Infra Red Tacho

Some time ago I built a tachometer for my mill from a published design using a PIC and a reflective sensor (IR emitter and photocell on one chip)

I have just converted my mill to belt drive and needed to change the sensor position. I printed out a new black & white strobe disk with six black segments and six white segments on a laser printer to fit on the spindle pulley and on a trial run it worked fine.

I then printed the strobe again on thin card using my inkjet (the inkjet handles card better than my laser), covered it with clear sticky plastic film as I had done on the original set up and the tacho would not work.

Tried again without the film and it still did not work.

Printed one strobe with the laser and one with the inkjet on the same type of paper and sure enough despite them looking equally dense the inkjet one does not work and gives no reading at all.

The moral of the story is that my black inkjet ink must reflect infra red the same as the white sections?

Regards

Regards

Interesting. My current Canon printer has 2 black cartridges - one is conventional inkjet ink for mixing with the colours, the other is a pigment ink for better quality, more permanent letter printing. My guess is that this second cartridge would behave more like a laser printer.

Rod

Lasers use carbon black.

Neil

Edited By Neil Wyatt on 04/07/2014 14:34:37

I don't think "cheaper" is even in the vocabulary of the printer manufacturers when it comes to ink cartridges,

The reason they use a separate black cartridge is because, although you can theoretically get black by mixing the other colours, it's just that - theoretical. The result is at best a sort of muddy colour.

But I doubt that they use "carbon" black as the laser printers do. Rather a black dye of some description.

Rod

I think your guess about the performance of the black cartridge in the Canon might be wrong.

The inkjet printer I used was a Canon with the two black cartridges and I tried the b&w setting as well.

regards

John.

Hi John,
From the clues you have given (Published design, PIC micro, ad 6 pulses per rev.) I am guessing it is the RevMaster design. I have just had a look at this design and there does not appear to be any sensitivity adjustment for the reflective opto. Adjusting the value of the 10K resistor in the collector of the photo transistor may get it to work. Monitor the voltage between the emitter and collector of the photo transistor to see if there is any difference between the reading when the white and the black part of the encoder is in front of it. The aim is for the voltage to be above about 2.5 volts when the black is in front of the sensor and and below when the white is front of it. Try adjusting (Probably reducing the value.) the value of the resistor so that the average of the two readings is about 2.5 volts. If you cannot get a voltage swing of more than about 1.5 volts centred on + 2.5v then it will not be reliable.

Les.

Les

You at right it is the remaster and it works perfectly as long as the strobe is printed on a laser printer.

No adjustments or modifications will make it work on a target that will not produce a pulse because of its response to IR.

Hi John,
Does the voltage on the collector of the photo transistor not change at all between the black and the white of the inkjet printed disk ?

Les.

No inkjets don't use carbon black, but there is variation in what they do use:

**LINK**

Good printers substitute black to make up for the part of the colour otehrwise made up of C+M+Y, i.e. they are a proper 4-colour process (CMYK) like litho printing. Photo printers often use extra colours.

Neil

Hi John,
I have done the test that I suggested that you do. There was hardly any change in the photo transistor emitter collector voltage between unprinted paper and solid black printed with an HP D5160 inkjet printer. (It does have a black cartridge installed.)

Les.

Hi John

I suspect it's not the quality of the ink that's the issue but the technology that's used to apply it. If you look at the printed pattern from an inkjet under a decent magnification you'll see that it is actually a set of dots with white spaces between whereas the output from the laser is pretty much a solid black - at least that's what I've found with my printers - both Brother the laser is an HL 2250DN and the inkjet a MFC-J5910DW. On the inkjet pattern there is more than enough white showing to reflect back the ir beam.

Many years ago (mid to late 70's) I was faced with much the same problem of trying to get an optical probe working for use on very high speed (80k plus rpm) rotary machinery. The surface that I was looking at with the probe was carbon fibre composite. We found that all the apparently obvious things to try to make the mark for sensing, white paint, snopake etc didn't really work as once you get into the infrared spectrum the colours that we see just don't apply. We got just as good results by having an area of the composite matted by abrasion the rest being satin to gloss from the overlaying epoxy resin. The biggest problem that we had though was that we were working at the very limit of the rise time of the devices at that time and could loose the signal simply because the photosensor didn't respond fast enough.

Keith

Edited By Keith Long on 04/07/2014 23:27:14

interesting point about the technology rather than the ink.

Some random thoughts about experiences with IR from a photographic perspective:

35mm IR film comes in a metal cassette and can rather than plastic ones to keep it light tight and even then it is recommended that the camera is loaded in the dark.

I have tried various black plastics as cheap IR pass filters. Despite the plastic sheets being made for the same type of decorative application, it depended on manufacturer if it worked or not. (There were no clues in the specs).

Black plastic bin bags pass IR without any apparent attenuation.

So, for my strobe, it might be the 'gaps' in the application of the ink, it might be the type of pigment or it could be a combination of both?

I would like to explore further but the bottom line is that if you want something that is opaque to IR don't print it on an inkjet!

It could also be the image you are working with and the resolution the printer is set to, it gets a bit complicated when you resize images as it can alter the DPI of the image and what may be fine from a visual point of view may well not be good enough for your needs. Then there is the setting of the printer which you can normally alter but have a maximum normally around 600dpi.

Bob

From the perspective of my work with Infra red measuring Theodolites the Infra Red beam initially was reflected through a total internal reflecting prism which had a known constant for the distance measured. Through development we now have reflectorless IR diodes in the Theodolites. The original targets could be reflecting surface materiel like the glass bead road sign plastic. Also we used a yellow target that allowed us to reset focus and centering of IR optics. Again this had an active surface which looked like glass beads?

The application of reflectorless measuring is to measure Dam wall deformation and any inaccessable measuring point.. Profiling is another application where rough hewn tunnels are measured before coating with wire and cement.

Clive

Hi Keith,
I do not agree with your theory that the laser print is solid black and the inkjet print is just dots. If this was the case I would have expected to see maybe a 50% reduction in the reflected infrared. In fact there was almost no change. On the inkjet sample the collector voltage changed from around 0.2 volts to 0.21 volts. On the laser sample it changed from about 0.2 volts to about 4.5 volts. I was using the Maplin SY-CR102 reflective sensor with a 270R resistor to set the current in the photodiode and a10k collector load resistor. The supply voltage was 5 volts. (I think this is the same configuration that John is using.) When I did the tests I was expecting to see a noticeable change in collector voltage with the inkjet sample that could have been made to cross the threashold level by changing the value of the 10K resistor. I have added close up pictures of the print samples.

Laser print.

Inkjet print

Les.

Les,

That's an excellent illustration

I think the answer will be found when we look at the IR absorption characteristics of carbon black, rather than the transmission characteristics of inkjet.

MichaelG.