micropython-servo-pdm-360 0.0.3

This is a micropython library for control continuous servo by PDM (PWM).

This is an automatic translation, may be incorrect in some places.

Servo PDM Continuous

A library for controlling continuous rotation servo drives via the PWM interface of the Raspberry Pi Pico microcontroller in MicroPython.

Library features:

• Rotation with a given force and direction
• Rotation within the specified time
• Possibility of soft start and stop
• Ability to use the smooth-servo library to change the smooth start and stop algorithm
• All pending work happens in the background, with two processing options:
  • Using an asyncio task (recommended)
  • Interrupt timer

If you don't need all the functionality listed above, then you should take a look at the micropython-servo library. It is much smaller and better suited for simple tasks.

The following materials were used in the development of the library:

• Material Principle of operation of constant rotation servos, author amperka.ru
• Material Servo Motor with Raspberry Pi Pico using MicroPython by microcontrollerslab.com
• Material Hobby Servo Tutorial by MIKEGRUSIN and BYRON J. (sparkfun.com)

Compatibility

• MicroPython 1.19.1
• Raspberry Pi Pico

On the hardware above the library has been tested and works correctly.

ATTENTION

You use this module at your own risk. I am new to MicroPython programming. So there may be nuances that I did not take into account. If you notice an error or have suggestions for improvement, please write to Issues.

Content

• Installation
• Initialization
• Documentation
• Examples
• Bugs and feedback

Installation

• Install the library via pip (Thonny -> Manage Packages) by name micropython-servo-pdm-360
• Or manual installation:
  • Download library from GitHub
  • take the micropython_servo_pdm_360 folder from the archive.
  • upload to the root of the microcontroller or to the lib folder.

If you want to play around with the logic of the library, then the 2nd installation option is preferable. :)

Initialization

Initializing the base library

from machine import Pin, PWM
from micropython_servo_pdm_360 import ServoPDM360

# create a PWM servo controller (21 - pin Pico)
servo_pwm = PWM(Pin(21))

# Set the parameters of the servo pulses, more details in the "Documentation" section
freq = 50
min_us = 400
max_us = 2550
dead_zone_us = 150

# create a servo object
servo = ServoPDM360(pwm=servo_pwm, min_us=min_us, max_us=max_us, dead_zone_us=dead_zone_us, freq=freq)

After that, basic methods of controlling the servo will be available to you, which do not require pending tasks.

To access additional methods that require deferred execution, you need to initialize one of the child classes. Depending on which of the ways you prefer to handle pending tasks:

Using the uasyncio library

This is the best option for most projects.

from machine import Pin, PWM
from micropython_servo_pdm_360 import ServoPDM360RP2Async

# create a PWM servo controller (21 - pin Pico)
servo_pwm = PWM(Pin(21))

# Set the parameters of the servo pulses, more details in the "Documentation" section
freq = 50
min_us = 1350
max_us = 8450
dead_zone_us = 300

# create a servo object
servo = ServoPDM360RP2Async(pwm=servo_pwm, min_us=min_us, max_us=max_us, dead_zone_us=dead_zone_us, freq=freq)

Using timer interrupts

You can read more about timers here For Raspberry Pi Pico here Be careful, although this is the easiest option, it is not optimal. Since the handling of servo events occurs in a timer interrupt, other interrupts will be delayed.

from machine import Pin, PWM
from micropython_servo_pdm_360 import ServoPDM360RP2Irq

# create a PWM servo controller (21 - pin Pico)
servo_pwm = PWM(Pin(21))

# Set the parameters of the servo pulses, more details in the "Documentation" section
freq = 50
min_us = 400
max_us = 2550
dead_zone_us = 150

# create a servo object
servo = ServoPDM360RP2Irq(pwm=servo_pwm, min_us=min_us, max_us=max_us, dead_zone_us=dead_zone_us, freq=freq)

Documentation

A little about PDM

PDM(pulse-duration modulation) is a process of power control by pulsing the power consumer on and off. By Wikipedia® In our case, it is used to control the servo. According to the pulse time, you can set the force and direction of rotation of the servo. ATTENTION: Unlike PWM, the control is not based on frequency, but on the duration of the pulse. You can read more here (with pictures): wiki.amperka.ru

For the correct operation of the servo, we need to set the following parameters:

• freq - pulse frequency, for analog servos 50 Hz. For digital 300 Hz or more.
• min_us - minimum pulse time at which the servo begins to rotate in one direction.
• max_us - maximum pulse time at which the servo stops rotating in the other direction.
• dead_zone_us - duration of the central zone in which the servo does not rotate. This is the transition zone between the two directions of rotation.

Below I tried to graphically represent these parameters for the MG995_360 analog servo:

As we can see in the figure, the servo begins to rotate at a pulse width of 400 µs, and stops rotating at 2600 µs. Thus, we get the parameters min_us=400 and max_us=2550 (2600 - 50, since at 2600 the drive has already stopped). Now we need to find the duration of the central zone. The figure shows that at a pulse length of 1450 µs, the servo stops rotating clockwise and stops. Further, at a pulse length of 1600 μs, the rotation in the opposite direction begins. Thus the parameter dead_zone_us=150 (1600-1450).

Where can I get these parameters for a specific servo? It all depends on the manufacturer. But in most cases they are specified in the documentation. If not, we pick them up manually using the set_duty method and giving different values from 0 to 3000 µs, in 50 µs increments. An example of such a configuration can be found in the examples folder file manual_config.py.

List of parameters for servos:

• MG995_360 - min_us=400, max_us=2550, dead_zone_us=150, freq=50

PLEASE: If you find parameters for a servo that are not listed, submit them to me via issue.

ServoPDM360 constructor parameters

ServoPDM360RP2Async and ServoPDM360RP2Irq inherit it and have the same parameters

Parameter	Type	Default	Description
pwm	PWM	None	PWM controller
min_us	int	500	Minimum impulse time
max_us	int	3000	Maximum pulse time
dead_zone_us	int	150	Duration of the central zone
freq	int	50	Pulse frequency
invert	bool	false	Direction inversion

• pwm - PWM controller object.
• min_us - Minimum pulse time (duty cycle) More.
• max_us - Maximum pulse time (duty cycle) More
• dead_zone_us - Central zone duration More
• freq - Pulse frequency, for analog drives it is 50. Digital drives are usually 300 or more.
• invert - Invert direction (maybe needed for some drives, maybe not :) )

ServoPDM360 base class methods

• set_duty(duty: int) - Sets an arbitrary value of the duty cycle in the range from 0 to 65000. This method is intended for manual search of the minimum and maximum duty cycle values. More
• turn_cv(force: int) - Starts rotation of the drive clockwise with the given acceleration.
• turn_ccv(force: int) - Starts the drive turning counterclockwise at the given acceleration.
• stop() - Stops the rotation of the drive.
• deinit() - Disables the PWM generator.

ServoPDM360RP2Async and ServoPDM360RP2Irq class methods

• turn_cv_ms(...) - Rotates the drive clockwise with various parameters.
• turn_ccv_ms(...) - Rotates the drive counterclockwise with various parameters.

Parameter	Type	Default	Description
time_ms	int	0	Rotation Time (0 - Infinite)
force	int	None	Effort, if not specified, is taken from start_smoothing
start_smoothing	ServoSmoothBase	None	Start deceleration, if the start is not specified, it will be instantaneous
end_smoothing	ServoSmoothBase	None	Decelerate stop, if not specified stop will be instant
callback	callable	None	The function that will be called after the end of the command. If time_ms == 0 then after overclocking

• smooth - Smooth stop of the drive.

Parameter	Type	Default	Description
end_smoothing	ServoSmoothBase	None	Stop slowdown
callback	callable	None	The function that will be called after the end of the command.

Slowdowns

To control slowdowns, you can use the ServoSmoothBase classes and its descendants. This library only has SmoothLinear linear deceleration, if you need more, install the smooth-servo library. An example of using built-in easing:

from machine import Pin, PWM
from micropython_servo_pdm_360 import ServoPDM360RP2Async, SmoothLinear

# create a PWM servo controller (21 - pin Pico)
servo_pwm = PWM(Pin(21))

# Set the parameters of the servo pulses, more details in the "Documentation" section
freq = 50
min_us = 400
max_us = 2550
dead_zone_us = 150

# create a servo object
servo = ServoPDM360RP2Async(pwm=servo_pwm, min_us=min_us, max_us=max_us, dead_zone_us=dead_zone_us, freq=freq)

# Rotate the actuator clockwise for two seconds with a force of 50 and decelerate at the beginning. After output to the console "callback cv"
servo.turn_cv_ms(2000, 50, start_smoothing=SmoothLinear(50, 1000),  callback=lambda: print("callback cv"))

Details about the parameters and types of slowdowns can be found in the smooth_servo documentation.

Examples

Usage examples can be found in the examples folder.

Bugs and feedback

If you find bugs, create issue The library is open for further development and your pull requests!