Wake-up circuit in an electric steering system

In one aspect, a wake-up circuit is provided for triggering a determination of an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle. The wake-up circuit includes a plurality of inputs coupled to separate phases of the motor. A voltage boosting circuit is operable to increase a back electromotive force (EMF) voltage induced on one or more of the inputs by rotation of the rotatable shaft of the motor and produce at least one voltage-boosted back EMF voltage. A comparator circuit is operable to compare the at least one voltage-boosted back EMF voltage to a reference voltage and trigger a monitoring system to perform the determination of the absolute rotational position of the rotatable shaft of the motor in the electric power steering system based on a result of the compare during an ignition off state of the vehicle.

FIELD OF THE INVENTION

The subject application relates to a wake-up circuit for triggering a determination of an absolute rotational position of a motor shaft in an electric steering system when a vehicle has an ignition off state.

BACKGROUND

A vehicle electric power steering system can utilize a monitoring system that monitors a motor shaft position in the electric power steering system. The monitoring system, however, is turned off when the ignition of the vehicle is off (i.e., an ignition off state of the vehicle). Accordingly, if a vehicle operator turns the steering wheel (i.e., handwheel) during the ignition off state, when the monitoring system is subsequently turned on during an ignition on state, the monitoring system may not be able to accurately determine an absolute rotational position of the motor shaft.

Accordingly, it is desirable to provide a system and a method for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system during an ignition off state of the vehicle. While a system that determines absolute rotational position in an electric power steering system during an ignition off state of the vehicle can be effective by periodically checking the rotatable shaft position of the motor, abrupt steering wheel motion can be difficult to detect when monitoring is performed on a fixed periodic basis.

SUMMARY OF THE INVENTION

According to an embodiment, a wake-up circuit is provided for triggering a determination of an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle. The wake-up circuit includes a plurality of inputs coupled to separate phases of the motor. A voltage boosting circuit of the wake-up circuit is operable to increase a back electromotive force (EMF) voltage induced on one or more of the inputs by rotation of the rotatable shaft of the motor and produce at least one voltage-boosted back EMF voltage. A comparator circuit of the wake-up circuit is operable to compare the at least one voltage-boosted back EMF voltage to a reference voltage and trigger a monitoring system to perform the determination of the absolute rotational position of the rotatable shaft of the motor in the electric power steering system of the vehicle based on a result of the compare during an ignition off state of the vehicle.

According to another embodiment, a system is provided for determining an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle. The system includes a wake-up circuit with a voltage boosting circuit and a comparator circuit. The voltage boosting circuit is operable to increase a back EMF voltage induced on one or more inputs by rotation of the rotatable shaft of the motor and produce at least one voltage-boosted back EMF voltage. The comparator circuit is operable to compare the at least one voltage-boosted back EMF voltage to a reference voltage and output a control signal based on a result of the compare during an ignition off state of the vehicle. The system also includes a monitoring system having a microprocessor and first and second position sensors operable to generate signals indicative of a relative rotational position of the rotatable shaft. The microprocessor is configured to activate in response to the control signal and determine a current absolute position value indicating a current absolute rotational position of the rotatable shaft based on a previously stored absolute position value and an amount of relative rotation of the rotatable shaft based on the signals from the first and second position sensors.

According to a further embodiment, a method is provided for triggering a determination of an absolute rotational position of a rotatable shaft of a motor in an electric power steering system of a vehicle. The method includes increasing a back EMF voltage induced on one or more inputs from separate phases of the motor by rotation of the rotatable shaft to produce at least one voltage-boosted back EMF voltage. The at least one voltage-boosted back EMF voltage is compared to a reference voltage. A monitoring system is triggered to perform the determination of the absolute rotational position of the rotatable shaft of the motor in the electric power steering system of the vehicle based on a result of the comparison during an ignition off state of the vehicle.

DESCRIPTION OF THE EMBODIMENTS

Referring now toFIGS. 1 and 2, a vehicle10having a handwheel20, an electric power steering system24, and a position determination system30in accordance with an exemplary embodiment is illustrated. The term “ignition off state” used herein corresponds to a power off state of an electric power steering system, and the term “ignition on state” corresponds to a power on state of an electric power steering system.

The handwheel20is operably coupled to the electric power steering system24. Rotation of the handwheel20induces the electric power steering system24to cause rotation of a rotatable motor shaft42operably coupled to a rack-and-pinion assembly to move an operational position of vehicle wheels.

The electric power steering system24includes an electric motor40having the rotatable shaft42and a magnet44coupled to the rotatable shaft42. In one embodiment, the rotatable shaft42is operably coupled via a gear assembly to a rack-and-pinion assembly for controlling an operational position of vehicle wheels.

The position determination system30is provided to determine an absolute rotational position of the rotatable shaft42of the motor40when the vehicle10has an ignition off state. The system30includes first and second position sensors60,62, a microprocessor66, a timer circuit70, a memory device74, a power source80, a voltage regulator82, a switch84, a wake-up circuit90, and a main controller92. The combination of the first and second position sensors60,62and the microprocessor66may also be referred to as a monitoring system68. The monitoring system68can also include other elements of the position determination system30.

Referring toFIGS. 2, 4 and 5, the first and second position sensors60,62are configured to generate first and second signals indicative of a relative position of the rotatable shaft42of the motor40. In one exemplary embodiment, the first and second position sensors60,62are Hall effect sensors that generate the first and second signals, respectively, in response to detecting a magnetic field from the magnet44coupled to the rotatable shaft42. In one exemplary embodiment, the first and second position sensors60,62are disposed 90 degrees apart from one another about a central axis99of the rotatable shaft42. As shown inFIG. 4, a graph120illustrates that the first position sensor60can generate a first signal over time represented by signal curve122as the rotatable shaft42and the magnet44are rotated. Also, the second position sensor62can generate a second signal over time represented by signal curve124as the rotatable shaft42and the magnet44are rotated.

The timer circuit70is operably coupled to the microprocessor66. The timer circuit70is configured to periodically generate a control signal that activates the microprocessor66when the vehicle10has an ignition off state. In one exemplary embodiment, the timer circuit70generates the control signal every 256 milliseconds to activate the microprocessor66. Of course, other time intervals are contemplated.

The memory device74is operably coupled to the microprocessor66. The microprocessor66is configured to store data values in the memory device74as will be explained in greater detail below. The memory device74can also hold instructions for execution by the microprocessor. The microprocessor66can be any type of processing circuitry operable to execute instructions, such as a microcontroller, a digital signal processor, a general purpose processor, a gate array, an application-specific integrated circuit, and the like.

The power source80is configured to output a voltage which is regulated utilizing the voltage regulator82. The voltage regulator82outputs an operational voltage that is received by the microprocessor66for powering the microprocessor66.

The switch84is coupled between the voltage regulator82and the position sensors60,62. When the switch84has a closed operational position, an operational voltage from the voltage regulator82is supplied to the first and second position sensors60,62to energize the position sensors60,62. Alternately, when the switch84has an open operational position, an operational voltage from the voltage regulator82is removed from the first and second position sensors60,62to de-energize the position sensors60,62. In one exemplary embodiment, the switch84is a p-channel MOSFET that is switched to either the closed operational position or the open operational position by control signals from the microprocessor66.

The wake-up circuit90is configured to compare the first, second, and third back electromotive force voltages from first, second, and third phases, respectively, of the motor40to a reference voltage. The wake-up circuit90outputs an interrupt/control signal that is received by the microprocessor66of the monitoring system68when either the first back electromotive force voltage is greater than the reference voltage, or the second back electromotive force voltage is greater than the reference voltage, or the third back electromotive force voltage is greater than the reference voltage, indicating that a rotational speed of the shaft42is greater than a threshold rotational speed. Of course, in an alternative embodiment, wake-up circuit90is configured to compare the first and second back electromotive force voltages from first and second phases, respectively, of the motor40to the reference voltage. When the microprocessor66receives the interrupt/control signal from the wake-up circuit90, the microprocessor66wakes up from a low power sleep mode to determine the absolute rotational position of the shaft42.

According to an embodiment, an example of the wake-up circuit90is depicted in greater detail inFIG. 9, which is described with continued reference toFIGS. 1 and 2. The wake-up circuit90can include a plurality of inputs302A,302B, and302C coupled to separate phases of the motor40, e.g., phase A, phase B, and phase C, offset by about 120 degrees. The wake-up circuit90also includes a voltage boosting circuit304operable to increase a back EMF voltage induced on one or more of the inputs302A-302C by rotation of the rotatable shaft42of the motor40and produces at least one voltage-boosted back EMF voltage.

The voltage boosting circuit304includes at least one capacitor in-line with each of the inputs302A-302C. In the example ofFIG. 9, capacitor C1is in-line with input302A, capacitor C2is in-line with input302B, and capacitor C3is in-line with input302C. The voltage boosting circuit304can charge the capacitors C1-C3on a first cycle polarity of each of the respective inputs302A-302C and effectively doubles the back EMF voltage on a second cycle polarity opposite the first cycle polarity to produce voltage-boosted back EMF voltage. For example, when input302A is negative, C1gets charged. When input302A is positive, C1is fully charged and the instantaneous forward line voltage is substantially doubled at phase output node308A to produce a voltage-boosted back EMF voltage. The speed of motor40, diode drops, resistances, as well as component tolerances, may result in an effective voltage doubling but not precise doubling. The positive/negative cycle relationship can be reversed if the input302A is inverted. Similarly, assuming a non-inverting sinusoidal signal, when input302B is negative, C2gets charged. When input302B is positive, C2is fully charged and the instantaneous forward line voltage is substantially doubled at phase output node308B to produce a voltage-boosted back EMF voltage. Likewise, assuming a non-inverting sinusoidal signal, when input302C is negative, C3gets charged. When input302C is positive, C3is fully charged and the instantaneous forward line voltage is substantially doubled at phase output node308C to produce a voltage-boosted back EMF voltage. The effective voltage doubling of the voltage boosting circuit304can increase sensitivity of the wake-up circuit90to movement of the rotatable shaft42such that rapid wake-up of the monitoring system68may be performed. Additionally, the effective voltage doubling of the voltage boosting circuit304can support a wider variety of motors as the motor40, such as motors having a lower back-EMF constant Ke.

Blocking diodes can be positioned between pairs of the inputs302A-302C to prevent the capacitors C1-C3in-line with each of the inputs302A-302C from discharging into the phases of the motor40. As depicted inFIG. 9, blocking diode Dl is coupled to an input side of capacitor C1and an output side of capacitor C3in relation to inputs302A and302C. Blocking diode D2is coupled to an input side of capacitor C2and an output side of capacitor C1in relation to inputs302B and302A. Blocking diode D3is coupled to an input side of capacitor C3and an output side of capacitor C2in relation to inputs302C and302B.

The wake-up circuit90can also include a full-wave rectifier circuit310, where at least one of the capacitors C1-C3on each of the inputs302A-302C of the voltage boosting circuit304is positioned between a diode pair of the full-wave rectifier circuit310for each of the inputs302A-302C. Examples of diode pairs include diodes D4and D7relative to input phase302A, diodes D5and D8relative to input phase302B, and diodes D6and D9relative to input phase302C. A first diode of a diode pair establishes a ground reference for one of the phases of the motor40, and a second diode of the diode pair establishes an instantaneous forward voltage at a phase output node308for a comparator circuit316. In the example ofFIG. 9, diode D4is connected between ground312and an input side of capacitor C1on input302A, while diode D7is connected between an output side of capacitor C1and phase output node308A. Diode D5is connected between ground312and an input side of capacitor C2on input302B, while diode D8is connected between an output side of capacitor C2and phase output node308B. Diode D6is connected between ground312and an input side of capacitor C3on input302C, while diode D9is connected between an output side of capacitor C3and phase output node308C. It will be understood that additional components can be included, such as current limiting resistors between diodes D4-D6and ground312.

The wake-up circuit90can also include a scaling and protection circuit306. In the example ofFIG. 9, the scaling and protection circuit306includes a voltage divider circuit314and a voltage clamp D10. Resistors R1, R2, R3, and R4form a voltage divider relative to the voltage at phase output nodes308A-308C. The scaling and protection circuit306may also include at least one current limiting resistor R5positioned between the voltage divider circuit314and the voltage clamp D10. The voltage clamp D10may be a Zener diode. The voltage clamp D10may be selected based on voltage requirements of the monitoring system68, such as an expected voltage level for an input to the microprocessor66. A comparator input node315is established between the current limiting resistor R5and the voltage clamp D10in the embodiment ofFIG. 9.

The wake-up circuit90can also include a comparator circuit316operable to compare the at least one voltage-boosted back EMF voltage to a reference voltage and trigger the monitoring system68to perform the determination of the absolute rotational position of the rotatable shaft42of the motor40based on a result of the compare during an ignition off state of the vehicle10. The comparator circuit316can include a comparator or equivalent circuit that compares voltage at comparator input node315to a reference voltage and outputs a control signal318to trigger the monitoring system68, for instance, based on the voltage at comparator input node315exceeding the reference voltage. The voltage at comparator node315can be a scaled version of the output of the voltage boosting circuit304and indicative of a voltage-boosted back EMF voltage. Additionally, the voltage at the comparator node315can be limited by voltage clamp D10. The reference voltage may be an absolute or relative voltage. In some embodiments, the reference voltage is configurable. While the comparator circuit316is depicted as located within a bounded box defining the wake-up circuit90, it will be understood that the comparator circuit316can be located at a distance from other elements of the wake-up circuit90. For example, the comparator circuit316can be located in whole or in part within the microprocessor66.

Referring toFIGS. 2 and 3, the microprocessor66is operably coupled to the voltage regulator82, the switch84, the first and second position sensors60,62, the timer circuit70, the memory device74, the wake-up circuit90, and the main controller92. The microprocessor66determines the absolute rotational position of the shaft42during the vehicle ignition off state by being periodically activated by a control signal from the timer circuit70or by being activated by an interrupt/control signal from the wake-up circuit90. In particular, in one exemplary embodiment, the microprocessor66is periodically activated by the timer circuit70to periodically monitor the first and second position signals from the first and second position sensors60,62during the ignition off state of the vehicle10. For example, the timer circuit70can wake up or activate the microprocessor66at times T1and T2representing a256millisecond time interval between activations. After, the microprocessor66is activated, the microprocessor66measures the first and second position signals from the position sensors60,62for 50-100 μsecond and then is de-activated. The microprocessor66has an activation duty cycle that is defined by a desired quiescent current draw of the microprocessor66and a desired maximum speed of the shaft42. Also, the microprocessor66can be activated at a time T3by an interrupt/control signal from the wake-up circuit90. The operation of the microprocessor66will be discussed in greater detail below.

Referring toFIGS. 2, 6 and 7, a flowchart of a method for determining an absolute rotational position of the rotatable shaft42of the motor40in the electric power steering system24in accordance with an exemplary embodiment will be explained.

At step200, the timer circuit70generates a control signal to activate the microprocessor66during an ignition off state of the vehicle10.

At step202, the microprocessor66generates a control signal that induces the switch84to supply an operational voltage to the first and second position sensors60,62to energize the first and second position sensors60,62at a first time when the microprocessor66is activated.

At step204, the first and second position sensors60,62generate first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft42at the first time.

At step206, the microprocessor66measures the first and second signals and determines a first relative position value indicating the relative rotational position of the rotatable shaft42at the first time and stores the first relative position value in the memory device74. In one exemplary embodiment, the first relative position value is determined utilizing the following equation: first relative position value=ArcTan (amplitude of signal curve124at the first time/amplitude of signal curve122at the first time) wherein signal curves122,124are shown inFIG. 4.

At step208, the microprocessor66deactivates itself after storing the first relative position value in the memory device74.

At step210, the timer circuit70generates a control signal to activate the microprocessor66during the ignition off state of the vehicle10.

At step212, the microprocessor66generates a control signal that induces the switch84to supply an operational voltage to the first and second position sensors60,62to energize the first and second position sensors60,62at a second time after the first time and after the microprocessor66was deactivated.

At step124, the first and second position sensors60,62generate third and fourth signals, respectively, indicative of a relative rotational position of the rotatable shaft42at the second time.

At step216, the microprocessor66measures the third and fourth signals and determines a second relative position value indicating the relative rotational position of the rotatable shaft42at the second time based on the third and fourth signals, and stores the second relative position value in the memory device74.

At step218, the microprocessor66determines an amount of relative rotation of the rotatable shaft42during the ignition off state based on the first and second relative position values, and stores the amount of relative rotation in the memory device74. In one exemplary embodiment, the amount of relative rotation is determined utilizing the following equation: amount of relative rotation=first relative position value−second relative position value.

At step220, the microprocessor66determines a current absolute position value indicating a current absolute rotational position of the rotatable shaft42based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft42, and stores the current absolute position value in the memory device74. In particular, the current absolute position value is calculated utilizing the following equation: current absolute position value=previously stored absolute position value+amount of relative rotation of the rotatable shaft42.

At step222, the microprocessor66determines a total number of turns of the rotatable shaft42of the motor40by dividing the current absolute rotational position of the rotatable shaft42by 360 degrees, and stores the total number of turns of the rotatable shaft42of the motor40in the memory device74. In particular, the total number of turns of the rotatable shaft42is calculated utilizing the following equation: total number of turns of the rotatable shaft42=current absolute rotational position of the rotatable shaft42/360 degrees.

At step224, the microprocessor66determines a total number of vehicle handwheel turns based on the total number of turns of the rotatable shaft42of the motor40and a gear ratio associated with the electric power steering system, and stores the total number of vehicle handwheel turns in the memory device74. In one exemplary embodiment, the total number of vehicle handwheel turns is determined utilizing the following equation: total number of vehicle handwheel turns =the total number of turns of the rotatable shaft42of the motor40/gear ratio associated with the electric power steering system.

Referring toFIGS. 2, 8, and 9, a flowchart of another method for determining an absolute rotational position of the rotatable shaft42of the motor40in the electric power steering system24in accordance with another exemplary embodiment will be explained.

At step250, the wake-up circuit90compares first and second back electromotive force voltages from first and second phases, respectively, of the motor40to a reference voltage, and outputs a control signal when either the first back electromotive force voltage is greater than the reference voltage or the second back electromotive force voltage is greater than the reference voltage.

The back EMF voltage induced on one or more inputs302A-302C from separate phases of the motor40by rotation of the rotatable shaft42is increased to produce at least one voltage-boosted back EMF voltage. Increasing the back EMF voltage can be performed by a voltage boosting circuit304that includes at least one capacitor C1-C3in-line with each of the inputs302A-302C. The capacitors C1-C3can be charged on a first cycle polarity of each of the inputs302A-302C. The back EMF voltage may be effectively doubled on a second cycle polarity opposite the first cycle polarity to produce the voltage-boosted back EMF voltage, and the voltage-boosted back EMF voltage can be further scaled and limited prior to comparison by the comparator circuit316. A blocking diode, such as blocking diodes D1-D3, can be used to prevent the capacitors C1-C3in-line with each of the inputs302A-302C from discharging into the phases of the motor40. The voltage-boosted back EMF voltage is compared to a reference voltage. The wake-up circuit90triggers a control signal to the monitoring system68based on the comparison.

At step252, the microprocessor66is activated in response to the control signal during an ignition off state of the vehicle10.

At step254, the microprocessor66generates a control signal that induces the switch84to supply an operational voltage to the first and second position sensors60,62to energize the first and second position sensors60,62at a first time when the microprocessor66is activated.

At step256, the first and second position sensors60,62generate first and second signals, respectively, indicative of a relative rotational position of the rotatable shaft42over time.

At step258, the microprocessor66measures the first and second signals and determines a first relative position value indicating a relative rotational position of the rotatable shaft42at the first time, and stores the first relative position value in the memory device74.

At step260, the microprocessor66measures the first and second signals and determines a second relative position value indicating a relative position of the rotatable shaft42at a second time and stores the second relative position value in the memory device74. The second time is after the first time.

At step262, the microprocessor66determines an amount of relative rotation of the rotatable shaft42during the ignition off state based on the first and second relative position values, and stores the amount of relative rotation in the memory device74.

At step264, the microprocessor66determines a current absolute position value indicating a current absolute rotational position of the rotatable shaft42based on a previously stored absolute position value and the amount of relative rotation of the rotatable shaft42, and stores the current absolute position value in the memory device74.

At step266, the microprocessor66determines a total number of turns of the rotatable shaft42of the motor40by dividing the current absolute rotational position of the rotatable shaft42by 360 degrees, and stores the total number of turns of the rotatable shaft42of the motor40in the memory device74.

At step268, the microprocessor66determines a total number of vehicle handwheel turns based on the total number of turns of the rotatable shaft42of the motor40and a gear ratio associated with the electric power steering system, and stores the total number of vehicle handwheel turns in the memory device74.

It should be noted that the microprocessor66can operate in a low power mode drawing less than 70 μA when there is no movement of the shaft42. The microprocessor66can also monitor rotational speeds of the shaft42up to11,000RPM and has a resolution of one-half of a mechanical revolution of the shaft42. In addition, the microprocessor66can determine +/−1080° of handwheel movement (e.g., three handwheel revolutions).