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Measuring RPM via Photo reflector
Computer
Interface
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Once this was accomplished, the sensors were introduced (Figure 1). Two pair of visible light emitter detector pairs were positioned at the front of the robot. The light emitters are high intensity red LEDs with a low viewing angle. The low viewing angle makes the light more unidirectional, like a laser and reduces interference between the two sensors. The detectors are photo resistors with black plastic surrounding them so that only a small circular region directly below the sensor will effect the detection of the paper color. The sensor pair work very well with a variety of imperfect paper surfaces. The photo resistors are set up in a voltage divider circuit to so that the output voltage varies about 1 volt under standard ambient lighting conditions and so that the trip point (1.4v) of the digital input to the microprocessor is in the center of this swing. When one diode detects a black area, the PIC’s program decides which way to steer the robot such that the detectors will both see white paper. For instance if the left sensor sees black, it tell the processor to turn left. However if both resistors detect a black area, the motor stops until this condition changes. A “light shield” was added at the front of the robot to minimize interference from the lighting of the room (see the attached program flow, and assembly code). Analysis and results: The robot successfully navigated along a solid black line and came to rest when it reached an all-black area. The “light shield” was added as a result of sensitivity problems when the room’s lighting interfered with the sensing of the black line. There are a few things that could be done differently to increase the accuracy and ability of the robot. A differential drive robot would increase the ability of the robot to navigate around sharper turns. As it is currently, the turning radius is quite large. If a standard steering scheme were used then a servo motor would be better suited for steering the robot so that gentle turns could be implemented instead of full left and full right being the only options. At least two more sensors placed directly in front of the original two in order to create a 2D image of the work space would allow for greater accuracy, the ability to turn at 90 degrees (with differential steering), and higher speeds. Conclusion While the idea of a robot that steers itself to follow a solid line may at first seem like a difficult project, it can be accomplished with relative ease by using a PIC microcontroller to run the motor and control the steering of the robot based on the status of the two sensors. Asembly Code: ; LINE FOLOWING ROBOT _CONFIG _CP_OFF & _WDT_OFF & _BODEN_OFF & _PWRTE_OFFFF & CBLOCK 0x20 ;********************* Port b Bits ********************* ;********************* Port C Bits ********************* ;************* INTERNAL RC CALIBRATION ****************** ORG h'0000' ;*********************************************** ;NOTE: ALL MAIN PROGRAM DATA IS IN THE SECOND HALF OF THE "PAGE" ;****************** INITIALIZE ********************* INITIALIZE: BCF STATUS,RP1 BSF STATUS,RP0 MOVLW B'01100010' ;Set GPIO data direction register, MOVWF TRISB MOVLW B'00000000' ;Set GPIO data direction register, MOVWF TRISC MOVLW B'10011111' ;watchdog timer assigned to prescaler, MOVWF OPTION_REG ;prescaler set to max (2.3 sec) BCF STATUS,RP0 ;set file register bank to ZERO BCF PORTB,STEER_1 ;STER MOTOR OFF BSF PORTB,STEER_2 BCF PORTB,STEER_3 BSF PORTB,STEER_4 BCF PORTB,MOTOR CLRF PORTC HOLD: BSF PORTC,LED_L BSF PORTC,LED_R MOVLW .255 ;0.119mS DELAY SEQUENCE CALL DELAY21uS BCF PORTC,LED_L BCF PORTC,LED_R START: MOVLW .5 ;0.119mS DELAY SEQUENCE CALL DELAY21uS BTFSC PORTB,SENSOR_L CALL T_L_T BTFSC PORTB,SENSOR_R CALL T_R_T MOVLW .12 MOVWF ON_TIME MOVLW .1 CALL MOTOR_FOWARD GOTO START T_L_T: BTFSC PORTB,SENSOR_R GOTO HOLD CALL TURN_LEFT RETURN T_R_T: BTFSC PORTB,SENSOR_L GOTO HOLD CALL TURN_RIGHT RETURN TURN_LEFT: BSF PORTC,LED_L BCF PORTB,STEER_1 ;MOTOR LEFT BCF PORTB,STEER_2 BSF PORTB,STEER_3 BSF PORTB,STEER_4 MOVLW .14 MOVWF ON_TIME MOVLW .4 CALL MOTOR_FOWARD BCF PORTB,STEER_1 ;MOTOR OFF BSF PORTB,STEER_2 BCF PORTB,STEER_3 BSF PORTB,STEER_4 BCF PORTC,LED_L RETURN TURN_RIGHT: BSF PORTC,LED_R BSF PORTB,STEER_1 ;MOTOR LEFT BSF PORTB,STEER_2 ;PNP BCF PORTB,STEER_3 BCF PORTB,STEER_4 ;PNP MOVLW .14 MOVWF ON_TIME MOVLW .4 CALL MOTOR_FOWARD BCF PORTB,STEER_1 ;MOTOR OFF BSF PORTB,STEER_2 BCF PORTB,STEER_3 BSF PORTB,STEER_4 BCF PORTC,LED_R RETURN ;****************************************************** ;DUTY CYCLE CREATION OF MOTOR (W*2.4mS = CALL) *** ; SET DUTY CYCLE BY MODIFYING "ON_TIME" ********* ;********************* PASS ON TIME THOUGH "W" REGISTER * ;****************************************************** MOTOR_FOWARD: MOVWF CYCLES DECF ON_TIME,F ;0 COUNTS AS A LOOP LOOP_DELAYS: MOVF ON_TIME,W ;SAVE INPUT W REGISTER DATA AS ON TIME (ON_TIME/20 = DC) MOVWF ON_TIME_COUNT MOVLW .19 ;20 IS TOTAL LOOPES USED (20*.119mS=2.4mS) MOVWF TOTAL_DUTY ;THUS TOTAL DUTY IS 20 ON_TIME_LOOP: BSF PORTB,MOTOR ;TURN ON OUTPUT LOAD MOVLW .1 ;0.119mS DELAY SEQUENCE CALL DELAY6uSA BCF PORTB,MOTOR ;TURN OFF OUTPUT LOAD (inverted) DECF TOTAL_DUTY,F ;WE HAVE X = X-1 TOTAL LOOPS LEFT DECFSZ ON_TIME_COUNT,F ;WE HAVE Y = Y-1 ON LOOPS LEFT GOTO ON_TIME_LOOP OFF_TIME_LOOP: BCF PORTB,MOTOR ;TURN OFF OUTPUT LOAD (inverted) MOVLW .1 ;0.119mS DELAY SEQUENCE CALL DELAY6uSA DECFSZ TOTAL_DUTY,F ;WE HAVE X = X-1 TOTAL LOOPS LEFT GOTO OFF_TIME_LOOP DECFSZ CYCLES,F ;WE HAVE X = X-1 TOTAL LOOPS LEFT GOTO LOOP_DELAYS RETLW .0 ;******************************************************** ;***** 21 uS DELAY SUBROUTINE * ;********************************************************* DELAY21uS: MOVWF COUNTER3 ;1uS WAIT3D: MOVLW .1 ;1uS MOVWF COUNTER2 ;1uS WAIT2D: MOVLW .7 ;1uS MOVWF COUNTER1 ;1uS WAIT1D: DECFSZ COUNTER1,1 ;1 or 2uS GOTO WAIT1D ;2uS DECFSZ COUNTER2,1 ;1 or 2uS GOTO WAIT2D ;2uS DECFSZ COUNTER3,1 ;1 or 2uS GOTO WAIT3D ;2uS CLRWDT ;1uS RETLW .0 ;*********************************************** ;************************ 6.0uS DELAY SUBROUTINE ;*********************************************** DELAY6uSA: NOP NOP NOP NOP NOP NOP NOP NOP ;2uS RETURN END |
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