Using switch and stick on one channel? Or via software?

[JensR]

Hi all!

I have modified two Turnigy 9x transmitters to control a model of an earth-moving machine.
To help me with the difficult control, I am thinking of the following, but couldn't find a description of whether this is possible and/or whether it has been done before. So I#d be grateful for your assistance.

I do need proportional speed control in at least some parts of the operation of the machine. For this, I have attached one of these "x-box" mini joysticks to the 9x.
The ministick is a potentiometer with a range of 0 to 10kOhm, with 5kOhm/5kOhm being the centre position.
This works very nicely.

However, it is very hard to maintain a constant speed while I control the other functions of the machine.
So, I would like to be alternate between a certain forward speed and a certain reverse speed (slower than maximum) - and still have the option for full proportional speed control.
So, I'd like to be able to switch between three modes:
1. chosen forward speed
2. free proportional mode = ministick active
3. chosen reverse speed
I was thinking of using a three position switch to choose the respective options by:
1. adding a certain resistance into the one branch of the potentiometer, making it 5kOhm vs 5kOhm + xkOhm
2. doing nothing (leave ministick voltage devider as is).
3. adding a resistor as in 1., but to the other branch of the potentiometer.

As an even more refined option, I could dial in the xkOhm by using a second and third potentiometer for forward and reverse speed.

Would this work?

Would this better be done by using the programmable functions of the 9x?
Like using a switch to limit the "end stop" of the speed channel to for example 20% when the switch is active. (Dual Rate maybe?)
I would then have to simply move the ministick to its maximum displacement and hold it there - much easier than trying to maintain the reduced speed - and I would still get some proportional feel until that reduced speed.

I hope this was somewhat understandable, it feels quite complicated to explain...

Thanks for any pointers and ideas.
Jens

[Flash1940]

Jens....
It's experiments like yours that has moved our present-day software to where it is .... and.....advancing. Every day you wake up in the morning you will have a new idea. I have been in the same boat as you are....thinking of new and innovative things to do with my RC equipment. As a hint of elderly advice.....use the KISS technique when you can. From an electronic standpoint....when you place resistance across a pot being used to control the movement on a channel....it is quite likely that the servo center will be disturbed and you may end up using the maximum sub trim to re-center....
Interesting project....try it & let us all know what happens.

Flash

[JensR]

Thanks very much for your kind words and advice, Flash.

Maybe I have to explain myself better, as

"when you place resistance across a pot being used to control the movement on a channel....it is quite likely that the servo center will be disturbed"

is actually exactly what I want to do. (Unless I misunderstand you.)
Process of using the proposed control:
I leave the stick centered. 5k/5k
But I want to get a velocity output, without touching the stick, like a very primitive cruise control - a preset speed.
So I add a -say- 2k resistor to one side of the pot by flicking the extra switch.
The transmitter now sees the pot + resistor as a pot with a division of 7k/5k - the same ratio I would get if I had moved the stick a bit, without the resistor: 5.8/4.2
I assume that's how transmitters use the voltage divider?!
http://www.rcgroups.com/forums/showthread.php?t=803857

Your KISS comment made me think (I guess that was the intention )

If I added a mechanical "gate" to allow me to lock in place the stick at whichever position I move it to, that might be also very interesting.

I'll let you what I do and how it works.
I'm just worried I might fry my transmitter by connecting the potentiometer wires in a wrong way...

BTW, I posted two shots of the twin radio here:
viewtopic.php?f=47&t=794&start=10

Cheers
Jens

[ADI]

Your ideas will certainly work, but try to keep reasonably close to the pot's total resistance value. You are trying to simulate the original pot after all. Remember you don't even need the pot at all if that's what you want. It can all be done with switches and fixed value resistors if you wanted to, or switches and preset trimmer pots.

[JensR]

Thanks very much for your reply, ADI.
I haven't been able to do much with the model over the last weeks. I managed to imrpove some mechanical aspects and still have one or two mechanical issues to resolve.
For the transmitter, I didn't get any further yet. I am pretty sure that I will want proportional control in some conditions. I think most comfortable would be a mini joystick that I can instantaneously switch from friction to spring return...

[JensR]

Just an update on this, because I hate leaving loose ends behind.
I have contemplated various methods and wasn't satisfied with any completely.
One reason for that was that even if the desired speed is constant, the ESC would need to be adjusted to the different loads experienced.
The other reason was that for safety: I wanted to be able to always return quickly to "zero" or reverse the direction.

The best way for me to overcome my control issues was to extend the joystick's stick length.
I did this and formed a "hook" bent from a piece of wire to sit comfortably around my finger.
This works very well and provides a much smoother less fidgety operation.
The other option would have been to remove some windings of the joystick's spring or replace the spring with a softer one, but this would have involved opening the joystick.

I did two long sessions of grading this weekend and I am very happy with this for now.

Edit August 2011:
I have reconfigured the controls and now have added dual rate for the speed controller. I added a custom switch which I can easily activate and so far this seems to improve controls at low speeds. Thankfully, the motors are oversized, so the machine is powerful enough despite the reduced speed and torque.

Cheers
Jens