Rudder position sensor.

[GILow]

I’ve got an old TMQ AP55 autopilot I’d like to recommission.

The manual asks for a 2k ohm potentiometer for rudder position sensing, I’ve got an old 5k model, I suspect from an early Raymarine autopilot.

The combo works, but it’s a bit too sensitive. Even playing around with the limit settings I end up with a useable arc of about 30 degrees, not enough for good position resolution.

I’ve found a few rudder position sensors with a suitable resistance range online, but I can’t gauge their quality.

I also wondered if I couldn’t just substitute a decent quality potentiometer or are they not robust enough?

Maybe try and work with some kind of Hall effect sensor?

Thoughts? Recommended makes/models?

[Wotname]

Couldn't a smart fella retrofit a 2K commercial pot into the old 5K sensor?

[GILow]

Originally Posted by Wotname

Couldn't a smart fella retrofit a 2K commercial pot into the old 5K sensor?

I could if I knew which one to buy. This is more your field than mine, any leads on some good brands?

[wholybee]

You can use a 5k pot in place of a 2k in most cases by adding a couple resistors to it. See the following post:

https://www.reddit.com/r/AskElectron...r_act_like_2k/

[GILow]

Originally Posted by wholybee

You can use a 5k pot in place of a 2k in most cases by adding a couple resistors to it. See the following post:

https://www.reddit.com/r/AskElectron...r_act_like_2k/

Before I look I’m going to guess… I just realised that if I paralleled the pot with a 3k resistor I’d have 1/R = 1/3000 + 1/5000 with the added advantage that this would really dilute the resultant change on the wiper arm…

Going to have a look now..,

Edit: yeesh, my maths are horrible this morning… make that a 3k resistor, not 2.

[GILow]

Originally Posted by wholybee

You can use a 5k pot in place of a 2k in most cases by adding a couple resistors to it. See the following post:

https://www.reddit.com/r/AskElectron...r_act_like_2k/

Yes, I like this approach. Thank you.

I’m a bit sad that I didn’t think of this immediately, my electronics used to be ok and this is good basic resistor theory. I got all hung up on the device itself without thinking of the underlying electronics,

I’ll report back on the results.

[Wotname]

Using a parallel resistor was my first thought but I discarded it as I figured the AP wants to see a the full range of variation and simply adding a parallel resistor would decrease the operating range of resistance. But I know jackshite about AP designs; maybe there is some gain adjustment somewhere that would allow this 'fix' to work. Perhaps a suck it and see approach might be best. It is cheap and easy to implement solution.

[Wotname]

Originally Posted by GILow

I could if I knew which one to buy. This is more your field than mine, any leads on some good brands?

Step 1 - look at the existing one and see if there is markings on it. This may help to identify a brand or at least a type.

Step 2 - failing any markings, try to determine if it is wire wound, carbon, ceramic etc.

Step 3 - from whatever you can find from steps 1 & 2, search RS / Farnell / Mouser etc and go by obvious well known manufacturers and price.

[Dockhead]

The old Raytheon rudder angle sensor had different sets of wires for different resistance values. I see it's still made: https://www.svb24.com/en/raymarine-r...ransducer.html


Maybe that would work?


Or maybe the one you have has these alternative connections? Do you have the manual?

[GILow]

Originally Posted by Dockhead

The old Raytheon rudder angle sensor had different sets of wires for different resistance values. I see it's still made: https://www.svb24.com/en/raymarine-r...ransducer.html


Maybe that would work?


Or maybe the one you have has these alternative connections? Do you have the manual?

Yikes, I think I could buy a new autopilot for that amount of money. That’s just plain nasty. It might work, but I’m sure as hell never going to find out.

[GILow]

A quick follow up to all this.

So, I pulled out my tub of assorted resistors and got to work. For hours.

No good. It seems no matter what I did I couldn’t get the unit to respond over more than a small arc of the sensor.

I checked voltages with the multimeter and I got some changes, but nothing I could do made a noticeable difference.

So, in desperation I did the unthinkable.

I read the manual!

Well, I’d been referring to it all along, but this time I did a real deep dive on the rotten thing and found a curious reference to a “standard” or “heavy duty” sensor. There are a couple of dip switches inside to select either and they were set to standard.

I flicked them over to heavy duty and now the FULL range of the sender works.

No, I have absolutely no idea why, but it’s good enough for me.

Just putting it here for anyone who finds a similar problem and to say thanks for the great suggestions that were shared here.