small bugfixes, make variable instead of magic number

Author: AnselChang

course/robot_control/distance_tracker.rst

@@ -22,17 +22,18 @@ For this activity, let's use a target distance of 30 cm.
 
             rangefinder = Rangefinder.get_default_rangefinder()
 
+            distance = 30
             while True:
-                if rangefinder.distance() < 30:
+                if rangefinder.distance() < distance:
                     drivetrain.set_speed(-20, -20)
-                elif rangefinder.distance() > 30:
+                elif rangefinder.distance() > distance:
                     drivetrain.set_speed(20, 20)
 
 
     .. tab-item:: Blockly
 
         .. image:: media/SimpleStandoff.png
-            :width: 300
+            :width: 500
 
 You'll notice that this code causes the robot to move back and forth, or oscillate, as the sonar distance continuously swaps between being greater than and less than 30 cm.
 So what if we add a third case that tells the robot's motors to stop when sonar distance equals 30 cm?
@@ -47,10 +48,11 @@ So what if we add a third case that tells the robot's motors to stop when sonar
 
             rangefinder = Rangefinder.get_default_rangefinder()
 
+            distance = 30
             while True:
-                if rangefinder.distance() < 30:
+                if rangefinder.distance() < distance:
                     drivetrain.set_speed(-20, -20)
-                elif rangefinder.distance() > 30:
+                elif rangefinder.distance() > distance:
                     drivetrain.set_speed(20, 20)
                 else:
                     drivetrain.stop()
@@ -59,7 +61,7 @@ So what if we add a third case that tells the robot's motors to stop when sonar
     .. tab-item:: Blockly
 
         .. image:: media/SimpleStandoffStop.png
-            :width: 300
+            :width: 500
 
 Unfortunately, even with this code, our robot still doesn't stop! The issue is that the distance sensor is so precise that it
 never reads exactly 30 cm. We can combat this by making our robot stop when it's *close* to 30 cm instead of *exactly* 30 cm.
@@ -76,10 +78,12 @@ We can do this by creating a range in which our robot stops called a "deadband."
 
             rangefinder = Rangefinder.get_default_rangefinder()
 
+            distance = 30
+            tolerance = 2.5
             while True:
-                if rangefinder.distance() < 27.5:
+                if rangefinder.distance() < distance - tolerance:
                     drivetrain.set_speed(-20, -20)
-                elif rangefinder.distance() > 32.5:
+                elif rangefinder.distance() > distance + tolerance:
                     drivetrain.set_speed(20, 20)
                 else:
                     drivetrain.stop()
@@ -88,7 +92,14 @@ We can do this by creating a range in which our robot stops called a "deadband."
     .. tab-item:: Blockly
 
         .. image:: media/deadband.png
-            :width: 300
+            :width: 550
+
+.. note:: 
+    Notice how, instead of hardcoding numbers such as 27.5 and 32.5, we used variables. This gives us two benefits:
+    
+    1. We can easily change the desired distance and tolerance without having to change the code itself.
+    
+    2. It's much easier to decipher what the code is doing, using "magic" numbers like 27.5 and 32.5 can be confusing to read because the user has to figure out what those numbers mean.
 
 This code should allow the robot to stop when it senses a sonar distance of ~30 cm. Our issue now is that
-there is a potential error of +- 2.5 cm from our desired following distance. Luckily, there is a solution to this called "proportional control."
+there is a potential error of 2.5 cm from our desired following distance. Luckily, in the next section, we'll learn about something called "proportional control"...