DE102021101884A1 - Phase current measurement method and control circuit - Google Patents

Abstract

The invention relates to a method (100) for measuring the phase current of an electric motor (M) using a current measuring element (14) which is connected between a switching bridge (12) with at least one switchable switching element (S) for controlling at least one phase current (I_U, I_V, I_W) switching state (s1, s2) influencing a motor phase (U, V, W) of the electric motor (M) and a voltage source (V_DC) connected to the switching bridge (12) for feeding the phase current (I_U, I_V, I_W), with an at least one of two switching states (s1, s2) that can be changed by the switching bridge (12) has a bridging time (t_u) to avoid a short circuit and the phase current is measured by using the current measuring element (14) to measure at least one of a motor phase (U, V, W) uniquely assigned phase current (I_U, I_V, I_W) is detected at a calculated sampling time (t_i) during the bridging time (t_u). The invention also relates to a control circuit (10).

Description

The invention relates to a method according to claim 1. The invention also relates to a control circuit.

For example, a phase current measurement method is from EP 1 347 567 A1 known. In it, when a three-phase electric motor is controlled with SVPWM signals, the signals are shifted in order to obtain sufficient measurement time windows for a phase current measurement, because if the time periods for an active voltage vector are very small or non-existent, the phase current measurement cannot be carried out. At least two phase currents of the windings in the electric motor are required to calculate the phase current through all three motor phases.

The object of the present invention is to detect the phase current of an electric motor more precisely and reliably and to increase the performance and efficiency of the electric motor. Furthermore, the control circuit for activating the electric motor and for measuring the phase current should be implemented in a cost-effective manner and with as few components as possible. In particular, the phase current, preferably the entire phase current, is to be measured with a single, central current measuring element.

At least one of these objects is achieved by a method for measuring the phase current of an electric motor using a current measuring element, which is arranged between a switching bridge with at least one switchable switching element for controlling a switching state influencing a phase current of a motor phase of the electric motor and a voltage source connected to the switching bridge for feeding the phase current , wherein a bridging time to avoid a short circuit is adjacent to at least one of two switching states that can be changed by the switching bridge and the phase current measurement is carried out by using the current measuring element to detect at least one phase current clearly assigned to a motor phase at a calculated sampling time during the bridging time.

As a result, the phase current of an electric motor can be measured more accurately and reliably, and at the same time the performance and efficiency of the electric motor can be increased. Knowing the at least one measured phase current can enable more precise regulation of the electric motor via the control circuit. A change in the motor control for a phase current measurement can be omitted. The phase current measurement is preferably carried out with the sampling time during the bridging time if a phase current measurement cannot be carried out outside of the bridging time, in particular during the respective switching state. During the bridging time, precisely one phase current that is uniquely assigned to a motor phase can preferably be measured.

The electric motor can be a brushless direct current motor, in particular a BLDC motor. The electric motor can have several motor phases, preferably three motor phases U, V, W. The electric motor can act as a drive in an actuator. The electric motor can be effective in a vehicle, in particular in a motor vehicle.

The switching bridge can be a B6 bridge. The switching bridge can be an inverter. The switching bridge can have six switching elements, in particular for the first motor phase U a first high-side switching element S1.h and a first low-side switching element S1.I, for the second motor phase V a second high-side switching element S2.h and a second low-side switching element S2.I and for the third motor phase a third high-side switching element S3.h and a third low-side switching element S3.I. At least one of the switching elements S preferably consists of a transistor T with a defined forward direction and preferably a diode D, in particular an antiparallel body diode.

The switching bridge for controlling the at least one phase current can be activated via a PWM signal with an activation frequency. The phase bridge is preferably controlled via the voltage space vector modulation using modulation factors that define the voltage space vector during a PWM period. The control can take place, for example, by a flat-bottom control, a flat-top control or a THI control (Third Harmonic Injection).

The phase current is preferably dependent on a switch position of the at least one switching element. Which phase current can be measured via the current measuring element can depend on the switch positions of the switching elements in the switching bridge.

Within a switching period, in particular a PWM period, two of three phase currents of the three-phase electric motor can be measured via the current measuring element. In particular, one of these phase currents has a positive polarity and the other has a negative polarity. Knowing the two phase currents measured via the current measuring element, the third phase current can be calculated, for example using Kirchhoff's law.

If, for example, exactly one high-side switching element of a motor phase is switched on while the associated low-side switching element is open and the other two high-side switching elements of the other two motor phases are open and the two associated low-side switching elements are closed, the phase current of the motor phase with the closed Highside switching element are clearly measured via a phase current measurement with the current measuring element.

During a transition between two defined switching states, for example U, V, W = 0, 1, 0 to U, V, W = 0, 1, 1, a transitional switching can take place when switching over the motor phase W, in which both switching elements of the motor phase W open are, i.e. where S3.h and S3.I are open here. The duration of the transitional shift is preferably the bridging time (dead time). In the case of a current measurement via the current measuring element, the bridging time can be longer than a measurement time required for the phase current measurement. The bridging time thus reduces the possibility of a current measurement in the regular switching state. If the duration of the switching state is already short, this can make it impossible to measure the current via the current measuring element during the regular switching state.

The current sensing element may include a shunt resistor. The current to be measured present at the current measuring element can be calculated via a voltage drop across the shunt resistor.

In a preferred embodiment of the invention, it is advantageous if the phase current measurement via the current measuring element detects a first phase current uniquely assigned to at least one motor phase and a second phase current uniquely assigned to a further motor phase, and one of the measured phase currents during one of the two switching states and the other measured phase current is measured during the bridging time. As a result, the clearly assigned phase current can also be measured when a regular switching state does not allow a phase current measurement, for example due to the short switching duration. The first phase current can be positive and the second phase current can be negative or vice versa.

• Der bei einem positiven Abtastzeitpunkt gemessene Phasenstrom: I_p = I(t_i,p)
• Der bei einem negativen Abtastzeitpunkt gemessene Phasenstrom: I_n = -I(t_i,n)

In a special embodiment of the invention, it is advantageous if the polarity of the phase current to be measured is determined and assigned to it before the first and second phase currents are measured. The phase current measured during the at least one sampling instant can be determined directly as a positive or negative phase current, which means that the setting of the measuring point performs an implicit logical assignment of the polarity of the phase current at this point, which reduces the logic effort that actually has to be implemented. It can apply:

• The phase current measured at a positive sampling time: I _p = I(t _i,p )
• The phase current measured at a negative sampling time: I _n = -I(t _i,n )

$$\left(S1.h,S2.h,S3.h\right)=\left(\mathrm{0,1,0}\right)\to I_V=I_p$$

$$\left(S1.h,S2.h,S3.h\right)=\left(\mathrm{0,0,1}\right)\to I_W=I_p$$

This assignment can be used for the measurements within or outside the bridging time. As a result, the two measured values of the first and second phase currents, which have different polarities, can be assigned to the correct phase separately from one another. In particular when assigning the positive phase current, it is necessary that the corresponding motor phase is the only one that is currently connected to the positive potential of the DC circuit, i.e. only its high-side switching element switches through, so that one of the three following assignments applies: $$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{1.0.0}\right)\to I_u=I_p$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{0.1.0}\right)\to I_V=I_p$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{0.0.1}\right)\to I_W=I_p$$

In a complementary operation of the switching bridge, the condition that a switching element of a respective motor phase should be active, ie for example S1.h=1, also covers the condition that the corresponding other switching element of this motor phase must be inactive, ie for example S1.I = 0 to prevent a short circuit in the DC circuit, for example via the bridging time involved in the control of the switching bridge. Therefore, an explicit request for the status of the other switching element of a respective motor phase is unnecessary.

The query for the status of the low-side switching elements of the other two motor phases, for example whether they are connected to ground, for example S2.I = 1, or whether they are open during the bridging time t_u, such as S3.I = 0 be expendable. If both motor phases are switched and connected to ground, there is a regular switching state (first switching state, second switching state) and the phase current can be clearly assigned to a motor phase via the phase current measurement with the current measuring element. If one of the other two motor phases is open, for example during the bridging time, sampling can take place during the bridging time t_u. In this case, the positive phase current can be measured if the positive sampling time falls within the bridging time t_u is placed. The motor phase whose high-side switching element is active is the only one through which this measurable positive phase current can flow, which means that the assignment of the phase current with regard to this motor phase is unambiguous.

$$\left(S1.h,S2.h,S3.h\right)=\left(\mathrm{0,1,0}\right)\to I_V=I_n$$

$$\left(S1.h,S2.h,S3.h\right)=\left(\mathrm{0,0,1}\right)\to I_w=I_n$$

The condition required when assigning the negative phase current is that the corresponding motor phase is the only one that is currently connected to the negative potential of the DC circuit, i.e. to ground, i.e. only its low-side switching element switches through, so that one of the following three assignments applies: $$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{1.0.0}\right)\to I_u=I_p$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{0.1.0}\right)\to I_V=I_n$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{0.0.1}\right)\to I_w=I_n$$

Querying the status of the high-side switching elements of the other two motor phases, for example whether they are connected to the positive supply potential, for example S2.h = 1 or whether they are open during the bridging time t_u, for example S3.h = 0, can be dispensable with a complementary operation of the switching bridge. If both motor phases are switched and connected to the positive supply potential, there is a regular switching state (first switching state, second switching state) and the phase current can be clearly assigned to a motor phase via the phase current measurement with the current measuring element. If one of the other two motor phases is open, for example during the bridging time, sampling can take place during the bridging time t_u. The negative phase current can be measured if the negative sampling time is placed in the bridging time t_u. The motor phase whose low-side switching element is active is the only one through which this measurable negative phase current can flow, which means that the assignment of the phase current with regard to this motor phase is unambiguous.

In addition to extending the measurement time window during a PWM period, this also simplifies the assignment logic when measuring the phase current with the current measuring element.

A preferred embodiment of the invention provides that a first sampling time for measuring the first phase current uniquely assigned to a motor phase and a second sampling time for measuring the second phase current uniquely assigned to another motor phase are calculated before the measurement is carried out. This allows the first and second phase currents to be measured more accurately.

A preferred embodiment of the invention is advantageous, in which the first and second phase currents are both measured during a switching period of a control cycle, which has a control frequency, of the control of the switching bridge. As a result, the accuracy of the control of the electric motor can be increased. The electric motor can be more powerful and efficient.

In a preferred embodiment of the invention, provision is made for the calculation of the at least one sampling instant to be dependent on a condition in which modulation factors used for driving the switching bridge via voltage space vector modulation are taken into account. As a result, the sampling time for an unambiguous phase current measurement can be precisely defined or it can also be determined in advance whether a phase current measurement is possible for the unambiguous assignment of a phase current during the relevant switching period.

In a specific embodiment of the invention, it is advantageous if the calculation of the at least one sampling time is dependent on a previously calculated polarity of the phase current to be measured. As a result, the sampling time can be precisely defined in order to optimally measure the phase current.

A preferred embodiment of the invention is advantageous in which the current measuring element is arranged between the voltage source and the switching bridge and thus measures an electric current flowing between the voltage source and the switching bridge. As a result, both the entire operating current supplying the electric motor via the voltage source and the individual phase current clearly assigned to a motor phase can be measured with the individual current measuring element.

In an advantageous embodiment of the invention, it is provided that the current measuring element is arranged in the ground path between the voltage source and the switching bridge. The current measuring element can also be arranged in the supply line between the voltage source and the switching bridge. Only a single current measuring element is preferably arranged for phase current measurement.

Furthermore, at least one of the above-mentioned objects is achieved by a control circuit for controlling an electric motor and for measuring the phase current of the electric motor, having a switching bridge which can be connected to a voltage source and having at least one switchable switching element for controlling a switching state influencing a phase current of the electric motor and a current measuring element which is connected between the Switching bridge and the voltage source feeding the phase current is arranged and which is set up to carry out a phase current measurement with a method described above. The control circuit for driving the electric motor and for measuring the phase current can be implemented inexpensively and with as few components as possible, and an accurate phase current measurement can nevertheless be carried out.

Further advantages and advantageous configurations of the invention result from the description of the figures and the illustrations.

character list

• 1: Eine Steuerschaltung zur Ansteuerung eines Elektromotors in einer speziellen Ausführungsform der Erfindung.
• 2: Einen zeitlichen Ablauf von Schaltzuständen während eines Ansteuerungszyklus bei der Ansteuerung des Elektromotors in einer speziellen Ausführungsform der Erfindung.
• 3: Eine Schaltbrücke mit eingezeichnetem Strompfad bei einem ersten Schaltzustand.
• 4: Eine Schaltbrücke mit eingezeichnetem Strompfad bei einem Umschalten der W-Motorphase in einem ersten Zustand.
• 5: Eine Schaltbrücke mit eingezeichnetem Strompfad bei einem Umschalten der W-Motorphase in einem zweiten Zustand.
• 6: Ein Verfahren zur Phasenstrommessung eines Elektromotors in einer speziellen Ausführungsform der Erfindung.

The invention is described in detail below with reference to the figures. They show in detail:

• 1 : A control circuit for driving an electric motor in a special embodiment of the invention.
• 2 : A time sequence of switching states during a control cycle when controlling the electric motor in a special embodiment of the invention.
• 3 : A switching bridge with a drawn current path in a first switching state.
• 4 : A switching bridge with a drawn current path when switching the W motor phase in a first state.
• 5 : A jumper with a drawn current path when switching the W motor phase in a second state.
• 6 : A method for phase current measurement of an electric motor in a special embodiment of the invention.

1 shows a control circuit 10 for driving an electric motor M in a specific embodiment of the invention. The control circuit 10 has a switching bridge 12 and causes the control of a three-phase electric motor M, in particular, by voltage space vector modulation. The switching bridge 12 is preferably designed as a B6 bridge. These include a total of six switching elements S, a first high-side switching element S1.h and a first low-side switching element S1.I, which are assigned to the first motor phase U of the electric motor M, a second high-side switching element S2.h and a second low-side switching element S2.I, which are assigned to the second motor phase V of the electric motor M and also a third high-side switching element S3.h and a third low-side switching element S3.I, which are assigned to the third motor phase W of the electric motor M. These switching elements S preferably each consist of a transistor T with a defined forward direction and preferably a diode D, in particular an antiparallel body diode.

The switching bridge 12 is connected to a voltage source, in particular a DC voltage source with the supply voltage V_DC, which supplies the three motor phases U, V, W of the electric motor M, which is preferably designed as a BLDC motor, with electrical energy. A current measuring element 14, in particular a shunt resistor, is connected in the ground path between the switching bridge 12 and the voltage source and enables a phase current measurement.

2 shows a time sequence of switching states during a control cycle when controlling the electric motor in a special embodiment of the invention. The electric motor can be controlled in particular via a flat-top control, a flat-bottom control or a THI control (Third Harmonic Injection).

For example, in the case of a flat-bottom control, the time sequence of the switching states as shown here results within a control cycle, in particular a PWM period T_PWM in the case of a voltage space vector modulation. The control cycle begins, for example, with an introductory switching state s0, in which the motor phases U, V, W of the electric motor are short-circuited via the transistors of the first, second and third low-side switching elements S1.I, S2.1, S3.I and no direct current im Ground path of the control circuit can be measured. The logic state of the switching elements is therefore S1.h, S1.I, S2.h, S2.I, S3.h, S3.I = 0, 1, 0, 1, 0, 1.

This is followed by a first bridging time t_u_1, during which the high-side switching elements S1.h, S2.h, S3.h are not conducting, i.e. neither the transistors nor the diodes, and during a transition to a positive phase current in one of the motor phases U, V, W lies. Before the second high-side switching element S2.h is switched to logic 1, the second low-side switching element S2.I is first switched to 0 in order to prevent a short circuit across the second switching elements S2.I, S2.h. No direct current can be measured in the ground path during the first bridging time t_u_1 since the direct current circuit is still interrupted.

This is followed by a first switching state s1, in which one of the high-side switching elements, here S2.h, is switched on via the associated transistor, while the other two high-side switching elements S1.h, S3.h remain unswitched, ie logically at 0. During the first switching state s1, according to Table 1 below, the acc whose phase current can be clearly assigned to a motor phase U, V, W, in this example motor phase V. In Table 1, the first column is the switch position of the motor phase, the second column is the switch position of the switching elements and the third column is the measurable unambiguous phase current of the respective motor phase. Table 1 AND MANY MORE S1.h, S1.1, S2.h, S2.I, S3.h, S3.I I 0, 0, 0 0, 1, 0, 1, 0, 1 0, 0, 1 0, 1, 0, 1, 1, 0 I_W 0, 1, 0 0, 1, 1, 0, 0, 1 I_V 0, 1, 1 0, 1, 1, 0, 1, 0 -I_U 1, 0, 0 1, 0, 0, 1, 0, 1 I_U 1, 0, 1 1, 0, 0, 1, 1, 0 -I_V 1, 1, 0 1, 0, 1, 0, 0, 1 -I_W 1, 1, 1 1, 0, 1, 0, 1, 0

The measured phase current in the current measuring element corresponds to a positive phase current of that motor phase whose high-side switching element is switched on.

Then there is a second bridging time t_u_2, during which a second motor phase, here W, changes its state and during which the associated high-side switching element, here S3.h and low-side switching element, here S3.I is not switched on. During this second bridging time t_u_2, a phase current can basically be measured via the current measuring element, the phase-correct assignment of which will be explained in more detail later.

The first half of the PWM period T_PWM (with symmetrical PWM control) concludes with a second switching state s2, during which exactly two motor phases, here V, W, are connected to the positive potential of the DC circuit, i.e. have a logical 1, for example either U , V, W = 1, 1, 0 or U, V, W = 1, 0, 1 or U, V, W = 0, 1, 1. According to Table 1, there is already an in-phase assignment of the measured phase current for these states known and basically a negative phase current, here -i_U measurable. The second half of the PWM period runs in reverse order, starting from the second switching state s2 to the introductory switching state s0, in which U, V, W = 0, 0, 0 applies.

The current path through the jumper is for a clear switching state, for example S1.h, S1.I, S2.h, S2.I, S3.h, S3.I = 0, 1, 1, 0, 0, 1, corresponding to U , V, W = 0, 1, 0 are precisely defined by the switching states of the six switching elements, so that the current always flows through one of the two complementary transistors of a motor phase U, V, W. In the example mentioned, the current in the first switching state s1 flows through the first low-side switching element S1.I, the second high-side switching element S2.h and the third low-side switching element S3.I. In these switching states, the clear assignments of the measured phase current from Table 1 apply; in the example mentioned, the positive current through the V motor phase, corresponding to the current through the second high-side switching element S2.h, can be measured as phase current at the current measuring element.

3 shows a switching bridge with drawn current path in a first switching state. The current paths are shown as an example for the first switching state, in which the motor phase V with the second high-side switching element S2.h and the first low-side switching element S1.I and the third low-side switching element S3.I are switched on, corresponding to U, V , W = 0, 1.0 for the motor phases. The positive current I_V flows into the electric motor M and as a negative current I_U and I_W via the ground path.

• Bei Flat-Bottom-Ansteuerung:
  • Der positive Phasenstrom ist messbar wenn gilt: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (Bedingung C_1_fb_p)
  • Der negative Phasenstrom ist messbar, wenn gilt: m_mid >= (t_u + t_m) / T_PWM (Bedingung C_1_fb_n)
• Bei Flat-Top-Ansteuerung:
  • Der negative Phasenstrom ist messbar, wenn gilt: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (Bedingung C_1_ft_n)
  • Der positive Phasenstrom ist messbar, wenn gilt: m_mid >= (t_u + t_m) / T_PWM (Bedingung C_1_ft_p)
• Bei THI-Ansteuerung:
  • Der positive Phasenstrom ist messbar, wenn gilt: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (Bedingung C_1_thi_p)
  • Der negative Phasenstrom ist messbar, wenn gilt: m_mid - m_min >= 2 * (t_u + t_m) / T_PWM (Bedingung C_1_thi_n)

The phase-correct assignment during the second bridging time is required if it is not possible to set a measuring point during the first or second switching state, for example if the corresponding switching state is present for too short a time. Whether the measurement time windows possible in the first and second switching state are each sufficient for a practical current measurement can be determined for a symmetrical PWM control as a function of the control method. This requires the consideration of modulation factors of the voltage space vector modulation, which are used when driving the motor phases U, V, W. The modulation factors assigned to the motor phases are divided according to size into m_max, m_mid and m_min for a set PWM control. With the period duration T_PWM, the bridging time t_u and the required measuring time t_m for the current measurement, the condition for a practical current measurement in the first and second switching state can be defined as follows depending on the control method used:

• With flat bottom control:
  • The positive phase current can be measured if: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (condition C_1_fb_p)
  • The negative phase current can be measured if: m_mid >= (t_u + t_m) / T_PWM (condition C_1_fb_n)
• With flat-top control:
  • The negative phase current can be measured if: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (condition C_1_ft_n)
  • The positive phase current can be measured if: m_mid >= (t_u + t_m) / T_PWM (condition C_1_ft_p)
• With THI control:
  • The positive phase current can be measured if: m_max - m_mid >= 2 * (t_u + t_m) / T_PWM (condition C_1_thi_p)
  • The negative phase current can be measured if: m_mid - m_min >= 2 * (t_u + t_m) / T_PWM (condition C_1_thi_n)

• Bei Flat-Bottom-Ansteuerung:
  • Der Abtastzeitpunkt für den negativen Phasenstrom: t_i_n = (T_PWM + t_u + t_m) / 2
  • Der Abtastzeitpunkt für den positiven Phasenstrom: t_i_p = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
• Bei Flat-Top-Ansteuerung:
  • Der Abtastzeitpunkt für den positiven Phasenstrom: t_i_p = (T_PWM + t_u + t_m) / 2
  • Der Abtastzeitpunkt für den negativen Phasenstrom: t_i_n = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
• Bei THI-Ansteuerung:
  • Der Abtastzeitpunkt für den positiven Phasenstrom: t_i_p = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
  • Der Abtastzeitpunkt für den negativen Phasenstrom: t_i_n = (1 - (m_mid + m_min) / 2) . T_PWM / 2 + (t_m + t_u) / 2

If both conditions are met with regard to one of the control methods, two sampling times can be set for the phase current during the measurement window in the first and second switching state and assigned to the correct phase according to Table 1. The sampling times t_i_p, t_i_n for the positive and the negative phase current can be calculated analytically for this case as follows:

• With flat bottom control:
  • The sampling time for the negative phase current: t_i_n = (T_PWM + t_u + t_m) / 2
  • The sampling time for the positive phase current: t_i_p = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
• With flat-top control:
  • The sampling time for the positive phase current: t_i_p = (T_PWM + t_u + t_m) / 2
  • The sampling time for the negative phase current: t_i_n = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
• With THI control:
  • The sampling time for the positive phase current: t_i_p = (1 - (m_max + m_mid) / 2) . T_PWM / 2 + (t_m + t_u) / 2
  • The sampling time for the negative phase current: t_i_n = (1 - (m_mid + m_min) / 2) . T_PWM / 2 + (t_m + t_u) / 2

If at least one condition C_1 set up for one of the control methods is not met, at least one of the two phase currents measured during a PWM period cannot be assigned with the correct phase according to the table given above, since there is no clear switching state at the sampling time (first switching state, second switching state ) exists.

• Bei Flat-Bottom-Ansteuerung:
  • Der positive Phasenstrom ist messbar, wenn gilt: m_max - m_mid >= 2 * t_m / T_PWM (Bedingung C_2_fb_p)
  • Der negative Phasenstrom ist messbar, wenn gilt: m_mid >= t_m / T_PWM (Bedingung C_2_fb_n)
• Bei Flat-Top-Ansteuerung:
  • Der negative Phasenstrom ist messbar, wenn gilt: m_max - m_mid >= 2 * t_m / T_PWM (Bedingung C_2_ft_n)
  • Der positive Phasenstrom ist messbar, wenn gilt: m_mid >= t_m / T_PWM (Bedingung C_2_ft_p)
• Bei THI-Ansteuerung:
  • Der positive Phasenstrom ist messbar, wenn gilt: m_max - m_mid >= 2 * t_m / T_PWM (Bedingung C_2_thi_p)
  • Der negative Phasenstrom ist messbar, wenn gilt: m_mid - m_min >= 2 * t_m / T_PWM (Bedingung C_2_thi_n)
  • kann der nicht im regulären Messfenster messbare Phasenstrom während der zweiten Überbrückungszeit als Phasenstrom gemessen und phasenrichtig zugeordnet werden. Dazu ist der jeweilige Abtastzeitpunkt t_i anzupassen und wie folgt festzulegen, damit dieser in die zweite Überbrückungszeit fällt:
• Bei Flat-Bottom-Ansteuerung:
  • Der Abtastzeitpunkt für den negativen Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_n = (T_PWM + t_m) / 2
  • Der Abtastzeitpunkt für den positiven Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_p = (1 - (m_max + m_mid) / 2) · T_PWM / 2 + t_u + t_m/2
• Bei Flat-Top-Ansteuerung:
  • Der Abtastzeitpunkt für den positiven Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_p = (T_PWM + t_m) / 2
  • Der Abtastzeitpunkt für den negativen Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_n = (1 - (m_max + m_mid) / 2) · T_PWM / 2 + t_u + t_m/2
• Bei THI-Ansteuerung:
  • Der Abtastzeitpunkt für den positiven Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_p = (1 - (m_max + m_mid) / 2) · T_PWM / 2 + t_u + t_m/2
  • Der Abtastzeitpunkt für den negativen Phasenstrom, falls dieser während der zweiten Überbrückungszeit gemessen wird: t_i_n = (1 - (m_mid + m_min) / 2) · T_PWM / 2 + t_m / 2

However, if exactly one of the two conditions C_1 specified above is met with regard to one of the control methods and the other is not met, for example if C_1_fb_n is not met, but C_1_fb_p is met and both of the following conditions are still met for each control method:

• With flat bottom control:
  • The positive phase current can be measured if: m_max - m_mid >= 2 * t_m / T_PWM (condition C_2_fb_p)
  • The negative phase current can be measured if: m_mid >= t_m / T_PWM (condition C_2_fb_n)
• With flat-top control:
  • The negative phase current can be measured if: m_max - m_mid >= 2 * t_m / T_PWM (condition C_2_ft_n)
  • The positive phase current can be measured if: m_mid >= t_m / T_PWM (condition C_2_ft_p)
• With THI control:
  • The positive phase current can be measured if: m_max - m_mid >= 2 * t_m / T_PWM (condition C_2_thi_p)
  • The negative phase current can be measured if: m_mid - m_min >= 2 * t_m / T_PWM (condition C_2_thi_n)
  • the phase current that cannot be measured in the regular measurement window can be measured as phase current during the second bridging time and assigned to the correct phase. For this purpose, the respective sampling time t_i must be adjusted and defined as follows so that it falls within the second bridging time:
• With flat bottom control:
  • The sampling time for the negative phase current if this is measured during the second bridging time: t_i_n = (T_PWM + t_m) / 2
  • The sampling time for the positive phase current if this is measured during the second bridging time: t_i_p = (1 - (m_max + m_mid) / 2) T_PWM / 2 + t_u + t_m/2
• With flat-top control:
  • The sampling time for the positive phase current if this is measured during the second bridging time: t_i_p = (T_PWM + t_m) / 2
  • The sampling time for the negative phase current, if this occurs during the second over bridging time is measured: t_i_n = (1 - (m_max + m_mid) / 2) T_PWM / 2 + t_u + t_m/2
• With THI control:
  • The sampling time for the positive phase current if this is measured during the second bridging time: t_i_p = (1 - (m_max + m_mid) / 2) T_PWM / 2 + t_u + t_m/2
  • The sampling time for the negative phase current if this is measured during the second bridging time: t_i_n = (1 - (m_mid + m_min) / 2) T_PWM / 2 + t_m / 2

The sampling time of the other of the two phase currents to be measured, which is not (and should not) be measured during the second bridging time, is to be calculated according to the previously specified relationship for calculating the respective sampling time t_1.

If precisely one of the conditions C_2 is also not met, only one of the two possible phase currents can be measured in this PWM period. The two remaining phase currents can be determined, for example, with an extrapolation.

If all conditions are not met with regard to one of the control methods, no phase current measurement is possible at all during the PWM period, which means that control can take place instead of current regulation.

• Erster Zustand: der negative zu messende Phasenstrom ist betragsmäßig größer als der positive zu messende Phasenstrom. Dann ist der dritte nicht messbare Phasenstrom positiv.

If the phase current is sampled in the second bridging time, it must be assigned to the correct phase. Two fundamentally different possible states can be defined:

• First state: the negative phase current to be measured is greater in absolute value than the positive phase current to be measured. Then the third non-measurable phase current is positive.

Second state: the positive phase current to be measured is larger than the negative phase current to be measured. Then the third non-measurable phase current is negative.

4 shows a switching bridge with a drawn current path when the W motor phase is switched over in a first state. The diode of the third low-side switching element S3.I lets through a current that is transmitted as a positive phase current I_W of the third motor phase. The current introduced via the second high-side switching element S2.h is transmitted via the motor phase V as a positive phase current I_V and is superimposed on the positive phase current I_W. The current flow takes place via the first motor phase U as a negative phase current -I_U, which in turn is partially derived via the ground line, while part of it returns to the third switching element. This reduces the direct current flowing back through the ground resistance to the direct voltage source V_DC and corresponds to the positive phase current I_V, which is smaller in absolute value than the negative phase current I_U and is conducted through the closed second high-side switching element S_2_h to the motor phase V and is measured via the current measuring element 14 .

5 shows a switching bridge with a drawn current path when switching the W motor phase in a second state. In this case, the phase current of the motor phase W is negative and since both transistors of the motor phase W are blocking, the phase current I_W must flow through the diode of the third high-side switching element S3.h, since the diode of the third low-side switching element S3.I restricts the current flow into this blocks direction. The phase current I_W is thus added to the direct current flowing in from the direct voltage source V_DC and forms the positive phase current I_V, which flows into the motor M through the closed transistor of the second high-side switching element S2.h. This reduces the direct current flowing in from the direct voltage source V_DC and corresponds in terms of amount to the negative phase current I_U, which flows out of the motor phase U through the closed transistor of the first low-side switching element S1.I and can be measured in the ground path by the current measuring element 14.

The non-measurable phase current I_W reduces the direct current coming from the direct voltage source V_DC and flowing through the current measuring element 14, whereby the smaller of the two measurable phase currents can be measured as phase current during the second bridging time.

6 shows a method 100 for phase current measurement of an electric motor in a special embodiment of the invention. The method 100 for phase current measurement enables three phase currents to be assigned in the correct phase using two measured phase currents within a control cycle, preferably in a PWM period, for example in the case of a flat bottom control.

The method steps specified below can be carried out for each PWM period. Beginning with the start 102 of the PWM period, in a first step 104 the modulation factors m _U , m _V , m _W of the three motor phases U, V, W, depending on the control method, are sorted according to size in mm _min , m _mid , mmax with m _min ≤ m _mid ≤ m _max

Then, in a further step 106, the minimum necessary time windows t_min, ie the minimum necessary switching state times for unambiguous measurements of the phase current via the current measuring element, are defined depending on whether a positive or a negative phase current is to be measured.

• positive Phasenstrom: t_p,min,1 = 2 · (t_u + t_m)
• negative Phasenstrom: t_n,min,1 = t_u + t_m

und für Messungen während der zweiten Überbrückungszeit:

• positive Phasenstrom: t_p,min,2 = 2 · t_m
• negativer Phasenstrom: t_n,min,2 = t_m

These are for measurements during the regular measurement window, i.e. during the first or second switching state with the bridging time t_u and the measurement time t_m:

• positive phase current: t _p,min,1 = 2 (t _u + t _m )
• negative phase current: t _n,min,1 = t _u + t _m

and for measurements during the second bridging time:

• positive phase current: t _p,min,2 = 2 t _m
• negative phase current: t _n,min,2 = t _m

und $$m_{mid}-m_{min}\ge \frac{t_{n,min\mathrm{,1}}}{T_{PWM}}$$

In a further step 108, both regular measurement windows (for the positive and negative phase current) are checked according to the following condition: $$m_{max}-m_{mi\mathrm{i.e}}\ge \frac{t_{p,min\mathrm{,1}}}{T_{PWM}}$$

and $$m_{mi\mathrm{i.e}}-m_{min}\ge \frac{t_{n,min\mathrm{,1}}}{T_{PWM}}$$

(Condition C_3_1)

und ein Abtastzeitpunkt t_i,n für den negativen Phasenstrom $$t_{i,n}=\frac{T_{PWM}+t_u+t_m}{2}$$

festgelegt, die beide in den regulären Messfenstern liegen.If both measurement windows t _p,min,1 and t _n,min,1 are sufficient, ie condition (C_3_1) is met, in a further step 110 a sampling time t _i,p for the positive phase current $$t_{i,p}=\left(1-\frac{m_{max}+m_{mi\mathrm{i.e}}}{2}\right)\cdot \frac{T_{\mathrm{<figure-callout\; id="2"\; label="P\; W\; M"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">P</figure-callout>}WM}}{2}+\frac{t_m+t_{\mathrm{and}}}{2}$$

and a sampling time t _i,n for the negative phase current $$t_{i,n}=\frac{T_{PWM}+t_{\mathrm{and}}+{\mathrm{<figure-callout\; id="2"\; label="t\; m"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">t</figure-callout>}}_m}{2}$$

fixed, both of which are within the regular measurement windows.

und $$m_{mid}-m_{min}\ge \frac{t_{n,min\mathrm{,2}}}{T_{PWM}}$$

If at least one of the two measurement windows is not sufficient, i.e. if condition (C_3_1) is not met, it is also checked in step 112 whether the measurement window for the positive phase current is long enough for the determination in the first switching state and the measurement window for the negative one Phase current is long enough for assignment during the bridging time t_u. The following condition is queried: $$m_{max}-m_{mi\mathrm{i.e}}\ge \frac{t_{p,min\mathrm{,1}}}{T_{PWM}}$$

and $$m_{mi\mathrm{i.e}}-m_{min}\ge \frac{t_{n,min\mathrm{,2}}}{T_{PWM}}$$

(Condition C_3_2)

If applicable, in step 114 the sampling time for the positive phase current is determined according to 1.1 and for the negative phase current during the bridging time as follows: $$t_{in}=\frac{T_{PWM}+{\mathrm{<figure-callout\; id="2"\; label="t\; m"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">t</figure-callout>}}_m}{2}$$

und $$m_{mid}-m_{min}\ge \frac{t_{n,min\mathrm{,1}}}{T_{PWM}}$$

If condition C_3_2 does not apply, a check is also carried out in step 116 to determine whether the measurement window for the negative phase current is long enough for the assignment in the second switching state and the measurement window for the positive phase current is long enough for the assignment during the bridging time t_u. The following condition is queried: $$m_{max}-m_{mi\mathrm{i.e}}\ge \frac{t_{p,min\mathrm{,2}}}{T_{PWM}}$$

and $$m_{mi\mathrm{i.e}}-m_{min}\ge \frac{t_{n,min\mathrm{,1}}}{T_{PWM}}$$

(Condition C_3_3)

If applicable, in step 118 the sampling time for the negative phase current is determined according to 1.2 and for the positive phase current during the bridging time as follows: $$t_{i,p}=\left(1-\frac{m_{max}+m_{mi\mathrm{i.e}}}{2}\right)\cdot \frac{T_{\mathrm{<figure-callout\; id="2"\; label="P\; W\; M"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">P</figure-callout>}WM}}{2}+t_{\mathrm{and}}+\frac{{\mathrm{<figure-callout\; id="2"\; label="t\; m"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">t</figure-callout>}}_m}{2}$$

Die Messung während des negativen Abtastzeitpunktes t_i,n wird als negativer Phasenstrom I_n in dem Schritt 122 wie folgt zugewiesen $$I_n=-\left(t_{i,n}\right)$$

At the defined sampling times t _i,p , t _i,p , the phase current is sampled twice via the current measuring element within a PWM period. The measurement during the positive sample time t _i,p is assigned as the positive phase current I _p in step 120 as follows $$I_p=I\left(t_{i,p}\right)$$

The measurement during the negative sampling time t _i,n is assigned as the negative phase current I _n in step 122 as follows $$I_n=-\left(t_{i,n}\right)$$

As a result, the sign of the phase current is already assigned without an additional determination being necessary for this.

$$\left(S1.h,S2.h,S3.h\right)=\left(\mathrm{0,1,0}\right)\to I_V=I_p$$

$$\left(S1.h,S2.h,S3.l\right)=\left(\mathrm{0,0,1}\right)\to I_W=I_p$$

The positive phase current is then assigned to the correct motor phase U, V, W in step 124, depending on the high-side switching element of this motor phase. The following applies: $$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{1.0.0}\right)\to I_u=I_p$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.H\right)=\left(\mathrm{0.1.0}\right)\to I_V=I_p$$

$$\left(\mathrm{<figure-callout\; id="1"\; label="S"\; filenames="DE102021101884A1\_0029.png,DE102021101884A1\_0030.png"\; state="\{\{state\}\}">S</figure-callout>}1.H,\mathrm{<figure-callout\; id="2"\; label="S"\; filenames="DE102021101884A1\_0029.png"\; state="\{\{state\}\}">S</figure-callout>}2.H,S3.l\right)=\left(\mathrm{0.0.1}\right)\to I_W=I_p$$

$$\left(S1.l,S2.l,S3.l\right)=\left(\mathrm{0,1,0}\right)\to I_V=I_n$$

$$\left(S1.l,S2.l,S3.l\right)=\left(\mathrm{0,0,1}\right)\to I_W=I_n$$

The negative phase current is then assigned to the correct motor phase U, V, W in step 126, depending on the high-side switching element of this motor phase. The following applies: $$\left(S1.l,S2.l,S3.l\right)=\left(\mathrm{1.0.0}\right)\to I_u=I_n$$

$$\left(S1.l,S2.l,S3.l\right)=\left(\mathrm{0.1.0}\right)\to I_V=I_n$$

$$\left(S1.l,S2.l,S3.l\right)=\left(\mathrm{0.0.1}\right)\to I_W=I_n$$

$$I_V=-I_U-I_W$$

$$I_W=-I_U-I_V$$

Finally, in a step 128, the third phase current, which cannot be measured as a phase current in the current PWM period, is calculated according to Kirchhoff's law as the negative sum of the other two phase currents as follows: $$I_u=-I_V-I_W$$

$$I_V=-I_u-I_W$$

$$I_W=-I_u-I_V$$

After completion 130 of the PWM period, the method runs through again.

If the condition is also not met in step 116, step 132 determines that at least one of the phase currents cannot be measured in the first or second switching state or in the bridging time in the current PWM period, since its switching duration is too short. If the other phase current can nevertheless be measured, at least this one phase current can be measured so that the two remaining phase currents can then be extrapolated.

If both measurement windows are too short for measurements of the two phase currents, no phase current can be measured in the current PWM period and all three phase currents are unknown. Then the current regulation can be replaced by a controller.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 1347567 A1 [0002]

Claims (10)

Method (100) for measuring the phase current of an electric motor (M) via a current measuring element (14) which is connected between a switching bridge (12) with at least one switchable switching element (S) for controlling at least one phase current (I_U, I_V, I_W) of a motor phase (U , V, W) influencing the switching state (s1, s2) of the electric motor (M) and a voltage source (V_DC) connected to the switching bridge (12) for feeding the phase current (I_U, I_V, I_W), with at least one of two switching states (s1, s2) that can be changed by the switching bridge (12), a bridging time (t_u) to avoid a short circuit and the phase current measurement is carried out by using the current measuring element (14) to measure at least one phase current ( I_U, I_V, I_W) at a calculated sampling time (t_i) during the bridging time (t_u). Method (100) according to claim 1 , characterized in that the phase current measurement via the current measuring element (14) a first phase current (I_p, I_n) clearly assigned to at least one motor phase (U, V, W) and a second phase current (I_p, I_n) clearly assigned to a further motor phase (U, V, W). I_p, I_n) and one of the measured phase currents (I_p, I_n) is measured during one of the two switching states (s1, s2) and the other measured phase current (I_p, I_n) during the bridging time (t_u). Method (100) according to claim 2 , characterized in that before measuring the first and second phase current (I_p, I_n), the polarity of the phase current to be measured is already determined and assigned to it. Method (100) according to claim 3 , characterized in that a first sampling time (t_i_p, t_i_n) for measuring a motor phase (U, V, W) uniquely assigned first phase current (I_p, I_n) and a second sampling time (t_i_p, t_i_n) for measuring a further motor phase unique associated second phase current (I_p, I_n) is calculated before carrying out the measurement. Method (100) according to claim 3 or 4 , characterized in that the first and second phase current (I_p, I_n) are both measured during a switching period (T_PWM) of a drive cycle having a drive frequency for driving the switching bridge (12). Method (100) according to one of the preceding claims, characterized in that the calculation of the at least one sampling time (t_i_p, t_i_n) depends on a condition (C_1, C_2) in which a voltage space vector modulation is used to control the switching bridge (12). Modulation factors (m_U, m_V, m_W) are taken into account. Method (100) according to one of the preceding claims, characterized in that the calculation of the at least one sampling time (t_i_p, t_i_n) depends on a previously calculated polarity of the phase current (I_p, I_n) to be measured. Method (100) according to one of the preceding claims, characterized in that the current measuring element (14) is arranged between the voltage source (V_DC) and the switching bridge (12) and thus an electric current flowing between the voltage source (V_DC) and the switching bridge (12). measures current. Method (100) according to claim 8 , characterized in that the current measuring element (14) is arranged in the ground path between the voltage source (V_DC) and the switching bridge (12). Control circuit (10) for controlling an electric motor (M) and for measuring the phase current of the electric motor (M), having a switching bridge (12) that can be connected to a voltage source (V_DC) and has at least one switchable switching element (S) for controlling a phase current (I_U, I_V , I_W) of the electric motor (M) influencing the switching state (s1, s2) and a current measuring element (14) which is arranged between the switching bridge (12) and the phase current (I_U, I_V, I_W) and which feeds the voltage source (V_DC). is to carry out a phase current measurement with a method (100) according to one of the preceding claims.

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