Abstract:
A drive and a method, including an electric motor, which is supplied by a rectifier, the rectifier including a time-discrete closed-loop control structure, which regulates the stator current of the electric motor by setting the voltage applied at the motor, the current of the motor being acquired in time-discrete fashion, the closed-loop control structure including a closed-loop controller whose actual value is a first current component of the current, the setpoint input of the closed-loop controller being coupled with at least one upstream setpoint limiter.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a drive and to a method. 
       BACKGROUND INFORMATION 
       [0002]    EP 0 179 356 describes a method for controlling a polyphase machine. According to its  FIG. 2 , the current and the voltage are acquired at the motor. In the process, components of the actual value of the flow are formed from a closed-loop control structure that corresponds to a modeling of the machine. These are forwarded to a flow controller whose output controls the switching states of the inverter supplying the machine, in the form of actuating variables. 
         [0003]    DE 1 941 312 describes a method for controlling an asynchronous machine, in which Hall-effect sensors are used. 
         [0004]    DE 195 32 477 describes a method for starting an asynchronous machine, in which the setpoint torque value of zero is specified for the startup. 
         [0005]    DE 199 752 describes a method for connecting a rectifier to an asynchronous motor. 
       SUMMARY 
       [0006]    Example embodiments of the present invention provide for improving the control characteristics of an electric drive. 
         [0007]    According to example embodiments of the present invention, a drive includes an electric motor, which is supplied by a rectifier, the rectifier including a time-discrete closed-loop control structure, which regulates the stator current of the electric motor by setting the voltage applied at the motor, the current of the motor being acquired in time-discrete fashion, the closed-loop control structure including a closed-loop controller whose actual value is a first current component of the current, the setpoint input of the closed-loop controller being coupled with at least one upstream setpoint limiter. 
         [0008]    In this context it may be provided that the setpoint limiter is configured such that the actuating limit of the rectifier, i.e., for example, the voltage actuating limit resulting from the intermediate circuit voltage, is not exceeded. Furthermore, it may be implemented such that no substantial feedback occurs and thus also no oscillation tendency. In addition, despite this, the most rapid adjustment of setpoint changes is possible, and the control characteristics of the electric drive are therefore improved. In particular, it is possible to implement the closed-loop control structure such that the actual value progresses such that it reaches the setpoint value applied at the actual closed-loop controller after precisely one time step in each case, i.e., one time raster, this setpoint, however, being able to be generated from the output signal of the setpoint limiter. 
         [0009]    In example embodiments, a time-delay element is disposed between the setpoint input of the closed-loop controller and the output value of the setpoint limiter, in particular for a delay by a single time step. In this context it may be provided that the closed-loop controller is able to be assigned a pilot control, which substantially compensates the changes of the setpoint. Thus, only negligible deviations have to be compensated by the closed-loop controller. 
         [0010]    In example embodiments, the closed-loop controller is assigned a pilot control, which adds to the output value of the closed-loop controller a variable that is proportional to the change in the setpoint input. It may be provided in this context that a controlled system having a substantially integrating behavior is already well controllable by the pilot control, so that the closed-loop controller must compensate only for deviations and interference variables. 
         [0011]    In example embodiments, the setpoint limiter requires as input variable at least the non-delayed setpoint, the time-delayed setpoint and the output value of the closed-loop controller. 
         [0012]    It may be provided in this context that the setpoint limiter is implementable such that the actuating limit of the rectifier is able to be taken into account and also at least substantial components of the transfer function of the controlled system, including the motor. 
         [0013]    In example embodiments, the closed-loop controller is a dead beat controller, in particular one that is adapted to the motor disposed in the controlled system. In this context it may be provided that the actual value at the closed-loop controller input is able to reach the applied setpoint already in a single time step, i.e., time raster. 
         [0014]    In example embodiments, the closed-loop controller is a linear controller such as a P-controller, a PI-controller or a PID-controller, in particular with pilot control. In this context it may be provided that simple, cost-effective closed-loop control structures are able to be used. 
         [0015]    In example embodiments, the current components are determined in a coordinate system from the detected current. An advantage in this context is that an adapted coordinate system is able to be used, in particular one that rotates in tandem, so that the computational work is minimal. 
         [0016]    In example embodiments, the rectifier includes for each current component a closed-loop control structure, which is substantially identical to the closed-loop control structure for the first current component. In this context it may be provided that overswings are able to be avoided in all current components. 
         [0017]    In example embodiments, the pilot control is implemented in the form of a dead beat control, that is to say, it has an inverse behavior with respect to the motor disposed in the controlled system, the actuating limit of the rectifier, in particular, not being taken into account in this context. It may be provided that the pilot control may be provided such that the pilot control is able to make the substantial deviations of the actual variable from the setpoint variable disappear in one time step already. 
         [0018]    In example embodiments, setpoint limiter SB limits the setpoint such that the voltage actuating limit is not exceeded, this being carried out in particular such that no other disadvantages arise, such as instabilities of the closed-loop control circuit as a result of additional feedback. In this context it may be provided that no swing behavior occurs and that the setpoint is able to be attained as rapidly as possible nevertheless and without overswings. 
         [0019]    In example embodiments, the setpoint limiter is provided such that the restriction to the setpoint is effective only if the output variable, i.e., limited setpoint I_setpoint, runs in the direction of the unlimited setpoint. In this context it may be provided that feedback and thus also oscillatory behavior are suppressed. 
         [0020]    In example embodiments, the difference is determined between actuating variable U and actuating limits U_max and U_min of the rectifier, multiplied by a factor 1/K, and added to the time-delayed, i.e., earlier, setpoint I_setpointZ1, this sum being forwarded to a limiter  50  as maximum or minimum value (max, min), as limit value that acts on the non-delayed setpoint I_setpoint. It may be provided in this context that the setpoint is attainable as quickly as possible and without overswings. 
         [0021]    In example embodiments, the limiter includes a delay element, which delays its output value by at least one time step, i.e., one time raster, and uses it for variable determination within the limiter. It may be provided in this context that feedback and oscillatory behavior are able to be suppressed. 
         [0022]    In example embodiments, the limiter includes means for suppressing feedback. In this context it may be provided that the oscillation behavior is reduced. 
         [0023]    In example embodiments, the limiter has as input values the maximum and minimum values (max, min) as well as the non-delayed setpoint I_setpoint, the smaller value being determined from max and input, compared to the delayed output value of the limiter, and the larger value thereof becoming effective as upper limit of the limit element acting on the input, the larger value being determined from min and input, compared to the delayed output value of the limiter, and the smaller value thereof becoming effective as lower limit of the limit element acting on the input. It may be provided in this context that the limit values are a function of the output variable of the closed-loop controller, so that overswings are able to be prevented. 
         [0024]    In example embodiments, the upper limit of limiter  50  amounts 
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         [0000]    n numbering the time steps, (U max ) being the voltage actuating limit, (u r,n ) being the value of output variable U of the closed-loop controller, K having the same value as in pilot control V. It may be provided in this context that parameters of the controlled system, such as K and actuating limit U_max are able to be taken into account to the effect that overswinging is suppressible. 
         [0025]    Among features of the present invention in the method are that the method is provided for the closed-loop control of the motor current in an electric motor supplied by a rectifier, the method being implemented in time-discrete fashion, and the stator current of the motor being determined in time-discrete fashion, the stator current of the electric motor being regulated, the actual value of a first current component of the current being regulated to a setpoint input, the setpoint input being the output of a setpoint limiter, the actuating variable of the closed-loop controller being the voltage applied at the motor. 
         [0026]    It may be provided in this context that the setpoint limiter is able to be implemented such that it includes parameters that are related to the controlled system and the actuating limit. Overswings are therefore preventable. 
       LIST OF REFERENCE CHARACTERS 
       [0000]    
       
         I_setpoint setpoint value 
         I_actual actual value 
         I_setpointlim output variable of setpoint limiter SB 
         U voltage as actuating variable 
         SR current controller 
         SA current sampling 
         V pilot control 
         R closed-loop controller 
         M motor 
         SG voltage actuating limit 
         SB setpoint limiter 
           50  limiter 
           61  limit element 
       
     
     
    
     DETAILED DESCRIPTION 
       [0040]    Example embodiments of the present invention are explained in greater detail below with reference to the figures. 
         [0041]      FIG. 1  shows a schematic illustration of the closed-loop control structure according to example embodiments of the present invention. Actual current value I_actual is acquired at the electric machine, for instance an electric motor M. This acquisition is implemented in time-discrete manner using current sampling SA, that is to say, once for each time step of the time-discrete closed-loop controller. 
         [0042]    Since the current of the machine is a multi-dimensional variable if the machine has a three-phased supply, the current is to be represented as vector. 
         [0043]    This vector is representable in a coordinate system, such as a flow coordinate system, rotor coordinate system, etc. 
         [0044]    Actual current value I_ACTUAL of the figures is to be understood as current component in this coordinate system. 
         [0045]    In the case of multiple current components, one closed-loop control structure according to  FIG. 1  is able to be used for each of these current components. In any event, however, a closed-loop control structure according to  FIG. 1  must be used for the torque-generating current component as I_ACTUAL, in particular. 
         [0046]    Setpoint I_setpoint and actual value I_ACTUAL are forwarded to the current closed-loop controller, which modifies the actuating variable in accordance with the system deviation. In the case at hand, the mentioned actuating variable preferably is voltage U, i.e., the amount of the voltage vector of the supply of motor M. 
         [0047]    Important in example embodiments of the present invention is that the setpoint input for closed-loop controller SR is forwarded to the closed-loop controller with a time delay. Since the new setpoint input is already known at the new instant, this makes it possible for the closed-loop control structure to influence the output variable of closed-loop controller SR as desired in time synchronism with the new setpoint input via a pilot control V. According to the physical laws of the controlled system including motor M, this then causes a change in the detected current-component actual value I_ACTUAL, which is forwarded to closed-loop controller SR as actual value. Thus, when looking at the time-discrete closed-loop control structure, it is clearly established that the actual value and the setpoint input of closed-loop controller SR belong to the same time raster, i.e., time step. 
         [0048]    Example embodiments of the present invention have the feature that the controlled system may be seen as integration member in a rough approximation, because the motor in any event has an inductivity as important feature. According to  FIG. 1 , pilot control V receives the difference from the time-delayed and the undelayed setpoint input, that is to say, the change in the setpoint input. Thus, when proportionality factor Kv has been set correctly, the integrating behavior of the controlled system is simulated directly. Only small deviations and disturbance variables must then still be corrected by closed-loop controller SR. 
         [0049]    Speaking in general terms, the closed-loop control structure according to example embodiments of the present invention thus includes a closed-loop controller whose setpoint input becomes effective with a time delay, and a dead beat pilot control, which takes effect without time delay, is connected in parallel with the closed-loop controller. 
         [0050]    It is also important in example embodiments of the present invention that actuating variable U is subject to an actuating limit since it is impossible to generate randomly high voltages in the rectifier supplying the motor. Overswinging of the current value in example embodiments of the present invention is avoidable nevertheless and thus also the reaching of the current limit and the attendant emergency shut-off operations and error reports. 
         [0051]    Actual closed-loop controller SR receives only the delayed setpoint input and compares it to the actual value variable that is more recent by at least one time step. 
         [0052]    It is important in example embodiments of the present invention that specified setpoint I_setpoint is processed and that only the result of this processing is transmitted to the closed-loop controller. A setpoint limiter SB is used in this context. 
         [0053]    This setpoint limiter SB prevents the actuating limit of the actuating variable from being exceeded. In the process, the setpoint is restricted such that voltage actuating limit SG will not be exceeded, which is accomplished such that no other disadvantages result, for instance instabilities of the closed-loop control circuit by additional feedback. 
         [0054]    Setpoint limiter SB uses as input variables the output variable U of the closed-loop controller, non-delayed actual setpoint I_setpoint, and delayed, i.e., earlier, setpoint I_setpoint Z 1 . 
         [0055]    The setpoint limiter acts such that the limitation to the setpoint takes effect only when the output variable, i.e., limited setpoint I_setpoint, runs in the direction of the unlimited setpoint. The output variable of setpoint limiter SB is denoted by I_setpointlim. 
         [0056]    A detailed exemplary embodiment of the setpoint limiter is shown in  FIG. 2 , and  FIG. 3  shows an exemplary embodiment of limiter  50 . 
         [0057]      FIG. 2  illustrates that the difference between actuating variable U and actuating limits U_max and U_min of the rectifier is determined, multiplied by a factor 1/K, and added to the time-delayed, i.e., earlier, setpoint I_setpoint Z 1 . The result is forwarded to a limiter  50  as maximum or minimum value (max, min). Furthermore, the non-delayed setpoint I_setpoint is likewise forwarded to limiter  50 , as input variable input. 
         [0058]      FIG. 2  thus considers the correlation 
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         [0000]    n numbering the time steps, (U max ) being the voltage actuating limit, (u r,n ) being the value of output variable U of the closed-loop controller, K having the same value as in pilot control V. An analogous correlation applies to the lower limit. 
         [0059]    Limiter  50  is shown in greater detail in  FIG. 3 . 
         [0060]    The smaller value is determined from input and max. Then the larger value is determined therefrom and from the delayed output value of limiter  50 . This value then becomes effective as upper limit value. 
         [0061]    In the same manner the greater value is determined from input and min. Then the smaller value is determined therefrom and from the delayed output value of limiter  50 . This value then becomes effective as lower limit value. 
         [0062]    In the process, the input is acting on the upper and lower limit values in limit element  61 . 
         [0063]    In  FIG. 2 , it is also possible to use a known standard limit element as limiter  50 , which limits the input to the extreme values of max and min. However, in this case oscillatory behavior of the closed-loop control structure due to inherent feedback may arise. Therefore, it may be provided to implement limiter  50  according to  FIG. 3 . However, other oscillation-suppressing measures may be taken as well. 
         [0064]      FIG. 4  shows an exemplary curve of the values of the variables, the setpoint limitation taking effect in a sudden change of the reference variable. The limited setpoint behaves precisely such that the actual value attains it after one sampling raster. In this manner, the system deviation of closed-loop controller SR remains zero during the entire correction, and the integrator of closed-loop controller SR does not change its value. This prevents overswings of the actual value. 
         [0065]    In an exemplary embodiment according to the present invention, the time delay is implemented as precisely one time step, i.e., one time raster, of the time-discrete closed-loop control structure. 
         [0066]    The closed-loop controller is implementable in the form of a time-discrete dead beat controller. The closed-loop controller is realized such that it is able to compensate the system deviation already in a single time step. This is successful at least in those instances where the closed-loop controller has a transfer function of the Z-transformed form 1/(z*G(z)), G being the transfer function of the controlled system.