Level sensors go floatless

Many of the gas tanks found on new-model cars don't look like gas tanks at all. They have contorted dimensions that make them resemble modern-art sculpture rather than containers meant to hold liquid. Their oddball shape is necessitated by the need to fit in the space left over once vehicle designers have addressed high-priority features.

Measuring gas level in these containers can be problematic. There is often little room for a float, and ordinary measurements often need electronic compensation to get a reading accurate over the entire contents of the tank.

These are among the reasons for a renewed interest in sensors that register fluid levels without using a float. The typical approach to floatless sensing employs either ultrasound or radar frequencies to generate an echo from the top of the fluid as a measure of the level in the tank. Alternatively, capacitive sensors may read the change in capacitance caused by changing fluid levels. All these techniques have drawbacks. The sensor itself and the interfacing electronics for sonar and radar can be expensive because of the high frequencies used. Capacitive methods can be susceptible to nearby metal objects that may throw off readings.

A new technique, however, takes a different approach. A method recently patented by MTS Systems Inc., Cary, N.C. (www.mts.com), uses a sensing element that functions essentially as a signal-transmission line whose properties vary when there is liquid in the electromagnetic field it generates. The beauty of the technique, dubbed SEF for shaped electromagnetic field, is that it will work at relatively low frequencies, generally in the 6 MHz area. This means the electronics for exciting the sensor and for detecting level changes can be relatively inexpensive, consisting of little more than an ordinary op amp and some discrete components.

In a typical scenario, the sensor extends to the bottom of the tank. It emits an electromagnetic field that extends out about 2 cm. Fluid in the tank "pulls" the center frequency transmitted through the SEF sensor element to a degree proportional to the length of the sensor covered by fluid. A typical full-scale frequency change might be 2 MHz for a 6-MHz carrier frequency. Interface electronics converts the frequency difference into a measure of fluid level.

This frequency pulling can be quite sensitive to even tiny changes in fluid level. MTS officials say they have seen a 0.10-mm difference in fluid level produce a measurable change in SEF frequency. Moreover, the technique can handle small volumes of liquid. Any container is fair game whose dimensions are out of the 2-cm field the sensor generates.

The MTS sensor itself also uses materials that are relatively inexpensive. It consists basically of one or more thin-metal conductors etched in special patterns on ordinary circuit-board material.

The basics

The operating principle behind the MTS sensor stems from properties of radio-frequency transmission lines. A signal sent through a transmission line that is flat (the simplest case for an SEF sensor) produces an electromagnetic wave on the conductor surface. The conductor is patterned to increase the length of the path that the wave travels, delaying it slightly.

Introducing a dielectric liquid into the field of this wave, in turn, further reduces the velocity of the electromagnetic field propagating on the sensor surface. This "slowed" electromagnetic field pulls down the frequency of the signal transmitted through the sensor. The higher the dielectric constant of the liquid, the more pronounced the pulling. (Specifically, field propagation is inversely proportional to the square root of the dielectric constant.) Concurrently, the more liquid in the field of the sensor, the more the pulling.

It is the degree of frequency pulling that serves as a measure of how much fluid is in a tank. Interestingly enough, the sensor need not reside inside the tank itself if the tank is a nonmetallic material such as plastic. In such cases an SEF device can be pasted to the outside of the tank and still gauge fluid level accurately. And an SEF sensor can even be configured to reside on the top of the tank, away from the fluid, and still register levels. In the latter two cases, a metal screen that functions like a reflector orients the electromagnetic field toward the fluid to ensure there is an "SEF-like" interaction.

The main sensor element typically consists of metal a few millimeters thick that is laid out in a serpentine, meandering, or similar pattern. The shape of the pattern controls the shape of the surface electromagnetic wave. Consequently, the exact pattern used must vary depending on the dielectric constant of the fluid to be measured. Relatively conductive fluids, for example, may need sensors that look quite different from those for nonconductive fluids.

The need to optimize the sensor pattern for the liquid dielectric and container properties means that all SEF applications need some up-front development work. This takes them out of contention for jobs using 500 sensors/year or less. MTS figures that its new sensing technique generally becomes practical for applications in the 40,000 to 50,000/year range.

The electronics used to detect the SEF signal can take several different configurations. The simplest approach is to detect a difference in signal amplitude between the SEF and original signal, generating a resistive or voltage output for an analog meter. In other applications, the SEF might sit in the feedback loop of a frequency generator as a means of producing pulse-width-modulated outputs.

Dealing with odd shapes

One of the beauties of the SEF approach is how it lends itself to reading out the volume of oddly shaped tanks. A good example is the case of a tank that necks down appreciably at some point and then expands toward its bottom dimension. An SEF sensor designed to handle this situation might use one pattern on the portion of the element that resides in the narrow region, another pattern for elsewhere in the tank. The sensor pattern fashioned this way can produce an output directly proportional to occupied tank volume. In contrast, extra electronics would have to provide a corrective factor (usually by table lookup) to get a readout of volume from an ordinary level sensor.

SEF sensors can be relatively small to handle such instances, though the width of the sensing element is dictated by the sensitivity of the measurement. MTS has so far constructed one device 2 mm thick and 25 mm wide at its smallest dimension.

Finally, applications to date for SEF technology have emerged in a wide variety of areas besides the obvious needs of gasoline and diesel fuel tanks. Other uses in the works include monitoring oil levels in heavy equipment and checking water levels in appliances, medical implements, and agricultural equipment.