Laser distance sensors with 1-2k samples are in the range of 1000 to 1500 €.
From time to time you can find a useable laser for half the price on ebay (pay attention to the distance range).
Regards
Heinrich
From time to time you can find a useable laser for half the price on ebay (pay attention to the distance range).
Regards
Heinrich
I got a 2kHz sampling sensor for about 350 euro on AliExpress.
There's absolutely a ton of these.
There's absolutely a ton of these.
There are a couple of important things to keep an eye on when it comes down to accuracy and resolution. It's not as straightforward as you might think.
Not all with a proper sample rate are usable.
Hopefully I can explain this a little better next week or so, when I have some more time on my hands.
Not all with a proper sample rate are usable.
Hopefully I can explain this a little better next week or so, when I have some more time on my hands.
I bought my laser from Baumer in 2007 for approx. 400 € (https://artalabs.hr/AppNotes/AN7-Estimation_of_Linear_Displacement_with_STEPS.pdf). It has served me well ever since. Other than described in the application note, I use the current output.
Regards
heinrich
Regards
heinrich
Keep in mind that connecting such a LDS to a sound-card input, won't give you much information about offset etc. Since it's not capable of detecting DC.
So it's not really usable as a Klippel LSI system.
But I agree that's very useful for all kinds of things.
Thanks for the responses and links. That’s an excellent point about the AC coupled sound card, you really need a DAQ to determine large signal parameters since you need to know the DC offset (and how it changes with drive level and even frequency) otherwise you’d neglect creep, hysteresis, coil offset, etc.
I’m guessing the measurement resolution stated in the aliexpress laser’s datasheet is given for the slowest sampling rate/time, I wonder what the resolution/accuracy becomes for the fastest sampling rate? I’m bit skeptical of a no-name laser, in my experience no-name tools often don’t meet their stated specs. The only laser calibration I’ve preformed used static targets (stacked gauge blocks). With a Keyence laser I’ve always trusted the a static calibration is sufficient to cover all of my speaker use cases for moving targets due to the inherent margin (sub micron resolution at 100kHz), but when you're running right up the edge of the laser’s specs you can’t make such assumptions. I’d like to do a side by side measurement comparing a relatively cheap laser to a Keyence. I’d be happy with 0.1mm resolution up to 2kHz, which is good enough for woofers and mids.
I’m guessing the measurement resolution stated in the aliexpress laser’s datasheet is given for the slowest sampling rate/time, I wonder what the resolution/accuracy becomes for the fastest sampling rate? I’m bit skeptical of a no-name laser, in my experience no-name tools often don’t meet their stated specs. The only laser calibration I’ve preformed used static targets (stacked gauge blocks). With a Keyence laser I’ve always trusted the a static calibration is sufficient to cover all of my speaker use cases for moving targets due to the inherent margin (sub micron resolution at 100kHz), but when you're running right up the edge of the laser’s specs you can’t make such assumptions. I’d like to do a side by side measurement comparing a relatively cheap laser to a Keyence. I’d be happy with 0.1mm resolution up to 2kHz, which is good enough for woofers and mids.
I wouldn't really call these things "no name", they are just being used a lot in production and such.
One of the things to look out for is the full-scale linearity (% F.S.).
The "full scale" part here can already be tricky. In general, this means the total "reach" of the sensor.
But I have seen some manufacturers cheating by defining this in a slightly different way.
So if we take the ±60 mm as an example: 60 mm in both directions = 120 mm full scale.
With 0.1%, this would mean that the linearity is (0.1/100) * 120 mm = 0.12 mm.
For the ±20 mm version, this is 0.04 mm.
The ±10 mm → 0.02 mm.
Resolution only says something about what the display can show
(it's the same pitfall with digital calipers).
In practice, I don't really see the need to go for more than ±15-20 mm.
However, this is not even the full story.
I have to dig up some papers again when I have a bit more time, but the non-linearity increases as a function of the distance as well as sample rate.
This doesn't even include temp drift and averaging.
Most manufacturers define this linearity with some kind of averaging.
A lot of those LDS devices have averaging on by default, which can really mess up the measurements.
As a very rough rule of thumb, the averaging just lowers the sampling rate.
So if we have 2k samples/s, and apply 2 averages, this means we end up with 1000.
The theoretical useful frequency is just like the Nyquist frequency,
although the practicality is, for data acquisition, very debatable.
I don't think you really need a DAQ, just a proper ADC capable of doing DC is fine.
The DAC actually has to be DC-decoupled to make sure the driver only creates DC components by itself and nothing else.
(Although I guess this is also done by the power amplifier.)
Also, these kinds of DACs with enough performance will be very expensive compared to a standard delta-sigma DAC.
One downside of having a separate ADC and DAC is that clocks aren't synced, so you will get a bit of spectral leakage.
There are also RS485 LDS devices.
I'm not entirely sure if these can just be used through RS485 and therefore omit an ADC entirely?
Maybe somebody else knows?
As far as I know, the RS485 is only for device settings etc.
I'm also not sure how well that works with spectral leakage etc etc problems again
I also had one RS485 LDS before that was kinda vendor locked, so I stopped looking further into that route.
I have actually been working on an open-source Klippel LSI DAQ device.
Not sure when I can realize some more information just yet.
Last edited:
Getting a gauge block with good enough accuracy (tolerances) is one thing.
Making a jig that's equally accurate is a lot harder, especially when things need to move/changed.
There are many very good quality Chinese companies. Many more than you might think. And their work gets re-branded and sold on as X or Y companies. Where do you think the Keyance LASER's are made? I have one from Baumer that is rather slow as well, And a Matsushita, and a SICK. They are all well within the tolerances required for measuring loudspeaker X-max distance linearly. And as (b_force) mentioned the upper frequency limit is determined by the LASER and the system sampling rate.
If you calibrate via a gauge block a LASER range finder capable of 40 micron resolution you are working in 0.0001" resolution. Definitely killing a gnat with a howitzer territory. DC offset will only be possible with an instrumentation ADC, most Audio ADC's are not designed for this. I think you are already aware of this issue from your comments.
Last edited:
Truly easy. When you wring together two gauge blocks you can do so with half the bottom block exposed. That will give you a repeatable measurement step that is calibrated very very easily in excess of 0.02mm or 0.001". That being with dirty blocks and not so great wringing practice.
The cost of two gauge blocks of 0 quality on Aliexpress is trivial. All you require is repeatability. And an acceptable tolerance. As even 1mm difference in X-max is barely a db, or less depending on the driver a tenth of a mm is much greater resolution than truly required for measuring voice coil displacement and X-max. If you are chasing modal irregularities along a cone for purposes of find cone breakups you need the resolution and speed a zero to the right. As distortion testing should not be a small signal exercise, there will be movements in the cone that are much greater than 20 microns or 0.001" And those resolutions are not hard to come by in relatively inexpensive LASER units.
I think there's a far more simple method that has an inherent better result.
We're not interested in the absolute distance, something a gauge block provides.
We're interested in the relative travel across a certain range.
In that case a (digital) micrometer is much easier.
Mount some kind of reference block to the end of one of these micrometers that we measure with the LDS at the same time, and we'll have a much more control of the accuracy. (or maybe there are micrometers with a bigger end??)**
Most micrometers will give you an accuracy of about ±5 micron absolute worst case.
Often more in the ballpark of about ±2-3 micron.
This block moves in the same plane, so any errors in the surface don't matter.
I've seen micrometers around for about 40-50 bucks, probably even cheaper.
Or if you don't mind some kind of manual labor, an old fashioned mechanical one will do as well.
**Now I am thinking about it more, the surface area of a standard one, might be just big enough for the laser
https://www.aliexpress.com/item/100...2745e55b2!12000044115548738!sh!CA!122373966!X
An example if you are chasing the ability to scan cones for vibratory analysis.
An example if you are chasing the ability to scan cones for vibratory analysis.