试图在kivy中绑定我的arduino串行监视器输出和我的小部件动画

Question

我正在尝试让我的arduino串行监控器（图1）与速度计上的指针通信。例如，当数据为28.1时，指针将指向速度计中的28.1范围。现在，针刚好从左向右连续走。 Speed_text是包含arduino数据的变量。但是，我不将root.speed_text放在py文件中。

arduino数据

车速表

.py文件

import threading   
import serial
from kivy.animation import Animation
from kivy.clock import Clock, mainthread
from kivy.lang import Builder
from kivy.properties import StringProperty
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.app import App

class SpeedWindow(Screen):
    speed_text = StringProperty("")

    def __init__(self, **kwargs):
        super(SpeedWindow, self).__init__(**kwargs)
        threading.Thread(target=self.tm).start()

    def tm(self):
        while (1):
            with serial.Serial('COM4', 9600) as ser:
                value = ser.readline()
                ser.close()
                self.update_fix(value.decode('utf-8'))


    @mainthread
    def update_fix(self, value):
        self.speed_text = value

class WindowManager(ScreenManager):
    pass

KV = """

WindowManager:
    SpeedWindow:

<SpeedWindow>:
    angle: 90    
    name: 'speedometer'   

    Image:
        source: 'cadran.png'
        size_hint: None, None
        size: 400, 400
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}

    Image:
        source: 'needle.png'
        size_hint: None, None
        size: 170, 170
        pos_hint: {'center_x': 0.5, 'center_y': 0.5}   
        # apply rotation matrix to this Image                                                                                                                                            
        canvas.before:                                                                                                                                             
            PushMatrix                                                                                                                                             
            Rotate:                                                                                                                                                
                angle: root.angle                                                                                                                                  
                axis: (0, 0, 1)                                                                                                                                    
                origin: (self.center_x, self.center_y , 0)                                                                                                                              
        canvas.after:                                                                                                                                              
            PopMatrix                           
"""

class MyMainApp(App):
    def build(self):
        Clock.schedule_once(self.animate_needle, 1.0)  # start the animation in 2 seconds
        return  Builder.load_string(KV)

    def animate_needle(self, dt):
        # animate the needle from +90 to -90 and then back to +90
        self.anim = Animation(angle=-90.0) + Animation(angle=90)
        self.anim.repeat = True    # repeat forever
        speedometer = self.root.get_screen('speedometer')
        self.anim.start(speedometer)

if __name__ == "__main__":
    MyMainApp().run()