Abstract:
A method for controlling speed in a pulse-width-modulation-controlled motor powered by a load voltage source is provided. The method includes the steps of measuring the motor load voltage, and setting a pulse-width-modulation duty cycle based on the measured voltage.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to motor speed control and, more particularly, to systems for controlling fan motor speed in a refrigerator.  
           [0002]    Refrigeration systems typically use a variety of variable speed direct current (DC) fan motors for air movement and cooling. Fan motors and their associated mounting structures, sometimes referred to as fixtures, have mechanical resonance frequencies that are sometimes approximately equal to the frequency (or multiples and sub-multiples thereof) of the driving frequencies utilized in a pulse width modulation (PWM) based system. As a result, the motor will sometimes be modulated at one or more duty cycles which causes increased perceived noise to a consumer.  
           [0003]    Additionally, variations in fan noise can be undesirable and the speed of each fan motor in the refrigeration system is typically controlled to facilitate a reduction in noise variations. PWM is a known method for controlling variable-speed DC fan motors in refrigerators. One known PWM based system utilizes a non-regulated DC power supply with an open-loop control that allows motor speed to vary with the alternating current (AC) line voltage. Another known PWM based system achieves a constant fan speed by using a speed feedback sensor, e.g. a Hall effect device, with a non-regulated DC supply. Other known PWM based systems utilize a regulated DC supply or a voltage regulator circuit to achieve a constant motor speed.  
           [0004]    However, utilizing a speed feedback sensor can raise manufacturing costs. Additionally, the constant speed obtained using a regulated DC supply can vary from one motor to another motor due to manufacturing variations among the motors, and voltage regulator circuits are costly and typically have an energy efficiency of less than eighty percent.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In one aspect, a method for controlling speed in a pulse-width-modulation-controlled motor powered by a load voltage source is provided. The method comprises the steps of measuring the motor load voltage, and setting a pulse-width-modulation duty cycle based on the measured voltage.  
           [0006]    In another aspect, a method for controlling speed in a pulse-width-modulation-controlled motor powered by a load voltage supplied by a supply voltage is provided. The method comprises the steps of diagnosing motor functionality using a difference between the supply voltage and the load voltage, and switching from motor functionality diagnosis to motor speed control.  
           [0007]    In another aspect, a closed loop motor control system is provided. The system comprises a motor, a power source, a resistive element electrically coupling said motor to said power source, at least one switching element electrically coupling said motor to said power source in parallel to said resistive element, and a processor electrically connected to said switching element. The processor is configured to determine a load voltage and set a pulse width modulation duty cycle based on the determined voltage.  
           [0008]    In another aspect, a method for operating a motor configured to operate at a variable average speed under pulse-width modulation control is provided. The method comprises the steps of energizing the motor, and setting an average speed by superimposing a sweep frequency onto an average pulse-width modulation frequency.  
           [0009]    In another aspect, a motor is provided. The motor comprises a housing, and a stator mounted in said housing, said stator comprising a stator bore. A rotor is rotatably mounted at least partially within said stator bore, and a processor electrically connected to at least one of said stator and said rotor. The processor is configured to determine a load voltage, and set a pulse width modulation duty cycle based on the determined voltage.  
           [0010]    In another aspect, a motor comprises a housing, a stator mounted in said housing, said stator comprising a stator bore, and a rotor rotatably mounted at least partially within said stator bore. A processor is electrically connected to at least one of said stator and said rotor, and the processor is configured to set an average speed by superimposing a sweep frequency onto an average pulse-width modulation frequency.  
           [0011]    In another aspect, a refrigerator is provided which comprises a housing, a freezer section at least partially within said housing, a fresh food section at least partially within said housing, a motor at least partially within said housing; and a processor electrically connected to said motor, said processor configured to set an average speed by superimposing a sweep frequency onto an average pulse-width modulation frequency.  
           [0012]    In another aspect, a refrigerator is provided that comprises a housing, a freezer section at least partially within said housing, a fresh food section at least partially within said housing, a motor at least partially within said housing, and a processor electrically connected to said motor. The processor is configured to determine a load voltage; and set a pulse width modulation duty cycle based on the determined voltage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a diagram of one embodiment of a closed-loop motor control system.  
         [0014]    [0014]FIG. 2 is a representation of a waveform produced by a conventional PWM circuit for a fifty-percent duty cycle.  
         [0015]    [0015]FIG. 3 is a representation of a monotonically increasing waveform.  
         [0016]    [0016]FIG. 4 is a cross-sectional view of the closed-loop PWM controlled motor shown in FIG. 1.  
         [0017]    [0017]FIG. 5 is a front view of a refrigerator. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    [0018]FIG. 1 is a diagram of one embodiment of a closed-loop motor control system  10 . As explained in greater detail below, system  10  provides closed loop motor control without using a Hall effect device or a voltage regulator. Rather, system  10  utilizes a plurality of switching elements and resistive elements to provide closed-loop motor control.  
         [0019]    Control system  10  includes a fan motor  12  that operates in a refrigerator (not shown in FIG. 1), such as, for example, a condenser fan motor or an evaporator fan motor. Control system  10  is powered by an unregulated DC power supply  14 . Power supply  14  supplies power to other loads in addition to control system  10 . Additionally, analog voltage signals from supply  14  are transmitted via a first line  16  to an analog-to-digital converter (ADC) (not shown). A second line  18  is also connected to the ADC.  
         [0020]    Line  16  is electrically connected at a connection node  20  to a switching element  22 , e.g. a PNP transistor. PNP transistor  22  is also electrically connected to line  18  at a connection node  24  such that PNP transistor  22  is between lines  16  and  18  allowing for an opening and closing of current flow between nodes  20  and  24  through transistor  22 . For example, PNP transistor  22  emitter and collector terminals are connected to nodes  20  and  24  respectively. A resistive element  26 , e.g. a sense resistor in line  18 , is connected to nodes  20  and  24  in parallel with the emitter and collector terminals of transistor  22 . Fan motor  12  receives a load voltage at node  24 . A second switch  30  is connected between a base of transistor  22  and a processor  32 , e.g. a Hitachi model H8-3644 processor commercially available from Hitachi, Ltd., Tokyo, Japan. Processor  32  is operationally coupled to the ADC. In one embodiment, switch  30  is a NPN transistor including a collector terminal electrically connected to a base terminal of PNP transistor  22 . NPN transistor  30  further includes a base terminal electrically connected to processor  32 . A resistor  33  connects the base of PNP transistor  22  to the emitter of PNP transistor  22 . It should be understood that the present invention can be practiced with many alternative processors, and is not limited to practice in connection with just processor  32 . Therefore, and as used herein, the term “processor” is not limited to just those integrated circuits referred to in the art as processors, but broadly refers to computers, processors, microprocessors, microcontrollers, microcomputers, application specific integrated circuits, and other programmable circuits including programmable logic controllers (PLCs).  
         [0021]    Circuit  10  further includes a PWM control sub-circuit  34  connected to processor  32 . In one embodiment, PWM control sub-circuit  34  is a module within a Hitachi H8-3644 processor or other known microprocessor. PWM circuit  34  is electrically connected to fan motor  12  via a transistor  36 . Although control system  10  includes transistors including bipolar transistors, control system  10  can utilize many alternative switching and current- or voltage-controlling elements, e.g. relays and Field Effect Transistors (FETs), such as, for example, Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) and Junction FETs (JFETs).  
         [0022]    In use, control system  10  performs closed-loop speed control and diagnostic functions as directed by processor  32 . To control the speed of fan motor  12 , processor  32  bypasses sense resistor  26  by turning on PNP transistor  22 . Processor  32  measures the load voltage of motor  12  at node  24 , and a duty cycle for PWM circuit  34  is set based on the load voltage measured at node  24 .  
         [0023]    For example, in an illustrative embodiment, it may be appreciated that the power supply voltage is approximately equal to the sum of V in  at node  24  and the applied voltage of PWM control sub-circuit  34 . Thus, assuming a minimum power supply voltage of 12 volts, PWM sub-circuit voltage is approximately V in  minus 12 volts. An appropriate duty cycle may be therefore be mathematically derived according to known theoretical or empirically determined relationships between an applied voltage signal from PWM control sub-circuit  34  and motor voltage, motor voltage and A/D converter counts, and output voltage at node  24  in relation to input voltage from power supply  14 . For instance, in one exemplary embodiment, the duty cycle for PWM circuit  34  is governed by the following relationship:  
         DutyCycle=3×10 −5 ( V   in −12) 4 −0.0019( V   in −12) 3 +0.0433( V   in −12) 2 −0.4198( V   in −12)+1.4591  
         [0024]    In different embodiments, the duty cycle is calculated directly by processor  32  according to such a relationship, or a pre-calculated duty cycle value corresponding to the sensed voltage is selected from a plurality of pre-calculated values associated with the processor, such as in a lookup table familiar to those in the art.  
         [0025]    The above-described process is performed sequentially and repeatedly while motor  12  is in an on state.  
         [0026]    Processor  32  controls all devices receiving power from power supply  14 . To test the electrical functionality of fan motor  12 , processor  32  switches off all electrical loads on power supply  14 . After all loads are shed from power supply  14 , processor  32  switches PNP transistor  22  to an off state allowing a measurable voltage drop across sense resistor  26  whenever current flows from node  20  to node  24 . PWM circuit  34  then energizes motor  12  using a duty cycle of 100 percent (PWM signal is kept high). Processor  32  then measures respective analog voltages from lines  16  and  18  and determines power consumption by sense resistor  26 , in accordance with the following relationship:  
           [       (     Upper_A   /   D_Reading     )     -     (     Lower_A   /   D_Reading     )       ]     2     Rsense                         
 
         [0027]    where Upper_A/D_Reading is the supply voltage measured from line  16 , Lower_A/D_Reading is the motor load voltage measured from line  18 , and Rsence is a resistance in ohms of sense resistor  26 . Rsence, in one embodiment, is selected to produce current values of between about 1 mA and about 100 mA through resistor  26 . Processor  32  also provides for switching from motor functionality diagnosis to closed loop control. For example, after diagnosing that the motor functionality is within a predetermined operating range, i.e., that the motor is energized and not locked, processor  32  switches PNP transistor  22  to an on state and controls motor  12  as explained above.  
         [0028]    In a further embodiment, a filter  38  (shown in phantom in FIG. 1) is employed between motor  12  and ground to reduce undesirable disturbances attributable to effects caused by the PWM wave form.  
         [0029]    The above described motor speed control circuit provides for constant fan speed control with diagnostic capabilities using an unregulated power supply. Through reduction in parts, compared to at least one known speed sensor system, an increase in reliability is facilitated. Also, as explained in greater detail below, using a fast frequency sweep over a slowly adjustable average frequency in a PWM controlled variable speed fan motor control system facilitates a reduction in the inherent motor and fixture resonances which can cause noise.  
         [0030]    [0030]FIG. 2 is a representation of a waveform  50  produced by a conventional PWM circuit for a fifty percent duty cycle. Waveform  50  includes a plurality of individual waves  52 . Each wave  52  includes a leading edge  54 , a high portion  56 , a trailing edge  58 , and a low portion  60 . Each wave  52  is substantially identical to each other wave  52 .  
         [0031]    During operation of a PWM controlled motor, the motor is energized during high portions  56  and is not energized during low portions  60 . Since each high portion  56  constitutes one-half of each wave  52 , the motor is operating at a 50% duty cycle. Typically, in a PWM controlled system, the duty cycle is adjusted based on various factors, such as, for example, a desired cooling rate. As a result, the motor may be modulated at one or more mechanical resonance frequencies causing increased perceived noise to the consumer. For instance, a motor having a mechanical resonance frequency at a 50% duty cycle will resonate when controlled with waveform  50  and produce more noise than when operated at a duty cycle not corresponding to a mechanical resonance frequency.  
         [0032]    [0032]FIG. 3 is a representation of a monotonically increasing waveform  70 . Waveform  70  includes a first wave  72 , a second wave  74 , a third wave  76 , a fourth wave  78 , a fifth wave  80 , and a sixth wave  82 . Each wave  70 ,  72 ,  74 ,  76 ,  78 ,  80 , and  82  has a substantially similar period  84  and includes a leading edge  86 , a high portion  88 , a trailing edge  90 , and a low portion  92 . High portion  88  of first wave  72  is approximately 40% of period  84 . High portion  88  of second wave  74  is approximately 45% of period  84 . High portion  88  of third wave  76  is approximately 50% of period  84 . High portion  88  of fourth wave  78  is approximately 55% of period  84 . High portion  88  of fifth wave  80  is approximately 60% of period  84 . High portion  88  of sixth wave  82  is approximately 40% of period  84 . High portions  88  vary from 40% to 60% and average 50%, which is the duty cycle. Specifically, high portions  88  vary from a low value of approximately 10 percent below the average (50%) and monotonically increase to a high value of approximately 10 percent above the average forming a sweep action before returning to the low value and sweeping again. The average is the duty cycle. In an alternative embodiment, the high value is approximately 20% above the average and the low value is approximately 20% below the average. In another embodiment, the high and low values are approximately 5% above and below the average respectively. In yet another embodiment, the high and low values are more than 20% above and below the average respectively. In a further embodiment, the high and low values are less than 5% above and below the average respectively.  
         [0033]    During operation of a PWM controlled motor (not shown in FIG. 3), the motor is energized during high portions  88  and not energized during low portions  92 . Since an average of high portions  88  is 50%, the motor is operating at a 50% duty cycle. However, the sweep action distributes the excitation energy over a large frequency band i.e., a twenty-percent range from a 40% duty cycle to a 60% duty cycle. Accordingly, the resonance energy at any particular frequency is lowered and the resonant system has less time to build up an appreciable resonance and associated noise. Because a motor has a large inertia compared to the fast sweep rate, the speed of a motor controlled with waveform  70  is substantially similar to the speed of a motor controlled with waveform  50  (shown in FIG. 2). However, as explained above, waveform  70  distributes the excitation energy over multiple frequencies, facilitating a reduction in the occurrences of modulating the motor at a resonance frequency.  
         [0034]    In one embodiment, processor  32  determines an average speed and outputs a PWM waveform as is known in the art e.g. waveform  50 . PWM circuit  34  superimposes a plurality of sweep additions and subtractions while maintaining the average set by processor  32 . In another embodiment, processor  32  and PWM circuit  34  are integrated into a single chip (not shown). The single chip determines an average speed value and outputs a monotonically increasing waveform centered around the determined value. It is contemplated that the benefits of distributing the excitation energy over multiple frequencies to facilitate a reduction in resonations accrue to systems and methods utilizing a monotonically decreasing waveform centered around the average. For example, waveform  70  can be reflected about a horizontal axis and waves  72 ,  74 ,  76 ,  78 , and  80  sent in reverse order. The motor is sent fifth wave  80  followed by fourth wave  78 , third wave  76 , second wave  74 , and finally first wave  72  before starting again with fifth wave  80 . Accordingly, the motor receives a monotonically decreasing waveform while still maintaining a 50% duty cycle.  
         [0035]    Additionally, a random waveform centered around the average will also distribute the excitation energy over multiple frequencies and facilitate a reduction in resonations. For example, sending waves  72 ,  74 ,  76 ,  78 , and  80  randomly to a motor energizes the motor with a 50% duty cycle and facilitates a reduction in resonations by distributing the energy over different frequencies. In one embodiment, PWM circuit  34  includes a random number generator (not shown) and utilizes the random number generator to generate random numbers between a negative limit and a positive limit with the same absolute value as the negative limit. Each random number is added to the average and thus the motor is regulated at a duty cycle set by processor  32  and a reduction in resonations is facilitated by distributing the excitation energy over multiple frequencies.  
         [0036]    [0036]FIG. 4 is a cross-sectional view of closed-loop PWM controlled motor  12  (shown in FIG. 1) including a housing  102 , a stator assembly  104 , a rotor assembly  106 , and a commutator assembly  108 . Stator assembly  104  is located within housing  102  and includes a stator core  110  including a stator bore  112  for receiving rotor assembly  106 . Stator core  110  further includes a plurality of wound field poles  114 . Rotor assembly  106  includes rotor shaft  116  carrying commutator assembly  108  and an armature core  118 . Commutator assembly  108  includes a plurality of commutator bars  120  and a brush holder  122  including a plurality of brushes (not shown). Commutator assembly  86  further includes a plurality of insulator segments (not shown) arranged alternately with commutator bars  120  in a circumferential direction of rotor shaft  116 . In an alternative embodiment, motor  12  is an electronic DC motor and does not include commutator assembly  108 . Motor  12  is electrically connected to processor  32  and PWM control sub-circuit  34  as shown in FIG. 1.  
         [0037]    During motor operation, processor  32  controls motor  12  as explained above and outputs a PWM control signal. Additionally, PWM circuit  34  receives the PWM control signal and superimposes a plurality of sweep additions and subtractions to the PWM control signal while maintaining the average set by processor  32 . Accordingly, motor  12  is controlled with a closed loop motor control with out using a Hall effect device or a voltage regulator. Additionally, a reduction in resonance is facilitated by the superimposition of the sweep additions and subtractions.  
         [0038]    [0038]FIG. 5 is a front view of a refrigerator  140  including a housing  142 , a freezer section  144 , and a fresh food section  146 . Refrigerator  140  further includes motor  12  (shown in FIG. 4) mounted within housing  142 . Motor  12  is electrically connected to processor  32  and PWM control sub-circuit  34  as shown in FIG. 1.  
         [0039]    During operation of refrigerator  140 , processor  32  controls motor  12  as explained above and outputs a PWM control signal. Additionally, PWM circuit  34  receives the PWM control signal and superimposes a plurality of sweep additions and subtractions to the PWM control signal while maintaining the average set by processor  32 . Accordingly, motor  12  is controlled with a closed loop motor control with out using a Hall effect device or a voltage regulator. Additionally, a reduction in resonance is facilitated by the superimposition of the sweep additions and subtractions. Accordingly, a reduction of noise generated by refrigerator  140  is facilitated.  
         [0040]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.