74*245 Motor Driver

H-Bridge Driver

Simple PWM Gen.

Handy Method Measuring RPM

Measuring RPM via Photo reflector

Introduction to Robotic

DC, Stepper,and Servo Motor

Microcontroller Tutorial

Computer Interface
Tutorial
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Software

Using photo-reflectors

To detect a line to be followed, most contestants are using two or more number of poto-reflectors. Its output current that proportional to reflection rate of the floor is converted to voltage with a resister and tested it if the line is detected or not. However the threshold voltage cannot be fixed to any level because optical current 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, this is effective when the detector is working in linear region.

In this project, pulsed light is used to cancel ambient light. This is suitable for arraied sensors that scanned in sequence to avoid interference from next sensor. The microcontroller starts to scan the sensor status, sample an output voltage, turn on LED and sample again the output voltage. The difference between the two samples is the optical current by LED, output voltage by the ambient light is canceled. The other sensors are also scanned the same avobe in sequence.

Signal processing of line detection

Right image shows the actual line posisiton vs detected line position in center value of 640. The microcontroller scans six sensors and calcurates 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 value that proportioning to reflection ratio, and the sensitivity have variety between each one of them. In this system, to remove the variations from the outputs, calibration parameters for each sensor can be held into 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.

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