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!

 

[IRstuff]

Not completely. An iPad is 7.3" x 9.5" Nothing the OP has said precludes that. Of course, the OP hasn't said much of anything.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[FeX32]

Very true IRstuff


Fe

 

[ritchie888]

Hi all,

Sorry for the delay in response, wasn't working yesterday.

Lots of good suggestions, thanks all for that. I've looked into hall effect sensors, which look promising, but unfortunately due to magnetometers on-board the IMU (although I'm currently not using them at the moment, they will be incorporated in the future) the magnets will completely throw off the results.

I think the iPad is rather impracticable for my application, one because of its size, and two because it uses accelerometers and gyroscopes and would only be as accurate as my IMU anyway. Using accelerometers and gyroscopes to measure the accuracy of accelerometers and gyroscopes seems a little redundant to me.

The gear suggestion, linear displacement, and use of trig, are also good suggestions, but ideally I want a digital result which can be recorded and then compared with the IMU results using some sort of software.

Apologies for my lack of knowledge in terms of resolution and range that I need at this time, truth of the matter is I just don't know. Will continue to look into it and will let you know when I know a bit more.

From my searches as what you guys have said I still think an encoder of some sort is the way forward; absolute rotary or optical.

 

[asimpson]

Check out these guys for modular optical encoder parts. Quick delivery and easy to set up.

http://www.usdigital.com/

For higher precision:

http://www.microesys.com/

 

[dinjin]

For linear measurement, couldn't you use a gear
and a belt in tension. No backlash that way.

 

[rb1957]

"but ideally I want a digital result which can be recorded" ... you could video yourself taking the measurements, doing the calcs !

 

[ritchie888]

Thanks for the links, asimpsons, but I'm British and I don't think the American site would be all that fast for me. I'm generally using Farnell and RS.

This looks very promising:

http://uk.farnell.com/bourns/eaw0j-b24-ae0128l/encoder-rotary/dp/9358234

I'll have to do the calculations and see if it'll work.

 

[Walterke]

"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.

trigonometry is the exact reason you'll get a change in accuracy.

Assume you measure the horizontal displacement of a rotating beam. What you'd actually be measuring is the cosine of the angle, multiplied by it's length. Since the relation between cosine and length isn't linear, your accuracy will be bigger near the middle (=vertical positioning) of the swing, and smaller near the extremities.

Or am I missing something?

NX 7.5
Teamcenter 8

 

[btrueblood]

So use two transducers at 90 deg. Assumes the OP will be swinging his pendulum at fairly high angles. Within small angles, say +/- 15 degrees, the cosine error can darn near be neglected.

 

[sreid]

Optical enoders are hard to beat. Avago is an exellent source.

http://www.avagotech.com/pages/en/motion_control_encoder_products/

 

[IRstuff]

You haaven't even described what parameter you're trying to measure.

And again, unless you've got a strategic missile class IMU, there are plenty of IMUs that are capable of being used as transfer standards. Even the Earth can be used as a rate standard at 72.7 urad/s rotation rate. If your IMU cannot measure Earth rate or see Earth rate, then there are indeed tons of IMUs and gyros that can be used as transfer standards.

And if you're truly serious about using a pendulum for measurement, then your IMU is not as good as you infer.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[ritchie888]

What parameters? What more do you need to know? I've stated I'm interested in measuring rotation angles. It's as simple as that. At this time I don't know resolution but am merely asking for suggestions on technologies that could be up to the job.

An IMU will be strapped to a pendulum arm and swung, by hand, to any random degree, 1DOF to start with. As I have stated, I don't expect it to do a full rotation, but the device would ideally be able to do one full rotation. For example, I push the pendulum arm in one direction, it records all the angles, from 0 (pointing downwards with gravity), to a maximum angle, say 67 degrees (depending on how hard I push it, obviously), the results will be digitally read and recorded on a computer. The IMU will also be recording at the same time, then the results will be compared, to see if they both obtained the same maximum angle at the same time.

Again, I'm not interested in measuring the IMU with accelerometers and gyroscopes, they'll be subjective to exactly the same real-world problems the IMU will be applied to, it doesn't make sense, what would I then use to calibrate the accelerometers/gyroscopes which are then suppose to calibrate the IMU?

I see no reason why the pendulum idea wouldn't be a good idea, even if it's considered an initial test. What would you suggest to determine the accuracy of my IMU's, IRstuff? Chances are my IMU's are very accurate already, but I won't know that until I compare them with another reliable source.

Many thanks, sreid. From my findings and peers recommendations I agree that optical encoders are the way to go. I'll check that link out and get back to you.

 

[BrianE22]

BEI sells readout units as well as the encoders:

http://www.bei-industrial-products.com/index.html

 

[byrdj]

I was looking at this

http://www.asm-sensor.com/asm/product_detail.php?lang=en&det=win_abs

to measure cam angle on steam turbine. they apear pretty ruggly made
I was thinking the the ones with housing and shaft bearing could be used to support the test pedulam
If I recall the quote was $700 for transducer and display. even the display had an output to send to recorder

 

[ScottyUK]

You might want to explore RVDTs, an angular version of the more common LVDT position transducer and which uses the same basic principles.

As far as I know Penny & Giles, Moog, Schaevitz, maybe Yokogawa are still making them, and possibly a few aero industry suppliers too. They can form the basis of a very accurate measurement system when married up with suitable signal conditioning.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[asimpson]

Hi richie888. I'm in Ireland and used USDigital stuff on numerous ocasions. Only a few days delivery.

The Farnell one may low resolution and high friction. Also USDigital has easy to use modules to convert encoder signals to what ever you want.

Also take a look a Renishaw magnetic encoders. Dont think you will get interference from edternal magnetic fields.

 

[CastMetal]

The biggest question is still a resolution you are looking for. When you say you want to see if the IMU and measuring device hit the maximum angle at the "same time" that doesn't mean much.

If you use a rotary encoder that measures angle every tenth of a second and an IMU that is measuring every hundredth, reaching maximum angle at the "same time" is only accurate within 0.1 seconds. To test/calibrate the IMU to be as accurate as possible you need to have a device capable of a resolution greater than the IMU. Said another way, the IMU tells you the peak angle is at 3.56 seconds into the test and the encoder on the pendulum output says its sometime between 3.5 seconds and 3.6 seconds its not much of a calibration test.

Comprehension is not understanding. Understanding is not wisdom. And it is wisdom that gives us the ability to apply what we know, to our real world situations

 

[Walterke]

So use two transducers at 90 deg. Assumes the OP will be swinging his pendulum at fairly high angles. Within small angles, say +/- 15 degrees, the cosine error can darn near be neglected.

I'm pretty sure he mentioned 360° rotation. Also, since you don't know the accuracy he's willing to measure (since he doesn't even know so himself) there is no certain way to say whether or not the cosine error can be neglected.

2 transducers might solve this though, as you suggested.

It may be a lot cheaper then whatever he's looking into now.

NX 7.5
Teamcenter 8

 

[btrueblood]

Um, may have missed the 360 degree requirement. I was thinking of a swinging pendulum, not a fully rotating device.

 

[IFRs]

use a stepper motor in reverse