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Title:
PRESSURE CONTROL IN A DEAD-HEADED HYDRAULIC SYSTEM USING PUMP MOTION CONTROL
Document Type and Number:
WIPO Patent Application WO/2018/207157
Kind Code:
A2
Abstract:
A method for controlling pressure and flow of hydraulic fluid through a hydraulic control unit comprises receiving a pressure command from an electronic control unit. The received pressure command is processed to generate a pressure- based torque command and to generate one of a shaft angle-based torque command or a volume-based torque command. The generated pressure-based torque command and the generated one of the shaft angle-based torque command or the volume-based torque command are processed to further generate a duty cycle for a field oriented controlled motor. Motor phase wires of the field oriented controlled motor are modulated according to the generated duty cycle to operate a bidirectional positive displacement pump connected to the field oriented controlled motor. The flow rate and pressure of fluid supplied by the bidirectional positive displacement pump are adjusted by updating the generated duty cycle.

Inventors:
BUSDIECKER, Matthew R. (31650 Westlady, Beverly Hills, Michigan, 48025, US)
Application Number:
IB2018/053320
Publication Date:
November 15, 2018
Filing Date:
May 11, 2018
Export Citation:
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Assignee:
EATON INTELLIGENT POWER LIMITED (30 Pembroke Road, Dublin, 4, 4, IE)
International Classes:
H02P21/00; F04B17/03; F04B49/06; F16D31/00; F16D48/06
Other References:
None
Download PDF:
Claims:
WHAT IS CLAIMED IS:

1. A method for controlling pressure and flow of hydraulic fluid through a hydraulic control unit, comprising:

receiving a pressure command from an electronic control unit;

processing the received pressure command to generate a pressure-based torque command and to generate one of a shaft angle-based torque command or a volume-based torque command;

processing the generated pressure-based torque command and processing the generated one of the shaft angle-based torque command or the volume-based torque command to further generate a duty cycle for a field oriented controlled motor;

modulating motor phase wires of the field oriented controlled motor according to the generated duty cycle to operate a bidirectional positive

displacement pump connected to the field oriented controlled motor; and adjusting the flow rate and pressure of fluid supplied by the bidirectional positive displacement pump by updating the generated duty cycle.

2. The method of claim 1 , wherein the hydraulic control unit is coupled to a dead headed hydraulic system, and wherein the method does not comprise actuating a pressure control valve between the hydraulic control unit and the dead headed hydraulic system.

3. The method of claim 1 , further comprising selecting a first pressure-based feedforward in the form of an actual pressure-based torque command estimate; and processing in a sum block the selected pressure-based feedforward with measured pressure data and the received pressure command from the electronic control unit to generate the pressure-based torque command.

4. The method of claim 1 or 3, further comprising selecting a second pressure- based feedforward in the form of a volume-based target or a shaft angle-based target; and generating a motor speed command by processing one of the volume- based target or the shaft angle-based target.

5. The method of claim 4, further comprising measuring a shaft angle associated with a rotor of the motor to know the location of the rotor with respect to its stator.

6. The method of claim 5, further comprising implementing a counter to count the rotations of the rotor with respect to its stator; and deriving a volume of fluid pumped by the pump based on the counted rotations of the rotor and based on a known displacement of the pump.

7. The method of claim 5, further comprising selecting a third pressure-based feedforward in the form of an estimated pump speed.

8. The method of claim 7, further comprising generating a motor speed-based toque command by processing the third pressure-based feedforward and the generated motor speed command.

9. The method of claim 7, further comprising processing the generated motor speed-based torque command and the generated actual pressure-based torque command to generate the duty cycle for the field oriented controlled motor.

10. The method of claim 1 , wherein receiving the pressure command from the electronic control unit comprises receiving a selection of one of a forward condition, reverse condition or stall condition for the bidirectional positive displacement pump.

1 1. The method of claim 1 or 10, wherein processing the received pressure command to generate a pressure-based torque command comprises applying a pressure servo loop, and wherein generating one of the shaft angle-based torque command or the volume-based torque command comprises respectively applying one of a shaft angle servo loop or a volume servo loop.

12. The method of claim 1 or 10, wherein processing the generated pressure- based torque command and processing the generated one of the shaft angle-based torque command or the volume-based torque command to further generate a duty cycle for a field oriented controlled motor comprises applying a speed servo loop and applying a current servo loop.

13. The method of claim 1 , comprising:

modulating the motor phase wires of the field oriented controlled motor

according to the generated duty cycle to operate the bidirectional positive displacement pump connected to the field oriented controlled motor in a zero speed condition; and

adjusting the flow rate and pressure of fluid supplied by the bidirectional positive displacement pump by updating the torque command applied to the generated duty cycle.

14. A hydraulic control unit for controlling pressure and flow of hydraulic fluid to a dead headed downstream device, comprising:

a pump unit, comprising:

a bidirectional positive displacement pump comprising an output path; a pressure sensor coupled to the output path to sense fluid pressure; a field oriented controlled motor connected to the bidirectional positive displacement pump, the motor comprising a rotor and a stator; and a sensed device coupled to the rotor;

control electronics, comprising:

an angle measurement sensor coupled to collect location data of the

rotor;

a shaft angle servo loop or a volume servo loop coupled to receive and process the collected location data and configured to issue a speed command;

a pressure servo loop coupled to receive and process the sensed fluid pressure and configured to issue a pressure-based torque command; a speed servo loop coupled to receive and process the collected location data and the issued speed command and configured to issue a speed- based torque command; and

a current servo loop coupled to receive and process the issued pressure- based torque command and the issued speed-based torque command and configured to issue current commands for field oriented control of a speed and a torque of the motor.

15. The hydraulic control unit of claim 14, further comprising at least one feedforward map stored in the control electronics, the at least one feedforward map comprising data for detecting an error in the location data.

Description:
PRESSURE CONTROL IN A DEAD-HEADED HYDRAULIC SYSTEM USING

PUMP MOTION CONTROL

Field

[001] This application provides a system and method for controlling hydraulic pressure and flow pumped to a downstream device.

Background

[002] Field Oriented Control ("FOC") is a motor control technique whereby the angle of the rotor is used to align the flux vector of the stator to produce maximum motor torque. The angle of the rotor can either be directly measured, or estimated by a variety of techniques. Direct measurement of the rotor angle is called "sensored FOC". Estimation of the rotor angle is called "sensorless FOC".

[003] Since the angle sensor adds cost, there is a desire to use sensorless methods. However, sensorless schemes have drawbacks at low speeds. Most rotor angle estimation methods make use of the voltage appearing in the stator windings (back-EMF voltage method). These methods do not work well at low motor speeds, due to low back EMF being developed at these speeds. Often there is an open loop control method used to get the motor rotating at a certain minimum speed before the sensorless observer is used. This minimum speed can cause a torque ripple and shudder in the downstream device.

[004] Another control technique uses one, or more, Hall effect sensors to trigger the motor commutation. Often called "6 step control," or BLDC control, this method is used with pumps and fans that do not have significant starting torque requirements. In these applications the motor easily accelerates from a stop since there is no load torque. Hall effect sensors are used to switch (energize) the stator windings. In this scheme, when the rotor is completely stopped, the angle cannot be measured if the rotor stops between the Hall sensors. This technique can also lead to torque ripple or downstream device shuddering because the rotor must be spun to face the Hall sensor in order to detect the location of the rotor.

SUMMARY

[005] The systems and methods disclosed herein overcome the above disadvantages and improve the art by way of a pressure control system and pressure control method that limits torque ripple and downstream device

shuddering.

[006] A method for controlling pressure and flow of hydraulic fluid through a hydraulic control unit comprises receiving a pressure command from an electronic control unit. The received pressure command is processed to generate a pressure- based torque command and to generate one of a shaft angle-based torque command or a volume-based torque command. The generated pressure-based torque command and the generated one of the shaft angle-based torque command or the volume-based torque command are processed to further generate a duty cycle for a field oriented controlled motor. Motor phase wires of the field oriented controlled motor are modulated according to the generated duty cycle to operate a bidirectional positive displacement pump connected to the field oriented controlled motor. The flow rate and pressure of fluid supplied by the bidirectional positive displacement pump are adjusted by updating the generated duty cycle.

[007] A hydraulic control unit for controlling pressure and flow of hydraulic fluid to a dead headed downstream device is also disclosed. A pump unit comprises a bidirectional positive displacement pump comprising an output path, a pressure sensor coupled to the output path to sense fluid pressure, a field oriented controlled motor connected to the bidirectional positive displacement pump, the motor comprising a rotor and a stator, and a sensed device coupled to the rotor. Control electronics comprise an angle measurement sensor coupled to collect location data of the rotor. A shaft angle servo loop or a volume servo loop is coupled to receive and process the collected location data and is configured to issue a speed command. A pressure servo loop is coupled to receive and process the sensed fluid pressure and is configured to issue a pressure-based torque command. A speed servo loop is coupled to receive and process the collected location data and the issued speed command and is configured to issue a speed-based torque command. A current servo loop is coupled to receive and process the issued pressure-based torque command and the issued speed-based torque command and is configured to issue current commands for field oriented control of a speed and a torque of the motor.

[008] The hydraulic control unit can further comprising at least one feedforward map stored in the control electronics, the at least one feedforward map comprising data for detecting an error in the location data. The hydraulic control unit can comprise three feedforward maps and four servo loops.

[009] Additional objects and advantages will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure. The objects and advantages will also be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] Figure 1 is a schematic of a motor control loop connected between an ECU, a pump motor, and a downstream device.

[011] Figure 2A is a schematic of a hydraulic control system having a pump with an internal leak path. An shaft angle servo loop is shown.

[012] Figure 2B is a schematic of a hydraulic control system having a pump with an internal leak path. A volume servo loop is shown.

[013] Figure 3A is a schematic of a rotor and stator assembly comprising a sensed mechanism for sensing by an angle measurement sensor.

[014] Figure 3B is a schematic of a 3-phase motor connected to aspects of the power electronics unit.

[015] Figures 4-6 are flow diagrams for methods of implementing pressure control in a dead-headed hydraulic system using pump motion control.

DETAILED DESCRIPTION

[016] Reference will now be made in detail to the examples which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Directional references such as "left" and "right" are for ease of reference to the figures.

[017] A "dead headed hydraulic system" can be seen in Figures 1 , 2A & 2B. In some cases, it is possible to dead head a pump, as by installing a valve at its output to cease flow through the pump. Installing a controlled valve at the output of the pump permits pressure control using a relatively simple control system. In this architecture it is common for the pump to provide continuous flow, while a proportional valve provides a restriction to achieve the desired pressure. Proportional valves have a direct relationship between solenoid current and pressure so the control system can be simplified to a pressure feedback loop.

[018] A dead headed hydraulic system as drawn has no flow control valve at the output of the pump 100 (except perhaps a pressure relief valve or check valve). Instead, the pump 100 connects to a pump output path 201 via tubing or porting or the like directly to a downstream device 150. The downstream device 150 provides the dead-heading which prevents further fluid flow, as when the downstream device 150 is fully actuated.

[019] The downstream device 150 can be, for example, a differential, a power take off unit, other clutch-containing device, or other hydraulically controlled device. As drawn, a cylinder 151 of the downstream device 150 can comprise a plenum 152 on a first side for receiving fluid from the pump 100. A piston 153 or the like can move in the cylinder 151 as fluid fills the plenum 152. The piston 153 can be arranged to exert force directly or indirectly on a clutch 154. The clutch 154 can comprise, for example, one or more packs of clutch discs that are part of a limited- slip differential (LSD) or electronically-controlled limited slip differential (eLSD).

[020] The hydraulic control system comprises a hydraulic control unit comprising pump motor 110 connected to control electronics (e.g. circuit board 1000). Motor 300 drives the pump 100. The pump 100 applies pressure to the piston 153, the piston exerts force in response to the pressure, and the applied pressure controls the downstream device 150. The motor torque controls the pressure applied by the pump 100 to the piston 153. The force on the clutch 154 is managed by the motor 300 via the pump 100. In the case of an eLSD, the torque transmitted by the clutch 154, and hence the slip of the differential, is controlled by the motor torque output to the pump. The reaction time of the clutch 154 is controlled by the speed of the motor shaft 310 as it actuates to the pump 100.

[021] There are several alternative motors 300 that can be used with the instant disclosure and there are several alternative pumps 100. The alternative motors can comprise aspects that permit bidirectional fluid flow and field-oriented control ("FOC") for the ability to control the motion electronically. Any type of motor that can develop torque at zero speed and dissipate the thermal losses can be used. Such motors can comprise a brushless DC motor ("BLDC"), induction motor, switched reluctance motor, PMSM (permanent magnet synchronous motor), as examples. A brush motor cannot be used due to its inability to maintain constant torque in a stall condition. A bi-directional motor can be used for the purpose of quickly reducing pressure.

[022] The contemplated motors can be combined with a controller to achieve speed and torque control. The motor 300 can interact with the pump 100 to pump fluid bi-directionally. Discreet motions can be controlled and longer operational periods are enabled. In the working example described herein, the motor 300 comprises a brushless direct current motor comprising a rotor 330 and stator 320 assembly, and the motor is turned in a first direction, such as a forward direction, and in a second direction, such as a reverse direction, by modulating electricity to the rotor and stator assembly. The pump 100 is driven by the motor 300, and so the hydraulic fluid flows in a first direction when the motor is turned in the first direction, and the hydraulic fluid flows in an opposite second direction when the motor is turned in the second direction. If the motor is "stalled" or held in a neutral position, then fluid flows according to an internal leak path through the pump. When the temperature of the hydraulic fluid is low, sometimes little or no leakage occurs and the pump can be stalled (zero speed). In some instances, it can be beneficial to apply a torque to the motor, even though it is at zero speed, to hold the pump position.

[023] Many hydraulic control systems run the pump continuously and use a valve to control the pressure of the downstream device, but the disclosed pump 100 can be run in forward or in reverse or can be stalled to control pressure in the plenum 152. A controlled leak path, such as an internal leakage path can permit hydraulic fluid to leak back from the outlet side 103 of the pump 100 to the sump 10 or inlet side 101 of the pump. The leakage can be accounted for as below. In some instances, it can be possible to measure the leak rate of the pump over a temperature range by using accurate shaft feedback. And, by using accurate motion control of the shaft 310, it can be possible to eliminate an external bleed orifice around the pump, relying on the bi-directionality of the pump motor and the internal leak path to increase or decrease hydraulic fluid pressure and flow to the downstream device 150.

[024] The pump 100 can comprise a positive displacement pump, such as a gear pump. The pump 100 can operate in a wide positive and negative speed range, which is contrary to traditional pumps that work in one direction (positive or negative) only. A bidirectional hydraulic pump motor 110 can be formed as a single unit. The pump motor can comprise a pump 100 integrally formed with the motor 300 as a single unit, or as drawn, the motor 300 can be linked to the pump 100 via a direct drive arrangement or one or more couplers such as motor shaft 310. The coupler can comprise, for example, shafts and linkages such as gears, or other drive mechanisms. In the disclosure, a shaft angle is discussed. When the rotor 330 is coupled via shaft 310, the angle of the shaft is referenced. It is to be understood that direct drive arrangement can result in no shaft between the rotor and pump mechanism with some other coupler in place. Or, a shaft or the sensed mechanism 503 can protrude from the rotor 330 away from the pump 100 and towards the angle measurement sensor 501 , and that shaft or other protrusion can comprise the shaft resulting in the shaft angle discussed herein. In any case, the precise location of the rotor 330 with respect to the stator coils Φ1 , Φ2, Φ3 (Phil , Phi2, Phi3) can be known precisely, and the rotor position with respect to the pump is known so the affiliated pump motion can be controlled with precision. As indicated by the double- headed arrow, the rotor 330 and hence the motor shaft 310 moves bidirectionally, here rotationally. The pump can comprise, for example, an internal gear pump or gerotor, or an external gear pump such as one or more spur gear, as alternatives.

[025] Sensored field-oriented control ("FOC") is necessary since the pump 100 will stop rotating under many conditions, and a significant torque is required to hold pressure to the downstream device 150. For the purpose of this discussion, some terms are explained. High resolution angle feedback can also be called angle feedback, or rotor shaft angle measurement, or resolver or encoder position sensing, or position sensing with sufficient resolution to be used for FOC, or position sensing that can always determine the rotor angle, even when the rotor is stopped. This is in contrast to measuring rotor position with Hall effect sensors, which cannot determine rotor angle when the rotor is stopped, and which have a relatively low angular resolution, perhaps having an error or tolerance of 30 degrees or more. The disclosed systems and methods permit determination of rotor position even when the rotor is stopped and can do so with better than 30 degrees of angular resolution. The disclosed system and method increases the cost for shaft angle detection and increases computational complexity, but reduces part count by eliminating pressure control valves and bleed valves or orifices, reduces torque ripple, and improves the precision of the hydraulic control.

[026] In order to use sensored FOC, an angle measurement sensor 501 is attached to the pump motor 110. The angle measurement sensor can be directly coupled to the pump motor 110 and read out to a circuit board 1000 or the angle measurement sensor 501 can be coupled to the circuit board 1000 and plugged in or otherwise coupled to the pump motor 1 10 to sense the location of a sensed mechanism 503. The angle measurement of the stator 320 can be determined via analog or digital techniques, as by using a resolver or encoder or the like. Sensed mechanism 503 can be attached to the rotor 330 of the motor 300. As one example, a magnet can be the sensed mechanism 503 and the angle measurement sensor 501 can be positioned to sense the location of the magnet. Other examples can comprise one or more optical sensor as the angle measurement sensor and a marked disc as the sensed mechanism 503. A resolver comprising a coil as the angle measurement sensor 501 can sense the coil windings of the stator as the sensed mechanism 503. Using one of the disclosed techniques or an equivalent thereto, the shaft angle can be determined no matter where the shaft 310 is in its 360 degrees of rotation. The electrical output of the angle measurement sensor 501 can be proportional to the angular position of the sensed mechanism 501. If an encoder is used, the encoder can electromechanically convert the angular motion or position of the magnet or other sensed mechanism 503 on the rotor into an analog or digital code that represents that motion or position. Sensed mechanism 503 can extend on a shaft from the rotor 330 to be positioned relative to angle measurement sensor 501.

[027] Circuit board 1000 can comprise a number of subunits described further below. Numerous options exist to integrate one or more circuit boards or chipsets in to a single control unit. Allocation programming of one or more processors, hardwiring between on or more integrated circuits, or processor handshakes, among other options, can be used in the alternative to result in the described system and to implement the disclosed methods. At least one processor and associated storage device executes algorithms stored in the storage device to implement the methods disclosed herein. In a vehicle application, typically a central or main ECU 2000 coordinates with other electronically controlled components of the vehicle via a controller area network (CAN). For example, an all-wheel drive vehicle can convert from single axle to dual axle torque transmission to the wheels via a drive unit. The drive unit can be coupled to a differential such as a limited slip differential. Both the drive unit and the differential can comprise their own electronic control unit. In such an instance, the drive unit could receive a command from the main ECU 2000 and forward it to the differential ECU. The differential ECU (e.g. eLSD ECU 2100) can in turn issue commands to a hydraulic control unit. The hydraulic control unit can, as illustrated herein, comprise additional control electronics in the form of circuit board 1000. Such electrical component stack-up is for example purposes, it is possible that main ECU 2000 integrate many

intermediary component electronics, including subsuming the circuit board 1000. More or fewer intermediary electronics are possible. So too, other vehicle implementations are also possible.

[028] Using a prior art Hall effect technique, the rotor position is only known when the sensor is pointed directly at the corresponding sensed device on the rotor. Out of 360 degrees, the rotor position is known for only 30 or so of those degrees. The six step commutation technique requires relatively few calculations to be done in the controller to rotate the motor. The instant disclosure provides a higher resolution of the angle at which the rotor 320 is positioned, and the rotor position can be determined even at zero speed of the rotor (stalled condition). The resolution required to implement FOC is a function of the number of poles on the stator 320 of the motor 300, the accuracy desired, and the efficiency of the control algorithm. The technique is more computationally intense, but the result is less torque ripple, and ability to control the motor while stopped..

[029] If pump 100 is lobed, as by using a gear or gerotor pump, fixed volumes of hydraulic fluid are conveyed as the lobes rotate. Fractional rotations of the motor can be significant to pump pressure control. So, angle feedback is used to have information for control of the pump 100. Torque ripple, caused by adversely actuating the clutch pack 154 of the downstream device, can be avoided by using the angle feedback to know where the rotor is and extrapolating that rotor information to know where any coupler or shaft 310 of the pump motor is at all times. Knowing the location of the sensed mechanism 503 permits the system to determine which coils Φ1 , Φ2, Φ3 (Phil , Phi2, Phi3) to actuate in the stator 320 in order to precisely rotate the rotor 330, which in turn rotates the shaft 310 coupled to the pump 100. Precise control of the pump 100, which can comprise positive displacement elements such as gears, precisely controls the hydraulic pressure conveyed by the pump 100 to actuate the downstream device 150. As above, the motors can comprise a BLDC, induction motor, switched reluctance motor, PMSM, as examples. One schematic example is shown in Figure 3A. The stator can comprise three poles with coil windings. The rotor can comprise embedded magnets to enable a PMSM, or the rotor can comprise additional coil windings to enable an induction motor. A location for an exemplary sensed mechanism 503 is also shown. A 3-phase 2-pole motor is shown, though other examples are possible, such as 3- phase 3-pole motors, among others.

[030] FOC permits motion control of the stator 310 and hence the pump 100 to provide a forward pumping condition, a reverse pumping condition, and a hold condition. The hold condition is a zero-motion condition with a torque signal applied to prevent the pump 100 from moving. For example, sometimes the backwards leaking of fluid internally can cause the pump internals to move. For a gear pump, this would mean that the teeth and therefor fixed volumes of fluid would leak backwards in addition to that quantity of fluid designed to leak via the leak path. But, in the instant disclosure, the backwards leakage is accounted for. The total leakage determines how fast the pump will rotate, and torque commands are used to control the pressure provided. The pump will have some leakage, given the mechanical tolerance of the pump, and the viscosity of the fluid. The present disclosure can operate the motor in a torque control mode. While in torque control mode, the motor maintains the pump pressure regardless of the pump speed (the pump speed will be determined by the leakage volume). The pump may stop rotating depending on conditions, but the pressure can be maintained independent of rotational speed. Now, the pump does not need to be continuously rotating and there can be periods where the pump is stalled while a downstream device pressure is held. The clutch pack 154 can provide a desired torque without constant pump 100 motion because the pump is "stalled."

[031] Additional trajectory generation is outlined below to explain the paths to reaching the desired goals, including fine control of the pump to control the pressure and flow rate of the hydraulic control unit. Several techniques can be implemented, including prefill events, where the rotations of the shaft 310 are counted to know when prefill of the plenum 152 is complete. Pressure increases and pressure decreases in the downstream device can be derived from the angle of shaft 310. The rate for the pressure increases and pressure decreases can also be measured.

[032] Precise control of the motor shaft torque, speed, and angle provides hydraulic pressure control of the downstream device 150. Further, embodiments can include a measurement of shaft speed or acceleration and subsequent estimation of angle feedback. Precise control of shaft angle of shaft 310 lends itself to a desired motion profile that is desirable for controlling pressure in a dead headed hydraulic system. Shaft angle control also enables feedforward control of flow and a resulting improvement in pressure control. So, using shaft angle control enables the hydraulic control system to add or subtract a precise volume of fluid from the output of the pump. By including the compliance of the system in the control algorithm, the system can then know ahead of time how much fluid needs to be added to the circuit to achieve a desired pressure.

[033] In a first aspect, shown in Figure 1 , a trajectory generation subsystem creates a set of commands based on an error between a clutch pressure command and a pressure measured by a pressure sensor 333. The clutch pressure command can issue from an on-board computer, such as an electronic control unit ("ECU"). In the case of a downstream device being incorporated in to a vehicle, the ECU can be a main ECU 2000 or the downstream device electronic control unit such as an eLSD ECU 2100. For purposed of discussion, a clutch torque target can be generated and commanded by a vehicle system ECU 4001. For example, a main ECU can determine that a vehicle is turning and that differentiated wheel speeds are desired. Since a differential comprises gears that permit wheels on an axle of a vehicle to rotate at different rotations per minute, and since a limited slip differential can permit some desired additional slip in the torque transfer through the differential, the main ECU can issue a command to permit a limited amount of slip at the clutch pack 154 of the differential between two or more wheels on an axle of the vehicle. An amount of torque should pass through a clutch pack in the differential, and so the clutch should be closed or opened as necessary to reach that torque target passing through the differential. So, either the main ECU or an ECU dedicated to controlling the eLSD, such as an ELSD ECU 2100 processes the clutch torque target to determine a clutch pressure command to control the pressure to the clutch pack 154. The clutch pressure command can be fed to a motor control loop 4002 to ultimately provide a motor current to motor 300.

[034] Motor control loop 4002 includes a trajectory generation unit 4003 to create a set of commands based on the error between the clutch pressure command and the pressure measured by the pressure sensor 333. The commands comprise a desired motion trajectory for the pump shaft 310. The trajectory generation unit 4002 uses knowledge about the system physics to define an ideal motion profile for the pump 100. The motor torque controller 4004 receives speed, angle, and acceleration target data from the trajectory generation unit and is then responsible for providing the appropriate motor torque commands to achieve the motion profile. Both the trajectory generation unit 4003 and the motor torque controller 4004 receive feedback data from a pump and motor physical model unit 4005. The data regarding the state of the pump 100 and motor 300 can comprise the feedback data. The pump and motor physical model unit 4005 can receive sensor data from each of the angle measurement sensor 501 , a temperature sensor 331 connected to path 201 , and a pressure sensor 333 connected to path 201.

[035] The motor control loop 4002 is described in more detail with respect to Figures 2A & 2B to disclose a servo system and servo control techniques for hydraulic pump operation and operation of a limited slip differential. By using a higher resolution shaft angle feedback than is typically used in eLSD hyrdraulic control units, a physical model of the hydraulic system is more accurately updated leading to better observations of the pressure versus volume relationship and then more accurate feedforward control.

[036] Figures 2A & 2B explain the motor control loop 4002 in more detail. As pressure increases, so does the volume. So, a pressure to angle map can manifest in an angle feedforward lookup unit 2200. Either data relating the pressure to the shaft angle can comprise the LUT (lookup table) of the lookup unit 2200 (Fig. 2A), or data relating pressure versus volume can comprise the LUT (Fig. 2B). In any case, knowing the angle of shaft 310 and plenum pressure via respective sensors 501 and 333, one can determine feedforward data to transmit to the angle controller 507. This map could be determined during an end-of-line calibration procedure, and/or adapted in real time as the system runs.

[037] Alternatively, by knowing plenum pressure via sensor 333 and by deriving fluid volume in the plenum 152, as by counting rotations of the pump 100 and accounting for backwards leaking through the pump, one can determine feedforward data to transmit to the angle controller 507. For the alternative map of volume vs. pressure, as the pressure in the system increases there is some additional oil volume pumped due to compliance in the system. As pressure increases, an additive compliance causes a volume change in the system. If the volume versus pressure dependence is known, then the number of pump rotations needed to get from one pressure to another pressure can be calculated, assuming the pump provides a known fluid volume per revolution. Knowledge of the number of pump rotations required to achieve the target pressure can be used as a feedforward in the pressure control system. That is, instead of using only the measured pressure (from the pressure sensor) as a feedback to know when the target pressure is achieved, servo techniques can be used as well. The

complementary servo loops are described below and can comprise a loop for each of motor current, motor shaft speed, shaft angle, and plenum pressure. Servo loops can be used to control angular position of a shaft, in addition to speed, and acceleration, but the combination and synergy disclosed herein is novel in the field of limited slip differentials. Using the disclosed servo loops, a specific angular differential (change in position of shaft 310 and thus pump 100) can be

commanded, which (by using the volume vs. pressure map) should be the number of rotations needed to achieve the target pressure.

[038] As the hydraulic control system of Figures 2A & 2B operates over the commanded pressure ranges, it is possible to save a map of pressure versus total rotation angle of the shaft 310. But, because at least an internal leakage exists in the pump 100, the collected map data can be adjusted to remove an offset due to the internal leak rate. This map can be stored in a leak rate subtraction unit 505 and could be used to know the number of rotations needed to achieve a pressure, providing a degree of phase lead over the pressure sensor 333, as drawn in Figure 2A. In Figure 2B, the leak rate subtraction unit 505 is replaced by a leak rate compensation unit 901 which instead provides an estimated volume as part of a volume servo loop. When the pump displaces a fixed volume of fluid, the shaft angle can be mathematically related to the volume in the shaft angle servo loop of Figure 2A. However, Figure 2B is an alternative for processing using terms of volume variables in lieu of terms of shaft angle variables.

[039] So, the motor angle servo-loop follows paths 510, 520, & 530 and merges with other loops to reach the power electronics unit 800. The angle measurement sensor 501 sends a data signal comprising angle feedback along path 510 to leak rate subtraction unit 505. The leak rate subtraction unit 505 provides data indicating what angle change in the rotor 330 is attributable to the leak rate of the pump 100. The leak rate subtraction unit 505 is responsible for removing, from the measured angle signal, the rotation offset that is due to the leak rate of the pump. The speed estimating unit 401 , could be used to assist in this process. Leak rate subtraction unit 505 can subtract out angle changes in the shaft angle due to the leak rate of the pump 100 to get the change in shaft angle that remains with the leak subtracted out. Angle controller 507 receives this data along path 520 and processes it along with angle feedforward data output to path 540 from a pressure to angle map 2200. Wth the leak accounted for, and the angle feedforward designated, a speed command can be issued along path 530, and the speed command can reflect changes in the shaft rotation due to the volume change in the system (as by compliance).

[040] While a temperature sensor 331 is shown, it can be limited in its usage. For example, temperature data can be transmitted on path 3310 to the leak rate subtraction unit 505. Since the temperature of the hydraulic fluid can impact the leak rate through the pump, the temperature data can be processed in the leak rate subtraction unit 505. In some instances, it can be possible to measure the leak rate of the pump over a temperature range by using accurate shaft feedback. Otherwise, the system can work without temperature dependency in the servo loops. This is a departure from the prior art, which takes fluid temperature in to greater

consideration. Because of the field oriented control (FOC), the system can be designed without temperature dependency. Thus, sensored FOC provides temperature independence as the pump speed is determined by the leak rate. So, by including temperature sensor 331 , it can be used to improve the leak rate estimate. For example, it can be used to build up a map of leak rate versus temperature. If there is no temperature sensor, then the system can still perform well, since it is operating the motor in torque control mode, and torque is directly proportional to pump pressure. Prior art systems that are based purely on speed control would be more affected by temperature changes because the leak rate increases with temperature, and this will change the mapping between speed and pressure.

[041] Since the pump speed depends on the leak rate, and since the pump speed is proportional to the voltage to the motor, the shaft angle data processed in the shaft angle servo loop results in the speed command due to the volume change, and that speed command is then fed to the next servo loop, which is a speed servo loop comprised of paths 410, 420, 430.

[042] A leak rate map can be stored in a leak rate map unit 404 to provide another feedforward enhancement to the hydraulic control strategy. It is possible to develop a map 404 of rotation speed of the shaft 310 versus pressure in the plenum 152 by observing the pump speed while holding steady state pressures. This map will have a temperature dependence, but should be relatively consistent from run to run (run means warmup cycle). This map could be determined during an end-of-line calibration procedure, and/or adapted in real time as the system runs. A pressure versus speed dependency can be looked up in a LUT. With pressure sensor data from pressure sensor 333 and temperature sensor data from temperature sensor 331 as inputs, leak rate map unit 404 can output an estimated speed that the pump should be running at for the current conditions. The estimate can be fed via path 440 to speed controller 403 for further processing to derive a speed command. The feedforward estimate can be used to compensate for the leak in the pump 100 in order to find the amount of pump adjustment needed because of the leak rate.

[043] Speed controller 403 functions like a sum block, adding in the speed command due to the volume change from the shaft angle servo loop, adding in the estimated leak speed for the current conditions, and subtracting out the speed estimate from speed estimation unit 401. Speed estimation unit 401 considers the shaft angle feedback from angle measurement device 501 and can comprise a counter for counting the number of rotations of the shaft 310 to estimate the speed of the shaft 310. Speed controller 403 ultimately outputs a speed and shaft angle- based torque command to path 430, which is fed to the pressure servo loop. In the case of Figure 2B, speed controller outputs a volume-based torque command to path 430.

[044] Since torque is proportional to pressure, and the hydraulic control unit controls torque to control pressure at the plenum 152, the pressure servo loop utilizes the torque command issued from the speed controller 403 and the torque command issued from the pressure sum block 703.

[045] Pressure servo loop comprises paths 710, 720, 730, 740. Path 710 can be eliminated if temperature sensor 331 and pressure sensor 333 are direct- coupled to pump output path 201. At least pressure data is forwarded from pressure sensor along path 720 to pressure controller 701. Pressure controller 701 acts as a sum block to process the pressure commands received from the pressure sensor 333 and the branched path 2300 connected to the eLSD ECU 2100, yielding a difference, or error, between the two pressure commands. The difference is further processed at sum block 703, where it is amalgamated with an output from a pressure-to-torque feedforward map unit 705 to yield a pressure-based torque command transmitted along path 750.

[046] The pressure command from eLSD ECU 2100 is also transmitted along path 2300 and is used as an input to the pressure-to-torque feedforward map unit 705 to select an appropriate feedforward torque command for the motor 300 based on the pressure command. Feedforward map 705 is used to predict the torque required to achieve a desired pressure using a specified pump displacement. The feedforward torque command is adjusted in sum block 703 to more accurately reflect the torque adjustment needed for the present conditions. For example, based on the signal from the pressure sensor 333, it can be necessary to increase or decrease the torque commanded over that sent from the eLSD ECU 2100 due to volume changes related to system compliance, or due to downstream effects of the other servo loops. By running the pressure servo loop with at least one feedforward operation, it is possible to achieve a target pressure faster than by relying on a pressure loop alone.

[047] The pressure-based torque command from sum block 703 is forwarded along path 740 to Field Oriented Control ("FOC") & Current Controller 704, which processes also the shaft angle based torque command or the volume based torque command along with the speed-based torque command from path 430 from speed controller 403. The motor 300 operation can be processed in a current servo loop and taken in to consideration with the pressure servo loop information, the speed servo loop information, and the shaft angle servo loop information. Since each of the servo loops impacts the inputs to the FOC & current controller 704, this controller amalgamates the influence of all of the hydraulic control unit

feedforwards, feedbacks and commands to generate the ultimate torque command for the motor. This ultimate torque command for the motor manifests as duty cycle commands fed by path 760 to the power electronics unit 800.

[048] Power electronics unit 800 is arranged with respect to the motor 300 as schematically shown in Figure 3B. Power electronics unit permits FOC via coordinate transformations between a 3-phase system and a 2-coordinate system based on an angular measurement of the motor shaft 310. Power electronics unit can comprise, for example, 6 power devices, which can be FETs (field effect transistors) or 6 IGBTs (insulated gate bipolar transistors) or the like. Figure 3B shows that there are 6 power devices connected to three windings of a Y-connected motor. The power devices, PD1 , PD3, PD5 are the "high side" switches. Power devices PD2, PD4, PD5 are the "low side" switches. The control of the 6 power devices can be completed as known in the art. At least the control comprises the power electronics unit 800 commanding the power devices to alternately switch power to the coils of the stator coils Φ1 , Φ2, Φ3 (Phil , Phi2, Phi3) to precisely turn the rotor 330.

[049] The current servo loop comprises at least path 610. At least two current sensors 601 , 602 are attached respectively to two of the three motor phase wires to detect the current thereon. Since all of the currents must sum to zero, the current of the third motor phase wire can be derived from the measured and known currents. Other methods comprising a single current sensor are known, and other methods comprising voltage sensing are known, and these other methods can be substituted for the current feedback loop shown in the Figures. The current feedback is fed to FOC & current controller 704 to adjust the outputted duty cycle in view of the other torque commands inputted to the FOC & current controller 704. The current servo loop manages the motor torque using knowledge of the motor torque constant and the electrical parameters of the motor. By controlling motor current, motor torque can be known with reasonable accuracy. [050] If the desired pressure has not been achieved, the combination of servo loops enables a technique to determine how much torque, and therefor current, should be added or subtracted to the motor 300 to reach the desired pressure. The direction of the rotor 330, forward or reverse, can also be derived, to increase or decrease the pressure to plenum 152 to reach the desired pressure. With the disclosed shaft angle servo loop, fine control of the rotor 330 can be achieved, with minimal torque ripple, because the appropriate coils Φ1 , Φ2, Φ3 (Phil , Phi2, Phi3) to actuate can be determined without moving the rotor 330 to align the sensed mechanism 503 with the angle measurement sensor 501. Knowing the shaft angle at zero speed eliminates a pressure ripple caused by rotating the rotor until its position is known. By using the disclosed torque control, the bidirectional motor can also be commanded to hold a zero speed or "stalled" condition. This is not possible in hydraulic systems that focus solely on the hydraulic fluid flow rate as it relates to the speed of the pump; these systems go faster or slower but do not comprise zero speed.

[051] Figure 2A describes the cooperation of four servo loops, using shaft angle control and shaft angle terms for the shaft angle servo loop. Figure 2B still relies on shaft angle data, but uses a volume servo loop. The volume servo loop relies on the collected data to process feedforwards and feedbacks in terms of volume. Volume servo loop comprises paths 930, 940, 950 & 510. Aspects that are identical to Figure 2A are not repeated hereinbelow.

[052] In lieu of the pressure to angle map unit 2200, a pressure versus volume map unit 9200 is used. As pressure increases, so does the volume, so volume changes due to compliance in the system can be accounted for in a LUT of the pressure versus volume map unit 9200. A pressure to volume map can manifest as a feedforward lookup unit. Data relating pressure versus volume can comprise the LUT. Based on the received pressure command from the respective ECU 2000 or 2100, a volume target can be a feedforward term, the volume target being a fixed property of the working system. This map could be determined during an end-of-line calibration procedure, and/or adapted in real time as the system runs.

[053] The volume target can be forwarded along path 950 to an angle controller unit 903. The volume target can be converted to a pump angle, the pump angle being a function of the shaft angle. As above, leak rate compensation unit 901 provides an estimated volume as part of a volume servo loop. Leak rate

compensation unit 901 receives temperature data along path 3310 from

temperature sensor 331 , and receives angle feedback data along path 510 from the angle measurement device 501. The volume servo loop comprises an additional data transfer along path 940 from the speed servo loop, as the estimated speed from the speed estimator 401 is also fed to the leak rate compensation unit 901. Speed estimator 401 processes angle feedback data related to shaft angle along with time data to estimate the speed of the motor, which can be translated in to the speed of the pump. Pump displacement parameters known, the volume output for the pump can be derived. So too, the flow rate can be derived, knowing the volume and speed of the pump 100. Leak rate compensation unit 901 yields the estimated volume in the system with knowledge of the leak rate of the pump 100. Angle controller receives the volume target from the pressure versus volume map unit 9200 along path 950 and the estimated volume from the leak rate compensation unit 901 along path 920 and processes the received data. Because the pump displacement is known, the volume is known, and the angle controller 903 can apply a volume error term to result in an outputted speed command due to the volume change in the system. This output can be forwarded along path 930 to the speed controller 403 for further processing consistent with that outlined above.

[054] Figure 4 shows a method for controlling pressure and flow of hydraulic fluid through a hydraulic control unit. Step 41 comprises receiving a pressure command from an electronic control unit. This can comprise the main ECU 2000 or a secondary manifestation of that command issued through eLSD ECU2100 or like secondary ECU. In step 42, the received pressure command is processed to generate a pressure-based torque command. In step 43, processing comprises generating one of a shaft angle-based torque command or a volume-based torque command. This can comprise the operation of the pressure servo loop and one of the shaft angle servo loop or volume servo loop. The speed servo loop can provide additional processing, as can the current servo loop. The generated pressure-based torque command and the generated one of the shaft angle-based torque command or the volume-based torque command are processed as by FOC & current controller 704 to further generate a duty cycle for a field oriented controlled motor 300, as by step 44. In step 45, motor phase wires of the field oriented controlled motor are modulated according to the generated duty cycle to operate the bidirectional positive displacement pump 100 connected to the field oriented controlled motor 300. The flow rate and pressure of fluid supplied by the

bidirectional positive displacement pump are adjusted by updating the generated duty cycle.

[055] Consistent with the disclosure, the physical layout and method can be such that the hydraulic control unit is coupled to a dead headed hydraulic system, and the method does not comprise actuating a pressure control valve between the hydraulic control unit and the dead headed hydraulic system.

[056] Figure 5 elucidates additional steps that can be applied as part of a control algorithm. After receiving a pressure command from a main ECU 2000 or the like in step 51 , control can further comprise selecting a first pressure-based feedforward in the form of an actual pressure-based torque command estimate in step 52. Then, with processing in a sum block the selected pressure-based feedforward with measured pressure data and the received pressure command from the electronic control unit, in step 54, the hydraulic control unit can generate the actual pressure-based torque command.

[057] In step 53, the hydraulic control unit control algorithm can further comprise selecting a second pressure-based feedforward in the form of a volume- based target or a shaft angle-based target. In step 57, the method generates a motor speed command by processing one of the volume-based target or the shaft angle-based target. Intervening step 55 can comprise measuring a shaft angle associated with a rotor 330 of the motor 300 to know the location of the rotor 330 with respect to its stator 320. It is possible to further implement a counter to count the rotations of the rotor with respect to its stator. Thereby, the control unit can derive a volume of fluid pumped by the pump based on the counted rotations of the rotor and based on a known displacement of the pump.

[058] In step 59, the algorithm can select a third pressure- based

feedforward in the form of an estimated pump speed. In step 61 , generating a motor speed-based toque command can be achieved by processing the third pressure- based feedforward and the generated motor speed command.

[059] In step 63, processing the generated motor speed-based torque command and the generated actual pressure-based torque command can result in generating the duty cycle for the field oriented controlled motor. Then, in step 65, it is possible to adjust the flow rate and pressure of fluid supplied by the bidirectional pump.

[060] The method of claim 1 , wherein receiving the pressure command from the electronic control unit comprises receiving a selection of one of a forward condition, reverse condition or stall condition for the bidirectional positive

displacement pump.

[061] Step 54 can comprise processing the received pressure command to generate a pressure-based torque command by additionally applying a pressure servo loop. Step 57 can comprise generating one of the shaft angle-based torque command or the volume-based torque command by additionally applying one of a shaft angle servo loop or a volume servo loop. Additional servo loops can be applied, as described above, such that processing the generated pressure-based torque command and processing the generated one of the shaft angle-based torque command or the volume-based torque command to further generate a duty cycle for a field oriented controlled motor can comprise applying a speed servo loop and applying a current servo loop.

[062] There are times when the pump 100 is not moving, and the motor is at zero speed (stalled), but the motor is powered to hold the pump position. So, control algorithms can further comprise modulating the motor phase wires of the field oriented controlled motor according to the generated duty cycle to operate the bidirectional positive displacement pump connected to the field oriented controlled motor in a zero speed condition. To keep the pump from moving, a torque can be applied to the motor. So, adjusting the flow rate and pressure of fluid supplied by the bidirectional positive displacement pump can be achieved by updating the torque command applied to the generated duty cycle.

[063] Figure 6 outlines the use of the 4 disclosed servo loops. Some aspects of the servo loop are processed serially, while other aspects can be processed in parallel. In step 71 , the control algorithm can respond to the selection of a forward condition, a reverse condition, or a stall condition for the FOC motor. A pressure and flow rate for the pump can also be selected. Depending on algorithm design and circuit 1000 design, either a shaft angle servo loop or a volume servo loop is selected and applied in step 73. A speed servo loop can be applied in step 75, then the pressure servo loop in step 77, and also the current servo loop in step 79. The servo loops can be applied to derive the torque and speed commands in step 81. Coordinate transformations can be applied to translate the derived torque and speed commands from the 2-coordinate system to the 3-phase system. Such transformations are enabled by including the angular measurements as disclosed. In step 83, the motor phase wires are modulated using the power electronics 800 such as those shown in Figure 3B. The duty cycle is applied to the FOC motor 300 and the pump 100 is consequently driven to adjust the flow rate and pressure of fluid supplied by the bidirectional pump.

[064] Other implementations will be apparent to those skilled in the art from consideration of the specification and practice of the examples disclosed herein.