How to accurately measure rotation? 1

[ritchie888]

I need to know the type of component I should use to determine the rotation of a shaft. Basically, I'll be building a pendulum which will hang down at 0' (due to gravity) with an axle at the top of it where I'll also mount the sensor (basically I'll look like the face of a clock with only one arm that always points to 6o'clock, due to gravity as I say, it'll only need to move in 2D). If I was to push that pendulum in an anti-clockwise manner, I want to calculate the positive rotation angle as it moves, likewise the rotation angle will be negative when it passes the 0' point in the other direction.

I'd like to use a mechanical method, such as a rotary encoder, rather than optical methods which don't seem to be as applicable to my application.

My problem here is that I don't know what attributes of a rotary encoder I need. Being able to measure 360' would be a plus, although I don't expect the pendulum to do a full rotation, being able to measure at least every 1' is necessary, anything more accurate would be a plus, and I need the shaft to rotate freely, not to 'click' round like I've seen in my switches which come in the same physical package.

I've been looking at something like this:

http://uk.farnell.com/bourns/pec11-4220f-s0024/incremental-encoder/dp/1653381?Ntt=PEC11-4220F-S0024

Would that be viable?

Cheers!

 

[MintJulep]

My problem here is that I don't know what attributes of a rotary encoder I need.

Yep, that's a problem.

No one can help you until you figure out what you need in a quantifiable way.

Encoders are pretty much commodity items. Start reading data sheets and ask yourself "Is that feature useful or necessary for me? How about that feature?"

 

[IRstuff]

So, at the minimum, we can at least write down the various sensors that could do some sort of rotation measurement job:
> gyros
> optical encoders
> capacitive encoders
> synchros
> other inductive sensors
> resistors
> magnetometers
> accelerometers

Once you figure out what it is that you're trying to measure, resolution, accuracy, etc., you can narrow down the range, or not.


TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

Oh, I forgot to note that the iPad2 has compass, gyro, and accelerometer onboard, along with an app that does recording. You could tape the iPad to the shaft and record away.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ritchie888]

Cheers for the response, guys. Basically I'm actually using the pendulum as a 'golden standard' to determine how accurate some IMU's I have are. So using gyro/accelerometers wouldn't make for a very good comparison.

I've been looking at mechanical methods for determining the angle as my mind is making me believe that nothing could be more accurate than a physical representation giving the angle. I'm not concerned with velocity nor acceleration, just the angle.

In terms of numbers for resolution I'm still clueless at this time, doing lots of Googling, but I basically want to be able to calculate the angle several times a second as the pendulum swings.

A friend of mine has suggested using potentiometers as they are apparently the most accurate and are susceptible to poor resolution and bad calculations of velocity/acceleration (not that I'm concerned about them anyway), he's also suggested servo-potentiometers.

I think he's slightly biased towards this method as he's used it before, but the calculations to determine the angle seem pointless and unnecessary as using a rotation encoder works almost identically to potentiometers (in terms of mechanics and physics) but can determine the rotation angle much easier.

I've been looking at absolute rotation encoders as they seem to be just what I need, but what do you guys think?

 

[rb1957]

would it help to convert the rotation motion into a linear motion ?

how about sine/cosine ? ie measure one (or both) sides of the triangle ??

 

[tygerdawg]

You have a couple of not-too-expensive options:
[ul]
[li]A simple pointer mechanically attached to your rotary shaft and indicating onto a scale, or alternatively a scale on a disk attached to the shaft with a stationary pointer. Both with appropriate graduations.[/li]
[li]An electronic rotary encoder with sufficient pulses-per-revolution specification, directly connected to rotary shaft or attached with something like timing belts & pulleys to give you a higher resolution. You'd have to output the encoder to an appropriate device of some sort to read the pulses.[/li]
[li]An analog potentiometer attached to shaft. Also would require sufficient hardware to read the signal and convert to something useful.[/li]
[li]...others, I'm sure, but....[/li]
[/ul]

First things first, though. Before you go any further, you need to do your homework and scope your requirements:
[ul]
[li]range[/li]
[li]resolution[/li]
[/ul]



TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Compositepro]

Use a rotary table intended for milling machines. It's already calibrated and will do everything you require including supporting your IMU. You just turn a hand wheel to whatever angle you desire and it will hold that angle very accurately. It's as compact and portable as any alternative and you can easily resell it later.

 

[1gibson]

Rotation -> linear... Use a gear on the pendulum pivot point coupled with a linear "gear" (think rack and pinion.) Then you can decide on potentiometers or other means of measuring/recording the linear position.

The larger diamter gear on the pivot point, the higher linear displacement (analogous to resolution) you can achieve.

Not knowing the size of your pendulum, this might be a little too obtrusive due to the friction of that assembly.

 

[Cockroach]

This is quite easy and inexpensive to do, measure shaft rotation. Get a small magnet and embed it to the rotating shaft. In the housing, a Hall Effect Sensor measures the magnet passing by as an electrical spike, i.e. oscillating magnetic field generates an electrical field. Done.

Componets are bought off-the-shelf or online a website hardware stores. You can probably get to it by your website browser. This is a dead simple problem.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[IRstuff]

'golden standard' to determine how accurate some IMU's I have are"

But, you haven't said what accuracy level you want. What parameter are you even trying to measure? And, unless you're building a replacement for the Minuteman, it's highly unlikely that you've got an IMU that cannot be compared against other IMUs.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

An, given that the British measured the height of Mt. Everest to within 26 ft in 1852 using transits, a theodolite blows any other mechanical approach out of the water.

You need to cough up the resolution and accuracy of your IMU.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[TheTick]

You could possibly just meaure the horizontal displacement of the pendulum and apply a little trig. Linear encodoers are not hard to come by.

 

[Walterke]

if you measure the linear displacement the resolution/accuracy of your results changes with the angle. This may be a problem.

NX 7.5
Teamcenter 8

 

[WolfHR]

1gibson- using gears might be detrimental to accuracy because of the play between gears (esp. when pendulum changes direction). There are ways of fixing it, but might not be worth the trouble...

 

[btrueblood]

"if you measure the linear displacement the resolution/accuracy of your results changes with the angle. This may be a problem"

Yes. A problem solved by trigonometry, as Tick suggested.

Cockroach's idea is a good one. If you need to know positively which direction the pendulum is swinging, use two hall effect sensors at some angle to each other.

 

[FeX32]

I agree with Cockroach. I have done this before with much success. The more magnets you have the more accurate the reading is. The components can be all together less than 5 bucks too...


Fe

 

[IRstuff]

If all you're interested in is gross, static angles, then an inclinometer may be the way to go:

http://www.mcmaster.com/#catalog/117/2286/=f8wysd

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[paulb7]

>Oh, I forgot to note that the iPad2 has compass, gyro, and accelerometer onboard, along with an app that does recording. You could tape the iPad to the shaft and record away.

That seems quite inconvenient given the size of the iPad. How accurate is it?

 

[FeX32]

It was a joke


Fe