Abstract:
An assembly and a method determine the angular position of a rotating machine by way of an inductive sensor. From the excitation signal for a primary winding of a sensor and voltages induced in the two secondary windings of the sensor, three more signals are derived using phase shifters and polarity sign determination units. The six signals in total are sampled using a sample and hold sampling unit and provided to a processor for evaluation, which then calculates the current angular position of the rotating machine at the sampling time.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an assembly and a method for determining the angular position of a rotating object, for example. 
     The angular position for example of a rotating machine is often recorded by using a sensor, which by means of inductive coupling forms, from a temporally varying periodic input value, two output values dependent on the angular position of the for example rotating electrical machine, the amplitudes of the time-periodic output values depending in different ways on the angular position to be recorded. Preferred technical embodiments of such sensors are for example resolvers or variable-reluctance (VR) sensors, in which an excitation usually occurs by means of a temporally sinusoidal electrical signal, and the amplitudes of the two likewise temporally sinusoidal output signals are modulated, dependent on the angular position to be recorded, on the one hand sinusoidally for example, and on the other hand cosinoidally (as for example for resolvers with secondary windings arranged turned through 90 degrees to each other). 
     The sinusoidal voltage on the first secondary winding is calculated here as
 
 U   s ( t )= a·U   0 −sin( w   e   ·t +β)·sin(α), and
 
the cosinusoidal voltage on the second secondary winding is calculated as
 
 U   c ( t )= a·U   0 ·sin( w   e   ·t +β)·cos(α), where
 
w e  is the exciter circuit frequency, t the time, U 0  the excitation amplitude, α the rotation angle of the machine, β the phase angle between excitation and the taps on the secondary coils, and a the transfer factor of the amplitude between the primary winding and the secondary windings.
 
     By evaluation of the sensor voltages, the required angle α is determined from the values U S (t) and U C (t) measured on the secondary windings. Here:
 
 U   S ( t )/ U   C ( t )=sin(α)/cos(α)=tan(α),
 
which yields the current rotation angle α. A disadvantage here is that this approach only leads to suitable results if sin (w e ·t+β) is not close to the zero crossing. If sin (w e ·t+β) is close to the zero crossing, then U s (t) and U c (t) take on very small values or tend to zero, and a suitable evaluation is no longer possible for this angular position.
 
     One solution possibility according to the present state of the art is to choose a sampling time for the two output signals of the secondary windings such that sin (w e ·t+β) takes on a sufficiently large value. A consequent disadvantage is that the time for determining the angular position can therefore no longer be freely selected, but must be executed in a particular phase relationship (and hence in a particular time relation) to the excitation voltage, in order to obtain a sufficient signal amplitude for the evaluation. 
     A further solution possibility according to the present state of the art is to perform a calculation of the current angle, from values determined beforehand for U S (t) and U C (t), in an assembly preferably implemented as an integrated circuit, by means of phase-locked loops (PLL). A consequent disadvantage here is that so-called lag errors arise in PLL circuits with changing angular velocities. If angular accelerations are too great, the PLL no longer runs synchronously, and a suitable signal for determining an angle can no longer be found. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the present invention to specify an assembly and a method for determining the angular position, in which assembly and method the aforementioned disadvantages are avoided. 
     The object is achieved with an assembly for determining a current angular position of an object rotating around a stationary point. The assembly includes an inductive sensor with at least one primary winding being fed an excitation signal and at least two secondary windings inductively coupled thereto. The inductive coupling causes signals to be generated with a periodic and phase-shifted waveform in the two secondary windings depending on a rotation angle. The at least two secondary windings include a first secondary winding and a second secondary winding. The assembly further has three phase shifters each with one input and one output, and the phase shifters have the same phase shifts. The phase shifters each are electrically coupled on the input with the primary winding, the first secondary winding or the second secondary winding respectively. An evaluation unit is provided for determining the current angular position from the signals at the inputs and at the outputs of the phase shifters. 
     The object is achieved in particular by an assembly and a method for determining the angular position by means of an inductive sensor, in which assembly and method three further signals are formed from the excitation signal of the primary winding of the sensor and the signals tapped in its secondary windings by phase shifter units, and are supplied after a sampling to an evaluation circuit which has for example polarity sign determination units. For example, in the evaluation circuit, the current angular position of the rotating machine can be determined at any time, for example using an appropriate calculation rule, from the now six available values, without the aforementioned disadvantages appearing. 
     The invention will now be described in detail with reference to the embodiments represented in the figures of the drawings, in which 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows in a block diagram an embodiment of the assembly according to the invention; and 
         FIG. 2  shows in a flow diagram, an embodiment of the method according to the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows in a block diagram an embodiment of the assembly according to the invention. The associated method for determining the angular position is explained with reference to the mathematical relations which are usually present on a resolver/VR sensor, but it can be applied to any sensor as initially mentioned. The assembly in the example has an inductively coupled angle sensor  1 , which has at least one primary input winding  2  and at least two secondary output windings  3  and  4 , the inductive coupling between the primary side and the two secondary sides having a sinusoidal or cosinusoidal waveform, depending on the rotation angle of a rotating machine which is not shown in any further detail. A customary resolver is described here as an example. 
     The assembly further possesses a generator unit  5  for excitation of the primary winding  2  with an excitation voltage  17  which is generally temporally sinusoidal. The assembly further possesses three subtractor units  6 ,  7  and  8 , three phase shifters  9 ,  10  and  11 , two polarity sign determination units  12  and  13 , a sample and hold unit  14  provided for signal sampling, and a processor  15 . 
     As shown in  FIG. 1 , the generator unit  5  is connected to the primary winding  2  and the subtractor unit  6 . The connections of the secondary winding  3  are connected to inputs of the subtractor unit  7 . The connections of the secondary winding  4  are connected to inputs of the subtractor unit  8 . The output of the subtractor unit  6  is coupled with the input of the phase shifter  9  and with the input of the polarity sign determination unit  12 . The output of the subtractor unit  7  is coupled with the input of the phase shifter  10  and with an input of the sample and hold sampling unit  14 . The output of the subtractor unit  8  is coupled with the input of the phase shifter  11  and with an input of the sample and hold sampling unit  14 . The output of the phase shifter  9  is coupled with the input of the polarity sign determination unit  13 . Finally, the outputs of the phase shifters  10  and  11  and the outputs of the polarity sign determination units  12  and  13  are coupled with further inputs of the sample and hold sampling unit  14 . 
     A sinusoidal voltage  17  fed from the generator unit  5  into the primary winding  2  generates via electromagnetic coupling a floating sinusoidal voltage  18  in the secondary winding  3  and a floating sinusoidal voltage  19  in the secondary winding  4 , the amplitude of  18  being modulated as a function of the rotation angle according to a sine function and the amplitude of  19  being modulated as a function of the rotation angle according to a cosine function, i.e. according to the terminal relationships on a resolver or a variable reluctance sensor respectively. By means of the subtractor unit  6 , the sinusoidal excitation voltage  17  is converted into a sinusoidal excitation voltage  20  referred to ground. By means of the subtractor unit  7 , the temporally sinusoidal voltage  19  of the secondary winding  3  is converted into a temporally sinusoidal voltage  21  referred to ground of the assembly. 
     By means of the subtractor unit  8 , the temporally sinusoidal voltage  20  of the secondary winding  4  is converted into a temporally sinusoidal voltage  22  referred to ground. 
     Furthermore, the temporally sinusoidal excitation voltage  20  referred to ground is shifted in phase by the phase shifter  9  through 90 degrees, the temporally sinusoidal signal  21  is shifted in phase by the phase shifter  10  through 90 degrees, and the temporally sinusoidal signal  22  is shifted in phase by the phase shifter  11  through 90 degrees. In the polarity sign determination unit  12 , a signal is generated with value 1 when the excitation signal  20  is greater than zero, otherwise a signal with value 0 is generated. In the polarity sign determination unit  13 , a signal is generated with value 1 when the excitation signal of the phase shifter  9 , referred to ground and shifted in phase through 90 degrees, is greater than zero, otherwise a signal with value 0 is generated. 
     In the sample and hold sampling unit  14 , samples of the signals supplied from the subtractor units  7  and  8 , the phase shifters  10  and  11  and the polarity sign determination units  12  and  13  are formed at the time of receiving a trigger signal  16  from the processor  15 . These six signals are supplied for further processing to the processor  15  as a computing device, which calculates therefrom the current angular position of the rotating machine at the time of receipt of the trigger signal  16  by the sample and hold unit  14 . The current angular position is for example calculated as
 
α=arctan( ab   1   /ab   2 )+π·sign( ab   2 )· VZ _EXITS,
         if a value G1 is greater than or equal to a value G2, and as
 
α=arctan( ab   3   /ab   4 )+π·sign( ab   4 )· VZ _EXITE90,
 
if G1 is smaller than G2. Here α denotes the current angular position of the rotating machine, π pi (e.g. 3.14159), VZ_EXITS the sampled signal value of the first polarity sign determination unit  12  and VZ_EXITE90 the sampled signal value at the output of the second polarity sign determination unit  13 . The value G1=ab 1   2 +ab 2   2  and the value G2=ab 3   2 +ab 4   2 , where ab 1  is the sampled signal value at the output of the subtractor unit  7 , ab 2  is the sampled signal value at the output of the subtractor unit  8 , ab 3  is the sampled signal value at the output of the phase shifter  10  and ab 4  is the sampled signal value at the output of the phase shifter  11 . By means of the selection of (ab 1 ; ab 2 ) or (ab 3 ; ab 4 ) based on G1 and G2, a signal which can readily be evaluated can always be accessed. This allows the mentioned disadvantages according to the state of the art to be avoided, and the current angular position of a rotating machine to be calculated at any time without lag errors. Thus for example with a phase shift of 90 degrees (and odd-numbered multiples thereof), a maximum (positive or negative) is when the original signal is zero.
       

       FIGS. 2   a  and  2   b  show as an example a flow diagram of the method for determining the current angular position of a rotating machine. In a step  1 , a primary winding of a resolver is energized with a temporally sinusoidal excitation signal. In a step  2 , this excitation signal is converted by means of a first subtractor unit into an excitation signal referred to ground. In a step  3 , a temporally sinusoidal floating signal induced in a first secondary coil of the resolver is converted by means of a second subtractor unit into a temporally sinusoidal signal referred to ground. In a step  4 , a temporally sinusoidal floating signal induced in a second secondary coil of the resolver is converted by means of a third subtractor unit into a temporally sinusoidal signal referred to ground. 
     In a step  5  of the method, the excitation signal referred to ground is shifted in phase through 90 degrees by means of a first phase shifter. In a step  6 , the temporally sinusoidal signal referred to ground is shifted in phase through 90 degrees by means of a second phase shifter. The temporally sinusoidal signal referred to ground is shifted in phase through 90 degrees by means of a third phase shifter in a step  7 . By means of a first polarity sign determination unit, in step  8  a signal with the value 1 is formed if the excitation signal referred to ground is greater than zero, otherwise a signal with value 0 is generated. In a step  9 , by means of a second polarity sign determination unit, a signal is generated with value 1 when the excitation signal, referred to ground and shifted in phase through 90 degrees, is greater than zero, otherwise a signal with value 0 is generated. 
     In a step  10  of the method, sample values of the signals formed from the second and third subtractor units, the second and third phase shifters and the first and second polarity sign determination units are formed at the time of the trigger signal by a sample and hold unit on receipt of a trigger signal, the point in time of which is freely selectable. In a step  11 , the sample values formed by the sample and hold sampling unit are received by a processor, and the current angular position of the rotating machine is calculated from these sample values according to the mathematical procedure described above. 
     In the present method described as an example, the order shown for the steps is not mandatory, and can be varied as desired. In the previously explained example of an assembly according to the invention, the subtractor units  6 ,  7 , and  8  can also be omitted, for example, if a floating measured value acquisition is not required.