而且看视频教程在给云台上电后没有发生机位重启的现象，我上完电后云台总是回归到固定机位.这又是怎么回事呢
mport sensor, image, time

from pid import PID
from pyb import Servo

pan_servo=Servo(1)
tilt_servo=Servo(2)

pan_servo.calibration(500,2500,500)
tilt_servo.calibration(500,2500,500)

#red_threshold = (13, 49, 18, 61, 6, 47)

pan_pid = PID(p=0.07, i=0, imax=90) #脱机运行或者禁用图像传输，使用这个PID
tilt_pid = PID(p=0.05, i=0, imax=90) #脱机运行或者禁用图像传输，使用这个PID
#pan_pid = PID(p=0.1, i=0, imax=90)#在线调试使用这个PID
#tilt_pid = PID(p=0.1, i=0, imax=90)#在线调试使用这个PID

sensor.reset() # Initialize the camera sensor.
sensor.set_contrast(3)
sensor.set_gainceiling(16)
sensor.set_pixformat(sensor.GRAYSCALE) # use RGB565.
sensor.set_vflip(True)
sensor.set_framesize(sensor.QQVGA) # use QQVGA for speed.
sensor.skip_frames(10) # Let new settings take affect.
sensor.set_auto_whitebal(False) # turn this off.
clock = time.clock() # Tracks FPS.

face_cascade = image.HaarCascade("frontalface", stages=25)

def find_max(blobs):
max_size=0
for blob in blobs:
if blob[2]*blob[3] > max_size:
max_blob=blob
max_size = blob[2]*blob[3]
return max_blob

while(True):
clock.tick() # Track elapsed milliseconds between snapshots().
img = sensor.snapshot() # Take a picture and return the image.

blobs = img.find_features(face_cascade, threshold=0.75, scale=1.35)
if blobs:
    max_blob = find_max(blobs)
    pan_error = max_blob[0]+max_blob[2]/2-img.width()/2
    tilt_error = max_blob[1]+max_blob[3]/2-img.height()/2

    print("pan_error: ", pan_error)

    img.draw_rectangle(max_blob) # rect
    img.draw_cross(int(max_blob[0]+max_blob[2]/2), int(max_blob[1]+max_blob[3]/2)) # cx, cy
    pan_output=pan_pid.get_pid(pan_error,1)/2
    tilt_output=tilt_pid.get_pid(tilt_error,1)
    print("pan_output",pan_output)
    pan_servo.angle(pan_servo.angle()+pan_output)
    tilt_servo.angle(tilt_servo.angle()-tilt_output)