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
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
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
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
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
According to this invention an apparatus for measuring the magnitude
of an electrostatic field comprising:
is characterised in that the vibratory element is a balanced
beam vibratory element.
- 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;
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
- 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
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.