tilt_servo.angle(tilt_servo.angle()-tilt_output) 和这个·类似 都是舵机控制的 那tilt_servo.angle为什么又嵌套在tilt_servo.angle(tilt_servo.angle()-tilt_output) 这里面？

	pan_servo.angle(pan_servo.angle()+pan_output) 这个函数pan_servo.angle()什么意思？  为什么嵌套了？

import sensor, image, time

from pid import PID
from pyb import Servo

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

red_threshold = (0, 49, 13, 56, 4, 40)

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_pixformat(sensor.RGB565) # use RGB565.
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.

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_blobs([red_threshold])
if blobs:
    max_blob = find_max(blobs)
    pan_error = max_blob.cx()-img.width()/2
  #  tilt_error = max_blob.cy()-img.height()/2

 #   print("pan_error: ", pan_error)

    img.draw_rectangle(max_blob.rect()) # rect
    img.draw_cross(max_blob.cx(), max_blob.cy()) # cx, cy

    pan_output=pan_pid.get_pid(pan_error,1)/2
   # tilt_output=tilt_pid.get_pid(tilt_error,1)
    print("pan_error: ", pan_output)
    pan_servo.angle(45+25*pan_output)  
 # print("pan_error: ", pan_output)
 #   tilt_servo.angle(tilt_servo.angle()-tilt_output)