11
Dec
10

How Heavy is a Lightning Bolt?

I am about to embark on a new phase of my Electronics Design Engineering career, with Mid-Continent Instruments. My very lengthy hiatus and search for employment gave me the opportunity to reflect on my work. This is one small example for which I can feel that pride is justifiable.

Cardinal Scale Company, while I worked there, was faced with significant new competition for truck scales. Toledo Scale (Now Mettler-Toledo(TM)) had developed one of the first ‘digital’ load cells. Load cells are the transducers which convert weight into electrical signals which can be processed by instruments. Every highway weigh scale uses them. The signals are so tiny – susceptible to error if anything gets between the load cells and the instruments – that no devices can be connected to them which provide effective protection from electrical upsets (such as lightning strikes) without degrading the signal. Digital load cells place the instrumentation within the cell, and communicate to other instruments with high-level computer signals.

Cardinal wanted to be able to quote a digital product when Toledo was a competing bidder for a truck scale. Cardinal’s load cell consultant was confident that he could have no confidence in Hillbilly Engineering, so he asked a couple of fellows he knew at Analog Devices (a major electronics company) to take a look. The two, a Manager and a Senior Engineer, visited and were (since the consultant wasn’t in town then) shown around by me, Big R, my Engineering buddy, and Brownie, our excellent Mechanical Engineer. We toured the facility, talked shop, and went to lunch. We talked about the digital load cell project and some more electronics esoterica. After lunch, Big R stopped me in the Cardinal parking lot and asked, “Jim, did you hear him (the Senior Engineer) tell you that he left his PORSCHE at the airport?” Sometimes I simply need to be slapped – the Engineer had been dropping hints to me. I asked him, “So, how much is a good Engineer worth?” His reply made the entire visit memorable: “A good Engineer is worth a king’s ransom.” I wasn’t smart enough to pursue that bounty at that time.

Analog Devices declined the opportunity to design the load cell. Despite our lack of competence to do so previously, we were mysteriously chosen to design the new Cardinal product. Big R would handle testing (which was rigorous but achievable, because he built the test equipment), Brownie would design the core mechanics, and I had the electronics. The project went as smoothly as could be realistically expected, and the new Smartcell(TM) – a trademarked name which I created – was excellent and gained ‘type certification’ for use in commercial weighing. The excellence of the Smartcell was to be demonstrated very soon.

Truck scales, typically used in outdoor installations, are subject to ugly weather. That includes lightning. A lightning strike near an analog-load-cell scale sometimes destroys the load cells. The flimsy protection which can be provided for analog cells is, itself, often damaged along with the delicate transducers welded inside the cells’ stainless-steel housings. My design of the Smartcell provided the robust protection that digital technology allowed. It was difficult, however, to test that protection during design. It’s hard to simulate lightning on a cheap budget. Maybe I should have asked Big R to make the tester instead of doing it myself.

Our first installation, at a landfill in Florida (Lightning Capital of the World!), had a close strike about a month after installation. Every piece of equipment in the scale house was destroyed. One Smartcell was damaged, and it was returned to Cardinal for the attention of my failure analysis skills.

The Smartcell electronics were inside a housing of 1/4 inch stainless-steel pipe. External connections were made via a hermetically sealed, welded feedthru. Transient protection was in an external extension pipe with a press-in plug. Removal of the plug revealed a mess. I extracted the detritus of an electrical calamity, and installed new transient protection components.

Big R tested the repaired Smartcell. It had been restored to flawless performance, even remaining precisely calibrated.

There were many Engineering decisions which resulted in the transient protection being both very simple and (obviously!) robust. It was a ‘no-brainer’, once some serious thinking was applied. The Smartcell had several other innovations which may appear in a future blog.

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1 Response to “How Heavy is a Lightning Bolt?”


  1. December 12, 2010 at 11:11 am

    Jim,

    I find your post interesting because I had some tangential experience with load-cell measurement in my second career as a battery engineer. We made nickel-hydrogen battery cells (to power satellites while in the earth’s shadow) which operate in a thin, sealed nickel alloy shell. When charged the cells produce an internal hydrogen pressure that is linearly proportional to the state of charge (up to 1,200 psi). We used strain gauges epoxied to the shell to read the pressure so that the state of charge can be relayed to the operators on the ground.

    When we first started it was tricky getting the tiny gauges to adhere properly to the hemispherical dome of the cells – they are baked on and later calibrated to each cell individually. I learned at that time through literature what a wide variety of remote sensing this kind of technology applies to. The world of engineering is amazingly complex, driven by understanding science, mixed with cut-and-try, and extendable by the imagination.

    Your post should be informative to the laity on the vast possibilities of engineering and as a cautionary tale to management as to the value of patience. And, the worth of Engineers. Thanks. 🙂

    Jim W.


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All original material Copyright James R. Stone 2010, except where specifically noted. Some images licensed under Creative Commons, or GNU Free Documentation License, or unlicensed and public domain.

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