adding wall follower

docs/tutorials/wall-follower.md

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+# Wall Follower
+
+## Introduction @unplugged
+
+This tutorial shows you how to use the ultrasonic sensor to 
+move a [EV3 Driving Base](https://le-www-live-s.legocdn.com/sc/media/lessons/mindstorms-ev3/building-instructions/ev3-rem-driving-base-79bebfc16bd491186ea9c9069842155e.pdf)
+along a wall.
+
+Your ultrasonic sensor should be placed horizontally, near the driving wheel, facing the wall.
+
+## Step 1 Measure distance
+
+Declare a new variable ``distance`` and store the distance from 
+the ultrasonic sensor on port 4.
+
+```blocks
+let distance = 0
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+})
+```
+
+## Step 2 Show distance
+
+Use a ``||brick:show value||`` block to display the distance value on the screen.
+This is **very** helpful when you are debugging your code on the robot.
+
+Once your code is ready, download it to your robot and check that the measured distance looks ok.
+
+```blocks
+let distance = 0
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+})
+```
+
+## Step 3 Goal
+
+Declare a new variable ``goal`` and assign it to ``10`` in ``on start``. 
+The value should be the distance in centimeters between your robot and the wall.
+
+```blocks
+let goal = 0
+goal = 10
+```
+
+## Step 4 Compute Error
+
+Declare a new variable ``error`` and assign a difference between ``distance`` and ``goal``.
+We will use this value to determine how much the robot needs to correct its trajectory.
+
+```blocks
+let distance = 0
+let goal = 0
+let error = 0
+goal = 10
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+    error = distance - goal
+    brick.showValue("error", error, 2)
+})
+```
+
+## Step 5 Show Error
+
+Just like ``distance``, use ``||brick:show value||`` to display the value of the error (line 2).
+This will allow you to debug your code while it is running on the robot.
+
+Download your program to the robot and check that the error goes to ``0`` when
+the robot is around 10cm from the wall.
+
+```blocks
+let distance = 0
+let goal = 0
+let error = 0
+goal = 10
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+    error = distance - goal
+    brick.showValue("error", error, 2)
+})
+```
+
+## Step 6 Kp
+
+Declare a new variable ``kp`` and assign it to ``1``.
+This number determines how to convert the error into a ``turn ratio`` for the steer block.
+For starter, set it to 1 and we will go through the steps to tune its value later on.
+As usual, also use ``||brick:show value||`` to display the value of ``kp`` on the screen (line 3).
+
+```blocks
+let distance = 0
+let goal = 0
+let error = 0
+let kp = 0
+goal = 10
+kp = 1
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+    error = distance - goal
+    brick.showValue("error", error, 2)
+    brick.showValue("kp", kp, 3)
+})
+```
+
+## Step 7 Turn ratio
+
+Declare a new variable ``turnratio`` and store the product of ``error`` and ``kp`` in it.
+Also use ``||brick:show value||`` to display its value on screen.
+
+Download the program on the robot and try moving the robot around the wall. You should see
+the value of ``turnratio`` change similarly to ``error``.
+
+```blocks
+let distance = 0
+let goal = 0
+let error = 0
+let kp = 0
+let turnratio = 0
+goal = 10
+kp = 1
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+    error = distance - goal
+    brick.showValue("error", error, 2)
+    brick.showValue("kp", kp, 3)
+    turnratio = error * kp
+    brick.showValue("turn", turnratio, 4)
+})
+```
+
+## Step 8 Steering
+
+Add a ``||motors:steer motors||`` block for ``large B+C`` at 35% and place the ``turnratio``
+variable for the turn value.
+
+Download the code to your robot and try it out. Does it follow the wall?... 
+Not really, this is because we need to tune the ``kp`` variable.
+
+```blocks
+let distance = 0
+let goal = 0
+let error = 0
+let kp = 0
+let turnratio = 0
+goal = 10
+kp = 1
+forever(function () {
+    distance = sensors.ultrasonic4.distance()
+    brick.showValue("distance", distance, 1)
+    error = distance - goal
+    brick.showValue("error", error, 2)
+    brick.showValue("kp", kp, 3)
+    turnratio = error * kp
+    brick.showValue("turn", turnratio, 4)
+    motors.largeBC.steer(turnratio, 35)
+})
+```
+
+## Step 9 Tuning kp
+
+As mentioned in a previous step, we need to find the right value for kp so that the robot
+follows the wall properly. This tuning can be tedious so we are going to the brick buttons
+to speed up the process.
+
+Add ``||brick:on button||`` blocks to handle the left and right button pressed. When left is pressed, change ``kp`` by ``-1``. When right is pressed, change ``kp`` by 1.
+
+Download your code to the robot and change the values of ``kp`` until the robot follows the wall. (Tip try something around -5 / -10).
+
+```blocks
+let kp = 0
+brick.buttonLeft.onEvent(ButtonEvent.Pressed, function () {
+    kp += -1
+})
+brick.buttonRight.onEvent(ButtonEvent.Pressed, function () {
+    kp += 1
+})
+```
+
+## Step 10 @unplugged
+
+Well done! Your robot is using the ultrasonic distance
+to correct is trajectory using a proportional controller!
+
+The robot will be more precise if it goes slow... Try using a variable
+and the brick up and down events to control the speed as well.