Patent Application
20080079764
Turning now to
The present invention is concerned with operating in a test mode for determining if the heating resistors Ri are defective. In a test mode, the controller 30 provides an input to the variable power supply 34 which causes it to operate in a second condition and produce a test voltage Vt. The test voltage Vt is lower than the operating voltage Vo. A low voltage sensing circuit 40 senses the reduction in the voltage level when the variable power supply 34 has switched to a test mode and opens switch 42. This action removes a low resistance bypass to a reference resistor Rr. Switch 42 consists for example of a field effect transistor (FET) having an on-resistance which is much less than reference resistance Rr when the switch is on. The step of removing the low resistance bypass of Rr will also be referred to as inserting reference resistor Rr into the circuit. Although Rr is in the circuit even when switch 42 is off, if the on-resistance of switch 42 is less than Rr/3 (and more preferably is less than Rr/10), the circuit behaves approximately as if Rr is not in the circuit, which minimizes the power wasted during the printing operation, particularly if the on-resistance is much lower than the nominal resistance of the heating resistors. A second low voltage sensing circuit 43 responds to reduction in the voltage at the junction between reference resistor Rr and a particular heating resistor Ri and opens switch 44, thereby open circuiting the capacitor C. Switch 32 is closed at this time and there is a serial connection between the resistors Rr and Ri. An analog to digital converter 46 senses the voltage VAD and converts it to a digital signal which is applied to the processor 14. While switch 32 is closed and switches 42 and 44 are open, the same current passes through reference resistor Rr and the particular heating resistor Ri and the voltage at the A/D converter is defined as VAD=Vi. When all of the switches 32 are open (so that none of the heating resistors are in the circuit), and also while switches 42 and 44 are open, the voltage measured at the A/D converter is given by good approximation as VAD˜Vt. This is because the only current flow through Rr is that allowed by the high input impedance of the A/D converter, so that the voltage drop across the reference resistor is negligible. The voltage Vi is a function of the resistor Ri since the same current flows through resistors Rr and Ri. The resistance of Ri is given by the following relationship:
R
i
=R
r
V
i/(Vt−Vi)
The processor 14 can compute the value of the resistance of each resistor Ri and provide the values to the display 18. Alternatively, the value of Vi can be compared with an acceptable range of values and the processor 14 can cause the display 18 to visibly indicate that a particular defective resistor is outside of that acceptable range. Also alternatively, the computation of the value of the resistance can be performed in the host computer.
Turning, now to
Switch 44 is provided by an N-channel FET in the low voltage sensing circuit 40. A simple voltage divider circuit can be provided by resistors 60 and 62, but it is preferable to put a Zener diode 64 in series with resistor 60 as shown in
Another issue with the N-channel FET is to prevent it from turning on when the heating resistor Ri is tested. When a heating resistor Ri is energized, the source voltage of switch 44 goes below 0 V (due to capacitor C). The gate is connected such that it will stay above 0 V. This may cause the N-channel FET to start to turn on. To prevent this from happening, it is preferable to put a diode 66 with a small resistor 68 in series between the source and the gate to pull the gate down with the source. A capacitor 70 is connected between resistors 60 and 62 and causes a delay in the N-channel FET turning on and off. Diode 69 takes resistor 62 out of the circuit when the source of the N-FET goes below 0V allowing the resistor 68 diode 66 combination to be more effective without loading down the gate voltage during printing mode (Vo). It is also important to lower the voltage slowly enough to bleed most of the charge off of C before FET turns off C. This process will be described when
The apparatus and method for monitoring the status of individual circuit elements while isolating them from the driving circuitry can be modified for applications other than an inkjet printer having heater resistors. Applications of interest might include, for example, lights on a scoreboard, an array of light emitting diodes in a display, or a group of relays in a switching system. As will be readily apparent to one skilled in the art, the circuit elements to be monitored will have some electrical characteristic that must be operational, or within a certain range of measurement, if the circuit is to operate properly in the operating mode. This electrical characteristic may be compared to a known reference circuit element. The reference circuit element may be of the same general type as the circuit elements to be monitored (in the same way that reference circuit element Rr is a resistor, similar to the heater resistors). Alternatively, the reference circuit element may be a different type of circuit element than circuit elements to be monitored. For example, suppose the circuit elements to be monitored were transistors or diodes or relays which have an effective resistance in some mode, and the reference circuit element were a resistor.
A common feature in applications of the invention is the effective removal of the known reference circuit element from the circuit in the operating mode, just as switch 42 bypasses the reference circuit element Rr in the first embodiment during operation of the printhead, so that power wastage and voltage drops in Rr are minimized, for example.
Another common feature in applications of this invention is a circuit element of a second type which is connected to the circuit elements to be monitored. This circuit element of the second type, like capacitor C in the first embodiment, is needed for proper operation of the circuit in the operating mode, but would interfere with an accurate monitoring of the circuit elements in a test mode. It is necessary to isolate the circuit element of the second type from the circuit elements to be monitored when in a test mode. This is accomplished by using a switch, analogous to switch 44 from the first embodiment.
Still another common feature in applications of this invention is a variable power supply which is effective in a first condition to produce a first operating voltage, and in a second condition, to produce a second known test voltage. In some embodiments, this voltage will be DC, as in the case of the first embodiment. However, in some other embodiments, the proper operation of the circuit requires an AC voltage from the variable power supply. As will be readily apparent to one skilled in the art, for embodiments having an AC test voltage, additional circuitry (73, 74, 75) such as a peak detector (80) may be incorporated into the measuring circuit, so that the AC voltage can be measured during the test mode.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
