Patent Publication Number: US-2020295682-A1

Title: Motor controller with power feedback loop

Description:
FIELD 
     This disclosure relates to motor control systems and circuits. 
     BACKGROUND 
     Circuits to precisely control, drive, and regulate brushless DC (“BLDC”) electric motors are required in many applications. These circuits often create pulse-width modulated (“PWM”) drive signals that are used to control power to the motor. 
     BLDC motors may include multiple coils. These coils, when energized, cause the motor to turn. However, for the motor to continuously turn, a motor controller circuit may have to energize one or more (but not all) of the coils at a time, energize the coils in a particular order, energize the coils in a forward and backward direction at different times, etc. The periods of time in which the coils are energized are often referred to as so-called “phases” of the motor. The coil (or coils) that are energized during a phase may be referred to as phase coils. 
     The sequence and timing of which coils are energized is dependent upon the design of the BLDC motor. As an example, one BLDC motor may have three coils that must be energized in sequence, i.e. a round-robin fashion, in order to turn the motor. Such a motor may have three “phases.” In each phase, a different one or more of the three coils is energized. As the motor turns, the phase will change, and the motor driver will energize the next one or more coils in order to keep the motor spinning. 
     As each phase is energized, it physically drives the motor&#39;s rotor. The amount of power supplied to the coils may be directly proportional to the amount of torque produced by the motor. In many BLDC motors, the amount of power provided to the coils rises and falls over time as the coils are energized. As a result, the motor does not produce a constant torque output. 
     Various electric motor drive circuits are described in U.S. Pat. No. 7,590,334 (filed Aug. 8, 2007); U.S. Pat. No. 7,747,146 (filed Aug. 8, 2007), U.S. Pat. No. 8,729,841 (filed Oct. 12, 2011); U.S. patent application Ser. No. 13/595,430 (filed Aug. 27, 2012); U.S. Pat. No. 9,088,233 (filed Dec. 18, 2012); U.S. Pat. No. 9,291,876 (filed May 29, 2013); and U.S. patent application Ser. No. 15/967,841 (filed May 1, 2018), each of which is incorporated here by reference, and each of which is assigned to the assignee of this patent. 
     SUMMARY 
     In an embodiment, a system comprises: a motor; a motor and a control circuit coupled to the motor to provide power to the motor. The motor control circuit comprises a power reference circuit to provide a reference power level and a power control circuit configured to provide a constant power level to the motor so that the motor operates with a substantially constant torque at a substantially constant motor speed. The constant power level is proportional to the reference power level. 
     One or more of the following features may be included. 
     The motor control circuit may include a power feedback loop. 
     The power feedback loop may include a difference circuit that produces a signal representing a difference between the reference power level and a power level applied to the motor. 
     The power level applied to the motor may be calculated by multiplying a voltage and a current applied to the motor. 
     The power level applied to the motor may be calculated by multiplying an input voltage to a motor driver circuit with an input current to the motor driver circuit. 
     The system may include a circuit to measure the voltage applied to the motor. 
     The system may include a circuit to measure the current applied to the motor. 
     The signal may be provided as an input to the power input controller circuit. 
     The motor may have three phases. 
     The motor control circuit may include a speed feedback loop. 
     The speed feedback loop may include a difference circuit that produces a signal representing a difference between a reference speed value and a measured speed of the motor. 
     In another embodiment, a circuit comprises a motor driver circuit comprising a plurality of switches coupled to provide power to a motor and a motor control circuit. The motor control circuit comprises a power reference circuit to provide a reference power level and a motor control circuit configured to control the motor driver circuit to apply a constant power level to the motor so that the motor operates with a substantially constant torque at a substantially constant speed. The constant power level is proportional to the reference power level. 
     One or more of the following features may be included. 
     The motor control circuit may include a power feedback loop. 
     The power feedback loop may include a difference circuit that produces a signal representing a difference between the reference power level and a power level applied to the motor. 
     The power level applied to the motor may be calculated by multiplying a voltage and a current applied to the motor. 
     A circuit to measure the voltage applied to the motor may be included. 
     A circuit to measure the current applied to the motor may be included. 
     The signal may be provided as an input to the power input controller circuit. 
     The motor control circuit may include a speed feedback loop. 
     The speed feedback loop may include a difference circuit that produces a signal representing a difference between a reference speed value and a measured speed of the motor. 
     In another embodiment, a method of driving a motor with constant torque includes measuring a voltage applied to a motor, measuring a current applied to the motor, calculating an instantaneous power applied to the motor based on the measured voltage and current, determining a difference between the instantaneous power and a reference power level, and adjusting the voltage and/or the current applied to the motor so that the instantaneous power matches the reference power level. 
     In another embodiment, a system comprises: a motor and means for driving the motor to achieve a constant power so that a torque output of the motor is substantially constant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features may be more fully understood from the following description of the drawings. The drawings aid in explaining and understanding the disclosed technology. Since it is often impractical or impossible to illustrate and describe every possible embodiment, the provided figures depict one or more exemplary embodiments. Accordingly, the figures are not intended to limit the scope of the invention. Like numbers in the figures denote like elements. 
         FIG. 1  is a block diagram of a system for controlling a motor. 
         FIG. 2  is a block diagram of a system for controlling a motor. 
         FIG. 3  is a graph of motor power for a motor driven by a motor control circuit of the prior art. 
         FIG. 4  is a graph of motor power for a motor driven by a motor control circuit that provides substantially constant torque output. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a circuit diagram of a motor control system  100  for controlling motor  102 . Motor control system  100  includes a motor control circuit  104  coupled to motor driver circuit  106 . Motor driver circuit  106  is coupled to motor  102  and provides power to motor  102 . In embodiments, motor control circuit may be a non-sinusoidal brushless DC motor control circuit. 
     In the example shown in  FIG. 1 , motor  102  is a three-phase motor. Accordingly, motor driver circuit  106  has six field effect transistor (FET) switches coupled in pairs between power line  108  and return line  110 . The nodes between the pairs (i.e. nodes A, B, and C) are coupled to the coils of motor  102 . As the FET switches open and close, they provide power to motor  102  and provide a return path from motor  102 . For example, if FET switch  112  and FET switch  114  are closed (e.g. in a conducting state) while the other FET switches are open (e.g. in a non-conducting state), current may flow from power line  108 , through FET switch  112  to node A, from node A through the internal coils of motor  102  to node B, and from node B through FET switch  114  to ground. 
     For ease of illustration, only the gate of FET switch  112  is shown coupled to motor control circuit  104 . However, in embodiments, the gate of each FET switch within motor driver circuit  106  may be coupled to motor control circuit  104 . Motor control circuit  104  may drive the gates of each FET switch with signal  104   a  to selectively open and close the FET switches. This effectively drives motor  102  by directing power to the coils of motor  102 . One skilled in the art will recognize that, in other embodiments, the FET switches may be replaced by any device that can act as a switch such as a bipolar junction transistor (“BJT”), a relay, etc. 
     In embodiments, signal  104   a  may be a pulse width modulated (“PWM”) signal. As the PWM on-time increases from zero to one hundred percent, the amount of current supplied to the motor increases proportionally from zero to its maximum value. Thus, motor control circuit  104  can control the amount of current supplied to motor  102  by altering the pulse width of signal  104   a.    
     Motor control system  100  may include sensors to measure the voltage and current supplied to motor  102 . As an example, to measure the voltage supplied to motor  102 , analog-to-digital converter (“ADC”)  116  may be coupled to nodes A, B, and/or C through multiplexor  118 . ADC  116  may measure the voltage at the node that is supplying power to motor  102  and generate signal  116   a  representing the voltage supplied to motor  102 . Multiplexor  118  may be controlled by motor control circuit  104  (or another control circuit) to connect the node (e.g. A, B, or C) that is currently providing power to motor  102  to ADC  116 . In embodiments, a processor or circuit may receive signal  116   a  and use it to calculate an average or RMS value of the voltage supplied to motor  102 . 
     As another example, to measure the current flowing through motor  102 , motor control system  100  may include a shunt resistor  120  in the current path. The inputs of differential amplifier  122  may be coupled across shunt resistor  120 . Thus, amplified signal  122   a  (i.e. the output of differential amplifier  122 ) may represent the voltage across shunt resistor  120 . ADC  124  may convert amplified signal  122   a  into digital signal  124   a,  which may be used by motor control circuit  104  (or another circuit) to calculate the current flowing through motor  102 . In embodiments, shunt resistor  120  may be According to Kirchhoff s current rule, the calculated current through shunt resistor  120  may represent the input current and/or the output current to motor  102 . 
     Because the resistance of shunt resistor  120  is known, motor control circuit  104  may use the voltage across shunt resistor  120  to measure the current flowing through motor  102 . Thus, digital signal  124   a  may also represent a measured current. In embodiments, the shunt resistor  120  may have a very small resistance so that it does not greatly impede the current flow and also does not dissipate a large amount of power. A typical value for shunt resistor  120  may be 0.1 Ohms or less. Also, although shunt resistor  120  is shown coupled to return line  110  to measure the current returning from motor  102  (I out ), shunt resistor could be coupled to power line  108  to measure the current flowing into motor  102  (I in ). 
     Referring to  FIG. 2 , motor control system  200  includes a motor control circuit  202 , which may be the same as or similar to motor control circuit  104 . Motor control circuit  202  is coupled to motor driver circuit  204 , which may be the same as or similar to motor driver circuit  106 . Like motor driver circuit  106 , motor driver circuit  204  may be coupled to motor  102  to provide power to motor  102 . 
     Motor control circuit  202  may receive reference speed signal  206   a,  which represents a desired speed of motor  102 , as an input from reference speed circuit  206 . Reference speed circuit  206  may be any external circuit that can generate reference speed signal  206   a  to represent a desired speed of motor  102 . In embodiments, reference speed circuit  206  may be an external control circuit, processor circuit, or the like. In other embodiments, reference speed signal  206   a  may be generated internally by motor control circuit  202 . 
     Motor control circuit  202  may also include power reference circuit  208 , which may generate power reference signal  208   a.  Power reference signal  208   a  may represent an amount of power to be applied to motor  102 . In embodiments, power reference signal  208   a  may be a constant or variable signal and/or may be generated internally by power reference circuit  208 . In other embodiments, power reference circuit  208  may receive an external power control signal  208   b  from an external source, which may represent a desired power level. In this case, power reference signal  208   a  may be proportional to or based on external power control signal  208   b.    
     Reference speed signal  206   a  may be generated externally to motor control circuit  202 . It can be fixed or change during operation to control the speed of motor  102 . Power reference signal  208   a  may be calculated based on the error (e.g. difference) between reference speed signal  206   a  (e.g. the desired speed) and signal  102   a  (the actual speed). Signal  212   a  may represent the error between reference speed signal  206   a  and signal  102   a.  Power reference circuit  208  may then use the error to calculate power reference signal  208   a.    
     For example, assume the reference speed signal  206   a  calls for 1000 rpm motor speed, and the measured speed  102   a  is 980 rpm. Signal  212   a  may represent the error (e.g.  20  rpm), which will be used by the PI loop circuit  208  to increase power reference signal  208   a  so that the motor accelerates to  1000  rpm at a predetermined power level. 
     Motor control system  200  may include two feedback loops: a speed feedback loop and a power feedback loop. The speed feedback loop may include difference circuit  212  and proportional integral (“PI”) controller  214 . Difference circuit  212  may receive reference speed signal  206   a  and back-EMF signal  102   a  and generate difference signal  212   a  representing the difference or “error” between reference speed signal  206   a  and back-EMF signal  102   a.  Back-EMF may be a signal representing the back-EMF voltage of motor  102  or any other type of signal that can represent the speed of motor  102 . 
     PI controller circuit  214  may receive difference signal  212   a  and generate control signal  214   a.  In embodiments, control signal  214   a  may be a pulse-width modulated signal to control motor  102 . For example, PI controller circuit  214  may increase the pulse width of control signal  214   a  to increase the speed of motor  102  and decrease the pulse width of control signal  214   a  to decrease the speed of motor  102 . By doing so, PI controller circuit  214  can match the speed of motor  102  to the desired speed represented by reference speed signal  206   a.    
     Although not shown, motor control system  100  may include signal shaping circuits such as amplifiers, filter, and the like, to condition back-EMF signal  102   a  before it is received by difference circuit  212 . 
     The power feedback loop may include difference circuit  216 , PI controller  218 , and multiplier circuit  220 . Multiplier circuit  220  may multiply a signal representing the voltage of motor  102  (e.g. signal  116   a  in  FIG. 1 ) by a signal representing the current through motor  102  (e.g. digital signal  124   a  in  FIG. 1 ). Power signal  220   a  of multiplier circuit  220  may represent the instantaneous power (e.g. the current power) applied to motor  102 . In embodiments, multiplier circuit  220  may multiply the input voltage (V in ) to motor  102  by the input current (I in ) to motor  102  to generate power signal  220   a,  as shown in  FIG. 1 . In other embodiments, multiplier circuit  220  may multiply the output voltage (V out ) from motor  102  by the output current (I out ) from motor  102  to generate power signal  220   a.    
     Difference circuit  216  may receive the current (e.g. instantaneous) power signal  220   a  and the desired power reference signal  208   a  and produce difference signal  216   a , representing the difference or “error” between power signal  220   a  and power reference signal  208   a.  PI controller  218  may receive difference signal  216   a  and produce control signal  218   a . Control signal  218   a  may be a pulse-width modulated signal to control motor  102 . For example, PI controller  218  may increase the pulse width of control signal  218   a  to increase the power applied to motor  102  and decrease the pulse width of control signal  218   a  to decrease the power applied to motor  102 . By doing so, PI controller  218  can match the power applied to motor  102  to the desired power represented by power reference signal  208   a.  In embodiments, control signal  218   a  may be the same as or similar to signal  104   a.  (See  FIG. 1 ). 
     In embodiments, the power supplied to motor  102  may be proportional to power reference signal  218   a.  Proportional means that changes in the value of power reference signal  218   a  may result in changes to the power applied to motor  102 . For example, if power reference signal  218   a  increases, the power applied to motor  102  may increase. In embodiments, power reference signal may have a constant value so that the power provided to motor  102  is a constant power level. In some cases, the power supplied to motor  102  may be a scalar multiple of the value of power reference signal  218   a.    
     Referring to  FIG. 3 , graph  300  illustrates a motor power curve of the prior art. The horizontal axis represents arbitrary units of time and the vertical axis represents arbitrary units of power. Graph  300  illustrates the torque profile of a three-phase motor driven by a motor control circuit of the prior art. 
     The bottom waveforms  304 ,  306 , and  308  each represent the output power of the motor when driven by one of the three motor phases. As each phase becomes active (e.g. as current is driven through the phase coil), the power increases to a peak, then drops off as the phase becomes inactive. Adding these three torque curves results in curve  310 , which represents the overall output power of the motor for a three-phase motor of the prior art. As shown, the overall power curve is not constant; it peaks and falls in a function similar to the amplitude (or absolute value) of a sine wave over time. This may occur, for example, if the motor is driven by a non-sinusoidal brushless DC motor controller. Because motor power is the product of torque times motor speed, the motor&#39;s torque will not remain constant when the motor reaches a steady-state motor speed when drive by a power curve like curve  310 . 
     Referring to  FIG. 4 , graph  400  illustrates a power output curve of motor  102 . The horizontal axis represents arbitrary units of time and the vertical axis represents arbitrary units of power. The waveforms are experimental results of motor input power and motor output power as the motor is controlled by motor control circuit  104  (or motor control circuit  202 ). 
     Waveform  402  represents the power input to motor  102 , which may be calculated by multiplying the input voltage and current, as discussed above. Output power waveform  404  represents the output from motor  102 . As shown, output power waveform  404  is relatively constant through all three motor phases A, B, and C and shows a reduced sinusoidal pattern. As a result, since torque is the quotient of power divided by motor speed, the motor&#39;s output torque is also constant for any constant motor speed. 
     Various embodiments are described in this patent. However, the scope of the patent should not be limited to the described embodiments, but rather should be limited only by the spirit and scope of the following claims. All references cited in this patent are incorporated by reference in their entirety.