Motor controller with accurate current measurement

Abstract

A motor control system for reducing or eliminating current ripple error during current sampling includes: a motor having a plurality of phase coils, a motor driver circuit having a plurality of switches coupled to the plurality of phase coils to drive current through the phase coils, and a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches. A phase circuit modifies a first output control signal to produce a modified control signal that is out of phase with a second output control signal. A current measurement circuit is included to measure the current through the motor by measuring the current during a first time period when the first output control signal is active and measuring the current during a second time period when the modified control signal is active.

Description

Technical Field

[0001] The present disclosure relates to motor controllers and, more particularly, to motor controllers that measure current flowing through a motor. Background Art

[0002] Circuits that accurately control, drive, and regulate brushless DC ("BLDC") motors are required in many applications. These circuits typically generate a pulse width modulated ("PWM") drive signal that is used to control the power to the motor.

[0003] A BLDC motor may include multiple coils. These coils, when energized, cause the motor to turn. However, in order for the motor to turn continuously, the motor controller circuit may have to energize one or more (but not all) coils at a time, energize the coils in a specific order, energize the coils at different times in the forward and reverse directions, etc. The time period during which the coils are energized is often referred to as the so-called "phase" of the motor. The coil (or coils) energized during a phase may be referred to as a phase coil.

[0004] The order and duration (timing) in which the coils are energized depends on the design of the BLDC motor. For example, a BLDC motor may have three coils that must be energized in sequence (i.e., in a cyclic manner) in order to turn the motor. Other orders of energizing the coils may also be used. 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 to keep the motor spinning.

[0005] When each phase is energized, it physically drives the motor's rotor. The amount of power supplied to the coils can be directly proportional to the amount of torque produced by the motor. In many BLDC motors, the amount of power supplied to the coils rises and falls over time as the coils are energized. Therefore, the motor does not produce a constant torque output.

[0006] Various motor drive circuits are described in U.S. Patent No. 7,590,334 (filed on August 8, 2007); U.S. Patent No. 7,747,146 (filed on August 8, 2007), U.S. Patent No. 8,729,841 (filed on October 12, 2011); U.S. Patent Application No. 13 / 595,430 (filed on August 27, 2012); U.S. Patent No. 9,088,233 (filed on December 18, 2012); U.S. Patent No. 9,291,876 (filed on May 29, 2013); and U.S. Patent Application No. 15 / 967,841 (filed on May 1, 2018), each of which is incorporated herein by reference and each has been assigned to the assignee of this patent. Summary of the invention

[0007] In one embodiment, a system includes: a motor having a plurality of phase coils; a motor driver circuit having a plurality of switches coupled to the plurality of phase coils to drive current through the phase coils; and a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches. Each output control signal is associated with a corresponding phase coil so as to control the switch to drive current through the corresponding phase coil. A phase circuit is included to modify a first output control signal of the plurality of output control signals to generate a modified control signal that is out of phase with a second output control signal of the plurality of output control signals. A current measurement circuit is included to measure the current through at least one phase coil by measuring the current during a first time period when the first output control signal is valid (active) and measuring the current during a second time period when the modified control signal is valid.

[0008] May include one or more of the following features:

[0009] The current measurement circuit may also be configured to average the current measured during the first time period and the current measured during the second time period.

[0010] The plurality of phase coils may include three phase coils.

[0011] The plurality of output control signals may include three output control signals.

[0012] The plurality of output control signals may be pulse width modulated control signals.

[0013] The first time period may correspond to a time period when the modification control signal is high, and the second time period corresponds to a time period when the second output control signal is high.

[0014] The modified control signal and the second output control signal may be approximately 180 degrees out of phase.

[0015] The current measurement circuit may be configured to measure the current at a midpoint of the first time period and a midpoint of the second time period.

[0016] A single shunt resistor may be coupled to the current measurement circuit.

[0017] The phase circuit may include a phase shift circuit that shifts the center of the first output control signal by half a cycle (time period).

[0018] In another embodiment, a circuit includes: a motor driver circuit having a plurality of switches configured to be coupled to a plurality of phase coils of a motor to drive current through the plurality of phase coils of the motor; and a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches. Each output control signal is associated with a corresponding phase coil to control a switch to drive current through the corresponding phase coil. A phase circuit is provided to modify a first output control signal of the plurality of output control signals to generate a modified control signal that is out of phase with a second output control signal of the plurality of output control signals. A current measurement circuit is provided to measure the current through at least one phase coil in the following manner: measuring the current during a first time period when the modified control signal is valid; and measuring the current during a second time period when the second output control signal is valid.

[0019] May include one or more of the following features:

[0020] The current measurement circuit may also be configured to average the current measured during the first time period and the current measured during the second time period.

[0021] The plurality of phase coils may include three phase coils.

[0022] The plurality of output control signals may include three output control signals.

[0023] The output control signal may be a pulse width modulated control signal.

[0024] The first time period may correspond to a time period when the modification control signal is high, and the second time period corresponds to a time period when the second output control signal is high.

[0025] The modified control signal and the second output control signal may be approximately 180 degrees out of phase.

[0026] The current measurement circuit may be configured to measure the current at a midpoint of the first time period and a midpoint of the second time period.

[0027] A single shunt resistor may be coupled to the current measurement circuit.

[0028] The phase circuit may include an inverter.

[0029] In another embodiment, a circuit includes: a motor driver circuit having a plurality of switches configured to couple to a plurality of phase coils of a motor to drive current through the plurality of phase coils; and means for measuring an average current through at least one phase coil. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The foregoing features may be more fully understood from the following description of the accompanying drawings. The accompanying drawings assist in explaining and understanding the disclosed technology. Since it is generally impractical or impossible to illustrate and describe every possible embodiment, the accompanying drawings provided depict one or more exemplary embodiments. Therefore, the accompanying drawings are not intended to limit the scope of the present invention. The same numbers in the figures represent the same elements.

[0031] Figure 1 It is the circuit diagram of the motor control system.

[0032] FIG. 2 is a graph of motor current and phase signals.

[0033] FIG3 is a graph of actual motor current and sampled motor current.

[0034] Figure 4 It is the timing diagram of the motor phase signal.

[0035] Figure 5 is a graph of the motor phase signal and the motor current.

[0036] Figure 6 It is a graph of actual motor current and sampled motor current. DETAILED DESCRIPTION

[0037] Figure 1 is a circuit diagram of a motor control system 100 for controlling a motor 102. The motor control system 100 includes a motor control circuit 104 coupled to a motor driver circuit 106. The motor driver circuit 106 is coupled to the motor 102 and provides electrical power to the motor 102. In an embodiment, the motor control circuit 104 may be a non-sinusoidal brushless direct current (DC) motor control circuit.

[0038] exist Figure 1 In the example shown, the motor 102 is a three-phase motor. Therefore, the motor driver circuit 106 has six field effect transistor (FET) switches coupled in pairs between the power line (power line) 108 and the return line 110. The nodes between the pairs of field effect transistor switches (i.e., nodes A, B, and C) are coupled to the coils of the motor 102. When the FET switches are opened and closed, they provide power to the motor 102 and provide a path for returning from the motor 102. For example, if FET switch 112 and FET switch 114 are closed (e.g., in a conductive state) and the other FET switches are open (e.g., in a non-conductive state), then current can flow from the power line 108 through the FET switch 112 to node A, from node A through the internal coil of the motor 102 to node B, and from node B through the FET switch 114 to ground.

[0039] For ease of illustration, only the gates of the FET switches 112, 113, and 115 coupled to the motor control circuit 104 are shown. However, in an embodiment, the gate of each FET switch within the motor driver circuit 106 can be coupled to the motor control circuit 104. The motor control circuit 104 can drive the gate of each FET switch using various signals (e.g., signals 104a, 104b, and 104c) to selectively open and close the FET switch. This effectively drives the motor 102 by directing power to the coils of the motor 102. Those skilled in the art will recognize that in other embodiments, the FET switches can be replaced by any device that can act as a switch (e.g., a bipolar junction transistor ("BJT"), a relay, etc.).

[0040] In an embodiment, the motor control signals 104a, 104b, and 104c may be pulse width modulated ("PWM") signals. 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, the motor control circuit 104 may control the amount of current supplied to the motor 102 by varying the pulse width of the signals 104a-c.

[0041] In an embodiment, motor control circuit 104 may include phase circuit 105 that modifies signal 104a so that signal 104a (and therefore phase A) is 180 degrees out of phase with signal 104b (ie, phase B). The timing and phase of phase signals 104a-104c will be discussed in more detail below.

[0042] The motor control system 100 may include a sensor to measure the current supplied to the motor 102 (or returned from the motor 102). In order to measure the current flowing through the motor 102, the motor control system 100 may include a shunt resistor 120 in the current path. The input of the differential amplifier 122 may be coupled across the shunt resistor 120. Therefore, the amplified signal 122a (i.e., the output of the differential amplifier 122) may represent the voltage across the shunt resistor 120 (i.e., on the shunt resistor 120). The ADC 124 may convert the amplified signal 122a into a digital signal 124a, which may also represent the voltage across the shunt resistor 120. Because the resistance of the shunt resistor 120 is known, the motor control circuit 104 may use the voltage across the shunt resistor 120 to measure the current flowing through the motor 102. Therefore, the digital signal 124a may also represent the measured current.

[0043] In an embodiment, the shunt resistor 120 may have a very small resistance so that it does not significantly impede the flow of current and does not dissipate a large amount of power. A typical value for the shunt resistor 120 may be 0.1 ohms or less. In addition, although the shunt resistor 120 is shown coupled to the return line 110 to measure the current (Iout) returned from the motor 102, the shunt resistor 120 may be coupled to the power line 108 to measure the current (Iin) flowing into the motor 102.

[0044] In an embodiment, the signal 124a can be coupled to and received by the motor control circuit 104, which can allow the motor control circuit 104 to measure and calculate the current through the motor 102. For example, during operation, the motor control circuit 104 can periodically sample the signal 124a at different times. The motor control circuit 104 can use the sampled values ​​directly or can perform mathematical operations on the samples (e.g., calculate the average of the samples) to determine the magnitude of the current flowing through the motor 102. The motor control circuit 104 can then use the measured current as a parameter for controlling the motor 102.

[0045] Although the motor 102 may have multiple phases, a single shunt resistor 120 may be used to measure current. Because the shunt resistor is located on the return path 108 (or alternatively on the power line 108), the current through the shunt resistor 120 will reflect the current through the active (energized, effective) motor phase. In order to sense the current associated with each of the motor phases A, B, and C (whose sum is zero), it is only necessary to sample the current during two of the three motor phases, from which the third motor phase current can be calculated. In other words, the current through the shunt resistor 120 can be sampled during the A phase to detect the A phase current, and then the current through the shunt resistor 120 can also be sampled during the B phase to detect the B phase current, and then the C phase motor current can be calculated from the detected A phase current and the B phase current according to the equation IA+IB+IC=0.

[0046] 2, a graph 200 illustrates potential errors in calculating average motor phase current when the motor is driven by a prior art motor control circuit. Waveform 202 represents the actual current of the motor phase. Waveform 204 represents the average A-phase current through the motor. Waveforms 206, 208, and 210 represent control signals that activate each phase of the motor.

[0047] Waveforms 206, 208, and 210 illustrate conventional two-phase modulation for controlling a motor. In this type of modulation, two of the three phases switch in each PWM cycle, and the centers of the high pulses of each active phase are aligned (i.e., at time 216). In the example shown, the A-phase motor control signal 206 transitions from low to high at time 212, and then the B-phase motor control signal 208 transitions high at time 214, while the A-phase signal 206 remains high. Sampling of the A-phase current occurs at point S1 between time 212 and 214, and again at point S2 after time 214.

[0048] As shown in waveform 202, the phase current through motor 102 has a ripple (ripple). In an embodiment, the ripple may have a frequency equal to the PWM frequency of the phase and an amplitude related to the inductance of the motor winding. Randomly sampling the current over time may introduce errors that may be as large as the ripple amplitude. Averaging samples S1 and S2 over time periods T1 and T2 and using S1 and S2 to calculate the average current does not guarantee that the calculated average current will be accurate (i.e., will reflect average current 204) because S1 and S2 are not sampled at the time when average current 204 intersects actual current 202.

[0049] 3, a graph 300 includes a waveform 302 representing the actual current through the motor phase and a waveform 304 representing the sampled current in the motor control circuit of the prior art. The vertical axis represents arbitrary current units and the horizontal axis represents arbitrary time units. As shown in the figure, the sampled current 304 is not centered relative to the ripple of the actual current 302, which indicates that the sampled current contains errors and does not accurately indicate the average motor current.

[0050] refer to Figure 4 , graph 400 is a timing diagram illustrating two-phase modulation for controlling motor 102 according to the present disclosure. The vertical axis represents voltage and the horizontal axis represents time. Waveform 400 represents the A-phase motor control signal, waveform 402 represents the B-phase motor control signal, and waveform 404 represents the C-phase motor control signal (these signals can be respectively Figure 1 The motor control signals 104a, 104b, 104c are the same or similar).

[0051] According to the present disclosure, the motor control circuit 104 (e.g., the phase shift circuit 105) can modify the A-phase waveform 400 by phase shifting the A-phase waveform 400 by 180 degrees. In other words, the A-phase and B-phase control signals 400, 402 can be inversely center-aligned so that the center of the A-phase high pulse 412 is aligned with the center of the B-phase low pulse 410. It should be understood that inverse center alignment of the motor control signals can be achieved by phase shifting either the A-phase signal 400 or the B-phase signal 402, as long as the signals 400, 402 can be phase shifted 180 degrees relative to each other so that the center of the A-phase high pulse 412 is aligned with the center of the B-phase low pulse 410.

[0052] In general, at time 420, waveform 400 (Phase A) may be low and waveform 402 (Phase B) may be high. At time 422, waveform 400 (Phase A) may be high and waveform 402 (Phase B) may be low. More specifically, high portion 406 of waveform 402 may be centered within low portion 408 of waveform 400, and high portion 412 of waveform 400 may be centered within low portion 410 of waveform 402. The cycle continues at time 424 (in which the high portion of waveform 402 is again centered within the low portion of waveform 400) and continues beyond time 424. Thus, the center of the A-phase high pulse is aligned with the center of the B-phase low pulse, and the center of the B-phase high pulse is aligned with the center of the A-phase low pulse, thereby achieving reverse center alignment of the A-phase and B-phase motor control signals 400, 402, respectively.

[0053] refer to Figure 5 Graph 500 is a timing diagram of the various phases of motor control system 100 and the current through one phase of motor 102. The horizontal axis represents time. The vertical axis of waveforms 500, 502, and 504 represents voltage. The vertical axis of waveform 505 represents current. For example, waveform 505 may represent the current through shunt resistor 120 (see Figure 1 ) current.

[0054] Waveform 501 represents the A phase motor control signal, waveform 502 represents the B phase motor control signal, and waveform 504 represents the C phase motor control signal (these signals can be respectively Figure 1 The motor control signals 104a, 104b, 104c are the same or similar. In this example, Figure 4 In the example of FIG. 1 , the phase shift circuit 105 of the motor control circuit 104 can phase shift the A-phase waveform 501 by 180 degrees. In other words, at time S1, the waveform 501 (A-phase) can be low, while the waveform 502 (B-phase) can be high. At time S2, the waveform 501 (A-phase) can be high, while the waveform 502 (B-phase) can be low.

[0055] Additionally, high portion 506 of waveform 502 may be centered within low portion 508 of waveform 501, and high portion 512 of waveform 501 may be centered within low portion 510 of waveform 502. Thus, the center of the Phase A high pulse is aligned with the center of the Phase B low pulse, and the center of the Phase B high pulse is aligned with the center of the Phase A low pulse.

[0056] When sampling the current through the motor 102, the ripple of the current waveform 505 may cause errors. For example, the average current is shown by line 507. Therefore, if sampling occurs at, for example, the beginning of time T1 and the beginning of time T3, the calculated average current may be higher than the actual average current 507. However, the current at S1 is approximately in the middle of the falling edge of the ripple, and the current at S2 is approximately in the middle of the rising edge of the ripple. Therefore, in an embodiment, the motor control circuit 104 can sample the output (or input) current of the motor 102 at times S1 and S2 to minimize or eliminate the error introduced into the motor current calculation by the ripple of the current waveform 505.

[0057] In general, the current waveform 505 will fall during time periods T1, T2, and T3 (e.g., during the low portion 508 of the waveform 501) when phase A is inactive, and rise during time period T4 (e.g., during the high portion 512 of the waveform 501) when phase A is active. Because the high portion 506 is centered within the low portion 508, and because sample S1 is centered within the high portion 506, sample S1 may also be centered within the low portion 508. Thus, sample S1 may represent the center point of the falling portion of the current waveform 505. Additionally, because sample S2 is centered within the high portion 512, sample S2 may represent the center point of the rising portion of the current waveform 505.

[0058] In addition, the phase output pattern is the same in time periods T1 and T3. In other words, during T1 and T3, the A phase waveform 501 is low, the B phase waveform 502 is low, and the C phase waveform 504 is low. Therefore, if the motor is running in a steady state, the phase current will be the same in time periods T1 and T3. In other words, I1-I2=I3-I4. In addition, the following formula applies:

[0059]

[0060] Where I1, I2, I3, I4 represent the phase current levels at the beginning of time periods T1, T2, T3, T4, respectively, and S1A and S2A represent the actual A-phase current at time S1 and S2, respectively. Obviously, sampling the A-phase current at S2 will have no error. In some embodiments, the processor circuit can generate an average of samples S1, S2, S3, etc. The average value can also represent the average current through the motor 102. Therefore, sampling S1 and S2 at the center of the high portion 506 and the high portion 512, respectively, can reduce or eliminate the error caused by the current ripple of the waveform 505 when calculating the average current through the motor 102.

[0061] In an embodiment, the frequency of the ripple of waveform 505 and the sampling rate at which the current is sampled can be greater than the frequency of the average current 507 through the motor 102. In an embodiment, the sampling rate can be twice the Nyquist frequency (or greater than) the frequency of waveform 505. Furthermore, in an embodiment, the frequency of waveform 505 can be twice the Nyquist frequency (or greater than) the frequency of waveform 507. As a corollary, the PWM frequency of waveforms 501 and 502 can be greater than the motor frequency.

[0062] refer to Figure 6 , the graph 600 includes a waveform 602 representing the actual current through the motor phase and a waveform 604 representing the current sampled by the motor control circuit 104. The vertical axis represents arbitrary current units, while the horizontal axis represents arbitrary time units. As shown, the sampled current 604 is centered relative to the ripple of the actual current 602, which indicates that the sampled current contains no error or contains minimal error (i.e., represents the actual average value of the motor current).

[0063] The above example illustrates the operation of the system 100 when two-phase modulation is used. However, one of ordinary skill in the art will recognize that if the system 100 uses three-phase modulation to control the motor 102, the same system and techniques can be used to measure current. Figure 5 , if three-phase modulation is used, waveform 504 will have a rising edge at some time after time 514. However, under three-phase modulation, waveforms 501 and 502 can remain unchanged (or similar), so sampling at S1 and S2 can still reduce the error caused by the ripple of waveform 505.

[0064] Various embodiments are described in this patent. However, the scope of this patent should not be limited to the described embodiments, but should only be limited by the spirit and scope of the appended claims. All references cited in this patent are incorporated by reference in their entirety.

Claims

1. A motor system, comprising: A motor having a plurality of phase coils; a motor driver circuit having a plurality of switches coupled to the plurality of phase coils to drive current through the phase coils; a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches, each output control signal being associated with a respective phase coil to control the switch to drive current through the respective phase coil; a phase circuit for modifying a first output control signal of the plurality of output control signals to generate a modified control signal that is out of phase with a second output control signal of the plurality of output control signals so that a center of a high pulse of the modified control signal is aligned with a center of a low pulse of the second output control signal, and a center of a low pulse of the modified control signal is aligned with a center of a high pulse of the second output control signal; and A current measurement circuit, wherein the current measurement circuit measures the current through at least one phase coil in the following manner: measuring current during a first time period when the modification control signal is pulsed high; and The current is measured during a second time period when the second output control signal is a high pulse.

2. The motor system according to claim 1, wherein: The current measurement circuit is further configured to average the current measured during the first time period and the current measured during the second time period.

3. The motor system according to claim 1, wherein: The plurality of phase coils include three phase coils.

4. The motor system according to claim 3, wherein: The plurality of output control signals include three output control signals.

5. The motor system according to claim 1, wherein: The plurality of output control signals are pulse width modulated control signals.

6. The motor system according to claim 1, wherein: The current measurement circuit is configured to measure current at a midpoint of the first time period and a midpoint of the second time period. 7 . The electric machine system of claim 1 , further comprising a single shunt resistor coupled to the current measurement circuit.

8. The motor system according to claim 1, wherein: The phase circuit includes a phase shift circuit that shifts the center of the first output control signal by half a period.

9. A circuit comprising: a motor driver circuit having a plurality of switches configured to be coupled to a plurality of phase coils of a motor to drive current through the plurality of phase coils of the motor; a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches, each output control signal being associated with a respective phase coil to control the switch to drive current through the respective phase coil; a phase circuit for modifying a first output control signal of the plurality of output control signals to generate a modified control signal that is out of phase with a second output control signal of the plurality of output control signals so that a center of a high pulse of the modified control signal is aligned with a center of a low pulse of the second output control signal, and a center of a low pulse of the modified control signal is aligned with a center of a high pulse of the second output control signal; and A current measurement circuit, wherein the current measurement circuit measures the current through at least one phase coil in the following manner: measuring current during a first time period when the modification control signal is pulsed high; and The current is measured during a second time period when the second output control signal is a high pulse.

10. The circuit according to claim 9, wherein: The current measurement circuit is further configured to average the current measured during the first time period and the current measured during the second time period.

11. The circuit according to claim 9, wherein: The plurality of phase coils include three phase coils.

12. The circuit according to claim 11, wherein The plurality of output control signals include three output control signals.

13. The circuit according to claim 9, wherein: The output control signal is a pulse width modulated control signal.

14. The circuit according to claim 9, wherein: The current measurement circuit is configured to measure current at a midpoint of the first time period and a midpoint of the second time period.

15. The circuit of claim 9 further comprising a single shunt resistor coupled to the current measurement circuit.

16. The circuit of claim 9, wherein: The phase circuit includes an inverter.

17. A circuit comprising: a motor driver circuit having a plurality of switches configured to be coupled to a plurality of phase coils of a motor to drive current through the plurality of phase coils, wherein the current includes a ripple; a motor controller circuit configured to provide a plurality of output control signals coupled to the plurality of switches, each output control signal being associated with a respective phase coil to control the switch to drive current through the respective phase coil; a phase circuit for modifying a first output control signal among the plurality of output control signals to generate a modified control signal out of phase with a second output control signal among the plurality of output control signals so that a center of a high pulse of the modified control signal is aligned with a center of a low pulse of the second output control signal, and a center of a low pulse of the modified control signal is aligned with a center of a high pulse of the second output control signal, wherein the high pulse of the modified control signal generates a rising edge of the ripple, and the low pulse of the modified control signal generates a falling edge of the ripple; and Means for making a first measurement of current at a midpoint of a rising edge of the ripple and a second measurement of current at a midpoint of a falling edge of the ripple.

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