Desktop Line Following Robot
By ChaN
Recently many kind of robot contests have being opened and some interesting reports of the challenge are found on the web. Line Following is a kind of robot contests which measures running speed on the line. I built a tiny line following robot which can run on the desk-only moving the key board aside will do. It is for only a personal toy as it is reduced in size less than one fifth compared to typical line following robots. I believe that it is suitable for home use in small houses and apartments.
About Line Follower
The line follower is a self operating robot that follows a line drawn on the floor. The basic operations of line following are as follows:
- Capture line position with optical sensors mounted at the front end of the robot. Most competitors are using several photo-reflectors, and some leading contestants are using an image sensor for image processing. The line sensing procss requires high resolution and high robustness.
- Steear robot to track the line with any steearing mechanism. This is just a servo operation, any phase compensation will be required to stabilize tracking motion by applying digital PID filter or any other servo argolithm.
- Control speed according to the lane condition. Running speed is limited during passing a curve due to friction of the tire and the floor.
There are two line styles, white line on the black floor and black line on the white floor. Most contest are adopting the first one in line width of between 15 and 25 millimeters.
Hardware
Mechanics
Right image shows bottom view and side view of the built line following robot. All mechanical and electrical parts are mounted on a proto board, and it also constitutes the chassis.
The line following robot is upheld in three points of two driving wheels and a free wheel. The driving wheels are made with a 7 mm dia ball bearing and a rubber tire. The free wheel is a 5 mm dia ball bearing attached loosely. To power driving wheels, two tiny vibration motors that are used for a cellular phone, pager or any mobile equipment. Each shaft is pressed onto the tire with a spring plate, and the output torque is transferred to the wheels.
The steering mechanism is realized in differential drive that steers the robot by difference in rotation speed between the left wheel and the right wheel. It does not require any additional actuator, only controlling the wheel speed will do.
Electronics
| Controller | ATmega8 (Atmel) |
| Line sensor | Six photo-reflectors |
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Power supply
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Two CR2032 lithium cells (One is for controller, the other is for motors) |
| Motor | Two micromotors for left wheel and right wheel |
| Dimensions | 45(L), 33(W), 12.5(H) [mm] |
| Weight | 15 grams (Body:8g, Cells:7g) |
| Performance | 53 centimeters per second at oval course |
An Atmel ATmega8 is used for the controller and it is powered by a lithium coin cell. The other lithium coin cell is for only motors. Separating the power supply into two cells is to avoid accidental reset of the microcontroller due to voltage dip by motor start current. Six photo-reflectors are mounted at front end of the chasis. They sense reflection rate of the floor under them. Motors are driven in PWM to control rotation speed lineary. The latest circuit diagram is shown below:
Software
Using photo-reflectors
To detect a line to be followed, most contestants are using two or more photo-reflectors. The output current-proportional to reflection rate of the floor-is converted to voltage with a resister and tested to determine if the line is detected or not. However the threshold voltage cannot be fixed to any level because optical current created by ambent light is added to the output current like the image shown right.
Most photo-detecting modules for industrial use are using modurated light to avoid interference by the ambient light. The detected signal is filtered with a band pass filter and disused signals are filtered out. Therefore only the modurated signal from the light emitter can be detected. Of course the detector must not be saturated by ambient light because it is effective when the detector is working in its’ linear region.
In this project, pulsed light is used to cancel ambient light. This is suitable for arrayed sensors that are scanned in sequence to avoid interference from the next sensor. The microcontroller starts to scan the sensor status, sample the output voltage, turns on the LED and samples again the output voltage. The difference between the two samples is the optical current created by the LED, as the output voltage produced by the ambient light is canceled. The other sensors are also scanned the same way in sequence.
Signal processing of line detection
The image at right shows the actual line posisiton vs detected line position with a center value of 640. The microcontroller scans six sensors and calculates the line position by output ratio of two sensors near the line. Thus the line position can be detected lineary with only six sensors. All the sensor outputs are captured as analog values that are proportionate to each sensors’ reflection ratio, and the sensitivity varys between each one of them. In this system, to remove the variations from the outputs, calibration parameters for each sensor can be held in non-volatile memory. This can be done with online mode. The microcontroler enters the online mode when an ISP cable is attached, and it can be controlled with a terminal program in serial format of N81 38.4kbps. S1 command monitors sensor values, and S2 command calibrates variation of sensor gain on the reference surface (white paper). The ATmega8 must be set to 8MHz internal osc.
Tracking control
The line position is compared to the center value to be tracked, and the position error is processed with Proportional/Integral/Diffential filters to generate steering command. The line folloing robot tracks the line in PID control that is the most popular argolithm for servo control.
The proportional term is the common process in the servo system. It is only a gain amplifier without time dependent process. The differential term is applied in order to improve the responce to disturbance, and it also compensate phase lag at the controlled object. The D term will be required in most case to stabilize tracking motion. The I term is not used in this project from following reasons. The I term that boosts DC gain is applied in order to remove left offset error, however, it often decrease servo stability due to its phase lag. The line following operation can ignore such tracking offset so that the I term is not required.
When any line sensing error has occured for a time due to getting out of line or end of line, the motors are stopped and the microcontroller enters sleep state of zero power consumption.
NOTES:
The Firmware can be found here and the include file is here.
Line following motion with only proportional correction. This is a video of the robot following the line with only proportional control. Notice that it leaves the line entirely at the end.
Line following motion with both proportional and differential control. This video shows the smooth operation attained with both proportional and differential control.
