Sheet-feed switch : Why do you think it's designed this way ?

Because they don't have a clue?

snipped-for-privacy@rediffmail.com wrote in news:1163512735.801646.123520 @b28g2000cwb.googlegroups.com:

to determine paper size?

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Could you eleborate on that ? If you're thinking of a minimum number of switches that have to be opened to start the process, the answer is no. As I already explained, the process can be activated by opening *any* or more of the switches.

It sure looks that way. It's just that I found it hard to believe that the design team of a very prominent European company would make that kind of mistake.

As an example, on some printing machines, the current that has to be fed to replace the charge on the drum, and ink useage, varies with the area that is being printed. If they have a timer on each switch, the sum of the times will be a good measure of the paper area.

Best Wishes

An interesting tidbit of info. But in this case, the sheet is simply passed through a chemical bath by a set of rollers. The chemical is of standard strength, and the two variables - time and temperature - are manually programmed by the operator.

The row of switches is arranged in a line perpendicular to the direction of travel so that the leading edge of the sheet passes through them at the same time.

maybe not to 'determine' paper size, but so that you could put any size piece of paper in in any position

Colin =^.^=

As far as the use of multiple sensors is concerned, yes. (The sheet is actually a photosensitive plastic film, previously exposed to UV light and cut from a roll to whatever size and length is necessary).

What puzzles me is the seemingly unnecessary use of a long multi-cored cable and complex logic circuitry, just to detect the presence of the film at the entry point and start the transport motor.

Could be for width. There may be a movable edge guide that is moved to the widest position, based on the switches that are open. Also, could be for skew or uneven front edge detection.

Ed

Can you be absolutely certain that that is all the switches do, namely start the transport motor?

What if they signal some other process inside the machine as to length and/or width of the film? What if the film is, say, 3 meters long?

I explained this in the third para of my first post. I quote :

"What is evident is that the process can be activated by opening any one, more than one, or all of the switches. There is nothing to prevent a skewed feed or to abort from an uneven leading edge."

This is confirmed by the operator who works with it everyday. The motor can be started by lifting the lever of ANY one of the sensor switches.

There is no adjustable edge guide.

Sounds like a 'pro' machine where cost is not really important.

There might have been a plan to detect uneven feeding by the mechanics guys, and by the time the electronics were designed, no-one cared anymore or the cure was worse than the symptom?

Thomas

I simplified that a bit, but not in a way to change its significance. See below.

Of course, much of what I said is reverse engineering based on observation without the benefit of a schematic or manual. But I don't see how the width would make a difference. The film is passed through the chemical bath at a constant speed set by the operator.

The length is probably important, but again not in a way that would make separate detection of the switches necessary. It probably works this way :

The machine in not in operating condition now, but the operator told me that the transport motor stops by itself some time after the film exits from the last stage of processing - the dryer.

I could not find any kind of sensor to detect the exit status. My theory is that the feed sensors signal when the trailing edge of the film has passed the entry point, and a counter starts counting the number of pulses from an optical tachometer fitted with the transport motor. It then stops the motor after a predetermined number of pulses.

*I* would design it that way, and in fact, I have. I worked out the speed reduction ratio from motor to roller, measured the roller diameter and the total path length, and calculated the required number of pulses. I added a certain number of pulses to compensate for roller-film slippage and I've worked out a simple circuit using only a few NAND gates and a garden-variety CMOS counter IC. The whole process is really quite simple.

It would be even simpler if they'd provided an exit sensor.

The area of the film being processed would determine the amount of chemical replensishment required to keep the process within spec.

It *is* a pro machine.

That's a distinct possibility. With all due respect to the many excellent products coming from Europe, I've often felt that European engineers tend to come up with over-complicated designs. This seems to be the result of a habit of adding more and more stuff to correct something that's still not quite right, instead of revising it.

There's an automatic replenisher pump, but it works by simple level detection with an immersed conductor. It detects only low level. The quantity to be pumped in one go is set by the operator.

Is there a timer that tells when it's time to change/clean the chemical tanks, or have a regular maintenance performed?

After reading through this thread it is obvious that everyone is simply guessing... and not getting anywhere. And why? Simply because you haven't posted the most basic information which is necessary if anyone is to provide some semblance of a correct answer. Its the same as asking a question like "I have this TV which is not going. Can anyone tell me what the little black thing with colored stripes on the printed board does?"

WHAT IS THE MAKE AND MODEL OF THIS MACHINE???

Who knows, somebody might even recognise the machine and give you the benefit of their knowledge.

No, nothing in that direction.