FANCY recalling for your grandchildren the flourishes you once made with the special (oily) rag and foil-like automobile dipstick, lunging (and cajoling) the dipstick into the narrow sleeve to measure the oil level before embarking on the family vacation. Already, a young face looks back at you with disbelief or rolled back eyes because that old dipstick and other fluid measuring devices were replaced way back in the mid-1990s with Tom McEwan's invention--the electronic dipstick.|
One result of a string of spin-off technology developments in the Lawrence Livermore Laser Programs, the electronic dipstick is a device that measures the time it takes for an electrical impulse to reflect from the surface liquid in a container, so fluid level can be calculated. At better than 0.1% accuracy, extremely low power, and a cost of less than ten dollars, applications include measuring fluid levels in cars, oil levels in supertankers, and even corn in a grain elevator. Unlike ultrasound and infrared measurement devices, the electronic dipstick is not tripped up by foam or vapor, extreme temperature or pressure, or corrosive materials. Over time, the technology will make other fluid-level sensing devices obsolete.
Spin-Offs from Digitizing to Measuring
Lawrence Livermore is home to the 100-trillion-watt Nova laser. Developed for nuclear fusion research, the ten-beam pulsed Nova laser generates subnanosecond events that must be accurately recorded. In the late 1980s, Laboratory engineers began to develop a new high-speed data acquisition system to capture the data generated by Nova and the next-generation laser system, the National Ignition Facility. The result was a single-shot transient digitizer--itself a 1993 R&D 100 Award winner described in the April 1994 issue of Energy & Technology Review.
The LLNL transient digitizer, which is the world's fastest, functions as a high-speed oscilloscope combined with a digital-readout device. The instrument records many samples from single electrical events (a brief signal called a "transient"), each lasting only 5 nanoseconds (5 billionths of a second). Compared to competitive products, such as the best oscilloscopes, the transient digitizer is much smaller and more robust, consumes less power, and costs far less.
While developing the transient digitizer, project engineer McEwan had an important insight. The sampling circuits developed for it could form the basis of a sensitive receiver for an extremely small, low-power radar system. What ensued was the development of micropower impulse radar (MIR). (For more MIR information, see January-February 1996 Science & Technology Review.)
The principal MIR components are a transmitter with a pulse generator, a receiver with a pulse detector, timing circuitry, a signal processor, and antennas. The MIR transmitter emits rapid, wideband radar pulses at a nominal rate of 2 million per second. This rate is randomized intentionally to create a distinctive pattern at a single location, which enables the system to recognize its own echo, even with other radars nearby. The components making up the transmitter can send out shortened and sharpened electrical pulses with rise times as short as 50 trillionths of a second (50 picoseconds). The receiver, which uses a pulse-detector circuit, only accepts echoes from objects within a preset distance (round-trip delay time)--from a few centimeters to many tens of meters.
The MIR antenna determines many of the device's operating characteristics. A single-wire monopole antenna only 4 centimeters long is used for standard MIR motion sensors, but larger antenna systems can provide a longer range, greater directionality, and better penetration of some materials such as water, ice, and mud. Currently, the maximum range in air for these low-power devices is about 50 meters. With an omnidirectional antenna, MIR can look for echoes in an invisible radar bubble of adjustable radius surrounding the unit. Directional antennas can aim pulses in a specific direction and add gain to the signals. The transmitter and receiver antennas, for example, may also be separated by an electronic "trip-line" so that targets or intruders crossing the line will trigger a warning. Other geometries, with multiple sensors and overlapping regions of coverage, are also being explored.
The first application McEwan dreamed possible was a burglar alarm, but other popular spin-offs of the MIR technology have been the electronic dipstick, auto safety devices such as an anticrash trigger, a heart monitor that measures muscle contractions instead of electrical impulses, mine-detecting sensors for the military, and corrosion detectors for rebar buried within concrete bridges. Within the next few years, the MIR technology may well become one of the top royalty revenue-generating licenses connected with any U.S. university or national laboratory. So far, over a dozen companies have entered into license agreements with the MIR technology, generating nearly $2 million in licensing agreements with the Laboratory, and soon royalties will add to that amount. To date, most of these licenses (9 of 15) are for the electronic dipstick.
How the Electronic Dipstick Works
The electronic dipstick uses the MIR fast-pulse technology to launch a signal--from a launch plate rather than an antenna--along a single metal wire rather than through air and measures the transit time of reflected electromagnetic pulses from the top of the dipstick down to a liquid surface. The air-liquid boundary is the discontinuity that reflects the pulse; the time difference between a pulse reflection at the top of the dipstick and a reflection at the air-liquid boundary indicates the distance along the line. The liquid level is thus measured from the top of the tank (the dielectric is air, which for all practical purposes does not vary with temperature or vapor content). The transmission line for the dipstick may be configured as microstrip, coaxial cable, or twin lead, whichever suits the application.
The strength of the pulse reflected from the air-liquid boundary and from the subsurface liquid-liquid boundary can be measured. When the liquid has a low relative dielectric constant, such as JP-3 jet fuel, only a portion of the pulse is reflected at the air-liquid boundary, and the remaining portion continues into the liquid until another discontinuity is reached, such as an oil-water boundary or the tank bottom itself. Thus, the dipstick can also provide additional information about conditions within the tank. The photo above shows the entire dipstick assembly with its simple digital output display, although the output could be connected directly to an analog meter. The dipstick's 14-bit, high-resolution output provides continuous readout that is accurate to within 0.1% of the wire's maximum length, and it functions at temperatures from -55 to 85°C (-67 to 185°F). Already, companies have shipped products that use this technology of the future.