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 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.
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.
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.
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.
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.
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.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 4
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
4 
