K210上用MaixPy写个简单的云台程序: 高性价比的人脸跟随云台

效果视频

硬件

• 可以运行 MaixPy 的 k210 开发板（此处用了maixbit 开发板)
• 屏幕，是买 maixbit 套餐里面带的，屏幕排线容易折断，要小心使用
• 两轴舵机，扭力尽量大一点，会更稳，上面视频使用的 20KG 扭力的舵机， 参数：
  • 频率: 50 (Hz)
  • 周期 T: 1/50 = 0.02s = 20ms
  • 占空比 duty: [0.5ms, 2.5ms] -> [0.025, 0.125] -> [2.5%, 12.5%]
• 硬件连接
  • IO24 <—> pitch（舵机俯仰轴）
  • IO25 <—> roll （舵机横滚轴，这里实际是控制的航向轴)

原理

1. 使用人脸检测模型来检测人脸，获取到人脸的中心坐标。
2. 将当前的坐标与中心点坐标进行对比，获得当前与目标的差值。
3. 根据这个与目标的差值，k210 驱动舵机给舵机一定的驱动量，使其将摄像头转动，达到中心对准人头。

这里的难点在第三步，根据与目标位置的差值如何驱动舵机转动使摄像头中心对准人头：

• 如果没有任何控制理论的同学会想，用判断语句，一点一点移动就好了。但这样看起来舵机非常僵硬，一卡一卡的。
• 稍微有点控制理论基础的应该都知道，这种情况使用 经典 PID 控制即可，简单地说就是根据误差值，通过一个带有三个参数（P I D）的公式得到一个输出值，将这个值作为增量输出到舵机控制舵机转动，比如公式这样：
  
  这里的 Kp Tt Td 就是 PID 参数，看起来很复杂，实际可以先从简单的 只用 P 控制（其它两个值为0）的情况做起就比较简单，即输出增量= 和目标的差值 * Kp。I是积分即把每次的误差累积起来，对于小误差很有用，随着时间累积会放大误差作用，比如离目标差几个像素，但是输出量太小不能让舵机转动，随着时间累积，输出量会越来越大直到让舵机动起来；D是微分，可以防止过冲，这里是将本次的误差和上次的误差变化参与输出值的计算，来让转动更加平滑。
  这里只是大概解释一下，具体可以自行搜索 经典PID控制 学习，结合看代码就很好懂。

实现

• 先学会如何在 k210(maixbit) 上使用 maixpy 开发，可以看官方文档，或者是我的另一篇快速入门教程
• 使用人脸检测模型， 可以在模型库下载到
• 使用 kflash_gui 烧录模型到 0x300000 处
• 运行下面的代码即可

源代码

代码在这里: https://github.com/sipeed/MaixPy_scripts/tree/master/application/gimbal

‘’‘
    @author neucrack
    @license MIT
’‘’
import time, sys
from machine import Timer,PWM
from fpioa_manager import board_info
from math import pi
from machine import UART
class Servo:
    def __init__(self, pwm, dir=50, duty_min=2.5, duty_max=12.5):
        self.value = dir
        self.pwm = pwm
        self.duty_min = duty_min
        self.duty_max = duty_max
        self.duty_range = duty_max -duty_min
        self.enable(True)
        self.pwm.duty(self.value/100*self.duty_range+self.duty_min)
    def enable(self, en):
        if en:
            self.pwm.enable()
        else:
            self.pwm.disable()
    def dir(self, percentage):
        if percentage > 100:
            percentage = 100
        elif percentage < 0:
            percentage = 0
        self.pwm.duty(percentage/100*self.duty_range+self.duty_min)
    def drive(self, inc):
        self.value += inc
        if self.value > 100:
            self.value = 100
        elif self.value < 0:
            self.value = 0
        self.pwm.duty(self.value/100*self.duty_range+self.duty_min)
class PID:
    _kp = _ki = _kd = _integrator = _imax = 0
    _last_error = _last_t = 0
    _RC = 1/(2 * pi * 20)
    def __init__(self, p=0, i=0, d=0, imax=0):
        self._kp = float(p)
        self._ki = float(i)
        self._kd = float(d)
        self._imax = abs(imax)
        self._last_derivative = None
    def get_pid(self, error, scaler):
        tnow = time.ticks_ms()
        dt = tnow - self._last_t
        output = 0
        if self._last_t == 0 or dt > 1000:
            dt = 0
            self.reset_I()
        self._last_t = tnow
        delta_time = float(dt) / float(1000)
        output += error * self._kp
        if abs(self._kd) > 0 and dt > 0:
            if self._last_derivative == None:
                derivative = 0
                self._last_derivative = 0
            else:
                derivative = (error - self._last_error) / delta_time
            derivative = self._last_derivative + \
                                     ((delta_time / (self._RC + delta_time)) * \
                                        (derivative - self._last_derivative))
            self._last_error = error
            self._last_derivative = derivative
            output += self._kd * derivative
        output *= scaler
        if abs(self._ki) > 0 and dt > 0:
            self._integrator += (error * self._ki) * scaler * delta_time
            if self._integrator < -self._imax: self._integrator = -self._imax
            elif self._integrator > self._imax: self._integrator = self._imax
            output += self._integrator
        return output
    def reset_I(self):
        self._integrator = 0
        self._last_derivative = None
class Gimbal:
    def __init__(self, pitch, pid_pitch, roll=None, pid_roll=None, yaw=None, pid_yaw=None):
        self._pitch = pitch
        self._roll = roll
        self._yaw = yaw
        self._pid_pitch = pid_pitch
        self._pid_roll = pid_roll
        self._pid_yaw = pid_yaw
    def set_out(self, pitch, roll, yaw=None):
        pass
    def run(self, pitch_err, roll_err=50, yaw_err=50, pitch_reverse=False, roll_reverse=False, yaw_reverse=False):
        out = self._pid_pitch.get_pid(pitch_err, 1)
        # print("err: {}, out: {}".format(pitch_err, out))
        if pitch_reverse:
            out = - out
        self._pitch.drive(out)
        if self._roll:
            out = self._pid_roll.get_pid(roll_err, 1)
            if roll_reverse:
                out = - out
            self._roll.drive(out)
        if self._yaw:
            out = self._pid_yaw.get_pid(yaw_err, 1)
            if yaw_reverse:
                out = - out
            self._yaw.drive(out)
if __name__ == "__main__":
    '''
        servo:
            freq: 50 (Hz)
            T:    1/50 = 0.02s = 20ms
            duty: [0.5ms, 2.5ms] -> [0.025, 0.125] -> [2.5%, 12.5%]
        pin:
            IO24 <--> pitch
            IO25 <--> roll
    '''
    init_pitch = 80       # init position, value: [0, 100], means minimum angle to maxmum angle of servo
    init_roll = 50        # 50 means middle
    sensor_hmirror = False
    sensor_vflip = False
    lcd_rotation = 2
    lcd_mirror = True
    pitch_pid = [0.23, 0, 0.015, 0]  # P I D I_max
    roll_pid  = [0.23, 0, 0.015, 0]  # P I D I_max
    target_err_range = 10            # target error output range, default [0, 10]
    target_ignore_limit = 0.02       # when target error < target_err_range*target_ignore_limit , set target error to 0
    pitch_reverse = False # reverse out value direction
    roll_reverse = True   # ..
    import sensor,image,lcd
    import KPU as kpu
    class Target():
        def __init__(self, out_range=10, ignore_limit=0.02, hmirror=False, vflip=False, lcd_rotation=2, lcd_mirror=True):
            self.pitch = 0
            self.roll = 0
            self.out_range = out_range
            self.ignore = ignore_limit
            self.task_fd = kpu.load(0x300000) # face model addr in flash
            anchor = (1.889, 2.5245, 2.9465, 3.94056, 3.99987, 5.3658, 5.155437, 6.92275, 6.718375, 9.01025)
            kpu.init_yolo2(self.task_fd, 0.5, 0.3, 5, anchor)
            lcd.init()
            lcd.rotation(lcd_rotation)
            lcd.mirror(lcd_mirror)
            sensor.reset()
            sensor.set_pixformat(sensor.RGB565)
            sensor.set_framesize(sensor.QVGA)
            if hmirror:
                sensor.set_hmirror(1)
            if vflip:
                sensor.set_vflip(1)
        def get_target_err(self):
            img = sensor.snapshot()
            code = kpu.run_yolo2(self.task_fd, img)
            if code:
                max_area = 0
                max_i = 0
                for i, j in enumerate(code):
                    a = j.w()*j.h()
                    if a > max_area:
                        max_i = i
                        max_area = a
                img = img.draw_rectangle(code[max_i].rect())
                self.pitch = (code[max_i].y() + code[max_i].h() / 2)/240*self.out_range*2 - self.out_range
                self.roll = (code[max_i].x() + code[max_i].w() / 2)/320*self.out_range*2 - self.out_range
                # limit
                if abs(self.pitch) < self.out_range*self.ignore:
                    self.pitch = 0
                if abs(self.roll) < self.out_range*self.ignore:
                    self.roll = 0
                img = img.draw_cross(160, 120)
                lcd.display(img)
                return (self.pitch, self.roll)
            else:
                img = img.draw_cross(160, 120)
                lcd.display(img)
                return (0, 0)
    target = Target(target_err_range, target_ignore_limit, sensor_hmirror, sensor_vflip, lcd_rotation, lcd_mirror)
    tim0 = Timer(Timer.TIMER0, Timer.CHANNEL0, mode=Timer.MODE_PWM)
    tim1 = Timer(Timer.TIMER0, Timer.CHANNEL1, mode=Timer.MODE_PWM)
    pitch_pwm = PWM(tim0, freq=50, duty=0, pin=24)
    roll_pwm  = PWM(tim1, freq=50, duty=0, pin=25)
    pitch = Servo(pitch_pwm, dir=init_pitch)
    roll = Servo(roll_pwm, dir=init_roll)
    pid_pitch = PID(p=pitch_pid[0], i=pitch_pid[1], d=pitch_pid[2], imax=pitch_pid[3])
    pid_roll = PID(p=roll_pid[0], i=roll_pid[1], d=roll_pid[2], imax=roll_pid[3])
    gimbal = Gimbal(pitch, pid_pitch, roll, pid_roll)
    target_pitch = init_pitch
    target_roll = init_roll
    t = time.ticks_ms()
    _dir = 0
    t0 = time.ticks_ms()
    stdin = UART.repl_uart()
    while 1:
        # get target error
        err_pitch, err_roll = target.get_target_err()
        # interval limit to > 10ms
        if time.ticks_ms() - t0 < 10:
            continue
        t0 = time.ticks_ms()
        # run
        gimbal.run(err_pitch, err_roll, pitch_reverse = pitch_reverse, roll_reverse=roll_reverse)