PWM voltage clamp for driver circuit of an electric fluid dispensing gun and method

Abstract

An electric fluid dispenser for dispensing a fluid onto a substrate. A power switching circuit is connected to an unregulated power supply providing a varying voltage. A solenoid connected to the power switching circuit operates a dispensing valve to move between open and closed positions. A control circuit is responsive to the varying voltage from the power supply and provides a drive signal to the power switching circuit having a time variable component determined by the varying voltage. The power switching circuit, in response to the drive signal, provides an output signal to the solenoid that causes the dispensing valve to move between the open and closed positions substantially independent of the varying voltage from the unregulated power supply.

1 . A driver circuit for an electrically operated fluid dispenser dispensing a fluid onto a substrate, the fluid dispenser having a dispensing valve movable between open and closed positions and a solenoid coil operative to cause the dispensing valve to move between the open and closed positions, the driver circuit comprising: 
 a power switching circuit operably connected to the solenoid coil;    a power source providing an unregulated voltage to said power switching circuit; and    a power switch control operable to cause said power switching circuit to provide a stepped current waveform to the solenoid coil, said stepped waveform comprising an initial peak current followed by a lesser hold current, said initial peak current having a rate of current flow represented by a slope of a leading edge of said initial peak current, said power switch control operable to maintain the slope of the leading edge of said initial peak current substantially constant in response to changes in said unregulated output voltage.    
   
   
       2 . The driver circuit of  claim 1  wherein said power switch control comprises a line voltage compensator responsive to said unregulated output voltage and being operable to cause said power switch control to maintain the slope of the leading edge of said initial peak current substantially constant in response to said unregulated output voltage changing in magnitude.  
   
   
       3 . The driver circuit of  claim 2  wherein said line voltage compensator causes the power switching circuit to provide a first slope of a leading edge of said initial peak current corresponding to a desired magnitude of said unregulated output voltage.  
   
   
       4 . The driver circuit of  claim 3  wherein said line voltage compensator maintains the first slope of the leading edge of said initial peak current substantially constant in response to said unregulated output voltage deviating from said desired magnitude.  
   
   
       5 . The driver circuit of  claim 3  wherein said first slope of the leading edge of said initial peak current corresponds to a lowest expected magnitude of the unregulated output voltage.  
   
   
       6 . The driver circuit of  claim 2  wherein said line voltage compensator is operable to cause said power switch control to modulate the leading edge of said initial peak current with a duty cycle determined as an inverse function of the unregulated output voltage, thereby maintaining a time required to move said valve to an open position substantially constant.  
   
   
       7 . The driver circuit of claim I wherein said hold current provides a rate of current flow represented by a slope of a trailing edge of said hold current, the slope of the trailing edge being produced in response to said unregulated output voltage, said line voltage compensator being operable to cause said power switch control to modulate the trailing edge of said hold current with a duty cycle determined as an inverse function of the unregulated output voltage, thereby maintaining a time required to move said valve to a closed position substantially constant.  
   
   
       8 . The driver circuit of  claim 2  wherein said power switch control further comprises: 
 a waveform generator producing a stepped waveform representative of said initial peak current followed by said lesser hold current;    a current sensor providing a current feedback signal representing current flow in the solenoid coil;    a summing node responsive to said stepped waveform from said waveform generator and said current feedback signal;    a hysteresis modulator connected to an output of said summing node;    a pulse width modulator; and    a first logic circuit having inputs connected to an output of said hysteresis modulator and an output of said line voltage compensator and causing the leading edge of said initial peak current to be modulated with a duty cycle determined as an inverse function of the unregulated output voltage.    
   
   
       9 . The driver circuit  claim 8  wherein the power switch control further comprises a second logic circuit having an input connected to said output of said line voltage compensator and causing the trailing edge of said hold current to be modulated with a duty cycle determined as an inverse function of the unregulated output voltage.  
   
   
       10 . The driver circuit of  claim 2  wherein said line voltage compensator comprises a pulse width modulator.  
   
   
       11 . The driver circuit of  claim 10  wherein said pulse width modulator is a fixed frequency pulse width modulator.  
   
   
       12 . The driver circuit of claim I further comprising a system control providing a trigger signal to said waveform generator for initiating a generation of said stepped waveform.  
   
   
       13 . A driver circuit for an electrically operated fluid dispenser dispensing a fluid onto a substrate, the fluid dispenser having a dispensing valve movable between open and closed positions and a solenoid coil operative to cause the dispensing valve to move between the open and closed positions, the driver circuit comprising: 
 switching means for alternately supplying and interrupting a flow of current to the solenoid coil;    means for providing an unregulated voltage to said switching means; and    means for providing a stepped current waveform to said switching means, said stepped waveform comprising an initial peak current followed by a lesser hold current, said initial peak current having a rate of current flow represented by a slope of a leading edge of said initial peak current, said providing means operable to maintain the slope of the leading edge of said initial peak current substantially constant in response to changes in said unregulated output voltage.    
   
   
       14 . A method of operating a driver circuit for an electric fluid dispenser operable to dispense a fluid onto a substrate, the fluid dispenser having a dispensing valve operatively connected to a solenoid coil, the solenoid coil being operative to move the dispensing valve between open and closed positions for controlling a flow of the fluid from the electric fluid dispenser, the method comprising: 
 providing a power switching circuit connected to the solenoid coil;    providing a power source supplying an unregulated output voltage to the power switching circuit;    producing with the power switching circuit a stepped current waveform having an initial peak current followed by a hold current, the initial peak current providing a rate of current flow represented by a slope of a leading edge of the initial peak current, the slope of the leading edge being produced in response to the unregulated output voltage;    maintaining the slope of the leading edge of the initial peak current substantially constant in response to the unregulated output voltage changing in magnitude; and    applying the stepped current waveform to the solenoid coil to operate the solenoid coil and the dispensing valve with an operational speed substantially independent of changes in the unregulated output voltage.    
   
   
       15 . The method of  claim 13  further comprising providing a slope of a leading edge of said initial peak current corresponding to a desired magnitude of the unregulated output voltage.  
   
   
       16 . The method of  claim 14  further comprising maintaining the slope of the leading edge of said initial peak current substantially constant in response to the unregulated output voltage deviating from said desired magnitude.  
   
   
       17 . The method of  claim 15  wherein said desired magnitude of the unregulated output voltage is a lowest expected magnitude of the unregulated output voltage.  
   
   
       18 . The method of  claim 13  further comprising: 
 modulating the leading edge of the initial peak current with a duty cycle determined as an inverse function of the unregulated output voltage to maintain the slope of the leading edge of the initial peak current substantially constant; and    applying the initial peak current to the solenoid coil to maintain the actuation time to open the dispensing valve substantially constant and independent of changes in the unregulated output voltage.    
   
   
       19 . The method of  claim 13  wherein the hold current provides a rate of current flow represented by a slope of a trailing edge of the hold current, the slope of the trailing edge being produced in response to the unregulated output voltage, the method further comprising: 
 maintaining the slope of the trailing edge of the hold current substantially constant in response to the unregulated output voltage changing in magnitude; and    applying the hold current to the solenoid coil to operate the solenoid coil and the dispensing valve with an actuation time to close the dispensing valve substantially independent of changes in the unregulated output voltage.    
   
   
       20 . The method of  claim 18  further comprising modulating the trailing edge of the hold current with a duty cycle determined as an inverse function of the unregulated output voltage.