Abstract:
A fault-handling system for a 2-phase motor. When an electric motor is used for power assist in a steering system in a vehicle, malfunctions can cause loss of assist, and detectable vibration. The invention utilizes a 2-phase motor in such an application, and implements alternate modes of operation when certain malfunctions occur, thereby maintaining the assist function in situations wherein the function would otherwise be lost or reduced.

Description:
[0001]     This application is related to an application entitled “Electric Power System for a Vehicle,” which is concurrently filed herewith on Oct. 31, 2003, and which is hereby incorporated by reference. 
     
    
       [0002]     The invention relates to electric power-steering systems in vehicles, and to approaches for handling malfunctions which may occur in such systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Modern electric motors, while extremely reliable, are nevertheless not perfect. They can experience malfunctions, particularly after they have been in service for extended periods of time, and especially if they have experienced abusive operation.  
         [0004]     When an electric motor is used in a power steering system in a vehicle, a malfunction can cause loss of the power assist which the motor otherwise provides. The invention provides approaches to detecting malfunctions, and taking corrective action.  
       OBJECTS OF THE INVENTION  
       [0005]     An object of the invention is to provide an improved power steering system in a vehicle.  
       SUMMARY OF THE INVENTION  
       [0006]     In one form of the invention, a short is detected across a coil in a stator of a motor, and current is terminated to that coil.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  illustrates a vehicle  3  in which one form of the invention is installed.  
         [0008]      FIG. 2  is a schematic of a stator in a 3-phase motor.  
         [0009]      FIG. 3  illustrates a relay  60  added to the stator of  FIG. 2 .  
         [0010]      FIG. 4  illustrates a short across coil  36  in the 3-phase stator, and a response taken to the short.  
         [0011]      FIG. 5  illustrates a short between coils  36  and  33  in the 3-phase stator, and a response taken to the short.  
         [0012]      FIG. 6  illustrates a short between coil  36  and ground in the 3-phase stator, and a response taken to the short.  
         [0013]      FIG. 7  illustrates shorts in two transistors  83  or  89  in a circuit which drives a coil in the three-phase stator, and a response taken to the short.  
         [0014]      FIG. 8  illustrates open circuits in two transistors  93  or  99  in a circuit which drives a coil in the three-phase stator, and a response taken to the open circuit.  
         [0015]      FIG. 9  is a schematic of the two coils  115  and  120  in a two-phase stator.  
         [0016]      FIG. 10  illustrates two coils Cx and Cy, used in a two-phase electric motor, together with an H-bridge of transistors which delivers current to each coil.  FIG. 10  illustrates how the transistors are switched on and off in each quadrant of operation of the motor. Quadrants I, II, III, and IV refer to the angular position of a reference on the rotor of the motor.  
         [0017]      FIG. 11  illustrates a short occurring in coil  120  of the two-phase stator, and a response taken to the short.  
         [0018]      FIG. 12  illustrates a short occurring between coils  120  and  115  of the two-phase stator, and a response taken to the short.  
         [0019]      FIG. 13  illustrates a short occurring between coil  120  and ground of the two-phase stator, and a response taken to the short.  
         [0020]      FIG. 14  illustrates shorts occurring in transistors T 5  and T 6  in the H-bridges driving the coils of the two-phase stator, and a response taken to the shorts.  
         [0021]      FIG. 15  illustrates open circuits occurring in transistors T 2  and T 3  in the H-bridges driving the coils of the two-phase stator, and a response taken to the open circuits.  
         [0022]      FIG. 16  is a flow chart illustrating processes undertaken by one form of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  illustrates a motor vehicle  3 , which contains an electric motor  6 , and a linkage  9  connecting between the motor  6  and the steerable wheels  12 . Such linkages  9  are well-known.  
         [0024]     Also shown is a control system  15 , which implements processes utilized by the invention. The control system includes a microprocessor  18 , which runs one or more programs  21 . A fault-diagnostic system  24  measures various voltages and currents in the motor  6  and associated components, and possibly other parameters such as motor speed. The diagnostic system  24  then either (1) infers the presence of certain malfunctions and transmits data indicating the malfunctions to the control  15  or (2) delivers data indicating the voltages, currents, and parameters to the control  15 , allowing the latter to infer malfunctions, or (3) some combination of (1) and (2).  
         [0025]     In response to detection of malfunctions, the control system  15  takes corrective actions. This discussion will discuss corrective actions which may be taken in a system powered by a 3-phase motor, and then corrective actions which may be taken in a system powered by a 2-phase motor.  
         [0026]      FIG. 2  is a schematic of the stator  27  of a 3-phase motor, which stator contains coils, or phases,  30 ,  33 , and  36 . Switches  39 ,  42 , and  45  control currents to the phases. Blocks T represent transistors. A significant feature is that, because of Kirchoff&#39;s Current Law, the three currents I 1 , I 2 , and I 3  in the coils must sum to zero. That is, the three currents I 1 , I 2 , and I 3  are not independent: once two currents are specified, or generated, they determine the third.  
         [0027]     If the motor of  FIG. 2  is used in a motor vehicle, the neutral point N often remains unconnected. That is, only three wires  48 ,  51 , and  54  enter the housing (not shown) of the motor. No neutral wire enters the housing.  
         [0028]     In some situations, a relay  60  in  FIG. 3  can be provided. In certain situations, the relay  60  is opened, thereby terminating operation of the motor. Various situations in which this termination may occur will now be discussed.  
         [0029]      FIG. 4  shows the three coils  30 ,  33 , and  36 , which individually produce three magnetic fields (not shown), which add vectorially to produce a rotating magnetic field, called the stator field, which rotates in the direction of arrow  63 .  
         [0030]     The rotor (not shown) of the motor is equipped with its own magnetic field  66 , and, because of the laws of physics, the rotor field  66  attempts to align itself with the rotating stator field, thereby causing the rotor to rotate.  
         [0031]     If coil  36  should become shorted, as by a connection  69  occurring between its two ends, or a connection (not shown) which shorts a significant number of the turns, then an undesirable event can occur within coil  36 . When the rotating rotor field  66  crosses the now-shorted coil  36 , it generates a short-circuit current IS. That current passes through the internal resistance of the coil  36 , thereby generating heat, and absorbing energy from the rotating rotor field  66 . This energy absorption creates a drag on the rotor. Explained another way, the rotating field of the rotor generates a voltage in the short-circuited coil which produces current through the short, and hence torque which opposes motion much as in a synchronous generator.  
         [0032]     One response to detection of the short  69  is to continue operation in the usual manner. The two remaining coils  30  and  33  provide a type of rotating stator field, which is effective to continue to rotate the rotor, but in the presence of the drag just discussed.  
         [0033]      FIG. 5 , left side, illustrates another type of fault, wherein a short  72  occurs between two coils  33  and  36 . Again, the rotating rotor field will generate short-circuit current IS, causing a drag on the rotor. When this type of fault is detected, a control system (not shown) opens the relay  60 , as indicated on the right side of the Figure. Now current IS is terminated, as is operation of the motor. If the motor were used in a power steering system, the power assist of the motor would, of course, terminate.  
         [0034]      FIG. 6 , left side, illustrates a short  80  between a coil  36  and ground. This fault will cause a short-circuit current IS, and drag on the rotor, as discussed above. In addition, if the location of the point of contact of the short  80  is sufficiently close to point P 1 , then a low-resistance, or zero-resistance, path may exist between transistor  83  and ground. When transistor  83  conducts, a large current may be created, which may damage the transistor  83 .  
         [0035]     When this fault is detected, if relay  60  is opened, as indicated on the right side of  FIG. 6 , the motor becomes non-functional, but the short-circuit current IS does not terminate.  
         [0036]     Alternately, the transistors  42  in  FIG. 2  which feed the shorted coil  36  in  FIG. 6  can be turned off, and the other two coils  30  and  33  can be used to generate a rotating stator field, to sustain rotation of the motor by appropriately adjusting the switching sequence.  
         [0037]      FIG. 7 , left side, illustrates situations wherein transistor  83 , or transistor  89 , become short-circuited. In either case, short-circuit current IS can be generated, causing the drag discussed above. When a shorted transistor is detected, relay  60  is opened, as indicated on the right side of the Figure. The motor becomes non-functional, and the short-circuit current IS terminates. Alternatively, the switching sequence of the inverter may be altered to allow partial production of torque while preventing a direct short to ground through the affected leg of the inverter.  
         [0038]      FIG. 8 , left side, illustrates situations wherein transistor  93 , or transistor  99 , become open-circuited. In either case, the relay  60  remains closed, as indicated, and the other phases are utilized to generate a rotating stator field. In addition, since transistor  93 A may be operational, it may be used to drive current in the direction of arrow  101  through coil  36 . However, current in the opposite direction cannot flow, because transistor  99  is open. Thus, the net effect of the opening of transistor  99  is to prevent generation of current in coil  36  in the direction opposite arrow  101 .  
         [0039]      FIG. 9  is a schematic of stator coils  110  of a 2-phase motor. In one embodiment, the stator is of the synchronous type. Two H-bridges A and B control currents through coils  115  and  120 .  
         [0040]      FIG. 10  illustrates how the H-bridges A and B generate currents in coils Cx and Cy, in order to generate a rotating stator field. In quadrant I, coil Cx generates a magnetic field Bx pointing to the right, and coil Cy generates a magnetic field By pointing upward. The stator field generated (not shown) is the vector sum of Bx and By.  
         [0041]     In quadrant II, coil Cx generates a magnetic field Bx pointing to the left, and coil Cy generates a magnetic field By pointing upward. The stator field generated (not shown) is the vector sum of Bx and By.  
         [0042]     In quadrant III, coil Cx generates a magnetic field Bx pointing to the left, and coil Cy generates a magnetic field By pointing downward. The stator field generated (not shown) is the vector sum of Bx and By.  
         [0043]     In quadrant IV, coil Cx generates a magnetic field Bx pointing to the right, and coil Cy generates a magnetic field By pointing downward. The stator field generated (not shown) is the vector sum of Bx and By.  
         [0044]     The magnetic fields Bx and By are coordinated so that their vector sum is a stator field vector which rotates at a constant speed.  
         [0045]      FIG. 11 , left side, illustrates a short  125  across coil  120 . This short causes drag on the rotor, as discussed above. When the short  135  is detected, the invention increases the current in the other coil  115 , as indicated on the right side of the Figure. The Inventors have observed that, even though the magnetic field produced by coil  115  is always parallel with the x-axis, as shown in  FIG. 10 , nevertheless, that field will sustain rotation of the rotor (not shown).  
         [0046]     In one embodiment, the current applied to coil  115  follows the function I( 115 )=COS (T), wherein COS refers to the cosine. The current applied to coil  120  follows the function I( 120 )=SIN(T), wherein SIN refers to the sine. These sinusoidal currents can be applied through Pulse Width Modulation, PWM, techniques, as known in the art.  
         [0047]     The magnetic fields Bx and By will be nearly proportional to the currents, in the absence of saturation. Because coils  115  and  120  are orthogonal, these two fields Bx and By will sum to a stator vector which rotates about the center of the stator, as time T increases.  
         [0048]     Under the fault condition of  FIG. 11 , the field By is present but oscillates with such a phase shift (in generating mode) that the resulting torque opposes the direction of rotation. The field Bx must be increased so that it dominates That field Bx either points in the positive x-direction, or the negative x-direction, and follows the time-function Bx=k COS (T), wherein k is a constant. As stated above, the current is increased in coil  115  after the fault, so that field Bx is larger than it was previously.  
         [0049]     Even though the field Bx does not rotate, the Inventors have found that field Bx, by itself, will sustain rotation of the rotor. One reason can be explained by an example. Assume that Bx initially points to the east. The rotor field will rotate the rotor, attempting to align with Bx. Since the phase of current Cx is synchronized to the position of the rotor, as it reaches the alignment point the current Cx, and hence the field Bx, go to zero. At the instant that alignment is about to occur, two events happen.  
         [0050]     One is that inertia of the rotating rotor carries the rotor past the alignment point with Bx. The second event is that field Bx changes in polarity by 180 degrees, and now points west. The rotor continues to rotate, now seeking to align with the west-pointing Bx. The two events just described repeat themselves, causing continued rotation.  
         [0051]     If a situation should arise wherein (1) the rotor field is parallel with the field produced by coil Cx, and (2) the rotor is stationary, movement of the rotor can be initiated by movement of the steering wheel  130  in  FIG. 1 . The rotation induced by the two events described above can occur. Also, it is unlikely that the rotor field will be exactly parallel with the x-axis, with the result that the reversing field of coil Cx will probably induce rotation.  
         [0052]      FIG. 12 , left side, illustrates a short  140  between coils  115  and  120 . This short provides various current paths, depending on which transistors are closed at any given time. When this type of fault is detected, all phases are shut down, as indicated on the right side of the Figure. That is, all transistors in the H-bridges A and B are opened, that is, turned off.  
         [0053]     It is observed that, in one embodiment, a snubber diode, such as diode  145 , can be provided in parallel with each transistor or the transistor may have an inherent body diode with the same electrical orientation. Those diodes provide a possible current path between the two phases, such as path  150 , depending on the polarity of the voltage induced in the coils  115  and  120 .  
         [0054]     In one form of the invention, those possible current paths are not terminated by the invention. In this form, the current must return back through the battery in order to complete the circuit. Since it is not likely that the voltage induced in the coils will be sufficient for this to happen, these diodes have little effect on the motor operation. In another embodiment, the diodes are, in effect, removed from the circuit, as by opening a switches at point  155 , at point  156 , or both.  
         [0055]      FIG. 13  illustrates a short  160  in coil  120 , running to ground. This fault will cause a short-circuit current IS, depending on which transistors are closed at any given time, with resultant drag on the rotor, as discussed above. In addition, if the location of the point of contact of the short  160  is sufficiently close to point P 2 , then a low-resistance, or zero-resistance, path may exist between transistor T 3  and ground. When transistor T 3  conducts, a large current may be created, which may damage the transistor. A similar comment applies to transistor T 1 , if the point of contact of short  160  is sufficiently close to point P 3 .  
         [0056]     When the short  160  is detected, the invention shuts down the affected phase coil  120  during quadrants wherein dangerous currents can flow. More than one approach is possible. In this particular example, the transistors below transistors T 1  and T 3  can be permanently opened, to prevent circulating currents. Transistors T 1  and T 3  operate normally: the motor operates with reduced performance and greater torque ripple. Of course, if the short is positioned so that transistor T 1  or T 3  faces a fault-to-ground, that transistor will be shut down.  
         [0057]     The fault-detection system can, in effect, detect the relative closeness of the point of contact of short  160  to points P 2  or P 3 . For example, if it is discovered that the current being passed by transistor T 1  is much larger than that through T 3 , then it may be inferred that the point of contact is closer to point P 3 . Thus, transistor T 1  may be opened, but transistor T 3  remains operative. Thus, in  FIG. 10 , the conduction of coil Cy in quadrants III and IV would occur with current conducting through T 3  and the short circuit, but not that shown in quadrants I and II so that a direct short circuit of the switch T 1  does not occur.  
         [0058]      FIG. 14 , left side, illustrates transistor T 5  as being shorted.  FIG. 10  indicates that, under this condition, coil Cx (corresponding to coil  115  in  FIG. 14 ) cannot produce the magnetic field Bx of quadrants II or III, because the upper-left transistor T 5  feeding coil Cx in each of those quadrants is now short-circuited. The left side of coil Cx cannot be grounded.  
         [0059]     However, coil Cx can still be powered using transistor T 8  in  FIG. 14 , in quadrants I and IV in  FIG. 10 , and operation in those quadrants persists as usual. Thus, operation in the two quadrants where the shorted transistor must be opened is terminated. Restated, the affected coil, coil  115  in this example, is inoperative for 180 degrees every rotation.  
         [0060]     In another embodiment, all operation of transistors T 5  and T 8  can be terminated in this fault condition.  
         [0061]      FIG. 14 , right side, illustrates transistor T 6  as being shorted.  FIG. 10  indicates that, under this condition, coil Cx (corresponding to coil  115  in  FIG. 14 ) cannot produce the magnetic field Bx of quadrants I or IV, because the lower-left transistor T 6  in each of those quadrants is now short-circuited. The left side of coil Cx cannot be pulled to 12 volts.  
         [0062]     However, coil Cx can still be powered by transistor T 7  in  FIG. 14 , in quadrants II and III in  FIG. 10 , and operation in those quadrants persists as usual. Thus, again, operation in the two quadrants where the shorted transistor must be opened is terminated. Restated, the affected coil, coil  115  in this example, is inoperative for 180 degrees every rotation.  
         [0063]     In another embodiment, all operation of transistors T 6  and T 7  can be terminated in this fault condition.  
         [0064]      FIG. 15 , left side, illustrates transistor T 3  as being open-circuited.  FIG. 10  indicates that, under this condition, coil Cy (corresponding to coil  120  in  FIG. 15 ) cannot produce a magnetic field By in quadrants III or IV, because the upper-right transistor T 3  in each of those quadrants is now open-circuited. The right side of coil Cy cannot be connected to 12 volts.  
         [0065]     However, coil Cy can still be powered by the mirror-image transistor T 1  in  FIG. 15 , in quadrants I and II in  FIG. 10 , and operation in those quadrants persists as usual.  
         [0066]      FIG. 15 , right side, illustrates transistor T 2  as being open-circuited.  FIG. 10  indicates that, under this condition, coil Cy (corresponding to coil  120  in  FIG. 15 ) cannot produce a magnetic field By in quadrants III or IV. The left side of coil Cy cannot be connected to ground.  
         [0067]     However, coil Cy can still be powered by the mirror-image transistor T 4  in  FIG. 14 , in quadrants I and II in  FIG. 10 , and operation in those quadrants persists as usual.  
         [0068]     It is of course recognized that the preceding discussion of  FIGS. 14 and 15  only focused on faults in four transistors. The discussion applies to all eight transistors in the H-bridges A and B.  
         [0069]      FIG. 16  is a flow chart illustrating processes undertaken by one form of the invention, using the hardware and software represented in  FIG. 1 . Block  200  in  FIG. 16  indicates that fault data is received from a monitoring system  24 , which measures selected voltages, currents, and other parameters in the circuit of  FIG. 9 , and other locations. Either the monitoring system  24 , or the invention, utilizes the fault data to infer the presence of faults, such as the shorts, open-circuits, and short-circuits discussed above.  
         [0070]     Block  205  inquires whether a short across a phase is detected, such as that of  FIG. 11 , left side. If so, the YES branch is taken in  FIG. 16 , wherein the current is increased to the other phase, and current to the shorted phase is terminated. If not, the NO branch is taken from inquiry block  205 , and inquiry block  215  is reached, wherein inquiry is made whether a phase-phase short, as in  FIG. 12 , is detected. If so, the YES branch is taken, and block  220  shuts down all phases. If not, the NO branch is taken, and inquiry block  225  is reached.  
         [0071]     Block  225  inquires whether a phase-ground short is detected, such as that of  FIG. 13 . If so, the NO branch is taken in  FIG. 16 , and block  230  is reached, wherein the affected phase is shut down in the proper quadrants, as discussed above. If not, the NO branch is taken, which leads to inquiry block  235 . If the short to ground is near the middle of the phase, the measures discussed above can be taken. In the case of block  225 , the short is probably at or near one end of the phase.  
         [0072]     Block  235  inquires whether an FET is shorted, as shown in  FIG. 14 . If so, the YES branch is taken, and block  240  is reached, wherein the available phases are powered in the quadrants where available. If not, the NO branch is taken, and inquiry block  245  is reached.  
         [0073]     Block  245  inquires whether an FET is open, as shown in  FIG. 15 . If so, the YES branch is taken, and block  250  is reached, wherein the available phases are powered in the quadrants where available. If not, the NO branch is taken, and the logic returns to block  200 .  
         [0000]     Additional Considerations  
         [0074]     1. A two-phase motor is not merely a motor which contains two phases, but which contains no more than two phases which produce torque. Some motors contain stator coils which assist in commutation, and other functions largely unrelated to driving the rotor. The presence of such coils in a two-phase system does not change the two-phase nature of the motor.  
         [0075]     From another perspective, a two-phase motor utilizes two spatially orthogonal coils to generate the rotating stator field.  
         [0076]     2. One advantage of the 2-phase system described herein is that the relay  60  of  FIGS. 3-9  is eliminated. This elimination provides several advantages. One is that the expense of the relay is eliminated, and the expense is not trivial because the relay is a high-current device, handling currents in the range of 100 Amps.  
         [0077]     A second advantage is that the relay, being a mechanical device, has inherent reliability issues. A third is that the environment in which the relay resides creates its own reliability issues: for practical reasons, to reduce external wiring, the relay must be located within the motor. However, the internal temperatures in the motor can reach more than 125 C. Relays which are rated to operate at that temperature are difficult to find, and expensive when found.  
         [0078]     A fourth advantage is that the relay is energized whenever the motor is operating, and thus consumes power. It may be thought that a normally closed relay could be used, but fail-safe considerations preclude this usage.  
         [0079]     3. The magnetic field produced by coil  115  in  FIG. 9  does not induce a significant voltage, if any voltage, in coil  120 . Similarly, the magnetic field produced by coil  120  in  FIG. 9  does not induce a significant voltage, if any voltage, in coil  115 . The reason is that the fields (except for stray fields) are orthogonal to the planes of the windings in the coils.  
         [0080]     As a consequence, there is no mutual coupling of the coils, unlike the 3-phase case, where coupling occurs.  
         [0081]     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.