Abstract:
The invention concerns a method for determining a numerical value for the duration of a periodically repeating pulsed signal. This method comprises the following steps: a) at time intervals, the period length of the signal is determined; b) at time intervals, a characteristic magnitude for the length of a pulse of that signal is determined; c) a numerical value that characterizes the signal is ascertained from the period length and the characteristic magnitude. Because of its shortness and accuracy, the method is particularly suitable for use in electric motors. A corresponding arrangement is also presented and described.

Description:
[0001]    This application is a §371 of PCT/EP02/07204, filed 29 Jun. 2002 and published 13 Feb. 2003 as WO 03-012971-A. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention concerns a method for determining a numerical value for the duration of a periodically repeating pulsed signal, and an apparatus for carrying out such a method.  
         BACKGROUND  
         [0003]    In the context of controlling the rotation speed of electric motors, a so-called rated speed signal is usually specified to a rotation speed controller, i.e. the rotation speed controller is instructed by means of a suitable signal that the motor is to run at, for example, 32,246 or at 1100.5 rpm.  
           [0004]    That rated speed signal can be, for example, a voltage between 0 and 5 V, or a frequency, or the ratio between signal length and period length (also called the pulse duty factor) in the case of a periodically recurring signal as depicted at 68 in FIG. 5. A signal of this kind then has a specific frequency that will usually lie somewhere between 2000 and 5000 Hz, and the magnitude of the pulse duty factor, which can lie between 0% and 100%, indicates to the rotation speed controller the speed at which the motor is to run at that moment; for example, a pulse duty factor of 0% can mean that the motor is stationary, and 100% can mean a high rotation speed.  
           [0005]    Microprocessors (μP) or microcontrollers (μC) are often used for the control and regulation of such motors, the term “microprocessor” (μP) being used in the claims as a general term for both. A μP of this kind usually controls the commutation of the motor—assuming the latter is an electronically commutated motor (ECM)—and it also serves to regulate the rotation speed and to perform other functions as applicable.  
           [0006]    A μP of this kind requires as a target value for its controller a numerical value, e.g. “37” or “214”; in other words, in a signal of the kind described above, the ratio of signal length to period length, or the ratio of signal off-time to period length, must be converted into a suitable numerical value that lies, for example, in the numerical range from 0 to 255 or 0 to 1023. This requires that both the period length and the signal length be sensed as accurately as possible, to the extent possible with an inexpensive μP such as those used for cost reasons in motors.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the invention to provide a novel method for determining a numerical value for the duration of a periodically repeating pulsed signal, and an apparatus for carrying out such a method.  
           [0008]    According to the invention, this object is achieved by the subject matter of claim  1 . Because the period length and the characteristic magnitude are ascertained at time intervals, those values can be ascertained sequentially and then processed further. This allows the measurement to be split up into relatively short routines that can be executed at suitable points in time, preferably at specific rotational positions of the rotor of a motor.  
           [0009]    It is particularly advantageous in this context to proceed in accordance with claim  2 , i.e. the type of measurement accomplished is dependent on a previously determined numerical value for the pulse duty factor of the signal. This permits an optimization of the measurement.  
           [0010]    A particularly advantageous embodiment of an optimized measurement of this kind is the subject matter of claims  3  and  4 . This type of measurement of the pulse duration can be performed provided a pulse is long with respect to the period length; and a measurement of the off-time length between two pulses can be performed provided the pulse duration is short and the off-time length is consequently long in relation to the period length.  
           [0011]    A particularly advantageous embodiment is the subject matter of claim  5 , since a switching hysteresis is obtained here so that frequent switching between the various measurement types, which might reduce measurement accuracy, does not occur.  
           [0012]    A further approach to achieving the stated object is the subject matter of claims  13  and  14 , yielding very simple and fast-working solutions. 
       
    
    
     BRIEF FIGURE DESCRIPTION  
       [0013]    Further details and advantageous developments of the invention are evident from the exemplary embodiments, which in no way are to be understood as a limitation of the invention, that are described below and depicted in the drawings.  
         [0014]    [0014]FIG. 1 is an overview circuit diagram for an electronically commutated motor  30  to which is conveyed, from outside, a periodic pulse train  68  whose pulse duty factor contains a datum for operation of the motor;  
         [0015]    [0015]FIG. 2 is a schematic diagram showing interactions in a motor according to FIG. 1;  
         [0016]    [0016]FIG. 3 is an overview depiction of a preferred program sequence in the motor according to FIGS. 1 and 2;  
         [0017]    [0017]FIG. 4 schematically depicts the processes occurring in the motor according to FIG. 1 as a function of the rotational position of the rotor;  
         [0018]    [0018]FIG. 5 depicts a first pulse train having periodically recurring pulses of period length T and a pulse duty factor of approx. 33%;  
         [0019]    [0019]FIG. 6 is a depiction similar to FIG. 5 with the same period length T but very short pulses and a pulse duty factor of approx. 5%;  
         [0020]    [0020]FIG. 7 is a depiction similar to FIGS. 5 and 6 with the same period length T but with very long pulses and a pulse duty factor of approx. 93%;  
         [0021]    [0021]FIG. 8 is a depiction in which the signal is continuously high and consequently has a pulse duty factor of 100%;  
         [0022]    [0022]FIG. 9 is a depiction in which the signal is continuously low and consequently has a pulse duty factor of 0%;  
         [0023]    [0023]FIG. 10 is a diagram to explain the interrupts that occur in normal circumstances upon measurement of a pulse  69 ;  
         [0024]    [0024]FIG. 11 is a diagram to explain the situation in which a pulse is so short that an interrupt is possible only at the beginning of the pulse, but not at the end of the pulse;  
         [0025]    [0025]FIG. 12 is a diagram to explain the situation in which a pulse off-time is so short that an interrupt is possible only at its beginning, but not at its end;  
         [0026]    [0026]FIG. 13 depicts a state machine preferably used in the context of the invention;  
         [0027]    [0027]FIG. 14 depicts the basic processes in the context of the measurement and evaluation of pulses;  
         [0028]    [0028]FIG. 15 is an expanded version of FIG. 14 showing the evaluation of special cases (PWM=0% or 100%) and the handling of errors;  
         [0029]    [0029]FIG. 16 is a diagram showing basic processes that are recorded and controlled by the state machine;  
         [0030]    [0030]FIG. 17 is a diagram showing how the various states of the state machine are cycled through in normal circumstances;  
         [0031]    [0031]FIG. 18 is a flow chart of an interrupt routine that is triggered by a settable edge of a signal;  
         [0032]    [0032]FIG. 19 is a flow chart of a “Meas. Signal” measurement routine which serves to determine the duration of pulsed signals or signal off-times; and  
         [0033]    [0033]FIG. 20 is a flow chart of an EVAL routine which serves to evaluate the data sensed in the routine according to FIG. 19.  
     
    
     DETAILED DESCRIPTION  
       [0034]    [0034]FIG. 1 shows, in order to illustrate the invention, an electronically commutated motor (ECM)  30  having two stator winding phases  32 ,  34  and a permanent-magnet rotor  36  which is depicted here as having four poles and in whose vicinity is arranged a Hall generator  38  that, in operation, generates at its output  40  a rectangular HALL signal whose edges are labeled, for example,  1 ,  2 ,  3 ,  4 .  
         [0035]    Motor  30  has an EMI filter  42  and a filter capacitor  44  for delivery of a DC voltage U B . A transistor  46  that serves as a first output stage (PS 1 ) is connected in series with phase  32 , and a transistor  48  that serves as a second output stage (PS 2 ) is connected in series with phase  34 . When transistor  46  is switched on, phase  32  receives current. When transistor  48  is switched on, phase  34  receives current.  
         [0036]    A microcontroller (μC)  54  serves to control transistors  46 ,  48 . Various modules are depicted symbolically in this microcontroller, including a module  56  for commutation, a ROM  62  (inside or outside μC  54 ) to store the program for motor  30 , a module n_CTL  64  for rotation speed regulation that regulates the rotation speed of motor  30  via module  56 , and also a module SW_CALC  60  for calculating target value SW that is conveyed to controller  64 . The present value of the rotation speed is conveyed in the form of the HALL signal to controller  64 , and also to modules  56  and  64 . AC  54  further contains a timer  66 , which may be thought of as a clock that furnishes, for each desired point in time, a so-called baseline time. This timer coacts with modules  60  and  64 .  
         [0037]    As shown in FIG. 1, a target value is conveyed from outside to μC  54 , at an input  67 , as a periodic pulse train  68 ; and the information concerning the desired rotation speed (SW) is contained in the pulse duty factor of pulses  68 . This is explained below in further detail.  
         [0038]    As is evident from FIG. 1, μC  54  must process two different pulse trains namely, on the one hand, the HALL pulse train and, on the other hand, pulse train  68 . Since the HALL pulse train is critical for operation of the motor, its processing usually takes precedence over the processing of pulse train  68 , except that at certain critical junctures the processing of pulse train  68  must not be interrupted, and pulse train  68  then takes precedence.  
         [0039]    [0039]FIG. 2 shows the manner in which the individual functions intermesh in the context of such a motor. Pulse train  68  is delivered at  68 , and is processed in module  60  to yield the value SW. The ON/OFF signals for switching motor  30  on or off are delivered at  72 , and they also pass through module  60 . At  74 , operating voltage UB is delivered; this can be taken into account, for example, in such a way that the motor is switched off if the operating voltage is too low, or so that certain changes in the program are made if the operating voltage is too high. Hall IC  38  that generates the HALL signal is depicted at  38 ; that signal is processed in a processing module  76  and furnishes information about the instantaneous position and rotation speed of rotor  36 .  
         [0040]    Lastly, commutation module  56  that controls the two output stages PS 1 , PS 2  in motor  30  is provided.  
         [0041]    [0041]FIG. 2 shows that there exist among the individual modules interactions that must be taken into account, as applicable, when configuring the program to be used in an ECM.  
         [0042]    [0042]FIG. 3 shows a typical basic structure of the program that is used to control the various functions of motor  30 . In step S 84  (Power-on initialize), at start-up an initialization is performed, in which various parameters are set to initial values. In step S 86  (Reset watchdog) the computer&#39;s watchdog is reset, and in step S 88  (Re-initialize) a reinitialization of certain values is performed at each pass in order to prevent μC  54  from crashing.  
         [0043]    At S 90  (Commutation control), the commutation is controlled, and in step S 92  (Flag_DoFcts?), a flag is polled: if its value is 0, the program goes back to S 86 ; if its value is 1, this flag is set to 0 in step S 94  (Flag_DoFcts=0). The subsequent step S 98  (Hall_CNT even?) asks whether the Hall counter is even or odd. This refers to the depiction of the HALL signal in FIG. 1. As depicted there, at each change in this signal a counter is advanced by one value, for example in the sequence 1-2-3-4-1-2-3-4; if the counter has an even value, then in S 100  (Target value sensing) a portion of the target value calculation SW_CALC is performed, and then in step S 102  (Flag_Actual Value_sensed=0) a flag for sensing the actual value is set to zero. The program then goes back to step S 86 .  
         [0044]    If the response in S 98  is No, the program goes to step S 104  (Flag_Actual Value_sensed?) which inquires whether the flag for the actual value has a value of 1 or 0. If its value is  1 , the program then goes to step S 106  (Target value sensing) where a portion of the SW_CALC calculation is performed; and if the response in S 104  is 0, the present rotation speed is then sensed in S 108  (Actual value sensing) by evaluating the HALL signal. In S 112  (Flag_Actual Value_sensed=1), the flag for the actual value is then set to 1 in order to indicate that the actual value has been sensed and that step S 106  (target value calculation) can be performed at the next pass through S 104 .  
         [0045]    The diagram shows that calculation of the target value is distributed between steps S 100  and S 106  since calculation of the target value requires quite a lot of time and, without that distribution, would not be compatible with the other functions of the motor.  
         [0046]    [0046]FIG. 4 is an overview depicting the sequence of program steps in motor  30  as a function of the rotational position of rotor  36 . An electric motor that is controlled by a μC  54  can have numerous additional functions depending on how it is used, e.g. rotation speed regulation, rotation speed limitation, current limitation, regulation to constant current, arrangements for outputting alarm signals, error handling routines, etc.  
         [0047]    In the present exemplary embodiment, the rotation speed of motor  30  is regulated to a target value, e.g. to 3000 rpm. That target value must therefore be updated for the control program at relative frequent intervals.  
         [0048]    A knowledge of the motor&#39;s instantaneous rotation speed, e.g. 2990 rpm, is also necessary for rotation speed regulation. This actual value of the rotation speed must also be updated at relatively frequent intervals.  
         [0049]    In addition, certain parameters must be reinitialized from time to time in order to ensure stable motor operation, and μC  54  must switch the current to motor  30  on and off, in accordance with the calculations of the rotation speed controller and also switch over the direction of the motor current depending on the instantaneous rotational position. All these operations are referred to in electrical engineering as “commutation.” This should be accomplished with great precision, since a motor runs quietly only if the commutation instructions are executed very exactly. This means that the program must check very frequently whether a program instruction for commutation is present and needs to be executed.  
         [0050]    [0050]FIG. 4 a  shows the profile of the HALL signal, and FIG. 4 b  symbolically shows the loops through which the program passes.  
         [0051]    As shown in FIG. 4, immediately after an edge  120 ,  122  of the HALL signal there is a large calculation loop  124 ,  126 ,  128  (FIG. 3) in which longer calculation procedures are performed depending on the value of counter HALL_CNT, and then there are many short calculation loops  130  in which commutation is merely checked and, if applicable, controlled. Since these short loops  130  contain very few steps and therefore follow one another very closely, they result in high resolution; in other words, every 60 to 100 μs the program checks whether anything needs to be modified in terms of commutation.  
         [0052]    [0052]FIG. 4 shows that, for example, directly after a rising edge  120  of the HALL signal, a long loop  124  is executed in which, as shown in legend  134 , the target value (SW) for regulating the rotation speed is calculated and the commutation is also checked. Large loop  124  is followed by many short loops  130  in which, as shown in legend  136 , commutation is simply checked and modified if necessary.  
         [0053]    In this example, a falling edge  122  of the HALL signal is followed by a long loop  126  in which, as shown in legend  138 , the following calculation steps are performed:  
         [0054]    actual value calculation (IW)  
         [0055]    commutation (Komm.).  
         [0056]    This long loop  126  is once again followed by short loops  130  for monitoring and controlling commutation.  
         [0057]    The next rising edge  120  of the HALL signal is once again followed by a long loop  124  of the kind already described. The result is that certain values are ascertained in the region of each Hall edge  120 ,  122 , i.e. at certain rotor positions, so that, for example, in the case of a four-pole rotor  36 , in the course of one complete revolution, a target value calculation is performed twice and an actual value calculation is performed twice. As is evident from FIG. 3, when counter HALL_CNT yields an “odd” result in step S 98 , either loop  126  or loop  128  can be run through, depending on whether the flag at step S 112  was set to  1  during the previous pass through that step. The calculation operations are thus distributed in time, and the distribution is controlled by factors that include the position of rotor  36 .  
         [0058]    This rotor-position-dependent sensing of values is possible in a motor because the rotation speed usually changes only slightly in the course of one rotor revolution.  
         [0059]    [0059]FIG. 5 is an enlarged depiction of signal  68  of FIG. 1, by means of which controller n_CTL  64  is informed of the desired rotation speed. This signal  68  has pulses  69  and pulse off-times  70 . Since it has a periodic profile, it has a period length T that can be, for example, 1 ms=0.001 s, and in that case signal  68  has a frequency of 1/0.001=1000 Hz. The length of pulses  69  is labeled t, and the length of the pulse off-times is labeled t′. Therefore  
           T=t+t′   (1).  
         [0060]    [0060]FIG. 6 shows that pulses  69  can be very short, resulting in long pulse off-times  70 ; and FIG. 7 shows conversely that pulses  69  can be very long and pulse off-times  70  consequently can be very short.  
         [0061]    As shown in FIG. 8, signal  68  can also continuously have a HIGH value, which corresponds to a pulse duty factor of 100%; and conversely, according to FIG. 9 signal  68  can also continuously have a LOW value, corresponding to a pulse duty factor of 0%.  
         [0062]    All the situations shown in FIGS. 5 through 9 must be correctly interpreted by the software. In the situations according to FIGS. 5 through 7, period length T must be measured in every case. In the situations of FIGS. 8 and 9, period length T is equal to infinity, and that fact must be correctly interpreted by the program.  
         [0063]    In the situation shown in FIG. 7, according to the present invention not only T but also the value t is measured, i.e. the length of pulses  69 , which in this instance is not much less than T.  
         [0064]    In FIG. 6, pulses  69  are very short and therefore difficult to measure, since pulse measurement is accomplished by measuring, by means of timer  66  (FIG. 1), the point in time of the rising edge of a pulse by means of a first interrupt, and the point in time of the falling edge of pulse  69  by means of a second interrupt, and ascertaining the difference between those two points in time. Since each interrupt requires a certain amount of time, e.g. 30 μs, this is difficult if pulse length t is very short, and the measurement then becomes very inaccurate or indeed impossible.  
         [0065]    According to the invention, therefore, with short pulses  69  (as depicted in FIGS. 5 and 6) time t′ for a pulse off-time is measured and calculated, and the formula  
           t=T−t′( 2)  
         [0066]    is then used to calculate pulse length t indirectly from that time.  
         [0067]    Since pulses  69  are in all cases long when motor  30  is switched on, a FlagPM (=pulse measurement) that defines the type of measurement is set, at the initialization after switching on, to FlagPM=1, which means a measurement of pulse length t; the ratio t/T (or alternatively t′/T) is then continuously monitored, and if the former ratio drops below 46%, FlagPM is set to 0 in order to switch over to measurement of the duration t′ of pulse off-times  70 .  
         [0068]    Conversely, if FlagPM=0, the program checks whether the ratio t/T becomes greater than 51%, in which case FlagPM=1 is set and pulse length t is measured. The difference between 46 and 51% results in a hysteresis, i.e. at a ratio t/T of 50% either the pulse length or the duration of the pulse off-times is measured, and the type of measurement changes only when the value either exceeds 51% or falls below 46%. The numbers 46 and 51 are, of course, merely examples indicated for better comprehension of the invention.  
         [0069]    [0069]FIG. 10 and FIG. 11 explain a problem that occurs in the measurement of pulses. FIG. 10 shows a pulse  69 . The latter has a rising edge  144 , and at a time previous to that edge, during a measurement of pulse length t, the sensitivity of input  67  (FIG. 1) of μC  54  is set so that the rising edge of a signal there triggers an interrupt  146  that results in a measurement of time t 01  in TIMER  66 .  
         [0070]    An interrupt comprises a plurality of instructions, and an interrupt routine of this kind requires a certain length of time for execution, ending e.g. at time t 02 . Its duration is, for example, between 60 and 100 ps.  
         [0071]    In FIG. 10, a check is made at time t 02  as to whether input  67  is high or low. In this case the input is high, i.e. pulse  69  has not yet ended. At time t 02  the sensitivity of input  67  is therefore switched over so that at the falling edge  148  that will then follow, an interrupt  150  is triggered and results in measurement of time t 03  in timer  66 .  
         [0072]    The length of pulse  68  is then calculated (t=t 03 −t 01 ). This is therefore a measurement in the context of pulses  69  that are longer than interrupt routine  146 . The polling of input  67  at time t 02  confirms that pulse  69  is still continuing, and therefore that its end can subsequently be measured.  
         [0073]    [0073]FIG. 11 shows the analogous situation for a very short pulse  69 ′ that is shorter than interrupt routine  146 , i.e. for example only 30 μs. In this case as well, prior to rising edge  144  the sensitivity of input  67  is set so that at edge  144 , an interrupt  146  is triggered and time t 01  is measured.  
         [0074]    Here as well, in similar fashion, input  67  is polled at the end of interrupt routine  146  (i.e. at time t 02 ), and it is found that this input has a LOW value. This means that pulse  69 ′ has already ended, that value t 03  therefore cannot be measured, and that it is necessary to switch over to measurement of pulse off-time t′, i.e. FlagPM=0 is set here, and this measurement is not evaluated.  
         [0075]    [0075]FIG. 12 shows a situation analogous to FIG. 11, i.e. measuring a very short pulse off-time  70  whose length t′ is shorter than that of the interrupt routine.  
         [0076]    In this case the sensitivity of input  67  is set so that falling edge  152  at the beginning of pulse off-time  70  triggers an interrupt  154  which lasts longer than pulse off-time  70 . That interrupt  154  ends at time t 05 , and at that time the signal at input  67  is polled and is found to be High at that time. This means that the pulse off-time has already ended and therefore cannot be measured. The measurement is therefore discarded, and the setting is switched over to FlagPM=1, i.e. measurement of the pulse length. If, conversely, input  67  were low at time t 05  in FIG. 12, then pulse off-time  70  would not yet have ended and would be measured by triggering, at the rising edge following falling edge  152 , a new interrupt with a time measurement.  
         [0077]    [0077]FIG. 13 shows a so-called “state machine” that is used in the present exemplary embodiment. This is a variable, namely a register in the RAM of μC  54 , that here can assume values from 1 to 8. Depending on the routine that is presently being executed, this register has different values that can be polled in the program. The individual states of FIG. 13 have the following meanings:  
         [0078]    State  1  “T_Start”. This means that the first interrupt for sensing period length T is expected.  
         [0079]    State  2  “t 1 _Start”. This means that the first interrupt for pulse sensing (edge  144  of FIG. 10, edge  152  of FIG. 12) is expected. This can therefore be both sensing of a pulse and sensing of a pulse off-time.  
         [0080]    State  3  “T_End”. This means that the second interrupt for sensing period length T is expected, i.e. the interrupt at point  156  in FIG. 5.  
         [0081]    State  4  “t 1 _End”. This means that the second interrupt for sensing the pulse length (interrupt  148  in FIG. 10), or the second interrupt for sensing the off-time length, is expected.  
         [0082]    State  5  “T_Over”. This means that the sensing of period length T is complete, and that sensing of pulse length t or sensing of off-time length t′ now follows.  
         [0083]    State  6  “t 1 _Over”. This means that sensing of the pulse length (t in FIG. 7) or sensing of the off-time length (t′ in FIG. 6) is complete, and that evaluation of the measured data now follows.  
         [0084]    State  7  “Limit”. This means that signal  68  contains no edges, as depicted in FIGS. 8 and 9, so that no interrupts are being generated. Signal  68  is then either statically High (FIG. 8) or statically Low (FIG. 9). This state is processed in steps S 296  through S 300  in FIG. 19, and SM=7 is therefore then set in S 302 .  
         [0085]    State  8  “Error”. This means a sensing error in the sensing of the pulse length as described in FIG. 11, or a sensing error in the sensing of the off-time length as described in FIG. 12. In other words, only one of the two interrupts pertaining to a measurement could be sensed, but an error occurred in the case of the second interrupt, as follows:  
         [0086]    a) Either the pulse was too short, so that the second interrupt could not be sensed, as described with reference to FIGS. 11 and 12. This state is processed in FIG. 18 at S 232 , S 234 , S 236 , and S 244 . SM=8 is then set in S 238  or S 246 , and the measurement type is automatically switched over (S 242 , S 252  in FIG. 18).  
         [0087]    b) Or the second interrupt came too late (after the Timeout in FIG. 19 had elapsed), so that once again it could not be sensed; SM=8 is then set in FIG. 19, S 294 , and the measurement is discarded and restarted.  
         [0088]    [0088]FIG. 14 shows the general sequence of routine S 160  (Digital PWM Sensing). At S 162  (Wait for period length starting edge “1”), the program is in state  1  and is waiting for the starting edge in order to measure period length T, i.e. edge  144  in FIG. 5.  
         [0089]    At S 164  (Wait for period length end “3”), the program is in state  3  and is waiting for edge  156  (FIG. 5), i.e. the end of period length T.  
         [0090]    At S 166  (Period length successfully sensed “5”), the program then goes into state SM=5, which means that period length T has been successfully sensed.  
         [0091]    At S 168  (FlagPM=1?), the program polls the value of FlagPM. If that value is 1, then at S 170  (Wait for pulse width starting edge “2”) the program goes into state  2 , i.e. waiting for the starting edge ( 144  in FIG. 10) of a pulse  69 .  
         [0092]    The program then goes to S 172  (Wait for pulse width end “4”), i.e. state  4 , where it waits for edge  148  (FIG. 10), i.e. the end of pulse  69 .  
         [0093]    The program then goes to S 174  (Pulse width successfully sensed “6”), i.e. into state  6 , meaning that pulse width t has been successfully sensed.  
         [0094]    If the response in S 168  is No, then a measurement of off-time length t′ is performed. In that case, in S 176  (Wait for off-time length starting edge “2”) the program goes into state  2 , i.e. expecting starting edge  152  (FIG. 12).  
         [0095]    In S 178  (Wait for off-time length end “4”) the program then goes into state  4 , where it waits for the end of the pulse off-time, e.g. edge  156  in FIG. 5.  
         [0096]    In S 180  (Off-time length successfully sensed “6”) the program then goes into state  6 , meaning the off-time length t′ has been successfully sensed.  
         [0097]    Subsequent to the sensing of pulse width t (S 174 ) or off-time length t′ (S 180 ), the program goes to S 182  (Calculations: pulse duty factor etc.) where the necessary calculations are performed, for example calculating the value t/T (which is referred to as the “pulse duty factor” of signal  68 ) or the frequency of signal  68  at output  67 . This is followed by a Return in step S 184 .  
         [0098]    The state machine according to FIGS. 13 and 14 is constructed so that firstly period length T is sensed in states  1 ,  3 , and  5 , then either pulse length t or off-time length t′ in states  2 ,  4 , and  6 , and then the various calculations are performed in S 182 . The target value module, shown in principle in FIG. 14, is therefore called in FIG. 3 at S 100  and at S 106 , and it must be called a total of three times before a new valid target value exists. The reason for doing this is to divide the calculation time for this module, which is fairly long, into manageable smaller portions that do not interfere with the commutation of motor  30 . If no consideration needs to be given to the commutation of a motor, such a division is of course unnecessary.  
         [0099]    As is evident from FIG. 14, data sensing is complete only after the processing of states  1 - 3 - 5  AND states  2 - 4 - 6 . For example, if the state machine (FIG. 13) is in state  1  or  2 , then data sensing is not yet complete and a sensing function must be performed. The value of the state machine is greater than  2  only when no further sensing needs to be performed. Execution cannot leave the state machine if the value is  3  or  4 . It ends instead in one of the states  6 ,  7 , or  8 , since in S 310 , FIG. 19 the state SM=5 is automatically switched over to SM=2 so that either pulse sensing or off-time sensing (depending on the value of FlagPM) can be started at the next pass.  
         [0100]    [0100]FIG. 14 shows only the basic structure of the measurement operation. FIG. 15 is a somewhat more detailed depiction in which states  7  and  8  are also shown. Steps S 160  through S 184  are identical to FIG. 14, and will be labeled in the same way as therein and not described again.  
         [0101]    If it is found in S 162  that an edge cannot be measured within a determined time (PWM pulse width is either 0% or 100%), then in S 186  (Limit signal without edges “7”) the program goes into state  7 , i.e. either the situation according to FIG. 8 exists, and the pulse duty factor is then set to 100% in S 182 ; or the situation according to FIG. 9 exists, and in that case the pulse duty factor is set to 0% in S 182 .  
         [0102]    If, in S 164  (Wait for period length end “3”), the interrupt for the end of period length T (interrupt  156  of FIG. 5) arrives too late or not at all, the program then goes to S 188  (Sensing error, second interrupt missing or too late “8”), i.e. into state  8 . The measurement is discarded, and the program begins a new measurement at S 162  (Wait for period length starting edge “1”) in state  1 .  
         [0103]    The same thing happens if, in S 172  (Wait for pulse width end “4”) or S 178  (Wait for off-time length end “4”), the second interrupt is absent or comes too late. In this case as well, the measurement is discarded, the program goes back to the start at S 162  (Wait for period length starting edge “1”) and begins a new measurement, and the previous measurement continues to be used until a new one is available.  
         [0104]    [0104]FIG. 16 shows the simple basic structure of the program that can be achieved using the state machine. After the beginning S 194 , step S 196  polls for the existence of state  8  (Error), which is depicted in FIG. 15 at S 188  and described there. If an error is present, the measurement is discarded and a new measurement begins, i.e. at S 198  the program goes to state SM=1 of the state machine and waits for the starting edge for measurement of period length T, labeled S 162  in FIG. 15.  
         [0105]    If no error is identified in step S 196 , the program goes to step S 200  and asks whether a state greater than 2 is present. A state of  1  or  2  means that at least one of the measurements still needs to be started and that consequently the measurement of signal  68  is not yet complete; if the response is No, measurement of the signal is therefore performed in module S 202  (FIG. 19). If the response in S 200  is Yes, however, this means that no further measurement needs to be started, and the measured data are then evaluated in the subsequent module S 204  EVAL (FIG. 20). Module S 202  or S 204  is followed by step S 206  (Return).  
         [0106]    As already explained, the state machine cannot be left at values SM=3 or 4 but instead must always end in one of states  6 ,  7 , or  8 , so that values greater than 2 mean that the measurements are complete.  
         [0107]    [0107]FIG. 17 shows the procedure for measuring the target value that results from the program of FIG. 16.  
         [0108]    With rotor  36  at a rotational position I, the program begins at state SM=1, passes through states  3  and  5  (i.e. measurement of period length), and halts at state SM=2 of the state machine, as depicted at S 310  of FIG. 19.  
         [0109]    With rotor  36  at a rotational position II, query S 200  in FIG. 16 generates a No response because the state machine initially has a value of 2, and states  2 ,  4 , and  6  are now cycled through, i.e. either the signal length or the off-time length is measured. At the end of this measurement, the state machine halts at SM=6.  
         [0110]    With rotor  36  at a rotational position III, query S 200  of FIG. 16 yields a response of Yes (Y), and evaluation (EVAL) of the data in module S 204  (FIG. 20) then follows. The state machine then goes into state SM=1 (S 334  in FIG. 20). This occurs in step S 334  of FIG. 20. From there the cycle then begins afresh, as indicated by the dashed line in FIG. 17.  
         [0111]    In this fashion, calculation of the target value can be distributed in time over several rotational positions. In each normal cycle, the left branch S 202  (signal measurement) in FIG. 16 is run through twice, and the right branch S 204  (EVAL) only once.  
         [0112]    This is controlled by Hall counter HALL_CNT in S 98  of FIG. 3, which is advanced at each edge of the HALL signal and continuously cycles through the values 0 through 7. A four-pole rotor  36  results in four edges of the HALL signal for each revolution. The pattern is then:  
                                                       HALL_CNT   Path   Sensing of                           0   124   Target value           1   128   Target value           2   124   Target value           3   126   Actual value           4   124   Target value           5   128   Target value           6   124   Target value           7   126   Actual value                      
 
         [0113]    A new target value and a new actual value are thus obtained here after each revolution, i.e. 60 times per second at 60 revolutions per second.  
         [0114]    [0114]FIG. 18 shows interrupt routine S 210  that is triggered by an edge of signal  68 . In step S 212  (Interrupt sensitivity=rising edge), μC  54  is set so that it reacts with an interrupt to a rising signal edge (e.g.  144  in FIG. 5) at its input  67 . A flag for the interrupt is then canceled in S 214 , and at S 216  the present value of timer  66  is copied into a 16-bit variable PWM_End.  
         [0115]    S 218  then identifies which of the interrupts has arrived. This is done by checking whether SM is less than  3 . If Yes, this implies either SM=1 or SM=2, i.e. the first of the two expected interrupts, and in S 220  the value from S 216  is therefore copied into the variable PWM_Start. If SM is greater than 2, however, then the interrupt sensed is already the second interrupt, execution leaves S 218  via the No branch, and in S 222  further interrupts are blocked.  
         [0116]    Following S 220 , in S 224  the value of SM is increased by 2. Likewise, following S 222 , in S 226  the value of SM is increased by 2. For example, at S 224  the value SM=1 is increased to SM=3, and at S 226  SM=3 is increased to SM=5, if period length T was being measured.  
         [0117]    If pulse length t or off-time length t′ was being measured, however, then in S 224  the value SM=2 is raised to SM=4, and in S 226  the value SM=4 is increased to SM=6. After S 226  the routine then goes to S 228  Return.  
         [0118]    Following S 224 , two special cases are dealt with. S 230  asks whether SM is now equal to 4. This means that in the context of a measurement of pulse length t or off-time length t′, the second interrupt is expected next, i.e. in FIG. 10 execution is, for example, in interrupt  146 , and interrupt  150  is expected next. If No, the program goes directly to S 228  Return. If Yes, the interrupt must be the second one, and the program goes to S 232  which asks whether Flag PM=1. A response of Yes means measurement of the pulse length, i.e. the length of a pulse  69  is presently being measured. At S 234  (Interrupt sensitivity=falling edge), the sensitivity of input  67  (FIG. 1) is therefore switched over to a falling edge.  
         [0119]    If, as depicted in FIG. 11, pulse  69 ′ is very narrow (only a few μs), a second interrupt cannot be generated, as described with reference to FIG. 11. S 236  therefore checks whether signal  68  has already assumed a value of 0. This is depicted graphically in FIG. 11 a:  if pulse  69 ′ has already ended, then signal  68 =Low and the second interrupt was missed. In this case SM=8 (Error) is set in S 238 , the interrupt is blocked at S 240 , and at S 242  FlagPM=0 is set, i.e. execution is switched over to off-time measurement since the pulses are too short for measurement. S 228  then follows.  
         [0120]    If, however, signal  68 =1 in S 236 , then the situation is as depicted in FIG. 10, i.e. it is found at time t 02  that signal  68 =High, the program goes directly to S 228 , and the measurement is continued.  
         [0121]    If FlagPM=0 in S 232  (off-time measurement), step S 244  then checks whether the off-time has already ended. This is depicted in FIG. 12, where at the end of interrupt  154  (at time t 05 ) signal  68  is already High again, i.e. the second interrupt was missed.  
         [0122]    If signal  68 =0 in S 244 , the program goes directly to S 228 . If the response in S 244  is Yes, then in S 246  SM=8 (Error) is set, in S 250  the interrupt at input  67  is blocked, and in S 252  execution is switched over to FlagPM =1, i.e. to pulse measurement, since the off-times have become too short and can no longer be measured.  
         [0123]    [0123]FIG. 19 shows the “Meas. Signal” routine S 202  for signal sensing. SM=1 if period length T is being sensed, and SM=2 if pulse length or off-time length is being sensed. For SM=1, the sensitivity of input  67  is set to a rising edge in S 264  (Interrupt sensitivity=rising edge). If SM=2, step S 266  checks whether FlagPM =0, meaning off-time measurement. If Yes, the sensitivity of input  67  is set in S 268  (Interrupt sensitivity=falling edge) to a falling edge, i.e. to the beginning of an off-time measurement.  
         [0124]    If the response in S 266  is No, a pulse measurement then follows, the previously set sensitivity (rising edge) is retained without change, and the program goes (as it does after S 264  and S 268 ) to S 272 , where a Timeout variable is set to a determined value, in this case  150 . This variable is then decremented in a wait loop. Each loop requires, for example, 10 μs, and since it is cycled through 150 times, the maximum delay time is 1500 microseconds=1.5 milliseconds. This is sufficient for dependable sensing of a signal  68  having a frequency of 2000 Hz, i.e. a period length T of 0.5 ms.  
         [0125]    Since sensing can begin at random at any point in time within signal  68 , allowance must be made for the possibility that measurement of the first edge was just missed, so the duration of the wait loop must be at least 2×0.5 ms, or 3×0.5 ms=1.5 ms including a safety factor.  
         [0126]    At S 274 , an interrupt by the HALL signal (FIG. 1) is then temporarily blocked, since no change in the HALL signal is to be expected during the routine shown in FIG. 19. At S 276  and S 278  the interrupt at input  67  is prepared and activated, and the program then goes into the aforementioned wait loop and awaits the arrival of the interrupt. This is done using interrupt routine S 210 , already described with reference to FIG. 18, which is triggered by the signal edge that was set in S 212 , S 264 , or S 268 .  
         [0127]    S 280  checks whether the aforesaid timeout time of, for example, 1.5 ms has elapsed. If it has not yet elapsed, the program goes to step S 282  (Decrement timeout) where the Timeout variable is decremented at each pass.  
         [0128]    During this period—at whatever points in time—two interrupt routines according to FIG. 18 are executing successively, thereby advancing the state machine (by means of step S 224  or S 226  of FIG. 18) either to SM=5 or to SM=6. This is checked at each pass in S 284 , and if such is the case, the program leaves the wait loop and goes to S 286 , where further interrupts at input  67  are blocked and, at S 288 , the blocked Hall interrupt is unblocked again.  
         [0129]    If it is found in step S 280  that Timeout=0 before all the interrupts have occurred, an inquiry is made as to the cause. This involves firstly, in S 290 , blocking input  67  (FIG. 1) for interrupts. S 292  then checks whether SM has one of the values 3, 4, 5, or 6, which are explained in FIG. 13. If so, the interrupt routine (FIG. 8) has detected only a first interrupt but not the second. SM=8 (Error) is therefore set in S 294 , the measurement is not used, and a new measurement begins. The program then goes to S 284  and from there to S 286 , etc.  
         [0130]    If the response in S 292  is No, then SM must be equal to 1 or 2, i.e. after 1.5 ms the program is still waiting for the first interrupt. This is the situation according to FIG. 8 or  9 , i.e. signal  68  at input  67  is either continuously high, corresponding to a PWM=100%, or continuously low, corresponding to a PWM=0%. This is checked in S 296 , and if signal  68  has a value of 1, then in S 298  execution switches over to FlagPM=1, i.e. to pulse measurement.  
         [0131]    If signal  68  has a value of 0 in S 296 , execution switches in S 300  to FlagPM=0, i.e. to off-time measurement. Subsequent to S 298  or S 300  at S 302 , the state machine is set to SM=7. The program then continues via S 284  to S 286  and S 288 .  
         [0132]    Subsequent to S 288 , in S 304 , the new PWM value NewPVal is calculated from the values PWM_End and PWM_Start that were stored during the two interrupt routines, as follows:  
         NewPVal= PWM _End− PWM _Start   (3).  
         [0133]    This new value can be period length T, or pulse length t, or off-time t′, depending on which value was measured previously. This is checked in the subsequent steps. This involves checking in S 306  whether SM=5. That means completion of a measurement of period length T, i.e. the new value is period length T. If that is the case, then in S 308  the new PWM value NewPVal is therefore stored as a new period length T, and in S 310  the new value SM=2 is stored in the state machine, as also indicated in FIG. 17.  
         [0134]    [0134]FIG. 20 shows, using an example, how the data ascertained are further evaluated in the EVAL routine S 204 . The target value is first normalized to an 8-bit value NorPVal, corresponding to the value range of an 8-bit variable. As a result, it has a value in a range from 0 to 255, 0 corresponding to a pulse duty factor of 0% and 255 to a pulse duty factor of 100%. Any other normalizations are of course also possible, e.g. 255=0% and 0=100% pulse duty factor, as are larger value ranges such as 0 to 1023. This will depend, for example, on particular requirements and on the accuracy of the rotation speed controller that is used.  
         [0135]    In FIG. 20, step S 322  checks whether the new PWM value NewPVal (from S 304 ) is longer than period length T. If so, the measurement was inaccurate, but it can be stated with certainty that pulse length t (or off-time length t′, in the case of an off-time measurement) was almost as long as T. If the response in S 322  is Yes, the program goes to S 324  where the normalized (i.e. standardized according to a fixed rule) setpoint NorPVal is set on a preliminary basis to 255, without calculation. This definition in S 324  is appropriate, however, only if a measurement of pulse length t was being performed at the time. S 326  therefore then checks whether FlagPM=1. If No, then the value in question was an off-time measurement, and the normalized value NorPVal is then set in S 328  to  
         255−255=0.  
         [0136]    S 322  also asks whether the state machine has the value SM=7. This implies one of the two situations according to FIG. 8 or  9 . In this case as well, the program goes to S 324  and sets the normalized target (setpoint) value NorPVal in preliminary fashion to 255. If an off-time measurement was made, that preliminary value is then corrected to 0 in the same fashion using steps S 326  and S 328 .  
         [0137]    If the response in S 322  is No, the program goes to S 328 A, where a preliminary calculation is made:  
         NorPVal=(255*NewPVal)/ T    (4).  
         [0138]    For example, if the new PWM value NewPVal is 100 μs and T=300 μs, then the preliminary normalized value NorPVal is  
         255*100/300=85.  
         [0139]    This value is applicable only to a pulse measurement. S 326  therefore once again checks whether a pulse measurement or an off-time measurement was made, and in the latter case the normalized value NorPVal is corrected in S 328 . The value calculated in the example was 85, and if this refers to an off-time measurement it is corrected to  
         255−85=170,  
         [0140]    i.e. the normalized PWM value NorPVal would then be 170.  
         [0141]    The program then checks whether, in the context of the measured periodic pulses  68 , pulse length t is greater than off-time length t′ or vice versa, and the measurement method is adapted accordingly. For a pulse measurement (Yes in S 326 ) the program therefore goes to a step S 330  which checks whether the normalized PWM value NorPVal is less than 120, meaning that the pulse length is less than 46% of period length T. If Yes, then in S 332  execution switches over to off-time measurement, i.e. FlagPM=0. The program then goes to step S 334  where SM=1 is set, i.e. the state machine is reset to “T_Start” (as also depicted in FIG. 17 at S 334 ) so that a new target value calculation begins again at SM=1. The program then goes to S 336  (Return).  
         [0142]    Following S 328 , S 338  checks whether the off-time length is greater than the pulse length. This involves checking whether the normalized PWM value NorPVal calculated in S 328 A is greater than 132, i.e. greater than 51% of period length T. If No, the measurement type remains unchanged (just as at S 330 ). If Yes, then in S 340  execution switches over to pulse measurement, i.e. FlagPM=1. The program then also goes to steps S 334  and S 336 , the measurement is completed, and a target value is available in the form of a normalized PWM value NorPVal that (in this example) can have a value in the range from 0 to 255, and that defines the rotation speed.  
         [0143]    NorPVal is compared with the value 120 in step S 330 , and with the value 132 in step S 338 . This results in a switching hysteresis, i.e. for a pulse duty factor between 46 and 51% both types of measurement—pulse or off-time—can take place.  
         [0144]    The present invention can also be used to make numbers from 0 to 255, contained in coded fashion in a pulse train, readable. In an electronically commutated motor, calculation of the target value is preferably performed in such a way that calculation is distributed among several rotation positions of the motor. The reason is that this calculation requires a great deal of time, and therefore might interfere with other operations in the motor—especially with commutation—if the calculation were performed in “concentrated” fashion, i.e. all at once.  
         [0145]    Automatically switching the measurement method to pulse measurement or to off-time measurement yields higher accuracy, and higher frequencies can be measured. The automatic error detection explained with reference to FIGS. 10 through 12 allows incorrect measurement results to be discarded and, in the cases described therein, permits a quick switchover to a better measurement method (as described in FIG. 18) so that a new and better target value can then be obtained quickly.  
         [0146]    Many variants and modifications are of course possible within the scope of the present invention.