Mixer for 3 brushed motors?

[bfgstew]

Hi again all, long time no see................

I am after some advice/pointers/help.

I am planning to build a scale model PT boat and it requires 3 brushed motors to run it, now I came across this website during my searches for info and was wondering if a Picaxe could do the same job? Scroll down to 'Motivating the model', to see explanation.

Many thanks in advance.

Stewart

 

[Rick100]

Hello bfgstew,

That's a very interesting project. It uses a Basic Stamp for the mixer. If you could get the code from him, it would probably run with very little or no modification on a Picaxe. It probably reads the receiver pulses with pulsin commands and sends the mixed signals to the ESC's with the pulsout command. The Basic Stamp doesn't have the servo command. I would use pulsout to avoid the possibility of glitches. Even if you couldn't get his code it should be possible to write it yourself.

Here's a link for a posting on RCGroups by the PT-41 builder. Post 20 provides a little more info.
http://www.rcgroups.com/forums/showthread.php?t=975836&highlight=stamp&page=2

Good luck,
Rick

 

[goom]

I did exactly that for a large icebreaker model with 3 motors. I used a -14M,and it works exceptionally well at both high and low speeds.
I used 2 potentiometers, one vary the rudder to outboard motors mix proportion and one to vary the center motor to outboard motors mix.
I also added an interrupt triggered setup to read the 2 potentiometers and the throttle neutral pulse width.
Full code attached. I have PCB layout and an post it if useful.
I do not seem to be able to attach the code. Will try again in a new post.

 

[goom]

Mixer Code

Here is another attempt to attach the .BAS file:

 

[goom]

And here is the schematic and PCB layout.

 

[rossko57]

I would use pulsout to avoid the possibility of glitches.

Glitches (which manifest as twitches on regular servos) probably don't matter much on a propulsion motor drive. If its easier to develop or understand with SERVOPOS, go fer it.

 

[bfgstew]

Thanks for the files Goom, gives me a solid starting point, this looks easier than I thought.........................for the moment......

 

[goom]

The main difficulty that I had was the port/starboard mixing as a function of throttle position.
For example, at full throttle, I did not want the motor on the inside side of the turn to go below zero thrust. At zero throttle each motor can be from 0 to 100%, forward or backward. In reverse, the rudder to throttle mix needs to be in the opposite sense to be coordinated with the rudder.
The 2 potentiometers controlling the mix proportions are there to fine tune to suit the boat/motor/propeller characteristics. Since values are sored in non-volatile memory, once everything is set up, it remains set.