Patent Grant
12088233
This patent application claims priority from Italian patent application no. 102021000018689 filed on Jul. 15, 2021, the entire disclosure of which is incorporated herein by reference.
The invention relates to a process and an apparatus for controlling an electric motor, in particular an electric motor driving a vehicle.
According to the state of the art, the electric motor driving an electric or hybrid vehicle can be powered with a supply voltage set by means of a static converter.
In this case, the electric motor has a mechanical characteristic, namely a relationship between torque and speed of the electric motor, which depends on the supply voltage.
Typically, the mechanical characteristic can conveniently be represented by means of a curve in a torque-speed diagram.
Generally speaking, the area delimited by the curve and by the axes of the diagram defines a zone of controllability of the electric motor, in which the motor correctly responds to the control signals.
The electric motor can be controlled in a torque-based manner, namely according to a control law that pursues a target signal of the torque delivered by the motor.
In this case, the target signal is determined as a function of a request of the driver of the vehicle, for example expressed by operating an accelerator pedal of the vehicle.
In some operating conditions, based on the instantaneous state of the vehicle and of the electric motor, the driver could express a request exceeding the zone of controllability of the electric motor.
This clearly causes a disadvantageous condition, since the electric motor would have to operate in a substantially uncontrollable manner.
Therefore, in general, there is a need to avoid the aforesaid disadvantageous condition as much as possible.
An object of the invention is to fulfill the need discussed above.
The aforesaid object is reached by a process and an apparatus for controlling an electric motor as claimed in the appended claims.
The dependent claims define special embodiments of the invention.
Hereinafter, embodiments of the invention will be described, in order to allow the latter to be better understood, by way of non-limiting example and with reference to the accompanying drawings, wherein:
In
The vehicle 1 comprises, in particular, two pairs of wheels 4 and an electric motor 2 coupled to at least one of the two pairs of wheels 4 for driving the vehicle 1. Therefore, the motor 2 is an electric motor driving the vehicle 1.
Furthermore, the vehicle 1 comprises an apparatus 3 configured to control the motor 2. The apparatus 3 comprises at least one control unit ECU configured to actuate a process for controlling the motor 2.
The control unit ECU provides or stores a functional relationship, which associates a first and a second quantity T, Vdc with a speed parameter ωlim of the motor 2.
The first quantity T is indicative of a torque delivered by the motor 2; the second quantity Vdc is indicative of a supply voltage of the motor 2. For example, the quantity Vdc can be indicative of a direct voltage, in particular adjustable by means of a power converter, more in particular a static converter. In other words, the motor 2 may comprise a power converter to adjust the supply voltage of the motor 2.
The parameter ωlim corresponds to a limit output speed for the motor 2.
More precisely, the parameter ωlim represents a controllability limit of the motor 2, which means that exceeding the output speed corresponding to this limit at the values of the quantities T, Vdc associated with the latter leads to an operation of the motor 2 under conditions of substantial uncontrollability.
More in detail, the functional relationship comprises two further functional relationships between the parameter ωlim and the quantities T, Vdc, respectively. More precisely, the composition of the two further functional relationships defines the functional relationship provided or stored by the control unit ECU.
Each one of the functional relationships can be expressed by means of a table, a diagram, an equation and the like, or in any other suitable form.
The functional relationship, depending, in turn, on the quantity Vdc, between the parameter ωlim and the quantity T defines what is normally known as mechanical characteristic of the motor 2. For example,
The mechanical characteristic of
The decreasing segment can, in some cases, have an extremely sharp inclination, so that the decreasing segment can be neglected, namely omitted. In this case, the mechanical characteristic is considered as entirely defined by its constant segment. The constant segment associates the critical threshold value ωc with any value of the quantity T.
Therefore, the parameter ωlim is constant with respect to the quantity T, in particular constantly equal to the critical threshold value ωc, or it increases as the quantity T decreases, in particular increases from the critical threshold value ωc up to an escape value ωf associated with a zero value for the quantity T.
Clearly, the first case mentioned above occurs when the decreasing segment is omitted; otherwise, the last case occurs.
In the first case, the parameter ωlim, namely, in this specific case, the critical threshold value ωc, will practically depends on the sole quantity Vdc. In the last case, the parameter ωlim will depend on both quantities T, Vdc. In particular, both the critical threshold and the escape value ωc, ωf will depend on the quantity Vdc and more in particular will increase with the latter.
More in general, the mechanical characteristic could be different from the one shown in
Hence, the mechanical characteristic can be represented by a curve in a diagram similar to
The functional relationship between the parameter ωlim and the quantity Vdc implies that the parameter ωlim increases as the quantity Vdc increases, in particular in a linear manner.
Specifically,
So far, the description relates to the absolute values of the quantities T, Vdc and of the parameter ωlim. However, the quantities T, Vdc and the parameter ωlim can also be considered with their own sign, in the light of adaptive considerations resulting in an evident manner for a person skilled in the art.
The control unit ECU is configured to sample the acquired or received signals according to a sampling time, for example a constant sampling time, in particular in the order of 1 ms.
The control unit ECU is further configured to emit control signals, for example for the motor 2, in particular according to the sampling time.
More in detail, the control unit ECU emits control signals of the quantities T, Vdc. In particular, the quantity T is controlled by the control unit ECU according to a control law, for example in closed loop, as explained more in detail below. For example, the control signal of the quantity T can be a control signal.
Hence, the control unit ECU is configured to determine a pair of values T(t), Vdc(t) of the relative quantities T, Vdc with reference to the current time instant t. In particular, the control unit ECU extracts the values T(t), Vdc(t) from the emitted control signals.
Alternatively or in addition, the apparatus 3 comprises transducers S1, S2 configured to detect the quantities T, Vdc, respectively, or further quantities indicative thereof, as well as to generate respective signals associated with the detected quantities. For example, the transducer S1 detects a quantity a indicative of an output acceleration of the motor 2. Clearly, barring an inertial factor (for example, stored or provided by the control unit ECU), the acceleration is indicative of the torque delivered by the motor 2. In other words, the quantity a is indicative of the quantity T. Therefore, the quantity T is derivable from the quantity a or extractable from the signal generated by the transducer S1.
The control unit ECU is coupled to the transducers S1, S2 and is configured to acquire the signals generated therefrom. Therefore, the control unit ECU can determine or extract the values T(t), Vdc(t) from the acquired signals coming from the transducers S1, S2.
The transducers S1, S2 should be considered as described independently of one another, so that each transducer S1, S2 may independently be present or absent.
Once the control unit ECU has determined the values T(t), Vdc(t), the control unit ECU is configured to determine a value ωlim(t) of the parameter ωlim that corresponds to the values T(t), Vdc(t) according to the provided or stored functional relationship.
Furthermore, the control unit ECU is configured to determine a value ω(t), with reference to the current instant t, of a quantity ω comparable with the parameter ωlim and indicative of an output speed of the motor 2. For the sake of clarity, the term “comparable” may be meant as having the same unit of measurement and/or deductible relative to one another.
For example, the apparatus 3 could comprise a transducer S3 configured to detect the quantity ω and to generate an associated signal. The control unit ECU, in this case, would be coupled to the transducer S3 and configured to acquire the signal generated from the latter, as well as extract the value ω(t) from it.
Alternatively or in addition, the control unit ECU is advantageously configured to predict a value ω(t+k) of the quantity ω starting from the signal generated by the transducer S1 and acquired by the control unit ECU.
The expression t+k indicates a future time instant; in particular, the parameter k can indicate one or more time samples. More precisely, the time interval elapsing between the instants t, t+k, which has a duration corresponding to the parameter k, lasts at least three times the sampling time (namely, at least three time samples), preferably at least five times the sampling time. Specifically, the parameter k is worth 5 ms.
Hence, the predicted value ω(t+k) may be assigned to the value ω(t), basically advancing the future measurement to the current time instant t.
More in detail, the prediction of the value ω(t+k) is based on the solution, in particular numerical solution, of a first-order ordinary differential equation. Said differential equation has an unknown function, which has to be determined, and a known term. The unknown function corresponds to the quantity ω; the known term can be extracted starting from the signal acquired at the current instant t.
For the sake of clarity, the numerical solution of the differential equation specifically corresponds, herein, to the solution of a finite difference equation. Therefore, the expression “differential equation” must be interpreted, herein, with a wide meaning, which covers both an actual differential equation in a continuous time domain and a finite difference equation in a discrete time domain.
In particular, the differential equation is numerically solved by means of an explicit method, for example the forward Euler method.
Therefore, the following equation applies:
ω(t+k)=ω(t)+ka(t)
wherein a(t) represents the value of the quantity a detected by the transducer S1 at the current time instant t. The control unit ECU may extract the value a(t) starting from the acquired signal coming from the transducer S1.
Preferably, the acquired signal coming from the transducer S1 may be subjected to a filtering step carried out by the control unit ECU before the extraction of the value a(t) or, anyway, before the prediction of the value ω(t+k). The filtering takes place by means of one or more suitable filters, for example of a known type.
The value ω(t) in the equation shown above may be extracted by the control unit ECU from the acquired signal coming from the transducer S3, but this is not limiting. As a matter of fact, the value ω(t) may constantly be updated by the control unit ECU through the solution of the differential equation starting from an initial condition ω(0) equal to zero or to any other known value.
At the end of the prediction, the value ω(t) takes on the predicted value ω(t+k) through assignation by the control unit ECU.
Once the control unit ECU has determined the values ω(t), ωlim(t), the control unit ECU is configured to determine a value Δω(t) of a quantity Δω indicative of a difference between the values ω(t), ωlim(t).
Herein, the term “difference” has to be interpreted in a wide manner; for instance, the quantity Δω is the actual subtraction between the quantities ω, ωlim, but a ratio between them would also be suited to indicate their difference. Furthermore, the standard deviation of the quantities ω, ωlim, the difference between the moduli of the quantities ω, ωlim and the like are suited to indicate their difference. More in general, the term “difference” can be interpreted as a general concept of difference or diversity or deviation or inequality between the quantities ω, ωlim.
The control unit ECU is configured to determine a target value Ttgt(t) for the quantity T as a function of the value Δω(t), in particular so that the quantity Δω indicate an increase in the modulus difference, namely so that the quantity ω strays away from the quantity ωlim, namely in order to cause the output speed of the electric motor 2 to stray away from the controllability limit. The target value Ttgt(t) is associated with or relates to the current instant t.
The control unit ECU is configured to control the motor according to the determined target value Ttgt(t). More precisely, as mentioned above, the control unit ECU controls the motor 2 in a closed loop. Therefore, the target value Ttgt(t) is compared with a feedback value Tfb(t) of the quantity T, for example extracted starting from the acquired signal coming from the transducer S1. The comparison leads to an error, on which a control law is based, for example of the PID type, to determine the control signal of the quantity T. In addition, an open loop control based on the target value Ttgt(t) is also possible.
More in detail, the target value Ttgt(t) is determined as result of a correction of a base value Tb(t), in particular representative of a request of a driver of the vehicle 1 concerning the torque delivered by the motor 2. For example, the vehicle 1 comprises an accelerator device, specifically a pedal (not shown herein), which can be operated by the driver in order to express the request for torque delivered by the motor 2. The control unit ECU is coupled to the accelerator device and is configured to extract an item of information associated with the request expressed by the driver through the accelerator device itself. By so doing, the control unit ECU determines the base value Tb(t) as a function of the extracted item of information. The determination of the base value Tb(t) is indicated by reference number 104 in
More precisely, though not necessarily, the correction is not always carried out, but it is performed at least when the value Δω(t) fulfills a given condition. When it is performed, the correction can be based on the value Δω(t), although this is not necessarily the case. For example, the condition can be completely arbitrary, even though it is predetermined or established.
For the sake of clarity, the term “correction” should be interpreted as a change in the base value Tb(t), for example by means of a mathematical operation.
Specifically, the correction can take place by multiplying a gain G(t) by the base value Tb(t), as shown in
Alternatively, the correction could be carried out by algebraically adding a sum parameter to the base value Tb(t); in this case, the sum parameter could still be determined as a function of the value Δω(t), although not necessarily.
Generally speaking, the correction may comprise the application of a numerical corrective factor (for example, the gain G(t) or the sum parameter) to the base value Tb(t) by means of a mathematical operator. The corrective factor can be determined by the control unit ECU as a function of the value Δω(t).
In particular, the correction reduces the modulus of the base value Tb(t).
More in particular, the modulus of the base value Tb(t) is decreased to a greater extent the lower the value Δω is, for example in a proportional manner.
Otherwise, the reduction of the modulus of the base value Tb(t) or, more in general, the correction can take place by assigning an arbitrary value to the target value Ttgt(t), for example equal to zero or to a value having an opposite sign with respect to the one of the base value Tb(t) or smaller than a threshold. The value having an opposite sign can be assigned as a function of the value Δω(t) or independently thereof.
For example, the arbitrary value may be sufficiently low to ensure that a modulus of Δω(t) becomes or is greater than a threshold.
In other words, the target value Ttgt(t) is determined by assigning the arbitrary value, for example when the value Δω(t) fulfills said condition.
For example, the multiplying gain G(t) could be a coefficient, for example positive and smaller than or equal to one, or smaller than and equal to one with no sign limits, or preferably ranging from −1 to 1, in particular with the extremes includes.
In addition, for example, the coefficient could decrease as the value Δω(t) decreases, for example in a proportional manner. Otherwise, the coefficient could take on a specific value, preferably independently of the value Δω(t); for example, the coefficient could be worth zero or also take on a negative value, if necessary even smaller than −1.
Preferably, the aforesaid condition is fulfilled when the value Δω(t) indicates that the difference between the values ω(t), ωlim(t) fulfills a predetermined relationship with a threshold.
For example, the condition is fulfilled when the value Δω(t) indicates that the subtraction between the values ω(t), ωlim(t) is equal to a zero threshold or that the modulus of the value ω(t) minus the modulus of the value ωlim(t) is smaller than a non-zero threshold.
In other words, this means that the controllability limit of the motor 2 is reached or exceeded. Hence, a quick intervention to restore the controllability of the motor 2 is appropriate.
In this case, according to a first example, the target value Ttgt(t) is determined as equal to zero, regardless of the value Δω(t). Otherwise, according to a second example, the target value Ttgt(t) is determined as equal to a value having an opposite sign with respect to the one of the base value Tb(t); in this case, there is a target torque reversal strategy.
According to an alternative example, the aforesaid condition is fulfilled when the value Δω(t) indicates that the subtraction or difference between the values ω(t), ωlim(t) has a modulus that is smaller than a non-zero threshold.
In other words, this means that the difference or distance between the values ω(t), ωlim(t) is relatively small. Namely, the value ω(t) is close to the value ωlim(t).
When the condition is not fulfilled, which means that the value ω(t) is relatively far from the value ωlim(t), the correction is not applied. Hence, more in particular, the target value Ttgt(t) coincides with the base value Tb(t), specifically as a function of the torque request of the driver.
Specifically, the multiplying gain G(t) of the correction is one when the condition is not fulfilled.
Herein, the determination takes place according to the specific first example discussed above: the target value Ttgt(t) is set equal to zero when the condition is fulfilled, namely when the controllability limit of the motor 2 is reached or exceeded. This condition is represented by the intersection between curve 200 or 201 and one of the lines 203.
More precisely, curve 300 in
Curves 300, 301 show how the correction takes place in advance for curve 301 with respect to curve 300, thus revealing the effectiveness of the prediction.
Furthermore,
When the correction takes place by means of the multiplying gain G(t), the aspect of the sign of the quantity ω becomes irrelevant for the purposes of the correction. Otherwise, when the correction takes place by means of the sum parameter, the latter has to be such as to increase the target value Ttgt(t) with respect to the base value Tb(t) for negative values ω(t), or such as to decrease the target value Ttgt(t) with respect to the base value Tb(t) for positive values ω(t).
More precisely, curves 500, 501 in
The control unit ECU performs a process for controlling the motor 2 powered with the supply voltage.
The process comprises at least the steps of:
Owing to the above, the advantages of the apparatus 3 and the process according to the invention are evident.
The apparatus 3 and the process substantially avoid an uncontrolled operation of the motor 2. As a matter of fact, the control unit ECU forcedly decreases the torque delivered by the motor 2 when the output speed thereof gets close to the controllability limits.
Furthermore, the prediction of the quantity ω allows the forced decrease to be advanced; by so doing, the motor uncontrollability risks are significantly reduced.
Finally, the apparatus 3 and the process according to the invention can be subjected to changes and variants, which, though, do not go beyond the scope of protection set forth in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102021000018689 | Jul 2021 | IT | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20110241583 | He | Oct 2011 | A1 |
| 20120063922 | Sano et al. | Mar 2012 | A1 |
| 20160134218 | Yamaguchi | May 2016 | A1 |
| 20160329852 | Bouallaga | Nov 2016 | A1 |
| 20200259431 | Sawada | Aug 2020 | A1 |
| Number | Date | Country |
|---|---|---|
| 2662974 | Nov 2013 | EP |
| 2020188693 | Sep 2020 | WO |
| Entry |
|---|
| Italian Search Report for Application No. 102021000018689 completed Mar. 31, 2022. |
| Number | Date | Country | |
|---|---|---|---|
| 20230024889 A1 | Jan 2023 | US |
