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What Can Basic RLC Sensing Tell You?

It's easy to get so enamored of complex, sophisticated measurements that we forget that basic resistance, inductance, and capacitance readings can tell us something — and often, quite a bit. It's like running to the MRI for tests without first checking pulse and blood pressure.

I was reminded of this when I saw a short item in a recent issue of NASA Tech Briefs , “Integrity Sensing With Smart Polymer and Rubber Components on Vehicles,” which looked at standard vehicle tires and electrical capacitance.

Tires, of course, are actually complex structures in both their design and their manufacturing steps; the days of a simple rubber “doughnut” (a torus) are long gone. But once you recognize that tires are layered constructions of steel bands and rubber, you see you have a basic capacitor. That's what the cited study tried to explore and exploit.

By adding some lead wires during fabrication, they were able to measure that capacitance. Even better, by seeing variation in capacitance, they could determine some defects such as improper curing, internal air gaps and voids, layer thickness variations, and even punctures. The article unfortunately did not have hard data or representative numbers (I think they wanted you to buy the full technical paper from the SAE) but there was a mention of picofarads.

Of course, this in-process technique may turn out to be mostly an interesting theoretical exercise, and not practical in mass-production or actual practice (soldering wires to production tires may be a non-starter). Still, it's an interesting way to look at a mechanical, non-electronic product.

It’s not just tires and capacitance that are potential parameters for basic measurements. Many personal-health monitors now check pulse rate and some other exercise-related factors by measuring skin resistance and changes.

What makes these measurements practical is a combination of factors. First, fairly accurate and consistent measurement subsystems are now available at low cost and small size. In many cases, their incremental cost is zero or near zero, because they need only a very small analog front end, relying mostly on a processor and software to extract the reading (quick self test for you, no cheating: How would you do this for at least one, or two, of the three parameters: inductance, resistance, or capacitance?).

Processor-based techniques are viable here since the measurement is at DC or very low frequencies, well within the capability of even a modest microcontroller. Further, the availability of the processor and software means that the measurement function can also invoke a calibration routine, which is often needed to compensate for unit-to-unit variations and offsets in either the electronics or the UUT (unit under test). Even better, you may be able to set up a transducer scheme that lets you sense the L, C, or R parameter with minimal or no direct or invasive connection — which is possible, since there are often “coupling” techniques via EM fields — so you could measure the value without disturbing the normal production, operation, or process flow.

Have you ever used a basic L, R, C measurement to tell you quite a bit about a system's condition or performance, and with low impact or cost?

Can you come up with a better design?

Can you come up with a better design?

And if you have some extra time on your hands, and want to keep those basic-engineering skills and your grey cells sharp, develop a simplified mathematical model of a typical tire and do a “back of the envelope” (see Figure 1) estimate of its capacitance… and let us know what number you come up with.

1 comment on “What Can Basic RLC Sensing Tell You?

  1. RedDerek
    February 6, 2014

    OK, I had a few moments to kill….

    C = e0 * er * A / d

        e0 = 8.8542e-12 F/m

        er = 7 for rubber

        A and d are area and plate separation in meters

    For a P215/65R15, one gets the following information

        215 is the width in mm

        65 is side wall height as percentage of total width; if >200 it is the diameter of the tire; if empty, it is 82

        15 is the wheel diameter

    Now, if one assumes two belts and the separation is quite small – say 1mm, the result for the size above comes out to 0.034 uF.

    I put a small spreadsheet together to crunch the numbers.

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