Until quite recently, accurate measurement of the level of fluid in a container was remotely obtained using systems that required cumbersome cabling and hardware.
Now, Senior Scientist Stan Woodard of NASA's Langley Research Center in Hampton, Virginia, and Bryant Taylor, an ATK (an aerospace and defence company ) Space Division electronics technician at Langley, have created a wireless fluid-level measurement system.
It eliminates the need for any electrical component or circuit to be in contact with combustible fuel or fuel vapours. To date, designing electronics and electrical devices has had every component dependent upon systems using closed circuits and electrical connections.
The wireless measurement system is simple to use and install. It is already in use by commercial and recreational boaters.
Evolving since 2000
This fundamental technology could be used to design an unlimited number of sensors for a variety of measurements. This technology has been evolving at NASA since about 2000.
There are three critical components: 1) the method of powering and interrogating the sensors 2) design of the sensors as inductive-capacitive resonant circuits and 3) developing the sensors as open-circuits thereby eliminating the need to have electrical connections to the sensor and to make the sensor.
Power is supplied to the sensor via a series of harmonic magnetic fields (magnetic wave forms whose frequencies are integer multiples of the frequency of the first wave) from the interrogating system (Faraday induction). The sensor is a resonant capacitance-inductance circuit. Because the sensor is a resonant capacitance-inductance circuit, it will respond with its own magnetic field once powered.
As the frequency of the harmonic from the interrogating system gets closer to its resonant frequency, the responding field amplitude will grow.
The series of increasing oscillating magnetic field harmonics is transmitted from the interrogating circuit's antenna. Each harmonic corresponds to a discrete measurement value. The resonant frequency of the sensor varies according to the level of the fluid in which it is immersed. At each harmonic, the sensor is electrically excited as a result of Faraday induction.
Once electrically active, the sensor responds with a harmonic magnetic field. The resonant frequency corresponds to the amount of fluid that the sensor plates have been exposed to. The antenna ceases transmission so that the sensor magnetic field response can be received by it or another antenna.
When the same antenna is used, it must be switched from a transmitting antenna to a receiving antenna. The sensor response to each transmitted harmonic is compared to the response created by the prior transmitted harmonic. If the response amplitude is higher than the previous one, the new amplitude and the frequency of the harmonic that produced it are stored. The next harmonic is then transmitted by the antenna. The repetition of “transmit-receive-compare” is continued until the transmitted frequency that produces the highest sensor response amplitude is identified. The frequency producing the highest response is the frequency nearest in value to the sensor's resonant frequency.
Resonant frequency
Resonant frequency is the frequency of the harmonic produced when the sensor circuit's inductive reactance and capacitive reactance cancel each other out. The amplitude of the waveform produced is the highest in value. This frequency is then used to calibrate the measured value of the fluid in which the sensor is immersed. In this way a complete set of values is obtained for all levels of the fluid.
This method of powering the sensor and acquiring the measurement from the sensor does not require a physical connection to a power source or to data acquisition hardware. The method is applicable to any passive inductive–capacitive resonant circuit such as the fluid-level sensors discussed in this article.
When multiple sensors are used for example, in a situation where there are more than one fluid tank each with its own sensor, how does the interrogating system know which response is from which sensor?
Answering this query, Mr. Stan Woodard noted in an email to this Correspondent that each sensor has a dedicated range of frequencies (measurement bands). The sensors are designed such that their frequency ranges do not overlap but are within the antenna frequency spectrum. Multiple sensors can be interrogated using a single data acquisition channel when a switching transmitting/receiving antenna is used.
The sensor of this device requires two components electrically connected. One component is a capacitor that can be electroplates. The second component is an inductor that stores and releases magnetic energy at a harmonic rate being that of the resonant frequency of the circuit formed by the inductor electrically connected to the capacitor. When the capacitor is immersed in a fluid, its capacitance will change proportional to the area of the capacitor plates exposed to the fluid. If the width of the electroplates is constant the sensor's response frequency is dependent upon the level of fluid that the capacitor is exposed to.
Using mathematics it can be shown that with a completely encased sensor circuit in a harsh fluid environment, (such as an acid) one can determine the sensor response with respect to the fractional level that the sensor is immersed into the fluid by knowing only the response when the container is empty, and the response when the container is full.
