K.I.S.S.
In other words measure the variable that is closest to what
you are controlling.
Since your issue seems to be one of keeping the drive units
aligned, why not just directly measure (with a pot, encoder, LVDT (they're
waterproof), hall effect pot) the rotation angle and take and display
  the difference? If the angles are the same the units are aligned,
assuming that the position gains and calibrations are identical.

You can apply whatever sin/cos function you need to display a
'distance' but I think that what the user really wants is 'alignment'
since you said in an earlier post that you wanted a display
with ' 0, plus/minus error'

There are also commercial units available that already do this.
The jet boats we were on in Alaska had 4 independently steerable
jets, and a control box that allowed them to be moved in unison,
with whatever 'offset' for individual drives that gave maxiumum speed.
This happened to be one of their new boat's first few runs, so they were
'tweaking' the settings as we went. It certainly did make a difference
to be off by a few degrees.
Once 'locked together' only one steering control needed to be used.

Robert

Tim N9PUZ wrote:
> Bob Blick wrote:
> 
>>>The task is to measure the distance between two drive
>>>units on a boat. The drives both propel and steer the boat.
>>
>>
>>I'm having trouble picturing what this is. The phrase "drive unit" brings
>>to mind an engine and transmission. Why would the distance between them
>>change, aren't they bolted down? This is a serious question, I'm not
>>kidding.
>>
>>Thanks,
>>
>>Bob
> 
> 
> The boat has 4 engines coupled to a pair of drive units. The owner 
> tells me that the two individual drives are not solidly linked 
> together by design (I do not know a lot about the mechanics and/or 
> boats.) Each drive unit moves and/or can be adjusted to both steer and 
> set the trim of the boat. The distance between them out near the props 
> can change based on their position. The measurement and adjustment 
> function will be more of a trimming feature and is not a part of a 
> real-time control. It just needs to be done at speed instead of 
> statically when the boat is not moving.
> 
> With regard specifically to ultrasonic sensors don't they typically 
> have elements that would be open and hence adversely effected by the 
> water? I had looked at literature for a couple. It seemed like sealing 
> them up would render them inoperable.
> 
> As far as problems from spalshing goes for either ultrasonic or laser 
> based sensors I believe that sort of interference could be filtered 
> out since this isn't a control system and the nature of the 
> adjustments rules out big changes in a short period of time.
> 
> Tim
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