Abstract:
The invention relates to a method for controlling a current fed to an electric motor during an operation to mount a tire on a rim of a wheel including the tire and the rim, or to demount the tire from the rim. The wheel is rotated by an electric motor about an axis, and the motor current fed to the electric motor is controlled dependent on the rotational speed and torque required for the mounting and/or demounting operation. The motor current is automatically changed to cause the electric motor to apply a high torque peak to rotate the wheel, when the motor current is measured to be greater than a preset limit at a low rotational speed during a preset time. Furthermore, the invention relates to an apparatus for mounting a tire on a rim or demounting the tire from the rim.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) or (f) to European Patent Application No. 10 000 720.2-2425, filed on Jan. 25, 2010, in the European Patent Office, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention concerns a method for mounting a tyre on a rim or demounting a tyre from a rim. Furthermore, the invention concerns an apparatus for mounting a tyre on a rim or demounting a tyre from a rim. 
     DESCRIPTION OF RELATED ART 
     An apparatus for mounting a tyre on a rim or demounting a tyre from a rim with an electric motor which drives a shaft of a rotary disc is already known from the prior art. For example EP 0 911 190 discloses such an apparatus. Thereby, an electric motor is used which can be powered with different supply voltages as the supply voltage differs for different countries. An apparatus for mounting a tyre on a rim or demounting a tyre from a rim according to the preamble of claims  1  and  7  is shown in DE 42 05 045; especially DE 42 05 045 shows a rim or a tyre being positioned on the rotary disc. By means of a tyre fitting or removal tool the tyre is mounted on the rim or demounted from the rim. The electric motor is activatable by a foot pedal. By a corresponding more or less activation of the foot pedal an operator is able to change the motor torque and thus the movement of the rotary disc. 
     SUMMARY 
     The invention provides a method for mounting a tyre on a rim or demounting a tyre from a rim, wherein the wheel (tyre/rim assembly) is rotated by an electric motor about an axis, wherein the motor current fed to the electric motor is controlled dependent on the rotational speed and torque required for the mounting and/or demounting operation, and wherein the motor current is automatically changed to apply a high torque peak, when the measured motor current is greater than a preset limit at a low rotational speed during a preset time. According to the invention, it is autonomously detected when a high torque peak is required and such a high torque peak is automatically applied. The high torque peak is achieved in changing the motor current, in particular in changing the frequency and/or the amplitude of the motor current. The applied high torque peak is also called “boost” function in the following. The “boost” function is synonymous with an activation of a pedal with which the electric motor is started. That means the “boost” function simulates a pedal retry of an operator in increasing the motor current fed to the electric motor. Such a motor current changing is useful as speed and torque requirements during mounting and/or demounting operations are different depending on the kind of the mounted and/or demounted tyre. In particular, the side walls of the tyres have different stiffness. For tyres with a high stiffness, e.g. run flat tyres, low speed and high torque are required, however for basic operations high speed and low torque are required. Advantageously, the method according to the invention prevents the tyres from damage during mounting and/or demounting process. The high torque peak is applied to the electric motor when the present current is greater than a preset limit at a low rotational speed during a preset time. The actual low speed corresponds e.g. to the nominal speed of the motor at 50 Hz (which approximately corresponds to 1500 rpm for a standard four-pole motor). The speed of the motor is reduced by means of a gearbox to about 7 rpm at the main shaft. That means when the shaft rotated by the electric motor is slowed down due to a high request of torque at a low rotational speed and this high request of torque exists during a preset time, the motor current is changed. 
     Preferably, the preset time is longer than the period during which the high torque peak is applied. 
     In particular, the preset time is three to ten times longer than the period of applying the torque peak. 
     The motor current is preferably set back to normal condition after applying the torque peak. After applying the torque peak, the method for mounting or demounting a tyre is continued with the motor current for normal conditions until a further high request of torque is detected by the converter. For example, the actual low speed corresponds to the nominal speed of the motor at 50 Hz—which approximately corresponds to 1500 rpm for a standard four-pole motor—and the actual high speed corresponds to the nominal speed of the motor at 100 Hz—which approximately corresponds to 3000 rpm for a standard four-pole motor. The actual low speed of the motor is reduced by means of a gearbox to about 7 rpm at the main shaft, and the actual high speed of the motor is reduced by means of a gearbox to about 14 rpm at the main shaft. 
     Preferably, the frequency and/or the amplitude of the motor current is increased within a preset maximum limit to apply a torque peak. Such a preset limit is given, e.g. stored, and prevents that the electric motor is driven with an excessive current frequency which leads to a damage of the electric motor. 
     The cycle of applying the torque peak and setting back to normal condition can be repeated. 
     Furthermore, the invention relates to an apparatus for mounting a tyre on a rim or demounting a tyre from a rim comprising an electric motor for rotating the wheel (rim/tyre assembly) about an axis, a controlled device for delivering the motor current to the electric motor and a sensing device for sensing the motor current and transmitting corresponding signals to a control device controlling the motor current dependent on the rotational speed and torque requirement for the mounting and/or demounting operation, wherein the control device is able to change the motor current to apply a high torque peak, when the measured motor current is greater than a preset limit at a low rotational speed during a preset time. With respect to the advantages it is referred to the above mentioned explanations. 
     Preferably, a time counter is connected to the control device to predetermine a time limit interval during which a motor current greater than a preset limit is measured. Thereby, the time counter can be a separate part or the time counter can be integrated into the control device. 
     The control device can comprise a microprocessor which adjusts a power driving circuit with power switches to control the motor current. 
     The electro motor is a two-phase or a three-phase electric motor, i.e. the electric motor is driven with two-phase current or three-phase current. 
     Preferably, the apparatus comprises a low voltage circuit providing the control device with low voltage. 
    
    
     
       BRIEF DESCRIPTION 
       The invention will be described in greater detail herein after by means of embodiments by way of example with reference to the Figures in which: 
         FIG. 1  schematically shows an apparatus in which a method according to the invention can be conducted, and 
         FIG. 2  shows a flow chart of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  a tyre changer is diagrammatically shown, the tyre changer  10  comprising an electric motor  12 , in particular three-phase motor, for driving a drive shaft  14  of a rotary disc  16 . Alternatively, a one-phase motor or a two-phase motor, especially induction motor, can be used. In particular a four-pole motor is used. On the rotary disc  16  tyres being centred to the rotary axis A of the rotary disc  16  can be positioned. A tyre fitting or removal head  18  is fixed on a pivotable arm  20 , the arm  20  being attached to a housing  22 . 
     The motor current, especially the frequency and/or the amplitude of the motor current fed to the electric motor  12  is changeable which results in a different speed of the driven shaft  14 . In order to detect the motor current fed to the electric motor  12 , a sensing device  24  is connected with the electric motor  12 . Furthermore, the sensing device  24  is connected with a control device  26 . Signals from the sensing device  24  are transmitted to the control device  26 . Depending on the signals detected from the sensing device  24 , the control device  26  is able to control the motor current dependent on the rotational speed and torque requirement for the mounting and/or demounting operation. In particular, the control device  26  is able to change the frequency and/or the amplitude of the motor current fed to the electric motor  12 . Such a changing in the motor current is useful as speed and torque requirements during mounting and/or demounting operations are different depending on the kind of the mounted and/or demounted tyre. In particular, the side walls of the tyres have different stiffness. For tyres with a high stiffness, e.g. run flat tyres, low speed and high torque are required, however for basic operations high speed and low torque are required. The control device  26  is able to autonomously detect, when the rotation of the shaft  14  is slowed down due to high request of torque, and is able to automatically start a so-called “boost” function. This “boost” function is synonymous with an activation of a pedal with which the electric motor  12  is started. That means the “boost” function simulates a pedal retry of an operator. This provides a torque peak of the electric motor  12 . Thereby the current fed to the electric motor  12  is changed in increasing the motor current fed to the electric motor  12  within given limit values. The control device  26  autonomously detects a requirement of changing the motor current fed to the electric motor  12  to apply a high torque peak. After detecting the requirement, the motor current is automatically changed. An action of an operator is thereby not necessary. 
     The control device  26  comprises a microprocessor  28 , an interface circuit  30  and a controlled device  32 . The sensing device  24  is connected with the microprocessor  28 , signals from the sensing device  24  indicating the motor current fed to the electric motor  12  being transmitted from the sensing device  24  to the microprocessor  28 . Depending on these signals, the microprocessor  28  being able to adjust the controlled device  32  comprising power switches to control the motor current. The interface circuit  30  positioned between the microprocessor  28  and the controlled device  32  serves for converting and forwarding the commands of the microprocessor  28  to the controlled device  32 . The interface circuit  30  provides voltage interface conversions, supplies dead time protection and shut down function protection. The controlled device  32  comprising power switches which are used to drive the electric motor  12  are positioned between the motor current sensing device  24  and a high voltage circuit  34 , the high voltage circuit  34  powers the electric motor  12 . 
     Furthermore, a low voltage circuit  36  is provided comprising a transformer to reduce the voltage, a bridge rectifier, filter and a regulator to provide low voltage levels being provided to the interface circuit  30  and to the circuit of the microprocessor  28 . 
     Furthermore, a time counter  37  is provided, the time counter  37  being connected to the control device  26 , especially the microprocessor  28 , to predetermine a time limit interval during which a motor current which is greater than a preset limit is measured. When the measured motor current is greater than a preset limit during the preset time, a high torque peak is applied. Alternatively, the time counter  37  can be integrated into the control device (not shown). 
     A flow chart of the method according to the invention which is conducted with the apparatus as explained above is shown in  FIG. 2 . 
     In a first step, which is indicated with reference numeral  38 , it is inspected if the foot pedal (not shown) is activated. 
     Only if the pedal is activated, that means if the first interrogation  38  can be answered with “yes”, it is checked, in a second step  40 , if the present speed of the electric motor  12  and thus of the drive shaft  14  is low. Thereby, a “low” speed means that the present speed is lower than a speed which is present with “normal” conditions. Values which represent the speed of several kinds of tyres with “normal” conditions can be entered and recalled. A “low” speed indicates a torque which exceeds the corresponding limit value. A “low” speed can be detected by the motor current sensing device  24  in an increase of the current of the electric motor  12 . Alternatively, a “low” speed can be detected by a speed sensor (not shown) observing the speed of the drive shaft  14  or of the electric motor  12 . Corresponding signals are sent by the motor current sensing device  24  or the speed sensor to the converter  26 , in particular to the microprocessor  28  being part of the converter  26 . For example, the actual low speed corresponds to the nominal speed of the motor at 50 Hz—which approximately corresponds to 1500 rpm for a standard four-pole motor—and the actual high speed corresponds to the nominal speed of the motor at 100 Hz—which approximately corresponds to 3000 rpm for a standard four-pole motor. The actual low speed of the motor is reduced by means of a gearbox to about 7 rpm at the main shaft, and the actual high speed of the motor is reduced by means of a gearbox to about 14 rpm at the main shaft. 
     If the present speed of the electric motor  12  and thus of the drive shaft  14  is low, e.g. the second interrogation  40  is answered with “yes”, the present current of the electric motor  12  is compared with a maximum current in a third step  42 . Thereby, it is checked, if the present current is greater than a maximum current. The present current of the electric motor  12  is measured by the motor current sensing device  24  and a corresponding signal is transmitted to the microprocessor  28 . The microprocessor  28  compares the signal of the motor current sensing device  24 , which corresponds to the measured current, with a stored limit value for the present kind of tyre and detects if the present current of the electric motor  12  is greater than the maximum current. Limit values for several kinds of tyres are stored in the microprocessor  28 . 
     If the measured current of the electric motor  12  is greater than a preset limit, a time counter  37  starts (see step  44 ). If the measured current of the electric motor  12  persists for a preset time, e.g. for one second (see step  46 ), the “boost” function is started for about 0.2 seconds. In other words, the “boost” function is conducted as long as the interrogation “Count &gt;1.2 s” can be answered with “no”, i.e. as long as the elapsed time is smaller than 1.2 seconds the “boost” function is conducted (see steps  48 ,  50 ). Is the elapsed time greater than 1.2 seconds, the motor current is set back to normal condition (see step  52 ). As already explained above, the “boost” function is synonymous with an activation of the pedal with which the electric motor  12  is started. That means the “boost” function simulates a pedal retry of an operator which provides a torque peak of the electric motor  12 . The frequency and/or the amplitude of the motor current fed to the electric motor  12  is changed, especially increased, within given limit values. 
     If there are detected any faults, the method is stopped (see steps  54 ,  56 ). Otherwise, the method starts again (see steps  54 ,  58 ). 
     Furthermore, the motor is driven with normal condition (see step  52 ), if the present speed of the electric motor  12  and thus of the drive shaft  14  is not low (i.d. interrogation  40  is answered with “no”), if the present current is lower than a maximum current (i.d. interrogation  42  is answered with “no”) and if the measured current is for less than 1 s greater than the maximum current (i.d. interrogation  46  is answered with “no”). Furthermore, if the pedal is not activated (see step  38 ), the method is stopped (see step  56 ). 
     According to the inventive method, it is autonomously detected, when the speed of the electric motor  12  and thus of the drive shaft is slowed down due to high request of torque, and the “boost” function is automatically started. Thereby, the frequency and/or the amplitude of the current fed to the electric motor  12  is increased within maximum limits. This “boost” function simulates a pedal retry of an operator and provides a torque peak of the electric motor  12 .