×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Large shaft speed measurement
4

Large shaft speed measurement

Large shaft speed measurement

(OP)
We need to measure  a rotational speed of the shaft diameter 1200 mm, and only the shaft surface is available for speed pick-up. Preferred output is 10000 PPR with full quadrature. The first idea was to use a magnetic band with incrementally coded scale (used in linear encoders), but how to make accurate joint to complete the circle? Or, maybe there are some other methods? Please advise.

RE: Large shaft speed measurement

Non-contact speed sensors with reflective tapes or proxmimity probes with a notch/keyway/welded pad in the shaft.

Muthu
www.edison.co.in

RE: Large shaft speed measurement

That would give one pulse per revolution which is good for some purposes, but apparently this guy needs 10000 PPR

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Large shaft speed measurement

10000 PPR ? Positional control ?

Muthu
www.edison.co.in

RE: Large shaft speed measurement

2
One magnetizing head working on the steel surface like the recording head of a tape recorder) and two pick-up coils positioned 1/4 period away (for quadrature) from each other does it.

The tricky 'make ends meet' thing can be fixed quite easily by looking at the picked up signals and adjusting the signal frequency so that the last period ends where the first period starts.

It takes several rotations before you have everything in place, after that you can enjoy a stable high-resolution speed signal.

If you only need to measure speed and do not need to have an update more than once per revolution, then reciprocal measurement is a no-brainer.

It may be tricky to have 10 kPPR. That will make the period of the magnetizing cycle equal to around a third of a millimeter. That can be done, but it takes a nice shaft surface. Putting a lower resolution pattern, like 1 or 2 mm period is quite easy.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

Gunnar's solution is very elegant.

However if I understand quadrature signals correctly, we only need 2500 magnetic periods to create 10000 PPR, so the magnetic period will be around 1mm on the circumference of a 1200mm diameter shaft.

I'm further curious if anyone knows how fast the shaft can travel beneath such a magnetizing head and pick-up coils (leading to the max rpm for this solution).

Thanks,
Sean

RE: Large shaft speed measurement

It is understood that you get four times the PPR. If you, for instance, specify a 1024 PPR encoder, you get an effective resolution equal to 4096 increments. That is how all encoder vendors specify their devices.

I do not think that the speed of a 1200 mm shaft is more than one or two hundred RPM - at most. So the frequency will not be extremely high. Many unknowns, the biggest problem is to keep the distance between pick-up and shaft surface close and constant.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

(OP)
Gentlemen, thanks for your responses. The max speed of the shaft is 30 RPM, this is a large gearbox output.

RE: Large shaft speed measurement

Gunnar / Mobius - can you explain the principle more?

We apply AC at the stationary head I assume.  If shaft was stationary we create a traveling wave which result in ac sensed a the stationary pickups.

When the shaft starts rotating, I can imagine that eddy currents would change the pattern seen at the pickups, but it's hard for me to understand what relationship it will have to speed or position.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Large shaft speed measurement

You record a periodic pattern on the shaft surface with the recording head. When you are satisfied with the pattern you remove the recording head. Then read the signals with the other two heads. Similar to a tape recorder.

One problem with this methd is that you need to use either hall devices or flux gates to read signals near zero speed. But if you only need to measure speed above a certain limit, then you can use inductive pick-ups.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

Aha interesting. That makes more sense.  Thanks.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Large shaft speed measurement

You mean zero pulse?

That is not needed when measuring speed. Only when positioning. It can be done in the same way. But you need an extra track, just like you need in an encoder.

I shall add that I have not used this technique IRL. But a guy named Orvar Dale did when he was experimenting with what later came to be the Torductor/Pressductor. This was back in the fifties and electronics was vacuum-tube based. I shall google the thing to see if it is being used today.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

Google was not my friend here. 'Tape recording' doesn't seem to be used any more. Laser correlation techniques have taken over. There are several ready-to-use systems available.

That would, of course, be a faster and easier solution. But probably less fun sad

Accuracy and time delay can be a problem in laser techniques. It depends what the specs are.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

Can anyone tell me what is the need for 10000 PPR ?

Muthu
www.edison.co.in

RE: Large shaft speed measurement

Sometimes it is only about specmanship. Sometimes, there is a real need for high resolution.

High PPR numbers are necessary when positioning, but seldom for speed measurement. There are simple encoder functions built into standard DGBBs and they seldom have more than a few hundred PPR. That has proven adequate for many speed measurement purposes.I did some work for SKF selling their 'Sensor Unit' to the salesmen and the main problem was to make them understand that high PPR is not always necessary - sometimes not even desirable. There are a few slides about that in http://www.gke.org/presentationer/files/SKF%20Sensor%20Bearing%201a.pdf

 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Large shaft speed measurement

2
Both Heidenhain and Renishaw make tape scale encoders that can can be wrapped around a shaft.  The tape scale comes with an adhesive backing.  The read head is mounted separately.  The optical pitch is 20 microns or 50,000 cycles per meter, 200,000 pulses/meter.  The A or B channel maximum frequency is above 200,000 Hz.  A 1.2 meter diameter at 30 RPM will be less than that.  There is an Index pulse.

You will have 750,000 PPR.

RE: Large shaft speed measurement

LPS for Gunnar and sreid. Thanks for useful tips.

Muthu
www.edison.co.in

RE: Large shaft speed measurement

I think high pulses per revolution can be useful for elaborate torsional vibration measurement systems also.

One thing monitored is speed oscillation. If machine nominal speed is fmachine and modulating speed is fmodulating, then the resulting spectra has components at frequencies |fmachine +/-fmodulating|, and the higher-frequency of these is at frequency of fmachine+fmodulating.  Based on Nyquist criteria, we need to "sample" the speed (computed from position) at  a rate exceeding 2*(Fmodulating+Fmachine) in order to avoid aliasing.  A common relatively high modulating frequency is twice line frequency (this frequency excitation occurs as result of electrical unbalance and can excite resonances such as turbine blades if not properly detuned).  So for this case we may need pulses at > 2*(7200+30)cpm. ~ 15,000 pulses per minute.  Since the machine is rotating at 30cpm, we would need 500 pulses per revolution (15000 ppm/30 rpm = 500 ppr) to be able to sense that particular speed oscilation.  Note gear mesh can also be sources of high frequency torsional excitation.

I believe even tighter requirements can apply if it is desired to measure torsional strain (twist) by comparing position at two opposite ends of a shaft... because we are trying to measure a small angle which may be changing over time.  The angle associated with pulses must be much smaller.   

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Large shaft speed measurement

By the way when I used the term "sample" above I was referring to a pulse as a sample of speed or position.  There is also the likelihood that a computer will be sampling the pulse output...that is another relevant "sampling" rate but a different useage of the term.  In any case no matter how fast the digital sampling, you get at most one "sample" per pulse from which to compute the position and speed.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources