The CheapBot Line Follower v2.0 is a plug-in single-board sensor for almost any programmable robot brain. With it, a robot can detect the presence of a black or white zone beneath its two sensors. In its simplest use, the board provides the feedback a robot needs to navigate a black line on a white surface. In addition, it also lets a robot detect the boundary of a robotic playing field. The line follower is a great first sensor for a CheapBot robot. Onwards and Upwards, Your near space guide Figure 1. CheapBot Line Follower Tools Needed to Assemble a NearSys Line Follower While there are literally one hundred tools for soldering, testing, and fixing electronic circuits, you only need these few tools to make the line follower. Safety Glasses Screwdrivers (straight slot and Phillips) Soldering Iron Wire Cutters 1
Pictorial List of Line Follower Components Figure 2. Printed Circuit Board Figure 3. Resistors (four of them) Figure 4. IR LEDs (two of them) Figure 5. Phototransistor with hood (two of them) 2
Figure 6. Header Cable Figure 7. Plastic Block (two of them) Figure 8. Sheet Metal Screw (four of them) 3
Figure 9. Nylon Spacers (three of them) Figure 10. #2-56 bolt and nylock (three of them) 4
Theory of Operation Figure 11. Schematic of the CheapBot Line Follower. The infrared LEDs (IREDs) in the line follower emit two infrared beams at a wavelength of 950 nanometers (just within the infrared portion of the spectrum and just outside our visible range). If there is a white surface beneath the IR beams, the beams reflect back up to the phototransistors. The phototransistors, with a peak sensitivity of 850 nanometers (nm) have enough bandwidth to detect the 950 nm IRED radiation. At the detection of the reflected beam, the phototransistor conducts and there is no voltage drop across the phototransistor. When there is no voltage drop, the robot controller detects a low signal. Dark surfaces on the other hand, like black electricians tape, do not reflect the IR beam back up to the phototransistors. Without the reflected beam, the phototransistor does not conduct and there is a five volt drop across the phototransistor. This results in the robot controller detecting a high signal. With phototransistor/ired pairs on the left and the right side of the line follower, black and white regions can be detected on the left, right, or on both sides of the robot. Two 330 ohm resistors control the brightness of the IREDs. Since each 330 ohm resistor and IRED series circuit is supplied with +5 volts and each IRED has a forward voltage of 1.5 volts, only 3.5 volts is dropped across each resistor. According to Ohm s Law, the current flowing through the resistors and therefore the IREDs is 3.5 volts/330 ohms or 0.011 amps (11 milliamps). 5
Figure 12. The IR LED (IRED) emits a beam of infrared that is reflected from a white surface and detected by the phototransistor (PhT) as in situation A. When the IRED is over black tape, the infrared is absorbed and none is reflected back to the phototransistor and detected, as in situation B. The sensitivity of the phototransistors is sufficient to detect the reflected beam of an IRED from several inches. Notice each phototransistor has a 47k ohm resistor connecting its collector to five volts. The resistor limits the maximum current flowing through the phototransistor when it conducts and it also acts like a voltage divider in conjunction with the conducting phototransistor. Assembly Directions The following components are polarized and must be inserted with the proper orientation. Q1 and Q2 Align the flat side of the plastic case (or short lead) with the A on the top silk D1 and D2 Align the long lead with the A in the top silk Note: D1 and D2 can also be aligned with flat of LED lens on pad opposite the A in the top silk Mount the resistors, IREDs, and wires flush with the PCB surface. The phototransistors stand approximately 1/8 inches above the PCB so they can be bent into proper alignment. This means the bottom of phototransistor s black jacket sits flush on the surface of the PCB. 6
Figure 13. Placement of Components on the line follower PCB Suggested Order of Assembly R1-330 ohm resistor (orange,orange,brown,gold) R2-47k ohm resistor (yellow,violet,orange,gold) R3-330 ohm resistor (orange,orange,brown,gold) R4-47k ohm resistor (yellow,violet,orange,gold) D1 IRED (anode lead in pad closest to A) D2 IRED (anode lead in pad closest to A) Q1 Phototransistor (collector lead in pad closest to A) Q2 Phototransistor (collector lead in pad closest to A) Figure 14. Identity of IRED leads Figure 15. Identity of phototransistor leads Cabling Steps The wires in the Interface Cable are soldered to the PCB using the strain relief holes as illustrated below. 7
Figure 16. A strain relieved wire ready for soldering Solder the Interface Cable to the PCB with the following connections Red wire to +5V pad Green or black wire to GND pad Remaining two wires to Left and Right pads One side of the plastic jacket covering the end of the Interface Cable is colored black. This is the side with the ground wire. The pins in the 2 by 3 header terminating the Interface Cable have the following function, based on their position within the header. Figure 17. The function of the pins in a 2 by 3 header Solder the green wire to any one of the GND header pins 8
Installing the Plastic Blocks The plastic blocks are designed to protect the IRED and phototransistor from being knocked out of alignment. The hole in their center is where the IRED and phototransistor are placed. The opening at the bottom of the block was cut in order that the Line Follower PCB can be mounted to the bottom of a robot. Therefore, the plastic blocks must be oriented so that the slots cut into them are facing the bottom of the PCB, or over the PCBs mounting holes. Use two sheet metal screws to attach each plastic block. Figure 18. Two plastic blocks bolted to the Line Follower PCB. Test Procedure Before inserting the CheapBot Line Follower into a robot controller I/O port, make the following measurements to verify that the PCB is properly assembled. Inspect the bottom of the PCB and verify there are no soldered connections overflowing their pads to neighboring pads Measure continuity between the +5V and GND pins in the header and make sure there is no continuity between the positive and negative terminals in the header Plug the header into the C0 and C1 pins of the CheapBot robot controller I/O port Plug a programming cable into the controller Start the PICAXE program Editor Start the robot Prop the robot up so the line follower points off into space Set the Editor to the correct PICAXE version and proper Com Port Type and download the following program Test: B0 = 0 BIT0 = PINC.0 BIT1 = PINC.1 DEBUG GOTO Test 9
The Debug terminal will pop up so you can observe the value of byte B0. * B0 will have a value of 3 when nothing reflective is in front of the line follower * B0 will have a value of 0 when a white index card passes in front of both phototransistor/ired pairs * B0 will have a value of either 1 or 2 when a white index card passes in front of either one of the phototransistor/ired pairs. If you do not observe variable B0 changing between values 0 and 3, then bend the leads of the phototransistor until it is pointed where it can see the infrared beam reflected off the white surface. Add a stripe of electrician s tape to the index card and pass the line in front of the phototransistor/ired pairs and observe that B0 changes values to correspond to the position of the black stripe. Using the CheapBot Line Follower The next step is to mount your CheapBot Line Follower to the robot body and plug in its cable. The Line Follower kit contains three #2-56 bolts and nylocks that attach the line follower to the underside of a robot. The kit also has three nylon spacers to place over the #2-56 bolts and between the line follower PCB and bottom of the robot. Figure 19. Underside view of the line follower mounted beneath the nose of a robot. The cables from the line follower PCB plug into two channels of an input port. Be sure that the black-colored side of the block on the end of the Interface Cable is on the ground side of the I/O port. 10
Line Following Board Figure 20. A CheapBot robot on a line follower board. The board is a sheet of melamine and the black lines are electrician s tape. Using the line follower requires that the robot be moving. So program the robot to drive forward. Then while the robot drives, monitor the voltage from the left and right phototransistors. When both are high, the phototransistors are detecting a white surface. When one or both voltages go low, the IRED/phototransistors are passing over a black surface. When a black line is detected, stop the robot and determine the direction the robot must steer in order to realign on the black line. Then drive the robot forward again while monitoring the line follower outputs. Here s some sample code for the PICAXE. SYMBOL Detectors = B0 SYMBOL RightDetect = BIT0 SYMBOL LeftDetect = BIT1 Line_Follower: GOSUB Forwards Check_Line_Follower: GOSUB Line_Finder_Check IF Detectors = 1 THEN Line_Right IF Detectors = 2 THEN Line_Left 11
IF Detectors = 3 THEN Line_in_Front GOTO Check_Line_Follower Line_in_Front: GOSUB Freeze GOSUB Right PAUSE 600 GOSUB Freeze GOSUB Forward GOTO Line_Follower Line_Right: GOSUB Left PAUSE 120 GOTO Line_Follower Line_Left: GOSUB Right PAUSE 120 GOTO Line_Follower Line_Finder_Check: Detectors = 0 RightDetect = PIN0 LeftDetect = PIN1 RETURN 18 March 2013 12