I was once consulted to resolve a problem with the small in-house-designed BLDC (brushless DC) motor in one of our company's products. A significant number of the newly assembled motors were behaving erratically or locking up altogether, but no failed electrical components were found.
The motors had optical sensing for commutation, with a plastic shutter wheel between the planes of the three LEDs and the three phototransistors. The motors were enclosed, but the cover was removed for inspection.
Ambient light was my first thought, but I was assured that these particular motors had never worked correctly, even with the cover on. Just to be sure, I tried shading it with my hand. If my finger was very close to one of the sensors, the commutation state would change and the rotor would rotate to another position. It returned to its former position when I withdrew my finger. Shading the whole works with a clipboard or similar object had no effect.
My memory flashed back to times when the polystyrene cover of my VOM (volt-ohm meter) had acquired a static charge. The pointer would rest well upscale from zero, and would respond to my hand if I brought it near the cover. To discharge the plastic, I only had to breathe on it. I exhaled slowly into the motor. It started up and continued running. My colleagues thought I was playing a joke on them -- I'd just breathed life into a dead motor. I explained my hypothesis, which the experiment had confirmed.
The next day, I brought in a piezoelectric air ionizer which I'd bought some years earlier to discharge phonograph records when cleaning them. It had two sharp points slightly recessed in a plastic housing, a brush on the bottom, and a handle which one squeezed and released to generate the high voltage. In a dark room, the corona could be seen at each tip. One squeeze of the handle sufficed to resuscitate a motor. I recommended several, to avoid potential RMAs (returned materials authorizations).
My record discharging gadget became official production equipment until we acquired ionized air blowers for production. I suggested some design improvements to reduce the susceptibility to static charge and also prevent its retention. Once discharged and assembled into the product, however, the motors worked fine with no indication of electrostatic issues. The design therefore remained static.
This entry was submitted by Dick Neubert and edited by Rob Spiegel.
Dick Neubert has a long and diverse history in electronics and (mostly real-time) programming. His design work ranges from high-performance disk head servo systems to computerized automation systems for sawmills. He has an MS in engineering sciences.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Sherlock, quiet sometimes back (1998) I had designed similar BLDC motor using hall-effect sensors. There also we had used a photo voltaic transistor for smooth rotating of the motor, but initially it seems that the motor is rotating for a moment then slow down and then rotates. I mean a regular slowdown in between the rotation. Later we identified that at certain instances, the amplitude of the sensor output is not enough for driving the motor. So what we had done is, just injected an external pulse at regular intervals using a timer chip for making the signal strength constant and hence a continuous rotation.
Sounds interesting, Mydesign. Just so you know, we're always looking for fresh Sherlock Ohms stories. We're also eager to see any Made by Monkey stories of designs gone haywire.
Sounds as though the motor designers inadvertently created static-electricity generator similar to a Van de Graaff generator often seen in high-school science fairs. Nice solution to the problem.
The Van de Graaff generator phenomenon is a well-known problem in belt-driven apparatus with no ground connections to the pulleys or to some object near the belt, and in paper-feed or similar mechanisms. These motors, however, didn't accumulate charge while running. Once discharged, they showed no further symptoms. It was during assembly that the charge was introduced. It might have happened as parts were removed from their stockroom packaging (the shutter wheel in particular), or handed by someone in Assembly with static-prone clothing.
Great fix! Always love it when we 'more experienced folk' use our oft wide ranging experience to come up with a somewhat out of the box fix for a problem.
Notarboca, you are right. I think if we are listening to the comments and blogs, we can avoid most of design flaws and debugging issues up to an extent. Most of the designs may be theoretically correct, but may not work in practical. This may be due to some ignorance, unseen mistakes or sometimes by complication from components.
I would liked to have seen the expressions on the faces of his colleagues when the author brought it back to life by breathing on it. If ever there was a story that deserved the "Sherlock Ohms" designation, this one is it.
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.
The simple fix would have been to change the material and use a plastic that was a bit conductive. ESD-dissipative plastics Are a good fix for the static build up problem. Even though they are not good enough for shielding against interference or EMI radiation, they can reduce the static buildup. Of course not all molders are able to provide this material.
This story is an EXCELLENT example of a real life engineering problem which should be mandatory in EVERY science, physics, chemistry, engineering curriculum, whether at the high school OR college level. Too many students learm the popular linear equations of phenomena (F=ma, e=ir, etc.), but fail to grasp that life is NOT solvable in linear terms. That's why mathematicians like TAYLOR, LAURENT, La Place, Newton, Leibniz, etal. spent countless hours developing higher order mathematical processes to explain and quantize these phenomena.
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.
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!
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!
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.
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.
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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