Method and device for identifying a faulty rotor position angle signal

[0001] This application claims the priority of German Patent Application, Serial No. 103 55 423.8, filed Nov. 27, 2003, pursuant to 35 U.S.C. 119(a)–(d).

[0002] The present invention relates to a system and method for identifying a faulty rotor position angle signal of a synchronous motor powered by a converter. The invention also relates to a computer with a computer program stored on a data carrier for carrying out the method and to a machine tool or production machine using the system and method.

[0003] Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.

[0004] Motors of electric drives are frequently powered and controlled by a converter. The motor control can include a field-based controller which decouples the field-producing and torque-producing current components. The current components can be decoupled, for example, by computing the current components in a coordinate system that rotates with the rotor flux of the motor. Such field-based controller requires information about the rotor position angle of the rotor for computing the flux angle in the motor. The rotor position angle can be measured by a transducer disposed on the motor and transmitted in form of a rotor position angle signal. However, when a transducer malfunctions with a field-based controller, the controller loses the information above the angle position of the rotating field in the motor. A reliable control of the motor with a field-based controller is then no longer possible.

[0005] The rotor position angle is frequently measured by the transducer incrementally without relying on an absolute reference position. The absolute position of the rotor in a stationary state is determined directly only at the time the motor is switched on. If an error occurs, the motor may continue to accelerate under certain unfavorable conditions (angle error between 90° and 270°) which can endanger the equipment or personnel. The same problem can occur if the incremental position indicator loses position markers.

[0006] To prevent endangering equipment or personnel, the drive must therefore be stopped as soon as possible, in particular when using linear motors with a limited range of travel, if a faulty rotor position angle signal is detected, or other measures must be taken.

[0007] It would therefore be desirable and advantageous to provide a system and method for identifying a faulty rotor position angle signal of a synchronous motor, to obviate prior art shortcomings and to allow safe operation of a synchronous motor powered by a converter if a faulty rotor position angle signal is detected.

[0008] According to one aspect of the invention, a method for identifying a faulty rotor position angle signal of a synchronous motor powered by a converter includes the steps of measuring on the synchronous motor a rotor position angle signal, determining from the measured rotor position angle signal a first flux angle, measuring on the synchronous motor at least one motor current, determining from the measured motor current or motor currents a current pointer, measuring on the synchronous motor at least one motor voltage, and determining from the measured motor voltage or motor voltages a voltage pointer. The method employs a flux modeler of the synchronous motor to determine from the voltage pointer and the current pointer a second flux angle, and identifies a faulty rotor position angle signal based on an agreement, or lack thereof, between the first and second flux angles.

[0009] With the method of the invention, a faulty rotor rotation angle signal can be already recognized at a very low rotation speed of the synchronous motor, which is only a fraction of the nominal rotation speed of the synchronous motor.

[0010] According to another feature of the invention, the flux modeler can compute an induction voltage pointer by using the relationship ${\stackrel{\rightharpoonup }{U}}_{\mathrm{ind}}=\stackrel{\rightharpoonup }{U}-R_A·\stackrel{\rightharpoonup }{I}-\frac{d\stackrel{\rightharpoonup }{I}}{dt}·L_A.$
The flux modeler can then compute from the induction voltage pointer an angle of the induction voltage pointer, and determine the second flux angle by subtracting from the angle of the induction voltage pointer an angle of 90° for a positive angular rotor frequency ω of the synchronous motor, or by subtracting from the angle of the induction voltage pointer a subtraction angle of 270° for a negative angular rotor frequency ω of the synchronous motor. {right arrow over (U)} is herein the voltage pointer, {right arrow over (I)} the current pointer, R_A the stator winding resistance, L_A the stator winding inductance, and {right arrow over (U)}_ind the induction voltage pointer. The flux modeler can hence determine the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind in a particularly simple manner.

[0011] According to another feature of the invention, the current pointer and voltage pointer can be referenced to a stationary coordinate system that is fixed relative to a stator of the synchronous motor. Alternatively, the current pointer and voltage pointer can be referenced to a coordinate system that rotates with a rotor flux of the synchronous motor, since the corresponding current and voltage pointers are already computed by the field-based controller, so that no additional computing resources are required for computing the pointer.

[0012] According to another feature of the invention, the voltage pointer of the motor voltage or voltages of the synchronous motor can be determined from measured terminal voltages of the synchronous motor. With this approach, the voltage pointer of the motor voltages of the synchronous motor can be reliably determined independent of the controller.

[0013] According to another feature of the invention, the voltage pointer can be derived from voltages that are defined by a controller of the synchronous motor and supplied to the synchronous motor by the converter. This obviates the need to measure in addition terminal voltages of the synchronous motor.

[0014] According to another feature of the invention, the derived voltage pointer can be used as an input value of the voltage pointer for the flux modeler. With a field-based controller, the applied voltage pointer is typically provided to the converter as a desired value and is therefore already computed by the field-based controller, so that this voltage pointer can be used as a voltage pointer input value of the flux modeler.

[0015] Suitably, the synchronous motor can be switched off if a faulty rotor position angle signal is identified, because the risk for equipment and/or personnel can then be safely eliminated. The motor can be switched off, for example, by blocking the pulses to the converter. As an additional measure, for example, a short-circuit braking action of the rotor can be initiated.

[0016] According to another feature of the present invention, the synchronous motor can be switched to a controlled safe state if a faulty rotor position angle signal is identified. With this approach, the production or manufacturing process can be continued even when a faulty rotor position angle signal is detected.

[0017] A method according to the present invention is particularly suited for identifying a faulty rotor position angle signal of a synchronous motor powered by a converter in machine tools or production machines, because in these applications damages resulting from faulty rotor position angle signals can be quite substantial. However, it should be mentioned at this point that the method of the invention can also employed in other technical areas. The motor control also need not be a field-based controller, and the method and system of the invention can be applied to other types of controllers.

[0018] Advantageously, a data carrier with a computer program stored on the data carrier can be used for carrying out the method of the invention. A computer with a program memory can execute the computer program to carry out the method of the invention.

[0024] Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted.

[0025] Turning now to the drawing, and in particular to FIG. 1, there is shown in form of a schematic block diagram a field-based controller 21 that controls a synchronous motor 5 using a flux modeler 20. The synchronous motor 5 can be implemented as a rotary motor or a linear motor. Three-phase power is supplied to the synchronous motor 5 by a converter 4 by way of three current phases i₁, i₂, and i₃. The corresponding voltages between the phases are indicated by u₁, u₂, and u₃. The three motor currents i₁, i₂, and i₃ are measured and supplied to a 3-2 converter 7. The 3-2 converter 7 computes from the three motor currents i₁, i₂, and i₃ the orthogonal current components i_αist and i_βist as so-called current pointer values within a stationary coordinate system affixed to the stator by using the equation (1): $\begin{array}{cc}a.\left(\begin{array}{c}{i}_{\alpha \mathrm{ist}}\\ i_{\beta \mathrm{ist}}\end{array}\right)=\left(\begin{array}{ccc}\frac{2}{3}& -\frac{1}{3}& -\frac{1}{3}\\ 0& \frac{1}{\sqrt{3}}& -\frac{1}{\sqrt{3}}\end{array}\right)·\left(\begin{array}{c}{i}_1\\ i_2\\ i_3\end{array}\right)& \left(1\right)\end{array}$

[0026] A vector rotation unit 8 transforms the actual current component i_αist as well as the actual current component i_βist via the complex relationship (2)
(i_dist+ji_qist)=(i_αist+ji_βist)·e^{−j{circumflex over (φ)}Ψ}  (2)
wherein

• [0027] e: Euler number
• [0028] j: imaginary unit (j²=−1)
  into the torque-producing actual current component i_qist and the field-producing actual current component i_dist in an orthogonal coordinate system that rotates with the rotor flux of the motor 5. A flux angle computing unit 9 computes the required flux angle {circumflex over (φ)}_Ψ from a motor position angle signal φ_r received from a transducer 6 connected to the motor 5.

[0029] In the depicted embodiment, the transducer 6 provides to the flux angle computing unit 9 a rotor position angle φ_r of the rotor of the synchronous motor 5 as the rotor position angle signal φ_r. The flux angle computing unit 9 computes from the rotor position angle φ_r of the synchronous motor 5 the flux angle {circumflex over (φ)}_Ψ of the rotor flux of the synchronous motor 5, using existing information about the mechanical and electrical configuration of the synchronous motor 5.

[0030] The actual current components i_dist and i_qist in the rotating coordinate system are controlled by supplying the difference between the desired current component i_dsoll and the actual current components i_dist to a proportional-integral (PI) controller 2. The desired current components i_dsoll and i_qsoll in the rotating coordinate system can be supplied either directly or by a master controller, for example a master position controller (not shown for sake of clarity). The PI-controller 1 outputs the field-producing desired voltage component i_dsoll to a vector rotation unit 3 as a first input value. The PI-controller 2 outputs the desired torque-producing voltage component U_qsoll to the vector rotation unit 3 as a second input value. The vector rotation unit 3 computes from the afore-described desired voltage component u_dsoll and u_qsoll obtained in the rotating coordinate system as well as from the flux angle {circumflex over (φ)}_Ψ, using the relationship (3)
(u_αsoll+ju_βsoll)=(u_dsoll+ju_qsoll)·e^{j{circumflex over (φ)}Ψ}  (3)
the desired voltage components u_αsoll and u_βsoll in the stationary coordinate system as so-called voltage pointers. The desired voltage components u_αsoll and u_βsoll are supplied to a control set disposed inside the converter 3 (not shown in FIG. 1). The control set computes from the desired voltage components u_αsoll and u_βsoll the required trigger angles for the converter 4 and the trigger pulses that control the valves of the converter.

[0031] If the transducer 6 fails in a field-based controller illustrated in the embodiment of FIG. 1, or if the rotor position angle signal φ_r is faulty in general, then the correct flux angle {circumflex over (φ)}_Ψ can no longer be computed. The field-based controller is then unable to obtain the orientation of the rotation field of the synchronous motor 5. The field-based controller is then no longer able to control, in particular brake, the synchronous motor 5. This applies not only to the field-based controller depicted in the embodiment of FIG. 1, but can in principle apply to any field-based controller, if a rotor position angle signal φ_r is faulty.

[0032] However, with the method of the invention, a flux modeler 20 can be used to compute from the motor voltages and motor currents a second flux angle φ_Ψ, which is then compared with the first measured flux angle {circumflex over (φ)}_Ψ. If there is insufficient agreement between the first flux angle and the second flux angle, then the rotation position angle signal φ_r is identified as being faulty.

[0033] For a better understanding of the flux modeler 20 shown schematically in FIG. 1, an equivalent electric circuit diagram of a single-phase synchronous motor is shown schematically in FIG. 2. When a voltage u is applied to the terminals of the synchronous motor, a current i begins to flow through the stator winding resistor R_A and the stator winding inductance L_A. An induction voltage u_ind with the opposite polarity of the terminal voltage u of the synchronous motor is induced, when the current i causes a rotation of the rotor of the synchronous motor, or for a linear motor, a linear motion of the rotor.

[0034] FIG. 3 shows the pointers derived from the equivalent electric circuit diagram of FIG. 2 in a stationary coordinate system affixed to the stator with a real part axis Re and an imaginary part axis Im. The depicted pointers rotate with the angular frequency ω of the rotor. The voltage pointer {right arrow over (U)} of the terminal voltage is obtained by vector or complex addition from the induction voltage pointer {right arrow over (U)}_ind of the induction voltages, from the voltages across the stator winding resistors in form of a voltage resistance pointer R_A·{right arrow over (I)}, wherein {right arrow over (I)} represents the current pointer of the motor currents i₁, i₂, and i₃, and wherein the voltages across the stator winding inductances are present as a stator winding voltage pointer jωL_A{right arrow over (I)}.

[0035] Accordingly, the stationary induction voltage pointer {right arrow over (U)}_ind is obtained as
{right arrow over (U)}_ind={right arrow over (U)}−R_A·{right arrow over (I)}−jωL_A{right arrow over (I)}  (4)
and for the general case $\begin{array}{cc}{\stackrel{\rightharpoonup }{U}}_{\mathrm{ind}}={\stackrel{\rightharpoonup }{U}}_{\mathrm{ind}}·e^{j{\phi }_{\mathrm{ind}}}=\stackrel{\rightharpoonup }{U}-R_A·\stackrel{\rightharpoonup }{I}-\frac{d\stackrel{\rightharpoonup }{I}}{dt}·L_A=j\omega \stackrel{\rightharpoonup }{\Psi }·e^{j{\phi }_{\Psi }},& \left(5\right)\end{array}$
wherein

• [0036] {right arrow over (U)}: voltage pointer
• [0037] {right arrow over (I)}: current pointer
• [0038] R_A: stator winding resistance
• [0039] L_A: stator winding inductance
• [0040] {right arrow over (U)}_ind: induction voltage pointer
• [0041] φ_ind: angle of the induction voltage pointer
• [0042] t: time
• [0043] ω: angular frequency of the rotor
• [0044] {right arrow over (Ψ)}: flux pointer

[0045] Based on the electrical situation in the motor, the flux angle φ_Ψ of the flux pointer {right arrow over (Ψ)} of the rotor flux is ideally advanced according to equation (5) for a positive angular frequency ω by 90° relative to the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind, i.e.
φ_Ψ=φ_ind−90° (for ω>0)  (7)

[0046] For a negative angular frequency ω, the flux angle φ_Ψ of the flux pointer {right arrow over (Ψ)} of the rotor flux is advanced according to equation (5) by 270° relative to the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind, i.e.
φ_Ψ=φ_ind−270° (for ω<0)  (8)

[0047] FIG. 3 shows for a positive angular frequency ω the resulting flux pointer {right arrow over (Ψ)} with the angle φ_Ψ computed by the flux modeler. The magnitude of the flux pointer {right arrow over (Ψ)} is not computed by the flux modeler, since information about the magnitude of the flux pointer {right arrow over (Ψ)} is not required for carrying out the method of the invention. The flux angle {circumflex over (φ)}_Ψ determined by the flux computation unit 9 from the rotor position angle signal φ_r together with the associated rotor flux {right arrow over (Ψ)} are indicated in FIG. 3 by a dotted line. A difference angle Δφ is obtained from the difference between the flux angle φ_Ψ determined with the flux modeler 20, which in the context of the present invention is also referred to as second flux angle φ_Ψ, and the flux angle {circumflex over (φ)}_Ψ determined by measuring the rotor position angle signal φ_r, which in the context of the present invention is also referred to as first flux angle {circumflex over (φ)}_Ψ. If there is insufficient agreement between the first flux angle {circumflex over (φ)}_Ψ and the second flux angle φ_Ψ, i.e., is the magnitude of the difference angle Δφ is too large, then it will be assumed that a faulty rotor position angle signal φ_r was supplied to the flux angle computation unit 9 as input value, and the rotor position angle signal φ_r is identified as being faulty. Considerations similar to those of FIG. 3 apply to a negative angular frequency ω.

[0048] FIG. 1 shows an embodiment of the method of the invention, with the flux modeler 20 representing the core of the method. In the depicted embodiment, the voltage pointer {right arrow over (U)} is already represented by the corresponding real and imaginary parts of the desired voltage components u_αsoll and u_βsoll. The voltage pointer {right arrow over (U)} is computed by a pointer computing unit 13 according to the relationship (8)
{right arrow over (U)}=u_αsoll+ju_βsoll  (8)

[0049] Likewise, the current pointer {right arrow over (I)} is computed from the actual current components i_αist and i_βist according to the relationship
{right arrow over (I)}=i_αist+ji_βist  (9)

[0050] The voltage pointer {right arrow over (U)} and the current pointer {right arrow over (I)} are supplied to a unit 14 for calculating the induction voltage {right arrow over (U)}_ind. The unit 14 computes the induction voltage pointer {right arrow over (U)}_ind from the voltage pointer {right arrow over (U)}, the current pointer {right arrow over (I)}, the stator winding resistance R_A and the stator winding inductance L_A. The stator winding resistance R_A and the stator winding inductance L_A are known from the manufacturer's data sheet for the synchronous motor. The computed induction voltage pointer {right arrow over (U)}_ind is supplied as input value to a polar coordinate converter 15, which computes the magnitude of {right arrow over (U)}_ind and the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind according to the first part of equation (5) and outputs the angle φ_ind. In a following subtracter 16, for a positive angular frequency ω, a subtraction angle β of preferably 90° is subtracted from the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind, or for a negative angular frequency ω, a subtraction angle β of preferably 270° is subtracted from the angle φ_ind of the induction voltage pointer {right arrow over (U)}_ind, thereby calculating a second flux angle φ_Ψ. A subtracter 17 subsequently subtracts the second flux angle φ_Ψ from the first flux angle {circumflex over (φ)}_Ψ, and the computed in difference is supplied to an absolute value unit 18, which outputs the difference angle Δφ as output value. The difference angle Δφ is then supplied as input value to a limiter 19. If the difference angle Δφ exceeds a critical difference angle Δφ_crit supplied by the user, then a signal S at the output of the limiter 19 is set to a logical high level, indicating an identified faulty rotor position signal φ_r.

[0051] In the described embodiment, the signal S is supplied as input value to the converter 4, which blocks the pulses when a logical high level of the signal S is detected, thereby safely and rapidly switching off the synchronous motor.

[0052] Alternatively, the signal S can also be used to control the motor when a logical high level of the signal S is detected, by using a so-called transmitter-less control, i.e. a control that does not require a rotor position angle measured directly by a transducer to control the motor. In this case, the manufacturing or production process can advantageously continue, and only an alarm message is provided to the user.

[0053] It is easy to determine if the angular frequency ω is positive or negative by evaluating the rotor position angle signal φ_r. An increasing or decreasing value of the rotor position angle can be associated with a positive or negative angular frequency, respectively, depending on a defined reference direction. It will be understood that the rotation direction of the angular frequency can also be derived from the pointer values.

[0054] It is also possible to use for the flux modeler 20 current and voltage pointers that are referenced to a coordinate system that rotates with the rotor flux of the synchronous motor, instead of current and voltage pointers that are referenced to a stationary coordinate system affixed to the stator. In this case, the desired voltage component u_dsoll and the desired voltage component u_qsoll, or the actual voltage components u_dist and u_qist, would be supplied in FIG. 1 to the pointer computing unit 12, instead of the actual current components i_αist and i_βist, or the desired voltage components u_αsoll and u_βsoll.

[0055] In the afore-described embodiment, the voltage pointer {right arrow over (U)} is advantageously derived from voltages that are provided to a controller of the synchronous motor and are then supplied to the synchronous motor by a converter controlled by the controller. The supplied voltages are her present in the form of the desired voltage components u_αsoll and u_βsoll and represent the real and imaginary part of the voltage pointer {right arrow over (U)} supplied by the converter. Alternatively, the terminal voltage of the synchronous motor can also be measured by suitable transducers, and the real and imaginary part of the voltage pointer {right arrow over (U)} can be determined by another 3-2 converter (not shown) that is independent of the controller. The flux modeler then operates more independently from the controller 21 so that the second flux angle φ_Ψ can be determined more accurately.

[0056] As seen in FIG. 4, a computer 22 with a program memory 23 can be provided for executing the method of the invention. The program memory 23 can store a computer program 24 that can be called by the computer 22 to execute the method. The computer 22 can be implemented as a controller 22 which can be an element of a machine tool or production machine 25. The machine tool or production machine 25 can include additional elements, which are not essential for an understanding of the invention and are not shown in FIG. 4 for sake of clarity.

[0057] The computer program 24 can also be stored on a data carrier 27 implemented, for example, as a flash card, diskette, CD-ROM, DVD, a hard drive and the like.

[0058] Machine tools in the context of the present invention can also include, for example, uniaxial or multi-axis lathes, milling machines, as well as drilling or grinding machines. Machine tools can further include processing centers, linear and rotary transfer machines, laser machines, rolling machines and/or gear cutters. These machines have in common that the material is machined along several axes. Production machines in the context of the present invention can include textile, paper, plastic, wood, glass, ceramic or stone processing machines, as well as machines used for forming, packaging, printing, conveying, lifting, pumping, transporting. Furthermore, fans, blowers, wind turbines, lifting gear, cranes, robots, production and assembly lines are also included under the term production machines in the context of the present invention.

[0059] While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.