# Thread: Continuous Potentiometer and SIOC for HSI

1. ## Continuous Potentiometer and SIOC for HSI

Hi everyone,

I have an almost fully-functioning HSI instrument that is powered by DC Motors. I have obtained an Opencockpits DC Motors Card to interface the instrument to FSX with.

Now, all I need to sort out now is the SIOC coding. But before this, I need to know a little about continuous potentiometers... Can anyone help me?

The instrument have 9 potentiometers (ALL of them are feedback pots coming from the motors). These potentiometers, obviously, are continously rotating (as the compass of a HSI must be able to move infinately in either direction).

Can these "continuous pots" be interfaced to SIOC? I assume so, because I have seen videos of people using compasses with SIOC; there must be a way.

A weird thing about these Potentiometers. They only have a 340 degree operating range, before nothing happens, and it resets to zero. EG:

0-340 degrees = Resistance increases from 0 to 5K Ohms

340-360 degrees = Nothing. "Open Circuit" reading from Multimeter

0 degrees = The cycle starts again. 0 Ohms going up to 5K, and so on.

These 20 degree "dead zone" is obviously there to prevent a short circuit. As we all know, no single potentiometer can have an operating range of more than 359 degrees (unless it is special), and it would just be a complete circle of wire; providing a perfect short circuit.

So, my questions are as follows:

1.) Can these "continous pots", with the dead zone, be interfaced using SIOC?

2.) IF YES to 1, how?

3.) What can I do about the 20 degree dead zone? Obviously, this zone provides no feedback. Any ideas?

Kindest regards and thanks for your time,

Jack

2. ## Re: Continuous Potentiometer and SIOC for HSI

Hi Im Not quite sure but you may want to check that your not doing damage to the pot wiper when you pass through the end stop ( at 340 degrees) dont quote me on this but i belive the null zone you refer to may be the gap where the carbon track starts and ends . so not sure you'll get the best life out of the pot if used this way just a thought , i may well have misundertood the potentiometer construction of this type .

3. ## Re: Continuous Potentiometer and SIOC for HSI

HI there,

Of course, the potentiometer is meant to turn infinitely in both directions, as it is for a compass motor, which equally turns forever in either direction.

So, regardless of design, the potentiometers are designed to pass through 340 degrees.

Jack

4. ## Re: Continuous Potentiometer and SIOC for HSI

Jack

I have no experience of continuous pots, but if it were me, then I would write a little test sioc script to gear the 360deg needle rotation to 340deg rotation of the feedback pot and have a little test bit of code to see if the feedback value was in the null range, if so skip through it back to a value of 1. Rough and ready, but you will learn what is happening and adapt accordingly

David

5. ## Re: Continuous Potentiometer and SIOC for HSI

Hi David,

Sorry, I'm a bit slow as you know. Please may you expand a little?

I think, if I'm right, you are referring to what I've been wondering; how to get feedback from the 20 degree "null range"?

Jack

6. ## Re: Continuous Potentiometer and SIOC for HSI

I would sudjest you to look at Simkits compass manuals on their site.
They using two pots to achieve 360 degrees coverage as the single pot cant have more then 300 degrees, or so ,turn , as you already noticed

Regards
Joseph

7. ## Re: Continuous Potentiometer and SIOC for HSI

Jack

You say 9 feedback pots. How many motors then? Presumably more feedback pots than motors, if so, they could be operating as Yoss suggests

Regards

David

8. ## Re: Continuous Potentiometer and SIOC for HSI

Yoss; I had thought about that yesterday. You seem to have confirmed my thoughts; seems plausible doesn't it?

If you guys want, take a look at these two PDF documents. I spent hours today testing (this is just ONE motor; three Pots are wired to one motor). They show my results, and show how I wired them up:

How I wired the Multimeter up: http://www.mediafire.com/?ir6172zep6dc2dt

My results: http://www.mediafire.com/?lzfh9whq7q2f6zz

Note: The constant refernce to "degrees" in the documents are because the item is a HSI COMPASS, note the references to South, North, etc.

Jack

9. ## Re: Continuous Potentiometer and SIOC for HSI

Jack

You have a good challenge here
Looking at your docs, I see that there is little change in ohm value on both pots in the range 130 to 180deg. Could this suggest that either the pots are interconnected the wrong way or the third pot provides the missing incemental value? I am out of my comfort zone here, but wonder whether the pots originally provided feedback based on a sum of all the values, if so, was this done by making the pots into one "physical" pot, or electronically summing the values of all three.

Hopefully, an electronic tyro or someone with specific knowledge can help - I am stumped (not difficult)

Regards

David