System and method for overcoming dead bands in switched reluctance motors

Abstract

A system and method for overcoming dead zones in switched reluctance motors is provided. The work machine (100) includes a frame (110), a traction system (120) supporting the frame (110), a power source (130) mounted to the frame (110), a switched reluctance motor (205), an inverter (220) configured to control power from the power source (130) to the motor (205), and a controller (210). The controller (210) is configured to receive a signal indicative of a desired torque (430) and determine whether the desired torque (430) is between an upper threshold (410) and a lower threshold (420). If the desired torque (430) is between the upper threshold (410) and the lower threshold (420), a PWM modulated torque command (450) is generated using pulse width modulation and the motor (205) is commanded based on the PWM modulated torque command (450). The PWM modulating torque command (450) is configured to cycle between an upper threshold (410) and a lower threshold (420) to produce a desired torque (430).

Description

[Technical Field]

[0001] The present disclosure relates generally to motors and generators, and more particularly to switched reluctance motors. [Background technology]

[0002] Many work machines, such as track tractors, excavators, and the like, may include a transmission connected to a power source that allows the work machine to be repositioned or moved between locations. With increasing interest in energy conservation and avoiding the use of fossil fuels, the use of electric motors as a power source has become more common. Electric motors convert electrical energy from an energy source, such as a battery, into mechanical energy to drive the work machine.

[0003] Electric motors known as switched reluctance motors (SRMs) are widely used in a variety of applications, including the aforementioned work machines, due to their robust and rugged construction. Switched reluctance motors comprise a rotor and multiple stators. Unlike conventional brushed DC motors, power is transferred to the stator windings rather than the rotor windings. This significantly simplifies mechanical design by eliminating the need to distribute power to moving parts, but complicates the electrical design by requiring a switching system to route power to the different windings. Some SRMs have sensorless motion control systems that estimate the rotor position and speed relative to the stator without using direct position sensors. Sensorless operation is crucial in many applications due to the need for minimal package size, high reliability, and low cost. Accurately determining the rotor's position at rest or at low speeds is essential to the motor's performance and efficiency.

[0004] Such a sensorless control system is described in U.S. Patent No. 10,079,566. However, at low loads, the stator may not have enough current to estimate the rotor position, producing minimal torque. Therefore, there is still a need for a control system for switched reluctance motors that provides sensorless operation when low torque is required. Summary of the Invention

[0005] According to one aspect of the present disclosure, a work machine is disclosed. The work machine includes a frame, a traction system supporting the frame, a power source attached to the frame, a switched reluctance motor, an inverter configured to control power from the power source to the motor, and a controller. The controller is configured to receive a signal indicative of a desired torque and determine whether the desired torque is between an upper threshold and a lower threshold. If the desired torque is between the upper threshold and the lower threshold, the controller generates a PWM adjusted torque command using pulse width modulation. The PWM adjusted torque command is configured to cycle between the upper threshold and the lower threshold to generate the desired torque.

[0006] According to another aspect of the present disclosure, an electric drive system is disclosed. The system includes a switched reluctance motor, an inverter configured to control power to the motor from a power source, and a controller. The controller is configured to receive a signal indicative of a desired torque and determine whether the desired torque is between an upper threshold and a lower threshold. If the desired torque is between the upper and lower thresholds, the controller generates a PWM modulating torque command using pulse width modulation and commands the motor based on the PWM modulating torque command. The PWM modulating torque command is configured to cycle between the upper and lower thresholds to generate the desired torque.

[0007] According to yet another aspect of the present disclosure, a method for overcoming deadband in a switched reluctance motor is disclosed. The method includes receiving a signal indicative of a desired torque and determining whether the desired torque is between an upper threshold and a lower threshold. If the desired torque is between the upper and lower thresholds, generating a PWM modulating torque command using pulse width modulation and commanding the motor based on the PWM modulating torque command. The PWM modulating torque command is configured to cycle between the upper and lower thresholds to generate the desired torque.

[0008] These and other aspects and features of the present disclosure will be more readily understood after reading the following detailed description in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a side view of a work machine according to one aspect of the present disclosure; [Figure 2] FIG. 2 is a block diagram of an electric drive system usable with the work machine of FIG. 1 according to one aspect of the disclosure. [Figure 3] FIG. 1 is a cross-sectional view of a switched reluctance motor according to one aspect of the present disclosure. [Figure 4] FIG. 1 illustrates pulse width modulation according to an aspect of the present disclosure. [Figure 5] FIG. 10 is a torque command diagram with pulse width modulation according to one aspect of the present disclosure. [Figure 6] 1 is a flowchart of a method for overcoming dead zones according to an aspect of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1 is a perspective view of a work machine including a switched reluctance motor control system according to an embodiment of the present invention; FIG. 2 is a perspective view of a work machine including a switched reluctance motor control system according to an embodiment of the present invention; FIG. 3 is a perspective view of a work machine including a switched reluctance motor control system according to an embodiment of the present invention; FIG. 4 is a perspective view of a work machine including a switched reluctance motor control system according to an embodiment of the present invention;

[0011] As shown, work machine 100 includes a frame 110, a traction system 120 supporting frame 110, a power source 130 supported by frame 110, and an electric drive system 200 configured to transfer energy from power source 130 to traction system 120. A work implement 140, such as, but not limited to, a bucket as shown, may be mounted to frame 110 and may be powered by electric drive system 220. Work machine 100 may further include an operating room 150. Power source 130 is configured to provide electrical power to work machine 100 and to provide operating power for propulsion of power drive system 200. Power source 130 may be a direct current (DC) power source, an Otto-cycle or Diesel-cycle engine, or the like. Power source 130 is operably positioned to receive control signals from an operator control (not shown) within operating room 150. Power source 130 is also operably configured to provide electrical power to other systems of work machine 100.

[0012] Electric drive system 200 may be operatively configured with power source 130 to selectively propel work machine 100 via control signals from an operator in operating room 150. Electric drive system 200 is operatively connected to traction system 120, which may be movably connected to work machine 100 via axles, driveshafts, transmissions, and / or other components. In some embodiments, traction system 120 may be provided in the form of the illustrated track drive system, although a wheel drive system or any other type of drive system configured to engage the ground and propel work machine 100 is also possible.

[0013] In some embodiments, electric drive system 200 may additionally or alternatively be configured to selectively operate work machine 100 and implement 140, which may be movably connected to electric drive system 200. The illustrated implement 154 is a blade attached to work machine 100 in the form of a tractor loader, although of course other embodiments may include any other suitable implement for various tasks, such as dozing, brushing, compacting, digging, grading, lifting, tearing, tilling, etc.

[0014] As noted above, Figure 1 is provided as one example of a work machine 100 that may utilize the switched reluctance motor and control system of the present disclosure. Other examples are possible and may differ from the example described in connection with Figure 1.

[0015] As shown in Figure 2, electric drive system 200 includes a motor 205, a controller 210, an inverter 220, and a current sensor 230. Motor 205 is a switched reluctance (SR) motor, as shown in Figure 3 and described in more detail below.

[0016] Controller 210 is configured to receive inputs from sensors 230, an operator, and / or other systems of work machine 100. Based on these inputs, controller 210 commands inverter 220 to provide power to SR motor 205 as needed, including start-up commands at start-up, switching commands during operation, and other commands required for the application, as described herein.

[0017] The inverter 220 is electrically connected to the power source 130 and the motor 205. The inverter 220 supplies current to the SR motor 205 according to instructions from the controller 210. In some embodiments, the inverter 220 can receive direct current from the power source 130 and control the phase of the direct current to supply alternating current to the SR motor 205. The current sensor 230 monitors the current in the SR motor 205 and communicates information regarding the current in each of several phases of the SR motor 205 to the controller 210.

[0018] An example of an SR motor 205 is shown in cross section in Figure 3. The SR motor 205 includes a stator 320 having multiple stator teeth 310. The stator teeth 310 are arranged in pairs on opposite sides of the stator, as indicated by A, B, and C in Figure 3. Unlike brushed DC motors, power is transferred to the stator 310 instead of the rotor 340. This significantly simplifies the mechanical design by eliminating the need to supply power to moving parts, but complicates the electrical design by requiring the use of some kind of switching system to sequentially transfer power to the different stators 310.

[0019] Each stator tooth 310 has a winding (not shown) that allows current to be applied to the stator tooth 310. Each stator tooth 310 may be energized as a different phase. Located within the center of the stator 320 is a rotor 340 that also has a number of rotor poles 350 arranged in pairs. The example shown has six stator teeth 310 and four rotor poles 350, although other numbers of stator teeth 310 and rotor poles 350 are possible.

[0020] The SR motor 205 operates by tending to move the rotor 340 toward a position of minimum reluctance relative to the stator 320. This position of minimum reluctance occurs when a pair of rotor poles 350 aligns with a pair of energized stator teeth 310. This magnetic attraction generates torque that rotates the rotor 340 and moves it toward the position of minimum reluctance. As power is transferred to each pair of stator teeth 310 and the rotor 340 is aligned, the next successive stator tooth 310 is energized, continuing the rotor 340's movement and maintaining angular momentum. Thus, the switching pattern of which stator teeth 310 are energized, which stator teeth 310 are not energized, and which phases complicate the operation of such a motor. Proper operation of the motor 310 depends on the proper timing of energizing each stator tooth 310. This timing is driven by the angular position of the rotor 340 relative to the stator 320.

[0021] While some prior art SR motors 205 use sensors to detect the position of the rotor poles 350 relative to the stator 320, the need for minimal package size, high reliability, and low cost of SR motors makes sensorless operation important for a variety of applications. Reliable and accurate position detection of switched reluctance motors is a key step toward developing low-cost, high-performance SR work machine drives.

[0022] One method for estimating the position of the rotor 340 involves injecting delta current pulses into one or more idle phases of the stator 310. The stator currents flowing through the idle phases are then estimated using an observer-based estimation method. The controller 310 compares the estimated stator currents with the actual stator currents received from the current sensors to generate an error signal. The error signal is used to calculate an estimated rotor position and an estimated rotor speed.

[0023] However, this sensorless method of operation requires a minimum current from the motor. At very low torque loads, the current required for the injection, measurement, and comparison process may be too low to operate correctly, creating a dead zone where the controller cannot determine the rotor position.

[0024] Therefore, this disclosure proposes overcoming the dead zone by further configuring the controller to apply pulse width modulation when the dead zone occurs. Similar disclosures may be applied to other dead zones of electric machines, including but not limited to, SR generators, SR motors with position sensors, induction motors, permanent magnet motors, and other electric motors, including motors or generators.

[0025] To overcome the deadband, the controller must first determine whether the amount of torque required is between upper and lower thresholds 410, 420 for positive and negative torque loads that correspond to the minimum current required for sensorless operation to occur. In some embodiments, upper threshold 410 may be 250 N·m and lower threshold 420 may be −250 N·m, although of course the particular thresholds will depend on the application.

[0026] If the desired torque or "desired torque command 430" is not between the thresholds 410, 420, the controller commands the inverter to produce the desired torque as normal.

[0027] On the other hand, if torque command 430 is desired to be between threshold 410 and threshold 420, the controller applies pulse width modulation (PWM) to generate PWM adjusted torque command 450. PWM is a method of reducing the average power delivered by an electrical signal by effectively dividing the signal into discrete portions. This allows the motor to use current outside the deadband limits while still producing the desired torque 430.

[0028] As shown in FIG. 4, the PWM adjusted torque command 450 can be generated by generating a PWM reference signal 440 and comparing it to a desired torque 430. The PWM reference signal 440 can be a sawtooth or triangular wave that oscillates between an upper threshold 410 and a lower threshold 420. The frequency of the reference signal must be faster than the mechanism of the SR motor 205 can respond, but not so fast as to interfere with position sensing. In some embodiments, the frequency can be 250 Hz, 500 Hz, or any other frequency suitable for a particular application. The same system can use different frequencies under different conditions.

[0029] If the PWM reference signal 440 is less than the desired torque 430, the PWM adjusted torque command 450 is equal to the upper threshold 410. If the PWM reference signal 440 is greater than the desired torque 430, the PWM adjusted torque command 450 is equal to the lower threshold 420. The PWM adjusted torque command 450 produces the same torque from the SR motor as the desired torque 450.

[0030] As mentioned above, the PWM adjusting torque command 450 is generated only when the desired torque 430 is between the threshold levels. Figure 5 shows a typical example where the desired torque 430 forms a sine wave. If the torque is near 0 N·m, the PWM adjusting torque command 450 is only applied within a narrow range. [Industrial Applicability]

[0031] In general, the present disclosure can be applied to many different industries, including, but not limited to, earthmoving equipment, construction, agriculture, mining, and the like. More specifically, the deadband-free sensorless operation disclosed herein is important for various applications due to the need for minimal package size, high reliability, and low cost for SR work machine drive actuators. Reliable and accurate position detection of switched reluctance motors is a key step toward developing low-cost, high-performance SR work machine drives. At very low torque loads, the required current may be too low to function properly, resulting in a deadband that prevents the control system from functioning properly. Therefore, the present disclosure proposes a method to avoid sensor deadbands in switched reluctance motors. As shown in FIG. 6 and described in further detail below, the present method can be used to correct deadbands in any SR motor with sensorless operation in various work machines and applications. These can include track-type tractors, excavators, graders, and any other electric-drive work machine 100 that can utilize an SR motor 205. Similar methods can also be applied to other deadbands of electric machines, including but not limited to SR generators, SR motors with position sensors, induction motors, permanent magnet motors, and other electric machines, including motors or generators.

[0032] Referring now to FIG. 6, as represented by block 610, method 600 first requires receiving a signal 430 indicative of a desired torque. The controller 210 may input the received signal from another system on the work machine 100 or from an operator. Next, the controller 210 determines whether the desired torque 430 is between an upper threshold 410 and a lower threshold 420, as shown in block 620. The upper threshold 410 and the lower threshold 420 correspond to the deadband limits of sensorless operation. If the desired torque is not between the upper threshold 410 and the lower threshold 420, the controller 210 commands the SR motor 205 to generate the desired torque normally (block 630).

[0033] On the other hand, if the desired torque 430 is between the upper threshold 410 and the lower threshold 420, the controller generates a PWM adjusted torque command 450 using pulse width modulation (block 640) and commands the SR motor 205 based on the PWM adjusted torque command 450, as shown in block 650. The PWM adjusted torque command 450 is configured to cycle between the upper threshold 410 and the lower threshold 420 to generate the desired torque 430.

[0034] The PWM adjusted torque command 450 can be generated by generating a PWM reference signal 440 and comparing this signal to the desired torque 430. The PWM reference signal 450 can be a sawtooth or triangular wave that oscillates between an upper threshold 410 and a lower threshold 420. The frequency of the reference signal must be faster than the mechanism can respond, but not so fast as to interfere with position sensing.

[0035] Although the foregoing provides detailed descriptions of many different embodiments, it should be understood that the scope of legal protection is defined by the claims set forth at the end of this patent. The detailed description should be construed as exemplary only and does not describe every possible embodiment, as describing every possible embodiment would be impractical, if not impossible. Many alternative embodiments can be implemented using current technology or technology developed after the filing date of this patent, and they would still be within the scope of the claims that define the scope of protection.

Claims

1. A work machine (100), A frame (110); a traction system (120) supporting said frame (110); a power supply (130) attached to the frame (110); A switched reluctance motor (205); an inverter (220) configured to control power from the power source (130) to the motor (205); and a controller (210), wherein the controller (210) receiving a signal indicative of a desired torque (430); determining whether the desired torque (430) is between an upper threshold (410) and a lower threshold (420); If the desired torque (430) is between the upper threshold (410) and the lower threshold (420), generating a PWM adjusted torque command (450) using pulse width modulation configured to cycle between the upper threshold (410) and the lower threshold (420) to generate the desired torque (430); The work machine (100) is configured to command the motor (205) based on the PWM adjusted torque command (450).

2. The work machine (100) of any preceding claim, wherein the PWM adjusted torque command (450) is generated by generating a PWM reference signal (440) and comparing the signal to the desired torque (430).

3. The work machine (100) of any preceding claim, wherein the PWM reference signal (440) is a sawtooth wave.

4. 2. The work machine of claim 1, wherein the upper and lower thresholds correspond to positive and negative torque loads that produce minimum currents required for sensorless operation of the switched reluctance motor.

5. The work machine (100) of any preceding claim, wherein the desired torque (430) generates a sine wave.

6. A method (600) for overcoming dead zones in a switched reluctance motor (205), comprising: receiving a signal indicative of a desired torque (430); determining whether the desired torque (430) is between an upper threshold (410) and a lower threshold (420); If the desired torque (430) is between the upper threshold (410) and the lower threshold (420), generating a PWM adjusted torque command (450) using pulse width modulation configured to cycle between the upper threshold (410) and the lower threshold (420) to generate the desired torque (430); and commanding the motor (205) based on the PWM adjusted torque command (450).

7. 7. The method (600) of claim 6, wherein the PWM adjusted torque command (450) is generated by generating a PWM reference signal (440) and comparing this signal to the desired torque (430).

8. The method (600) of claim 6, wherein the PWM reference signal (440) is a triangle wave.

9. 7. The method of claim 6, wherein the upper and lower thresholds correspond to positive and negative torque loads that produce minimum currents required for sensorless operation of the switched reluctance motor.

10. The method (600) of claim 6, wherein the desired torque (430) generates a sine wave.

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