Non-contact measurement & inspection

Figure 7, to create space for the pump motor. This introduces a measurement nonlinearity that could be easily corrected in the system firmware if necessary.

Figure 5. Discrete balun and terminated transmission line, before affixing to the tank.

affixed to an empty tank due to the detuning effect of the tank wall material as an additional dielectric layer on the transmission line.

EXAMPLE TEST RESULTS Figure 7 shows a complete test setup. The transmission line is affixed to the side of a tank, and the tank has provision for filling and draining in a controlled manner. Analog Devices’ evaluation kit DC2847A is used

to easily read ADL5920 reflectometer measurement results. This evaluation kit includes a mixed-signal MCU to read the forward and reflected detector analog voltages. PC software will automatically load and display results in graphical format vs. time. Return loss is easily calculated as the difference between forward and reflected power measurements. Figure 7 shows the complete test setup for the design example. In this design example, fluid-level conditions are

established by activating a pump on one of the two tanks. Mass flow rate is relatively constant when a pump is running, so ideally the fluid level in the tank ramps linearly with respect to time. In practice, the tank cross-section is not fully consistent from top to bottom. Figure 8 shows the test results as fluid level goes from full to empty. As fluid is pumped out of the tank, forward power holds constant, while reflected power falls relatively linearly. At t = 33 seconds, a visible change in slope

occurs. This is believed to be due to the design of the tank. The cross-sectional area of the tank is reduced at the lower end of the tank, as seen in

CALIBRATION For best accuracy, reflectometer calibration is required. Calibration will correct for the manufacturing variation of the RF detectors within the reflectometer— namely slope and

intercept. The DC2847A evaluation kit supports individual calibration, as seen in Figure 8. At a higher level, the fluid level vs. return loss also needs calibration. This can be due to the following sources of uncertainty:

Manufacturing variation of the distance between transmission line and tank wall.

Variation in tank wall thickness.

Fluid and/or tank wall dielectric properties could vary vs. temperature.

Systematic nonlinearities may exist, for example, the change in slope observed in Figure 8. If linear interpolation is used, a three-or-more point calibration becomes necessary in this case.

Figure 8. Example test results vs. fluid level. Fluid-level measurement is linear and monotonic, with an exception due to tank design as noted in the text.

APPLICATION EXTENSIONS For some applications, the basic contactless fluid- level measurement principle can be extended in several ways. For example:

The measurement may be performed at low duty cycle to conserve power.

If the fluid level is held constant, return loss measurement may correlate to another fluid property of interest; for example, viscosity or pH.

Each application is unique. For example, there are some techniques that might offer better accuracy up at the top end of the scale, compared to the bottom end, or vice versa, depending on the application.

If the tank is metallic, the transmission line will need to go inside the tank. Depending on the application, the transmission line may be submersed.

Figure 7. Complete test setup for the design example.

All calibration coefficients will typically be stored in the system’s nonvolatile memory, which could be unused code space in an embedded processor application, or a dedicated nonvolatile memory device.

FLUID-LEVEL MEASUREMENT LIMITATIONS Directivity of any reflectometer is a key specification. Neglecting balun losses, when the transmission line is precisely

terminated with its own ZO, reflected power goes to zero, and the reflectometer measures its own directivity specification. The higher the directivity specification, the better the ability of the reflectometer to accurately separate the magnitudes of incident and reflected waves. For the ADL5920, directivity is

Figure 6. Example design showing the transmission line affixed to the side of the tank.


specified as 20 dB typical at 1 GHz, increasing to approximately 43 dB typical at 100 MHz or lower. This makes ADL5920 well suited for fluid-level measurements where tank height is about 30 mm or higher (see Figure 3).

Measurements at more than one RF power level can help identify if external RF interference is a contributing error. Many single-chip PLL devices support this feature, which becomes a confidence test for the system, or a self-test.

Transmission line sensors on two or four sides of the tank can compensate for container tilt along one axis or two axes, respectively.

If fluid-level threshold measurement is the goal, one or more shorter transmission lines operated at higher frequency can be a good solution.

CONCLUSION The development of a single-chip reflectometer device such as the ADL5920 brings with it new types of applications, such as fluid-level instrumentation. Eliminating moving parts, such as a mechanical float that has been used for years, will result in a huge reliability increase. Oil- and fuel-level monitoring may also be possible, opening up many new industrial and automotive applications.

Analog Devices June 2020 Instrumentation Monthly

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