CHAPTER 25 ■ LAUNCHING THE LINE-FOLLOWER
- The lower resistance of the photoresistors on the right side has a voltage-reducing
effect on TP2. However, light reflecting off the surface in front of the right headlight
also seems to be reaching the left sensors depending on the surface material. Note
the fairly low TP1 voltage, but fairly high TP2 voltage, for aluminum. Perhaps a
physical barrier between the two sets of sensors might improve performance?
1-inch wide blue masking tape beneath right sensors on white paper,
TP1 and TP2: 3.604 V and 2.065 V
1-inch wide strip of black paper beneath right sensors on white paper,
TP1 and TP2: 4.93 and 2.075 VAcceptable Ranges of Voltages at the Sensor Test Points
The comparator will not switch correctly if both test-point voltages exceed the positive power bus’s voltage
minus 1.5 V. For example, a 9.07 V bus permits comparisons up to 7.57 V (9.07 V – 1.5 V). The sensors are not
getting enough light if a line exists but both test-point voltages are that high.
In Chapter 23 , the section “Tweaking for Better Performance” includes instructions for optimizing R9
and R1 for your robot’s white LEDs and sensors. If you previously chose to stay with the default 150 W values,
but your sensor test points routinely exceed the upper 1.5 V limit, then you probably need to remove those
resistors. Replace R9 with 100 W for increased brightness and replace R1 with 22 kW for a larger voltage drop.
The comparator won’t differentiate precisely if both test-point voltages are too similar to each other.
That’s desirable if the surface has the same brightness (a blank piece of paper) and the sensors are properly
balanced. But, that’s a big problem if the line to follow is beneath one set of the sensors! Look for a difference
of at least ±1 V between the set of sensors with the line and the set of sensors without the line.
If both sensors are below 1 V most of the time, the sensors are getting too much light.
After fine-tuning the trimpots and the circuit board’s position, be sure to move the robot into the
brightest, darkest, most shadowy, most contrasting, and least contrasting situations to ensure the voltages
meet the comparator’s requirements under all conditions.
Does This Look Straight to You?
Now is a good opportunity to observe and explain that the robot doesn’t quite drive straight.
Except for brief moments when both sets of sensors are receiving exactly the same amount of light, both
motors are not turned on at the same time. This means the robot rarely drives straight forward. Instead, the
robot shuffles left and shuffles right so the overall direction averages out to be straight.
Following a Dark Line
Figure 25-7 consists of three frames showing the motion of the robot following a dark line. The robot’s angles
in these pictures are exaggerated for illustrative purposes.