Achieving analog precision via components and design, or just trim and go

I finally managed to get to a book that has been on my “to read” list for quite a while: “Beyond Measure: The Hidden History of Measurement from Cubits to Quantum Constant” (2022) by James Vincent (Figure 1). It traces the evolution of measurement, its uses, and its motivations, and how measurement has shaped our world, from ancient civilizations to the modern day. On a personal note, I found the first two-thirds of the book a great read, but in my opinion, the last third wandered and meandering off topic. Regardless, it was a worthwhile book overall.

Figure 1 This book provides a fascinating journey through the history of measurement and how different advances combined over the centuries to get us to our present levels. Source: W. W. Norton & Company Inc.

Several chapters deal with the way measurement and the nascent science of metrology were used in two leading manufacturing entities of the early 20^th century: Rolls-Royce and Ford Motor Company, and the manufacturing differences between them.

Before looking at the differences, you have to “reset” your frame of reference and recognize that even low-to-moderate volume production in those days involved a lot of manual fabrication of component parts, as the “mass production” machinery we now take as a given often didn’t exist or could only do rough work.

Rolls-Royce made (and still makes) fine motor cars, of course. Much of their quality was not just in the finish and accessories; the car was entirely mechanical except for the ignition system. They featured a finely crafted and tuned powertrain. It’s not a myth that you could balance a filled wine glass on the hood (bonnet) while the engine was running and not see any ripples in the liquid’s surface. Furthermore, you could barely hear the engine at all at a time when cars were fairly noisy.

They achieved this level of performance using careful and laborious manual adjustments, trimming, filing, and balancing of the appropriate components to achieve that near-perfect balance and operation. Clearly, this was a time-consuming process requiring skilled and experienced craftspeople. It was “mass production” only in terms of volume, but not in terms of production flow as we understand it today.

In contrast, Henry Fiord focused on mass production with interchangeable parts that would work to design objective immediately when assembled. Doing so required advances in measurement of the components at Ford’s factory to weed out incoming substandard parts and statistical analysis of quality, conformance, and deviations. Ford also sent specialists to suppliers’ factories to improve both their production processes and their own metrology.

Those were the days

Of course, those were different times in terms of industrial production. When Wright brothers needed a gasoline engine for the 1903 Flyer, few “standard” engine choices were available, and none came close to their size, weight, and output-power needs.

So, their in-house mechanic Charlie Taylor machined an aluminum engine block, fabricated most parts, and assembled an engine (1903 Wright Engine) in just six weeks using a drill press and a lathe; it produced 12 horsepower, 50% above the 8 horsepower that their calculations indicated they needed (Figure 2).

Figure 2 Perhaps in an ultimate do-it-yourself project, Charlie Taylor, mechanic for Wright brothers, machined the aluminum engine block and fabricated most of the parts and then assembled the complete engine in six weeks (reportedly working only from rough sketches). Source: Wright Brothers Aeroplane Company

Which approach is better—fine adjusting and trims, or use of a better design and superior components? There’s little doubt that the “quality by components” approach is the better tactic in today’s world where even customized cars make use of many off-the-shelf parts.

Moreover, the required volume for a successful car-production line mandates avoiding hand-tuning of individual vehicles to make their components plug-and-play properly. Even Rolls-Royce now uses the Ford approach, of course; the alternative is impractical for modern vehicles except for special trim and accessories.

Single unit “perfection” uses both approaches

In some cases, both calibration and use of better topology and superior components combine for a unique design. Not surprisingly, a classic example is one of the first EDN articles by late analog-design genius Jim Williams, “This 30-ppm scale proves that analog designs aren’t dead yet”. Yes, I have cited it in previous blogs, and that’s no accident (Figure 3).

Figure 3 This 1976 EDN article by Jim Williams set a standard for analog signal-chain technical expertise and insight that has rarely been equaled. Source: EDN

In the article, he describes his step-by-step design concept and fabrication process for a portable weigh scale that would offer 0.02% absolute accuracy (0.01 lb over a 300-pound range). Yet, it would never need adjustment to be put into use. Even though this article is nearly 50 years old, it still has relevant lessons for our very different world.

I believe that Jim did a follow-up article about 20 years later, where he revisited and upgraded that design using newer components, but I can’t find it online.

Today’s requirements were unimaginable—until recently

Use of in-process calibration is advancing due to techniques such as the use of laser-based interferometry. For example, the positional accuracy of the carriage, which moves over the wafer, needs to be in the sub-micrometer range.

While this level of performance can be achieved with friction-free air bearings, they cannot be used in extreme-ultraviolet (EUV) systems since those operate in an ultravacuum environment. Instead, high-performance mechanical bearings must be used, even though they are inferior to air bearings.

There are micrometer-level errors in the x-axis and y-axis, and the two axes are also not perfectly orthogonal, resulting in a system-level error typically greater than several micrometers across the 300 × 300-mm plane. To compensate, manufacturers add interferometry-based calibration of the mechanical positioning systems to determine the error topography of a mechanical platform.

For example, with a 300-mm wafer, the grid is scanned in 10-mm steps, and the interferometer determines the actual position. This value is compared against the motion-encoder value to determine a corrective offset. After this mapping, the system accuracy is improved by a factor of 10 and can achieve an absolute accuracy of better than 0.5 µm in the x-y plane

Maybe too smart?

Of course, there are times when you can be a little too clever in the selection of components when working to improve system performance. Many years ago, I worked for a company making controllers for large machines, and there was one circuit function that needed coarse adjustment for changes in ambient temperature. The obvious way would have been to use a thermistor or a similar component in the circuit.

But our lead designer—a circuit genius by any measure—had a “better” idea. Since the design used a lot of cheap, 100-kΩ pullup resistors with poor temperature coefficient, he decided to use one of those instead of the thermistor in the stabilization loop, as they were already on the bill of materials. The bench prototype and pilot-run units worked as expected, but the regular production units had poor performance.

Long story short: our brilliant designer had based circuit stabilization on the deliberate poor tempco of these re-purposed pull-up resistors and associated loop dynamic range. However, our in-house purchasing agent got a good deal on some resistors of the same value and size, but with a much tighter tempco. Additionally, getting a better component that was functionally and physically identical for less money seemed like a win-win.

That was fine for the pull-up role, but it meant that the transfer function of temperature to resistance was severely compressed. Identifying that problem took a lot of aggravation and time.

What’s your preferred approach to achieving a high-precision, accurate, stable analog-signal chain and front-end? Have you used both methods, or are you inherently partial to one over the other? Why?

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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