Capactive sensor for sensing the amount of material in a container

Abstract

One embodiment of the invention is a capacitive sensor for sensing the amount of material in a container. This embodiment includes an oscillator that generates an oscillating signal. This oscillator includes an integrating amplifier that is formed by (1) an operational amplifier that has a low-impedance output and a virtually-grounded input, and (2) a capacitor that has a capacitance which depends on the amount of material in the container. The capacitor connects between the low impedance output and the virtually-grounded input of the operational amplifier. In some embodiments, the operational amplifier includes a first amplifier and a second amplifier that are connected in series, with an output of the second amplifier fed back through the capacitor to the input of the first amplifier. Another embodiment of the invention is a capacitive toner-level sensor that senses the amount of toner material in a toner cartridge of a reprographic device. This embodiment also includes an oscillator that generates an oscillating signal. The oscillator includes a toner-capacitor, a capacitance multiplier, and a resistance multiplier. The toner-capacitor has a capacitance that depends on the toner level in the container. The toner-capacitor also has a first electrode and a second electrode. The capacitance multiplier couples to the second electrode in order to drive this electrode to a negative voltage, while the resistance multiplier couples to the first electrode in order to reduce the current flow to and from the first electrode. In some embodiments of the invention, one of the electrodes of the toner capacitor is a conductive component part of a toner cartridge, while the other electrode is positioned outside of the toner cartridge. In some of these embodiments, the conductive component part is a developer roller.

We claim:  
     
         1 . A capacitive sensor for sensing an amount of a material in a container, said sensor comprising: 
 an oscillator generating an oscillating signal, said oscillator having an operational amplifier and a capacitor,    the operational amplifier having a low-impedance output and a virtually-grounded input, and    the capacitor connected between the low impedance output and the virtually-grounded input, wherein the capacitor has a capacitance that depends on the amount of material in the container, and wherein the frequency of the generated oscillating signal depends on the capacitance of the capacitor.    
     
     
         2 . The capacitive sensor of  claim 1 , wherein the operational amplifier includes a first amplifier and a second amplifier that are connected in series, wherein an output of the second amplifier is fedback through said capacitor to an input of the first amplifier.  
     
     
         3 . The capacitive sensor of  claim 2 , wherein said first and second amplifiers have compensations that prevent spurious oscillations at the output of the second amplifier.  
     
     
         4 . The capacitive sensor of  claim 2  further comprising a frequency analyzing circuit that couples to the oscillator to receive the oscillating output of the oscillator, and generates a signal when the frequency of the oscillating signal crosses a threshold level.  
     
     
         5 . The capacitive sensor of  claim 4  wherein the frequency analyzing circuit couples to the oscillator through a Schmidt trigger.  
     
     
         6 . The capacitive sensor of  claim 1 , wherein the sensor is a toner sensor that senses the amount of a toner in a toner cartridge, wherein the capacitor is a toner capacitor that has two electrodes, wherein one of the electrodes is a conductive component part of the toner cartridge.  
     
     
         7 . The capacitive sensor of  claim 6 , wherein the conductive component part is a developer roller.  
     
     
         8 . The capacitive sensor of  claim 6 , wherein the other electrode is positioned outside the toner cartridge.  
     
     
         9 . The capacitive sensor of  claim 8 , wherein at least one of the electrodes is protected from potentials of neighboring objects by a guard electrode.  
     
     
         10 . A capacitive sensor for sensing an amount of toner material in a toner cartridge of a reprographic device, said sensor comprising 
 an oscillator generating an oscillating signal, the oscillator having a capacitor with a capacitance, the capacitance of the capacitor depending on the amount of toner in the cartridge,    the capacitor having a first electrode and a second electrode; and    a capacitance multiplier coupled to the second electrode, said capacitance multiplier driving the second electrode to a negative voltage.    
     
     
         11 . The capacitive sensor of  claim 10 , said sensor further comprising: 
 a resistance multiplier coupled to the first electrode, the resistance multiplier reducing the current flow to and from the first electrode.    
     
     
         12 . The capacitive sensor of  claim 10 , said sensor further comprising: 
 a comparator coupled to the capacitor to receive the potential on the first electrode,    wherein the comparator outputs a first potential when the potential on the first electrode reaches a first threshold level, and outputs a second potential when the potential on the first electrode reaches a second threshold level.    
     
     
         13 . The capacitive sensor of  claim 12 , said sensor further comprising: 
 a discharge transistor coupled to the first electrode, wherein said transistor turns on to discharge the potential on the first electrode when the comparator outputs the first potential, and turns off to allow the first electrode to charge when the comparator outputs the second potential.    
     
     
         14 . The capacitive sensor of  claim 13 , wherein the comparator and discharge transistor are part of a 555-timer.  
     
     
         15 . The capacitive sensor of  claim 10  further comprising a frequency analyzing circuit that couples to the oscillator to receive the oscillating output of the oscillator, and generates a signal when the frequency of the oscillating signal crosses a threshold level.  
     
     
         16 . A toner-level sensor for capacitively sensing an amount of toner material in a toner cartridge of a reprographic device, said sensor comprising: 
 a toner capacitor having two electrodes, wherein one of the electrodes is a conductive component part of the toner cartridge, and the other electrode is positioned outside of the toner cartridge.    
     
     
         17 . The capacitive sensor of  claim 16 , wherein the toner cartridge includes a toner container that stores the toner material, wherein the conductive component part is adjacent to a position where the toner material recedes as the toner material is depleted.  
     
     
         18 . The capacitive sensor of  claim 16 , wherein the toner cartridge includes a toner container that stores the toner material, said toner container having an interior surface, wherein the conductive component part partially forms the interior surface of the toner container.  
     
     
         19 . The capacitive sensor of  claim 16 , wherein the conductive component part is a developer roller.  
     
     
         20 . The capacitive sensor of  claim 16 , wherein one of the electrodes is protected from potentials of neighboring objects by a guard electrode.