Member #815726

Member Since: June 12, 2016

Country: United States

  • A couple of fine points on subtractive colors:

    The subtractive primaries are cyan, magenta, and yellow, not red, blue, and yellow. (Hence CMY printers.) These are the secondary colors in your diagram of the additive primaries. Magenta pigments absorb green and reflect blue and red light. Cyan absorbs red and reflects blue and green. So when you mix the two pigments, the only thing that gets reflected is blue. This means the subtractive secondaries are red, blue, and green. Mixing red and blue pigments does not result in a bright magenta as suggested in your pigment color wheel, but in a dull purple. Mixing magenta and cyan pigments gives a nice blue.

    This confusion continues in your example of the team colors. The dull gray in the middle would be true if you were mixing gold and blue pigments. The blue pigment would absorb everything but blue light, and the yellow/red pigment would absorb blue leaving nothing left. But if you are mixing LED light, the blue and gold (a mix of red and green light) would have all three primary light colors and would be whiter. Mixing two colors of light always results in a brighter, not a duller color. This will be more clear in your next installment on color spaces.

    All of this is a bit tangential to your main point which is additive mixing, but as both artist and engineer, I like to help people better understand this fascinating subject.

    The other thing worth noting is that the eye is more sensitive to green and yellow light than other colors. Because of that, as well as because of difference in conversion efficiency, you may need to adjust the current differently for different color LEDs to get a good color mix. In other words, 255 for your green LED may be 10 ma. where for your blue LED it might be better to use 20 ma. Again, this will be clearer in your next installment on color spaces.

  • I suggest starting with the physics of this problem. Dry air is a notoriously bad heat transfer medium and it is even worse at high altitude. It may be that the amount of air you need to move is just not feasible with a small solar array and small fans. Consider that the incoming air from a blacktop parking lot is likely to be in the 90s at best, and that the solar power falling on the car is in the range of 800 plus Watts per square meter. I'm guessing you are talking hundreds of cubic feet per minute needed to make a significant difference. An evaporative cooling mechanism might require much less airflow.

    Update: So I got curious and did a little research. A cubic meter of air under the conditions of this problem requires about 1300 Joules to raise its temperature one degree C (A Joule is one Watt for one second). I looked at a few fans in the 200 ma @ 12 volt power range and one cubic meter per minute is a reasonable airflow. This means that such a fan could remove roughly 22 watts for each degree (C) of temperature rise. If the incoming air is 90 F and the outgoing air is 110 F, that is about 11 degrees C and the fan could remove about 245 Watts - not much compared to the 800 or so Watts per square meter of solar power falling on the car.

    The hard part of the analysis is figuring out the incoming and outgoing power of the total system. Obviously even without the fans, the temperature of the car is not increasing without bounds. Some surfaces are absorbing energy and other surfaces are losing it through the same three mechanisms: conduction, radiation, and convection. It is the temperature rise above ambient, the very problem you are trying to solve, that is the ultimate driver of the balancing heat loss, and the overall solution requires reducing gain and increasing loss to minimize the temperature increase.

    Since the solar array is thin and flexible, and you need some way to mount the fans, and some kind of solar shading / insulation seems to be required, maybe a good solution would be to integrate the fans and solar array into a reflective insulating fabric car cover!

  • Sorry Nate, not quite right. The 300 bps was not because the telephone carbon mics couldn’t handle higher frequencies but more a result of the properties of copper wire and human hearing. In fact, the mics were not designed to pass frequencies below about 300 Hz because human hearing falls off rapidly below this frequency and lower frequencies add little to the intelligibility of speech. Similarly, above about 3500 Hz., cable attenuation rises sharply and again, hearing acuity drops off. So for decades, telephone systems were optimized for 300-3500 Hz. For this reason, the 300 bps modems used frequency shift keying at frequencies between 1070 and 2225 Hz. The bit rate limitation was mostly a result of the time it took to reliably detect the frequency shifts with the technology of that time. When the modem spec was released in 1962, the mainstream technology was discrete transistors (still a lot of germanium): no ICs, no microprocessors. I don't recall exactly, but the early modems might have even used LC filters. The UART (called a distributor in those days) in the Teletype Model 33, which was the workhorse as your photo suggests, was a mechanical affair with one electromagnet and some clutches and gears. It was limited to about 110 bps, so there wasn’t much economic incentive to have higher speed modems.

  • I won’t have the time to code up an entry, but in keeping with Nate’s desire to learn from each other, and to talk about approaches to the problem, I’ll share what wisdom I have. I think the essence of good engineering starts with a clear understanding of the physical principles at work in the system under consideration. At the level of the accelerometer data, things are pretty chaotic with all the shaking and bouncing around and all. But if you take a step back, it is much simpler. The bag and platform is a system with an under-damped (oscillatory) response that stores and dissipates energy. A “hit” is the way that energy is added to the system. The boxer’s job is to periodically keep adding energy to the system at a time and in an amount necessary to keep things going with some degree of consistency. So if you can somehow measure the energy contained in the system, then you can look for periodic increases in the amount of energy. The choice of sensors, mounting locations, and the mathematics of processing the data should be evaluated according to the ability to measure the energy in the system, subject to the other constraints of cost, simplicity, processing capacity, etc. And no matter what it looks like in the video, if there is no significant increase in the energy in the system, it probably shouldn’t be considered a hit.

    So how do you measure energy in the system? Some of the energy is kinetic, 1/2 MV^2: movement of the bag, movement of the platform and brackets, etc. Part of it is potential: compression of the air in the bag, elastic deformation of the platform, height of the bag above rest position. As the system is bouncing and shaking, some of the energy is being converted from one form to another, but except for the boxer's punch, the overall energy will always decrease. Assuming that platform mounted accelerometers are a reasonable choice of sensor, one can integrate the signals from the accelerometers to get a profile of the velocities of the platform in each direction at any given time. By squaring these velocities, we should be able to get a snapshot of the kinetic energy profile of the system at that time, and then look for increases and decreases. I think the solution proposed by #284237 gets at this with the analysis of decreasing peaks and the variable thresholds that account for the current level of energy in the system. Todd's solution similarly looks for an pulse above a decreasing threshold that one could interpret as the decay of energy in the system. Similarly, other measures suggested, velocity of the bag, air pressure in the bag, etc. are also related to the energy content of the system.

    It’s great that Nate is trying to bring the benefits of technology to the sport of boxing!

    One last thought. A good solution might be to find an ultrasonic or radar doppler sensor. It could be mounted on the platform just out of reach of the bag, or on the back post or wall and aimed at the bag. Otherwise, it requires minimal modifications and should work for any speedbag. The output would be an FM signal, responsive to the velocity of the bag, which should be easier to filter and process. It would directly measure the velocity of the primary energy storage element of the system.

  • Hi Nate. I'm coming into this a little late, but it was good to see all the different insights to this (I think very difficult) problem from the respondents. But I have a very basic question. What are you really trying to measure? Is it hits per minute? Can a good boxer really make things go faster or is the speed pretty much determined by the natural resonance of the system? Is it a matter of getting into the rhythm of the system and keeping it consistently going - in other words, are you trying to measure "flubs" where the boxer looses sync and has to start over? Are you trying to measure how accurately the boxer hits the bag, and if so, how much of a glancing blow counts as a miss?

    This is tough because what you are measuring, vibrations of the platform, are pretty far removed from the act of hitting the bag. The initial punch probably transfers some energy to the overall system (platform, brackets, etc.) which makes it respond in its own natural rhythm. Then, as others have observed, the bag appears to hit the platform multiple times per punch, each time making the platform vibrate in its own natural frequencies. To make matters worse, discs like the platform like to vibrate in many complex, non-integer harmonic modes (think of the crash of a cymbal). Then, acceleration, as a second derivative function, tends to amplify noise. On top of that, there is the natural variation in the force of each punch. Finally, there are issues with the sensors themselves, like the occasional "timeouts" in the data, and quantizing and sampling errors. To be able to find the "signal" in all that noise, I would suggest that you need to know as much about the signal as possible - hence my questions.

    Theoretically, I suppose you could measure the flow of a mountain stream by measuring the noise it makes, but it would be better to measure cross section and flow velocity. Similarly, there are probably better ways to measure a boxer's skill if we can decide what that actually means.

    In the meantime, what a great signal processing challenge!

    Some additional thoughts: How about putting two accelerometers on wristbands? Then you could gather all kinds of info that might be of use to a boxer: punch velocity and reach, contacts and contact force, missed punches. And, this could be gathered while sparring or in a match, as well as in the speedbag workouts. Another interesting experiment might be to tape a single accelerometer to your body, like maybe on your sternum. Theoretically, punches and hits should cause corresponding movements in a boxer's body and these may be easier to detect and provide more information than the vibrations of the speedbag platform.

No public wish lists :(