TRANSISTOR BASED MOBILE AUTONOMOUS LINE FOLLOWING ROBOT
István – Mihály NAGY
University of Oradea,
Keywords: transistor based, microcontroller less, line follower, autonomous robot
Abstract:This paper will describe a line following robot with no microcontroller, only based on transistors and future way of improvement for speed and reliability. The mobile line following robot (short form is known as “LFR”) is a type of mobile robots with single task which is to follow a black line on a white surface or a white line on a black surface. The one described in this paper will follow a black line on a white surface.
1. INTRODUCTION
The autonomous mobile line following became really popular maybe because its simplicity, maybe because it can be used to teach how could be the industrial PID (proportional integral differential) control in this type of robots or even because the number of annual tournaments in many countries all over the world. The next robot will be a “brainless” robot, with no microcontroller.
Organization of this paper will be as following. First subheading will be the introduction for this scientific work and will describe the content, motivation and organization of this paper. First part of the second subheading will describe the principle of function of a simple LED and LDR based robot, while the second part will describe an improved robot using Fairchild Semiconductors QRD1114 Line sensors. In the third subheading will be described the principle of navigation for both robots making a comparation between them. The fourth subheading will describe the principle of motion which is the same for both type of robots because this robot was firstly built with LEDs and LDRs than, the LEDs and LDRs was removed for an improved circuit of the robot. The seventh subheading will describe the stage of construction. And finally, the sixth subheading consist of comments and conclusions.
2. PRINCIPLE OF FUNCTION
2.1 LED and LDR based robot
The robot will use two light dependent photoresistors, and two white LEDs for illuminating and sensing the track. The two LEDs will illuminate the surface upon whitch the line is drawn. The line that should be tracked is black, or at lease is a significant color diference between the background color and the line color. On the simple physic rule that the while light is reflected on white floors and absorbed on the black floor we could use this assemble to track the black line. As knowing this rule and knowing that the LDR needs light to reduce his resistance, we can conclude that when he LDR and LED pair will be on the black line, the LDR will increase his resistance and the motor will slow down making the robot to turn slowly in the direction of the line. A third LED will be used for the “Hello World” type function so we know when the robot is turned on and be able to debug it if there is a problem. The NPN transistors will be operated in its linear region, so they should allow a little bit more current on its collector then the motors will draw in stall.
Two Solarbotics GM8 gear motors where use, which draw 670mA at stall so the 2N2222 transistor which allows 800mA on their collector was chosen. The diode is needed to protect the transistors from the EMF (Electromotive force) which is generated by the DC motors when is turned off. The variable resistor or trimpot of 20Kohm is rather enough to adjust the speed of the motors in case that the motors will have a faster speed and is used even for correcting the forward direction problems due to the errors given by the LDRs.
Table 1 - Characteristics of the 2N2222A transistor
Model / Vce / Ic / PD / fT2N2222A / 40 V / 800 mA / 500 mW/1.8 W / 300 MHz
Due to high amount of light outside, the LDRs could react to it so the robot will fail following the line and it could be used only indoor and the sensor board should be covered with ducttape or other material which is not transparent. Fig. 1 shows the block diagram of the robot.
For electronics this robot will need the following components:
2x 5mm white LED / 2x 8mm LDR2x 2N2222A NPN tranzistor / 2x Diode 1N4148
2x 10 or 20KOhm trimpots / 1x Autoflashing LED
3x 330Ohm resistor / 4.5V or 6V batteries
2.2 Improved robot with new QRD1114 line sensors
As described earlier the robot based on LDR and LED has errors due to high amount of outside light and was needed some opaque material to cover the sensor array which reduces its visual aspect.
Instead, for improving this error and to have a better response for the line and surface the LED and LDR were replaced with Farichild Semiconductor’s QRD1114 line sensor. These sensors consist of an IR emitter and a IR NPN phototranzistor and it have an effective range of approximately 6mm, which make it ideal for line detection. These sensors has a built in daylight filter which improves the stability and makes the protecting opaque material unnecesary. It operates like an NPN transistor, but the base here is is replaced with the IR reciever.
As it is acting like an NPN transistor, this could be wired as a Darlington pair with the 2N2222A transistor. A Darlington pair operates like an amplifier. The principle of functions si described below for the QRD1114 IR Sensor and 2N222A Darlington Wiring: The IR emites signal, and in function of how much amount of IR is reflected the NPN transistor from inside the QRD1114 will open for current. This current will be applied on the Base of the 2N2222A transistor which allow up to 800mA on its Collector, where the motors are mounted. In total 2 Darlington pairs were used, one for each motor.
The block diagram of the robot using QRD1114 is the same as for the robot with LED and LDRs. The only modification needed is replacing the LED and LDRs with QRD1114 in the following way: The emiter will be wired instead of the LED (the emitter is a flat smaller sized LED but with IR) taking a look at the polarity of these LEDs, while reciever from the QRD1114 will be wired instead of the LDR (note: this has a polarity so the Collector will go to the positive side and the emmiter will be wired to the Base of the 2N2222A transistor).
Fig. 2 QRD1114 IR Sensor
For the electronic of this new robot these additional components were required:
· 2x QRD1114 sensor
3. PRINCIPLE OF NAVIGATION
As described earlier, the robot will use a simple principle based on LDRs and LEDs or based on QRD1114 sensors. This robot will track only black line on a white surface. These colors were selected for the best contrast and it is used too in all Line Following Competitons.
The two LED will illuminate the floor and the LEDs will react on the amount of the light reflected. If a lot of light is reflected, this is when the sensors are on the white surface because a white surface reflects white light, the LDRs will reduce their resistance value and more current will flow to the base of the 2N2222A tranzistor which operates each motor. (note: the LDRs should be placed near the LED and their head should be coaxial as shown in Fig. 5 otherwise the LDR won’t detect the surface as it needs). When a black line is detected, one of the LED will be on the black line, the amount of the light reflected will be less than on the white line so the LDRs will increase his resistance value so less current will be applied on the base of the 2N2222A tranzistor so the motor will slow down making the robot to turn slowly presented in Fig. 5.
Fig. 5 Navigation the robot based on the two sensors
To prevent problems with turnings which happens when the left sided LDR is on left but the operated motor is right-sided, the LDR will control the tranzistor and motor from its own side.
To suprimate the problems caused by the position of the LDR and LED and even for correcting the slow response of the LDR there will be used the QRD1114 Line detector sensor which comes in a package so there will be no need for calculating distances between LED and LDR so the only distance need to be respeced is the distance between the sensor and the floor because the QRD1114 has an effective distance of approximately 6 mm.
Figure 6 shows the positioning of the LED and the LDR based sensor array.
4. PRINCIPLE OF MOTION
For navigation this robot uses two readymade gear motors: the Solarbotics GM2. This little motor has small dimensions which allow it to fit almost in every little project. It has a 69 mm wheels attached and with an RPM of 29 so the maximum speed is of aproximative 10cm/s which is rather enough for this robot, remember we needed a slow RPM gear motor. Due to its 224:1 gear ratio his little motor doesn’t have a high speed, but it has 3500gm*mm torque. It is has more than needed for carrying a bigger case like this. And doesn’t really consume a lot of current, at 3V draws 400mA in stall. We will operate this gear motor at 3,6V (3x1, 2AAA researchable batteries). A third wheel was used in form of a caster wheel of 13 mm made from plastic. It was used to help the robot balancing on the end to maintain the constant distance between sensor-floor. Trying to remove this third will the robot still had the balance, but when it slowed down or accelerated it started swinging and the distance between sensors and floor wasn’t constant inducing the sensor to an always open status leading to a continuous forward movement ignoring the line.
5. STAGE OF CONSTRUCTION
Pre-made chassis for robots are usually expensive so making a good and cheap robot chassis could be the most challenging stage. In case, the chassis consists of two protective CD films that come with a package of bulk CDs. There was used two protective CDs for both upper and lower part.
5.1 The chassis
The upper parts hold the control PCB while the lower part holds the sensor array, the caster wheel and the batteries. There were driven four metric 3 holes for mounting the two chassis part together.
The motors are kept in place by the strength of the four screws which holds the two chassis parts together.
5.2 The PCB
The printed circuit boards, aka known as PCBs, were designed with Yenka PCB, great free licensed software for home use. Then the design was printed with a laser printer and there was used a toner transfer method to put the schematic on the board.
The toner transfer method consists of transferring the toner from the printed paper to the board by heating it up, using an iron. The toner reacts at high temperature resulting in a transfer to the board. After this step the board is sunk in water to remove the melted paper.
Next step is to remove unnecessary copper from the board. This is achieved by sinking the PCB with the design into a solution of ferric chloride which is a corrosive solution. After the copper is removed, the tonner should be removed with acetone (Fig. 8).
After the PCB is cleared the remaining steps are making the holes and mounting the electronic components.
5.3 Other elements
Other elements that were used in designing of the chassis:
· standard bolts and nuts (metric 3)
· 1 3xAAA battery holder
· counter weight
Materials used for this robot could be bought or found easily. The electronic parts was bought from a local electronic store, excepting the QRD1114 line sensor, because this was found only on an online store, and even the motion units consisting of the motors, wheels and caster wheel was bought online. The mechanical unit was prebuilt but it could be made from old RC cars, etc.
The following table shows all the parts that were used for making this improved robot using QRD1114 Line sensor.
2x QRD1114 sensor / 2x2N2222A transistor / 1x auto flashing LED / 1x switch3x 330R resistor / 2x 20K trimpots / 3x 2PIN header / 1x battery holder
3x 1.2V AAA Battery / 1x ¾” Caster wheel / 2x Wheels / 2x GM2 Motors
2x Diode 1N4148 / 5wire ribbon cable / 2wire cable / bolts M3
nuts 3M / washers M3 / 4x protective CD / PCB
Table 2. – Components required by the Line Follower Robot
Fig. 8 Making process of the custom PCB
6. COMMENTS AND CONCLUSION
As described in this paper, a line follower robot is easy to obtain and it comes in a large variety from simple transistor based to more complex industrial PID controlled robot. The easiest and cheapest robot is the transistor based robot whose sensor array consists by LED and LDR or by QRD1114 Line sensor.
As an advantage of this robot is the reduced price and its simplicity of build, but there are major disadvantages like the slow speed and instability on different line thickness or hard angles.
In conclusion, from these two types of transistor based robots, the QRD1114 Line sensor based robot is faster and has a faster response to environment change. This robot could have some advantages in line following competitions because it doesn’t have a microcontroller in which resets can occur a and loose all settings.
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