Abstract:
An electronically commutated motor (ECM) often employs a Hall sensor for reliable operation. Even when a Hall sensor is omitted from a motor structure, one can assure reliable startup, in a preferred rotation direction, if the motor ( 20 ) is designed with an auxiliary reluctance torque (T rel ) which, when the motor is running in the preferred rotation direction (DIR=1), has a driving branch ( 130 ) that is effective where a gap ( 136 ) exists in an electromagnetic torque (T el ) between two successive driving portions of that electromagnetic torque (T el ), and by using the steps of (a) upon starting, controlling application of electrical energy to the motor ( 20 ) in such a way that, in the event of a start in the wrong rotation direction, the motor cannot overcome the braking reluctance torque ( 130 ′) which is then effective; and (b) monitoring rotor movement to determine whether the rotor ( 22 ) is rotating in the desired rotation direction (DIR=1).

Description:
CROSS-REFERENCES  
       [0001]     This application incorporates by reference the Müller patents, U.S. Pat. No. 4,119,895 and corresponding DE 23 46 380-C2. This application claims priority from German application DE 10 2004 024 638.6, filed 12 May 2004, the entire content of which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a starting method for an electronically commutated motor (ECM) that, prior to starting, is rotated into a predefined rest position by a reluctance torque built into the motor, as is the case, for example, in fans that are driven by such motors.  
       BACKGROUND  
       [0003]     Motors of this kind usually use a rotor position sensor, for example a Hall sensor, to ensure starting in the desired rotation direction. A Hall sensor of this kind requires precise mechanical placement, which is difficult especially with small motors. The permissible maximum temperature of a Hall sensor is also limited, and problems can result when it is used in an aggressive atmosphere. It is also often desirable for the electronics to be at a distance from the motor, e.g. for applications in an environment where explosion protection is necessary.  
         [0004]     The so-called “sensorless” principle is therefore utilized in such cases, in order to enable dependable starting of the motor in the correct rotation direction. Once the motor has started, continued operation in the desired rotation direction does not constitute a problem.  
       SUMMARY OF THE INVENTION  
       [0005]     It is therefore an object of the invention to provide a new starting method and a corresponding motor structure, in which reliable operation is achieved, even in the absence of a Hall sensor.  
         [0006]     According to the invention, this object is achieved by controlling electrical energy applied to the motor so that, if it tries to start in the non-preferred direction, the motor cannot overcome a braking portion of auxiliary reluctance torque, and monitoring rotor movement to detect whether the rotor is rotating in the desired rotation direction. The result thereof is that the motor reliably starts up in the desired direction.  
         [0007]     Further details and advantageous refinements of the invention are evident from the exemplary embodiments, in no way to be understood as a limitation of the invention, that are described below and shown in the drawings.  
     
    
     BRIEF FIGURE DESCRIPTION  
       [0008]      FIG. 1  is a circuit diagram of a motor adapted for carrying out a method according to the present invention;  
         [0009]      FIG. 2  is an overview diagram to illustrate a method according to the present invention;  
         [0010]      FIG. 3  depicts the torques that occur during operation, in a motor according to the invention, when that motor is rotating in the desired rotation direction;  
         [0011]      FIG. 4  is similar to  FIG. 3 , but depicts the torques for the case in which the rotor is not rotating in the desired rotation direction;  
         [0012]      FIG. 5  is a flow chart to explain a preferred method sequence;  
         [0013]      FIG. 6  depicts the currents that can occur upon starting;  
         [0014]      FIG. 7  is a depiction similar to  FIG. 6 , in which the current through the motor is lower after starting than during starting;  
         [0015]      FIG. 8  shows the profile of the induced voltage during operation of the motor in its preferred rotation direction;  
         [0016]      FIG. 9  shows the profile of the induced voltage during operation of the motor opposite to its preferred rotation direction;  
         [0017]      FIG. 10  is a basic circuit diagram to explain the considerations underlying “sensorless” sensing of the rotation direction in a motor of this design;  
         [0018]      FIG. 11  schematically depicts sensing of the rotation direction for DIR=1; and  
         [0019]      FIG. 12  schematically depicts sensing of the rotation direction for DIR=0.  
     
    
     DETAILED DESCRIPTION  
       [0020]      FIG. 1  illustrates a circuit for operating a so-called “two-pulse” electronically commutated motor  20  (ECM) that has a permanent-magnet rotor  22  and a stator winding, the latter being shown here with two phases  24 ,  26  that are usually magnetically coupled to one another, via the iron of the stator lamination stack (not shown). A motor of this kind is called “two-pulse” because two stator current pulses flow in stator winding  24 ,  26  for each rotor rotation of 360° el. In many cases, the stator winding can have only one phase, and then a current pulse flows in it in one direction during one rotation of 180° el., and a current pulse flows in the opposite direction during the subsequent rotation of 180° el. There are many designs for these motors, which are produced in enormous quantities. A typical example is shown in Müller DE 23 46 380 C2 and corresponding U.S. Pat. No. 4,119,895. Such motors are often implemented as so-called “claw pole motors,” the claw poles then being implemented so that they generate a reluctance torque dependent on the rotational position.  
         [0021]     Most motors of this kind use a Hall sensor to sense the rotor position. When it is necessary to produce such motors for an extended temperature range, however, or when a considerable distance exists between ECM  20  and its electronic controller, the rotor position must be sensed using the so-called “sensorless” principle.  
         [0022]      FIG. 1  refers to a circuit based on the sensorless principle, i.e. having no Hall sensor. Rotor  22  is depicted with two poles, but can also have different numbers of poles. In a two-pole rotor, one complete revolution corresponds to a rotation through 360° electrical, i.e. in this case 
 360° mech.=360° el.   (1).  
 For a four-pole rotor: 
 360° mech.=720° el.  
 These relationships are familiar to those of ordinary skill in the art of electrical engineering. 
 
         [0023]     Motor  20  is controlled by a microcontroller (μC)  30  whose terminals are labeled  1  through  14 . These refer to a μC of the PIC16F676 type, details of which are available at the website  WWW.MICROCHIP.COM  maintained by Microchip Technology Inc. of Chandler, Ariz., USA. Terminal  1  is connected to a regulated voltage of +5 V, and terminal  14  to ground  32 . A capacitor  34  is connected between terminals  1  and  14 .  
         [0024]     Motor  20  is supplied with power by an operating voltage U B . The positive terminal is labeled  36 , and the first terminals  24 ′,  26 ′ of phases  24 ,  26  are connected to terminal  36 , as shown. Present between positive terminal  36  and ground  32  is, for example, a potential difference U B =13 V, i.e. the voltage of a typical vehicle battery (not shown).  
         [0025]     An n-channel MOSFET (Metal Oxide Field Effect Transistor)  40  serves to control the current in phase  24 , and an n-channel MOSFET  42  serves to control phase  26 . For that purpose, terminal  24 ″ of phase  24  is connected to drain terminal D of transistor  40 , and terminal  26 ″ of phase  26  to terminal D of transistor  42 . The source terminals S of the two transistors are connected to one another and to drain D of an n-channel MOSFET  44  which serves to generate a constant total current in phases  24 ,  26 . Source S of transistor  44  is connected to ground  32  via a resistor  46  serving for current measurement. Voltage u R  at resistor  46  is delivered via an RC filter element  48 ,  50  to input  3  of μC  30 . The μC furnishes, at its output  2 , a control signal  52 , corresponding to the magnitude of u R , which controls the working point of transistor  44  so as to yield the desired constant current, which can be adjusted by the program of μC  30 .  
         [0026]     Inexpensive controllers, such as those used in motors, usually have no hardware to generate a PWM (Pulse Width Modulation) signal. Such a signal would therefore need to be generated by a program, which would consume most of the resources of such a microcontroller.  
         [0027]     In this case, therefore, capacitor  56  is first charged through resistor  54 . As a result, transistor  44  operates so as to yield the desired constant current, and that current is consequently adjustable by the program of μC  30 . When capacitor  56  is charged, the terminal of μC  30  is switched to high resistance. When capacitor  56  discharges, its charge is “refreshed.” μC  30  usually requires only one clock cycle for this, i.e. 1 microsecond for the microcontroller indicated.  
         [0028]     Transistors  40 ,  42  are each driven by control transistor  44 , in the source region, in such a way that the current through phases  24 ,  26  is substantially constant, at least during commutation. Transistors  40 ,  42  are operated, for that purpose, as so-called “pinch-off” current sources. When transistor  40  is made conductive, for example, control transistor  44  acts as a resistor with respect to ground  32 . The current intensity through phase  24 ,  26 , and therefore the rotation speed of motor  20 , can therefore be set by means of signal  52 , and thus the voltage u 56  at capacitor  56 .  
         [0029]     The result of control transistor  44  is that the drain-source voltage U DS  in transistors  40  and  42  is modified, and the magnitude of the current through phases  24  and  26  is therefore also influenced. Another possible result of this is that transistors  40 , operate in the pinch-off range. All types of field-effect transistors exhibit a pinch-off range of this kind.  
         [0030]     When control transistor  44  is driven in such a way that it exhibits a high resistance, and therefore low conductivity, the potential at source S of the respectively conductive output-stage transistor  40 ,  42  then rises. As a result, less current flows through that transistor and it transitions into the pinch-off range.  
         [0031]     When control transistor  48  is driven in such a way that it has a low resistance and therefore high conductivity, the potential present at source S of the respectively conductive transistor  40  or  42  is therefore low. The high gate-source voltage associated therewith results in a correspondingly high current intensity in phases  24 ,  26 .  
         [0032]     In contrast to an ordinary commutation operation, the current in motor  20  is thus kept substantially constant, with the results that motor  20  runs very quietly, and the starting of motor  20  can be controlled.  
         [0033]     Transistor  40  is controlled by output  5  of μC  30 , and transistor  42  by output  6 . For that purpose, output  5  is connected via a resistor  60  to gate G of transistor  40 , which is connected via a resistor  62  to ground  32  and via the series circuit of a resistor  64  and a capacitor  66  to drain D. The latter is connected via a resistor  68  to a node  70 , which is connected via a capacitor  72  to ground  32  and via a resistor  74  to terminal  8  of μC  30 . (Terminals  4 ,  7 ,  9 , and  11  of μC  30  are not connected.)  
         [0034]     During operation, a voltage u 72  that is used to determine the rotation direction of rotor  22  occurs at capacitor  72 . This will be described below.  
         [0035]     Output  6  is connected via a resistor  80  to gate G of transistor  42 , which is connected via a resistor  82  to ground  32  and via the series circuit of a resistor  84  and a capacitor  86  to drain D.  
         [0036]     Terminal  24 ″ is connected via a capacitor  90  to a node  92 , which is connected via a resistor  94  to ground  32  and via a resistor  96  to input  12  of μC  30 , to which a filter capacitor  98  is also connected.  
         [0037]     Terminal  26 ″ is connected via a capacitor  100  to a node  102 , which is connected via a resistor  104  to ground  32  and via a resistor  106  to input  13  of μC  30 , to which a filter capacitor  108  is also connected.  
         [0038]     Elements  90  through  108  cause the point in time during a rotor rotation at which the current through phase  24  or  26  is switched on to be shifted to an earlier point in time with increasing rotation speed; borrowing from the terminology of a gasoline engine, this is usually referred to as “ignition advance,” even though of course nothing is being “ignited” in an electric motor  20 .  
         [0039]     Connected to terminal  36  via a resistor  112  is a node  114  that is connected via a capacitor  116  to ground  32 . A voltage u 116  dependent on the voltage at terminal  36  occurs during operation at capacitor  116 , and this voltage is delivered via a line  118  to input  10  of μC  30  and serves to eliminate, by computation, noise voltages that are contained in voltage u 72 . This will be described below.  
                                             PREFERRED VALUES OF COMPONENTS IN  FIG. 1  (for U B  = 13 V)                                    Transistors 40, 42, 44   ILRL3410           C72, 116    2 nF           C50, 66, 86, 98, 108    1 nF           C34, 56   100 nF           R62, 68, 82, 94, 96, 104, 106, 112   100 kilohm           R48, 54, 60, 80    10 kilohm           R74    0 ohm           R46    1.5 ohm           R64, 84    1 kilohm                      
 
         [0040]     Motor  20  has a rotation direction sensing system  72 ,  74  with which a determination can be made as to whether the motor, after a startup attempt, is rotating in the desired rotation direction. Motor  20  furthermore has a control system, namely μC  30 , with which current regulator  44  can be set to a desired starting current; this current regulator  44  acts on output stages  40 ,  42 , as described above, in such a way that motor  20  can be operated with a constant starting current that is adjusted precisely in accordance with requirements.  
         [0041]     Rotation direction sensing system  72 ,  74  allows a start in the wrong rotation direction to be detected and reported to control system  30 . The latter then stops motor  20  and makes another attempt to start in the correct direction.  
         [0042]      FIG. 3  shows the torques T that occur over the rotation angle alpha (α) of rotor  22  in a two-pulse motor  20  upon starting.  
         [0043]     A motor of this kind has a reluctance torque T rel  that is, so to speak, “built into” the motor and is therefore invariant. This torque has, for the rotation direction depicted in  FIG. 3 , a driving positive portion or branch  130  that is relatively short and has a high amplitude. T rel  additionally has a negative (i.e. braking) portion or branch  132  that has a low amplitude, but a longer duration.  
         [0044]     When rotor  22  is driven externally, it is braked between points A and F′ by negative branch  132  of reluctance torque T rel . Between points F′ and A′, T rel  becomes positive and thereby assists the rotation of rotor  22  in the desired rotation direction.  
         [0045]     When rotor  22  is driven in the opposite direction, as shown in  FIG. 4 , i.e. from A to F in  FIG. 4 , branch  130 ′ of T rel  then has a strongly braking effect between points A and F, and branch  132 ′ has a driving effect. The conditions are thus the reverse of those in  FIG. 3 .  
         [0046]     Also plotted in  FIG. 3  is the electromagnetic torque T el  that, for the rotation direction according to  FIG. 3 , has a driving effect in the manner depicted and thus overcomes negative branch  132  of T rel . Electromagnetic torque T el  has, as shown, gaps  136  that are bridged by positive branch  130  of T rel , as is directly evident from  FIG. 3 . The resultant torque T rel +T el  is consistently positive, and causes motor  20  to be driven continuously in the preferred direction, i.e. DIR=1.  
         [0047]      FIG. 4  shows the electromagnetic torque −T el  during operation in the opposite rotation direction. In this case, it is assisted by branch  132 ′ of T rel , while it is counteracted by branch  130 ′ (which is braking in this case) of T rel .  
         [0048]     A motor  20  of this kind thus has a preferred direction that is depicted in  FIG. 3 , in which torques T el  and T rel  complement one another very effectively; and it has a “bad” rotation direction shown in  FIG. 4 , in which torques T el  and T rel  coordinate very badly with one another, so that startup in this rotation direction is difficult. Startup in this rotation direction is not usually required.  
         [0049]     As shown in  FIG. 6 , in order to start in the rotation direction depicted in  FIG. 3 , the constant current I in the motor is set to a value I 1  , the rise in the current from I=0 to I=I 1  occurring substantially monotonically and within a short period.  
         [0050]     At starting, rotor  22  is usually in position A ( FIG. 3 ), because T rel  has a value of zero there and it is a stable rest position of rotor  22 .  
         [0051]     When rotor  22  starts from this rest position A in the correct rotation direction, the electromagnetic torque T el , which previously had a value of zero, then rises to point B ( FIG. 3 ), becomes greater than the braking branch  132  of T rel , and drives rotor  22  against braking branch  132  of T rel  so that rotor  22  rotates in the direction of arrow  140  ( FIG. 3 )  
         [0052]     Additional confirming actions would be superfluous in the context of startup in the preferred direction, but such actions are preferably performed in both rotation directions, so that the structure of the program used can be kept simple.  
         [0053]     As shown in  FIG. 6 , current I 1  is maintained for a time period Ta, i.e. between times t 1  and t 2 ; Ta can be, for example, between 0.5 and 2 seconds depending on the size of the motor.  
         [0054]     When motor  20  is then running normally, current regulator  44  sets current I to a value I 2  that corresponds to the desired rotation speed of motor  20 .  FIG. 6  shows the case in which I 2  is greater than I 1 .  FIG. 7  shows the opposite case, in which I 2  is less than I 1 . It is apparent, from this, that I 1  and time period Ta should be selected in accordance with the requirements for motor starting.  
         [0055]      FIG. 4  shows what happens if rotor  22  starts in the wrong direction. In this case, current I 1  generates a torque −T el  in the opposite direction, so that this electromagnetic torque −T el  drives rotor  22  in the direction of arrows  142  ( FIG. 4 ), in which context −T el  decreases in magnitude. A resultant total torque T el +T rel  is initially negative, and causes a small rotation opposite to the preferred direction. After passing through a point G, the resultant total torque T el +T rel  becomes positive, so that the rotation comes to a stop at point E.  
         [0056]     The profile and amplitude of −T el  are determined by the constant current I 1 . The latter is defined so that torque −T el  cannot overcome branch  130 ′ (which is braking in this case) of reluctance torque T rel  in the event of startup in the wrong rotation direction; in other words, rotor  22  starts from a point C and arrives at a point D. At point D a commutation occurs, i.e. the current is switched over either from phase  24  to phase  26  or vice versa. The result is that the direction of the electromagnetic torque is switched over to +T el , and a positive total torque (T el +T rel ) is produced which causes a rotation in the preferred direction, as indicated by an arrow  143 .  
         [0057]     The program of μC  30  contains the corresponding routines for this purpose.  
         [0058]      FIG. 5  is the corresponding flow chart, which begins at S 148 . At S 150  the rotation direction is set to DIR=1, and current regulator  44  is set to I=I 1 . The profile and duration of the ramp-up between values I=0 and I=I 1  can also be set.  
         [0059]     S 152  checks whether rotor  22  is, in fact, rotating in rotation DIR=1, i.e. whether a corresponding rotation direction signal is present. If NO, the program goes to S 154  and motor  20  is switched off.  
         [0060]     If DIR=1 in S 152 , S 156  then checks whether the time period Ta, e.g. one second, has already elapsed. If NO, energization with I 1  continues. If time period Ta has elapsed, the constant current is switched over to I 2  (see  FIG. 6  and  FIG. 7 ).  
         [0061]     Following S 154 , the program goes to S 156 , where the number N of starting attempts is counted. If this number is greater than 3, the program goes to S 158  and generates an alarm. If N is less than 4 in S 156 , a new attempt is made to start in the correct rotation direction.  
         [0000]     Ascertaining the Rotation Direction  
         [0062]     The rotation direction is ascertained by sensing and analyzing the voltages induced in the stator winding during operation. This is possible because, in a motor of the kind cited initially, these voltages have different profiles, depending on the rotation direction. From this, the desired information, regarding the rotation direction of the motor relative to the reluctance torque, can be derived.  
         [0063]      FIG. 8  shows the profile of the induced voltage u ind  during operation of motor  20  in its preferred rotation direction (DIR=1). It is apparent that the induced voltage u ind  shows a rising trend over a large rotation angle range  170  when the relevant phase is currentless. In rotation angle range  171  in which a current is flowing in the relevant phase, the voltage is lower and shows a decreasing trend.  
         [0064]      FIG. 9  shows, for comparison, the induced voltage u ind  during operation of motor  20  opposite to its preferred rotation direction (i.e. for DIR=0). It is apparent that the induced voltage decreases over a large rotation angle range  172  when the relevant phase is currentless. In rotation angle range  173  in which, for DIR=0, a current is flowing in the relevant phase, the voltage is lower and shows a rising trend.  
         [0065]     It should be noted that  FIGS. 8 and 9  are schematic depictions; in other words, the rise in ranges  170  and  173  and the decrease in ranges  171  and  172  may in reality be less pronounced. The differences are shown in exaggerated fashion, for didactic purposes.  
         [0066]      FIG. 10  shows a portion of  FIG. 1 , namely those elements of the measurement circuit that are essential for sensing the rotation direction.  
         [0067]     The potential at point  24 ″ of phase  24  is measured when transistor  40  is not conductive, i.e. when transistor  42  is carrying current. In this case, operating voltage U B  is present at point  36 , and added to this is the induced voltage u ind  in currentless phase  24 , so that the potential U at point  24 ″ is 
 
 U=U   B +u ind    (2). 
 
         [0068]     This potential is delivered through resistor  68  to capacitor  72 .  
         [0069]     Located in parallel with capacitor  72  is a switch S in μC  30 ; this switch S is closed most of the time—symbolized in  FIGS. 11 and 12  by “SC” (=switch closed)—thus keeping capacitor  72  discharged so that during this time, voltage u 72  has a value of zero.  
         [0070]     When a measurement M is to be performed, switch S is opened by the program of μC  30  so that the voltage u 72  at capacitor  72  rises to a value corresponding approximately to the instantaneous voltage U. This voltage at capacitor  72  is converted in A/D converter  120  into a digital value and temporarily stored.  
         [0071]     If the time interval between two commutations is designated Tp, this happens once, for example, after a time Tp/4, and at this point in time a first measurement M 1  is performed and a first value u_ 72 . 1  is stored.  
         [0072]     After a predetermined time period, e.g. after 0.5-0.6 Tp, a second measurement M 2  is then performed and the second value u_ 72 . 2  measured at that point is also stored.  
         [0073]     The difference Δ is then calculated, i.e.: 
 
Δ= u _ 72 . 2   −u _ 72 . 1    (3), 
 
 and the sign of the difference Δ is determined. 
 
         [0074]     In  FIG. 11 , the difference Δ is found to have a positive sign and, in  FIG. 12 , the sign is negative, since in the case of the rotation direction according to  FIG. 12  the voltage U has a decreasing characteristic (as in  FIG. 9 ) in the currentless phase, whereas in  FIG. 11  it has a rising characteristic (as in  FIG. 8 ). This is a property of these two-pulse motors that is exploited in the present case, in order to sense the rotation direction.  
         [0075]     It is very advantageous in this context that the current in resistor  46  is kept constant by control transistor  44 , i.e. phase  26  that is presently conducting current has substantially no influence on the voltage u ind  in phase  24 , in which the measurements are taking place, since the constant current in phase  26  causes no transformer coupling to phase  24 .  
         [0076]     Because motor  20  is running in DIR=1 after starting up correctly,  FIG. 11  shows that a positive Δ is obtained as confirmation of a correct startup.  
         [0077]     If motor  20  is rotating in direction DIR=0 after starting, a negative Δ is obtained as shown in  FIG. 12 ; starting is interrupted and a new starting attempt is made. This ensures that the motor starts in the correct rotation direction in every instance.  
         [0078]     The absolute measured values that are measured at the energized phase  24  or  26  are additionally used to generate a constant current. If u R  drops below 1 V, it becomes difficult to maintain a constant current.  
         [0079]     A great advantage of the present invention that a Hall sensor is not necessary, and that reliable startup in the desired rotation direction is nevertheless possible. ECMs (Electronically Commutated Motors) having a wider temperature range can thus be produced and, in an ECM of this kind, the motor can be physically separated from its control system.  
         [0080]     Many variants and modifications are of course possible within the scope of the present invention.