Support controlling servo motors about pingo-py

[Vido]

@felipesanches,

As we talked on Garoa,
We have plans to support PPM signals and Servos. But we are not sure, how this could be done.
To make a Servo work on each particular board is easy. However, to write a one-size-fits-all code is not so easy.

We could consider two options:

• A Pin can write and read a PPM signal. We code have just write for now, and think about read later.
• A Servo (from pingo.parts) uses a PWM pin to send a PPM signal.

There is almost a discusion on how to implement more Pin's functions:
https://groups.google.com/d/msgid/pingo-io/5a51d463-b853-4dfc-8da6-ffb93dd09577%40googlegroups.com?utm_medium=email&utm_source=footer

Maybe @ramalho could dispense some wisdom on this issue.

That's all folks.

[Vido]

After some googling:

Board	Servo support Status
Arduino	Firmata Prototocol suports Servos, so does PyMata
Raspberry	RPI.GPIO does not support Servos. It can be implemented via software with PWM http://razzpisampler.oreilly.com/ch05.html . Occidentalis v0.2 distribution support on GPIO#18
pcDuino	Support it via Arduino IDE or via pwm-sunxi Kernel Module with some PWM hack.
BeagleBone Black	Adafruit_BBIO and some PWM hack. https://learn.adafruit.com/controlling-a-servo-with-a-beaglebone-black/writing-a-program
Galileo	Via Arduino IDE, or PWM hack https://github.com/mikalhart/galileo-Servo

There is a recipe to get a Servo working on a PWM signal.

• Set your PWM frequency to 50Hz (to match the 20ms Servo's period)
• A duty-cycle of 2.5% represents 0.5ms, which is 0º for a Servo.
• A duty-cycle of 12.5% represents 2.5ms, which is 180º for a Servo.

We need to test it.

[ramalho]

Good research, thanks @Vido!

On Wed, Nov 26, 2014 at 4:37 PM, Lucas Vido [email protected]
wrote:

After some googling:
Board Servo support Status Arduino Firmata Prototocol suports Servos,
so does PyMata Raspberry RPI.GPIO does not support Servos. It can be
implemented via software with PWM
http://razzpisampler.oreilly.com/ch05.html . Occidentalis v0.2
distribution support on GPIO#18 pcDuino Support it via Arduino IDE or
via pwm-sunxi Kernel Module with some PWM hack. BeagleBone Black Adafruit_BBIO
and some PWM hack.
https://learn.adafruit.com/controlling-a-servo-with-a-beaglebone-black/writing-a-program
Galileo Via Arduino IDE, or PWM hack
https://github.com/mikalhart/galileo-Servo

There is a recipe to get a Servo working on a PWM signal.

• Set your PWM frequency to 50Hz (to match the 20ms Servo's period)
• A duty-cycle of 2.5% represents 0.5ms, which is 0º for a Servo.
• A duty-cycle of 12.5% represents 2.5ms, which is 180º for a Servo.

We need to test it.

—
Reply to this email directly or view it on GitHub
https://github.com/garoa/pingo/issues/53#issuecomment-64691479.

Luciano Ramalho
Twitter: @ramalhoorg

Professor em: http://python.pro.br
Twitter: @pythonprobr

[Vido]

• Erin (@Lana-chan) and Gabriel asked about this.
• Marcos from Telefonica asked too.
• Oda wants to build a PayPal paid foosball

So folks this feature is top priority.

[Vido]

Last Sunday, I was working on the "Nativity scene" software.
And I used a I2C PWM driver http://www.adafruit.com/datasheets/PCA9685.pdf
Although it is a "LED controller optimized for LCD Red/Green/Blue/Amber (RGBA) color backlighting applications", you can drive servos.

It provides a 12bits (4096 states) duty cycle. Here is the math:

At 50Hz, we have a 20ms period.

0º is 0.5ms -> 20ms/0.5ms = 40
4096 / 40 = 102.4 ~ 103

90º is 1.5ms -> 20ms/1.5ms = 13.3
4096 / 13.3 = 307.9 ~ 308

180º is 2.5ms -> 20ms/2.5ms = 8
4096 / 8 = 512

Yes, this trick works. Any board with a setable PWM frequency, with at least 40 states, will work

So far:

• There are some libraries that support servos: eg. PyMata (https://github.com/MrYsLab/PyMata/blob/master/examples/servo/servo_demo.py)
• Most of Pingo's drivers have no support.

Should Servo be a pingo.parts or should it be a Board.mode?
Well, if it's implementation hangs upon each Board, we can't have it as pure pingo.parts.
If a code is Board specific, than it should be within it's own Board driver.
From this point of view, it makes sense to have a SERVO mode.