Abstract:
An electromagnetic, pulse width modulated control device includes a solenoid powered by a varying supply voltage and having an actuator movable from an extended position to a pulled in position by a first power level and into a hold in position by a second power level less than the first power level. A pulse width modulated circuit is connected with the solenoid and is responsive to the varying supply voltage for adjusting the pulse width accordingly so as to provide a substantially constant second power level applied to the solenoid to maintain the hold in position.

Description:
FIELD OF THE INVENTION 
     The present invention relates broadly to the field of electromagnetic control devices and, more particularly, pertains to improvements in solenoids controlled by pulse width modulation (PWM). 
     BACKGROUND OF THE INVENTION 
     As is well known, a solenoid is essentially a coil of wire or winding that is wrapped around a hollow bobbin. There is normally a frame or enclosure made of a magnetically conductive material surrounding the unit. Floating inside the hollow area of the bobbin is a piece of magnetically conductive material designated an actuator or plunger. Typically, the plunger is designed with some sort of feature that allows the user to connect whatever they would need to actuate with it. At one end of the solenoid in the center of the hollow bobbin and attached to the frame is a slug of magnetic material known as a backstop. The backstop dictates the maximum pulling distance the plunger can achieve when the coil is energized. The coil is normally wound to parameters that will dictate the amount of magnetic forces placed upon the plunger based upon an application. 
     When a solenoid is energized, the magnetic energy causes the plunger to move toward the backstop. As the plunger approaches the backstop, its level of pull force increases. When the plunger bottoms out on the backstop, the maximum amount of pulling power is achieved. Once in this position, the typical application no longer requires the high level of pulling force to hold the plunger in as is needed to pull the solenoid in. Additionally, a solenoid only requires a fraction of the power in the hold in mode as it does for pulling. Since the wattage of the coil does not change, the solenoid simply wastes energy and produces heat when in the holding mode. Such heat can deleteriously affect the performance of the solenoid. 
     It is desirable to associate the solenoid with a circuit which will monitor source voltage and adjust the PWM duty cycle (ratio of ON time to OFF time) to maintain solenoid coil wattage accordingly. For instance, if a solenoid is designed to operate with a full voltage pull in level of 40 watts at 12 volts and a 5 watt level to hold, using PWM this can be achieved by selecting the proper hold in duty cycle. However, if the voltage should fluctuate to 16 volts, the solenoid would be operating at a value higher than the 5 watt level. Though the holding force would increase, so would the heat rise of the solenoid and potentially create an adverse condition. Likewise, should the source voltage drop to 8 volts, the solenoid would operate far under the 5 watt level and have much reduced holding power. By sampling the source voltage, the PWM duty cycle is increased should the voltage drop, and decreased as the voltage increases. The net result is that whatever the source voltage is, the solenoid coil current will remain constant. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a pulse width modulated solenoid for use in various applications. In the pulse width modulated solenoid, full voltage is applied to the solenoid to move the actuator from an extended position to a pulled in position. Once the solenoid is in the pulled in position, the voltage is pulse width modulated to reduce the power drawn from the power supply during the hold in state. During the hold in state, the circuit monitors the value of the source voltage and increases or decreases the pulse width based upon the source voltage. In this manner, the control circuit is able to maintain relatively constant power applied to the solenoid to maintain the hold in position. 
     It is one object of the present invention to provide a versatile, adaptable and highly efficient solenoid which maintains its holding power regardless of various supply voltage. 
     It is also an object of the present invention to provide a pulse width modulated solenoid with a relatively simple control circuit which will monitor source voltage and adjust duty cycle accordingly. 
     It is an additional object of the present invention to provide a solenoid with substantially higher pulling force rating which can be used in continuous duty applications. It is another object of the present invention to provide a solenoid having a single winding which is employed for both pull in and hold phases. 
     It is a further object of the present invention to provide a reduced size solenoid which is designed with adequate pull in and holding forces and is accommodated in confined spaces. 
     In one aspect of the invention, a circuit controlled, pulse width modulated solenoid is powered by a varying supply voltage. The invention is improved wherein the circuit monitors the varying supply voltage and adjusts the pulse width so that the current flow through the solenoid remains constant. 
     The invention contemplates a method of controlling a pulse width modulated solenoid powered by a varying supply voltage and having a movable actuator. The method includes the steps of applying power at a first level to move the solenoid actuator from an extended position to a pull in position; and utilizing a pulse generating circuit to define a pulse width mode for applying power at a substantially constant second level less than the first power level to move the solenoid actuator from the pull in position to a hold in position and maintain the hold in position. 
     In another aspect of the invention, an electromagnetic, pulse width modulated control device includes a solenoid powered by a varying supply voltage and having an actuator movable from an extended position to a pulled in position by a first power level and to a hold in position by a second power level less than the first power level. A pulse width modulated circuit is connected with the solenoid and is responsive to the varying supply voltage for adjusting the pulse width accordingly so as to provide a substantially constant second power level applied to the solenoid to maintain the hold in position. The circuit includes a microcontroller with A/D converter. A voltage regulator is connected between the varying supply voltage and the microcontroller. The microcontroller has an analog input connected to a reference percentage of the varying source voltage obtained through a pair of serially connected resistors. The microcontroller has a digital output connected to a MOSFET which selectively turns the circuit on to allow a substantially constant current flow through the solenoid. A flyback diode is connected in parallel across the solenoid to provide a BACK EMF current path flow in one direction only and prevent burn-out of the MOSFET. The microcontroller also has an internal A/D convertor for converting an analog input voltage into a digital output fed to the MOSFET. A resistor is connected between the voltage regulator and the microcontroller for initiating the microcontroller program sequence. 
     In yet another aspect of the invention, a control apparatus for regulating a duty cycle type solenoid is powered by a varying supply voltage. A pulse generating means is responsive to the varying supply voltage for generating a driving pulse to the duty cycle type solenoid at a predetermined duty cycle ratio so as to maintain a substantially constant current flow through the solenoid. 
     Yet another aspect of the invention relates to a method of controlling a pulse width modulated solenoid powered by a varying supply voltage and connected to a pulse width generating control circuit. The method includes the steps of monitoring the supply voltage with the control circuit; and adjusting the pulse width of the control circuit according to the source voltage so as to maintain a substantially constant current flow through the solenoid. 
    
    
     Various other objects, features and advantages of the invention will be made apparent from the following description taken together with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. 
     In the drawings: 
     FIG. 1 is a perspective view of a pulse width modulated solenoid embodying the present invention; 
     FIG. 2 is a circuit diagram used in conjunction with controlling the solenoid; 
     FIG. 3 is a graph depicting voltage/current versus time for the pulse width modulated solenoid assuming a substantially constant voltage source of 12 volts DC; and 
     FIG. 4 is a graph like FIG. 3 showing a varying voltage source between 8 and 16 volts DC. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 illustrates a pulse width modulated (PWM) solenoid  10  embodying the present invention. The solenoid  10  is retained in a mounting frame  12  and provided with a control circuit  14  having input leads  16 ,  18 . Although shown exposed for purposes of this description, it should be understood that the solenoid  10  and circuit  14  are suitably encased in a protective package when used in a desired application. It should further be appreciated that, although not shown, it is well known that the solenoid  10  includes an internal actuator or plunger  13  which is moved by magnetic energy when the solenoid  10  is energized. The plunger  13  is movable between an extended position shown in full lines and a pulled in position with an outer end of the actuator  13  shown in phantom lines. The plunger moves toward a backstop fixed on one end of the frame  12  for dictating the maximum pull in distance the plunger can achieve when the solenoid  10  is energized. As the plunger approaches the backstop, its level of pull force increases. When the plunger bottoms out on the backstop, the maximum amount of pulling force is achieved. Once in this position, the solenoid only requires a fraction of pulling power to maintain holding in the plunger. 
     Referring now to FIG. 2, control circuit  14  associated with solenoid  10  is comprised of a variable supply voltage of 8 to 16 VDC, a filter capacitor C 1 , a voltage regulator  20 , a first resistor R 1 , a microcontroller 22, second and third resistors R 2  and R 3 , a MOSFET (transistor) Q 1 , a flyback diode D 1  and solenoid  10 . 
     Voltage regulator  20  is designed to provide the microcontroller  22  with a source voltage of 5VDC with filter capacitor C 1  filtering the rectified voltage appropriately. Microcontroller  22  is an 8 pin, 8 bit, CMOS with A/D converter and EEPROM Data Memory such as manufactured by Microchip Technology, Inc. of Chandler, Ariz. and identified as P1C12C67 X. Pin  1  is connected to the output of voltage regulator  20 . R 1  is connected between the output of voltage regulator  20  and pin  4  and serves as a pull-up resistor to command the microcontroller to begin running the program. Pin  6  provides a digital output and is joined to the MOSFET Q 1  which functions as an ON/OFF switch. Pin  7  is coupled to a junction  24  between serially connected resistors R 2  and R 3  which forms a voltage divider across the voltage supply. The voltage V REF  is calculated as the voltage across R 3 . Pin  8  is connected to ground. Flyback diode D 1  has one end connected to source voltage lead  18  and another end connected to MOSFET Q 1 . Solenoid  10  is connected in parallel across the diode D 1  which functions to short BACK EMF and prevent burnout of MOSFET Q 1 . D 1  further ensures that a current will flow through the solenoid  10  in a direction shown by the arrows. Circuit  14  pulses voltage on and off at a frequency of 1,000 cycles per second. 
     In operation, upon the application of power, the microcontroller  22  immediately starts running a program stored inside the microcontroller flash RAM. Then, the output of the microcontroller  22  that controls power to the solenoid  10  is turned on for one-half second which provides the full on-time for solenoid  10  to pull in. The one-half second on pulse is arbitrarily selected as a typical time in addition to a safety factor for the plunger to fully seat against the backstop. After this one-half second, an analog input is queried. Connected to the analog input is a reference percentage of the source voltage obtained through resistors R 2  and R 3 . The analog value is compared to a number of value ranges in order to determine what the PWM duty cycle should be. The duty cycle is set accordingly and the solenoid output is fired with the correct PWM pulse train for given number of cycles. After the train of cycles is complete, the input is required and the duty cycle is reevaluated. This process will continue as long as there is power connected to the system. 
     The microcontroller  22  has an AID converter which assigns a numeric value between 1 and 255 based on an input voltage. Each V REF  is associated with a particular address or set using the formula: 
     
       
           V   REF ×51=Result 
       
     
     In the particular design set forth herein, V REF  for input voltages of 16 VDC, 12 VDC, and 8VDC is calculated as follows (where R 1  and R 2  are 4.7 kΩ and R 3  is 1 kΩ):          16                 v   ×       1      k                 Ω         1      k                 Ω     +     4.7      k                 Ω           =       2.8                 v     =       V   REF        16                 v                 12                 v   ×       1      k                 Ω         1      k                 Ω     +     4.7      k                 Ω           =       2.1                 v     =       V   REF        12                 v                 8                 v   ×       1      k                 Ω         1      k                 Ω     +     4.7      k                 Ω           =       1.4                 v     =       V   REF        8                 v                              
     All incremental voltages V REF  between 8 VDC and 16 VDC are calculated similarly to generate the following list: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 V DC   
                 V REF   
                 Result 
                   
                 Duty Cycle 
                 Duty Cycle Ratio 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  8 v 
                 1.4 v 
                   
                   
                   
                   
               
               
                   
                   
                   
                 &gt; 
                 125 
                 49.8% 
               
               
                   
                 1.49 v 
                 76 
               
               
                   
                   
                   
                 &gt; 
                 118 
                 46.3% 
               
               
                   
                 1.59 v 
                 81 
               
               
                   
                   
                   
                 &gt; 
                 115 
                 45.1% 
               
               
                   
                 1.67 v 
                 85 
               
               
                   
                   
                   
                 &gt; 
                 112 
                 43.9% 
               
               
                   
                 1.75 v 
                 89 
               
               
                   
                   
                   
                 &gt; 
                 109 
                 42.7% 
               
               
                   
                 1.84 v 
                 94 
               
               
                   
                   
                   
                 &gt; 
                 107 
                 42.0% 
               
               
                   
                 1.92 v 
                 98 
               
               
                   
                   
                   
                 &gt; 
                 105 
                 41.2% 
               
               
                   
                 2.02 v 
                 103 
               
               
                   
                   
                   
                 &gt; 
                 102 
                 40.0% 
               
               
                 12 v 
                 2.10 v 
                 107 
               
               
                   
                   
                   
                 &gt; 
                 100 
                 39.2% 
               
               
                   
                 2.20 v 
                 112 
               
               
                   
                   
                   
                 &gt; 
                 98 
                 38.4% 
               
               
                   
                 2.27 v 
                 116 
               
               
                   
                   
                   
                 &gt; 
                 96 
                 37.6% 
               
               
                   
                 2.37 v 
                 121 
               
               
                   
                   
                   
                 &gt; 
                 94 
                 36.8% 
               
               
                   
                 2.45 v 
                 125 
               
               
                   
                   
                   
                 &gt; 
                 92 
                 36.1% 
               
               
                   
                 2.55 v 
                 130 
               
               
                   
                   
                   
                 &gt; 
                 90 
                 35.3% 
               
               
                   
                 2.63 v 
                 134 
               
               
                   
                   
                   
                 &gt; 
                 89 
                 34.9% 
               
               
                   
                 2.73 v 
                 139 
               
               
                   
                   
                   
                 &gt; 
                 88 
                 34.5% 
               
               
                 16 v 
                 2.80 v 
                 143 
               
               
                   
               
             
          
         
       
     
     For each range of results, the program assigns the appropriate duty cycle so that the solenoid output is fired with the correct PWM pulse train. The duty cycle ratio is calculated by the duty cycle/255. It can be appreciated that if the source voltage drops below 12 VDC, the pulses are increased. If the source voltage rises above 12 VDC, the pulses are decreased. 
     FIG. 3 graphically portrays the operation of a PWM solenoid  10  designed to operate with a full voltage pull in level of 40 watts at 12 volts and a 5 watt level to hold. After the initial pull in time of one-half second, the duty cycle is adjusted so that the coil current remains constant. 
     FIG. 4 shows that the coil current continues to remain constant even though the supply voltage varies from 8 to 16 VDC. Because the hold and wattage of the solenoid  10  remains at 5 watts, the solenoid  10  will not overheat and burnout. 
     In the example described herein, the PWM solenoid  10  is particularly useful in controlling the mirror employed in the high/low beam design of a High intensity Discharge (HiD) headlight provided in an automobile. However, it should be understood that the solenoid  10  has widespread utility and many other applications having various voltage supply and can be easily designed to fit the necessary operating parameters. It should also be noted that the example described herein is for a DC voltage situation, but that the PWM solenoid  10  may likewise be applicable to high voltage AC situations where supply voltages are susceptible to brown out conditions. 
     The present invention thus provides a unique solenoid  10  wherein the voltage is pulse width modulated to reduce the power drawn during the hold in state. During the hold in state, circuit  14  monitors the varying supply voltage and adjusts the pulse width accordingly. In this manner, the circuit  14  is able to maintain relatively constant power applied to the solenoid  10  to maintain the hold in position. The PWM solenoid  10  can thus be rated with higher pull in ratings yet be used in continuous duty applications. Such a solenoid  10  can eliminate the need for dual windings and will allow users to have higher forces available in a smaller sized device. 
     While the invention has been described with reference to a preferred embodiment, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made without departing from the spirit thereof. Accordingly, the foregoing description is meant to be exemplary only, and should not be deemed limitative on the scope of the invention set forth with the following claims.