阅读说明：本技术 电动马达设备 (Electric motor apparatus ) 是由 V·科罗班-施拉默 于 2020-04-23 设计创作，主要内容包括：本发明公开了用于控制电动马达的设备,包括控制器,转矩需求极限发生器和驱动级,所述控制器被布置为接收指示所需马达转矩量的转矩需求信号作为输入,并产生一组马达电流需求信号作为输出；所述驱动级接收所述马达电流需求信号,并且被布置为使电流根据需要在所述马达的每个相中流动,以满足所需转矩；所述转矩需求极限发生器被布置为输出指示转矩需求极限的转矩需求极限信号,超过所述转矩需求极限,电池电流就将超过一个或多个极限。(An apparatus for controlling an electric motor comprises a controller, a torque demand limit generator and a drive stage, the controller being arranged to receive as an input a torque demand signal indicative of a required amount of motor torque and to generate as an output a set of motor current demand signals; the drive stage receiving the motor current demand signal and being arranged to cause current to flow as required in each phase of the motor to meet the required torque; the torque demand limit generator is arranged to output a torque demand limit signal indicative of a torque demand limit beyond which the battery current will exceed one or more limits.)

1. An apparatus (13) for controlling an electric motor, the electric motor (14) and the apparatus being powered by a battery power source (3), the apparatus comprising:

a controller (24) arranged to receive as input a torque demand signal (9) indicative of a required amount of torque of the motor (14) and to generate as output a set of motor current demand signals (25, 26); and

a drive stage (27) receiving the motor current demand signal (25, 26) and arranged to cause currents a, b, c to flow in each phase (17, 18, 19) of the motor as required to meet a required torque indicated by a torque demand signal (9);

wherein the apparatus further comprises a torque demand limit generator (20) arranged to output a torque demand limit signal (21) indicative of a torque demand limit beyond which the battery current i_{Battery with a battery cell}One or more limits will be exceeded.

2. The apparatus of claim 1, wherein the apparatus further comprises a torque demand generator (28) that generates a torque demand signal (9) indicative of a required amount of motor (14) torque Tm.

3. An apparatus as claimed in claim 2, wherein the torque demand generator (28) generates a torque demand signal (9) whose value depends on the required motor (14) assistance torque amount Tm and the torque demand limit signal (21) such that the value of the torque demand signal does not exceed a limit value.

4. An apparatus as claimed in claim 2, wherein the torque demand generator (28) generates a torque demand signal (9) independent of the torque demand limit signal (21), and subsequently modifies the torque demand signal (9) to produce a torque demand limit signal (21) which is fed to the controller if the signal (9) is to exceed a limit.

5. An apparatus as claimed in any preceding claim, wherein the torque demand generator (28) uses a model (33) of the motor (14) and the drive stage (27) to set the value of the torque demand limit signal (21).

6. The apparatus of any preceding claim wherein the torque demand limit generator sets the torque demand limit in dependence on the voltage of the battery, Vbatt.

7. An apparatus as claimed in any preceding claim, wherein the torque demand limit generator uses one or more of the following parameters in determining the torque limit:

a motor battery current limit (29);

a generator battery current limit (30);

a motor electrical power limit (31);

a generator electrical power limit (32).

8. An apparatus as claimed in any preceding claim, wherein the torque demand limit generator (20) generates one or more battery current limits, and these limits are fed into a model (33) of the motor (14) together with a battery voltage Vbatt and used by the torque demand limiter to determine the torque demand limit.

9. An apparatus as claimed in any preceding claim, wherein the torque demand limit generator (20) limits the rate of change of the demanded torque such that the current i drawn from the battery (3) during motoring operation or fed back into the battery (3) during generating operation is limited_{Battery with a battery cell}Is limited.

10. An apparatus as claimed in any preceding claim, further comprising a current monitor (34) which monitors or calculates an estimate of the actual current from a current controller or the actual current of the motor and the torque demand generator (28) reduces the torque demand limit if the current exceeds the current limit.

11. An apparatus as claimed in any preceding claim, wherein the torque demand generator (28) limits the rate of change of torque demand.

Technical Field

The present invention relates to an electric motor apparatus for controlling an electric motor.

Background

Electric motors are used in a wide variety of applications and are becoming more and more common in motor vehicles. For example, it is known to provide an electric power assisted steering system in which an electric motor apparatus applies an assist torque to a portion of the steering system to make it easier for the driver to turn the steering wheel of the vehicle. The magnitude of the assist torque is determined in accordance with a control algorithm that receives as input one or more parameters such as the torque applied to the steering column by the driver turning the steering wheel, the vehicle speed, etc.

In order to accurately control the electric motor torque, it is essential to control the current applied to the electric motor. Typically, a star-connected three-phase motor operating according to a pulse width modulation control/drive strategy is used, with each phase connected to an upper drive stage switch and a lower drive stage switch connected to battery power and ground, respectively. In a PWM strategy, each phase is driven with a periodic PWM drive signal having a first state and a second state and a duty cycle indicating the ratio of time spent in each state in one period. The requested motor torque is determined as a torque demand signal and the torque demand signal is then fed into a current controller which will generate appropriate d-q axis motor current demand signals which will cause the motor to produce that torque. These signals are then converted to three-phase currents in a static reference frame, which requires knowledge of the electrical position angle of the motor rotor, as required by the drive circuit. A position sensor may be provided which measures the rotor position, or the system may be of the sensorless type, such as taught in WO 2004/023639. Finally, using the measured values of the actual currents as feedback, the Pulse Width Modulation (PWM) duty cycle for each phase required to produce the required actual average current is calculated and used to drive the motor phases.

The motor draws current from the vehicle's electrical power source, which is typically a battery that is fully charged by an alternator that is driven by the vehicle's powertrain drawing power from the engine during braking or regenerative power. The current drawn by the motor is a function of the battery voltage and the duty cycle of the drive signal applied to each phase.

When large assistance is required, the duty cycle of the switch can be high and the total current drawn by the motor from the battery can be high. For a healthy vehicle electrical system, the alternator can typically meet high current demands so the battery does not drain. The maximum current draw of the motor should be set at a level that can be met by the alternator to prevent battery drain.

Disclosure of Invention

According to a first aspect, the present invention provides an apparatus for controlling an electric motor, the electric motor and the apparatus being powered by a battery power supply, the circuit comprising:

a controller arranged to receive as input a torque demand signal indicative of a required amount of motor torque and to generate as output a set of motor current demand signals; and

a drive stage receiving the motor current demand signal and arranged to cause current to flow as required in each phase of the motor to meet a required torque;

wherein the apparatus further comprises a torque demand limit generator arranged to output a torque demand limit signal indicative of a torque demand limit beyond which the battery current will exceed one or more limits.

The apparatus may further include a torque demand generator that generates a torque demand signal indicative of a desired amount of motor torque.

The torque demand generator may generate a torque demand signal, the value of which depends on the amount of assistance torque demanded by the motor and the torque demand limit signal, such that the value of the torque demand signal does not exceed the limit value. The torque demand limit signal may thus be fed into the torque demand generator.

In the alternative, the torque demand generator may generate a desired torque demand signal independent of the torque demand limit signal, and if the signal is to exceed the limit, then the torque demand signal is subsequently modified to produce a torque demand signal which is fed to the controller.

The torque demand limiter may set the value of the torque demand limit signal using a model of the motor and the drive stage. This machine model may fully or partially characterize the motor and drive stage, and as such may include many parameters.

The model may include as inputs one or more of the following parameters:

a motor stator equivalent resistance;

the motor temperature;

the mechanical speed of the motor;

a motor torque constant;

motor maximum bridge power;

motor bridge equivalent resistance.

In a preferred arrangement, the torque demand limiter may estimate a torque limit that limits the battery current to a given limit for a given battery current, a given phase current limit, and a given motor speed based on parameters of the motor circuit model.

Exceeding one or more limits refers to the current drawn from the battery and any associated alternator exceeding one or more maximum values when the motor is motoring, and may also refer to the current fed back to the battery exceeding one or more limits when the motor is motoring, preferably complying with limits for both motoring and generating operation.

By imposing a limit on the torque demand fed to the current controller, the current drawn by each phase of the motor can be very effectively limited, since the current is a function of the torque of the motor. This is a much simpler strategy than the prior art that directly limits the individual currents within the controller based on estimated or measured values of the currents. Limiting the torque demand allows any prior art current controller to be used without modification, as the controller is unaware that the torque demand fed to it has been so limited.

Typically, the power source will include a battery connected to an alternator that keeps the battery fully charged and provides power to the electric motor when the engine of the vehicle is running. Thus, as far as the present invention is concerned with drawing current from a power source, it should be interpreted broadly as the total current drawn from the battery, from the battery and the alternator, or from the alternator alone. The latter case applies when the battery has been disconnected.

The torque demand limit may have a value equal to an estimated maximum amount of torque that may be produced by the motor without the current exceeding the limit. This value may be used only as an estimate, since depending on the motor performance the current may exceed the limits of this torque value.

The torque demand limiter may set the torque demand limit according to the voltage of the battery. At lower voltages, the torque limit may be reduced compared to higher voltages.

The torque demand limiter may set the torque demand limit according to a predefined current maximum value predefined for the application.

The torque demand limiter may set the torque demand limit according to a mechanical speed of the motor.

In determining the torque limit, the torque demand limiter may calculate or otherwise access values for one or more of the following parameters:

a motor battery current limit;

generator battery current limit;

a motor electrical power limit;

generator electrical power limits.

The torque demand limit signal may include a plurality of parameters. In the case where multiple parameters are present, the torque demand signal should not exceed all of the limits defined by the multiple parameters.

One parameter of the torque demand limit may be set as the maximum torque.

Another parameter of the torque demand limit may be defined in newton-seconds, which is equal to the estimated current limit in ampere-seconds. For example, if this is set to a value of 5 ampere-seconds, the limit will be 5 amperes of current for 1 second or 1 ampere of current for 5 seconds. This value may be set between 5 and 80 ampere-seconds.

Thus, the torque demand limit may be set to an absolute value, and the torque demand generator may allow the torque to exceed the absolute value for a short period of time, perhaps as long as 1 millisecond or so.

The torque demand limiter may set one or more parameters for use when the motor is motoring (i.e., drawing current from the battery) and one or more other limits for use when the motor is generating (i.e., supplying current to the battery). In each case, the limit may vary depending on the speed.

The torque demand limiter may generate one or more battery current limits and these limits may be fed into a model of the motor arrangement together with the battery voltage and used by the torque demand limiter to determine these torque demand limits.

Thus, the generation of the torque demand limits may be a two-stage dynamic process, first calculating the current limits and then using the model to generate the required torque demand limits. If the torque demand exceeds the limit, the torque demand is limited accordingly.

The torque demand limiter may generate one or more of the limits from the battery terminal voltage, for example using a look-up table.

The torque demand limiter may be arranged to receive an override current limit value and, in the event that this value is less than the current limit value from the look-up table, this value may override the normal look-up value. This may be a customer preset limit that can never be exceeded and therefore may be static, or may be a dynamic limit provided from another system mounted to the vehicle. For example, if a critical system wants to ensure that battery charge is preserved, the critical system may direct the steering system to limit the current that the steering system can draw.

The torque demand limiter may comprise signal processing circuitry. The signal processing circuit may include a processor and a memory in which the determined values and parameters are stored.

The torque demand generator may additionally or alternatively limit the rate of change of the demanded torque such that the rate of change of current drawn from the power supply during motoring operation or fed back into the power supply during generating operation is limited.

Setting the torque demand limit may not always prevent the battery current from exceeding the allowable limit. This may be the case in situations where the motor circuit model is inaccurate, for example if the temperature changes rapidly and the model does not take into account temperature. This may occur where a model is pre-set for a batch of circuits and the motors and drive stages used in each circuit are slightly different.

Thus, the apparatus may include a current monitor that monitors or calculates an estimate of the actual current demand or motor actual current from the current controller, and in the event that these currents exceed the current limit, may direct the torque demand limiter to further reduce the torque demand limit.

Monitoring of the actual or estimated current or current demand may form part of a feedback control loop such that the torque demand limit is driven to an optimum value to limit the current drawn from or fed to the battery.

By monitoring the current and feeding back to the torque limiter in case the current limit is exceeded, any slight errors in the model can be accommodated. Since this is only the error that the correction should be small, the response time of the feedback loop may be relatively high.

In addition to limiting the total current, the motor circuit may also limit the rate of change of the torque demand. This enables limiting the rate of change of the current using the same model applied for determining the torque value limit.

The torque demand generator may set torque demand gradient limits, which are fixed or dynamic and in dynamic situations will vary with time. The limit may vary depending on one or more operating parameters of the vehicle or the motor and drive stage.

These parameters may include motor speed, vehicle speed, charge of a battery of the vehicle power supply, supply voltage, and other parameters from the vehicle stability system or braking system.

The drive stage may convert the current demand into a Pulse Width Modulated (PWM) waveform for each phase of the motor with a periodic PWM drive signal having first and second states and a duty cycle indicating the ratio of time taken by each state in a cycle, the drive stage may include determining an estimate of the current drawn from the power supply using the motor current demand signal and a signal indicative of the duty cycle of the PWM signal applied to each phase, and limiting the rate of change of the current drawn by the motor from the power supply by modifying the motor torque demand signal. The current draw estimate may be expressed as:

I＝da·Ia+db·Ib+dc·Ic+I_ECU

i_vatt＝d_ai_a+d_vi_v+d_ci_c+I_ECU

wherein I_ECUAn estimate of the current drawn;

da. db, dc are the duty ratios of the PWM signals for each of the phases a, b and c;

ia. Ib, Ic are instantaneous phase currents during the conducting part of the PWM cycle; and is

I_ECUIs an optional offset to account for the current drawn by the control circuit.

The current controller may comprise a PI (or PID) controller.

The apparatus may be used to control a motor of an electric power steering system. In such applications, the torque demand generator may receive as input a measure of torque in the steering portion of the electric power steering system, and may determine an initial torque demand value based on the measured torque. As is well known to those familiar with electric power steering system design, maps relating measured torque to torque demand may be used.

The skilled person will appreciate that the one or more limits of the battery current may be predefined limits defined during system design or varied during system use. Since power is defined as the product of battery current and battery voltage, a current limit may be set in conjunction with a measured or estimated value of battery voltage to effectively limit the power drawn from the battery.

The invention may also comprise a combination of the electric circuit of the first aspect and an electric motor.

According to a second aspect, the present invention provides a method of controlling an electric motor circuit of the type comprising an electric motor and control circuitry powered by a battery power supply, the control circuitry comprising a torque demand generator, which generates a torque demand signal dependent on the amount of torque demanded by the motor, a controller arranged to receive the torque demand signal as an input and to generate a set of motor current demand signals as an output, and a drive stage, which receives the motor current demand signals and is arranged to cause current to flow as required in each phase of the motor to meet the demanded torque,

the method comprises the following steps: generating a torque demand limit signal indicative of a torque demand limit beyond which the battery current will exceed one or more limits; and generating a torque demand signal, the value of which is dependent on the required amount of motor assist torque and the torque demand limit signal, such that the value of the torque demand signal does not exceed the limit value.

The method may generate the torque demand limit signal using a motor model including one or more motor parameters.

The method may further comprise measuring the current flowing in the motor and modifying the torque demand limit signal if the current exceeds a current limit.

The method may comprise any of the steps foreseen by the features of the apparatus of the first aspect of the invention.

The method may be used with motors in electric power steering systems, or with motors in various other systems (automotive and non-automotive).

Drawings

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a portion of a vehicle electrical system illustrating the connection of an electric power steering system to a power source;

FIG. 2 is a schematic representation of the critical portions of an exemplary electric power steering system to which the control strategy of the present invention may be applied;

FIG. 3 illustrates a dual channel motor and motor bridge that may be used in embodiments of the apparatus of the present invention;

FIG. 4(a) shows the prior art, while (b) shows the highest system level embodiment of the present invention, showing how to limit the torque demand, and thus the current drawn from or fed back into the battery;

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

FIG. 6 shows the input and output of a torque demand limiter stage to initially calculate a set of target current and power limits;

FIG. 7 illustrates the input and output of a torque demand limiter to set torque demand limits using the current limits and power limits used in motoring operation of the motor of FIG. 6;

FIG. 8 illustrates the input and output of the torque demand limiter to set the torque demand limit using the current and power limits used in the motoring operation of FIG. 6;

FIG. 9 is a functional block within the torque demand limiter that enables a customer to overrule a current limit;

FIG. 10 illustrates an interface between a current controller and a motor current controller for correcting errors in a motor model;

fig. 11 is an overview of a circuit for limiting the peak of the battery current to a predefined allowed peak current for operation, where fig. 11(a) explains the principle, and fig. 11(b) shows an algorithm implementation:

P_pred＝1.5(v_{d_DEm}i_d+v_{q_DEm}i_q)

P_{max_MOT}＝v_{DRV_STG}i_{MAX_MOT_ALW}

P_{max_GEN}＝v_{DRV_STG}i_{MAX_GEN_ALW}

FIG. 12 illustrates a suitable scaling of the torque demand limit based on excess charge accumulated in the battery over time due to exceeding the current limit;

FIG. 13 is a diagrammatic view of the function of the torque demand limiter when operating to limit the torque demand gradient;

FIG. 14 shows a typical plot of battery voltage versus battery current limit that may be used to populate an appropriate look-up table for use by the blocks of FIG. 6;

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

FIG. 16 is a graph covering the torque demand limits for all four quadrants of motor operation;

fig. 17 illustrates current gradients, showing an unacceptable (NOK) high gradient and an acceptable (OK) low gradient, also intermediate gradients that are feasible due to the short duration;

FIG. 18 is a more detailed diagram corresponding to FIG. 17, showing how the traces are not smooth and result in short spikes of increasing gradient; and

FIG. 19 is a block diagram showing the function of the motor control circuit and an optional charge limiting section; and

fig. 20 shows an alternative arrangement of the device.

Detailed Description

The following examples describe embodiments of the invention for use in automotive vehicle applications, but the reader will appreciate that the scope of the invention should not be limited to such applications.

As shown in FIG. 1, the vehicle is provided with an Electric Power Assisted Steering (EPAS) system that draws a current i from the vehicle power supply across the power rail 2_{Battery with a battery cell}. The power supply includes a battery 3 (typically rated at 12 vdc) which is in turn charged by an alternator 4. The battery also provides current to other vehicle accessories 5.

The EPAS system 1 is shown schematically in figure 2 of the drawings. The system comprises a steering column 10 attached to a steering wheel 11, a torque sensor 12 that measures the torque applied to the steering column 10 by the driver when turning 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 in series with the steering column 10, and the motor 14 may act on the steering column or other part of the steering system, typically through a gearbox 15.

The motor 14 typically comprises a three-phase wound stator element and a rotor having, for example, six embedded magnets therein, in which case the magnets are arranged to provide six poles that alternate between north and south poles about the rotor. Thus, the rotor defines three straight or d-axes evenly spaced about the rotor and three orthogonal or q-axes spaced from one another between the d-axes. The d-axis is aligned with the poles of the magnets with the lines of magnetic flux from the rotor in the radial direction and the q-axis is spaced between the d-axis with the lines of magnetic flux from the rotor in the tangential direction.

The three motor stator windings are connected in a star network. The motor is controlled by a device 13 according to an aspect of the invention, the device comprising a controller and a driving stage, the driving stage 27 of the device 13 comprising a three-phase bridge forming a switching stage. This is shown in figure 3. Each leg of the bridge comprises a pair of switches in the form of top transistors T connected in series between the battery supply rail 2 and ground₁、T₃、T₅And a bottom transistor T₂、T₄、T₆. The motor windings are each tapped from between a complementary pair of respective transistors. The transistors are turned on and off in a controlled manner by the control and drive circuit to provide Pulse Width Modulation (PWM) of the potential applied to each terminal, thereby controlling the potential difference applied across each winding and hence also the current a, b, c flowing through the windings depending on the duty cycle d of the respective phase 17, 18, 19 of the motor. This in turn controls the strength and orientation of the magnetic field generated by the windings, which in turn controls the motor torque. In practice, as shown in fig. 3, the motor has a double channel, so there are copies of three phases and a three-phase bridge. The two channels may be run in parallel, each providing half of the motor torque, or one channel at a time.

The torque signal output from the torque sensor 12 is fed to the input of the device 13. This is input to a torque demand generator 28 which generates an initial torque demand signal 9. The initial torque demand represents a desired torque demanded by the motor, such as to provide assist torque to the driver when the driver turns the steering wheel.

The initial torque demand signal 9 is fed into a torque demand limit generator 20 which is arranged to limit the torque demand signal 9 to ensure that the current drawn by the apparatus does not exceed a limit, or in the case of a motor which is operating in power generation, the current produced by the motor does not exceed a limit. If the initial torque demand signal exceeds the limit, it is held at the limit. If the limit is not exceeded, the initial torque demand passes through the torque demand limiter 20 without modification.

The modified torque demand limit signal 21 is fed to a current controller 24 which calculates the current demand of the motor. The current demand output from the controller 24 is in the form of two current demand signals 25, 26 in a d-q axis reference frame, one for each channel in the case of two channels.

In the final stage, the drive stage 27 converts the d-q axis current output from the current controller 24 into three current demand components in a static reference frame, one for each phase 17, 18, 19 of the motor. These demand currents a, b, c are then converted by the drive stage 27 in combination with the estimation of the rotor position into suitable PWM signals, which are supplied to the switching motor phases of the drive stage 27 by means of the PWM of the switches. A range of PWM switching strategies are known in the art and will therefore not be described in detail here. Switch arrangements are well known and described in documents such as EP 1083650a 2.

The measured values of the phase currents are fed to the apparatus 13 by a current monitor 34 to provide control feedback, such as the current shunt in the common ground path from the motor back to the battery as shown in fig. 3.

Applying the torque demand limit causes the torque of the motor to deviate from the ideal torque demanded by the torque demand generator, but it is clear that the optimum setting of this limit ensures that the motor produces the maximum possible torque at all times whilst ensuring that the current demanded by the battery and alternator does not exceed the system limits. Importantly, the modification of the torque demand makes the implementation of the current controller simpler than prior art arrangements, since all the restrictions are performed before the current controller.

The controller, torque demand generator and torque demand limiter may be implemented using an electronic control unit running software stored in a region of memory.

Fig. 5 is a block diagram showing the functions of the parts of the apparatus at the highest level 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 the initial torque demand is calculated. In the modification shown in fig. 20, the limit is used during generation of the initial torque demand.

The torque demand limiter includes an algorithm that executes two distinct phases:

phase 1) defining and applying maximum torque limits for motoring and generating operations and applying the limits to the initial torque demand; and

phase 2) defines the maximum torque gradient for motoring and generating operation and imposes limits on the initial torque demand.

These two phases are implemented as software executed by a signal processor. Fig. 6 to 13 show an overview of the key functional blocks of the software, showing each key functional phase of the software in more detail, as well as the input parameters and outputs.

Phase 1) maximum limit of torque

The purpose of this stage is to determine the torque limit corresponding to the maximum battery current during power generating operation and to do so separately while the motor is motoring. In other words, if the motor generates the same torque as the torque limit, the battery current limit for the power generating operation or the motor operation is not exceeded.

Fig. 14 shows a typical function of maximum torque plotted against battery voltage. It can be seen that at low battery terminal voltages, where the battery can be considered to be 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 kept at a constant upper limit. This is typically selected based on the maximum rate at which the alternator can replenish the charge drawn and to ensure that the battery does not drain over time. Setting this maximum limit may be related to other factors.

FIG. 15 illustrates the effect of limits on torque demand over time. The required motor current or the current generated by the motor is limited to this 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 upper limit, provided the algorithm knows the required limits and operating parameters of the motor.

The torque demand limiter in the first block shown in fig. 6 calculates a set of current limits before limiting the torque. As shown, there is a limit calculated for the motor battery when the motor is in run operation and drawing current. When the motor is in power generating operation, a limit of the battery current is set. A limit is also set for the motor drive stage bridge during motor operation and for the drive stage bridge during motor generating operation. The limits include a motor battery current limit 29, a generator battery current limit 30, a motor electric power limit 31, and a generator electric power limit 32.

These limits may be set as a maximum total current demand (which may be preset by the customer) for motoring and generating operations based on battery voltage using a map or look-up table (LUT) similar to that shown in fig. 14.

Next, based on these current limits, the torque demand limiter determines a corresponding torque demand limit value. This is performed by feeding motor parameters that can be used to model the motor behavior to a torque demand limiter. These parameters include instantaneous current demand, resistance of the motor phases and bridge, motor speed, and torque constants. The purpose is to feed the current limits into the model 33 to generate the appropriate torque demand limits.

Fig. 7 shows a function defining a model for calculating the torque limit when the motor is running.

For an electric motor with negligible core and mechanical losses, the power equation can be written as:

for a given battery current limit, the above equation becomes:

expanding the torque, the equation becomes:

solving the equation when the motor is running:

the above equation can be rearranged as a quadratic equation with one unknown quantity:

the solution to this equation is:

wherein: a ═ R_s，

And k is_Rel＝-1.5p(L_d-L_q)

These equations can be solved to arrive at a torque T of:

T_max＝(i_qmaxk_T+i_qmaxi_dDemk_Rel)

wherein

v_batt-a dc link/drive stage voltage;

i_batt-battery/supply current;

R_s-an equivalent stator resistance;

i_q-a q-axis current;

i_d-a d-axis current;

i_dDem-d-axis current demand;

w_mech-motor mechanical speed;

k_T-permanent magnet torque constant 1.5 p flux, wherein p-motor pole pair number, flux;

k_Rel-a reluctance torque constant.

Fig. 8 shows a similar model for calculating the torque demand limit when the motor is operating for generating electricity, which is defined by the following model function:

for power generation operation, the equation to be solved is:

note that: at this time i_minIs negative.

Note that in the case of motor generating operation, an additional input indicative of maximum electric power regeneration is provided.

The limits for motoring and generating operation are shown in FIG. 16.

Additionally, in at least one embodiment of the present invention, the current limit may be overruled by a user-defined current limit. This may be fed into a torque demand limiter as shown in fig. 9. It can be seen that the current limit is typically set from the battery voltage Vbatt using a look-up table LUT, but where the customer set limit is low, the current limit may be overruled by the customer set limit.

Stage 2) torque gradient limit.

In addition to setting a limit for the maximum current, it is also desirable to limit the rate of change of current drawn from or fed back to the motor. The applicant has realised that by using a suitable motor model this can be achieved by limiting the torque gradient.

To understand the meaning of the torque gradient, fig. 17 shows the change in battery current demand over time. Three different variations are shown, where one variation, indicated by a solid line, has an acceptable gradient, while another variation, indicated by a cross, is unacceptable. If the instantaneous gradient is within limits, a change between the two is also acceptable. Fig. 18 shows how, in a short duration, a line is not a straight line with gradient spikes in reality.

Applicants have found that the provision of a torque gradient limiter is useful in situations where the ability of the power supply to produce a high rate of change of current is compromised (as may occur if the battery is partially or fully depleted or disconnected). Its function is to ensure that the rate of change of the battery current drawn by the motor (battery gradient) does not exceed a predefined threshold. This is achieved by limiting the gradient of the torque demand signal.

The gradient limiter block is shown in fig. 13. It can be seen that this can be done after limiting the maximum value, but it can be done before limiting the maximum value.

Thus, the gradient limiter limits the rate of change of the torque demand, which in turn limits the rate of change of the current drawn by the motor from the battery when the rate of change exceeds a threshold. The proportional and integral terms of the PI controller are selected in such a way that transients are not under-or over-damped during the limited time to comply as closely as possible with the desired d-q axis current demand signal value.

Model error reduction

In addition to limiting the maximum torque demand and the gradient, applicants have also recognized that sometimes the model is not accurate enough to achieve current limits by limiting torque. In a perfect motor with a perfect model, a torque limit can be set that 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 estimate calculated for static conditions (speed constant and current limit constant) and there is no guarantee that the battery current limit will not be exceeded if the actual torque demand is limited to this torque limit. 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 extra current/power is needed to bring the actual current to the target value during the current ramp-up.

To accommodate this model error, the device may be configured to monitor the actual motor currents and, in the event that these currents do exceed the set limits, may direct a further reduction in the torque demand limit. This is illustrated in fig. 10 and 11. The loop formed by feeding back the measured current is also shown in fig. 5, where the current is input to the torque demand limit generator.

This error correction applied to the entire device can also be summarized as shown in figure 19 of the drawings. This error correction may be omitted if the model can be sufficiently accurate.

Excess charge

If there is a model error and the current demand will exceed the limit from time to time, excess charge will build up over time. This charge excess can be seen in the solid shaded area in fig. 15. This is undesirable because a non-zero charge will reduce the torque/power capability of the system.

The charge is defined as the integral of the positive battery current error (true-limit) during motoring and the negative battery current error (true-limit) during generating. The charge is calculated as a moving average over a predefined window (e.g. 1 second) with an update rate of e.g. 50 times/second. This strategy ensures that at any time during the last second (for example), the maximum charge limit does not exceed the battery current limit. If there is no charge increment in the next predefined window, the torque limit reduction factor will be set to one (no correction required) because the charge becomes zero.

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