Line Tracking Robot Project

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

Line Tracking Robot Project
This project is based on the MOVIT/OWIKIT LINE TRACKER kit. Each group of three students is provided with one kit. However, to make the project more meaningful, each group will construct and test each of the electrical subsystems of the robot on a breadboard and test it. After the entire system is put together in this fashion and its operation understood, each group will also put the pre-wired kit together and observe its behavior. Each robot has the following subsections: A. Sensor Section B. Comparison Section C. Amplification Section D. Power Source Before describing this section of the Robot we will go over the operation of some of the devices used in it. 1. A diode Diode is a two terminal semiconductor device made of two sections called n and p side. A terminal is connected to each of these two sides. If a positive voltage of more than a few tenths of a volt is applied to the p side with respect to the n side, the diode conducts current very easily. This is called forward-bias. On the other hand, if the p side is negative with respect to the n side, the

diode conducts very little current and is called reverse-biased.

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

2. Light Emitting Diode Certain diodes, under the condition of forward-bias, emit light. These could be of different colors. In our case it is infrared.

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

3. Transistor A transistor is a three-terminal device that is made of three sections called emitter, base and collector. A transistor is either npn or pnp.

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

A small base current, IB, is used to control a much larger collector current Ic . A small signal (small variations) superimposed on the IB results in similar but larger variations in Ic . This is called amplification. 3. Phototransistor A phototransistor is a transistor that is sensitive to light. When light shines on it, the collector current will greatly increase, while in the absence of light, Ic is very small. We can view the effect of light as controlling the effective resistance between the collector and emitter. In the presence of light this resistance is low, resulting in a large collector current, while in the absence of light the opposite happens.The phototransistor is functioning as a switch; no connection is made to the base. 4. Potentiometer This device is essentially a variable resistor. It has three terminals. Figure 3 shows the example of a 10kW potentiometer. The contact C can be moved along the 10kW resistor from A to B. The resistance between A and B is fixed (10kW in this case). The resistance between A and C can be changed from 0 to 10kW by moving the pointer from A to B. As this happens, the resistance between C and B changes from 10kW to 0.

A. Sensor Section
The sensing subsystem consists of two identical parts called Photo Interrupters. Each photo interrupters is made of a light emitting diode (LED) and a phototransistor.

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

The LED emits infrared radiation. If the Photo Interrupter passes over a white surface, the infrared light is reflected back and is detected by the phototransistor. Let us assume that this is the case for the right (shown in red) photo interrupter assembly. The result is that the emitter-collector resistance drops and a large current flows through the red 27kW resistor. This created a large voltage drop across this resistor and VR is reduced to near zero. If at the same time, the left photo interrupter assembly (shown in blue) is over the black line, the radiation is not reflected back to the phototransistor and the emitter-collector resistance remains high. This results in a small current through the blue 27kW resistor. The resulting small voltage drop across this resistor leads to a high value of ~9 V for VL. We send these two voltages to a comparator, which then makes one of the two wheels of the robot rotate in such a fashion that it turns to the left. The 10kW resistor is a potentiometer, and we adjust it so that when light strikes both photo-interrupters (or neither of them), the left motor will be rotating and the right motor stopped. We use the pre-assembled photo-interrupter units from the kit. The following picture shows the protoboard set-up for the sensor stage of the robot:

B. Comparison Section
Before describing the next section we will introduce a new element, the Operational Amplifier (OPAMP). 5. OPAMP This device (Fig. 4), has two inputs, VN (the inverting input), and VP (the non-inverting input).

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

It has one output, VO. The remaining two connections, +VCC and -VCC provide power to the OPAMP. VCC is usually between 12 and 25 V. In our circuit, +VCC is connected to the +9V power supply and -VCC to the ground. We use the OPAMP as a comparator:. If VP > VN , then V0 = +VCC. If VP < VN , then V0 = -VCC. In our case, +VCC=+9V and -VCC=0V(ground) The integrated circuit (IC) used in this kit contains two OPAMP’s used as comparators. The package (Fig. 5) has 8 pins: 4 inputs, 2 outputs and 2 more pins for +VCC and -VCC.

The robot moves in such a way as to locate itself above a dark line drawn on a white surface. The voltage at pin 3 of the comparator 1 is fixed at +3V because of the voltage divider (we have 9V across the series combination of the 10 and 20kW resistors). Now let us see what happens if the right photo-interrupter is on a dark region and the left photo-interrupter is on a light region. As we saw in Section A, under this condition VR > VL. This will result in the output of the comparator 2 (pin 7), to be low (a few tenths of a volt). The Amplifier Section connected to pin 7, i.e., TR1 and TR3 (as we will see later) actually work in such a way as to output a large current to the Left Motor when pin 7 is low! This allows the Left Motor to rotate. At the same time, pin 7 is fed into the pin 2 of comparator 1. Since the voltage at pin 3 of this comparator (the non-inverting input) is held fixed at +3 V, the output (pin 1) will go high (~9V), and the amplifier section, TR2 and TR4 out put very little current to the Right Motor. Therefore it does not rotate. If the left motor operates and the right motor does not, the robot move to the right, until the left photo-interrupter is located on the dark region and the right photo-interrupter on the light region. The 1k resistors connecting the outputs of the two compatators (pins 7 and 1) to +9V are temporarily there to simulate the rest of the circuit, and are removed when the amplifier section is installed.

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

This will result in VL > VR, which in turn makes the output of comparator 2 high (pin 7 becomes ~9V). As a result, the Left Motor stops but also, since the output of the comparator 1 goes low, the Right Motor rotates. So the robot will turn to left and find the dark line again. The resulting motion is a zig-zag around the dark line. The following two pictures shows the protoboard set-up for the sensor and the comparator stages of the robot:

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

D. Amplification Section:
This section (See Figure 4) is made of two amplifier subsections. One, consisting of transistors TR1 and TR3, connects pin 7 of the the second comparator to the Left Motor. The second, consisting of transistors TR2 and TR4, connects pin 1 of the first comparator to the Right Motor. Let us see how one of these, the first, operates. If the output of pin 7 is high (~9V), very little current will flow through the 68k resistor connected to the BASE of TR1. Therefore this transistor acts as a switch which is turned off. As a result very little current

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

will leave the COLLECTOR of TR1 which is in turn connected (through the 750W resistor) to the BASE of TR3. In turn, very little current will leave the COLLECTOR of TR3 (no amplification). Since the collector of TR3 is connected to the Left Motor, it does not rotate. The opposite is true when pin 7 is low. To summarize: Pin 7 high Left Motor stopped Pin 7 low Left Motor turns Pin 1 high Right Motor stopped Pin 1 low Right Motor turns The following two pictures show the completed protoboard:

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Line Tracking Robot Project

http://www.seas.upenn.edu/~ese111/robot/Robot.html

E. Power Source
There are two sources of power. A 9V battery provides power for the electronic circuits and two 1.5V cells in series power the motors. In the lab, you will use the 0 to 25V variable power supply set for 9V, and the 0 t0 6V variable supply set to 3V.

Created by: Sohrab Rabii and Siddharth Deliwala, November 1998.

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