Single balanced beam electrostatic voltmeter modulator

Abstract: An electrostatic measurement apparatus having a single balanced beam vibratory element supported and balanced at its center, so that it may be driven in a manner to cause a regular vibration of the beam. The apparatus also has an electrode suitable for sensing a capacitive coupling relationship with an electrostatic field to produce a signal indicative of the magnitude of the electrostatic field. The coupling of the electrode with the electrostatic field is modulated (coupled/decoupled) either by attaching the electrode to the end of the beam and vibrating the electrode past an aperture, or by affixing a vane to the end of the beam, whereupon the vibration of the beam causes the vane to occlude the electrode from capacitively coupling with the electrostatic field.

Description: This invention relates generally to the measurement of electrostatic potentials, and more particularly to an apparatus for achieving the modulation of a sensor or electrode used to measure an electrostatic field or electrostatic voltage. The electrostatic voltmeter which includes the present invention is a device capable of measuring electrostatic fields or potential without current flow through the device. Generally, these devices include a probe or sensor assembly working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential. Thus, the features of the present invention may be used in the printing arts and, more particularly, in an electroreprographic system to control a xerographic process. These electrostatic voltmeters are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality reprographic output. Heretofore, it has been established that a sensing electrode must be modulated with respect to the field being measured in order to accurately measure the field. Essentially, two methods of achieving the required modulation of the electrode are known. The first method requires that the electrode be stationary and that a vibrating element, or vane, be moved between a viewing port and the electrode itself to modulate the field which reaches the electrode. The second method utilizes a moving electrode which is vibrated relative to the surface being measured. US-A-4,763,078 relates to a sensor for an electrostatic voltmeter which consists of a vibratory element supported on one end in the manner of a cantilever beam, a sensitive electrode on the vibratory element for measuring the potential, a driver for vibrating the vibratory element in a direction to vary the capacitive coupling between the electrode and the electrical field being measured, and an amplifier mounted directly on the vibratory element so as to be in synchronous motion with the electrode. US-A-4,720,682 discloses a surface electric potential sensor for detecting the potential on a surface in a non-contacting fashion. US-A-4,625,176 to Champion et al. describes a vibrating probe for measuring electrostatic potential associated with electrophotographic copiers and print machines. US-A-4,614,908 to Daniele et al. relates to a probe for electrostatic voltmeters which measures the voltage on a photoconductive surface. The probe consists of a microdeflector which includes a base having a well and a flexible finger on the base, positioned over the well. US-A-4,318,042 to Eda et al. relates to an electrometer probe for measuring the electrostatic potential on the surface of a photoconductive drum, in an electrostatic machine. The probe includes an electrode which is in the form of a strip. US-A-4,149,119 to Buchheit teaches an electrostatic voltmeter or electrometer which includes a probe sensor for sensing electrostatic charge present in a test surface. The probe sensor is modulated using a rotating vane or shutter arrangement. US-A-3,921,087 to Vosteen discloses a capacitive electrostatic modulator for an electrostatic voltmeter. the modulator includes tines, or vanes, operatively associated with each of the ends of the tuning fork. US-A-3,852,667 to Williams et al. relates to a probe or sensor for an electrostatic voltmeter including a voltage sensitive electrode which is vibrated within a housing so as to vary the amount of the surface of the electrode which is directly exposed to an external electrical potential through an aperture in the housing. According to this invention an apparatus for measuring the magnitude of an electrostatic field comprising: a vibratory element; means for resiliently supporting the vibratory element; drive means for vibrating the vibratory element; and, an electrode for sensing a capacitive coupling relationship with the electrostatic field to be measured, the electrode, in use, cooperating with the vibratory element to produce a modulated signal indicative of the magnitude of the electrostatic field as modulated by the vibration of the vibratory element; is characterised in that the vibratory element is a balanced beam vibratory element. Preferably, the apparatus includes a housing enclosing the apparatus; and an aperture in the housing positioned so as to allow the electrode to become exposed to, and thereby coupled with, an external electrostatic field at least once during each cycle of the balanced vibratory element. Preferably, the drive means caused the vibration of the balanced beam vibratory element at a frequency of at least 1 kilohertz. Preferably, the apparatus further comprises amplifying means, affixed to the balanced beam vibratory element in close proximity to the electrode, for amplifying the signal produced by the electrode, and for outputting the signal to an electrostatic voltmeter. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an embodiment of the modulated sensor assembly of the present invention; Figure 2A is an enlarged perspective view of the single balanced beam vibratory element of Figure 2, where a sensor element is affixed to one end of the vibratory element; Figure 2B is an enlarged perspective view of the single balanced beam vibratory element of Figure 1 having a fixed sensor element and a vane attached to one end of the vibratory element in accordance with an alternate embodiment; and Figures 3A and 3B depict two illustrative bending modes for the vibratory element of the present invention. In the drawings, like reference numerals have been used throughout to designate identical elements. Figure 1 shows a sensor, 10, which may be used in an electrostatic voltmeter (ESV). Sensor 10 includes a housing having top 12a and sides 12b, a rigid substrate 14, and a modulator assembly 16, all of which are assembled as illustrated to form the complete sensor package. While not shown, the sensor may also include additional electrical components necessary for amplifying or filtering the signals produced by various elements of modulator assembly 16. In addition, substrate 14 is generally a substrate suitable for the patterning of electrical circuits thereon, and may further include pads 20 which provide solderable electrical contacts for a suitable multi-wire cable (not shown). Once connected to sensor 10, the multi-wire cable would provide paths for incoming power used by the elements of the modulator, as well as the output signals which are transmitted back to a receiving station (not shown) on the other end of the cable. The receiving station may include any commonly known circuit for the capture and/or characterization of the signals produced by sensor 10. Suitable electrostatic voltmeter circuitry is described in US-A-3,852,667 or US-A-3,921,087. Referring also to Figure 2A, where modulator assembly 16 is shown in greater detail, the assembly includes a one-piece vibration element, 30, which is rigidly affixed to substrate 14 via mounting blocks, or standoffs, 24. Vibration element 30 includes a longitudinal beam 32 having sensor electrode 34 permanently affixed to one end thereof, and resilient supports 36 located near the midpoint of the beam to support the beam yet allow oscillation or bending of the ends of the beam. Sensor electrode 34 may be any commonly known sensor element suitable for capacitively coupling with an electrostatic field external to housing 12, and thereby producing a signal indicative of the magnitude of said electrostatic field. The signal produced by sensor electrode 34 may be amplified by amplifier 80 so as to produce a signal suitable for transmission to an external voltmeter. Also, amplifier 80 may be positioned on beam 32 so as to reduce the cross-coupling of the signal generated by the electrode with other extraneous signals. A suitable amplifier arrangement is further described by Williams in US-A-4,763,078. The characteristic dimensions of beam 32, preferably made of Ni-Span-C® (a Nickel-Iron-Chromium alloy, available from the International Nickel Co., Inc.), and the location of supports 36 operate to define where the vibrational node and center of vibration of the beam will lie. While numerous materials may be used, those that are commonly used for the production of vibrational references, for example, tuning forks, exhibit the required mechanical characteristics. Moreover, the present embodiment employs a vibrating member made of a material having a high magnetic permeability so that it will be responsive to an applied magnetic field. Therefore, when beam 30 is induced to vibrate under the influence of magnetic coil 38, located beneath the end of the beam opposite the sensor element, sensor electrode 34 will be oscillated in the direction indicated by arrows 40. During the oscillations, the electrode is repeatedly coupled and decoupled to an electrostatic field as it passes aperture 42, located in a side wall of housing 12b, at a location proximate the resting or nominal position of the sensor electrode. In other words, when beam 32 is vibrated, the resulting motion causes sensor electrode 34 to swing back-and-forth across aperture 42. The oscillation of the sensor electrode causes it to be exposed to an external electrical field passing through the aperture whenever it passes its resting position. Thus, the sensor electrode is exposed twice during each vibratory cycle of the beam and the remainder of the time it is partially or fully occluded by the walls, 12b, of the housing, thereby producing a modulation frequency that is double the vibrational frequency of beam 32. As an alternative, sensor electrode 34 may also be positioned, with respect to aperture 42, so that the frequency is not doubled, but remains equal to the vibrational frequency of beam 32. As previously mentioned, the oscillation of beam 32 is directly influenced by magnetic coil 38, which acts as a driver for the one-piece vibration element. Piezoelectric pickup, 46, located across the vibrational node of beam 32, senses the vibrations of the beam, and provides signals to a feedback control circuit (not shown) so as to control the drive signal supplied to the magnetic coil and, thus, the frequency and mode of the beam vibrations. More specifically, the feedback control regulates the frequency of the AC voltage applied to magnetic coil 38 so as to achieve the desired harmonic vibration of the beam. In one embodiment, a beam having a total length of about 25 millimeters and a width of about two millimeters maintains a vibrational amplitude, measured at the end of the beam, of approximately 1 millimeter peak-to-peak at a frequency of about 1 kilohertz (kHz). At the harmonic frequency of the beam, the energy required to maintain the vibration is minimized, resulting in additional efficiency and lower driving current for the magnetic coil. Moreover, the dynamics of the single balanced beam design result in a sharper resonance curve, or higher Q, for the modulator. The higher Q factor in turn reflects a lower rate of decay for damped free vibration when compared to sensors which employ cantilever modulation means. Because of the efficiency of the single balanced beam modulator, low driving current is required to modulate the sensor element, resulting in the further reduction of the sensor error caused by the cross-coupling of the drive signal with the signal produced by sensor element 34. Furthermore, the height of sides 12b is reduced, as compared to the available tuning fork modulators, because the necessary clearance for the single balanced beam is about one-half that of a tuning fork. Therefore, the reduced size of sensor 10 will allow it to be used in equipment having limited space for access to the surface for which the electrostatic charge is to be measured. The embodiment depicted in Figure 2A indicates that piezoelectric pickup 46, which may be any suitable film-type element producing an electrical response to a deflection thereof, is used to monitor the vibration of beam 32, and magnetic coil 38 is used to drive the vibration of the beam. Alternatively, these two operations may be accomplished by, for example, replacing magnetic coil 38 with a second piezoelectric element placed on the underside of the beam, near the center, to drive the beam in response to the electrical drive signals supplied thereto. Conversely, a pair of magnetic coils may be positioned at opposite ends of the beam, one being used to drive the beam and the other being used to sense the vibration of the beam and provide the feedback necessary to control the vibration. Accordingly, the scope of the present invention is intended to include all such alternative methods of driving and monitoring the beam vibration. Referring now to Figure 2B, where an alternative sensor embodiment is displayed in detail, beam 32 has vane 142 attached to one end thereof. When beam 32 is vibrated, vane 142 moves in the direction indicated by arrows 140, thereby periodically occluding the direct coupling of sensor electrode 144 to the electric field passing through aperture 146. Thus, the illustrated embodiment utilizes a stationary sensor electrode, 144, and achieves the modulation of the electrode by obstructing the electric field with vane 142. As in the embodiment of Figure 2A, the inherent advantages of the single balanced beam modulator are again present, resulting in an efficient mechanical system, simple frequency doubling, higher operating frequencies, and more accurate measurement of the electrostatic field characteristics. Referring finally to Figures 3A and 3B, which illustrate the two fundamental bending modes for the single balanced beam modulator, beam 32 may be operated in the symmetrical bending mode of Figure 3A, or the asymmetrical mode of Figure 3B. The bending mode of beam 32 is controlled using feedback from piezoelectric pickup 46, to regulate magnetic coil 38 to achieve the desired mode. Commonly known feedback techniques are employed to characterize the signal generated by piezoelectric pickup, 46, and, subsequently, to generate the signals which drive coil 38. In the symmetrical bending mode illustrated in Figure 3A, the ends of the beam are generally traveling "in phase," or in the same direction at the same time. In the asymmetrical mode illustrated in Figure 3B, the ends go in opposite directions as indicated by arrows 180 and 182. Since the symmetrical mode is less dependent on the mounting structure or accurate positioning of the supports, 36, at the center of the beam and since the resulting vibrational frequency is higher, oscillation of the beam in this mode is generally considered to be desirable. In recapitulation, the present invention is a single balanced beam modulator suitable for use in a sensor assembly of an electrostatic voltmeter. The invention provides a highly efficient modulator as a result of the application of the single balanced beam design. The single beam design enables the increased separation of the driving and sensor signals which considerably reduces the undesirable cross-coupling of these signals. The invention further provides for the easy doubling of the modulation frequency by employing an arrangement where a sensor element is exposed to the external electrostatic field twice during each modulation cycle.

Claims: An apparatus for measuring the magnitude of an electrostatic field comprising: a vibratory element (30) ; means (36) for resiliently supporting the vibratory element (30); drive means (38) for vibrating the vibratory element (30); and, an electrode (34,144) for sensing a capacitive coupling relationship with the electrostatic field to be measured, the electrode, in use, cooperating with the vibratory element to produce a modulated signal indicative of the magnitude of the electrostatic field as modulated by the vibration of the vibratory element (30) ; characterised in that the vibratory element (30) is a balanced beam vibratory element. An apparatus according to claim 1, wherein the resilient support means (36) supports the balanced beam vibratory element (30) at a position in proximity to a vibrational node present at substantially the mid point of the element (30). An apparatus according to claim 1 or 2, wherein the electrode (34) is attached to one end of the balance beam vibratory element (30) so that the vibration of the element (30) results in the oscillation of the electrode (34) and modulation of the capacitive coupling relationship between the electrode (34) and the electrostatic field to be measured. An apparatus according to any one of the preceding claims, wherein the electrode (144) is positioned in a capacitive coupling relationship with the electrostatic field to be measured and wherein the apparatus further comprises a means (142) fixed to one end of the vibratory vibratory element (30) for modulating the capacitive coupling relationship between the electrode (144) and the electrostatic field to be measured. An apparatus according to claim 4, wherein the modulation means is an electrically conductive vane (142) attached to one end of the balanced beam vibratory element (30) so that the vibration of the element (30) results in the oscillation of the vane between the electrode and the electrostatic field to be measured and thereby varies the capacitive coupling relationship between the electrode (144) and the electrostatic field to be measured. An apparatus according to any one of the preceding claims, wherein the drive means includes a magnetic coil (38) disposed adjacent to an end of the balanced beam vibratory element (30), so as, in use, to cause deflection of the element (30) upon activation of the coil (38). An apparatus according to any one of claim 1 to 5, wherein the drive means includes a piezoelectric device operatively affixed to a surface of the balanced beam vibratory element (30), so as, in use, to cause a deflection of the element in response to an electrical signal supplied to the piezoelectric device. An apparatus in accordance with any one of the preceding claims, including means (46) for sensing the vibrations of the balanced beam vibratory element (30) and generating signal in response to it; and means responsive to the signal generated by the vibration sensing means (46) for controlling the drive means (38) so that the balanced beam vibratory element (30) is caused to vibrate at a predetermined frequency. An apparatus according to claim 8, wherein the sensing means include a magnetic coil disposed adjacent to an end of the balanced beam vibratory element (30), whereby the oscillation of the element (30) induces an electrical signal in the magnetic coil in response to the oscillation of that end of the vibratory element (30). An apparatus according to claim 8 or 9, wherein the sensing means includes a piezoelectrically responsive film (46) operatively affixed to one surface of the balanced beam vibratory element (30) so as to produce an electrical signal in response to the deflection of the vibratory element (30).

Inventors: WERNER ALAN J JR, WERNER, ALAN J., JR.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1993
Main classification: G01R29
IPC codes: G01R29, G01R15