Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-11-23
2002-08-27
Vo, Anh T. N. (Department: 2861)
Incremental printing of symbolic information
Ink jet
Controller
C347S010000
Reexamination Certificate
active
06439678
ABSTRACT:
BACKGROUND OF THE INVENTIONS
1. Field of Inventions
The present invention relates generally to a method and apparatus for controlling firing energy in a printer and, more specifically, to a method and apparatus for non-saturated switching for firing energy control in an inkjet printer.
2. Description of the Related Art
Thermal inkjet printers employ nozzle resistors to fire drops of ink. A sufficient amount of energy must be provided to each nozzle resistor to properly fire the drops of ink. If an amount of energy delivered to a nozzle resistor is too low, there may not be enough heat generated to eject an ink drop, or the velocity of the drop may be too low. Either condition may result in visible defects in the printed page. If the amount of energy delivered to a nozzle resistor is too high, the resistor may get too hot resulting in decreased pen life. For these reasons, accurate energy control is essential for proper operation of thermal inkjet pens.
Referring to
FIG. 1
, a control electronics/ inkjet pen system
100
of an inkjet printer includes a main electronics board
102
, an inkjet pen
104
, an interconnecting cable
106
and associated connectors
108
,
110
at each end of the cable
106
. An exemplary preferred electronics board
102
includes a voltage regulator circuit
112
for creating an accurate pen voltage and a pen driver integrated circuit (IC)
114
containing solid state switches for turning nozzle currents on and off.
When the driver switches are turned on, electrical current flows from the pen voltage supply at board
102
, through the cable
109
, through the nozzle resistors in the pen
104
, and returns back through the cable
106
to the ground side of the pen voltage supply. Since none of these components are ideal, there are losses associated with each of them. For instance, switches of the pen driver IC
114
have some resistance that creates a voltage drop when current flows through them. Likewise, the cable
106
and connectors
108
,
110
have resistances of their own resulting in further losses. Since these resistances are not exactly known and vary from printer to printer and over temperature, the amount of current flowing through the nozzle resistors is difficult to perfectly control. Other contributors to energy errors stem from the tolerance of the generated pen supply voltage and variations in the resistances of the nozzle resistors themselves.
FIG. 2
shows an electrical schematic representation of the system of
FIG. 1
including non-ideal parameters which contribute to errors in delivered energy. In this schematic, V
Supply
represents the voltage of the pen voltage supply, R
Series
represents the series combination of the cable and connector resistances, T
Fire
is the time for which the switch is closed, and V
Switch
is the voltage drop across the switch when current is flowing while the switch is closed. Energy variations due to the loss across the switch contribute significantly to the energy error and, for the electrical schematic of
FIG. 2
, are calculated as follows:
E
Fire
=
(
V
supply
-
V
switch
R
Series
+
R
Pen
)
2
×
R
Pen
×
T
Fire
In this equation, the current flowing through R
Pen
is given by the term in parentheses, which is equivalent to the voltage across both resistances divided by the sum of the resistances. Since the energy is proportional to the square of the current, the energy will change at approximately twice the rate the current changes. In other words, if the current is allowed to vary by ±1%, the energy will vary by ±2%. If the current varies by ±5%, the energy will vary by ±10%, etc. This is a result of the fact that a change in something is equivalent to its derivative, and the derivative of x
2
(with respect to x) is 2.
Since the term inside the parentheses is equal to current, the current is proportional to the quantity (V
Supply
−V
Switch
) As this quantity changes, the energy delivered to the pen changes at twice the rate. Assuming the supply voltage is known exactly, it is possible to determine how variations in the switch voltage affect the delivered energy. Since the supply voltage is greater than the switch voltage, a variation in the switch voltage will result in a smaller variation in the overall quantity (V
Supply
−V
Switch
). Thus, variation in current is determined by the following equation.
Variation in current=&Dgr;I=&Dgr;(V
Supply
−V
Switch
)=&Dgr;V
Switch
*(V
Switch
/V
Supply
−V
Switch
)) Eq. 1
where “&Dgr;” indicates a percent variation in the corresponding value. For instance, if V
Supply
is five times greater than V
Switch
, V
Switch
/(V
Supply
−V
Switch
) would be 0.25, and variations in V
Switch
would result in one fourth the variation in current. By way of example, where V
Supply
is 12.0 volts and V
Switch
is 1.3 volts ±30%:
Variation in current=&Dgr;I=30%*(1.3/(12.0−1.3))=3.6%.
Recall that variation (or tolerance) in the energy delivered to the pen is twice the variation in current since energy is proportional to the current squared. Therefore, the energy tolerance due to the switch voltage tolerance is doubled to 7.2%. By itself, this is already in violation of the specified limits for some inkjet pens. An understanding of each of the parameters in the electrical schematic of
FIG. 2
would be useful to the end of tightening all of the tolerances as much as possible. With respect to the switches in the pen driver IC
114
(FIG.
1
), it would be useful to be able to accurately characterize the voltage drop across the switches for improving the accuracy in delivered energy.
Past architectures have attempted to solve this problem by making the switch voltage drop as small as possible. In practice, these switches are transistors (field-effect or bipolar) that are designed to have very low resistance and voltage when they are turned on. By making this voltage very small, the overall error contributed by the switch voltage drop is less (see Equation 1). However, implementing such very low on-resistance transistors in an integrated circuit requires that the transistors occupy a relatively large area of the silicon die. When many of these transistors are contained on the same die (which is usually the case with typical pen driver ICs), the area of the die can become fairly large, resulting in increased cost for the IC. For instance, to reduce the on-resistance between the drain and source (R
DSon
) of a field effect transistor, many small transistors are connected in parallel to form a compound transistor such that the overall channel resistance reduction is proportional to the number of individual transistors used. The R
DSon
of these transistors in typical pen drivers is kept small enough that, when current passes through the switch, the voltage drop is small enough to yield an acceptable variation in energy. Notwithstanding, there remains a need for a method and apparatus for firing energy control in a printer that maintains an acceptable tolerance for the voltage drop across the driver transistors to precisely control the amount of energy provided to the nozzle resistors while keeping the size of the driver transistors relatively small.
SUMMARY OF THE INVENTION
According to the present invention, a method and apparatus for controlling firing energy in an inkjet printer reduces energy errors induced by the voltage drop across the switch by first accurately characterizing this voltage drop. Since the voltage drop across the switch is well characterized, the pen voltage can be increased to compensate for this loss (i.e. (V
Supply
−V
Switch
) is kept constant by increasing the supply voltage by an amount equal to the switch voltage drop). The firing energy control implementation of the present invention keeps the voltage across the pen and current well characterized; and the energy delivered to the pen is therefore controlled more accurately. Additionally, the firing energy control implementation of the present invention facilit
Hewlett--Packard Company
Vo Anh T. N.
LandOfFree
Method and apparatus for non-saturated switching for firing... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for non-saturated switching for firing..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for non-saturated switching for firing... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2908069