Device for optimizing the speed control behavior of a subfractional horsepower motor
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed to a device for optimizing the speed control of a subfractional horsepower motor such as those typically used in dental instruments. 
     2. Description of the Prior Art 
     Two conventional control devices of this type are known in the art and disclosed in DE 32 21 146. In one of these designs, the regulation of the speed is accomplished through compensation of the internal voltage drop of the motor. The motor current is measured dependent on the motor load. Given a reduction in speed as a result of a higher load, a higher motor current is measured. Conversely, the speed increases with a reduction in load, and consequently a lower motor current is measured. The measured difference in motor current is fed to a controller that adjusts the speed to the desired value. An advantage of this type of motor regulation is extremely quiet operation of the motor even into its upper speed range. One disadvantage however, is that the motor provides relatively low torque, especially in its lowest speed range. The increases in current are too slight to compensate by measurement; in contrast thereto, the collector losses are felt to a relatively large degree. 
     In the second conventional control device, the speed is calculated by measuring the voltage with the motor switched off. One advantage of this design is that it provides extremely good torque for the motor, even in its lowest speed range. The regulation can compensate the collector losses well, and as a result speeds having high torque can be regulated down to zero. One disadvantage of this control method is that rough running that can lead to vibration may occur under certain circumstances in the upper speed range. The reason for this is that system-deactivation of the motor is necessary for the voltage measurement. 
     One object of the present invention is thus to provide a device that makes it possible to operate a subfractional horsepower motor of the type primarily used for dental equipment in such a way that it can be optimally controlled in every desired speed range. The device will provide optimum speed control even with different load values in both the lower as well as the upper speed range. 
     SUMMARY OF THE INVENTION 
     The present invention is based on a system which embodies the two aforementioned control devices and which processes each measurement. The device which is set forth in greater detail below can, preferably automatically, undertake a sliding transition from the voltage control into the current-load control and vice versa on the basis of predetermined motor data. The decision as to when the user must switch between two types of regulation is thus eliminated. The motor data required for the input, i.e. the switching speed on the one hand and the maximum speed at which the motor is to be operated, can either be permanently prescribed or can be set by the user. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit block diagram of a first embodiment of the present invention; 
     FIG. 2 is a graph illustrating operation of the present invention; 
     FIG. 3 is a block diagram of a second embodiment of the present invention; 
     FIG. 4 is a graph illustrating operation of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first exemplary embodiment of the present invention is set forth in FIG. 1. An electric motor 1 is driven by an output stage 2 that converts data from a control and regulation computer 3 into motor speed and motor torque. The computer 3 contains a current-load module 4 and voltage module 5. The current-load module 4 processes the incoming motor data according to the initially described current-load compensation method. The voltage module 5 processes the incoming motor data according to the voltage measurement. The motor control module 6 is connected to the motor output stage 2 and drives the output stage according to either the current-load control method or according to the voltage control method. 
     The motor speed data required for voltage measurement is input into the computer via the line 7. Information for the current load compensation, which is obtained in a known way via a resistor 9, is input into the computer 3 via the line 8. Corresponding control information is also forwarded by the computer 3 to a current shut-off circuit 10 required for the voltage measurement. 
     An evaluation logic circuit 11 decides whether voltage or current load control is to be used. The information required for the decision is supplied by comparison logic 12 into which information about the desired, maximum speed is input via input 13. Information about the selected switching speed U at which the change is to be made from voltage control to current load control is input via an additional input 14. Comparison logic 12 decides whether the current speed, i.e. the actual speed, is higher or lower than the switching speed U and forwards the result to evaluation logic 11. The input of the switching speed U can be input by the operator at a panel or in some other manner. The same is true of the desired, maximum speed at which the motor is to be operated. 
     The switching event is set forth with reference to the diagram of FIG. 2. The switching speed U at which a switch is undertaken from voltage to current-load control and vice versa can be defined by the user. For example, this switching value should be at a rated value of 2,000 rpm. When the maximum speed N1max is above the switching speed U, for example at 40,000 rpm, then regulation ensues according to current-load compensation. When, in contrast, the maximum speed N2max lies below this value, for example at 800 rpm, then speed regulation is accomplished only with voltage measurement. The switching speed U can be advantageously set by the individual user. It is desirable, however, to permanently set this at the time of installation of a dental apparatus wherein the subfractional horsepower motor is utilized. The maximum desired speed can be automatically called in during the treatment or can also be manually input. When the desired speed is above the switching speed U, the current-load compensation is used for the entire, selected speed range. When, in contrast, procedures are to be carried out that require a speed in the lower speed range, then the user will select a maximum speed below the switching speed U. The voltage control is automatically activated for this selection, this resulting therein that the motor 1 outputs a relatively high torque even in the lower speed range. 
     FIG. 3 shows an embodiment wherein the adaptation to the two types of control or regulation ensues automatically. In contrast to the above-described exemplary embodiment, it is not the switching speed U but a switching range that is input via input 14 of comparison logic 12. The comparison logic 12 compares whether the current speed that is likewise input into the comparison logic 12 via the line 7 is above, below or within the switching range. When the current motor speed lies in the switching range, the evaluation logic 11 decides how many switching parts or, respectively, regulating parts should take effect. This information is supplied to the computer 3 that then selects either the current load module 4 or the voltage module 5 or corresponding portions of the two modules for control characteristics. Viewing the diagram of FIG. 4, it may be seen that all the more current load control parts must be activated the closer the current motor speed comes to the switching speed U. At the same time the voltage influences are slowly eliminated. When the speed in turn drops, the proportions shift in the opposite direction. When the electronics is in the current load range, the speed is only rarely measured, so that the motor exhibits extremely quiet running. In contrast, when the speed again approaches the transition range, the voltage regulation starts to become active. Below the transition range, only the voltage regulation takes effect. The interruption of the motor current for the speed measurement required in this lower speed range, however is unnoticeable in this speed range. 
     The present invention is subject to many variations modifications and changes in detail. It is intended that all matter described throughout the specification and shown in the accompanying drawings be considered illustrative only. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.