Sampling Hz increase
Sampling Hz increase
(OP)
Hey guys,
I have a straightforward question.
I have a DAQ system that can sample at 10,000 Hz max. I now have an application that requires 20,000Hz sampling.
There are several unused ports on the DAQ system. Is there a way to obtain an effective sampling of 20K Hz with the current system? Or an approximate method? Thanks,
Fe
I have a straightforward question.
I have a DAQ system that can sample at 10,000 Hz max. I now have an application that requires 20,000Hz sampling.
There are several unused ports on the DAQ system. Is there a way to obtain an effective sampling of 20K Hz with the current system? Or an approximate method? Thanks,
Fe
"Simplicity is the ultimate sophistication." L. da Vinci
- Gian





RE: Sampling Hz increase
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Sampling Hz increase
I thought someone might know of some other method. I know it is not likely though.
I am measuring rotary speed. My other alternative it to mechanically reduce the speed. Unless there is some electrical circuit I can use for this.
"Simplicity is the ultimate sophistication." L. da Vinci
- Gian
RE: Sampling Hz increase
Come to think of it, sampling is kind of an awkward way to measure rotational speed. It would be more direct a fit a one-bit encoder of some sort to give you a pulse at every revolution, and just hook that to a counter and a timer.
Or maybe I am misinterpreting the problem, as sometimes happens.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sampling Hz increase
It's rotation upwards of 20K rev/s. So Ideally I would need >40K sampling. But I would like more than one sample per revolution to measure small variations in angular acceleration. It's an automotive application. This is something I am doing on the side (not part of my job). So I didn't want to have to purchase a more advanced daq setup. I have something similar to this: http://performancetrends.com/dtm-hdwe.htm
I think I will just have have to step down the rpm, hopefully without too much loss of inertial accel.
My only issue is logging the very small time increments accurately.
"Simplicity is the ultimate sophistication." L. da Vinci
- Gian
RE: Sampling Hz increase
RE: Sampling Hz increase
FedX32; Your DAQ system is not going to work as you realize.
Are you just looking for data or actually trying to control with it?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sampling Hz increase
RE: Sampling Hz increase
TTFN

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RE: Sampling Hz increase
I was just thinking I that if there is a way for me to avoid making this extra reduction system, it would save some time. I will also then have to take into account the inertia of the extra system too. The prime mover is an IC engine.
My experience with daq systems thus far have only been at work where we have state of the art stuff. Sampling freq. was never a problem really.
Oh an I was also considering a pulley system at 1:5, but I am concerned about belt flexibility affecting the results.
After this I thought that if I use 2 (or more?) ports, say one on opposite ends of the rotating disc/shaft I could somehow effectively increase the sampling rate by a recursive averaging algorithm. Say taking into account which 1-bit input is triggered. For example, if they are triggered in order, then the rotation is below the max sampling of the system, or if one is triggered and then the second is not, following by the first, this would indicate that the speed is above the max sampling.
I thought that there might have been some work done on this before. I guess not really
Thanks guys.
RE: Sampling Hz increase
One option is a magnetoresistive rotation sensor: http://www.nxp.com/documents/application_note/AN98... that is then fed into a frequency to voltage converter than can be then read by your DAQ. Of course, by that time, you could just get a digital frequency counter to read the MR rotation sensor. An optical encoder would do the same, but there are constraints on lighting, contamination, etc.
Is there a way of tapping into whatever is causing the rotation? I'm thinking you have some sort of motor driving whatever it is. Most AC motors are commutated from the outside, so hanging a frequency counter on one of the windings and dividing accordingly would get you the rotation speed.
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RE: Sampling Hz increase
Well, now I'm curious.
20,000 r/sec is 1.2 million RPM.
Turbos don't go that fast.
Gyroscopes don't go that fast.
I know of no way to mechanically reduce that speed.
I must have missed another meeting.
Hall effect may be too slow, and you can't just strap a magnet on with a hose clamp.
If it has a single feature that's at all shiny, you can use an optical retroreflector to get a 1/rev signal. From there, I think you can put a phase lock loop on the pulses, and watch the error signal to see rotational accelerations.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sampling Hz increase
Thank you.
I'm looking to accurately measure the rotational speed of a shaft (and accompanying flywheel and other.) driven by an IC motor. It is geared so that the speed of the shaft is quite high. The end result is part of a side project I'm working on. I'm sorry, but I can't say too much more. Only that the instantaneous pulses of the IC motor are one of my interests.
Seems the main issue with this is accurately logging the rotational speeds. I guess if I directly use a rotational encoder, instead of sampling at a frequency, I will have to look into correlating the error in the signal to accelerations. Thanks.
IRStuff,
Thanks. The magnetoresistive rotation sensor option looks promising. I may just setup an optical encoder and try some post-processing.
I hope that the datamite can keep up with the high speeds of the system though. I would imagine that the datamite's circuitry may be the limiting factor. I guess I will have to see.
At least now I know that sampling at a frequency is not really the way to go.
Thanks guys!
RE: Sampling Hz increase
Fed have you considered an oscilloscope? That's a cake-walk for them. You could even see the dither you're seeking.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Sampling Hz increase
ramp = unwrap(angle(hilbert(signal))) / (nteeth*2*pi);
Missing teeth can cause a problem - an interesting one.
- Steve
RE: Sampling Hz increase
They produce quite a high voltage when close to a typical automotive gear (like a flywheel ring gear).
- Steve
RE: Sampling Hz increase
The word 'grenade' comes to mind. Look at the safety cage used on this recent 1 million RPM world record (2008) electric motor.
LiveScience 'New Spin Record Set: 1 Million RPM (November 14, 2008)
Now 1.2M RPM is being applied? Wow... F1 motorsports?
---
If one simply measures the period, counting a clock between one reference pulse (perhaps an optical sensor) per revolution and the next, then one would have a new data point every 50us. That's already a flood of data. If one needs more frequent data about the system, then arrange for multiple reference pulses per revolution.
One must consider the required clock frequency. If one uses (for example) a 10 MHz clock, along with ten pulses per revolution (as from gear teeth for example), then one is only counting about 50 clock pulses between the teeth. So the temporal resolution is terrible. One will need a much faster clock, hundreds of MHz minimum.
If one demands this sort of detail, then one will be dealing with pretty high clock frequencies, and a flood of data. It's all doable, but non-trivial.
A completely different approach would be to simply use an oscilloscope to view the reference pulses (N per revolution) signal from the shaft. If there's any significant acceleration variation, then the pulses would be moving around. If they're steady, then the RPM is stable even within one revolution. One might even be able to trigger from the drive for synchronous exploration. A very simple check using a cheap 'scope.
RE: Sampling Hz increase
Keith,
Thanks I have not considered an oscilloscope. I will look into it, as long as I can somehow gather the data.
Steve,
Thanks for the info on magnetoresistive and inductive sensors. I will for sure look at these. Seems like the main advantage would be to estimate the output at a frequency higher than sampling.
I think I underestimated the complexity of dealing with such high rotations. But, should be alright to at least estimate what I want.
VE1BLL,
Thanks. I also was thinking along these lines. I need to make sure the clock speed of the hardware is quick enough.
Oh and unfortunately this is not for F1
I think I have enough info to go upon now. Should I run into issues I will for sure chime back.
Worst case is I can add a low inertia mechanical reduction and correlate it back to the original speeds. Thanks guys.
RE: Sampling Hz increase
It is trivial, actually. There is a report on this here: http://www.gke.org/rapporter/files/Reciprok%20frek...
Sorry that it is in Swedish. But I think that the few formulae and pictures can be understood even without Googletranslate. With it you won't understand a word.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Sampling Hz increase
... until the gearing frags. I suspect you may have underestimated the cost/ difficulty/ technical challenges/ safety risks associated with transferring significant power at 1.2x10^6 r/min.
It's time to read up on scattershields. There was an article in the ME house rag maybe a decade ago about scattershields for energy storage flywheels. The post-scatter photos were seriously disturbing.
Mike Halloran
Pembroke Pines, FL, USA
RE: Sampling Hz increase
An "impossible" measurement might become trivial with the inverse approach. Or more accurately, using the wrong approach may make a relatively simple measurement impossible.
Of course, one could even do both schemes in parallel if required.
RE: Sampling Hz increase
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Sampling Hz increase
Thanks for your input Gunnar. That article may come in handy. Simply counting the number of revolution in a set time period will do the trick. I will have to try and correlate each of these intervals to obtain accel too.
VE1BLL, I think for this application counting number or revs in a time period may be simpler. But I can consider both for sure.
Dan, Thanks. I am unfamiliar with an electrical downcounter. I can consider it too however.
Happy Easter to everyone.
RE: Sampling Hz increase
TTFN

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RE: Sampling Hz increase
RE: Sampling Hz increase
In that case, one is dealing with low MHz signals. Trivial. One needs to balance update rate with numerical resolution (1s = 1 ppm, 1ms = 0.1%). And there are ways to blend and balance both.
--
Previously: "...would like more than one sample per revolution to measure small variations in angular acceleration."
Which is where the opposite approach would perhaps have been better.
RE: Sampling Hz increase