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IR Speed control
xti   4/25/2012 11:40:55 AM
I had a vaguely similar problem with an IR sensor on a prototype motion control system for a custom motorized TV mount we built about 15 years ago.

We made up our own shutter wheel out of an opaque plastic disc, as we couldn't find anything on the market that would fit our application.

When the system was assembled, the motor ran wild. After some head scratching, we discovered that there were no pulses coming from the LED sensor assembly.

How could this be? We tested the sensor, and it was fine. The wheel was definitely spinning in the gap in the sensor. We stared at it in disbelief.

Suddenly, the light went on. The one in my head, that is.

The plastic was only opaque to VISIBLE light. To infrared, it was clear as a bell.



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The bigger lesson
jeffbiss   4/25/2012 12:07:57 PM
I think that the bigger lesson is considering potential problems during design, and that includes those posed during manufacture. While not the same issue by any means, we have a whole house vacuum cleaner. One day I noticed that it was running and found that the hose was put away, which meant that something had turned it on other than a switch or shorting the outlet contacts. I had to go to the unit to shut it off by unplugging it. This happened a second time, which forced us to unplug it whenever we left the house for any length of time. I called the store owner who sold it to us and he contacted the manufacturer to find that no one else had reported this issue. So, what happened?

When I was rewiring my basement I had to locate the breaker for a circuit I needed to change and started cycling breakers, a quick off/on cycle turned on the vacuum cleaner. I then cycled the breaker for a count of 10. The vacuum cleaner did not turn on. This proved that the issue was one of bad design, that didn't account for a power droop/loss for a short period.

So, while not necessarily in the same category of problem as the static/motor control, a comprehensive spec may help avoid many odd problems, such as unforseen states. But then again, learning experiences are a fact of life precisely because we will always miss something.

Rob Spiegel
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Re: IR Speed control
Rob Spiegel   4/25/2012 4:17:59 PM
Your story could make a good Sherlock Ohms posting, Xti. Please consider sending it along with plenty of detail to rob.spiegel@ubm.com

If you send the story, please include a short bio - two to four sentences describing your background.


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bobjengr   4/25/2012 4:29:42 PM
I agree completly.  Engineers solve "real-life" problems and this should be transmitted to every high school student considering a career in engineering.  The most exciting part of my professional life has been solving problems--design and process problems.  The most difficult are ones in which the product is used incorrectly or abused.  When this happens, you almost never know the real story behind the failure and the "fix" is very alusive.


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Tool_maker   4/26/2012 6:46:44 AM
I am going to take your screen name literally and assume that we are of roughly equivalent ages, and therefore I can agree with your basic premise of practical experiences trumping classic bookwork often. However, there are an infinite number of field problems we run into and it is unrealistic to expect them to be taught in a classroom. It is equally unrealistic to expect the teacher to be versed in all the problems possible. Each field has its own unique set of problems and solutions.

That is why it is the moral obligation of people like you and me and the majority of people who post on this site to pass our knowledge on to the next generation of engineers, no matter how arrogant and smart alecky they are. Remember we were once new to our respective fields and knew everything and had a brand new sheepskin to prove it.

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OLD_CURMUDGEON   4/26/2012 8:15:26 AM
Tool_Maker:  It IS very obvious that there has never been a textbook nor course outline that can fully explain ALL the variables that enter into a dynamic system (whether closed loop or open).  However, my point was (and still is!) that engineers, designers, technicians MUST always be aware of higher order variables, and that the world does not adhere to the linear model which is too often the only model preached in school.

When I graduated college w/ my crisp, clean diploma, I was hired by a very large & stable radio communications company.  After going through the initial interview process, I IMMEDIATELY KNEW that once in the door, IF the CHIEF ENGINEER handed me a broom to swab down the Engineering Dept. floor daily, then that was precast to be my introduction into the fascinating world of product design.  So, the day before I started, I checked my bravado at the door, and walked in as a humbly as I knew how.  I was immediately assigned to work along side a senior engineer, who was understanding and very willing to show me how the FOURIER TRANSFORM of the course texts fit into the real world of design.  I have NEVER forgotten those sympathetic lessons in personal communication to this day, some 50 years later!

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Re: True Sherlock Ohms
Tool_maker   4/26/2012 12:34:23 PM
I also would have liked to see the expressions. If this happened today there would be someone with a camera, the breath would have been accompanied by some some incantations and the whole thing would have gone viral. It is a great story.

Charles Murray
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Re: True Sherlock Ohms
Charles Murray   4/26/2012 8:38:33 PM
It I agree it would go viral today, Tool_Maker. People all over the country would be breathing on their dead motors.

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Wildbush   5/4/2012 7:07:33 PM
When analog meters were the only way to go, the staticly charged meter solution was to wipe the meter face with a weak solution of dishwashing detergent and let it dry. The residue was conductive enough to bleed off the static charge and all was right with the world!


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