Counter-EMF detection circuit

Abstract

The invention relates to a method and a device for detecting the position of the rotor (5) of an electric motor (11) using the opposing electromagnetic force voltages (e a , e b , e c ) of the motor phases (3a, 3b, 3c) of the electric motor (5) by scanning the electromagnetic force voltages (e a , e b , e c ) of the motor phases (3a, 3b, 3c) and detection of a passage of a specific voltage level of the back EMF voltages (e a , e b , e c ), wherein the phases (3a, 3b, 3c) of the electric motor (5) are connected to each other via a common neutral point (12, 15). The motor phases (3a, 3b, 3c) are partially electrically decoupled from the neutral point (12, 15) by electrical decoupling devices (16).

Description

[0001] The invention relates to a method for detecting the position of a rotor of an electric motor. The invention also relates to an electrical circuit capable of detecting the position.

[0002] A rotary position determination system for brushless motors is known from the prior art, in particular from the generic document DE 10 2008 025 442 A1. Furthermore, a method and device for monitoring and controlling brushless motors are known from US Pat. No. 6,128,436 A, and a method and device for driving a brushless DC motor are known from US Pat. No. 5,923,134 A.

[0003] To control a brushless DC motor 11 (BLDCM), it is necessary to know the position of the rotor 5 of the machine 1. The position of the rotor 5 can be found using sensors. This is not practical in a compressor application due to the very harsh environment and also because of the price. Another approach is the sensorless approach. In the compressor industry, the most widely used sensorless position determination method is the "direct BEMF method" (direct back electromagnetic force method), in which the zero crossings of the induced machine voltage are detected by reading the motor feedback signals 2 after Fig. 1.

[0004] Fig. Figure 1 shows the motor feedback signals 2, which are read directly on the machine (motor) phases 3. The feedback signals 2 are processed by the control unit 4 to determine the appropriate commutation sequence to the motor 11.

[0005] With the “direct BEMF procedure”, which is Fig. 1 (BEMF = Back Electromagnetic Force), it is possible to read the position of the rotor 5 through every 60° electrical interval. A model 7 (a representative equivalent circuit) for a BLDCM is shown in Fig. 2, which shows a 3-phase full-bridge inverter 8 and model 7 of a BLDCM. The induced voltages are denoted by e a , e b and e c designated.

[0006] The BLDCM 11 is driven using a 2-phase conduction strategy. This means that ideally two phases 3a, 3b, 3c are conducting at a time. In reality, this is not entirely the case. During commutation between phase sequences, all three phases 3a, 3b, 3c conduct for a short period of time. The reason for this is that magnetic energy (flyback energy) is stored in the machine inductances 9. The "extra" phase 3a, 3b, 3c, which conducts for a short period of time (for the so-called demagnetization period) after commutation, plays an important role. From this phase 3a, 3b, 3c, the position of the rotor 5 is derived using the BEMF voltage (zero crossing of the BEMF). This event is essentially used to determine when phase commutation occurs.

[0007] The following describes the BLDCM drive. The relevant events associated with the drive are: BEMF voltage zero crossing (Z event), commutation (C event), and demagnetization (D event). These events are described below. Fig. Figure 3 shows these electrical events for one electrical revolution and is a rough sketch of some of the relevant signals when driving a BLDCM 11. The solid lines in Fig. 3 shows the back-electromagnetic force voltages of the motor for three phases 3a, 3b, 3c. The dashed lines show the three motor phase currents I a , I b , I c .

[0008] Fig.Figure 3 shows three events relevant to the drive of a BLDCM 11: zero crossing of the BEMF (Z event), commutation (C event), and demagnetization (D event). Furthermore, it can be seen that the phase BEMF voltages e a , e b , e c and the phase currents I a , I b , I c Repeat electrically after 360°. Fig. Figure 3 also shows that the three previously mentioned events (i.e., the Z event, C event, and D event) are electrically present every 60°. Driving the BLDCM with trapezoidal phase BEMF voltages e a , e b , e c is performed using a 6-step procedure, where traditionally two phases 3a, 3b, 3c conduct electrically over 60° each. This 6-step procedure is shown in Table 1. Table 1 Degree electric 60 120 180 240 300 360 Phase A current Positive current injection (+Ia) Negative current injection (-Ia) Negative current injection (-Ia) Positive current injection (+Ia) Phase B current Positive current injection (+Ib) Positive current injection (+Ib) Negative current injection (-Ib) Negative current injection (-Ib) Phase C current Negative current injection (-Ic) Negative current injection (-Ic) Positive current injection (+Ic) Positive current injection (+Ic)

[0009] Table 1 and Fig.3 show that this strategy is based on injecting 120° electrical current pulses into the BLDCM 11.

[0010] Studying phase B, one can see that the first event that takes place is the demagnetization (at the interval between 0° electrical - which is "identical" to 360° electrical - and 60° electrical directly following the previous interval, in particular the previous interval between 240° 360° electrical, where a negative current -I b This completed demagnetization interval occurs due to the energy storage in the inductance 9b. The current sequence goes from (+I a , -I b ) (between 300° and 360° electrical) to (+I a , -I c ) (between 0° = 360° electrical and 60° electrical). This means that at this point no further current I bis fed into phase B. The phase B inductance 9b contains magnetic field energy. This magnetic field energy is released (flyback effect) back into the three-phase full-bridge inverter 8. The end of the demagnetization can be seen as a jump in the motor potential voltages e b on the BLDCM 11, because the energy path is normally through the freewheeling diodes 10 in the three-phase full-bridge inverter 8 and this means that the motor potential voltages e a , e b , e c are initially on the positive or negative DC connecting rail (here, a voltage drop across the freewheeling diodes 10 is omitted from consideration) until the respective phase inductance 9a, 9b, 9c is de-energized. When phase 3a, 3b, 3c is de-energized, the respective motor potential e returns. a , e b , e c back to roughly half the DC rail voltage V dc .

[0011] The next event that is present is the zero crossing of the BEMF voltage e b of phase B (between 0° electrical and 60° electrical). This event can be easily seen on the motor potential e b of phase B, because phase B is de-energized (no current I b Here you can see that the slope of the B BEMF voltage e B increases. The zero point crossing is searched for. This zero point crossing is caused at a certain electrical angle (theoretically at 30° electrical) after the commutation event. This means that the current sequence of (+I a , -I c ) to (+I b , -I c ) etc. The result here is, I a , I b , I c to feed into the phase at the right time to generate optimal motor torque. It should be noted here that the instantaneous power at shaft 5 can be calculated as: PWave=Ia⋅ea+Ib⋅eb+Ic⋅ec

[0012] Wherein • I a , I b and I c is the instantaneous motor phase current (measured in amperes [A]). • e a , e b and e c is the instantaneous voltage per phase (measured in volts [V]).

[0013] It is possible to draw up the following table (i.e. Table 2) which shows a general picture for the control of a BLDC motor 11. Table 2 Signals\Degrees electrical [° electrical] 60 120 180 240 300 360 Phase A BEMF voltage Falling BEMF\ Rising BEMF / Phase B BEMF voltage Rising BEMF / Falling BEMF\ Phase C BEMF voltage Rising BEMF / Falling BEMF\ Phase A current Positive power feed Negative power feed-in Negative power feed-in Positive power feed Phase B current Positive power feed Positive power feed Negative power feed-in Negative power feed-in Phase C current Negative power feed-in Negative power feed-in Positive power feed Positive power feed

[0014] To generate optimal motor torque, it is essential that the events mentioned in Table 2 are properly handled. The two events that must be detected and validated are the demagnetization (D event) and the zero crossing (Z event) of the BEMF. The commutation event (C event) is based on the zero crossing of the BEMF. To detect and validate these events, some external circuitry is required, along with a processor with a built-in comparator (e.g., a control unit 4).

[0015] The detection is handled as a comparison between the motor potential voltages e a , e b , e c and the potential of the motor star point 12, 15. These aspects are described below.

[0016] The following should be remembered: • The 6-step operation is based on a two-phase conduction strategy, ignoring demagnetization. • The de-energized phases 3a, 3b, 3c are used to detect the zero point crossing of the BEMF to ensure optimal torque generation.

[0017] The magnitude of the BEMF stresses e a , e b , e c is machine speed dependent. This means that at low speeds of the motor 11, the BEMF voltages e a , e b , e c are small. Therefore, it is relatively difficult to start a BLDCM 11 due to the lack of BEMF voltages e a , e b , e c (or to be more precise: the BEMF voltages e a , e b , e c are not large enough to be processed with sufficient accuracy by the control unit 4). The motor feedback signals 2 in Fig.1 should be considered very carefully, as this is the key to success in detecting the zero crossing of the BEMF. Some examples of the motor feedback signals 2 are presented below. Examples of motor feedback signals:

[0018] New compressor technology—brushless direct current (BLDC) motors (motors) 11—with considerably higher moments of inertia have since been introduced. This raises some critical challenges. In particular, when using starting methods known in the art, it is necessary to build up machine speed (being able to detect the zero crossing of the BEMF voltage with sufficient accuracy) by running in a stepping mode for a longer period of time (a method for this initial start-up phase of a BLDC 11 is known per se in the art). This is very critical because there is a potential risk of losing the position of the rotor 5 due to the unknown compressor load (or other mechanical load).

[0019] A typical example of a previously used BEMF circuit 13 is shown in Fig. 4. With this circuit 13, which is shown in Fig. As shown in Figure 4, the motor feedback signals e a , e b , e c from phases 3a, 3b, 3c, damped by the voltage dividers 14 (R10 and R13; R11 and R14; R12 and R15). This is not ideal for these high-inertia BLDCMs 11 due to the longer step-mode operation and the possibility of "losing" the position of the rotor 5.

[0020] A possible solution to the challenge of high inertia could be to increase the ground resistances (R13, R14, R15 in Fig. 4 R4, R5, R6 are Fig. 5) and allow the control unit 4 to detect the non-suppressed BEMF signals. This procedure is described in Fig. 5. In this method, the detection of the zero crossing is performed synchronously with the pulse width modulation (PWM) of the three-phase full-bridge inverter 8 during the off-time, where the zero crossing of the BEMF voltage can be referenced to ground.

[0021] The disadvantage is that it is not possible to use 100% PWM, which results in a degradation in the maximum machine speed that could be achieved.

[0022] To get (essentially) 100% PWM at higher speeds, it is necessary to connect the lower resistors to ground (as done with R13, R14, R15 in Fig. 4) and suppress the motor feedback signals. The detection of the zero crossing of the BEMF voltage is then performed synchronously with the PWM during the on-time. The reference at high speeds will then be the actual machine star point 12, which Fig. 4 is generated.

[0023] The interruption / connection of the lower resistors (R4, R5, R6 in Fig. 5) with ground is done via some digital pins on the control unit, compare Fig. 5, which shows a modified BEMF circuit.

[0024] The structure, which in Fig. 5, however, will not work because signal on EMK_A is influenced by the signal level on EMK_B or EMK_C through the resistors (R7, R8, R9 in Fig. 5) used to generate the star point 12. 325 V DC will be present on one of the other motor phases 3a, 3b, 3c and this will significantly disturb the signal intended to be measured.

[0025] One solution could be to increase the resistance to the star point 12 to a level where this disturbance is no longer a problem, but since the upper voltage divider resistors (R1, R2, R3 in Fig. 5) are in the megaohm range, in order to limit losses, the resistors (R7, R8, R9 in Fig. 5) The resistance to star point 12 should be tens of megaohms to limit the influence. If this is done, star point 12 becomes very sensitive to coupled noise, and resistance values ​​in this range are not very stable when exposed to dust and moisture, making star point 12 unreliable.

[0026] Another solution is to create a shifted star point 15, which is Fig. 6 is shown.

[0027] The Fig. The solution shown in Figure 6 will work, but will result in twice as many losses in the BEMF circuit, making it unattractive if highly efficient circuits are required.

[0028] A third solution will be to generate the machine star point (not shown) using the DC bus voltage to create a shifted star point 15. However, this could potentially result in false detections of the BEMF voltage zero crossings.

[0029] Therefore, there is still a need for an improved method for detecting the position of an electric motor rotor. Similarly, there is a need for an improved circuit for detecting the position of an electric motor rotor.

[0030] It is therefore an object of the invention to provide a method for detecting the position of a rotor of an electric motor that is improved over prior art methods. Furthermore, it is an object of the invention to provide an electrical circuit capable of detecting the position of a rotor of an electric motor and that is improved over prior art electrical circuits.

[0031] The invention solves this problem.

[0032] It is proposed to implement a method for detecting the position of a rotor of an electric motor, which method at least partially uses a counter-electromagnetic force voltage from at least one motor phase of the electric motor by sampling at least one counter-electromagnetic force voltage of at least one motor phase and detecting a passage of a specific voltage level of the counter-magnetic force voltage, wherein the phases of the electric motor are connected to one another via a common star in such a way that at least one motor phase is at least temporarily partially electrically decoupled from the star point by at least one electrical decoupling device. The decoupling device is controlled by a microprocessor. Furthermore, the star point is formed from resistors separate from the motor phases of the motor. In principle, the electric motor can have an essentially arbitrary number of motor phases.As an example, it may have two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, or even more motor phases (the typical design being with an even number of motor phases). The sensing of the at least one counter-electromagnetic force voltage of at least one motor phase may be performed by essentially any means known in the art. For example, a voltage measurement across a resistor or the measurement of a magnetic field (strength) and / or a magnetic field (strength) and / or a magnetic field (strength) variation may be envisaged. The sensing may be read out and processed by an analog and / or digital circuit. In particular, digital processing units (such as a single-board computer or the like) may be used. Such devices may also be used to control the current supplied to the electric motor.As is common with electric motors, the different phases of the electric motor are connected to each other via a common star. This does not necessarily mean that they are directly connected, for example by (essentially) short-circuiting the respective lines. Instead, it is possible that resistors or other devices are placed in series with and / or parallel with the respective lines. The star point itself may be earthed (probably by a resistor or the like) and / or it may float (by being isolated from ground and / or by using a resistor with a high resistance value, for example in the range of megohms or even higher). According to the present proposal, at least one phase is at least temporarily partially electrically decoupled from the star point by at least one electrical decoupling device.Electrical decoupling can be accomplished in a wide variety of ways. In particular, it is possible for one direction of electrical current to be favored over the other (an extreme form would be a diode, where the preference is essentially 100%; however, smaller preferences are also possible). It is also possible for a connection (including a connection that favors one direction) to be established only during certain time intervals. Furthermore, it is possible for partial electrical decoupling to be achieved through a non-linear behavior of the electrical coupling (as a means of comparison, a standard electrical resistor should exhibit a linear behavior between the applied voltage and the current passing through the resistor). Non-linear behavior can also involve an amplifying device (including amplifying devices that favor one direction over another).

[0033] While it is in principle sufficient that at least one counter-electromagnetic force split is sampled and / or at least one motor phase is at least temporarily partially electrically decoupled from the star point, it is preferred if a plurality and / or (essentially) all counter-electromagnetic force voltages are sampled or if a plurality and / or (essentially) all motor phases are partially electrically decoupled. As a rule of thumb: the more counter-electromagnetic force voltages are sampled and / or the more motor phases are partially electrically decoupled, the better the position detection. Essentially all voltage levels can be used as the voltage level used for detection. In particular, certain levels are preferred, such as a level (close) to the occurring maximum and / or minimum voltage. Particularly preferably, a "zero point crossing" is detected.The "zero-point crossing" can be a zero ground level and / or a voltage level that is between two potentials, a DC voltage supply, and / or a voltage level that lies between a maximum voltage level and a minimum voltage level. Other potentials are, of course, also possible.

[0034] In particular, the method can be used in such a way that at least one counter-electromotive force voltage is compared with a reference voltage. In this way, a particularly simple embodiment can be envisaged. Reference voltages can normally be generated relatively easily. A comparison can be made using a "zero-difference" comparison. Such a comparison can be particularly precise and / or particularly simple.

[0035] Although it is possible to detect a wide variety of voltage levels, it is preferable to use the method in such a way that a zero crossing of at least one counter-electromagnetic force voltage is detected. Such a zero crossing is typically very significant for determining the position of the rotor. The term "zero crossing" does not necessarily mean a passage of a ground level potential. Instead, particularly if an electric motor is used that is galvanically isolated from the ground level, other potentials may be used. A particularly well-suited embodiment can be obtained if a medium voltage potential level between the two potentials of a power supply (in particular a DC power supply and / or the "extremes" of an AC power supply) is used as a zero level for detecting the zero crossing.

[0036] It is preferred if at least one electrical decoupling device is at least partially designed as a non-linear electrical decoupling device. Initial experiments have shown that such non-linear behavior can be sufficient to improve sensing. In particular, non-linearity can occur by having a non-linear behavior between applied voltage and the resulting current and / or when one direction is preferred over the other. To simply name a few typical electronic components that exhibit non-linear behavior, diodes, Zener diodes, DIACs, TRIACs, IGBTs, transistors, FETs, MOSFETs, operational amplifiers, or the like can be used.

[0037] In particular, it is preferred to carry out the method in such a way that at least one electrical decoupling device is at least partially designed as an amplification device, in particular as an operational amplifier device. In particular, such an amplification device can exhibit non-linear behavior (although a linear amplification behavior, at least over a wide interval), which can also be used to carry out the method. To simply name a few electronic components that exhibit amplification behavior, transistors, FETs, MOSFETs, IGBTs, operational amplifiers, or the like can be used. Experiments have shown that operational amplifiers (or electronic components that have a similar amplification behavior) in particular are particularly well suited for carrying out the presently proposed method.

[0038] It is further proposed that at least one counter electromagnetic force voltage be measured at least at times during a substantially full pulse width modulation sampling cycle. Nowadays (particularly when inverters are used to drive electric motors), electric motors are quite often controlled by pulse width modulation (PWM). Previously proposed methods for detecting the position of a rotor of an electric motor typically have the disadvantage that position detection cannot be performed (with sufficient precision or at all) when the motor is driven with (substantially) 100% pulse width modulation sampling cycle. However, with the method proposed here, this is possible. Therefore, the power and / or torque of the electric motor can be utilized to its full extent, which is particularly advantageous.

[0039] Furthermore, it is proposed to use the method at least temporarily during low-speed operation of the electric motor. This is a further advantage of the presently proposed method, since initial experiments have shown that the measured accuracy of the proposed method is comparatively high, even at low rotational speeds of the electric motor. This is particularly advantageous in combination with the use of a 100% PWM sampling cycle. As a rule of thumb, if the method is used very early, an initial start-up phase can be abbreviated (for example, using a stepper motor behavior), making it possible to shorten the start-up time.

[0040] Furthermore, an electrical circuit device for detecting the position of an electric motor using at least partially a counter-electromagnetic force voltage of at least one motor phase of the electric motor is proposed, wherein the phases of the electric motor are connected to one another by a common star point, which has at least one sampling device for sampling at least one counter-electromagnetic force voltage of at least one motor phase and at least one evaluation device for detecting a passage of a specific voltage level of the counter-electromagnetic force voltage, wherein at least one electrical decoupling device is used, which is used for at least one motor phase and which partially electrically decouples this motor phase from the star point at least temporarily.Using such an electrical circuit device, the features and advantages described above can usually also be achieved, at least in an analogous manner. Furthermore, it is also possible to modify the electrical circuit device in the sense described above, at least in an analogous manner.

[0041] According to the invention, the electrical circuit device is to be modified in such a way that the at least one electrical decoupling device is at least partially designed as an amplification device, in particular as an operational amplifier device. Using such an embodiment, the previously described features and advantages can usually also be utilized, at least in an analogous manner.

[0042] In particular, it is advantageous if the electrical circuit arrangement is "completed" by an electric motor. In particular, the electric motor can be connected to a compressor, which typically results in a particularly advantageous configuration. In particular, it is possible to market a preassembled group comprising an electrical circuit device and / or an electric motor and / or a compressor device as a prefabricated group and / or as a construction kit.

[0043] The present invention and its advantages will become clearer when considering the following description of possible embodiments of the invention, which are described with reference to the attached figures, which show: Fig. 1 a possible embodiment of an electric motor with a control unit in a schematic circuit diagram; Fig. 2 a possible embodiment of an electrical circuit of an electrical inverter for driving a three-phase electric motor; Fig. 3 a possible actuation scheme for a brushless DC motor and the feedback by the electric motor; Fig. 4 shows a first embodiment of an electrical circuit for obtaining a counter-electromagnetic force voltage signal; Fig. 5 shows a second embodiment of an electrical circuit for obtaining a counter-electromagnetic force voltage signal; Fig. 6 shows a third embodiment of an electrical circuit for obtaining a counter-electromagnetic force voltage signal; Fig. 7 shows a fourth embodiment of an electrical circuit for obtaining a counter-electromagnetic force voltage signal;

[0044] Fig. Figure 7 shows an embodiment of an improved circuit 17 for detecting the zero crossing (Z-event) of the BEMF. The proposed circuit 17, which is shown in Fig. 7, has operational amplifiers 16 for partially electrically decoupling the star point 12.

[0045] The Fig. Circuit 17 shown in Figure 7 makes it possible to combine the following. • Minimization of step-mode operation due to the ability to detect the zero crossing (Z event) of the BEMF voltage at low speeds. Detection of the zero crossing of the BEMF voltage at low speeds is critical for operating with high-inertia machines, such as compressors. • Low speed and high speed operation using 100% PWM, which results in no undue degradation in speed performance. • Circuit with low losses due to a non-isolated star point. • Maintaining a correct star point 12 to avoid incorrect detection of the zero crossing of the star point 12.

[0046] In particular, the invention can be considered as follows: A method for detecting a zero crossing of a back EMF, the method comprising: - Sampling of a back EMF voltage of a motor phase during 100% PWM sampling cycle, - comparing the sampled back-EMF voltage with a reference voltage, - Detection of a zero point crossing based on the comparison of the back EMF voltage and the reference voltage and - Decoupling a star point using amplifier, motor phases and the star point.

[0047] Additional information can be found in the applications filed under the reference numbers DE 10 2012 006 492 A1 and DE 10 2012 006 495 A1 on the same date as the present application.

Claims

[1] Method for detecting the position of the rotor (5) of an electric motor (11) which at least partially generates a counter-electromagnetic force voltage (e a , e b , e c ) at least one motor phase (3a, 3b, 3c) of the electric motor (5) used by sensing at least one counter-electromagnetic force voltage (e a , e b , e c ) at least one motor phase (3a, 3b, 3c) and detecting a crossing of a certain voltage level of the counter-electromagnetic force voltage (e a , e b , e c ), wherein the phases (3a, 3b, 3c) of the electric motor (5) are connected to each other by a common star point (12), characterized bythat the star point (12) is formed separately from motor phases of the motor (5) from resistors (R25, R26, R27), wherein at least one motor phase (3a, 3b, 3c) is at least temporarily partially electrically decoupled from the star point (12) by at least one electrical decoupling device (16), wherein the decoupling device (16) is controlled by a microprocessor, wherein the at least one electrical decoupling device (16) is an operational amplifier device, and wherein at least one counter-electromagnetic force voltage (e a , e b , e c ) is measured at least at times during a substantially full pulse width modulation sampling cycle. [2] Method according to claim 1, characterized by that at least one counter-electromagnetic force voltage (e a , e b , e c ) is compared with a reference voltage [3] Method according to claim 1 or 2, characterized bythat a zero point crossing of at least one counter-electromagnetic force voltage (e a , e b , e c ) is recorded. [4] Method according to one of the preceding claims, characterized by that at least one electrical decoupling device (16) is at least partially designed as a non-linear electrical decoupling device (16). [5] Method according to one of the preceding claims, characterized by that the method is used, at least partially, during low speed operation of the electric motor (5). [6] Electrical circuit device (1) for detecting the position of an electric motor (5) using, at least partially, a counter-electromagnetic force voltage (e a , e b , e c) of at least one motor phase (3a, 3b, 3c) of the electric motor (5), wherein the phases (3a, 3b, 3c) of the electric motor (5) are connected to one another by a common star point (12), the at least one scanning device (2) for scanning at least one counter-electromagnetic force voltage (e a , e b , e c) of at least one motor phase (3a, 3b, 3c) and at least one evaluation device (4) for detecting a passage of a specific voltage level of the counter-electromagnetic force voltage, characterized in that the star point (12) is formed separately from motor phases of the motor (5) from resistors (R25, R26, R27), wherein at least one electrical decoupling device (16) is used for at least one motor phase (3a, 3b, 3c) and which partially electrically, at least temporarily, decouples the motor phase (3a, 3b, 3c) from the star point (12), wherein the control of the decoupling device (16) is carried out by a microprocessor, wherein the at least one electrical decoupling device (16) is an operational amplifier device, and wherein at least one counter-electromagnetic force voltage (e a , e b , e c) is measured at least at times during a substantially full pulse width modulation sampling cycle. [7] Electrical circuit device (1) according to claim 6, characterized by that at least one electrical decoupling device is designed, at least partially, as an amplifying device (16), in particular as an operational amplifier device (16).

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