Electric field sensor

According to embodiments, an electric field sensor having a sensor electrode is constructed of an electrically conductive material and having one or more outwardly protruding pillars. A screen electrode overlies the sensor electrode and has one or more openings which register with the one or more pillars on the sensor electrode. At least one piezoelectric actuator is connected to the screen electrode so that, when excited by a voltage signal, the piezoelectric actuator modulates the screen electrode toward and away from the sensor electrode at the frequency of the periodic voltage signal. An output circuit configured to detect a voltage, a current output, or both, between the sensor electrode and the screen electrode which is proportional in magnitude to the strength of the electric field.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to electric field sensors and, more particularly, to an electric field sensor with piezoelectric actuated electrodes.

II. Description of Related Art

Low frequency electric field sensors have found numerous industrial, medical, scientific, and military uses. The field sources typically measured in these applications are electrostatically charged particles or surfaces, atmospheric electricity, or active voltage/charge sources such as brainwaves, printed circuit components, and power lines. While there are a variety of means of measuring an ambient electric field, those electric field sensors based on galvanic measurement principles offer the advantage of relative simplicity due primarily to the use of standard metallic conductor electrodes. In the discussion that follows, “galvanic measurement” means that a voltage or current is induced on the electrodes directly by the electric field, and is measured with a sensitive preamplifier.

There are two basic galvanic measurement techniques for E-field measurements in air. High impedance potential gradiometers require special attention to leakage currents and extremely high input impedances. Only recently have mass produced commercially available free space electric potential sensors become available. By comparison, low impedance charge induction (D-Dot) sensors may be constructed from simple and inexpensive off-the-shelf components such as metal plates and operational amplifier integrated circuits.

The low cost and simplicity makes D-Dot sensors attractive, but they have limitations and applications regarding both high sensitivity at low frequencies and small size because their sensitivity is proportional to both the rate of change of the ambient field and the electrode area used to collect the charge. Field mills have employed mechanical chopping of the E-field, typically at tens of hertz, and this modulation permits the ambient DC E-field to be sensed using an induction probe. However, the added size, weight, power, cost, and complexity of bulky drive motors, spinning shafts, and electromagnetic interference shielding of noisy drive components makes these field mills unappealing for small mobile applications or sensor arrays.

MEMS-technology offers the promise of low-cost production due to wafer-level processing. One problem with the previously known traditional MEMS E-field sensors lies in the unwanted interference signals generated by the high voltage electrostatic drive electronics. Thermal drives have been used, but these devices suffer from high power consumption.

A still further disadvantage of the previously known D-Dot sensors is that the output signal from the sensors often includes a high proportion of common mode signal. Thus, it is necessary to account for the common mode signals in order to obtain an accurate measurement of the magnitude of the E-field.

SUMMARY OF THE PRESENT INVENTION

The present invention provides an electric field sensor that overcomes all of the above-mentioned disadvantages of the previously known devices. In one embodiment, the electric field sensor of the present invention includes a substrate, made of any suitable material such as a silicon based material, which supports a sensor electrode constructed of an electrically conductive material. One or more pillars protrude outwardly from the sensor electrode, for instance, in a spaced-apart manner. The pillar(s) are constructed of electrically conductive material and can be formed as one piece with the sensor electrode.

A screen electrode overlies the sensor electrode. The screen electrode includes one or more openings so as to register with the one or more pillars, i.e., so that each pillar can extend through an associated opening on the screen electrode as the position of the screen electrode varies relative to the sensor electrode. The screen electrode is also constructed of an electrically conductive material.

At least one, and preferably several spaced piezoelectric actuators are connected between the substrate and the screen electrode. These piezoelectric actuators are powered or actuated by a voltage signal, such as a periodic voltage signal, so that the piezoelectric actuators modulate and displace the screen electrode toward and away from the sensor electrode at a frequency of the voltage signal. Consequently, as the piezoelectric actuators modulate the position of the screen electrode, the screen electrode moves from an upper position, in which the pillars are positioned beneath the screen electrode, and a lower position in which the pillars extend through the openings in the screen electrode. The use of piezoelectric actuators permits operation at low voltages, making this device easier to integrate with other low-power sensor circuitry. Moreover, the drive fields are contained in the piezoelectric material, so the drive fields are much lower than with conventional (electrostatic comb drive) techniques. And the piezoelectric actuators require very low power to drive as compared to thermal technique.

A circuit is electrically connected to both the screen electrode, which is electrically grounded, and the sensor electrode is configured to detect a voltage, a current output, or both, from the sensor electrode. In some embodiments, the circuit includes an operational amplifier having one input electrically connected to the screen electrode, its second input electrically connected to the sensor electrode, and a resistor electrically connected between the sensor electrode and an output from the operational amplifier so that the voltage output from the operational amplifier varies directly proportionally with the current from the sensor electrode.

A source of the electric signal modulates the position of the screen electrode relative to the sensor electrode preferably at a resonant frequency. Thus, when the electric field sensor of the present invention is positioned within an electric field, the current output from the sensor electrode would be lowest when the screen electrode is most spaced from the sensor electrode and effectively screening the sensor electrode from the electric field. Conversely, the current output from the sensor electrode will be maximum when the screen electrode is modulated to its closest position to the sensor electrode. When this occurs, the electric field acts directly on the pillars extending through the screen electrode thus inducing a current in the screen electrode which increases proportionally to the magnitude of the electric field. Consequently, the voltage output from the operational amplifier in the output circuit is proportional to the strength of the electric field.

At certain frequencies, the screen enters a mode of differential resonance. When this occurs, one area of the screen electrode moves in a direction toward the sensor electrode while a second area of the screen electrode moves away from the sensor electrode, and vice versa. Consequently, by splitting the sensor electrode into two separate sensor subelectrodes with one subelectrode aligned with the first screen electrode area and the second subelectrode aligned with the second screen electrode area, the differential outputs from the two subsensors are essentially doubled from that of a single electrode. Furthermore, by providing an output circuit for each sensor subelectrode, and combining those outputs together, the combined output of the electric field sensor effectively eliminates common mode signals.

In another embodiment of the invention, the sensor electrode includes a base having one or more fingers extending outwardly from one side of the base and across the top of the substrate, for instance, in a spaced apart and parallel manner. Similarly, the screen electrode also includes a base having one or more fingers which generally overlie the substrate and are registered, i.e., interdigitally positioned with respect to one or more fingers on the sensor electrode. A piezoelectric actuator is then coupled to at least the screen electrode, and preferably a second piezoelectric actuator coupled to the sensor electrode so that, when excited by an electric voltage signal, each piezoelectric actuator tilts its associated electrode relative to the base. When these two piezoelectric actuators are excited by a periodic voltage signal the sensor electrode and screen electrode pivot in opposite pivotal directions in synchronism with each other.

Thus, when the screen electrode is pivoted above the sensor electrode, the screen electrode effectively screens the sensor electrode from the electric field thus minimizing the signal output from the sensor electrode. Conversely, when the sensor electrode fingers are positioned above the screen electrode fingers, the current output from the sensor electrode is maximized. The screen electrode and sensor electrode are electrically connected to the output circuit in the previously described fashion.

With reference first toFIGS. 1-3, a first embodiment of the electric field sensor10according to the present invention is shown. The electric field sensor10includes a sensor electrode12constructed of an electrically conductive material and supported on a substrate14. The substrate14may be constructed of any conventional material, such as a silicon-based material.

The sensor electrode12includes a planar base16overlying and supported by the substrate14. A plurality of spaced apart pillars18protrude outwardly from the base16so that the pillars18are spaced apart to each other, such as in a parallel or an orthogonal arrangement. The pillars18and sensor base16may be all of a one piece construction and may be formed, for instance, as a part of a Micro-Electro-Mechanical System (MEMS) processing step. While a plurality of pillars18is illustrated here, it will be appreciated that, in other embodiments, the electric field sensor10may include only a single pillar18.

A screen electrode20, also constructed of an electrically conductive material, overlies the sensor electrode12. The screen electrode20also includes a plurality of openings22so that one opening22is aligned with each pillar18on the sensor electrode12. Furthermore, the openings22in the screen electrode20are preferably complementary in shape, but slightly larger in size than, the cross-sectional size of the pillars18. Consequently, each pillar18is able to pass, unobstructed, through its associated opening22in the screen electrode20as the screen electrode20moves from an upper position, illustrated inFIG. 2, and to a lower position, illustrated inFIG. 3. In its lower position, the pillars18protrude above the screen electrode20. Of course, if the sensor10includes only one pillar18, then only a single opening22will be needed.

In order to support the screen electrode20above the sensor electrode12, at least one, and preferably several piezoelectric actuators24are provided. Each piezoelectric actuator24may be in the form of an elongated flat strip having one end26attached to the substrate14and its other end28attached to the screen electrode20. The piezoelectric actuators24generally permit the screen electrode20to move relative to the sensor electrode12in an up and down manner. Although the number of piezoelectric actuators24may vary, as shown, the electric field sensor10is generally rectangular in shape with one piezoelectric actuator24attached adjacent each corner of the screen electrode20.

In order to modulate the screen electrode20relative to the sensor electrode12, i.e. to move the screen electrode20periodically between the position shown inFIGS. 2 and 3, a periodic voltage source29(FIG. 2) is electrically connected to piezoelectric electrodes30and32on opposed surfaces of the piezoelectric actuator24such as a sine wave, saw tooth wave, triangular wave, etc. Consequently, with the periodic voltage source29electrically connected to each of the piezoelectric electrodes30and32for each piezoelectric actuator24, the position of the screen20will modulate in synchronism with the frequency, i.e. −10 kHz or 20 kHz, of the voltage source29between the upper and lower positions illustrated inFIG. 2andFIG. 3.

With reference now toFIG. 1, an output circuit34is electrically connected to the sensor electrode12and screen electrode20which provides an output which varies in proportion with the strength of an electric field38(illustrated diagrammatically) acting on the electric field sensor10.

In some embodiments of the invention, the output circuit34may include an operational amplifier40having its grounded input electrically connected to the screen electrode20and its other input electrically connected to the sensor electrode12. A resistor42is electrically connected between the operational amplifier output36and the sensor electrode12so that the magnitude of the output voltage from the operational amplifier40is proportional to the current produced from the sensor electrode12.

In operation, the modulation of the screen electrode20relative to the sensor electrode12causes the screen electrode to periodically block the electric field38from the sensor electrode12when the screen electrode20is in its upper position (FIG. 2), and expose the pillars20of the first electrode12to the electric field38when the screen electrode20is in its lower position (FIG. 3). When the screen electrode20blocks the electric field38from the sensor electrode12, the current output from the sensor electrode12is minimized. Conversely, when the sensor electrode pillars18are exposed to the electric field when the screen electrode20is in its lower position (FIG. 3), the sensor electrode12will produce its maximum current output. By making many holes and pillars, the sensor area can be made larger (more sensitive), without increasing the distance that the screen travels to effectively modulate the field seen by the pillars. The magnitude of the current output from the sensor electrode12varies proportionally with the strength of the electric field38. This variable current signal, furthermore, results in a varying voltage signal from the operational amplifier40.

In order to determine the amount of current from the first sensor12, an electric charge induction probe (or “D-Dot” sensor) senses an induced current i generated by a moving source charge (or more generally, by any time-varying E-field). The surface charge density ρSinduced on a conducting electrode at a free space interface due to exposure to an ambient E-field Enis
ρS=Dn=ε0En(1)
Dnand Enare the normal components of the electric flux and E-field vectors just outside the surface and ε0is the permittivity of free space. The total charge Q on the conductor is obtained by integrating the induced ρSover the electrode area A:
Q=∫ρSdA=ε0EnAeff(2)
where Aeffis the effective area taking into account flux-concentration and modulation fraction effects. Equation (2) assumes 100% modulation of the measured field. If the screen is made to travel far enough to effectively shield and expose the pillars to the ambient field (near 100% modulation), then Aeffcan be slightly larger than the actual area A of the electrode. In any event, Aeffvaries with the distance from the nearest ground electrode. This charge induction effect may be measured as a current i:

i=d⁢⁢Qd⁢⁢t=d⁡(ɛ0⁢En⁢Aeff)d⁢⁢t=ɛ0⁢Aeff⁢d⁢⁢End⁢⁢t.(3)
Assuming the sensed field Enis sinusoidal at a frequency f
i=2πfε0Aeff|En|.  (4)

Equations (3) and (4) show how the measured current of the D-Dot sensor depends strongly on the area of the electrode Aeffand the rate of change of En(and hence, the frequency f of En). Therefore, additional sensitivity can be obtained by modulating and measuring an E-field at a higher frequency. The practical upper limit of the modulating frequency is limited by the size of the sensor, the stiffness of the materials, and the degree of vacuum packaging.

With reference now toFIG. 4, at certain frequencies, which may be determined mathematically or empirically, of the periodic voltage source28, the screen electrode20resonates in differential mode in which a first area46of the screen electrode20moves in a first direction, i.e. away from the sensor electrode12, while a second area48of the screen electrode20moves in the opposite direction, i.e. toward the sensor electrode12. In this event, the sensor electrode12may be divided into two subelectrodes50and52with the first area46of the screen electrode overlying the first subelectrode50while the second area48of the screen electrode20overlies the second electrode52. One output circuit34is then electrically connected between the screen electrode20and the sensor subelectrode52while a second output circuit34is electrically connected between the screen electrode20and the second sensor subelectrode50. The outputs from both output circuits34are then combined by a combiner circuit54to provide a combined signal on its output56.

The differential mode operation depicted inFIG. 4achieves two significant advantages over nondifferential resonance sensors. First, since the output current from the two sensor subelectrodes50and52is either increasing or decreasing in opposite directions when the electric field sensor is subjected to an electric field, the effective signal output from the combined sensor subelectrodes50and52essentially double the overall output from the individual sub-electrodes. In addition, however, since the areas46and48move in opposite directions, the coupling between the screen sensor areas46and48and the sensor subelectrodes50and52with their pillars18effectively cancel each other thus eliminating common mode noise from the output of the sensor. Although the combined effective area of the subelectrodes is reduced somewhat (relative to an equivalently-sized single electrode), the overall signal-to-noise ratio is expected to increase using this operational mode.

With reference now toFIGS. 5-7, a second embodiment of an electric field sensor60of the present invention is shown. The electric field sensor60includes a sensor electrode62having a base64and at least one, and preferably, a plurality of elongated spaced apart and parallel fingers66which extend outwardly from the base64. The fingers66may integrally formed as a one piece construction with the base64in some embodiments.

Similarly, a screen electrode68also includes a base70and at least one, and preferably, a plurality of elongated spaced apart and parallel fingers72which extend outwardly from the base70and are of a one piece construction with the base70. As best shown inFIG. 5, the fingers66and72of the sensor electrode62and screen electrode68are registered, i.e., interdigitally positioned with one screen electrode finger72positioned between each adjacent pair of sensor electrode fingers66. With reference now particularly toFIGS. 6 and 7, a first piezoelectric actuator74supports the base64of the sensor electrode62to a substrate76. Similarly, a second piezoelectric actuator78connects the base70of the screen electrode68to the substrate76.

The piezoelectric actuators74and78operate in the same fashion as the piezoelectric actuator24(FIG. 1) in which the piezoelectric actuators74and78move their associated electrodes62and68periodically with a periodic voltage source, such as the voltage source28shown inFIG. 2. However, unlike the piezoelectric actuator24, the piezoelectric actuators74and78cause the sensor electrode62and screen electrode68to tilt in opposite directions relative to each other.

For example, as shown inFIG. 6, when a voltage signal of one polarity is applied to the piezoelectric actuators74and78, the piezoelectric actuator74tilts the sensor electrode62upwardly while, simultaneously, an opposite voltage applied to the piezoelectric actuator78tilts the screen electrode68downwardly. In doing so, the sensor electrode62is exposed to the maximum electric field38thus producing maximum current from the sensor electrode62. This current is then received by the output circuit34(seeFIG. 2) to generate an output signal representative of the strength of the electric field38.

Conversely, when an opposite polarity signal is applied to the piezoelectric actuators74and78, the piezoelectric actuators74and78tilt their respective sensor electrode62and screen electrode68in the opposite direction. In doing so, the screen electrode68is positioned above the sensor electrode62thus screening the sensor electrode62from the electric field38and minimizing the current output from the sensor electrode62. This is shown inFIG. 7.

Still other configurations other than the sensor electrode and screen electrode configuration shown inFIGS. 1 and 5are, of course, possible such as side to side or multimode. Furthermore, it will be understood that the entire sensor60can be manufactured as a MEMS structure so that the components of the sensors are on a microscopic level, e.g., on the order of about 10−6meters. Furthermore, in order to provide a meaningful signal from the sensor, multiple sensors, either the electric field sensor60or the electric field sensor10, can be synchronized in frequency and phase, and ganged together to provide a larger aggregate output signal. It is preferable to synchronize several smaller structures at a higher operating frequency, instead of operating one larger structure at a lower frequency, because the overall sensitivity can be made higher.

From the foregoing, it can be seen that the present invention provides a simple, yet effective, electric field sensor. Having described our invention, however, many modifications thereto will become apparent to those skilled in the art to which it pertains without deviation from the spirit of the invention as defined by the scope of the appended claims.