Keeping the ball at a distance

Now that we have our COG_BOX_SIZE variable we can arbitrarily chose 85 as the size that BucketBot should try to maintain (this equates to a couple inches from the ball). Doing so will require forward and backwards movement based on the relative size of the green ball or COG_BOX_SIZE. We also need to steer the robot to keep the ball in the center of the screen. This involves rotating the robot from right to left and moving the camera from up to down.

To create these movement commands we'll add the SSC module (the BucketBot uses electronic speed controls compatible with PWM signals created by the SSC) to move the robot. Note that the SSC module communicates to the SSC board via serial commands.

The SSC module will send two variables to the SSC board that range from 0 to 255 with 0 meaning reverse, 128 neutral and 255 full forward. The BucketBot has two motors that control the left and right wheels independently. (This is nice since it can rotate in one spot.) There is one more servo that controls the tilt of the camera that also ranges from 0 to 255 with 220 meaning the camera is pointed about straight. Our next task is to translate the COG_BOX_SIZE variable into motor commands from 0 to 255 for each motor and for the tilt servo.

Doing this translation will require a bit of logic. So we add in the VBScript Program module into the current processing pipeline. The VBScript module allows us to insert this program

  ' get the current center of the image
  midx = GetVariable("IMAGE_WIDTH") / 2
  midy = GetVariable("IMAGE_HEIGHT") / 2

  ' set the movement neutral range, i.e. if the
  ' cog is with range of the center of the image
  ' then consider it ok and move straigh. This
  ' helps to prevent fine tune movements that
  ' can cause oscillations.
  range = 5

  ' determine the factor that scales image width
  ' to a range of 0 to 128 .. increase beyond 128
  ' if your robot turns too quickly
  horizFactor = 200 / midx

  ' vertical factor ... we don't have much
  ' range here so the number is larger than
  ' horizontal movement
  vertFactor = 300 / midy

  ' ball size when it is too close
  ballSize = 40

  ' initialize our start motor values to neutral
  left_base = 128
  right_base = 128

  ' get the size (width or height) of the current
  ' bounding box
  size = GetVariable("COG_BOX_SIZE")

  ' if it is equal to "" then no object was detected
  if size > 10 then

    ' get the horizontal center of gravity found by the COG
    ' module
    cogX = GetVariable("COG_X")
    ' if it is less than 75 the blob is on the left side
    ' of the screen so move that way
    if cogX < (midx - range) then
      left_base = 128-(midx-cogX)/horizFactor
      right_base = 128
    ' otherwise move to the right if above 85 pixels (and no
    ' movement if 75 < cogX < 85 )
    elseif cogX > (midx + range) then
      left_base = 128
      right_base = 128-(cogX-midx)/horizFactor
    end if

    ' if the ball is too close then we have to move backwards
    if size > ballSize then
      left_motor = left_base+((size-ballSize)*2)
      right_motor = right_base+((size-ballSize)*2)
    ' otherwise move forward
    else
      left_motor = left_base-((ballSize-size)*2)
      right_motor = right_base-((ballSize-size)*2)
    end if

    ' set the final motor values to be picked up by
    ' the SSC module
    SetVariable "LEFT_MOTOR", CInt(left_motor)
    SetVariable "RIGHT_MOTOR", Cint(right_motor)

    ' now lets work on the tilt ... grab the current
    ' tilt value
    tilt = GetVariable("TILT")
    ' if it was not set then default to 220 (in our case
    ' this is horizontal to the floor)
    if tilt = "" then
        tilt = 220
    end if

    ' if the blob is below 55 then tilt down up a bit more
    ' otherwise tilt up a little more
    cogY = GetVariable("COG_Y")
    if cogY < midy - range then
        tilt = tilt - (midy-cogY)/vertFactor
    elseif cogY > midy + range then
        tilt = tilt + (cogY-midy)/vertFactor
    end if

    ' we don't want to look up more than horizontal to the
    ' floor as we do not expect the
    ' ball to be on the ceiling
    if tilt > 220 then
      tilt = 220
    end if
    if tilt < 0 then
      tilt = 0
    end if

    ' and set that value for the SSC module too
    SetVariable "TILT", tilt

  else

    SetVariable "LEFT_MOTOR", 128
    SetVariable "RIGHT_MOTOR", 128

  end if

The program first grabs our size variable COG_BOX_SIZE and compares it with what size we'd like to maintain in view. If the object is too big we move backwards by a relative amount or if the object is smaller than we'd like we move forwards. However, we also need to change the relative amounts for the right and left motors to ensure that we can rotate towards the ball as appropriately.

The LEFT_MOTOR and RIGHT_MOTOR variables that are set are then mapped to the left and right motors using the SSC module. Likewise, the TILT variable is mapped to the camera tilt servo. Note that they all range from 0 to 255. If you change the above equation to be more aggressive you need to ensure that the values stay within the 0 to 255 range.