Steve,

Once again..... thanks for the input.  I kinda like the electromagnet idea........ IF the "quick spin" disconnect approach does not work ( Occam's Razor ...K.I.S.S. ... and so on ).

The electromagnet could be powered by one of the Phidget digital output boards and triggered by FS2Phidget reading the Whiskey Compass heading at 360 degrees.

A one minute 360 degree turn is 6 degrees per second or .16 seconds per degree.  So if your estimate of the "pickup time" of .3 seconds works out...... there is about a 2 degree "screw up" potential in there.  If the electromagnet fires off the 360 degree point for .16 seconds as the airctraft comes thru that exact point, that likely cuts the "error potential" down to about 1 degree.

Sounds like this is at least plausable either way.  I am dealing with other stuff right now.... but the first step I see in the near future is to do some experiments with an automotive compass and a bar magnet to see how it reacts.

Then get into designing the construction of the physical unit.... and then get into dealing with the servo stuff.

best,

.......................john

John, if all else fails: http://bellsouthpwp.net/b/o/bobpaige/images/Wet%20Compass.JPG

What's confusing you John is a bit of mis-direction by the servo manufacturers...

You will see many servos listed at 45° of travel...but it's not "stop-to-stop" travel. They're actually only listing "centre-to-stop".

Servos typically have "stop-to-stop" travel of 80° to 90°. You would need about a 4-1 gear ratio to cover 360°. I would fudge it a little and go a bit over, using the Phidget software to limit the travel down to what you actually need. It's my understanding that the software can do this easily.

If you check out: http://www.servocity.com/html/hs-322hd_standard_deluxe.html you will see that the travel is listed as Quote:


Count on the 80° travel and use 4.5 or 5 to 1 gearing.

Servocity also has extrenal gears available that should do the trick without any major heartburn. 

Direction of rotation (the "Clockwise" bit) refers to how the servo behaves to an PWM that is lengthening. If the PWM is shortening the servo will turn counterclockwise. It's a bit hard to figure out for the un-initiated...they get into some esoteric descriptions to provide the info:

Quote:


Quote:


At 1500 usec the servo is at neutral, at 1900 usec the servo is at full clockwise and at 1100 usec the servo is at full counterclockwise.

Basically a long-winded way of saying that your assumption of servo direction behaviour is correct.

Now for 180° servos...be very wary of these. The vast majority of 180° servos are not proportional. These are specialty devices generally used for things like landing gear retraction. I've found that it can be very hard to figure out if a servo is proportional or not when talking about 180° servos...they aren't very forthcoming. Total servo travel on these types is 180° to 190°.

One note about using even a proportional 180° servo...it would need to be geared to give you 360° rotation anyway. When you add up the cost of the gearing and the extra cost of the servo it's a more expensive solution.

The compass with the "aircraft style" compensator is a nice touch. You can actually adjust the compensator the same way it would be done in a real plane. There will be some inaccuracy in the system which you can partially correct using the compensator...meaning you will need a compass deviation card. Deviation cards are another "real plane" feature that you will then have added.

Adjusting a compensator can be a frustrating PITA (pain in the ***) if you are unsure how to do it. If the included instructions don't make sense or don't seem to work feel free to ask. Compass calibrations were perversely one of my favourite things after I got the hang of them.

I hope that helps.

I can't wait to see this develop John!

I know I'll be following your lead and putting one in my 'pit if it works as good as your initial tests seem to indicate.

As for the help...why sweat that minor detail? You know I'll get full value back as soon as my 'pit designs far enough that I'm running into problems that you have already solved.

That's the whole point of a place like this Forum, help each other and help yourself in the process.

I cant wait to see how things work out on this one. This was the only analog gauge that I wanted to include in my pit. Still havent built anything yet, still planning.

First Parts Of Whiskey Compass

Here are the first half of the parts for this part of the project:

The Phidgets servo controller (small enough for you?), the Hitek servo, and the automotove wet compass.



Soon to arrive from Servo City ......... the gears to give me the 360 degrees of rotation I need to accomplish this.

It all creeps forward.....s....l...o....w...l....y .  

best,

.......................john

PS:  That servo controller and servo package from Phidgets is the core makings of an analog panel glauge (like for flaps) "on the hoof".

Digital servos still have the rotation limiter built into the geartrain... so no 360 degree continuous rotation for them.

The growing robotics market may see that type of servo developed... but for now we're limited to what is available and cheap.