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This article shows you how to use raspberry pie to control the steering gear MG90D, the content is concise and easy to understand, can definitely brighten your eyes, through the detailed introduction of this article, I hope you can get something.

Introduction of steering gear

The steering gear is a position (angle) servo drive, which is suitable for control systems that need constant angle change and can be maintained. At present, it has been widely used in high-grade remote control toys, such as aircraft, submarine models and remote control robots. The MG90 steering gear we use here is common on Taobao. As shown in the following figure, it should be noted that different models have different steering angles and force distances, which can be selected according to their own needs.

working principle

The control signal of the steering gear enters the signal modulation chip from the channel of the receiver to obtain the DC bias voltage. There is a reference circuit inside it, which generates a reference signal with a period of 20ms and a width of 1.5ms. The DC bias voltage obtained is compared with the voltage of the potentiometer to obtain the voltage difference output. Finally, the positive and negative output of the voltage difference to the motor driver chip determines the positive and negative rotation of the motor. When the speed of the motor is constant, the potentiometer is rotated by cascade reduction gears, so that the voltage difference is 0 and the motor stops rotating.

The control of    steering gear generally needs a time base pulse of about 20ms. The high level part of this pulse is generally the angle control pulse part within the range of 0.5ms-2.5ms, and the total interval is 2ms. Take 180-degree angle servo as an example, then the corresponding control relationship is as follows:

Table of the relationship between pulse width, duty cycle and steering gear angle:

Pulse / ms duty cycle angle 0.52.5% 01.05% 451.57.5% 902.010% 1352.512.5% 180 data reckoning

Based on the above table, we need to calculate some key parameters.

According to the above table, if we want to turn the steering gear to the specified angle θ (0 ≤ θ ≤ 180) θ (0 ≤ θ ≤ 180), the pulse duty cycle that needs to be input is:

Dao 2.5 + (θ × (12.5 − 2.5)) / 180 × 2.5 + (θ × 10) / 180

It takes some time for the steering gear to rotate, and it should be given instructions to avoid conflict. The speed of the steering gear is about 0.2s every 60 degrees, or 0.003s/ °. The accuracy of the steering gear is 1024 ≈ 0.18 °, and the corresponding pulse duty cycle accuracy is 12.5 − 2.5 / 1024 ≈ 0.01.Therefore, in the step rotation (i.e. 0.18 °each turn, and then directly take 0.2 °), the instruction interval time sent to the steering gear should not be less than 0.2x0.003mm 0.0006s, rounded up that is 0.001s. If we specify any corner, then the interval between the two instructions should be longer than 180 × 0.260 degrees 0.4 s, which is the time from 0 degrees to 180 degrees.

Steering gear wiring

Connect the steering gear brown wire to the GND, the red wire to the + 5V pin, and the orange wire to the GPIO.12 (BCM18) pin.

Programming

Rotation.py

Import RPi.GPIO as GPIOimport time# this class represents a single SG90 module class Rotation: frequency=50 # Pulse Frequency (Hz) delta_theta=0.2 # step rotation interval (degree) min_delay=0.0006 # theoretical time spent turning delta_theta (s) max_delay=0.4 # time spent from 0 to 180 def _ _ init__ (self,channel,min_theta,max_theta Init_theta=0):''constructor: channel: raspberry pie pin number connected to the steering gear signal line (BCM code) min_theta: minimum angle of steering gear rotation max_theta: maximum angle of steering gear rotation init_theta: initial angle of steering gear'' Self.channel=channel if (min_theta180): self.min_theta=0 else: self.min_theta=min_theta if (max_theta180): self.max_theta=180 else: self.max_theta=max_theta if (init_thetamax_theta): self.init_theta= (self.min_theta+self.max_ Theta) / 2 else: self.init_theta=init_theta # initial angle # calculate the minimum angle, Duty cycle of the maximum angle self.min_dutycycle=2.5+self.min_theta*10/180 self.max_dutycycle=2.5+self.max_theta*10/180 def setup (self): 'initialize' GPIO.setmode (GPIO.BCM) GPIO.setwarnings (False) GPIO.setup (self.channel GPIO.OUT) self.pwm=GPIO.PWM (self.channel,Rotation.frequency) # PWM self.dutycycle=2.5+self.init_theta*10/180 # initial value of pulse duty cycle self.pwm.start (self.dutycycle) # turns the steering gear to the initial position time.sleep (Rotation.max_delay) def positiveRotation (self):''positive stepping rotation Each call only rotates the delta_ self.dutycycle=self.dutycycle+Rotation.delta_theta*10/180 if self.dutycycle > self.max_dutycycle: self.dutycycle=self.max_dutycycle self.pwm.ChangeDutyCycle (self.dutycycle) time.sleep (Rotation.min_delay) def reverseRotation (self):''reverse rotation Each call only rotates the delta_ Theta degree''self.dutycycle=self.dutycycle-Rotation.delta_theta*10/180 if self.dutycycle.

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