标签: calibration

A bicycle's front-and-rear derailleurs are an amazing mix of effectiveness, light weight, reliability, and tightly focused function, but they come with a frustrating initialisation phase. I was reminded of this a few days ago, when the cable that shifts the front derailleur on my bike snapped from wear. Fortunately, I had a replacement cable available so I could replace it; I had done this same fix several years ago and knew what was involved, but I had forgotten the fine points.   Replacing the cable is pretty quick and easy, less than five minutes -- but adjusting it so it shifts properly among the three front chainwheels takes time. In principle, it should be simple because there are only three (or sometimes four) parameters to adjust: the installed cable length; the high-shift limit screw; and the low-shift limit screw; many bicycles also have a "fine" adjustment for the cable length on the shift-lever itself.   There is a well defined procedure and sequence for making the adjustments to get the proper shifting performance, and there are many online videos describing it. Some are brief, around five minutes, but a few address the reality and run 30 minutes. The reason is that after the first pass through, you have to go back, tweak the screws, try again, tweak again, and so on: It's an iterative prices with interactive tweaks. Finding the precise combination of screw positions to get it all working right is time-consuming and aggravating, as improving one part of the derailleur's shifting usually degrades another part. (It's complicated by the fact that you want the shifter cage to overshoot slightly to ensure the chain snaps into position, and then settle back a little.   I don't fault the design of the derailleur -- it's a refined, elegant design that meets many conflicting objectives amazingly well. Certainly, it's not just mechanical designs that suffer from this malady. Many years ago, while involved in the design and implementation of an analog front end, we had a very senior designer on the team whose hallmark was elegance in circuitry. His designs almost always worked the first time (very impressive), and he also strived to minimize the number of parts on the BOM.   In this design, he got the number of mechanical trimpots down to about five, but in doing so, their adjustment was interactive (Figure 1) and recursive. While the whole BOM was shorter and less expensive, it was a time-consuming hassle to adjust the resultant circuit at the prototype stage to meet specs, and even more so for the production people doing the pilot run.   Figure 1 : When adjustments interact with each other, calibrating the system requires many iterative loops.   Pretty soon, the very legitimate complaints from the manufacturing folks were heard loud and clear. Management assigned an experienced and pragmatic engineer to redesign the front end. His design rearranged the circuit and needed two more trimpots, but none of their adjustments interacted (Figure 2). You could do it all in a couple of minutes, just by following a specific linear, non-recursive sequence. Problem solved, at modest cost in components.   Figure 2: When adjustments don't interact, calibration requires only one pass through the steps.   Now, of course, the interactive calibration and trim situation would be an entirely different story entirely. If physical circuit trimming were needed, designers would likely use digital pots (digitpots) that could be calibrated by an algorithm in a test fixture. Even if there were interrelated trims, the software could sweep through all the combinations to find the one that worked. Even more likely, the entire trim and calibration process would be done using software that would measure and record desired versus actual results at various points of the circuit's range, then put calibration factors or equations into memory to be used by the operational software as needed.   Have you ever been involved with the designing a circuit or function with interactive adjustments requiring integrative calibration steps? Was this an unavoidable situation, or was it due to lack of insight or concern? Or have you ever had to deal with a design that had this problem?

I am a young'un, being only in my early 30s. That is why I wasn't familiar with the VoltOhmyst. I have to be honest, I bought this thing purely because the name amused me. It was only $15, so even if it is only ever an interesting conversation piece, it was worth it.   The RCA VoltOhmyst wv-97A The 1953 RCA WV-97A VoltOhmyst is a meter that is considered "service-grade." This means it works well enough, but it isn't a precision instrument.   Senior!   The top two dials are fine adjustment, while the bottom two switch modes.   This unit has been used for sure. The worn face is a testament to that. Let's open this baby up!   Simple and plain construction. I like the integrated power cord storage.   ID sticker located on the inside of the case.     The internals of the unit removed from the case. You can see the calibration system here on the back.     Looking up at the bottom of the unit. A bank of resistors in wax sleeves are visible on the mode dial. The precision resistors are seen here encased in wax sleeves. These allow for you to switch between different circuits as you switch modes.   Looking downward at the top of the unit. I doubt that this battery is from 1953.   Look closely towards the centre-right of this image. There is a yellow wire that has been clipped. This combined with the newer battery being soldered in place and some random electrical tape make me believe this has been repaired at some point in the past.       Soldering iron burn on the brown wire?     Fine adjustment on one of the modes. I'd say this is an interesting piece of history to keep around. I don't know that I have enough of a use for it to bother replacing the caps so that I could fire it up. I was able to find the manual for this unit. Like most manuals of the time, it includes calibration instructions as well as a full schematic. You can download it here if you want, but it is a rather large pdf file at almost 30MB. That should help if I do decide to make any repairs! Caleb Kraft, Chief Community Editor, EE Times  

This article about the special type of screwdriver needed to open the latest iPhone reminded me of a similar incident from my distant past. Many years ago, I was "tasked" with opening up a handheld Nintendo Gameboy . No big deal, I figured , until I looked closely at the Phillips-like tiny screws used. I say "Phillips-like" because the screw-head had the same conical flare, but with three flutes instead of the four of a conventional Philips screw. I assumed this was done to prevent casual hackers from getting into the box. What to do? We could have purchased the required screwdriver by mail (I found out later that it was called a TriWing), but that would take a few days to arrive, and we were impatient (and maybe a little cheap, too). Plus, a "real" engineer doesn't let lack of tools stop him or her, but instead views this as a challenge. We took a standard Philips screwdriver with the appropriate body diameter, and cut the tip off with a hacksaw. Then we used a bench grinder to form the overall "conical" screwdriver-tip shape. Finally, we used a Dremel hand grinder with an abrasive disc to cut away material, leaving the three flutes (wings) we needed; the tool did the job. The whole process took about two hours, and we were quite satisfied and pleased with ourselves. And why shouldn't we have been? We had seen a problem, and improvised a solution: we had done real engineering and tool-making. (Isn't tool-making one of the factors which distinguishes humans from animals? That's a topic for another time.) That final point is what really struck home. Way, way back in the day, engineers and scientists often made their own instruments, tools, jigs, and fixtures. In fact, they also often made the tools needed to make the jigs and fixtures—hard to believe these days, but true. For example, in the recent excellent book "World in the Balance: The Historic Quest for an Absolute System of Measurement" by Robert Crease, there's a long section on the vital role which the diffraction grating has played in advances in physics in general, and precision metrology and standards in particular.   One thing which struck me was how the leading researchers, often amateurs working on their own, not only ruled their own gratings, but built the ruling engines needed, or enhanced available engines built by other experimenters to reduce their already tiny imperfections. Even earlier, John Harrison, solo maker of the legendary clock of the mid-1700s, not only had to cut his own high-precision gears, but he also had to make the gear-cutting machines (see Dava Sobel's Longitude ). Go to any high-end science, industry, and technology museum, such as Science Museum in London, the Galileo Museum of the History of Science in Florence, or the Collection of Historical Scientific Instruments at Harvard University, and you'll be humbled not only by the instruments themselves, but by the realisation that many were not simply bought by their users, but hand-made by them, often along with the tools and calibration tooling. In addition to tools, engineers often have to build fixtures and jigs for prototype and production test, assessment, and evaluation. When you are pushing the product envelope—whether just slightly or a lot—what you need may not be available or affordable, or is needed right away, so the ability to make that special fixture or jig is just as important to success as a good design and execution of the design. What's the most interesting, challenging, or clever tool, fixture, or jig that you have seen, or made yourself? Was there a simple, clever one that stands out in your recollection?

Reading this blog from DesignCon about the special type of screwdriver needed to open the latest iPhone reminded me of a similar incident from my distant past. Many years ago, I was "tasked" with opening up a handheld Nintendo Gameboy . No big deal, I figured , until I looked closely at the Phillips-like tiny screws used. I say "Phillips-like" because the screw-head had the same conical flare, but with three flutes instead of the four of a conventional Philips screw. I assumed this was done to prevent casual hackers from getting into the box. What to do? We could have purchased the required screwdriver by mail (I found out later that it was called a TriWing), but that would take a few days to arrive, and we were impatient (and maybe a little cheap, too). Plus, a "real" engineer doesn't let lack of tools stop him or her, but instead views this as a challenge. We took a standard Philips screwdriver with the appropriate body diameter, and cut the tip off with a hacksaw. Then we used a bench grinder to form the overall "conical" screwdriver-tip shape. Finally, we used a Dremel hand grinder with an abrasive disc to cut away material, leaving the three flutes (wings) we needed; the tool did the job. The whole process took about two hours, and we were quite satisfied and pleased with ourselves. And why shouldn't we have been? We had seen a problem, and improvised a solution: we had done real engineering and tool-making. (Isn't tool-making one of the factors which distinguishes humans from animals? That's a topic for another time.) That final point is what really struck home. Way, way back in the day, engineers and scientists often made their own instruments, tools, jigs, and fixtures. In fact, they also often made the tools needed to make the jigs and fixtures—hard to believe these days, but true. For example, in the recent excellent book "World in the Balance: The Historic Quest for an Absolute System of Measurement" by Robert Crease, there's a long section on the vital role which the diffraction grating has played in advances in physics in general, and precision metrology and standards in particular (also see " The search for ever-better primary standards is a captivating story ").   One thing which struck me was how the leading researchers, often amateurs working on their own, not only ruled their own gratings, but built the ruling engines needed, or enhanced available engines built by other experimenters to reduce their already tiny imperfections. Even earlier, John Harrison, solo maker of the legendary clock of the mid-1700s, not only had to cut his own high-precision gears, but he also had to make the gear-cutting machines (see Dava Sobel's Longitude ). Go to any high-end science, industry, and technology museum, such as Science Museum in London, the Galileo Museum of the History of Science in Florence, or the Collection of Historical Scientific Instruments at Harvard University, and you'll be humbled not only by the instruments themselves, but by the realisation that many were not simply bought by their users, but hand-made by them, often along with the tools and calibration tooling. In addition to tools, engineers often have to build fixtures and jigs for prototype and production test, assessment, and evaluation. When you are pushing the product envelope—whether just slightly or a lot—what you need may not be available or affordable, or is needed right away, so the ability to make that special fixture or jig is just as important to success as a good design and execution of the design. What's the most interesting, challenging, or clever tool, fixture, or jig that you have seen, or made yourself? Was there a simple, clever one that stands out in your recollection?