I do not own a meter but I will see if I can get my hands on one this week! Here is the code:

```
# Color Line Following Example with PID Steering
#
# For this script to work properly you should point the camera at a line at a
# 45 or so degree angle. Please make sure that only the line is within the
# camera's field of view.
import sensor, image, pyb, math, time
from pyb import LED
from pyb import Pin, Timer
tim = Timer(4, freq=1000) # Frequency in Hz
cruise_speed = 60 # how fast should the car drive, range from 0 to 100
steering_direction = -1 # use this to revers the steering if your car goes in the wrong direction
steering_gain = 1.7 # calibration for your car's steering sensitivity
steering_center = 60 # set to your car servo's center point
kp = 0.8 # P term of the PID
ki = 0.0 # I term of the PID
kd = 0.4 # D term of the PID
# Color Tracking Thresholds (L Min, L Max, A Min, A Max, B Min, B Max)
# The below thresholds track in general red/green things. You may wish to tune them...
# old thresholds = [(30, 100, 15, 127, 15, 127), # generic_red_thresholds
# (30, 100, -64, -8, -32, 32), # generic_green_thresholds
# (0, 15, 0, 40, -80, -20)] # generic_blue_thresholds
threshold_index = 2
# 0 for red, 1 for green, 2 for blue
thresholds = [(0, 100, -1, 127, -25, 127), # generic_red_thresholds
(0, 100, -87, 18, -128, 33), # generic_green_thresholds
(0, 100, -128, -10, -128, 51)] # generic_blue_thresholds
# You may pass up to 16 thresholds above. However, it's not really possible to segment any
# scene with 16 thresholds before color thresholds start to overlap heavily.
# Each roi is (x, y, w, h). The line detection algorithm will try to find the
# centroid of the largest blob in each roi. The x position of the centroids
# will then be averaged with different weights where the most weight is assigned
# to the roi near the bottom of the image and less to the next roi and so on.
ROIS = [ # [ROI, weight]
(38,1,90,38, 0.4),
(35,40,109,43,0.2),
(0,79,160,41,0.6)
]
blue_led = LED(3)
old_error = 0
measured_angle = 0
set_angle = 90 # this is the desired steering angle (straight ahead)
p_term = 0
i_term = 0
d_term = 0
old_time = pyb.millis()
radians_degrees = 57.3 # constant to convert from radians to degrees
def constrain(value, min, max):
if value < min :
return min
if value > max :
return max
else:
return value
ch1 = tim.channel(1, Timer.PWM, pin=Pin("P7"), pulse_width_percent=0)
ch2 = tim.channel(2, Timer.PWM, pin=Pin("P8"), pulse_width_percent=0)
def steer(angle):
global steering_gain, cruise_speed, steering_center
angle = int(round((angle+steering_center)*steering_gain))
angle = constrain(angle, 0, 180)
angle = 90 - angle
angle = radians_degrees * math.tan(angle/radians_degrees) # take the tangent to create a non-linear response curver
left = (90 - angle) * (cruise_speed/100)
left = constrain (left, 0, 100)
right = (90 + angle) * (cruise_speed/100)
right = constrain (right, 0, 100)
print ("left: ", left)
print ("right: ", right)
# Generate a 1KHz square wave on TIM4 with each channel
ch1.pulse_width_percent(left)
ch2.pulse_width_percent(right)
def update_pid():
global old_time, old_error, measured_angle, set_angle
global p_term, i_term, d_term
now = pyb.millis()
dt = now - old_time
error = set_angle - measured_angle
de = error - old_error
p_term = kp * error
i_term += ki * error
i_term = constrain(i_term, 0, 100)
d_term = (de / dt) * kd
old_error = error
output = steering_direction * (p_term + i_term + d_term)
output = constrain(output, -50, 50)
return output
# Compute the weight divisor (we're computing this so you don't have to make weights add to 1).
weight_sum = 0
for r in ROIS: weight_sum += r[4] # r[4] is the roi weight.
# Camera setup...
clock = time.clock() # Tracks FPS.
sensor.reset() # Initialize the camera sensor.
sensor.__write_reg(0x6B, 0x22) # switches camera into advanced calibration mode. See this for more: http://forums.openmv.io/viewtopic.php?p=1358#p1358
sensor.set_pixformat(sensor.RGB565)
sensor.set_framesize(sensor.QQVGA) # use QQVGA for speed.
sensor.set_vflip(True)
sensor.set_hmirror(True)
sensor.set_auto_gain(True) # do some calibration at the start
sensor.set_auto_whitebal(True)
sensor.skip_frames(time = 4000) # When you're inside, set time to 2000 to do a white balance calibration. Outside, this can be 0
sensor.set_auto_gain(False) # now turn off autocalibration before we start color tracking
sensor.set_auto_whitebal(False)
while(True):
clock.tick() # Track elapsed milliseconds between snapshots().
img = sensor.snapshot().histeq() # Take a picture and return the image. The "histeq()" function does a histogram equalization to compensate for lighting changes
print("FPS: ",clock.fps())
centroid_sum = 0
for r in ROIS:
blobs = img.find_blobs([thresholds[threshold_index]], roi=r[0:4], merge=True) # r[0:4] is roi tuple.
if blobs:
# Find the index of the blob with the most pixels.
most_pixels = 0
largest_blob = 0
for i in range(len(blobs)):
if blobs[i].pixels() > most_pixels:
most_pixels = blobs[i].pixels()
largest_blob = i
# Draw a rect around the blob.
img.draw_rectangle(blobs[largest_blob].rect())
img.draw_cross(blobs[largest_blob].cx(),
blobs[largest_blob].cy())
centroid_sum += blobs[largest_blob].cx() * r[4] # r[4] is the roi weight.
center_pos = (centroid_sum / weight_sum) # Determine center of line.
# Convert the center_pos to a deflection angle. We're using a non-linear
# operation so that the response gets stronger the farther off the line we
# are. Non-linear operations are good to use on the output of algorithms
# like this to cause a response "trigger".
deflection_angle = 0
# The 80 is from half the X res, the 60 is from half the Y res. The
# equation below is just computing the angle of a triangle where the
# opposite side of the triangle is the deviation of the center position
# from the center and the adjacent side is half the Y res. This limits
# the angle output to around -45 to 45. (It's not quite -45 and 45).
deflection_angle = -math.atan((center_pos-80)/60)
# Convert angle in radians to degrees.
deflection_angle = math.degrees(deflection_angle)
# Now you have an angle telling you how much to turn the robot by which
# incorporates the part of the line nearest to the robot and parts of
# the line farther away from the robot for a better prediction.
# print("Turn Angle: %f" % deflection_angle)
now = pyb.millis()
if now > old_time + 0.02 : # time has passed since last measurement; do the PID at 50hz
blue_led.on()
measured_angle = deflection_angle + 90
steer_angle = update_pid()
old_time = now
steer (steer_angle)
# print(str(measured_angle) + ', ' + str(set_angle) + ', ' + str(steer_angle))
blue_led.off()
```