Patent Publication Number: US-10308138-B2

Title: Hybrid electric vehicle creep control

Description:
TECHNICAL FIELD 
     The present disclosure relates to creep control in hybrid-electric vehicles (HEVs). 
     BACKGROUND 
     A hybrid-electric powertrain includes an engine and an electric machine. The torque (or power) produced by the engine and/or the electric machine can be transferred through a transmission to the driven wheels to propel the vehicle. A traction battery supplies energy to the electric machine. 
     SUMMARY 
     According to one embodiment, a vehicle includes an engine and a transmission. The transmission includes a torque converter having an impeller. The vehicle further includes an electric machine configured to provide drive torque to the impeller. The impeller is selectively coupled to the engine via a clutch. At least one vehicle controller is configured to, in response to the engine being OFF, the transmission being in DRIVE, a vehicle speed being zero and a brake pedal being released beyond a threshold position, command the electric machine to provide a torque to the impeller. The torque is a predetermined feedforward torque adjusted by a feedback torque that is based on a difference between actual and target speeds. The speeds may be the speeds of the electric machine. 
     According to another embodiment, a vehicle includes a powertrain having an electric machine driveably connected to an impeller of a torque converter. The vehicle also includes a controller configured to, in response to vehicle speed being zero and a brake pedal being released beyond a threshold position, command the electric machine to provide a torque to the impeller, the torque being a predetermined feedforward torque adjusted by a feedback torque that is based on a difference between measured and target speeds. 
     According to yet another embodiment, a method of restarting a torque-converter impeller driveably connected to an electric machine is presented. The method includes, in response to a vehicle speed being zero and a brake pedal being released beyond a threshold position, commanding the electric machine to provide a torque to the impeller, the torque being a predetermined feedforward torque proportionally adjusted according to a difference between measured and target electric machine speeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an example HEV. 
         FIG. 2  is a plot illustrating speed for an example impeller/motor that is controlled using feedback control. 
         FIG. 3A  is a plot illustrating speeds associated with an example electric machine that is controlled using feedforward and feedback controls. 
         FIG. 3B  is a plot illustrating the speed error of the electric machine from the example in  FIG. 3A . 
         FIG. 3C  is a plot illustrating torques associated with the electric machine from the example in  FIG. 3A . 
         FIG. 4  is a flow chart illustrating an algorithm for controlling motor/generator speed in an HEV according to one embodiment of the present disclosure. 
         FIG. 5  illustrates control for controlling motor/generator speed during impeller spin-up using feedforward and feedback controls. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     Referring to  FIG. 1 , a schematic diagram of a hybrid-electric vehicle (HEV)  10  is illustrated according to an embodiment of the present disclosure.  FIG. 1  illustrates representative relationships among the components. Physical placement and orientation of the components within the vehicle may vary. The HEV  10  includes a powertrain  12  having an engine  14  that drives a transmission  16 , which may be referred to as a modular-hybrid transmission (MHT). As will be described in further detail below, a transmission  16  includes an electric machine such as an electric motor/generator (M/G)  18 , an associated traction battery  20 , a torque converter  22 , and a multiple step-ratio automatic transmission, or gearbox  24 . 
     The engine  14  and the M/G  18  are both drive sources for the HEV  10 . The engine  14  generally represents a power source that may include an internal-combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine  14  generates an engine power and corresponding engine torque that is supplied to the M/G  18  when a disconnect clutch  26  between the engine  14  and the M/G  18  is at least partially engaged. The M/G  18  may be implemented by any one of a plurality of types of electric machines. For example, M/G  18  may be a permanent-magnet-synchronous motor. Power electronics  56  condition direct current (DC) provided by the battery  20  to the requirements of the M/G  18 , as will be described below. For example, power electronics may provide three-phase alternating current (AC) to the M/G  18 . 
     When the disconnect clutch  26  is at least partially engaged, power flows from the engine  14  to the M/G  18 . Power flow from the M/G  18  to the engine  14  is also possible. For example, the disconnect clutch  26  may be engaged and M/G  18  may operate as a generator to convert rotational energy provided by a crankshaft  28  and M/G shaft  30  into electrical energy to be stored in the battery  20 . The disconnect clutch  26  can also be disengaged to isolate the engine  14  from the remainder of the powertrain  12  such that the M/G  18  can act as the sole drive source for the HEV  10 . The shaft  30  extends through the M/G  18 . The rotor  19  of the M/G  18  is fixed on the shaft  30 , whereas the engine  14  is selectively driveably connected to the shaft  30  only when the disconnect clutch  26  is at least partially engaged. 
     The M/G  18  is driveably connected to the torque converter  22  via the shaft  30 . For example, the torque-converter housing may be fastened to the shaft  30 . The torque converter  22  is therefore driveably connected to the engine  14  when the disconnect clutch  26  is at least partially engaged. Two components are driveably connected if they are connected by a power flow path that constrains their rotational speeds to be directly proportional. The torque converter  22  includes an impeller  35  fixed to the torque-converter housing (and consequently, fixed to the rotor  19 ) and a turbine  37  fixed to a transmission input shaft  32 . The torque converter  22  provides a hydraulic coupling between the shaft  30  and the transmission input shaft  32 . The torque converter  22  transmits power from the impeller  35  to the turbine  37  when the impeller rotates faster than the turbine. The magnitude of the turbine torque and impeller torque generally depend upon the relative speeds. When the ratio of impeller speed to turbine speed is sufficiently high, the turbine torque is a multiple of the impeller torque. A torque converter bypass clutch  34  may be provided to, when engaged, frictionally or mechanically couple the impeller and the turbine of the torque converter  22 , permitting more efficient power transfer. The torque converter bypass clutch  34  may be operated as a launch clutch to provide smooth vehicle launch. Alternatively, or in combination, a launch clutch similar to disconnect clutch  26  may be provided between the M/G  18  and gearbox  24  for applications that do not include a torque converter  22  or a torque converter bypass clutch  34 . In some applications, disconnect clutch  26  is generally referred to as an upstream clutch and launch clutch  34  (which may be a torque converter bypass clutch) is generally referred to as a downstream clutch. 
     The gearbox  24  may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the ratio between a transmission output shaft  38  and the transmission input shaft  32 . The gearbox  24  is automatically shifted from one ratio to another based on various vehicle and ambient operating conditions by an associated controller, such as a powertrain-control unit (PCU)  50 . The gearbox  24  then provides powertrain output torque to output shaft  38 . The output shaft  38  may be connected to a driveline  37  (e.g., a driveshaft and universal joints) that connects the output shaft  38  to the differential  40 . 
     It should be understood that the hydraulically controlled gearbox  24  used with a torque converter  22  is but one example of a gearbox or transmission arrangement; any multiple-ratio gearbox that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present disclosure. For example, gearbox  24  may be implemented by an automated mechanical (or manual) transmission (AMT) that includes one or more servo motors to translate/rotate shift forks along a shift rail to select a desired gear ratio. As generally understood by those of ordinary skill in the art, an AMT may be used in applications with higher torque requirements, for example. 
     As shown in the representative embodiment of  FIG. 1 , the output shaft  38  may be connected to a driveline  37  that connects the output shaft  38  to the differential  40 . The differential  40  drives a pair of wheels  42  via respective axles  44  connected to the differential  40 . The differential transmits approximately equal torque to each wheel  42  while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example. 
     While illustrated as one controller, the controller  50  may be part of a larger control system and may be controlled by various other controllers throughout the vehicle  10 , such as a vehicle-system controller (VSC) and a high-voltage battery controller (BECM). It is to be understood that the powertrain-control unit  50  and one or more other controllers can collectively be referred to as a “controller” that controls various actuators in response to signals from various sensors to control functions such as starting/stopping engine  14 , operating M/G  18  to provide wheel torque or charge the battery  20 , select or schedule transmission shifts, etc. The controller  50  may include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine, traction battery, transmission, or other vehicle systems. 
     The controller communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment of  FIG. 1 , the controller  50  may communicate signals to and/or from the engine  14 , disconnect clutch  26 , M/G  18 , launch clutch  34 , transmission gearbox  24 , and power electronics  56 . Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by controller  50  within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an alternator, air-conditioning compressor, battery charging, regenerative braking, M/G operation, clutch pressures for disconnect clutch  26 , launch clutch  34 , and transmission gearbox  24 , and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure (if applicable), crankshaft position (PIP), engine rotational speed (RPM), wheel speeds (WS 1 , WS 2 ), vehicle speed (VSS), coolant temperature (ECT), intake-manifold pressure (MAP), accelerator-pedal position (PPS), ignition-switch position (IGN), throttle-valve position (TP), air temperature (TMP), exhaust-gas oxygen (EGO) or other exhaust gas component concentration or presence, intake-air flow (MAF), transmission gear, ratio, or mode, transmission-oil temperature (TOT), transmission-turbine speed (TS), torque converter bypass clutch  34  status (TCC), deceleration or shift mode (MDE), for example. 
     Control logic or functions performed by controller  50  may be represented by flow charts or similar diagrams in one or more figures. These figures provide representative control strategies and/or logic that may be implemented using one or more processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Although not always explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending upon the particular processing strategy being used. Similarly, the order of processing is not necessarily required to achieve the features and advantages described herein, but is provided for ease of illustration and description. The control logic may be implemented primarily in software executed by a microprocessor-based vehicle, engine, and/or powertrain controller, such as controller  50 . Of course, the control logic may be implemented in software, hardware, or a combination of software and hardware in one or more controllers depending upon the particular application. When implemented in software, the control logic may be provided in one or more computer-readable storage devices or media having stored data representing code or instructions executed by a computer to control the vehicle or its subsystems. The computer-readable storage devices or media may include one or more of a number of known physical devices which utilize electric, magnetic, and/or optical storage to keep executable instructions and associated calibration information, operating variables, and the like. 
     An accelerator pedal  52  is used by the driver of the vehicle to provide a demanded torque, power, or drive command to propel the vehicle. The pedal  52  may include a pedal position sensor. In general, depressing and releasing the pedal  52  causes the pedal sensor to generate an accelerator-pedal-position signal that may be interpreted by the controller  50  as a demand for increased power or decreased power, respectively. Based at least upon input from the pedal, the controller  50  commands torque from the engine  14  and/or the M/G  18 . The controller  50  also controls the timing of the gear shifts within the gearbox  24 , as well as engagement or disengagement of the disconnect clutch  26  and the torque converter bypass clutch  34 . Like the disconnect clutch  26 , the bypass clutch  34  can be modulated across a range between the engaged and disengaged positions. This produces a variable slip in the torque converter  22  in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the bypass clutch  34  may be operated as locked or open without using a modulated operating mode depending on the particular application. 
     To drive the vehicle with the engine  14 , the disconnect clutch  26  is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch  26  to the M/G  18 , and then from the M/G  18  through the torque converter  22  and gearbox  24 . When the engine  14  alone provides the torque necessary to propel the vehicle, this operation mode may be referred to as the “engine mode,” “engine-only mode,” or “mechanical mode.” 
     The M/G  18  may assist the engine  14  by providing additional power to turn the shaft  30 . This operation mode may be referred to as a “hybrid mode,” an “engine-motor mode,” or an “electric-assist mode.” 
     To drive the vehicle with the M/G  18  as the sole power source, the power flow remains the same except the disconnect clutch  26  isolates the engine  14  from the remainder of the powertrain  12 . Combustion in the engine  14  may be disabled or otherwise OFF during this time to conserve fuel. The traction battery  20  transmits stored electrical energy through wiring  54  to power electronics  56  that may include an inverter and a DC/DC converter, for example. The power electronics  56  convert DC voltage from the battery  20  into AC voltage to be used by the M/G  18 . The controller  50  commands the power electronics  56  to convert voltage from the battery  20  to an AC voltage provided to the M/G  18  to provide positive (e.g. drive) or negative (e.g. regenerative) torque to the shaft  30 . This operation mode may be referred to as an “electric only mode,” “EV (electric vehicle) mode,” or “motor mode.” 
     In any mode of operation, the M/G  18  may act as a motor and provide a driving force for the powertrain  12 . Alternatively, the M/G  18  may act as a generator and convert kinetic energy from the powertrain  12  into electric energy to be stored in the battery  20 . The M/G  18  may act as a generator while the engine  14  is providing propulsion power for the vehicle  10 , for example. The M/G  18  may additionally act as a generator during times of regenerative braking in which rotational energy from spinning wheels  42  is transferred back through the gearbox  24  and is converted into electrical energy for storage in the battery  20 . 
     It should be understood that the schematic illustrated in  FIG. 1  is merely exemplary and is not intended to be limited. Other configurations are contemplated that utilize selective engagement of both an engine and a motor to transmit through the transmission. For example, the M/G  18  may be offset from the crankshaft  28 , an additional motor may be provided to start the engine  14 , and/or the M/G  18  may be provided between the torque converter  22  and the gearbox  24 . Other configurations are contemplated without deviating from the scope of the present disclosure. 
     Most conventional vehicles with an automatic transmission have creep control that allows a driver to move the vehicle at low speeds by simply releasing the brake without pressing the accelerator pedal. Creep control utilizes the torque produced at engine-idle speed to propel the vehicle. Most automatic transmissions have a torque converter. The torque-converter impeller (connected to the crankshaft) transfers torque to the turbine (connected to the transmission input shaft) hydrodynamically. This hydrodynamic coupling only occurs when the impeller speed is above a turbine-stall speed. The torque converter is configured such that idle speed is above the stall speed. Thus, in a conventional vehicle, the impeller and turbine are hydrodynamically coupled when the engine is ON. The vehicle will be propelled, without lag, once the engine torque exceeds the braking torque, which typically occurs prior to full release of the brake pedal. 
     Drivers have grown accustom to creep control, as such, it may be advantageous to program electric and hybrid vehicles to mimic the creep control of conventional vehicles. Rather than using the engine, hybrid vehicles may provide creep torque with the electric machine. In hybrid vehicles with a torque converter (such as vehicle  10 ), the electric machine may be programmed to spin at a speed that approximates engine idle (e.g., 800 RPM) to maintain hydrodynamic coupling within the torque converter during times when creep control is expected. Unlike conventional vehicles, in which the impeller is spinning at idle speed when the vehicle is stopped with the engine running, hybrid vehicles typically turn OFF the electric machine when stopped to preserve the battery state of charge (SOC). Thus, hydrodynamic coupling is not maintained when the hybrid vehicle is stopped. Once creep torque is requested, the impeller must spin-up to the turbine-stall speed before any creep torque is provided to the driven wheels. Drivers have come to expect the immediate propulsion that conventional vehicles provide when the brakes are at least partially released. To limit the delay and provide a satisfactory driving experience, hybrid vehicles should spin-up the impeller to turbine-stall speed as quickly as possible to reduce the delay. When the turbine is stalled, reaction forces of the torque converter are low and a majority of the electric-machine torque is used to accelerate the impeller. Creating a control strategy that accurately and quickly accelerates the impeller to idle speed is difficult to execute. 
       FIG. 2  illustrates plots of impeller/motor speed associated with a feedback control for spinning-up the impeller. The feedback control may use proportional-integral-derivative (PID) control to adjust the torque applied by the electric machine. In this strategy, the controller calculates a target speed curve  60 . Using feedback control, the electric machine increase or decreases the torque to reduce the error between the target speed  60  and the measured speed  62 . The speeds may be that of the impeller or the electric machine. One problem with pure feedback control is that the speed may overshoot the target idle speed as shown at  64 . This creates jerky torque that is perceivable by the driver and is considered to be an unsatisfactory driving experience. Another problem is that the measured speed  62  lags behind the target speed  60 . In the illustrated example, the target-speed plot  60  reached idle speed 50 milliseconds faster than the actual-speed plot  62 . This time period is noticeable by the driver and may be perceived as unsatisfactory delay. One way to avoid overshoot using feedback control is to limit the slope of the target speed  60  near the idle speed. While this may reduce overshooting, it further increases the time it takes the impeller to reach idle speed, which may be undesirable. 
     In order to solve these and other problems, impeller spin-up may be controlled by a control strategy having a torque algorithm with feedforward and feedback components. Equation 1 is an example electric-machine-torque equation that includes both feedforward and feedback components. 
     
       
         
           
             
               
                 
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                             ⁢ 
                             
                                 
                             
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                             commanded 
                           
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     where: J is inertia of the impeller; ω is angular speed; e is the error between ω commanded  and ω measured ; and P, I, and D are constants of the PID control. 
     Equation 1 may be used to control torque to the M/G  18  during impeller spin-up. Impeller spin-up is typically initiated in response to the vehicle being stop, the impeller being below turbine-stall speed (idle speed), and creep control being requested by the driver. For example, impeller spin-up may be initiated when the brake torque requested by the driver approaches the wheel torque produced by the impeller when spinning at idle speed. The feedforward component includes one or more predetermined torques that are applied for a predetermined time. The predetermined time may be the expected time that it takes for the impeller to spin-up to the desired idle speed. The predetermined torques may be stored in one or more lookup tables stored in memory of the controller  50 . Different feedforward torques may be applied in different situations. For example, one feedforward torque may be used when the impeller is spun-up to support a transmission engagement from NEUTRAL to DRIVE, and another may be used when the brake pedal is released with the transmission in DRIVE. 
     The feedback component adjusts the feedforward component based on an error (e) between the commanded speed and the measured speed of the impeller  35  or the M/G  18 . The feedback loop may use PID controls or may use PD or PI controls. The feedback torque may be limited to plus or minus 40 newton meters (N·m) since a majority of the torque is being provided by the feedforward component. If the error requires a torque correction in excess of 40 N·m, a mechanical failure may have occurred and limiting the feedback may reduce potential damage since the feedforward torque is only supplied for a short duration of time. (In contrast, feedback control does not have this capability without using other checks.) The speeds may be the impeller speed or the M/G speed. In the vehicle  10 , the rotor  19  and the impeller  35  are fixed relative to each other. Since electric machines typically include a speed sensor, unlike torque converts which do not, the below example controls will use M/G speeds in Eq. 1 as it does not require additional speed sensors. 
       FIGS. 3A to 3C  illustrates a series of plots associated with a combined feedforward and feedback control strategy. The plots will be described in conjunction with the flowchart  100  shown in  FIG. 4 . As those of ordinary skill in the art will understand, the functions represented by the flow chart blocks may be performed by software and/or hardware. Depending upon the particular processing strategy, such as event-driven, interrupt-driven, etc., the various functions may be performed in an order or sequence other than that illustrated in the Figures. Similarly, one or more steps or functions may be repeatedly performed, although not explicitly illustrated. In one embodiment, the functions illustrated are primarily implemented by software, instructions, code or control logic stored in a computer-readable storage medium and executed by one or more microprocessor-based computers or controllers to control operation of the vehicle. All of the illustrated steps or functions are not necessarily required to provide various features and advantages according to the present disclosure. As such, some steps or functions may be omitted in some applications or implementations. The algorithm for controlling a motor in an HEV according to one embodiment of the present disclosure as illustrated in  FIG. 4  includes steps or functions that may be represented by control logic or software executed by one or more microprocessor-based controllers, such as controller  50 , for example. 
     At operation  102  the controller  50  determines if creep control is being requested by the driver. It can be determined that creep control is being requested in response to one or more of the following: the engine being OFF, the transmission being in DRIVE, the electric machine speed being zero, the vehicle speed being zero, a brake pedal being released beyond a threshold position (this threshold can vary depending on the grade of the road and vehicle weight), and the accelerator pedal is not depressed. If creep control is being requested, control passes to operation  103  and the controller determines if the M/G speed is below the turbine-stall speed. If no, control loops back to the start. If yes, the controller enters into impeller-spin-up control and control passes to operation  104 . At  104 , a target M/G speed plot  66  is calculated. The speed plot  66  may include one or more rates such as a first rate  68 , a second rate  70 , and a third rate  72 . Having more than one rate provides greater control of the acceleration of the impeller during spin-up. The first target rate  68  may be used between a M/G speed of zero up to a M/G speed that corresponds to a speed required to create line pressure within the transmission (e.g., 300 RPM). The first rate  68  may be the steepest rate in order to quickly generate line pressure within the transmission. The first rate  68  may also be the steepest because it is farthest away from the idle speed and poses less threat to overshooting the idle speed, which is to be avoided. The second rate  70  may be the second steepest rate and is used between the line pressure speed and another M/G speed that may vary depending upon operating conditions. The third rate  72  may have the shallowest scope to prevent overshooting the idle speed. Of course, the three-rate example is not limiting, and more or fewer rates may be used. 
     At operation  106  the controller calculates a feedforward torque  74  for each of the speed rates. Each of the feedforward torques are commanded for a predetermined time that corresponds with the expected time it takes the M/G (and impeller) to reach the target speed. In the illustrated embodiment, three feedforward torques are calculated because three rates are being used. A first feedforward torque  76  is commanded during the first rate  68 , a second feedforward torque  78  is commanded during the second rate  70 , and a third feedforward torque  80  is commended during the third rate  72 . Accelerating the impeller from rest requires more torque than further accelerating the impeller. Thus, the first feedforward torque  76  is the highest torque and the last feedforward torque  80  is the lowest torque. At operation  108  a torque command is generated and sent to the M/G  18 . 
     At operation  110  the M/G speed may be determined by a speed sensor the measures a component associated with the M/G. In one embodiment, the M/G  18  includes a speed sensor that measures the angular speed of the rotor  19 . The speed sensor may be an encoder sensor. The speed sensor is configured to output a signal to the controller indicating the speed of the rotor  19 . Since the impeller and rotor are fixed, the impeller speed can be inferred based on the rotor speed. The measured M/G speed  73  is shown as trace  73  in  FIG. 3A . 
     At operation  112 , the controller determines the relative weighting between the feedback torque relative to the feedforward torque. This could be achieved by adjusting the gains of the PID controller or by applying a scaling factor to the feedback torque. During the first rate when the feedforward torque is the highest, the feedback gains could be selected to be small values. As the impeller speed increases, for example, during the third rate and beyond, nominal values of the feedback gains could be selected (e.g., larger values than during rate  1 ). 
     At operation  114  the feedback torque is calculated based on a speed error  82 . The error  82  may be the difference between a commanded M/G speed  66  and the measured M/G speed  73 . The controller dynamically inserts the speed error  82  into the feedback component of Eq. 1, and adjusts the torque command up or down accordingly to reduce the speed error at operation  116 . The feedback torque is shown as trace  86  in  FIG. 3C . At operation  118  the controller provides the adjusted torque command  84  to the M/G  18 . 
       FIG. 5  illustrates a control structure that outputs a torque command to the M/G  18  according to an algorithm including feedforward and feedback components. In response to creep control being requested and the impeller being below the turbine-stall speed, the controller enters an impeller-spin-up-control mode where torque is supplied to the M/G  18  according to Equation  1 , for example. In this mode, a commanded M/G (or impeller) speed is generated based on mapping saved in the memory of the vehicle. Based on the commanded M/G speed, a feedforward-torque command  130 , which is proportional to the rate of change of the speed request, is generated using Equation 1. The feedforward torque may be weighted according to the inertia of the impeller, and then sent to the M/G  18 . The feedforward torque is adjust by an error (e) based on a difference between the commanded M/G speed and the measured M/G speed. A speed sensor  132  of the M/G  18  may send a speed signal to the controller  50  to determine the error between the commanded and the measured speeds. The error is fed into the feedback component  134  and a feedback torque is calculated. The feedback torque may be weighted. The feedforward torque is then adjusted by the feedback torque and a torque command  138  is output to the M/G  18 . This sequence may be repeated at a predetermined frequency until the controller exits impeller-spin-up control. 
     The processes, methods, or algorithms disclosed herein can be deliverable to/implemented by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, or algorithms can also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     While example embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.