCHEMATIC DIAGRAM
Fig.1.3. Line Following Robot with Front Collision Countermeasure Schematic
Diagram.
PRINTED CIRCUIT BOARD LAYOUT
Background
The Printed Circuit Board (PCB) was designed to have multiple modular parts
that had large copper traces so to compensate for the error in any kind of PCB
fabrication because ordinary PCB fabrication was not as good as factory standards,
and ordinary fabrication with small copper traces were usually dissolved or cut. Also,
the printed PCBs in this report were in 1:1 scale. Any printing error or blotting
resulted in an inaccuracy of PCB fabrication. So, to be safe, a large copper trace was
implored to create a safe margin for successful PCB fabrication.
Technicalities
The PCB was designed in three parts that were to be connected in cables that
ran through the robot and was connected via terminal blocks and speaker wires, which
can carry a high load of current and voltage while offering a sturdy and ductile
conductor with a safe kind of plastic insulation. Unlike using solid or stranded core
wires, it served both purposes of being sturdy and flexible.
The terminal blocks offered a versatile connector for any kind of wire that wa
used in the robot. In case a wire was to be replaced, it can be most likely compatible
with the terminal block connection. Unlike soldering the wires, which was a dirty
maneuver in this kind of modular circuit because it made replacement difficult,
terminal blocks minimized the chances of a wire breaking up. It lets the wire clip on
more tightly with an option to be easily removed, compared to soldering, which was a
permanent process.
These PCBs were the main board, which contained the switching transistors of
the motors, the relay with the relay driver, and other power supply connectors that
needed to be inside the enclosure. The other PCB was the LDR sensor PCB. It's
placed outside the robot. It consisted of a two pairs light sources-White Light
Emitting Diode (LEDs) closely attached to a Light dependent resistor. They were
spaced quite fairly apart so to give a bandwidth of detection for the line on a clean
surface. The last PCB was the front collision detector circuit. It was composed of one
pair of LED and LDR. It was just a sensor placed at the front of the robot. Its
detection distance of a white object was about 3-5cm from the robot's front.
These sensors were connected using 3-pin terminal blocks. These terminal
blocks had a convention to follow so not to confuse the user in case repair or
modification. Facing the user, an image below describes the terminal block
configuration.
Fig. 1.4. 3-Pin Terminal Block Convention.
2-pin terminal blocks were either dedicated for the motors or for the power
supply. The motors' terminal block were not polarity-sensitive, but the power supply
terminal blocks were.
RECOMMENDATIONS
-Make the switching transistors more modular, yet secure.
-Error or fault finding circuitry for error checking
-Reduction of weight by reducing metallic parts, and maximization of space and
minimizing the material used
-Use of bearings with a proper alignment of motors to the rim-driven wheel
-Use of a faster response photo transistor
-use of more LDRs for line detection a more accurate transistor logic
-smaller and space saving battery with a high current and charge rating
-use of pulsated or modulated infrared light to isolate the beams from the robot to
detect distance or a line instead of just using continuously lit LEDs so to make a more
accurate reading by removing the factor of ambient light. It's like creating a very fast
shutter speed that rejects ambient light and only detects the in-phase pulsated infrared
lights.
-use of pulse width modulation to control them motor's speed rather than use of
switching transistors to reduce the wasted power in heat, and also to remove the in
series trimmer resistor.
\
REFERENCES
-Buildciruit.net.(2010)."How to Use a Relay". Retrieved on July 10, 2015 from
http://www.buildcircuit.com/how-to-use-a-relay/
-Rwb. (2009). " Build Your Own Transistor Based Mobile Line Follower Robot
(LFR) – First Part". Retrieved on July 10, 2015 from
http://www.ermicro.com/blog/?p=1097
BILL OF MATERIALS
Table 1.1. Bill of Materials.
Part Description Quantity Price (PHP)
1/4W resistor- 10K Ohms 2 0.60
1/4W resistor- 100K Ohms 2 0.60
1/4W resistor- 220 Ohms 3 0.90
1/4W resistor- 300 Ohms 1 0.30
1/8" nut and bolts 3 25.00
10K Ohms Trimmer Resistor 2 10.00
100K Ohms Trimmer Resistor 1 5.00
1N4007 diode 3 3.00
2N3904 10 45.00
2-pin Terminal Block 2 24.00
3-Pin Terminal Block 6 90.00
3-6VDC Geared DC Motor 48:1 Speed Reduction 2 400.00
2x AA Battery Holder 1 15.00
4x AA Battery Holder 1 15.00
6V Relay 1 20.00
BC547 6 27.00
Drill bit 2 24.00
Ferric Chloride (small) 1 25.00
Green lunch box enclosure 1 30.00
Light Dependent Resistor (small) 3 18.00
LED (White) 4 32.00
Presensitized Printed Circuit Board (4"x6") 1 135.00
Rolling wheel 1 75.00
Speaker wires 2m 36.00
Toggle Switch SPST 1 15.00
Zip Ties 1 50.00
Total 1121.40
There were several parts such as the speaker wires, zip ties, 2xAA battery
holder, a few small 1/8" nuts and bolts that were found lying around the house, but the
market price as of July 2015 was placed there as reference. Wheels and their axles
were also made from parts lying around the household, but the market prices of these
objects are unknown.
DOCUMENTS
Figure 1.6. Basis Circuit: LDR Relay Light Switch.
Retrieved from http://www.buildcircuit.com/how-to-use-a-relay/
Figure 1.7. Basis Circuit: Line following Robot. Retrieved from
http://www.buildcircuit.com/how-to-use-a-relay/
Fig. 1.8. Final Line Following Robot with Front Collision Countermeasure.
Fig. 1.9. 3-6VDC Geared DC Motor 48:1 Speed Reduction.