Patent Publication Number: US-2011052420-A1

Title: Liquid-conducting pump with dc motor and power supply with closed-loop control

Description:
The invention relates to a pump for conveying liquids, in particular water, having a direct-current motor and a power supply unit for rectifying and/or converting the AC voltage or DC voltage which is applied to the power supply unit. 
     In addition to the use of pumps which have an alternating-current motor, direct-current motors are frequently used in particular for relatively small pumps which have to have only a low feed power. 
     In this case, the pumps which are used in the prior art with a direct-current motor have an external power supply unit, which rectifies the AC voltage applied to the power supply unit and, depending on the design of the power supply unit, transforms the voltage down, that is to say it steps it down. In this case, however, the output voltage of the power supply unit is constant, and therefore it is not possible to react, or is possible to react only in a complicated and cumbersome manner, to any change in the rotation speed of the pump motor, for example caused by a change in the viscosity of the liquid to be pumped, by wear to the pump, or other load-dependent factors. 
     The invention is based on the object of specifying a pump having a direct-current motor, which avoids the above disadvantages and makes it possible in an easy way to keep the rotation speed of the pump motor and/or the pump feed power substantially constant in the event of any load changes, or to allow the feed power to follow a predetermined profile. 
     In the case of the pump according to the invention, this is achieved in that the power supply unit has a closed control loop for controlling a controlled variable of the pump. 
     This control system makes it possible to change the controlled variable to a value which is predetermined by a reference variable, wherein the reference variable is in this case given, for example, by the pump feed power, and to maintain the controlled variable at this value, even in the event of disturbance influences. It is likewise possible to design the control system such that the controlled variable follows a predetermined profile. The closed control loop reacts to changes in the controlled variables, such as those caused by the so-called disturbance variables, by varying the manipulated variables, and in this case controls the controlled variable such that the corresponding desired profile of the controlled variable is achieved. In contrast to this, in the case of simple open-loop control of the pump without closed-loop control, there is no such feedback taking account of the actual value of the controlled variable. The reference variable can in this case itself be generated by an external appliance, or else can be set directly, for example at the pump. 
     Pumps such as these with a power supply unit and a closed control unit can be used particularly preferably in metering technology, wherein the closed control loop makes it possible to keep the controlled variable at a value which is predetermined by the reference variable, that is to say for example by the feed power. Specific application options for pumps according to the invention in this case arise, for example, in the domestic field, where washing machines or kitchen appliances such as dishwashers, fan ovens or tape machines can be provided with pumps such as these. Pumps according to the invention can also usefully be used in health facilities such as swimming baths or saunas, as well as for water processing installations. 
     In this case, the electronic components for rectification and step-down conversion of the AC voltage which is applied to the power supply unit, that is to say for transformation of the AC voltage applied to the power supply unit to a lower value are already known per se in the prior art, for example in the form of a switch-mode power supply, which is also referred to as a switching mode power supply (SMPS). Instead of step-down conversion of the AC voltage which is applied to the power supply unit, it is also possible in conjunction with high-voltage direct-current motors to provide the power supply unit with electronic components which carry out step-up conversion of the AC voltage which is applied to the power supply unit, that is to say transformation of the AC voltage applied to the power supply unit to a higher value. A combination of step-down and step-up converters can also be used. By way of example, converters such as these are already known in the prior art, in the form of so-called Buch boost converters. 
     Particularly for the situation in which the pump according to the invention is not operated from the public main electricity mains system but from an external electricity source or voltage source, it is also possible for the power supply unit not to rectify AC voltage, but to carry out only the step-down and/or step-up conversion of the DC voltage applied to the power supply unit. 
     In a closed control loop, a reference variable produces a nominal value of a variable to be controlled, the so-called controlled variable. The controlled variable should then follow the predetermined profile of the reference variable. However, generally the reference variable is only a simple (constant) value at which the controlled variable should be kept, even under the influence of disturbance factors. 
     Further advantageous embodiments of the invention are defined in the dependent claims, and will be explained in more detail in the following text. 
     In addition to direct-current motors with mechanical commutators, as has been known for a long time, and in which brushes composed of carbon or graphite can slide on the circumference of the commutator in order to supply the current, brushless-direct current motors (brushless DC motors) have also recently become known. These motors have been found to have the disadvantage that the commutation is carried out electronically, and that the rotation speed of a brushless motor such as this can thus be controlled in a similar manner to the rotation speed of an alternating-current motor via the frequency, where a frequency converter must be used for this purpose. However, such control is many times more complex than that for a direct-current motor with a mechanical commutator, where the rotation speed of the motor is directly proportional to the DC voltage applied to the armature of the motor. 
     Therefore, in one particularly preferred embodiment of the invention, a direct-current motor which has a mechanical commutator can be used in the pump. Since, in this case, the rotation speed of the motor is directly proportional to the DC voltage applied to the armature of the motor, the control system can be implemented particularly easily, in particular for characteristic variables which are dependent on the rotation speed, and there is no need for complicated electronic switching fittings, which are always also subject to defects. 
     In one preferred embodiment of the invention, the pump feed power in this case represents the controlled variable. The aim of the closed control loop is then to keep the feed power substantially constant. In this case, the pump feed power can be measured by a flow sensor. In another embodiment of the invention, the controlled variable is given by the rotation speed of the motor which, for example, can be measured directly on the motor shaft by means of a tachogenerator or an encoder. Further possible controlled variables are the electromotive force induced in the motor, the back-emf (back-induction voltage) or else the current drawn by the motor, which can be measured by means of a so-called shunt resistor in the motor circuit. A combination of a plurality of these measurement variables as controlled variable is also feasible. The controlled variable itself is in this case preferably measured continuously and is compared with the reference variable in a regulator. Feeding back the actual value of the controlled variable induces negative feedback, which influences the behavior of the control loop. 
     Specifically, if the regulator finds a discrepancy between the reference variable and the controlled variable, that is to say if the measured feed power differs from the nominal feed power for example because of changes in the feed pressure, an actuating element varies a manipulated variable in the control loop such that the controlled variable returns to the value of the reference variable. This type of closed-loop control is achieved by negative feedback, thus stabilizing the control loop. 
     In the case of the present invention, it may be particularly advantageous to use the analogue voltage on the output side of the power supply unit as a manipulated variable, since the rotation speed of the direct-current motor with a mechanical commutator is directly proportioned to this voltage, if this is applied as the input voltage to the motor and is transmitted to the commutator by the brushes. For example, a DC voltage of 0 volts to 5 volts may correspond to a pump feed power of 0 liters per hour to 8 liters per hour. In other exemplary embodiments of the invention, the manipulated variable may be implemented by means of a pulse-width-modulated signal, a resistance value or a digital signal. By way of example, the pulse-width-modulated signal can change between a DC voltage of 0 volts and 5 volts and in the process may have a variable duty ratio of 0% to 100%, which itself may in turn correspond to a pump feed power of 0 liters per hour to 8 liters per hour. A combination of a plurality of the abovementioned manipulated variables is also possible as the manipulated variable for the control loop according to the invention. 
     The essential feature is that a change in the manipulated variable results in a change in the controlled variable in the controlled system, that is to say for example the rotation speed of the motor or the pump feed power. In this case, the relationship between the manipulated variable and the controlled variable which results from the design and configuration of the controlled system, that is to say of the corresponding components of the pump, in other words the influence of the manipulated variable on the controlled variable and/or the effect of the negative feedback, may be known or can be modeled. However, it is also possible for the system identification to be carried out partially or exclusively using measured values of the input and output behavior of the pump. 
     There are various influencing factors on use of a pump according to the invention, which influence the operation of the pump and in the process have a disturbing influence on the controlled variable, and which are therefore included as disturbance variables in the control loop. These disturbance variables cause a change in the controlled variable, as a result of which the latter no longer matches the reference variable, or is no longer at the desired value. In this case, one or more of these influencing factors can be modeled mathematically or physically. In one embodiment of the invention, the disturbance variable of the closed control loop is given by a load-dependent rotation-speed change. Load-dependent rotation-speed changes such as these may have various causes which can result, for example, from the viscosity of the liquid to be pumped, the pump wear or further phenomena which influence the feed pressure. 
     Many different embodiments are available for the direct-current motor with a mechanical commutator as used in the pump according to the invention. The essential feature in this case is that of a relationship which is as simple as possible is produced between the controlled variable, that is to say for example the rotation speed of the motor, and the manipulated variable, that is to say for example the output voltage of the power supply unit. Possible exemplary embodiments envisage a series-wound motor, a shunt-wound motor or a so-called compound motor, that is to say a mixed form of series-wound and shunt-wound motor. However, it is also possible to use motors with external excitation, where the excitation current for the electromagnet is produced by an independent voltage source. Special forms, such as disk-rotor motors, which likewise have mechanical commutators, are likewise possible. 
     Motors which use permanent magnets instead of electromagnets are in this case included in the motors with external excitation. Special forms of direct-current motors with mechanical commutators, such as a disk-rotor motor, are likewise possible. 
     The power supply unit for the pump has the purpose on the one hand of rectifying the AC voltage supplied from the public mains system and on the other hand of transforming it to a lower or a higher value, that is to say to step it down or to step it up. During operation with direct current, there is no need for rectification. In one preferred embodiment of the invention, this power supply unit comprises an inductor which is in the form of a transformer and is also referred to as a constant-current transformer (energy-storage inductor). The inductor has a coil, which may be in the form of an air-cored coil or a coil with a ferrite or iron core. The ratio of the input voltage and output voltage is governed primarily by the circuit and the drive (pulsing) of the inductor. 
     In a further embodiment of the invention, the power supply unit of the pump comprises an RF transformer, wherein this RF transformer has an induction coil with primary and secondary windings which are coupled via a ferrite core or an iron core, wherein the ferrite core or iron core can be completely closed or may have an air gap. The ratio of the inductances, which are governed by the core material and the respective number of turns, is a major factor governing the ratio of the input voltage and output voltage of the transformer, and in addition the output voltage can also be adapted by pulsing of the primary coil. 
     Transformers such as these are known per se for operation of low-power electronic circuits, for example in the case of switch-mode power supplies (SMPS). In this case, it may be particularly advantageous to reduce the voltage by means of a so-called step-down converter (Buck converter). Electronic components such as these for power supply units and step-down converters without transformers are already known per se. Step-up converters can be used to increase the voltage, in which case it is possible to use converters which represent a combination of step-down and step-up converters, and which are known commercially via the expressions converter or, specifically Buck boost converter. 
     Particularly for use in small pumps, such as those used in aquariums, the power supply unit in one preferred embodiment of the invention includes the closed-loop control unit and, additionally or alternatively, the open-loop control unit which in the end controls the pump motor. In this case, the closed-loop control unit comprises a regulator for comparing the reference variable and the controlled variable, as well as an actuating element for preferably automatically setting the manipulated variable. 
     It is also possible for the open-loop control unit to be arranged in the regulator or in the closed-loop control unit. 
     A power supply unit as mentioned above, which comprises the closed-loop control unit and/or the open-loop control unit, has been found to be particularly advantageous for a pump according to the invention, because the power supply unit, and therefore also the pump, can be designed in a space-saving manner. It is particularly preferable for a power supply unit such as this to be accommodated in the pump housing, which means that there is no longer any need for external appliances, except for the electrical power supply. In this case, it is possible for this power supply unit to be integrated directly in the pump head. The type of pump is itself irrelevant in this case. The invention relates to all types of pumps, for example peristalstic pumps, membrane pumps, eccentric pumps, etc. 
     In one embodiment of the invention, the pump feed power is predetermined as an internal or external signal as a reference variable, which is intended to be kept at a constant value. The closed control loop according to the invention allows this maintenance of the feed power since the closed-loop control makes it possible to compensate for any change in the feed pressure and the viscosity. For this purpose, one embodiment of the invention provides that the pump has an apparatus for setting a desired feed power. However, it is also possible for the pump to have an interface by means of which a desired feed power can be transmitted from a separate apparatus. For example, it is possible to connect a computer or a processor unit to this interface, in which case this computer or this processor unit transmits data via the interface to the open-loop or closed-loop control unit. 
     The invention furthermore relates to a pump installation which has a plurality of pumps, wherein, as stated above, these pumps have a direct-current motor with mechanical commutators, as well as a power supply unit for rectification and/or transformation of the AC voltage or DC voltage applied to the power supply unit, and have a closed control loop for controlling a controlled variable of the pump. A pump installation such as this which, for example, can be used as a metering installation in metering technology, has the advantage that each pump has its own power supply unit, as a result of which the pumps cannot influence one another. 
     The invention furthermore relates to a method for operating a pump, as described above, wherein a desired pump feed power is selected and the pump is controlled such that the desired feed power is substantially maintained by variation of the manipulated variable for the control loop of the pump. 
    
    
     
       Further details and advantages of the present invention will be explained in more detail in the following text using the description of the figures and with reference to the drawings, in which: 
         FIG. 1  shows an exploded illustration of a pump with a power supply unit, which comprises a closed-loop control unit, integrated in the pump head, 
         FIG. 2  shows a schematic layout of a power supply unit according to the invention with a closed-loop control unit and open-loop control unit, as well as a connected pump motor and measurement apparatuses, and 
         FIG. 3  shows a diagram of the closed control loop according to the invention. 
     
    
    
       FIG. 1  shows an exploded illustration of the major components of a pump  1  according to the invention. The pump motor  6  is arranged in the pump head  14 . A board  16  is likewise integrated in the pump head  14 , on which board  16  the major electronic components are arranged, that is to say the power supply unit  2  with the closed-loop control unit  4  and the open-loop control unit  5 . The pump is closed by the pump housing  15 . For the sake of simplicity, further components and details of the pump  1 , which are less relevant for the invention, are not shown here. 
       FIG. 2  shows a schematic illustration of the power supply unit  2 , which has a power area  3  in which the AC voltage VAC provided by the public mains system or a DC voltage provided by an external voltage source is applied to the input side of the power supply unit  2 , and is possibly rectified and stepped down, that is to say converted downward. In the present case, for this purpose, an electronic component without a transformer, which is known per se, is used with a step-down converter  3 . This is followed by a closed-loop control unit  4  which has a regulator  12  and an actuating element. The closed-loop control unit  4  determines a manipulated variable u which in this case is given by the output voltage of the power supply unit  2 , and is emitted via an open-loop control unit  5  to the motor  6  of the pump  1 . It is also possible for the open-loop control unit  5  to be part of the closed-loop control unit  4 . The output voltage produced by the closed-loop control unit  4  is produced between the supply lines  10  and  10 ′ on the output side of the power supply unit  2  and may, for example, be between 0 V and 24 V (DC voltage). The pump  1  itself can be disturbed by widely differing factors, such as a load-dependent rotation-speed change, wherein these variables are included as disturbance variables d in the control loop  11 . In this case, the rotation speed of the motor  6  is used as the controlled variable y, and is monitored continuously by a tachogenerator  7  or an encoder, with the values of the rotation speed being transmitted by the supply line  8  to the closed-loop control unit  4 . A further possible controlled variable y would be the current drawn by the motor  6 , which is measured via a shunt resistor  9  in the circuit of the motor  6  and is likewise transmitted via the supply lines  8 ′ and  8  to the closed-loop control unit  4 . If the regulator  12  finds a difference between the reference variable w, that is to say the nominal value of the rotation speed, and the actual measured rotation speed, the controlled variable y, that is to say by way of example the rotation speed of or the current drawn by the motor  6 , is regulated in the controlled system  13 , which includes an actuating element, by varying the manipulated variable u, that is to say in this case by regulating the output voltage of the power supply unit  2 . Such closed-loop control allows the rotation speed of the motor  6  to be kept at a constant value, as a result of which the feed power of the pump  1  is likewise essentially constant and independent of external influences, because changes, for example in the feed pressure and the viscosity for the liquid or liquids to be pumped, can be compensated for. 
       FIG. 3  shows a diagram of the closed control loop  11  of the pump  1  according to the invention. The reference variable w is supplied to the open-loop control unit  5 , for example by generating it by means of an external appliance such as a metering appliance. A manual input is also feasible, and is then transmitted to the open-loop control unit  5 . In this case, the reference variable w may be the analogue voltage value of the output voltage of the power supply unit  2  or of an external appliance, which corresponds to a certain rotation speed of the motor  6  of the pump  1 , and therefore to a certain feed power. However, it is also possible for the reference variable w itself to be a predetermined feed power, and for the reference variable w or the controlled variable y to be converted in the regulator  12 , thus allowing these variables to be compared. A pulse-width-modulated value or a digital signal can also be used as the reference variable. The open-loop control system, which in this exemplary embodiment is arranged in the regulator  12 , calculates a control difference (control error, control discrepancy) e by comparison of the reference variable w with the manipulated variable y. The negative feedback z is also included in this, that is to say the actual value of the controlled variable y, which is transmitted via a return line in the closed control loop  11  to the closed-loop control unit  4 . The regulator  12  processes the control difference e and passes a manipulated variable u which has been modified on the basis of this result on via the controlled system  13 , in which the controlled variable y is regulated. The disturbance variables d are also included in the closed control loop  11 . 
     It is self-evident that the pump according to the invention is not restricted to the illustrated exemplary embodiments, nor is it intended to be restricted thereby.