Electric motor apparatus

Abstract

An apparatus for controlling an electric motor having a controller, a torque demand limit generator, and a drive stage. The controller may be arranged to receive as an input a torque demand signal indicative of the amount of torque demanded from the motor and to produce as an output a set of motor current demand signals. The drive stage may receive the motor current demand signals and is arranged to cause currents to flow in each phase of the motor as required to meet the demanded torque. The torque demand limit generator may be arranged to output a torque demand limit signal indicative of a torque demand limit above which the battery current would exceed one or more limits.

Claims

1. An apparatus for controlling an electric motor, the electrical motor and the apparatus being supplied by a battery source, the apparatus comprising: a controller arranged to receive as an input a torque demand signal indicative of an amount of torque demanded from the motor and to produce as an output a set of motor current demand signals; and a drive stage which receives the set of motor current demand signals and is arranged to cause currents to flow in each phase of the motor as required to meet the amount of torque demanded; wherein the apparatus further comprises a torque demand limit generator which is arranged to output a torque demand limit signal indicative of a torque demand limit, wherein the torque demand signal has a value that is dependent on an amount of assistance of torque demanded from the motor and the torque demand limit signal, such that the value of the torque demand signal does not exceed a limit value.

2. The apparatus as defined in claim 1, further comprising a torque demand generator which generates the torque demand signal indicative of the amount of torque demanded from the motor.

3. The apparatus as defined in claim 2, wherein the torque demand generator generates an ideal torque demand signal independent of the torque demand limit signal, and subsequently modifies the torque demand signal if the torque demand signal would exceed the limit value to produce the torque demand signal that is fed to the controller.

4. The apparatus as defined in claim 2, wherein the torque demand limit generator sets the value of the torque demand limit signal using a model of the motor and the drive stage via a torque demand limiter.

5. The apparatus as defined in claim 3, wherein the torque demand limit generator sets the torque demand limit as a function of a voltage of the battery via a torque demand limiter.

6. The apparatus as defined in claim 4, wherein the torque demand limit generator uses one or more parameters when determining the torque limit: a motor battery current limit; a generator battery current limits; a motor electric power limit; a generator electric power limit.

7. The apparatus according to claim 3, wherein the torque demand limit generator generates one or more battery current limits, and the one or more battery current limits are fed into a model of the motor apparatus along with the battery voltage and used by the torque demand limiter to determine the torque demand limit.

8. The apparatus as defined in claim 7, wherein the torque demand generator limits a rate of change of demanded torque such that a rate of change of current drawn from a power supply during motoring or fed back into the power supply during generating is limited.

9. The apparatus as defined in claim 8, further comprising a current monitor to monitor actual current demand values from the controller or monitor motor actual currents or calculate estimates of the currents, and in response to one of the actual current demand values, the motor actual currents, or the calculated estimates of the currents exceeding the one or more battery current limits the torque demand limiter reduces the torque demand limit.

10. The apparatus as defined in claim 9, wherein the torque demand generator limits the rate of change of the torque demand.

11. A method of controlling an electric motor circuit comprising an electric motor and a control circuit, the motor and control circuit being supplied by a battery source, the control circuit including a torque demand generator which generates a torque demand signal dependent on an amount of torque demanded from the motor, and a controller arranged to receive as an input the torque demand signal and to produce as an output a set of motor current demand signals, and a drive stage which receives the set of motor current demand signals and is arranged to cause currents to flow in each phase of the motor to meet the amount of torque demanded, the method comprising generating a torque demand limit signal indicative of a torque demand limit and generating a torque demand signal that has a value that is dependent on an amount of assistance torque demanded from the motor and the torque demand limit signal, such that the value of the torque demand signal does not exceed a limit value.

12. The method as defined in claim 11, further comprising the step of generating the torque demand limit signal using a model of the motor including one or more motor parameters.

13. The method as defined in claim 12, further comprising the step of measuring a current flowing in the motor and in response to the current exceeding a current limit modifying the torque demand limit signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) There will now be described by way of example only one embodiment of the present disclosure with reference to the accompanying drawings of which:

(2) FIG. 1 is an overview of a part of a vehicle electrical system showing the connection of an electric power steering system the electrical supply;

(3) FIG. 2 is a schematic representation of the key parts of an exemplary electric power steering system to which the control strategy of the present disclosure can be applied;

(4) FIG. 3 shows a dual lane motor and motor bridge that may be used within the embodiment of an apparatus of the disclosure;

(5) FIG. 4(a) shows a prior art electric motor apparatus;

(6) FIG. 4(b) shows an embodiment of the present disclosure at the highest system level, showing how torque demand is limited to in turn limit the current drawn from or fed back into the battery;

(7) FIG. 5 is a more detailed representation of one particular exemplary implementation of the strategy illustrated in FIG. 4(b);

(8) FIG. 6 shows the inputs and outputs of the torque demand limiter stage to initially calculate a target set of current and power limits;

(9) FIG. 7 shows the inputs and outputs of the torque demand limiter to set the torque demand limit using the current and power limits from FIG. 6 used when the motor is motoring;

(10) FIG. 8 shows the inputs and outputs of the torque demand limiter to set the torque demand limit using the current and power limits from FIG. 6 used when the motor is generating.

(11) FIG. 9 is a functional block within the torque demand limiter that enables a customer to overrule the current limits;

(12) FIG. 10 shows the interface between the current controller and the motor current controller for use in correcting errors in the model of the motor;

(13) FIG. 11 is an overview of a circuit used to limit the peak of the battery current to a predefined allowable peak current for motoring where FIG. 11 (a) explains the principle, and FIG. 11(b) shows the algorithm implementation:
P.sub.pred=1.5(v.sub.d_DEmi.sub.d+v.sub.q_DEmi.sub.q)
P.sub.max_MOT=v.sub.DRV_STGi.sub.MAX_MOT_ALW
P.sub.max_GEN=v.sub.DRV_STGi.sub.MAX_GEN_ALW

(14) FIG. 12 illustrates an appropriate scaling of the torque demand limit as a function of excess charge accumulated in the battery over time due to the current limits being exceeded;

(15) FIG. 13 is an overview of the function of the torque demand limiter when operating to limit the torque demand gradient;

(16) FIG. 14 shows a typical map of battery voltage against battery current limit which can be used to populate an appropriate lookup table for use by the block of FIG. 6;

(17) FIG. 15 is a sample plot of battery current over time showing the ideal current limit and overshoot due to inaccuracies in the model used in FIGS. 7 and 8 leading to charge accumulation;

(18) FIG. 16 is a plot of torque demand limit covering all four quadrants of the motor operation;

(19) FIG. 17 illustrates the current gradient showing both an unacceptable (NOK) high gradient and an acceptable (OK) lower gradient, and also an intermediate gradient which is OK due to the short duration;

(20) FIG. 18 is a more refined plot corresponding to FIG. 17 showing how the traces are not smooth and lead to short spikes of increased gradient; and

(21) FIG. 19 is a block diagram showing the function of and optional charge limiting part of the motor control circuit; and

(22) FIG. 20 shows an alternative arrangement of an apparatus.

DETAILED DESCRIPTION

(23) The following example describes an embodiment of the present disclosure used in an automotive application although the reader will understand that the present disclosure should not be limited in scope to such an application.

(24) As shown in FIG. 1 a vehicle is provided with an electric power assisted steering (EPAS) system that draws current i.sub.battery from the vehicle electrical supply across power rail 2. The supply comprises a battery 3, typically rated at 12 volts DC, which is in turn topped up by an alternator 4. The battery also provides current to other vehicle accessories 5.

(25) The EPAS system 1 is shown schematically in FIG. 2 of the drawings. It includes a steering column 10 attached to a steering wheel 11, a torque sensor 12 which measures the torque applied to the steering column 10 by the driver as they turn the steering wheel, a motor control and drive circuit 13 and an electric motor 14. The torque sensor 12 may be attached to a quill shaft in series with the column 10, and the motor 14 may act upon the steering column or other part of the steering system, typically through a gearbox 15.

(26) The motor 14 typically comprises a three phase wound stator element and a rotor having for example six embedded magnets within it which in this instance are arranged so as to provide six poles which alternate between north and south around the rotor. The rotor therefore defines three direct or d axes evenly spaced around the rotor and three quadrature or q axes interspaced between the d axes. The d axes are aligned with the magnetic poles of the magnets where the lines of magnetic flux from the rotor are in the radial direction, and the q axes are spaced between the d axes where the lines of magnetic flux from the rotor are in the tangential direction.

(27) The three motor stator windings are connected in a star network. The motor is controlled by an apparatus 13 in accordance with an aspect of the present disclosure that includes a controller and drive stage. The drive stage 27 of the apparatus 13 comprises a three phase bridge forming a switching stage. This is shown in FIG. 3. Each arm of the bridge comprises a pair of switches in the form of a top transistor T.sub.1, T.sub.3, T.sub.5 and a bottom transistor T.sub.2, T.sub.4, T.sub.6 connected in series between the battery supply rail 2 and ground line. The motor windings are each tapped off from between a respective complementary pair of transistors. The transistors are turned on and off in a controlled manner by a control and drive circuit to provide pulse width modulation (PWM) of the potential applied to each of the terminals, thereby to control the potential difference applied across each of the windings and hence also the currents a, b, c flowing through the windings which depends on the duty cycled for each phase 17,18, 19 of the motor. This in turn controls the strength and orientation of the magnetic field produced by the windings, and in turn the motor torque. In fact as shown in FIG. 3 the motor has dual lanes, so there is a duplicate of the three phases and the three phase bridge. The two lanes can be run in parallel, each providing one half of the motor torque, or used one at a time.

(28) The torque signal output from the torque sensor 12 is fed to an input of the apparatus 13. This is input to a torque demand generator 28 which generates an initial torque demand signal 9. The initial torque demand represents an ideal torque demanded from the motor, for instance to provide an assistance torque to the driver as they turn the steering wheel.

(29) The initial torque demand signal 19 is fed into a torque demand limit generator 20 which is arranged to limit the torque demand signal 19 in order to ensure that the current drawn by the apparatus does not exceed a limit, or in the case of a motor that is generating ensure that the current generated by the motor does not exceed a limit. If the initial torque demand signal value exceeds the limit it is held to the limit. If not, it passes through the torque demand limiter 20 without modification.

(30) The modified torque demand limit signal 21 is fed into a current controller 24 which calculates a current demand for the motor. The current demand output form the controller 24 is in the form of two current demand signals 25, 26 in the d-q axis reference frame, one for each lane where two lanes a provided.

(31) In a final stage, a drive circuit 27 converts the d-q axis currents output from the current controller 22 into three current demand components in a static reference frame, one for each phase of the motor 17, 18 or 19. These demand currents a, b, c are then converted by the drive stage 27, in combination with an estimate of the rotor position, into suitable PWM signals that are supplied to the switching motor phases of the drive stage 27 by PWM of the switches. A range of PWM switching strategies are known in the art and so will not be described in detail here. The switch arrangement is well known and described in such documents as EP 1083650A2. To provide control feedback a measurement of the phase currents is fed into the apparatus 13 from a current monitor 34, shown in FIG. 3 as a current shunt in the common ground path from the motor back to the battery.

(32) The application of the torque demand limit causes the torque of the motor to deviate from the ideal torque demanded by the torque demand generator, but as will be apparent optimal setting of the limits ensures that at all times the motor is producing the maximum possible torque whilst ensuring that the current demanded from the battery and alternator do not exceed system limits. Importantly, the modification of the torque demand makes the implementation of the current controller simpler than prior art arrangements as all limiting is performed prior to the current controller. The controller, the torque demand generator and the torque demand limiter may be implemented using an electronic control unit running software that is stored in an area of memory.

(33) FIG. 5 is a block diagram showing at the highest level the function of each part of the apparatus, from the initial input of the measured torque to the output of the d-q axis current demand signals 25, 26. Note that as shown the torque limit is applied after an initial torque demand has been calculated. In a modification, shown in FIG. 20, the limit is used during generation of the initial torque demand.

(34) The torque demand limiter comprises an algorithm which performs two distinct stages: Stage 1) Defines and applies a maximum torque limit for motoring and generating, and applies limit to initial torque demand; and Stage 2) Defines a maximum torque gradient for motoring and generating, and applies limit to initial torque demand. The two stages are embodied as software that is executed by a signal processor. An overview of the key function blocks of the software is shown in FIGS. 6 to 13 showing in more detail each of the key functional stages of the software and the input parameters and outputs.

(35) Stage 1) Torque maximum limit: The purpose of this stage is to determine the torque limit that corresponds to the maximum battery current during generating, and separately to do the same for when the motor is motoring. In other words, if the motor produces the same torque as the torque limit the battery current limit for generating or motoring is not exceeded.

(36) FIG. 14 shows a typical function of maximum torque plotted against the battery voltage. As can be seen, at low battery terminal voltages where it can be considered that the battery is partially depleted it is beneficial to greatly limit the current drawn by the motor. When the battery is fully charged, in this case above its nominal 12 volts, the current drawn can be held at a constant high limit. This is typically chosen as a function of the maximum rate at which the alternator can replenish the drawn charges, and ensures that over time the battery does not become depleted. Other factors may be relevant in the setting of this maximum limit.

(37) FIG. 15 shows the effect of the limit on the torque demand over time. The current that is demanded from the motor, or generated by the motor, is capped at the limit. Since the current demand is a function of the torque demand, this means that the torque demand can be limited to achieve the required cap provided that the algorithm is aware of the required limit and the operational parameters of the motor.

(38) Prior to limiting the torque, the torque demand limiter in a first block shown in FIG. 6 calculates a set of current limits. As shown there is a limit calculated for the motor battery when the motor is in a motoring operation and drawing current. A limit is set for the battery current when the motor is generating. Limits are also set for the motor drive stage bridge when motoring and the drive stage bridge when the motor is generating. The limits may include a motor battery current limit 29, a generator battery current limits 30, a motor electric power limit 31 and a generator electric power limit 32.

(39) These limits may be set as shown as a function of the battery voltage-using a map or lookup table (LUT) similar to the one shown in FIG. 14, as a maximum overall current demand for motoring and generating which may be pre-set by a customer. From these current limits, the torque demand limiter next determines corresponding torque demand limit values. This is performed by feeding into the torque demand limiter motor parameters that can be used to model the behaviour of the motor. These include the instantaneous current demand, the resistance of the motor phases and bridge, the velocity of the motor and a torque constant. The aim is to feed the current limits into a model 33 to generate appropriate torque demand limits.

(40) FIG. 7 shows the function defining the model that is used to calculate the torque limits when the motor is motoring. For an electric motor with negligible core and mechanical losses, the power equation can be written as:
v.sub.batti.sub.batt=R.sub.s(i.sub.d.sup.2+i.sub.q.sup.2)+T.sub.mω.sub.m

(41) For a given battery current limit the above equation becomes:
v.sub.batti.sub.max=R.sub.s(i.sub.d.sup.2+i.sub.qmax.sup.2)+T.sub.maxω.sub.m

(42) Expanding the torque, the equation becomes:
v.sub.batti.sub.max=R.sub.s(i.sub.dDem.sup.2+i.sub.qmax.sup.2)+(i.sub.qmaxk.sub.T+i.sub.qmaxi.sub.dDemk.sub.Rel)ω.sub.m

(43) The equation to be solved for motoring is:
P.sub.max=R.sub.s(i.sub.dDem.sup.2+i.sub.qmax.sup.2)+(i.sub.qmaxk.sub.T+i.sub.qmaxi.sub.dDemk.sub.Rel)ω.sub.m

(44) The above equation can be re-arranged in a quadratic equation with one unknown:
R.sub.si.sub.qmax.sup.2+i.sub.qmax(k.sub.T+i.sub.dDemk.sub.Rel)ω.sub.m+R.sub.si.sub.dDem.sup.2−P.sub.max=0

(45) The solution of this equation is:

(46) $i_{\mathrm{qma}x}=\frac{-b+\sqrt{b^2-4ac}}{2a}$
where: a=R.sub.s, b=(k.sub.T+i.sub.dDemk.sub.Rel)ω.sub.m and c=R.sub.si.sub.dDem.sup.2−P.sub.max;and k.sub.Rel=−1.5p(L.sub.d−L.sub.q)

(47) These equations can be solved to give the torque T as:
T.sub.max=(i.sub.qmaxk.sub.T+i.sub.qmaxi.sub.dDemk.sub.Rel)
where Vbatt=dc-link/drive stage voltage; Ibatt=battery/power supply current; Rs=equivalent stator resistance; Iq=q axis current; Id=d-axis current; idDem=d-axis current demand; wmech=motor mechanical velocity; kT=permanent magnet torque constant=1.5*p*FluxPM, where p-number of motor pole pairs; FluxPM=permanent magnet flux; kRel=reluctance torque constant.

(48) FIG. 8 shows the similar model used to calculate the torque demand limits when the motor is generating, defined by the model function:
v.sub.batti.sub.batt=R.sub.s(i.sub.d.sup.2+i.sub.q.sup.2)+(i.sub.qk.sub.T+i.sub.qi.sub.dk.sub.Rel)ω.sub.m

(49) For generating the equation to solve becomes:
v.sub.batti.sub.min=R.sub.s(i.sub.dDem.sup.2+i.sub.q.sup.2)+(i.sub.qmaxk.sub.T+i.sub.qmaxi.sub.dDemk.sub.Rel)ω.sub.m

(50) Note: this time i.sub.min is negative. Note that in the case of the motor generating an additional input is provided indicative of the maximum electric power regeneration.

(51) The limits for motoring and generating and shown in FIG. 16. In addition, the current limits may in at least one embodiment of the present disclosure be overruled by a user defined current limit. This may be fed into the torque demand limiter as shown in FIG. 9. As can be seen the current limits are normally set using the lookup table LUT as a function of battery voltage Vbatt, but can be overruled by a customer set limit where the customer set limit is lower.

(52) Function 2) Torque gradient limit: As well as setting a limit of the maximum current, it is desirable to limit the rate of change of current drawn from or fed back to the motor. The applicant has appreciated that through use of a suitable motor model this can be achieved by limiting the torque gradient.

(53) To understand what is meant by torque gradient, FIG. 17 shows a change in battery current demand over time. Three different changes are shown, with one in solid line having an acceptable gradient and one marked with crosses not acceptable. The one in between is also acceptable provided the instantaneous gradient is within limits. FIG. 18 shows how the lines are not in reality straight lines with spikes in gradient for short durations.

(54) The provision of the torque gradient limiter has been found by the applicant to be useful where the ability of the electrical supply to deliver high rates of change of current is compromised, as might happen if the battery is partially or fully depleted or becomes disconnected. Its function is to ensure that the rate of change of battery current drawn by the motor (the battery gradient) does not exceed a predefined threshold. This is achieved by limiting the gradient of the torque demand signal.

(55) The gradient limiter block is shown in FIG. 13. As can be seen, this limits the gradient in series after the maximum value has been limited, although it could be done before the maximum value is limited. The gradient limiter therefore limits the rate of change of torque demand, which in turn limit the rate of change of the current that is drawn by the motor from the battery at time where the rate of change would exceed a threshold. The PI controller proportional and integral terms are chosen in such a way that the transients are neither under or over damped during times of limiting, to follow as closely as possible the ideal d-q axis current demand signal values.

(56) Model Error reduction: In addition to limiting the maximum torque demand value and the gradient, the applicant has appreciated that there may be times when the model is insufficiently accurate for the current limit to be achieved by limiting the torque. In a perfect motor, with a perfect model, a torque limit can be set which gives a known current limit. In an imperfect motor or model, the actual current may still exceed the limit. The torque demand limit is an estimated value calculated for static conditions (constant speed and constant current limit) and there is no guarantee that if the actual torque demand is limited to this torque limit, the battery current limit is not exceeded. This is because some of the motor parameters used in this calculation chain are not known very accurately (e.g. due to stator resistance) and because during current ramp-up extra current/power is needed to bring the actual current to the target value.

(57) To accommodate this model error, the apparatus may be configured to monitor the actual motor currents and in the event that these do exceed the limits set may instruct a further reduction in the torque demand limit. This is shown in FIGS. 10 and 11. The loop formed by feeding back the measured currents is also shown in FIG. 5 where the currents are input to the torque demand limit generator. The application of this error correction to the complete apparatus can also be summarised as shown in FIG. 19 of the drawings. This error correction can be omitted if the model can is sufficiently accurate.

(58) Charge excess: Where there is a model error, and the current demand from time to time exceeds the limits, there will be a charge excess which is accumulated over time. This charge excess can be seen in FIG. 15 as the solid shaded regions. This is undesirable as it as a non-zero charge would reduce the torque/power capability of the system.

(59) The charge is defined as the integral of positive battery current error (actual-limit) for motoring and negative battery current error (actual-limit) for generating. The charge is calculated as a moving average over a predefined window (e.g. 1 second) with an update rate of, for example, 50 times/second. This strategy ensures that at any time in the last second (as an example) the battery current limit is not exceeded by the maximum charge limit. If in the next predefined window, no increment in the charge exists, the torque limits reduction factor will be set to unity (no correction needed) as the charge becomes zero.
Q.sub.motor[As]=∫.sub.0.sup.1s(i.sub.batt−i.sub.limmot).Math.[(i.sub.batt−i.sub.limmot)>0]
Q.sub.generator[As]=∫.sub.0.sup.1s(i.sub.batt−i.sub.limgen).Math.[(i.sub.batt−i.sub.limgen)<0]

(60) The apparatus may be arranged to measure this charge excess over time and to apply a charge multiplication factor to the torque demand limit as shown in FIG. 12.