Patent Publication Number: US-2004055395-A1

Title: Measurement of the rate of change of current in switched reluctance machines

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
     [0001] The subject matter of this application is related to the subject matter of British Patent Application No. 0220673.8, filed Sep. 5, 2002, priority to which is claimed under 35 U.S.C. § 119 and which is incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] This invention relates to the detection of a point on a current waveform where the gradient of the current changes sign. It is particularly applicable to the control of switched reluctance machines.  
       [0004] 2. Description of Related Art  
       [0005] The control and operation of switched reluctance machines generally are described in the paper “The Characteristics, Design and Applications of Switched Reluctance Motors and Drives”, by J. M. Stephenson and R. J. Blake, delivered at the PCIM&#39;93 Conference and Exhibition held in Nurnberg, Germany, Jun. 21-24, 1993, which paper is incorporated herein by reference. In that paper the “chopping” and “single-pulse” modes of energization of switched reluctance machines are described for operation of the machine at low and high speeds respectively.  
       [0006] A typical prior art drive is shown schematically in FIG. 1. This includes a DC power supply  11  that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply  11  is switched across phase windings  16  of the motor  12  by a power converter  13  under the control of the electronic control unit  14 . Some form of current transducer  18  is normally provided to give phase current feedback. The motor  12  is connected to a load  19 .  
       [0007] One of the many known converter topologies is shown in FIG. 2. The phase winding  16  of the machine is connected in series with two switching devices  21  and  22  across the busbars  26  and  27 . Busbars  26  and  27  are collectively described as the “DC link” of the converter. Energy recovery diodes  23  and  24  are connected to the winding to allow the winding current to flow back to the DC link when the switches  21  and  22  are opened. A low-value resistor  28  is connected in series with the lower switch  22  to act as a current-sense resistor and provide a current feedback signal. A capacitor  25 , known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor  25  may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.  
       [0008] At low speeds, switched reluctance systems generally operate in a current-controlled or chopping mode, while at higher speeds they typically operate in the single-pulse mode of energization. This mode is illustrated in FIG. 3 in which the linearized current and inductance waveforms are shown over a phase inductance period. The current rises when the voltage is applied to the phase winding at the switch-on angle θ on , reaches a peak and then rolls over as the rotor poles begin to overlap the stator poles and the inductance rises. The current is limited naturally by the back emf of the circuit. The voltage is reversed at the switch-off angle θ off , after a conduction angle θ c , and the current falls at a faster rate as energy is returned to the supply and ceases when it reaches zero. There is then a period of zero current before the cycle begins again. In practice, the inductance waveform has rounded corners due to flux fringing in the air, and this leads to corresponding rounding of the current waveform, as shown in FIG. 4.  
       [0009] An alternative switching strategy uses freewheeling for a period before θ off , whereby the switching circuit allows the current to re-circulate in the phase winding without an applied voltage. FIG. 4 illustrates this technique and the period of zero applied voltage during freewheeling is clearly shown. Again, the current falls to zero for a time before the cycle repeats.  
       [0010] While motoring operation has been illustrated in the above discussions, it is well-known that the machines operate equally well in the generating mode, in which the current waveforms are generally mirror images in time of the motoring waveforms.  
       [0011] The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. Detection of rotor position is conventionally achieved by using a position transducer  15 , shown schematically in FIG. 1, such as a rotating toothed disk mounted on the machine rotor, which co-operates with an optical or magnetic sensor mounted on the stator. A pulse train indicative of rotor position relative to the stator is generated and supplied to control circuitry, allowing accurate phase energization.  
       [0012] Alternative methods of position detection include the so-called “sensorless” methods, in which the position is deduced from measurements of another parameter of the machine. Such methods have the benefit that no dedicated position sensor is required on the machine, leading to reduced cost and increased reliability. A summary of many known sensorless methods is given in “Review of sensorless control methods in switched reluctance motor drives”, by Fahimi et al, IAS2000, Thirty-fifth Annual Meeting and World Conference on Industrial Applications of Electrical Energy, Oct. 8-12, 2000, Rome, Italy, Vol. 3, pp 1850-1857, incorporated herein by reference. Among the known methods are those which make use of the feature in an unsaturated phase current waveform where the peak current coincides with the onset of overlap between the rotor and stator poles, as shown in FIG. 3. These techniques are discussed in some detail in the paper “A New Indirect Rotor Position Detection Method For Switched Reluctance Drives”, by Kjaer et al, Proc International Conference on Electrical Machines, Sep. 5-8, 1994, Paris, Vol. 2, pp 555-560, incorporated herein by reference. A particular method of determining the onset of pole overlap is disclosed in European Patent Application No. 1,109,309 A2 to Moriarty, incorporated herein by reference.  
       [0013] All of these techniques require an accurate knowledge of the phase current, especially if the derivative of the current is required, since the process of differentiation automatically emphasizes any errors. Accurate current sensing can be performed by Hall-effect current transducers, which also provide electrical isolation from the phase current, but these are costly and, in low-power drives, take up a considerable amount of space. In low-cost applications, a current sense resistor is often used, as shown in FIG. 2, but these do not provide isolation and, in low-voltage applications, the extra voltage drop across the resistor often produces an unacceptable degradation in performance.  
       [0014] The Rogowski coil has enjoyed a return to use in recent years, and a recent paper “Voltage Profiles and Closures on Rogowski Coils” by Viana et al, Proc IEE, Electric Power Applications, Vol. 149, No  3 , May 2002, pp 223-227, incorporated herein by reference, details recent studies on the accuracy of the system. The Rogowski coil measures the field surrounding a conductor and gives a voltage proportional to the rate of change of the sensed current. This voltage is then integrated by a specially designed circuit to give a measure of the current. The design of the integrator and the calibration of the system are by no means trivial. While such systems, when well-designed, can give high accuracy over a wide bandwidth, they are generally regarded as high-cost laboratory tools rather than as suitable for low-cost, mass-produced applications. Attempts have been made to produce cheaper versions, without sacrificing too much accuracy, such as those discussed in U.S. Pat. No. 5,414,400 to Gris, incorporated herein by reference, but the need to place the conductor through the printed circuit board can lead to significant assembly problems.  
       [0015] U.S. Pat. No. 6,380,727 discloses a current sensor using Printed Circuit Board (PCB) trace-loops. Both the current sensor and the conductor are provided as PCB copper traces on a PCB board. The current flow in the conductor is orthogonal to the axis of the coil.  
       [0016] There is therefore a need for a low-cost current measuring system for a switched reluctance machine which can monitor the rate of change of phase current and detect when the roll-over of the current occurs.  
       SUMMARY OF THE INVENTION  
       [0017] In accordance with embodiments of the present invention there is provided a rate of change of current sensor comprising a coil for coupling the flux from a conductor in which the rate of change of current is to be sensed, the coil comprising a plurality of turns, each turn being a track on a printed circuit board, each turn being displaced from its neighboring turn in a direction parallel to the direction of the conductor. The flux linkage between the conductor and the coil is thereby increased.  
       [0018] The printed circuit board optionally has at least a first layer and a second layer, each turn comprising a first part of the turn on the first layer and a second part of the turn on the second layer, the first and second parts of the turn being connected by a via extending through the printed circuit board. The turns may be of any suitable shape e.g. rectangular, circular or hexagonal. Each turn of the coil is optionally of the same dimension as the other turns.  
       [0019] Means may be provided on the printed circuit board to hold the conductor in place relative to the coil. Alternatively the conductor may be formed on a layer of the printed circuit board or may comprise a layer of the printed circuit board. The conductor may be a split conductor having at least two limbs each of which runs close to the vias of either side of the coil.  
       [0020] In one embodiment, two coils are provided on the printed circuit board with the conductor extending between the two coils.  
       [0021] In a second aspect of the invention there is provided a switched reluctance drive including a motor having a rotor defining a plurality of rotor poles, a stator defining a plurality of stator poles, and at least one phase winding for exciting two or more of the poles, and a sensor according to an embodiment of the invention, the sensor being connected to sense the rate of change of current in the phase winding.  
       [0022] The output of the sensor optionally is fed to a circuit which detects the point at which the rate of change of current crosses zero. The output of the sensor may be used to provide rotor position information. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023] The invention can be put into practice in a number of ways, some of which will now be described by way of example and with reference to the accompanying drawings in which:  
     [0024]FIG. 1 shows a typical prior art switched reluctance drive;  
     [0025]FIG. 2 shows a known topology of one phase of the converter of FIG. 1;  
     [0026]FIG. 3 shows an ideal current waveform in single-pulse control;  
     [0027]FIG. 4 shows a typical current waveform in single-pulse control using freewheeling;  
     [0028]FIG. 5 shows a coil according to one aspect of the invention;  
     [0029]FIG. 6 shows a coil according to another aspect of the invention;  
     [0030]FIG. 7 shows a coil according to a yet further aspect of the invention;  
     [0031]FIG. 8 shows a typical output waveform from a coil; and  
     [0032]FIG. 9 shows an exemplary zero crossing detection circuit. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0033] One embodiment of the invention is shown in FIG. 5, where a conductor  50  carries a current to be sensed. In the field of switched reluctance machines, typically this conductor leads to the phase winding of the switched reluctance machine. The conductor is arranged parallel to a printed circuit board (PCB) and may be held in place by retaining means  52  which can take the form of features on the PCB, clips or other fasteners as commonly used. The PCB may be dedicated to current detection, as shown in FIG. 5, or it may carry some or all of the control system and power converter to control an electrical machine. For example, it may contain some or all of the items  13  and  14  to control a switched reluctance machine as shown in FIG. 1.  
     [0034] A planar coil is fabricated on the PCB by laying out conductive tracks, for example of copper, on the PCB. FIG. 5 shows the coil laid out on the upper (solid lines) and lower (dashed lines) sides of the PCB, the sides of each turn of the coil being connected by vias in the usual way. The coil begins at via  46  and the track  42  constitutes the first part of the first turn. This part is connected to the second part  43  of the first turn, on the other side of the board, by via  44 . The second part of the first turn is connected to the first part of the next turn by via  47  and the coil is continued in this fashion until the second part of the last turn is terminated at via  48 . The output voltage of the coil appears across vias  46  and  48 .  
     [0035] The number of turns in the coil is a variable influenced by, inter alia, the size of the current to be sensed, the available area on the board and the accuracy required. The current i in conductor  50  produces a voltage in the coil given by:  
       V   c   =A di/dt   (1)  
     [0036] where A is a constant whose value depends in a complex fashion on the geometry of the coil, the number of turns and the disposition of the conductor to the coil. It may best be determined empirically. In FIG. 5, the conductor is shown close to the vias at one side of the coil. If the conductor is moved away from the coil, the value of A, and hence V c , falls, since the coil links less of the field surrounding the coil. If the conductor is moved towards the center of the coil, the value of A will initially rise a little, then falls as the field to the right of the conductor begins to link the coil and cancels the original field. When the conductor is central to the coil the output falls virtually to zero.  
     [0037] As can be seen in FIG. 5, each turn of the coil is displaced from its neighboring turn in a direction which, in use, is parallel to the direction of the conductor and in particular the direction of the current flow in the conductor. Thus the second turn of the coil is displaced along the line A-A from the first turn, the third turn of the coil is displaced along the line A-A from the second turn and so on for the remainder of the turns of the coil. This arrangement of the coil increases the flux linkage between the conductor  50  and the coil. This can also be viewed as the conductor being arranged parallel to a line formed by the axes of the turns of the coil. In FIG. 5 the axis of each turn is orthogonal to the page.  
     [0038]FIG. 5 shows a hexagonal shaped coil, which is convenient for PCB manufacture, though other shapes are possible, e.g. rectangular or circular. Alternatively, the coil may be fabricated by depositing tracks on three or more layers and suitably interconnecting them. The conductor  50  can be constituted by a track or tracks on one of the layers of the PCB.  
     [0039] Another embodiment is shown in FIG. 6, where the conductor  50  is placed between two coils  61 ,  62 , each fabricated in a similar way to the coil of FIG. 5, so that, by appropriately connecting the two coils, the sensitivity of the output is enhanced.  
     [0040] A yet further embodiment is shown in FIG. 7, where two coils and a split conductor are laid out to maximize the sensitivity of the system. The conductor is formed by a track which may be provided on one or more layers of the PCB. This arrangement can conveniently be laid out with the coil sides on the top and bottom layers as before and the conductor track on a middle layer. The conductor track has pads or terminals  72 ,  74  to allow connection to an external circuit. If the current flows in at, say,  74 , it flows up limb  76  then splits into limbs  77 ,  78 . This split avoids the vias connecting the coil sides. The current then flows along limbs  79 ,  80  and returns to the bottom limb  70 , to exit the circuit at  72 . This arrangement allows each coil  61 ,  62  to be influenced by the current from two limbs, thus maximizing the sensitivity.  
     [0041] Note that the power conductor track in FIG. 7 is laid out for clarity and in practice limbs  77 , 78  could run closer to the coil side connections and the outer limbs could run closer to the outer connections.  
     [0042] The conductor  50  may form part of a bus bar that connects with the switch  22  and the phase winding  16 . Alternatively switch  22  may also be provided on the PCB and connected to the coil and the phase winding by means of the conductor  50 . Referring to FIG. 7, the phase winding may be connected directly to the terminals  72 ,  74 .  
     [0043] The output from a coil such as shown in FIGS. 5, 6 or  7  will depend on the shape of the current waveform. FIG. 8 shows the type of waveform produced by the current of FIG. 3, in accordance with Equation 1. The output from the coil is used to provide a detection signal for a position detection scheme in a switched reluctance drive. In order to use the coil output to detect the point of current roll-over, the point where the coil output crosses zero must be detected. This can be done in any convenient way, as well understood by those skilled in the art. An exemplary circuit is given in FIG. 9. The coil  90  is connected to a differential amplifier  92  which supplies an output to the Schmitt comparator  94 . The output  96  can be used to drive the interrupt of a microprocessor in known fashion. The microprocessor can then be programmed with a sensorless position detection algorithm, e.g. as described in European Patent Application No. 1,109,309 A2.  
     [0044] Note that it is not necessary to know the exact magnitude of the coil output (i.e. to know the value of the constant A in Equation 1) since it is the position of the zero crossing that is important, not the magnitude of the waveform. The output only needs to be large enough to drive the detection circuit reliably.  
     [0045] The skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of several embodiments is made by way of example and not for the purposes of limitation. It will be clear to the skilled person that minor modifications can be made without significant changes to the operation described above.