HK1070945A - Sensor for non-contacting electrostatic detector - Google Patents

Description

Sensor for non-contact electrostatic detector

Background

The present invention relates to electrical measurement technology, and more particularly, to a new and improved sensor or probe for use as a non-contact electrostatic field, electrostatic voltage or electrostatic charge sensing device.

Non-contact electrostatic detectors are used in applications where electrical charge transferred to a measuring device by physical contact with a surface, circuit or device being measured/monitored will alter or corrupt data. Such measurements include, for example, monitoring the voltage level of a photoresistor used in a copier or other electrophotographic process, monitoring the build-up of electrostatic charge on a process line such as a dielectric web used in a textile or plastic film manufacturing process, or monitoring areas in a semiconductor manufacturing/processing process where the build-up of charge is a source of product contamination or failure through the result of electrostatic discharge (arcing).

Heretofore, electrostatic sensors/probes of the type required to make high precision, high speed, low noise measurements, and also requiring small physical dimensions, have been very expensive to manufacture, precluding their use in higher number and lower cost processes and/or equipment, such as low end laser printers, copiers, or multi-point electrostatic charge build-up monitoring systems.

Summary of The Invention

It is therefore a first object of the present invention to provide a new and improved sensor/probe for electrostatic measurements which meets the above-mentioned requirements while being inexpensive to manufacture.

It is a further object of the present invention to provide a sensor/probe having electrical properties independent of the mass of the sensor/probe and/or the mass of the holding/fixing structure used to place the sensor/probe in its measurement position relative to the test surface or area.

It is a further object of this invention to provide such a sensor/probe which minimizes or readily eliminates the generation of noise and offset signals, thereby eliminating the need to provide a relatively large space between the electrical components of the sensor/probe amplifier circuitry and the sensor/probe housing components, thereby reducing the likelihood of contaminants entering the interior of the sensor/probe structure.

It is another object of the present invention to provide such a sensor/probe that can be easily configured to provide performance with a variety of spatial resolutions.

It is another object of the present invention to provide such a sensor/probe with improved electrical speed of response to accurately measure higher frequency electrostatic parameter variations.

It is a further object of the present invention to provide such a sensor/probe which is smaller in size, simpler in construction, more economical to manufacture and more reliable in operation.

The invention provides a sensor for an electrostatic detector comprising an elongate vibrating element supported in cantilever form at a mechanical node at one end, a sensitive electrode on the vibrating element adjacent the other end adapted to be disposed towards a charge, electric field or potential being measured, and a drive transducer on the vibrating element for vibrating the element in a direction to alter the capacitive coupling between the electrode and the charge, electric field or potential being measured. The drive means is positioned along the beam and operates at a vibration frequency such that a virtual mechanical node is displayed along the beam. This in turn reduces vibration at the mechanical attachment node so that the stiffness of the mechanical node does not affect the operating frequency and/or displacement at the free end of the cantilever beam. The cantilever beam vibration amplitude varies and therefore the amplitude of the motion of the detector at that end of the beam is independent of the mass of the sensor/probe and/or the mass, such as a clamping/holding fixture, used to place the sensor/probe in its measurement position relative to the test surface or area. Thus stabilizing the electrical performance independent of the particular clamping/securing structure used. An amplifier on the vibrating element has an input connected to the sensing electrode and an output adapted to be connected to a sensing/measuring device such as an electrostatic voltmeter. According to a preferred embodiment of the present invention, the vibration frequency corresponds to a second vibration node of the driving device, and the driving device is located on the cantilever beam at a position overlapping with a vibration peak near the middle of the cantilever beam.

The above and other advantages and features of performance of the present invention will become more apparent upon reading the detailed description and the included drawings.

Brief Description of Drawings

FIG. 1 is a perspective view of the prior art with parts removed;

FIGS. 2A and 2B are schematic diagrams illustrating the operation of a cantilever beam in the sensor of FIG. 1;

FIG. 3 is a schematic diagram of a tuning fork system employed in the sensor;

FIGS. 4A, 4B and 4C are graphs illustrating vibration modes of a cantilever beam of the type shown in FIGS. 2A and 2B;

FIG. 5 is a schematic view of a sensor employing a cantilever beam according to the present invention;

FIG. 6 is a schematic diagram further illustrating the present invention;

FIG. 7 is a schematic block diagram of circuitry for use with the sensor of the present invention;

detailed description of exemplary embodiments

Referring initially to FIG. 1, there is shown a prior art sensor generally designated 10 in which a vibrating element 40 in the form of a single cantilever beam has one end connected or integrated to a substrate 14, while the free end provides a mounting surface for an amplifier 90 and one sensitive electrode 50. For a more detailed description of the prior art Sensor 10, reference may be made to U.S. patent No. 4763078 entitled "Sensor for electronic Voltmeter" filed on 1988, 8/9, the disclosure of which is incorporated herein by reference.

As is conventionally known, a single cantilever beam can produce an unbalanced mechanical torque in vibration at a point attached or connected to a mechanical node structure or substrate structure in the case of the sensor of fig. 1. In contrast, in a dual cantilever beam structure such as a tuning fork, the mechanical forces generated by the motion of the tines of each tuning fork can be made equal and opposite such that no mechanical torque is generated at the common connection point (fulcrum) of the forks. Thus, the tuning fork operates at a stable amplitude and frequency as a function of the mass (or mounting stiffness) present at the fulcrum, whereas a single cantilever beam of the type shown in fig. 1 is unstable in the frequency and amplitude of vibration due to variations in the mechanical mass or mechanical stiffness at the point of attachment of the substrate. This instability is particularly troublesome when attempting to reduce the physical size of the substrate structure in an effort to reduce the size and/or weight of the sensor/probe.

Specifically, as the size of the probe is significantly reduced from the illustrative size shown and described in patent 4763078, the vibration amplitude of the free end of the cantilever beam can become unstable and depend on the fixture used to position the sensor/probe relative to the test surface/area. In addition, because of the unstable nature of the single cantilever beam, which involves the probe/sensor structure and mounting fixture to the mechanical vibration system of the cantilever, undesirable audible vibrations are generated and emitted out of the sensor/probe structure, causing objectionable audible noise. By using an inherently balanced tuning fork instead of a single cantilever beam, these mechanically undesirable effects can be eliminated. However, the manufacturing cost when using a double cantilever, i.e., a tuning fork, is prohibitive because the tuning fork, which is one of the bonded portions of the substrate 14, is difficult to manufacture by an inexpensive process such as chemical etching.

Fig. 2A illustrates the single cantilever operation of the device of fig. 1 described in patent 4763078. As shown, the cantilever beam 40A is supported at one end at the mechanical node (N), while the other end is free to vibrate. Figure 2A shows the cantilever beam 40A in an unpowered, i.e., rest, position. A line 70 drawn through the beam to the mounting node (N) defines the rest position. When power is applied to an oscillating circuit comprising a drive and feedback piezoceramic chip (not shown), the cantilever beam 40A is caused to oscillate at or near its fundamental resonant frequency (which is approximately 700hz for the dimensions shown in the illustrative device of patent 4763078).

Fig. 2B illustrates the limitation of the mechanical movement produced by the beam 40B, which produces an oscillating movement al at the free end of the beam. It should be noted that any torque generated by the motion of the cantilever beam at the mechanical mounting node N is not compensated by any equal and opposite torque that would be generated by, for example, the tuning fork system 74 as shown in fig. 3. In fig. 3, torque T2 is precisely compensated by torque T3. The T3 has the same number but opposite directions because the prong 76 associated with T3 always makes relative motion 180 ° out of phase with the prong 78 associated with T2, as is conventionally known. Thus, node N1 experiences equal and opposite torque, and therefore produces no net torque.

In the case of the system of fig. 2, the operation of the single cantilever is not compensated, node (N) is not bulky and therefore not mechanically "stiff", node (N) will vibrate and will enter a mechanical equilibrium which accounts for the movement of the cantilever at the free end of the cantilever with respect to the operating frequency and movement Δ L. Since the stiffness of the node (N) also depends on the mass of the sensor/probe holding fixture, the amplitude of oscillation of the free end and thus of the electrode 50 will also depend on the stiffness of the fixture and hence on the specific application of the sensor/probe. Any instability in the amplitude of the electrodes 50 will adversely affect the accuracy and stability of the amount of static electricity detected by the sensor/probe, particularly when attempting to reduce the size and/or weight of the sensor/probe structure, thereby resulting in a smaller bulk volume or less stiffness of the node (N).

The above is further illustrated by considering the dynamic structure of a thin cantilever beam. Any vibrating beam will exhibit a set of separate resonant modes, the shape and frequency of which depend on the nature of the boundary conditions applied. In fig. 4A, 4B and 4C, curves 80, 82 and 84 depict the shape of the three vibration modes (n equals 1, 2, 3, respectively) of the beginning of a cantilever beam with one clamped end and one free end. While only the first few modes may have any practical significance in a device of the type described herein, these modes are unlimited in number. The pattern of n-1 is very similar to the static displacement of a cantilever with a common load at the end. The pattern with n equal to 2 has a single inflection point and a zero crossing point, and the pattern with n equal to 3 has two inflection points and a zero crossing point. Successive higher modes are characterized by more inflection points and zero crossings.

The resonant frequencies of the modes have a defined ratio relative to each other. For a beam with ideal clamping, which is given the condition of zero motion and zero slope at the fixed end, the ratio is:

f₁∶f₂∶f₃… is 1: 6.2: 17.5: … so that the first resonance is at f₁400Hz, then f₂＝2.48kHz，f₃7.0kHz, and so on. In a metal beam, the mechanical losses are low and therefore the quality factor of the mode is high, i.e. Q₁，Q₂，Q₃Is greater than 50. In this way, the resonances can be considered well isolated.

In any practical approach ideal clamping is not possible, i.e. a manufacturable probe of the type shown in fig. 1 is not required. FIG. 1 shows that the resonant modes of the structure were found to have a shape very similar to the "ideal clamping" case shown in FIGS. 4A, 4B, 4C; however, the ratio of the resonance frequencies changes somewhat:

f₁∶f₂∶f₃another observed phenomenon at 1: 8: 22: …: … is that the vibration losses of these well-isolated resonances are still low, i.e. Q > 50, as long as the plates are reliably welded to their housings.

The different shapes of the modes indicate that a mode can be selectively and optimally excited to a given mode by appropriately arranging the piezoelectric actuators. Also, to detect the presence of a given mode with the maximum sensitivity available, the position of the piezoelectric element can be optimized. The piezoelectric transducer converts AC electrical excitation into mechanical deformation. When one such element is rigidly attached to the cantilever beam, the voltage-induced change in the lateral dimension of the element-along the surface of the cantilever beam-induces a shear that causes the beam to deflect. Thus, the optimal position for exciting (and also for detecting) any mode is at the point where the curvature of the deflected beam is greatest, as determined according to the present invention. This simple premise for optimal transducer position has been examined in the cantilever modes of n-1, 2 and 3.

Figure 5 illustrates the operation of the cantilever beam 100 of the sensor of the present invention. As shown, the cantilever beam (100) is supported at a mechanical node (N), also indicated at 102, while the other end 104 is free to vibrate. The arrangement of the oscillator circuit along with the driver 106 and feedback piezoceramic chip 108 along the cantilever beam 100 is different from the oscillator and chip arrangement in the sensor of patent 4763078, as will be explained herein. In the present invention, when power is applied to the oscillator circuit and piezo chips 106, 108, the cantilever beam 100 vibrates at a frequency that causes a virtual mechanical node, also indicated at 110, labeled (NS), to appear along the cantilever beam. When the boom 100 is operating at a frequency above its fundamental frequency, a substantially compensated torque is generated at the mechanical mounting node (N3) due to the virtual mechanical Node (NS) present, and thus substantially no net torque is applied at node (N3).

As shown in FIG. 5, the vibration of the cantilever beam 100, as defined by the limits of vibration illustrated by the solid and dashed line positions, produces a vibration of Δ LB at one end 104 of the beam. When the virtual Node (NS) remains stationary over the oscillation period of the oscillation, the portion of the cantilever beam between nodes (N3) and (NS) moves in a direction opposite to the direction of movement of the portion of the beam between Node (NS) and free end 104 at all times. The generation of the substantially compensated torque is shown graphically in fig. 5. In FIG. 5, the torque T4 resulting from the force F4 applied at a distance D4 from node N3 is substantially compensated by the torque T5 resulting from the force F5 applied at a distance D5 from node N3. Distance D4 is measured from node N3 to the center of mass of beam 100, and distance D5 is measured from node N3 to the center of the turn-back of the mass between Node (NS) and end point 104.

When the substantially compensated torque T5 is generated, substantially no net torque is applied to node N3. Because substantially no net torque is applied to node N3, node N3 is unable to vibrate and therefore its stiffness does not significantly affect the operating frequency of the cantilever beam 100 and/or the displacement Δ LB at the end point 104 of the cantilever beam 100. Thus, when the cantilever beam is operated at a frequency that produces a virtual mechanical node, or multiple virtual mechanical nodes at appropriate locations along the cantilever beam, the single cantilever beam 100 can be made to behave as if its mounting node were torque compensated, similar to the tuning fork case. Because its mounting node torque is substantially compensated for, there is a variation in the amplitude of oscillation Δ LB, and thus the amplitude of motion of the probe at the beam end point, independent of the mass of the sensor/probe and/or the mass of the clamping/holding fixture used to place the sensor/probe in its measurement position relative to the test surface or area. It is therefore an object of the present invention to stabilize the electrical performance independent of the particular clamping/securing device being used.

Considering the application of the second (n-2) vibration mode of cantilever beam 100, with drive transducer 106 positioned such that it overlaps the peak near the middle, the mechanical response of cantilever beam 100 will be maximized because its curvature is greatest at this point when deflected. The feedback piezoelectric transducer 108, which is used to detect motion and provide a signal to close the feedback loop, is positioned relatively close to the pinch point 112. Is placed at another location with a large curvature and the element is found to detect all appropriate signals. The capacitive electrode 114, which is the center of the non-contact ESV device, is attached very close to the end 104 of the cantilever beam 100 where the amplitude of motion is greatest.

The use of higher order modes in the cantilever beam improves the serious problems associated with vibration isolation between the beam and its mounting. To understand how this improvement is achieved, the shapes of the vibration modes of the first and second modes in fig. 4A and 4B may be compared. For the mode of n-1, half of each period of the cantilever beam is entirely above the equilibrium position and the other half of the period is entirely below the equilibrium position. As a result, the maximum torque is passed through the beam to the housing. This torque usually results in resonance of the carrier and the components to which the probe is attached, as there is in fact no sufficiently strong housing or carrier to prevent this. On the other hand, for the mode with n-2, the deflection always bisects the equilibrium position, providing some degree of balance between the outside and the inside of the beam. The degree of auto-balance increases when the pattern number n rises. Whether a mode exceeding the second (n-2) can be utilized cannot be demonstrated due to other practical limiting factors. Also, from a vibration isolation point of view, the higher mode is better.

Another advantage of using higher order modes is that when the probe is used to measure electrostatic surface potentials in important applications such as xerographic copiers/printers, the higher operating frequency of the probe improves the stiffness of the supporting electronics while also increasing the obtainable sampling rate. This latter feature would significantly improve the control of the xerographic process for a new class of low cost color copier printers (three primary colors + black), perhaps the most important, in the current context. If the current copy speed in color machines is close to that of black and white, higher speeds and increased spatial resolution of the photoreceptor charge will be of paramount importance. Increasing spatial resolution is crucial to achieving more restrictive registration requirements for color technology.

Consider the following example to illustrate such a modification. A second mode of a 1cm cantilever in a non-contact electrostatic voltmeter (ESV) converts the operating frequency from about 400Hz to about 2400 Hz. This six-fold increase greatly improves the effective bandwidth of the ESV. Another advantage is that the mechanical losses of the second mode are smaller than the first mode, i.e. Q₂＞Q₁. The higher Q value of the second mode is valuable in two ways. First, the electro-mechanical conversion efficiency of the piezoelectric drive transducer is improved, converting to more intense vibrational motion for a given AC excitation. Second, the higher Q value improves the stiffness of the feedback control circuit by improving the filtering of the noise signal from the output of the sensing piezoelectric element.

The foregoing is further illustrated by considering fig. 6 which illustrates first and second modes of a cantilever beam for an electrostatic voltmeter according to the present invention. Cantilever beam 100 'is equipped with drive and feedback piezoelectric transducers 106' and 108 ', respectively, and cantilever sensor 114'. Fig. 6 also shows displacement profiles for the first (n-1) and second (n-2) modes. The Euler-Bernoulli continuum beam mode gives a simple expression for the natural frequency of the first mode:

where L and t are the length and width of the beam and E and ρ are the Young's modulus and mass density of the material, respectively.

For a typical ESV probe scale, the n-1 mode provides an obtainable resonance frequency, which is about half the value required to provide sufficient sampling bandwidth for a color copier. In addition, the resonance peaks of the assembled ESV probe unit show unacceptably high sensitivity to the method of installation in a copier. The above formula shows that: once the continuous beam material is selected, the resonant frequency can be changed by merely changing the length L or the thickness t. Increasing the frequency in this way reduces the amplitude, which in turn reduces the voltage detection sensitivity of the probe.

Replacing the first mode with the second mode (n 2) alleviates both of the above problems by a small amount of redesign of the cantilever beam or housing. The resonant frequencies of the second mode shown in fig. 6 are:

from the two equations above, f₂/f₁6.3, thereby solving the first problem. Furthermore, it is experimentally known that the sensitivity of the mounting of the n-2 harmonic peak should be much lower than the n-1 mode, since the motion of the second mode tends to balance automatically. By comparison, for some mounting arrangements, the vibration of the first resonance almost disappears, while the amplitude of the second resonance only drops by 10% to 30%. It is important to note that efficient excitation and detection of the n-2 mode requires the piezoelectric transducer element to be located close to the maximum of curvature. Fig. 6 shows an optimized arrangement for the drive 106 'and motion detection 108' elements. Likewise, the capacitive (ESV) electrode 114' must be located at a position where local maximum displacement occurs near the end of the beam (as shown in fig. 6) or closer to the clamped end.

The amplifier 130 is mounted on the cantilever beam 100 and functions in a manner similar to the amplifier 100 of the invention of the above-mentioned patent 4763078. The circuit 140 for the operating sensor of fig. 5 and 6 is shown diagrammatically in fig. 7. The circuit 140 has a portion 142 operatively connected to the amplifier 130 for providing an operating voltage to the amplifier and for pre-processing signals received from the amplifier 130 and passing such signals to the signal processing/monitoring portion of the electrostatic voltmeter in a known manner. The circuit 140 also has a drive or oscillator section 144 for providing a drive signal to operate the drive transducer 106 to vibrate the cantilever. Feedback converter 108 is connected in controlling relation to section 144. The frequency of vibration of the beam 100 is sensed by the transducer 108, which emits a signal that is used by the circuit portion 144 to control the frequency of the generated drive signal, and thus the frequency of vibration of the beam 100. The vibrating cantilever 100 functions as a resonant component of the circuit and various electrical connections may be made in a manner similar to that of the invention of patent 4763078.

To generate the virtual node NS, indicated at 110 in fig. 5, to facilitate generation of the appropriate operating frequency, oscillator circuit 144 and piezoceramic chips 106, 108 operate together as a closed-loop frequency-selective positive feedback system that maintains electrical oscillation at a particular frequency that is synchronized with the mechanical frequency of the displayed virtual node. This frequency is related to the fundamental mechanical oscillation frequency of the single cantilever beam 100 by a factor of about six. Thus a cantilever having the dimensions illustrated in patent 4763078 when operated in accordance with the principles of the invention will operate at a frequency of about 4 kilohertz, replacing the frequency of about 700Hz at which the base node operates. Thus, this result in a probe operating at a frequency of 4 kilohertz increases the speed at which the probe can detect and track the electrostatic data being measured, thus achieving another object of the present invention.

The sensor of the present invention operates in the following manner. The sensor housing is positioned with the electrodes 114 facing a surface subjected to an electrostatic field or potential to be measured, i.e., a test surface. There is a capacitive coupling between the electrode 114 and the test surface through the electrode sensitive surface exposed to the test surface. The oscillator 144 applies an a.c. voltage to the piezoelectric element 106 causing it to vibrate in a direction substantially perpendicular to the plane of the plate from which the cantilever beam 100 extends at a frequency substantially equal to the oscillator output frequency. The cantilever beam 100 transmits the vibrations of the piezoelectric element 106 to the electrodes 114. The electrode 114 vibrates in a direction substantially perpendicular to its plane at a frequency determined by the oscillator output frequency and the mechanical properties of the element 100 as described above. When the electrode 114 vibrates, it moves toward and away from the test surface, thereby changing the capacitive coupling between the electrode sensitive surface and the test surface.

As the element 100 vibrates, its motion generates a voltage on the piezoelectric element 108 attached to the element 100 that is electrically fed back to the circuit 140 to complete a feedback loop, maintaining the oscillation. Specifically, the fed back signal, which represents the actual vibration or oscillation, is compared to a reference signal, which represents the desired oscillation or vibration, to control the output of the oscillator. The manner of doing so will be readily apparent to those skilled in the art and need not be described in any greater detail.

If the potential of the probe electrode 114, which is the system feedback potential, is different from the potential on the test surface, an a.c. signal is induced on the surface of the electrode 114. The induced a.c. signal is applied to the input of preamplifier 130. The amplitude and phase of the induced signal relative to the drive signal obtained from the oscillator depend on the magnitude and polarity, respectively, of the potential difference between the two surfaces. The output of amplifier 130 is applied to an electrostatic voltmeter and then to the input of a demodulator (not shown). The synchronous demodulator functions to remove amplitude and phase information from the induced a.c. signal using the drive signal obtained from the oscillator as a reference phase. The line frequency derived from the oscillator signal may be used to ensure that any environmentally induced signal modulated onto the detector electrode 114 after the demodulation process is not displayed as noise. The demodulated signal, which is already a d.c. voltage, can then be amplified in a high gain amplifier, the output of which is applied to the input of a suitable device, such as a meter, for continuous display and monitoring of the test surface potential.

It will thus be apparent that the invention achieves the desired objects thereof. The sensor according to the invention uses one of the higher order resonant modes of the cantilever beam. As described above, these modes include the n-2 mode and possibly higher order modes, which can be used in the design of new high performance probes for less expensive non-contact ESV devices. The improved performance results from the fact that the higher order modes resonate at frequencies 6 times (or more) higher than the 1 st order mode (n-1). Several advantages of operating the ESV at higher frequencies have long been recognized, but have never been considered seriously before to achieve this. Driving the cantilever at higher order resonances clearly puts the higher frequencies into practice.

Although the embodiments of the present invention have been described in detail, the practice of the invention for the purposes of illustration is not limiting.

Claims (16)

1. A sensor for an electrostatic detector, such as an electrostatic voltmeter, comprising:

(a) an elongated vibratory element having two ends;

(b) means for supporting said vibratory element at one of said end points as a cantilever beam and confining a mechanical mounting node at said end point;

(c) a sensitive electrode on said vibrating element near the other of said end points, the sensitive electrode being disposed toward the charge, field or potential being detected, the charge, field or potential being capacitively coupled to said sensitive electrode;

(d) a drive transducer rigidly attached to said vibrating element at a predetermined location for vibrating said element at a predetermined frequency in a direction to vary the capacitive coupling between said sensitive electrode and the charge, electric field or potential being detected;

(e) an oscillator operatively connected to said drive transducer for operating said transducer to vibrate said element; and

(f) selecting the position of said drive transducer and the frequency of vibration of said element to vibrate said element at a frequency above the fundamental vibration frequency of the cantilever beam to produce a compensating torque at the mechanical mounting node of the cantilever beam such that the net torque applied to said mounting node is substantially reduced and the mounting node is not sufficiently stiff to affect the operating frequency of the cantilever beam or the free end displacement of the cantilever beam; and

(g) means for deriving from said sensitive electrode an electrical signal comprising information about the charge, electric field or electric potential detected.

2. A transducer according to claim 1, wherein the drive transducer is located on the cantilever beam at a position of maximum curvature for a selected vibrational mode of the cantilever beam.

3. The sensor of claim 2, wherein the selected mode of vibration of the cantilever is a second mode.

4. The sensor of claim 1, wherein said means for obtaining an electrical signal comprises an amplifier.

5. The sensor of claim 1, further comprising a feedback transducer on said vibrating element for detecting the frequency of vibration of said element.

6. The sensor of claim 5, further comprising a controller coupled to said feedback transducer and to said oscillator for controlling said oscillator to vibrate said element at a predetermined frequency.

7. A sensor for an electrostatic detector, such as an electrostatic voltmeter, comprising:

(a) an elongated vibratory element having two ends;

(b) means for supporting said vibratory element at one of said end points as a cantilever beam and confining a mechanical mounting node at said end point;

(c) a sensitive electrode on said vibrating element near the other of said end points, the sensitive electrode being disposed toward the charge, field or potential being detected, the charge, field or potential being capacitively coupled to said sensitive electrode;

(d) a drive transducer rigidly attached to said vibrating element at a predetermined location for vibrating said element at a predetermined frequency in a direction to vary the capacitive coupling between said sensitive electrode and the charge, electric field or potential being detected;

(e) an oscillator operatively connected to said drive transducer for operating said transducer to vibrate said element; and

(f) the drive transducer operating at a frequency corresponding to a second mode of vibration of the cantilever beam and the transducer being located on the vibrating element in overlapping relationship to a mode peak corresponding to the second mode of vibration at a central portion thereof; and

(g) means for deriving from said sensitive electrode an electrical signal comprising information about the charge, electric field or electric potential being detected;

(h) thereby providing vibration isolation between the cantilever beam and the mechanical mounting node.

8. The sensor of claim 7, wherein the means for obtaining an electrical signal comprises an amplifier.

9. The sensor of claim 7, further comprising a feedback transducer on the vibrating element to detect vibration of the element.

10. The sensor of claim 9, further comprising a controller connected to the feedback transducer and the oscillator to control the oscillator to operate the drive transducer at the frequency corresponding to a second vibration mode.

11. A method for operating a sensor of an electrostatic detector, such as an electrostatic voltmeter, wherein the sensor comprises an elongated vibrating element supported at a mounting node as a cantilever beam and having a sensing electrode at a free end thereof, and wherein vibration of the element alters capacitive coupling between a sensitive electrode being detected and an electrical charge, field or potential, said method comprising:

(a) applying a vibratory force to said element at a location thereon and at a frequency to vibrate said element at a frequency above the fundamental vibration frequency of the cantilever beam, thereby producing a substantially compensating torque at the mounting node of the element, so that substantially no net torque is applied to the mounting node, so that the stiffness of the mounting node does not affect the operating frequency of the cantilever beam or affect the displacement of the free end of the cantilever beam; and

(b) an electrical signal containing information about the charge, electric field or electric potential being detected is obtained from the sensitive electrode.

12. The method of claim 11, wherein the vibratory force is applied to the cantilever beam at a location on the cantilever beam where the curvature of the selected mode of vibration of the cantilever beam is greatest.

13. The method of claim 12, wherein the selected mode of vibration of the cantilever beam is the second mode.

14. The method of claim 11, further comprising:

(a) detecting a vibration frequency of the element;

(b) the frequency of the vibratory force applied to the element is controlled using the detected vibration frequency.

15. A method for operating a sensor of an electrostatic detector, such as an electrostatic voltmeter, wherein the sensor comprises an elongated vibrating element supported at a mounting node as a cantilever beam and having a sensing electrode at a free end thereof, and wherein vibration of the element alters capacitive coupling between a sensitive electrode being detected and an electrical charge, field or potential, said method comprising:

(a) applying a vibrating force to the element at a frequency corresponding to the second vibration mode of the cantilever beam and at a position of the element in an overlapping relationship with the mode peak position of the second vibration mode at the middle portion; and

(b) obtaining an electrical signal from the sensing electrode containing information about the electrical charge, field or potential being detected;

(c) thereby providing vibration isolation between the cantilever beam and the mechanical mounting node.

16. The method of claim 15, further comprising:

(a) detecting a vibration frequency of the element;

(b) the frequency of the vibratory force applied to the element is controlled using the detected vibration frequency.