Vector subtraction

[Coryman]

To All,

This is not really a Labview  question, per se.  I am attempting to subtract one vector from another.  I have attached the vi.  The numbers look like they work fine for vectors that are directly in or out of phase.  However, when the vectors are something in between, the largest vector seems to be 'pulled' in the opposite direction from what I would expect.  For example, if the run vector is 4 @ 90 degrees, and the slow roll vector is 1 at 30 degrees, I would expect the resultant vector to be less than 90.  But the answer is more than 90, in this case 103.9 degrees.  I am missing something very simple here but can't see it.

[wiebe@CARYA]

The maths are right.

[RavensFan]

What does "run" vector and "slow roll" vector mean?

Have you drawn out the vectors on a piece of paper to see if they make sense?

For R=4@90 and S=1 @30,  I think S-R being 3.605 @ 103.9 is perfectly right.

 

[Coryman]

Run vector is vibration from rotor taken at running speed.  Slow roll is vibration at very low speed and is meant to be used to remove runout and shaft imperfections seen in the vibration data from the running speed data.  I noticed your example showed slow roll minus running speed.  It is supposed to be running speed minus slow roll.  I thought that's what my example shows.

Regards,

Ron

[Kevin_Price]

I'd venture that the problem is in how you're thinking about it.

Going back to basics, subtracting the slow roll vector is the same as adding its inverse.  The inverse of the slow roll vector will have the same magnitude but point in the exact opposite direction.  So its phase will be 180 degrees different.

So now visualize moving along the run vector (magnitude 4, angle 90 deg) and then *adding* the slow roll vector's *inverse* (magnitude 1, angle *210* deg).  Now you can see why the resultant vector is over there in the 2nd quadrant at ~104 deg.

-Kevin P

[wiebe@CARYA]

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@RavensFan wrote:

What does "run" vector and "slow roll" vector mean?

Have you drawn out the vectors on a piece of paper to see if they make sense?

For R=4@90 and S=1 @30,  I think S-R being 3.605 @ 103.9 is perfectly right.

 

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I think the green (4@90) should be R, R (1@30) should be S?

[wiebe@CARYA]

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@Coryman wrote:

Run vector is vibration from rotor taken at running speed.  Slow roll is vibration at very low speed and is meant to be used to remove runout and shaft imperfections seen in the vibration data from the running speed data.  I noticed your example showed slow roll minus running speed.  It is supposed to be running speed minus slow roll.  I thought that's what my example shows.

Regards,

Ron

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Please address whom you're speaking to. You've got two answers, your reply can apply to either. Then again, both answers are pretty much the same .

[Coryman]

Yes.  One would usually expect the slow roll data to have less vibration than the running speed vibration.  

[Coryman]

Sorry.  I was addressing Ravens Fan.

[RavensFan]

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@Coryman wrote:

Run vector is vibration from rotor taken at running speed.  Slow roll is vibration at very low speed and is meant to be used to remove runout and shaft imperfections seen in the vibration data from the running speed data.  I noticed your example showed slow roll minus running speed.  It is supposed to be running speed minus slow roll.  I thought that's what my example shows.

Regards,

Ron

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I'm sorry.  I think I swapped the letters in my diagram.  So swap R and S with each other throughout.

Here is that diagram fixed.