SICK sensor and AD wobble

AWOL:
Not sure what you mean by "wobble" (or SICK NT6 for that matter, hint),

So, just to be clear, this is the sensor we are talking about?

Sick NT6-03022 Overview
Registration Control Scanner NT6-03022 lens No.211 (1004936)

Sick NT6-03022 Description

LED light source with two switchselectable spectral ranges
Lens position selectable
Light- or dark-switching selectable
Rapid response times

datasheet

If so, it appears to be a contrast detector that switches between a high voltage and a low voltage output on detecting a dark (or light, depending how you set it) contrast change?

In principle this is the sensor but mine is a rare variant, NT6-03014 (from memory) with a 4m cable, which Sick has assigned a new part number to.

The beuaty about these great yester year devices (unlike many modern eyes) is that they were made with an anaolgue output too. If you dig into the specs you'll find references to it: They have a green light and a red one and this topic ties nicely into the figures I've just recieved from the analysis as promissed.

Following my rationalisation of what matters and what doesn't, I have the following (all relative to 1024 values [10 bits]):
Set to the green light...
A glossy white mark: 75..77
A glossy black mark: 15..17

Set to the red light...
A glossy white mark: 201..204
A glossy black mark: 37..39

The key to this project holding water is a matter of the ratio of distance moved (image movement beneath the sensor) to ADC change. The green gives around 0.1mm per count and the red around 0.01mm.

My conclusion is that 0.01mm is fine whereas 0.1mm isn't and unfortunately this is the best scenario as other colours produce a much smaller range. Therefore I feel I need to continue refining. I would be gratefull for peterH (author of 20 bit suggestion) or of course for anyone who knows some ins and outs of taking the 20 ADC bit route because I'm new to this and hoped my little Arduino would cope.

Thanks a lot all.

MarkT:

PeterH:
If your requirements are so close to the resolution of the ADC that a variation of one or two is important to you, you probably need to use a more precise ADC.

Under battery power the ADC is pretty well behaved (certainly noise below 1 lsb) - I suspect this is power supply noise - is the PSU a switching one? Extra decoupling can help.

MarkT sorry, I have little idea what a switching supply is but I can say that it is pretty cheap. It is surprisingly accurate though despite this (compared with other supplies I've measured), reading 11.94 for a 12v supply.

Cheers

The first thing I would try as a simple experiment would be to remove the external Aref voltage you are applying to the Aref pin and use the instead default internal Avcc reference and see if your wobble improves, gets worst, or remains the same.

Lefty

I tried a simple analogRead of a simple two resistor voltage divider and got "wobble", readings of 512 or 511. Your circuit is much more complex than mine!, so many more sources of wobble.

I think your in the area where real circuits and theory differ.

Possible sources of wobble a 1 part in a 1000 wobble.

  1. The output from your PS is not as good as you think. If you looked at it with a very good 'scope you will find it varies even with no load, at mains frequency.

  2. The load is never as constant as you think. A lot of chips draw more current on the rising/falling edge of the clock than in between.

  3. Ambient light changes, some bulbs flicker at the mains frequency, this may be, just, enough.

  4. Induction in that 4 meter cable (from near by mains cables) is the cable shielded?

  5. Small changes in temperature.

  6. Vibration.

Mark

retrolefty:
The first thing I would try as a simple experiment would be to remove the external Aref voltage you are applying to the Aref pin and use the instead default internal Avcc reference and see if your wobble improves, gets worst, or remains the same.

Lefty

Lefty, Aref was only tried after suffering with wobble and made very little perceivable difference compared with the Unos' default 5V. I should just check: When you say 'internal', do you mean something other than using the 'DEFAULT' Const when setting the reference in 'analogReference()'...this is how it was being used prior to using the Aref pin (although it was used by default, i.e. no specific call to 'analogReference()')...

Thanks Lefty

holmes4:
4. Induction in that 4 meter cable (from near by mains cables) is the cable shielded?

Mark

Mark after I mentioned 4m, I thought that it might lead (no pun intended) to trouble. In fact we have two NT6's in this embryonic system and the newer (recently bought - albeit an old model) was on my mind. The irony being that this is the eye that is providing the digital signal, whereas the other analogue one has a miserly 10" ± (254mm ± in new money) of cable - both Sick cables are shielded.

N.B. In terms of which model the analogue NT6 is, I think it might be the ...22 as discussed in an earlier post.

Thanks Mark

2Tricky:

retrolefty:
The first thing I would try as a simple experiment would be to remove the external Aref voltage you are applying to the Aref pin and use the instead default internal Avcc reference and see if your wobble improves, gets worst, or remains the same.

Lefty

Lefty, Aref was only tried after suffering with wobble and made very little perceivable difference compared with the Unos' default 5V. I should just check: When you say 'internal', do you mean something other than using the 'DEFAULT' Const when setting the reference in 'analogReference()'...this is how it was being used prior to using the Aref pin (although it was used by default, i.e. no specific call to 'analogReference()')...

Thanks Lefty

No, I meant the DEFAULT mode with no analogReference command issued.
I worked with lots of ADC and DAC stuff in a oil refinery before retirement, so have some experience with trying to make quality analog voltage conversions in the real world. My opinion of the AVR 10 bit ADC is that it is very handy for hobby type projects and applications but falls pretty short of being a instrumentation quality measurement device. In todays world there are so many really nice and affordable external ADC chips and modules available and many are easy to interface to an arduino via SPI or I2C. So if accuracy, resolution, repeatability, common noise cancellation via differential inputs are important to your application I would suggest you look into external ADC converters.

Lefty

Lefty just in case I've negated a negative (!?), the first implementation displayed similar wobble without the function 'analogReference()' being called.

It's interesting what you say about alternative chips and I wonder whether you have any models in mind. I admit that I'm trying to do this on half a shoe string, but a few quid of prudent expenditure is surely horses for courses.

Thanks for your view...

2Tricky:
Lefty just in case I've negated a negative (!?), the first implementation displayed similar wobble without the function 'analogReference()' being called.

It's interesting what you say about alternative chips and I wonder whether you have any models in mind. I admit that I'm trying to do this on half a shoe string, but a few quid of prudent expenditure is surely horses for courses.

Thanks for your view...

The only one I've had first hand experience with on a arduino is the TI ADS1115 16 bit 4 channel (or 2 differential channels). http://www.ebay.com/itm/4-Input-16-Bit-I2C-Analog-Digital-Breakout-Micro-Controllers-Arduino-/330761041957?pt=LH_DefaultDomain_0&hash=item4d02e4f825

I have a check out sketch code available if you actual try one of these out.

Lefty

Lefty I'm amazed at how small the chip is, no wonder it's offered with a breakout board. Do you happen to know of one that might be attachable to a breadboard so that I can experiment cheaply. I appreciate that this is the only one familiar to you in an Arduino context, but guessing that I2C is I2C is I2C (unless perhaps it's revision 4 - I don't know) and provided I can get insight into the 'quality' of the chip, perhaps I could use another without much code change...

N.B. I have now tested the current AREF setup with a 12V battery. Both Sicks were powered from it, as was the AREF (using a zener 4V7 to cut it down). The signal from the digitally configured Sick used a simple divider to reduce 12V to around 5V and the analogue output was just as nature intended because it is low anyway (even though the specs say max 6V, I've never seen it above 3V and only then when it was 'blinded' by perpendicular light I think). One thing though, the Uno was powered by USB which was still being used to give feedback to the PC.

Cheers

TI ADS1115 16 bit

Let me be the first to say that unless you are a very good engineer, using a 16-bit adc can be quite a challenge. And unless you are some analog god, using a 24-bit adc can be downright impossible.

Rather than increasing the bits, you should think if you need that much resolution and if that high of a resolution makes sense for your application (for example, is it even that accurate?).

dhenry:

TI ADS1115 16 bit

Let me be the first to say that unless you are a very good engineer, using a 16-bit adc can be quite a challenge. And unless you are some analog god, using a 24-bit adc can be downright impossible.

Rather than increasing the bits, you should think if you need that much resolution and if that high of a resolution makes sense for your application (for example, is it even that accurate?).

dhenry I think it will help if I clarify with a bit more context: The best scenario from the ratio of the 'image movement to ADC output' point of view is using the red light and comparing white with black marks. The technique used (so far) places the light half on one colour and half on the other colour, which provides the average greyscale as output. To move from fully on one colour to fully on the other the image needs to travel around 1.5mm which provides less and less resolution the lower the relative greyscale values get.

As mentioned, 0.01mm per integer from the ADC is adequate, but 0.1mm isn't. For example: If a small sheet (all sides are in centre vision) of paper has a black border of say 1mm, and a 0.1mm error occurs along one axis, one side will become 0.9mm and the other 1.1mm, meaning the smaller will be nearly 20% smaller which humans can see quite easily and may or may not hurt sensibilities (depending upon the nature of the onlooker!) and this assumes the conversion is accurate and doesn't suffer from noise etc.

It is also worth noting that my current quest is to measure any error rather than applying any correction (I'm treating subsequent correction as incidental).

You've got me worried though and I would appreciate some detail if you are in the position to do so...Thanks a lot

I don't know your setup that well but if you are using dividers or voltage references, think about what kind of resistors /Vref you will need in order to maintain the comparability of 16-bit adc from source to source.

With a good layout engineer and a good analog engineer, you may be able to get a 16-bit adc to work in the real world environment. But that requires a lot of work and luck. Most people end up doing is discarding the last few digits.

2Tricky:
The technique used (so far) places the light half on one colour and half on the other colour, which provides the average greyscale as output. To move from fully on one colour to fully on the other the image needs to travel around 1.5mm which provides less and less resolution the lower the relative greyscale values get.

This is the first time I've had any idea what you're using the A/D input for. Is it correct that you're using the dark/light contrast to find the position of a boundary between dark and light parts of an image?

How accurately can you position the sensor? Is it feasible to scan the sensor across the boundary? If so, I suspect you could get a much more precise position for the boundary by interpolating the position of the '50%' brightness point from a sequence of brightness readings in different positions rather than needing a single brightness reading to give you a hugely precise measurement of the proportion of dark/light areas within the image.

Lefty I'm amazed at how small the chip is, no wonder it's offered with a breakout board. Do you happen to know of one that might be attachable to a breadboard so that I can experiment cheaply.

Yes, that is why they supply those header pins pictured with the module, you solder the small pin ends to the module and then you can plug the module into any standard solderless or solder breadboard that uses .1" hole spacing.

PeterH:
..Is it correct that you're using the dark/light contrast to find the position of a boundary between dark and light parts of an image?

That's right.

How accurately can you position the sensor? Is it feasible to scan the sensor across the boundary? If so, I suspect you could get a much more precise position for the boundary by interpolating the position of the '50%' brightness point from a sequence of brightness readings in different positions rather than needing a single brightness reading to give you a hugely precise measurement of the proportion of dark/light areas within the image.

Unfortunately 'not really' possible given that a big problem would be the inertia in the hardware used to accomplish a scan. The material is moving at speeds of 20 fpm minimum (can be much faster) but in the direction that everything being equal (which of course cannot be so) will give a constant reading over the time taken to examine the analogue pin i.e. along the threshold of the two colours. Hence there is really just one shot possible and it has to be the best effort. In the current guise, a chart plots the result along the X axis as time passes.

I hasten to add: I have lashed together a temporary complete system to test the reliability of using a moving target and the result is indistinguishable from a completely stationary image and so I have no worries with a moving target in principle. I should also mention: The duration that a consistent image will be available for under the sensor is as long as a piece of string in that it could start to be very different within say 1mm movement (prorata time elapsed and speed).

Thanks a lot PeterH

Lefty I appreciate that but I'm sorry to say I have even shallower pockets than that - of course I have to get it here too. I'll see if I can get something for the fluff!

As mentioned, 0.01mm per integer from the ADC is adequate, but 0.1mm isn't.

I don't know what exactly you are trying to achieve but 0.1mm seems to be well below what a (untrained) human eye can detect.

Maybe you want to step back and see the minimum resolution / accuracy required and what that corresponds to a voltage signal for your adc and go from there.

dhenry:

As mentioned, 0.01mm per integer from the ADC is adequate, but 0.1mm isn't.

I don't know what exactly you are trying to achieve but 0.1mm seems to be well below what a (untrained) human eye can detect.

Maybe you want to step back and see the minimum resolution / accuracy required and what that corresponds to a voltage signal for your adc and go from there.

I posted a little explanation of how the error can be magnified just now if you get a chance to read it but I'll do a little working backwards as you suggest too.

Thanks a lot.