Abstract:
A floor cleaning apparatus driven by a switched reluctance drive that is soft-started to substantially avoid any transient torque reaction disturbing the body of the cleaner. The soft-start is produced by profiling the current demand for the drive system as a function of the rotor speed. Profiling is typically done with a continuous function, a series of steps or a pair of values. Related methods provide similar advantages.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to vacuum and floor cleaners that are controlled by electronic controllers. In particular, it relates to such cleaners where the speed of the appliance is controlled during a starting period. 
     2. Description of Related Art 
     Vacuum cleaners largely fall into two broad categories. Firstly, those in which the whole cleaner is moved across the surface to be cleaned: such cleaners are normally referred to as “upright” cleaners. Secondly, those in which the main body of the cleaner is connected by a flexible hose to a cleaning nozzle that is moved across the surface to be cleaned: such cleaners are normally classed as “cylinder” cleaners. A variant of this second category has a centrally located, static body and a system of ducts and hoses to provide local cleaning. In each case, cleaning action is in part provided by suction that is produced by a fan unit driven by an electric motor. 
     Traditionally, the electric motor used in vacuum cleaners is a series-commutated motor with a wound armature and either an energizable winding or a permanent magnetic field. Such motors are well documented in the art, e.g. in “Electric Motors and Drives”, Hughes, Heinemann Newnes, 1980, which is incorporated herein by reference. A typical torque vs speed characteristic of this type of motor is shown in FIG. 1, from which it can be seen that the torque is relatively high when the motor is initially connected to the supply and falls off as the speed rises. A typical torque vs speed curve for a vacuum cleaner fan is also shown in FIG. 1, showing that the load presented by the fan is low at low speed but rises rapidly with speed. The difference between the two curves represents the accelerating torque that is available at any speed to accelerate the load. Hence, it follows that the fan will accelerate rapidly when the motor is initially connected to the supply but that the acceleration will fall as the curves come together and the motor will run stably at the speed where the curves cross. 
     Because the accelerating torque is high at standstill, and because there is a significant inertia associated with the rotor of the motor and the fan unit, there is considerable torque reaction at start-up. While this is not so much a problem with upright cleaners (because the mechanical layout is generally such as to cope reasonably well with the reaction torque), it is more troublesome in cylinder cleaners since the torque reaction causes the body of the cleaner to rock sharply as the motor starts. This can be at least a cause for user annoyance or even a source of danger to the user if the cleaner rolls over. With increasing suction requirements in cleaners, and hence more powerful motors, this is becoming a greater problem. 
     Similar problems occur in rotary floor cleaners where a rotating brush or mop is used to clean or polish a floor surface. The transient torque reaction produced can be annoying or even dangerous for the user because the machine can swing out of control when it is started. 
     Rudimentary forms of speed control for cleaning appliances have been available for many years and generally take the form of thyristors or triacs which are used to phase control the alternating supply voltage over a limited range. Typically these will allow the user to reduce the speed from 100% to some lower level, e.g. 70%. However, because of the inherent high starting torque of the motor, these forms of speed control are not particularly effective in controlling the starting transient. 
     One solution adopted for rotary floor cleaners has been to replace the series motor with a 3-phase induction motor driven by an inverter, e.g. as described by Kumaki in U.S. Pat. No. 4,992,718, which is incorporated herein by reference. In order to avoid a high starting current being drawn, Kumaki proposes a complex method to reduce the starting current for a given period of time using a microprocessor. While this proposal for reducing the starting current will have the incidental effect of reducing the starting torque, the system has no way to measure the speed and therefore no control over the speed at which the torque of the motor is varied. 
     There is therefore a need for a simple system capable of starting a floor cleaning appliance so that the transient torque is reduced to an acceptable level over the speed range from standstill to the working speed. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a floor cleaning apparatus comprising cleaning means, a switched reluctance drive having a rotor for driving the cleaning means, a controller for controlling energization of the drive and means for determining rotor speed of the drive and feeding a signal indicative of the rotor speed to the controller, wherein when the apparatus is switched on initially the controller is operable to vary the energization of the drive as a function of the measured rotor speed from a first value to a second higher value, thereby to control the torque produced by the motor during start up. 
     The controller may be operable to vary the energization smoothly from the first value to the second. For example, the controller may be operable to vary the energization level according to the following relationship: 
     
       
         
           E=aω 
           n 
           +b  
         
       
     
     where E is the demanded energization level, ω is the rotor speed, a is an empirical constant, n is an index describing the shape of the torque curve of the fan, and b is a constant representing the required value of energization level at zero speed. 
     The controller may be operable to vary the energization in steps between the first value and the second value. The steps are preferably stored in a look-up table. 
     The energization level of the drive may vary from the first level to the operating level in a single step. In order to achieve this, the controller may comprise a first resistor connected to a reference voltage, a second resistor connected to the first resistor, a third resistor connected to the second resistor and another voltage, a microprocessor connected between the second and third resistors and an output between the first and second resistors for supplying a control signal to the drive, the microprocessor being operable to set its output impedance to substantially zero when the apparatus is switched on and subsequently set its output impedance to a high value when the rotor speed reaches a predetermined value. In this way, when the microprocessor has its impedance set to substantially zero, the controller provides a relatively low level of control signal at said output and so produces said first value of energization, and when microprocessor impedance is set to the high value, a control signal sufficient to produce the second value of energization of the drive is provided. 
     Preferably the controller is operable to vary the energization level by varying current demand of the drive. 
     Preferably, the apparatus is a vacuum cleaner. Typically, the cleaning means comprises a fan for creating suction and means operably connected to the fan for removing debris from a surface being cleaned. Alternatively, the apparatus may be a floor polisher. Typically, the cleaning means comprises a polishing pad or a brush head or a mop head. 
     According to another aspect of the invention, there is provided a method of operating a floor cleaning apparatus having cleaning means driven by a switched reluctance drive having a rotor operably connected with the cleaning means, the method comprising: supplying the drive with a first energization level to drive the apparatus at a first speed when the apparatus is switched on, measuring the speed of the rotor, and increasing the energization level supplied to the drive as a function of the measured rotor speed. 
     The energization level may be varied smoothly as a function of the measured rotor speed. The energization level may be increased according to the following relationship: 
     
       
         
           E=aω 
           n 
           +b  
         
       
     
     where E is the demanded energization level, ω is the rotor speed, a is an empirical constant, n is an index describing the torque curve of the fan, and b is a constant representing the required value of energization level at zero speed. 
     The method may involve increasing the energization level in steps. This may comprise reading from a look-up table containing information on the energization level as a function of rotor speed. The energization level may be increased in a single step from its initial value to that required for driving the apparatus at its full operating speed. Preferably the energization level is varied by varying the current demand of the drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various embodiments of the invention and appliances in which the invention is embodied will now be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 shows typical torque vs speed curves for a series commutator motor and for a vacuum cleaner fan unit; 
     FIG. 2 shows a schematic diagram of a vacuum cleaner having a switched reluctance drive coupled to a fan unit; 
     FIG. 3 shows a torque vs speed curve and a current demand vs speed curve for a switched reluctance drive for use in a floor cleaning apparatus in which the invention is embodied; 
     FIG. 4 shows a torque vs speed curve and a current demand vs speed curve for another switched reluctance drive for use in a floor cleaning apparatus in which the invention is embodied; 
     FIG. 5 shows a torque vs speed curve and a current demand vs speed curve for yet another switched reluctance drive for use in a floor cleaning apparatus in which the invention is embodied; and 
     FIG. 6 shows a comparator circuit that may be used in an apparatus in which the invention is embodied. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 2 shows, in schematic form, a vacuum cleaner fan unit  19  driven by the rotor  18  of a switched reluctance motor  12 . Connected to the fan unit is a flexible hose  21  with a cleaning nozzle  23  at its end. In use, the cleaning nozzle  23  is moved across the surface to be cleaned. 
     The motor  12  is powered by a DC power supply  11  that can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply  11  is switched across the phase windings  16  of the motor  12  by a power converter  13  under the control of the electronic control unit  14 . For proper operation of the drive, the switching must be correctly synchronized to the angle of rotation of the rotor. A rotor position detector  15  is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector  15  may also be used to generate a speed feedback signal. 
     The rotor position detector  15  may take many forms. For example it may take the form of hardware, as shown schematically in FIG. 2, or of a software algorithm that calculates the position from other monitored parameters of the drive system, as described in European Patent Application No. 0573198 (Ray), which is incorporated herein by reference. In some systems, the rotor position detector  15  can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter  13  is required. 
     The energization of the phase windings in a switched reluctance machine depends not only on detection of the angular position of the rotor but also on the level of current demanded in the phase winding during the conduction period. 
     Typically, one or more current sensors sense the actual phase winding current and feed that information back to the electronic controller. This is shown schematically in FIG. 2 by current sensor  17  sensing the current in a phase winding  16  and supplying the information to the electronic controller  14 . The actual phase current, as determined by the current sensor, can then be compared with a desired phase current and the appropriate control action can be taken by the controller  14  to operate the switches in the power converter  13 . In some drives, the current demand, and hence energization level, will be constant over the excitation period; in others, the current demand will be adjusted during a conduction period (so-called “current profiling”). In both cases, the level of current demanded is used as a measure of the torque being developed by the motor. The voltage applied to the windings is switched on and off to achieve the desired current level. The characteristics and operation of switched reluctance machines are well known in the art and are described in, for example, “The Characteristics, Design and Application of Switched Reluctance Motors and Drives” by Stephenson and Blake, PCIM &#39;93, Nürnberg, Jun. 21-24, 1993, incorporated herein by reference. 
     Whether or not current profiling is used over a conduction period, it is typically the case that the maximum value of the current demand is kept constant as the speed of the drive increases from zero to full load. This would give rise to a large accelerating torque at low speed and consequently a large transient torque reaction on the body of the cleaner. The inventors have recognized that by modifying the current demand as a function of speed, the accelerating torque can be reduced to a controlled amount over the speed range up to full load. This is shown in FIG. 3, where the current demand has been set so that only a small torque is available at low speed and the accelerating torque is kept at a level which will give more gradual acceleration of the fan to its working speed. This has the benefit of reducing stress on the motor, reducing the reaction torque on the cleaner body, reducing transient stresses on the fan unit itself, and possibly reducing the rating of the power switches. The more gradual the acceleration is, the greater these benefits will be. Eventually, there is a point at which a trade-off is reached with the reduced speed of response of the motor to a start-up command. 
     Since the switched reluctance drive operates by continuously monitoring the position (and hence the speed) of the rotor, various methods are available for making the current demand, and hence the applied voltage and developed torque, dependent on speed. The appropriate values can be calculated in real time using a simple algorithm, say of the form 
     
       
           I   d   =aω   n   +b   (1)  
       
     
     where: 
     I d  is the current demand 
     ω is the rotor speed 
     n is an index related to the torque curve of the fan and the saturation level of the magnetic circuits of the motor, typically around 2 
     a is an empirical constant 
     b is a constant representing the required value of current at zero speed. 
     A relationship of this general form will give a smooth change in current demand, and so torque, with rotor speed. 
     Depending on the ability of the cleaner to resist the transient torque reaction without tipping over or displaying other unwanted characteristics, it will be possible to approximate to the smooth curve of current demand by a series of steps. FIG. 4 shows such a series of steps. The values of current demand, and the rotor speeds at which they change, can be stored in a simple look-up table and accessed by using the measured rotor speed as a parameter. 
     In some applications, especially where cost is important, the series of steps can be reduced to two values, as shown in FIG.  5 . The current demand can again be stored as a function of speed in a look-up table, or, in a particularly cost-effective embodiment, can be produced as described below. 
     It should be noted in FIGS. 4 and 5 that, for the sake of simplicity, the diagrams have been drawn on the basis that constant current demand will produce constant torque. Those skilled in the art will recognize this as a commonly used simplification of the more complex relationship that exists in practical machines. 
     FIG. 6 shows part of a control system for a switched reluctance drive which includes a microprocessor  40  and which could be included in the controller  14  of FIG.  2 . One output  41  of the microprocessor is connected to a chain of resistors R 1 , R 2  and R 3  that act as voltage dividers for a reference voltage Vref. Signal Vcr from the junction of R 1  and R 2  is supplied to comparator  42  to represent a desired current reference. The second input to the comparator  42  is a current feedback signal  44  representative of the current in the phase winding of the motor. Signal  44  may be taken from a current transducer as shown in FIG. 2 or may be produced by an algorithm that predicts or deduces the phase winding current. The output  45  of the comparator  42  is used to control the firing of the switches in the power converter  13  of FIG.  2 . This is done in the conventional way, i.e. when the phase winding current rises above the level corresponding to current reference V cr , the comparator disables the switching of the power converter until the current falls below the required level. The cycle then repeats for as long as it is required to energize that phase winding. 
     When the drive is started, the microprocessor detects that the speed is at or near zero and is programmed to make the output  41  at or near zero volts. This shorts out R 3  and reduces V cr , causing the drive to start with a low current reference and hence a reduced level of torque. As the speed of the drive increases, it reaches a predetermined threshold at which the microprocessor sets the output  41  to a high impedance. The junction of R 2  and R 3  is no longer held at zero volts and rises to a steady-state value determined only by V ref  and R 1 , R 2  and R 3 . V cr  then rises to a level suitable for steady-state running at the required levels of current and torque. 
     This method provides a simple and cost-effective implementation of the invention where the appliance in which the drive is installed is able to cope with the step in torque associated with the sudden change in current reference level at the changeover point. 
     A simple modification to this technique is to include a capacitor connected across, say, R 2 , or R 2  and R 3 . As will be apparent to one skilled in the art, this will have the effect of turning the step in current demand into a ramp, thus softening the abrupt transition between the two levels of current demand. 
     It is important to realize that, in all the embodiments described, the current demand, and hence the developed torque, varies as a function of rotor speed and is not a timed function. Thus, if for any reason the load on the fan varies, e.g. because of dust build-up in a filter or suction orifice, and the time taken to reach a particular speed varies, the system will still operate correctly. In contrast, if the demand had been a function of time the performance of the system would have varied with varying load on the fan. 
     The skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Although the specific description has been based on a cylinder type vacuum cleaner, it will be apparent that the invention can be applied equally to other types of vacuum cleaners and to floor cleaners and polishers. Accordingly, the above description of several embodiments is made by way of example and not for the purposes of limitation.