control of 360 degree servos

Hi Bo,

A  servo capable of 360 degrees would be different from those "modified RC servos" that give continuous rotation. Are you planning on using this for a gymbal type mechanism to track?

If so, perhaps PWM is not the best way to do this, especially if you're going to the mechanical effort at producing a gymbal.

A better choice would be to use geared-down stepper motors for much higher resolution.

nope... it will be a simple up/down with one servo and left/right with another.... one limit will be that it can not go from 360/0 to 1... it will be told to go the other way round to not mess up wires

up/down will be limited to 90 degree up and no down...

and i will better stay on servos... cheaper... just need the big hobby servos... then a few bearings to take load off servos

Have you got a URL/datasheet to a 360 degree servo that you're planning to use? Modifying a 180 degree servo for continuous rotation is not the same thing as a servo capable of a controlled 360 degrees.

yes i have....

but the Q is not about the servos... its on how to control it.... ie python code

http://www.hobbyking.com/hobbyking/store/__18742__Vigor_VSD_11AYMB_MG_HV_Extra_Large_360_Degree_Winch_Servo_0_75sec_50kg_150g.html

Bo Herrmannsen wrote:

 

yes i have....

 

but the Q is not about the servos... its on how to control it.... ie python code

I understand. But you didn't supply enough information to give an answer. Hence the request for that URL.

You can use the same Python code as you would for a 180 degree servo. According to that URL, the pulse width needs to be between 800 and 2200usec for a rotation of 2160 degrees. So, the center position would be at 1500usec pulse width, and if you changed the pulse width by up to +- 116.6 usec then the servo would rotate by +-180 degrees, giving you a controlled rotation of 360 degrees.

The existing Python code most likely uses values of 1000us to 2000uec to control the servo, with the center being 1.5msec, which matches the calculations for your desired servo (1500usec).

So, you can continue to use the existing code, but don't instruct the servo to adjust more than +-116.6usec, whereas a normal 180 degree servo would be controlled by up to +-500usec to get the span from 1000usec to 2000usec.

So, the existing Python code/library will work. Just don't expect good granularity.

ok, what do you mean by excisting code? i have yet to find even that...

i dont want to rely on a librart

Without a library, you will need to spend some time  to write your own library for the processor that you've chosen.

Once you have decided on a processor, you'll need to research what PWM facilities exist for that processor (often there is an integrated peripheral inside the chip) and look up the register config (all this will be in the processor reference guide) and write your own driver/library that will control the PWM based on the custom interface that you may write.

That's just a quick summary of the process; some experience may be needed in writing driver code. You may get lucky and find the config needed to set up the PWM peripheral on the Internet, for your particular processor. Or, you may be able to see the source code of an existing PWM library for your processor and re-write, if you don't want to rely on any existing library.

processor = Beagle Bone Black

and i dont have the skills to code from scratch

Writing a Linux driver for PWM will need some experience. Another way is to use the PRU inside the chip, but that will take some studying too. The latter is not difficult, but expect to spend at least a day or more reading up on it and experiementing before you'll be able to write something useful, and it won't be in Python, but C and assembler. For now, it may be best to rely on the adafruit example code for the BeagleBone Black until you're confident to write a driver.

EDIT: this web page looks very good, it has a Linux PWM driver and a link to the source code, and how to compile the kernel if the built driver is outdated. So, although you'd still be using a driver,  you can see the source code.