BACKGROUND Throughout the history of writing instruments, efforts have been made to optimally control the flow of a writing fluid, e. g., an ink, from the tip of the writing instrument. In an effort to better control ink flow, writing instruments have evolved from quill pens to fountain pens to ball point, felt tip and roller ball pens. Efforts continue to further optimize the writing properties of ink pens, e. g., writing smoothness and ink laydown.

Correction fluids are often utilized to obscure typewritten or hand written markings on a surface, e. g., paper, and to allow a corrective marking to be applied to the surface. Correction fluids have been traditionally provided in a small bottle, and brushed onto the surface using a brush that is stored within the bottle. There is a continuing effort to develop other types of dispensers for correction fluids.

SUMMARY The inventors have discovered that writing fluids, such as inks and correction fluids, can be dispensed from a writing instrument with excellent flow control by providing an ink jet device to meter delivery of the fluid to the tip of the writing instrument. As a result, writing tips can be designed that have excellent writing smoothness without the design constraint that the tip must meter the flow of ink to the paper. Instead, the ink laydown during writing can be controlled by adjusting the ink jet device to deliver more or less ink to the writing tip. The use of an ink jet device to control ink flow also prevents leakage of low viscosity inks from roller ball type pens, many of which may exhibit a tendency to leak under increased pressure conditions.

In one aspect, the invention features a writing instrument that includes (a) an elongated body constructed to be held by a user, (b) a reservoir, within the body, containing a writing fluid, (c) at one end of the body, a writing tip constructed to form a marking with the writing

fluid on a surface when the user contacts the tip against the surface with a writing motion, and (d) an ink jet device for delivering the writing fluid to the writing tip.

Preferred embodiments may include one or more of the following features. The ink jet device is selected from the group consisting of thermal ink jet heads and piezoelectric ink jet heads. The writing fluid has a viscosity of less than 300 cps. The writing instrument has a viscosity of from about 1 to 10 cps. The writing tip is selected from the group consisting of ball points, fountain pen nibs, porous nibs and roller ball tips. The reservoir includes a balloon-like bag. Alternatively, the reservoir includes a foam plug. The writing fluid includes a correction fluid or an ink. The writing instrument includes a second reservoir within the body, containing a second writing fluid different from the writing fluid. The writing fluid is an ink and the second writing fluid is a correction fluid. The writing fluids are inks of different colors. The writing instrument further includes an actuator constructed to automatically activate the ink jet device when writing pressure is detected, and a pressure sensor constructed to detect writing pressure at the writing tip. The writing instrument further includes an accelerometer constructed to detect writing motion of the writing instrument. The writing instrument further includes an adjustment device constructed to allow adjustment of the laydown of writing fluid by the writing tip, e. g., by the user of the pen. The adjustment device includes an adjustment control, external to the writing instrument, constructed to be manually set by a user to obtain a desired laydown. The writing tip is removable and replaceable. The writing instrument includes a set of interchangeable writing tips.

In another aspect, the invention features a writing instrument that includes (a) a body, (b) a reservoir, within the body, containing a correction fluid, (c) at one end of the body, a writing tip constructed to form a corrective marking with the correction fluid on a surface, and (d) an ink jet device for delivering the correction fluid to the tip.

Preferred embodiments include one or more of the following features. The ink jet device is selected from the group consisting of thermal ink jet heads and piezoelectric ink jet heads. The writing tip includes a ball point. The reservoir includes a balloon-like bag, or, alternatively, a foam plug. The writing instrument further includes a second reservoir within the body, containing a writing fluid, e. g., an ink. The writing instrument further includes an actuator constructed to automatically activate the ink jet device when pressure at the writing tip is detected. The writing instrument further includes a pressure sensor constructed to detect

writing pressure at the writing tip. The writing instrument further includes an accelerometer for detecting writing motion of the writing instrument. The writing instrument further includes an adjustment device constructed to allow adjustment of the laydown of correction fluid onto the surface, e. g., by the user of the pen. The adjustment device includes an adjustment control, external to the writing instrument, constructed to be manually set by a user to obtain a desired laydown.

In a further aspect, the invention features a method of forming a written marking on a surface, including (a) supplying a writing fluid to the tip of a writing instrument using an ink jet device, and (b) applying the writing fluid from the tip to the surface by writing with the writing instrument.

Preferred embodiments include one or more of the following features. The writing fluid is an ink or a correction fluid. The writing instrument contains both a correction fluid and an ink, and the method further includes selecting the desired writing fluid for application to the surface. The writing instrument is constructed to allow the user to adjust the laydown of the writing fluid, and the method further includes adjusting the laydown.

In yet another aspect, the invention features a writing instrument that includes (a) a body, (b) a reservoir, within the body, containing a writing fluid, (c) at one end of the body, a tip constructed to form a marking with the writing fluid on a surface, (d) an ink jet device for delivering the writing fluid to the tip, and (e) an adjustment device constructed to allow the laydown of the writing fluid by the tip to be adjusted.

In preferred embodiments, the adjustment device includes a potentiometer for electronically adjusting the firing frequency of the ink jet device, and an actuator positioned to allow a user of the writing instrument to adjust the laydown by varying the setting of the potentiometer.

The invention also features a writing instrument that includes (a) a body, (b) a reservoir, within the body, containing a writing fluid, (c) at one end of the body, a tip constructed to form a marking with the writing fluid on a surface, (d) an ink jet device for delivering the writing fluid to the tip, and (e) an actuator constructed to activate the ink jet device in response to pressure at the writing tip.

In preferred embodiments, the actuator includes a pressure sensor, and is constructed to activate the ink jet when writing pressure is detected by the pressure sensor, and the actuator is

constructed to differentiate between writing pressure and non-writing pressure, and only activate the ink jet device in response to writing pressure.

In another embodiment, the invention features a writing instrument that includes (a) a body, (b) a reservoir, within the body, containing a writing fluid, (c) at one end of the body, a roller ball writing tip constructed to form a marking with the writing fluid on a surface, and (d) an ink jet device for delivering the writing fluid to the writing tip.

Preferred embodiments include one or more of the following features. The writing fluid has a viscosity of less than about 300 cps. The writing fluid has a viscosity of from about 1 to 10 cps. The writing instrument is constructed to be stored without a cap. The ink jet device is a thermal ink jet head.

The term"writing tip", as used herein, refers to any structure that is capable of forming a line of writing fluid on a substrate when the tip is used to write on the surface.

The term"correction fluid", as used herein, refers to a fluid that is capable of obscuring a marking, e. g., a typewritten, handwritten or printed marking, on a substrate and providing a dried film that is capable of receiving a corrective marking.

The term"written marking", as used herein, refers to a marking formed by the user of the writing instrument that includes one or more characters, e. g., letters and/or numbers.

Other features and advantages of the invention will be apparent from the description and drawings.

DESCRIPTION OF DRAWINGS Fig. 1 is a perspective view of a writing instrument according to one embodiment of the invention. Figs. 1A and 1B are cross-sectional views of the writing instrument of Fig. 1, taken along lines A-A and B-B, respectively.

Fig. 2 is an enlarged partial cross-sectional view of the front portion of the writing instrument of Fig. 1. Fig. 2A is an exploded view of the front portion of the writing instrument of Fig. 1. Fig. 2B is an enlarged perspective view of a strain gage mount and strain gage used in the writing instrument of Fig. 1.

Fig. 3 is a highly enlarged radial cross-sectional view of the writing instrument of Fig 1, with the ink jet head shown in dashed lines.

Figs. 4 and 5 are schematic diagrams of the circuitry used to control and power the writing instrument of Fig. 1.

Fig. 6 is an enlarged partial cross-sectional view of the front portion of a writing instrument according to an alternate embodiment of the invention.

Fig. 7 is an enlarged partial cross-sectional view of the front portion of a writing instrument according to another alternate embodiment of the invention.

Figs. 8-8B are enlarged partial cross-sectional views of removable nibs that can be press-fit onto a writing instrument according to an alternate embodiment of the invention, having a removable writing tip.

DETAILED DESCRIPTION Referring to Fig. 1, writing instrument 10 includes a body 12 and a writing tip 14. Body 12 includes a removable portion 16, through which batteries can be removed and replaced.

As shown in Figs. 1A and 1B, body 12 is hollow, and contains an ink reservoir 20, preferably a flexible, balloon-like bladder, in which ink is stored, and a printhead 18 that meters the flow of the ink to writing tip 14 as will be described in detail below. Body 12 also contains a printed circuit board 22, for controlling the operation of the printhead, and batteries 24, for supplying power to the printhead. The printed circuit board and batteries are connected to printhead 18 by leads (not shown). As shown in Fig. 2, the writing instrument 10 also includes a nib 11 that mounts the writing tip on the body 12, a strain gage mount 13, discussed further below, and a reservoir transition 15 and reservoir transition cover 17, between which the rim of the reservoir (not shown) is sandwiched.

Referring now to Fig. 2, when the pen is not in use printhead 18 acts as a barrier, preventing ink from flowing from reservoir 20 to writing tip 14. To enhance the seal, when the pen is assembled a small amount of ink is squeezed out of the reservoir to collapse the flexible walls, creating negative pressure within the reservoir. This negative pressure is maintained as ink is exhausted from the pen during use.

The flow of ink from the reservoir to the writing tip is controlled by the printhead 18. In the embodiment shown in Figs. 1A-2, printhead 18 is a thermal jet printhead. The thermal jet printhead, shown in detail, and highly enlarged, in Fig. 3, includes a glass substrate 26 having a feed reservoir 28 in one surface (the surface facing reservoir 20). Feed reservoir 28 holds a

small volume of ink, and communicates with an array of tiny apertures 30 (highly enlarged for clarity) that extend from the feed reservoir through the substrate and communicate with the back of the writing tip. Normally, due to the very small diameter of apertures 30 (typically less than 0.002\' and the surface tension of the ink, the ink will not pass through apertures 30. However, when the printhead 18 is actuated, by supplying current from batteries 24 to one or more of an array of resistors (not shown), ink in the immediate vicinity of the resistors is heated, causing it to expand and be forced through apertures 30. This procedure is a conventional thermal ink jet delivery technique that is well known in the ink jet field. A suitable thermal ink jet printhead is available from HP under the tradename THINKJET (the glass substrate for this printhead is generally too large to fit within a pen body, but this type of printhead can be modified to a smaller size by cutting the glass and microsoldering leads to the printhead).

Thus, when printhead 18 is actuated, droplets of ink are delivered from reservoir 20, through apertures 30, to the back of writing tip 14. The amount of ink that is delivered to the writing tip may be varied by changing the number of apertures through which ink is forced (by selecting the resistors to which current is delivered), and/or by changing the frequency at which current is delivered to the resistors (the"firing frequency"of the ink jet printhead). As a result, ink laydown, and thus the darkness of the line produced, can be easily controlled without changing the design of the pen. For example, a ball point having a given ball geometry can be used to form lines of varying darkness, depending on the amount of ink delivered to the point by the ink jet, whereas generally in conventional ball pens a small diameter ball will form a lighter line and a large diameter ball will form a darker line because of the relative amounts of ink that are metered by the balls. Thus, advantageously, a large diameter ball can be used to provide writing smoothness, without necessitating a dark line.

Preferred firing frequencies will depend upon the ink used and the laydown desired.

However, preferred firing frequencies are generally from about 600 to 1500 Hz for roller ball- tips. The firing frequency is adjustable within a predetermined range that is fixed by the electronics of the system. The firing frequency is adjusted within this range by a potentiometer that is controlled by a user-controlled actuator, e. g., a knob (not shown) at the end of the writing instrument.

Suitable circuitry for printed circuit board 22 is shown in Figs. 4 and 5.

Fig. 4 shows a feedback and control circuit that is used to control the firing of the ink jet.

The feedback and control circuit 100 includes an acceleration feedback circuit 102, an oscillator circuit 104, a pulse width modulating circuit 106, and an ink jet driver circuit 108.

The acceleration feedback circuit includes a two-axis accelerometer and a signal conditioning instrumentation amplifier. The acceleration feedback circuit provides all the necessary excitation for the accelerometer and, after conditioning, produces a clean amplified signal to be used by the oscillator circuit The accelerometer is mounted in such a way as to provide the best response in the x and y axes. The accelerometer detects when the pen is moving, to determine when the printhead should be fired.

The oscillator circuit is a voltage to frequency converter. This converter has the ability to scale the output frequency in a range acceptable for the ink jet head. The output of this circuit is a varying frequency of about 50% duty cycle. The output frequency is dependent on the signal received from the acceleration feedback circuit The output frequency is then fed into a pulse width modulating circuit 106. The pulse width is scaled to optimize the ink jet performance and is range selectable. This circuit will output a pulse signal on every falling edge of the input frequency. The output pulses from this circuit are fixed with varying frequency (the"firing frequency"of the printhead) determined by the accelerometer feedback circuit. The pulse width modulating circuit also has the ability to switch the output pulses off and on. This is accomplished by a strain gage sensor 21 (Fig. 2B) mounted strategically on strain gage mount 13 to sense forces applied to the tip of the pen and connected to the printed circuit board 22. If a force is detected, the output switches on. The signal from the pulse width modulating circuit is fed into an ink jet driver circuit.

In response to the signal, the ink jet driver circuit fires an electronic switch to turn on the ink jet when the pulse is high. The driver is off when the pulse signal is low. The current to the ink jet resistors is determined by controlling the voltage level applied to the ink jet (Vdd). Vdd is set by the power regulating and control circuits (logic power circuit) shown in Fig. 6 and described below.

As shown in Fig. 5, the power regulating and control circuits are designed to run from two 1.5 volt batteries connected to run in series to provide 3 volts. Suitable batteries will be of an appropriate size to fit within the pen housing. The circuit will supply all voltages and current capacities to apply circuit logic power and power the ink jet. The power supplied to the ink jet

is adjustable, to optimize ink jet performance by compensating for the reduction in power input as the batteries are drained during use.

In an alternative embodiment, shown in Fig. 6, printhead 18 is a piezoelectric ink jet printhead. In this embodiment, printhead 18 includes a nozzle plate 34 that has a single tiny aperture 37 in fluid communication with a frustoconical chamber 36 that is defined, in part, by a housing 38. A tube 40, formed of a piezoelectric flexible polymer, is positioned within housing 38 and defines a cylindrical side wall of chamber 36. Chamber 36 is normally filled with ink, with the remaining ink supply being stored in a reservoir, e. g., in an ink foam 42 as shown in Fig. 6. Suitable ink foams (also known as"porous media\') are commercially available from Aubex, Tokyo, Japan, and Teibow, Japan. (Alternatively, the reservoir may be a flexible bladder, as shown in Fig. 1A, or any other suitable reservoir. Similarly, an ink foam can be used as the reservoir in the embodiment shown in Fig. 1A.) Like the glass substrate discussed above, nozzle plate 34 prevents ink from flowing to the writing tip 14 until the printhead 18 is actuated. When the printhead 18 is actuated, current flows from the batteries 24, through leads 44, to tube 40. Because tube 40 is formed of a piezoelectric material, the walls of tube 40 contract in response to the current, forcing the incompressible ink out through the tiny aperture in nozzle plate 34 and thus to the writing tip 14. In this embodiment, ink laydown can be varied by changing the frequency of activation of the piezoelectric material by the current A suitable piezoelectric material is PZT-4 piezoelectric polymer, commercially available from Morgan Matroc Electro Ceramics. The piezoelectric ink jet printhead can be used with higher viscosity inks (e. g., 150 to 300 cps) than the thermal ink jet printhead, which is typically used with low viscosity (e. g. 1-10 cps) aqueous inks.

A suitable diameter for the aperture (s) in printhead 18 will depend on the surface tension and wetting characteristics of the ink, and can be determined as is well known in the ink jet field. Typically, the apertures will have a diameter of from about 0.002 inch to 0.0035 inch.

Other embodiments are within the claims.

For example, as shown in Fig. 7, the writing instrument may include two reservoirs 20A, 20B, for storing two different writing fluids, e. g., an ink and a correction fluid or inks of different colors. Advantageously, because the writing fluid is"fired"at the writing tip at high velocity, there is very little writing fluid on the interior walls of the writing tip, and thus there is little or no cross-contamination between the two fluids when a change is made by the user. A

change would be made by the user actuating a selection switch 50 that sends a signal to the printed circuit board indicating which resistors should be fired (this embodiment utilizes a multicolor thermal ink jet printhead). A writing instrument utilizing the piezoelectric printhead can also be provided with multiple writing fluids by providing separate reservoirs and apertures for the different fluids.

Moreover, as shown in Figs. 8-8B, the writing instrument may include a selection of interchangeable, removable writing tips, e. g., a ballpoint (Fig. 8), a roller point (Fig. 8A) or a porous nib (Fig. 8B). The removable writing tips may be press fit onto the body of the writing instrument, or attached in any suitable manner.