A DC/DC power supply

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DESCRIPTION

Control method of a mechanical commutated electric motor

The present invention refers to a method for controlling an electric motor by means of a microprocessor that controls the power supply to said motor. More specifically, the present invention relates to a method for controlling a mechanically commutated electric motor such as a direct current motor or a universal motor, and also to a microprocessor controlled DC power supply.

The electrical motors of mechanical commutation, that is to say, the motors where a system of brush and commutator changes the polarity of the current in the windings of the armature, while the armature rotates; within magnetic fields produced by the stator, are known in the art. Magnetic fields in such motors can be produced by permanent magnets or electromagnets. If the magnetic field is provided by electromagnets, the current in the windings, which produces the magnetic field of the stator, can be provided by a separate magnetizing current, or it can be provided by the same current that passes through the armature windings. The latter is commonly referred to as a universal motor, as it is also quite suitable for AC, as long as the AC frequency is not too high. Such a universal motor works very well at the most commonly used AC frequencies such as 16 2 / 3Hz, 50Hz or 60Hz.

The mechanical commutated permanent DC motor is the most commonly used motor for small and medium electrical and electronic devices. Therefore, such engines are produced in large numbers, which makes them very economical.

Switching or pulse width modulation (PWM) controlled electric motors are known in the art and are used in a variety of electric drive applications including one or more mechanically commutated DC electric motors. Typical applications, within which the present invention is encompassed, include drive systems to regulate the adjustable height of furniture components such as tops of tables, chairs and other support furniture for use in homes, offices and medical establishments, control movement of robotic devices, etc.

In such applications it is desirable to use several motors, for example two, one for each end of the table top. If two motors are used, they will often withstand different loads, and therefore can be synchronized in their operation in order to keep the table top level. In DE-A-3744188 it is suggested to control an electric motor on the basis of a counter, that is, the detection of pulsations of the supply voltage caused by the variable magnetic resistance, which, in turn, is caused by the change in size of the air gaps between the stator and the rotor as they move relative to each other. Against this background the invention aims to provide a method for controlling a mechanical commutation electric motor, in particular to allow a first and a second electric motor to operate synchronously.

According to a first aspect of the invention this is achieved by a method for controlling a mechanical commutation electric motor, wherein the rotational angle of the output of said motor is determined by detecting the change in the back electromotive force, when the motor electrical switches, and wherein the power supply to the electric motor is stopped after a predetermined number of detected switches.

The inventors noted that by performing this detection and stopping the general current source to the motor whenever necessary, it is possible to operate a common mechanical commutating electric motor such as a stepper motor. This is an important advantage, since such common mechanical commutated electric motors cost much less than a stepper motor.

According to a preferred embodiment of the invention, said number of detected switches is one. This allows the electric motor to operate in separate steps a stepper motor with a number of steps corresponding to the number of commutations per revolution.

According to another preferred embodiment said change in the back electromotive force is detected by measuring the voltage on an inductor and / or resistance connected in series with said motor on a supply line to it. This allows detection to occur directly on one of the power lines to it, which once again allows rotational angle detection by measurements made directly at the power source for the motors. This eliminates any need for additional tachometers and encoders, as well as separate lines to these.

According to another embodiment of the invention the mechanical commutation electric motor is a universal or series motor comprising at least one field coil, and wherein said at least one field coil comprises the inductor on which said voltage is measured. The use of the field coil avoids the use of additional inductors in the supply circuit, which reduces the necessary circuits.

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According to a specifically preferred embodiment the mechanical commutation electric motor is controlled by pulse width modulation by a microprocessor. This allows a good control of the supply current to the electric motor which allows the motor to stop, each time a desired amount of commutations has been detected.

According to another preferred embodiment, if a switch is detected, but said predetermined number of switches has not been reached, the supply current is reduced during the switch. This reduces arcing, which occurs during commutation, which leads to less wear on the electric motor, and less production of corrosive gases.

According to another specifically preferred embodiment, the method is adapted to control at least one other mechanical commutation electric motor by means of a microprocessor that controls the power supply to said first motor and said other motor, said method comprises the steps of determining the rotary angles of the outputs of said first motor and said other motor by detecting the changes in the back electromotive forces, when said first and said other mechanical commutation electric motor switch; when one of said first and said other motor reaches said predetermined angle, deactivating the current source to said motor; and again activating the current source to said one of said first or said other motor when the other of said first and said other motor reaches said predetermined angle.

With this method it is possible to operate common mechanical commutated electric motors in separate steps of operation, where the first motor to reach a given angle of rotation stops and waits for the other motor. Therefore, with appropriately chosen steps, the overall operation of the motors is synchronized.

According to a second aspect of the invention , the method is used for the control of motors in furniture.

The invention will now be described in more detail based on non-limiting exemplary embodiments and with reference to the drawings, in which Figure 1 shows a flow chart of a preferred embodiment of the method according to the invention, and Figure 2 shows a diagram of a power supply adapted to execute the method according to the invention.

The basic idea behind the present invention lies in the fact that the inventors noted that commutations of a conventional mechanical commutated electric motor can be detected by measuring the change in back electromotive force. This is the case specifically when using a current source for the pulse width modulated mechanical commutation electric motor, where the inventors noted that the voltage generated by the back EMF can be easily detected between the pulse width modulated pulses.

Switch detection by back electromotive force in combination with microprocessor controlled pulse width modulation of the supply current enables the supply current to be stopped to a motor, thus the motor effectively operates as a stepper motor with a resolution, which corresponds to the maximum number of commutations per revolution of the mechanical commutation electric motor.

Although this number is typically less than the resolution of a stepper motor, the fact that the mechanical commutated electric motor is much more economical makes the invention attractive in any application where stepped operation is desired or needed, but where higher resolution is not needed.

One such application is the synchronization of motors, for example, to lift the tops of tables or the like.

The flow chart of figure 1 illustrates a preferred embodiment of the method according to the invention in an implementation for the control of two mechanical commutating DC electric motors 22, 24 by means of a microprocessor 20, figure 2. A person skilled in the art will understand that other types of mechanical commutation electric motors, such as universal or series motors, can be used instead.

The method begins in step 100 and proceeds to the decision in step 101. If no commutation of motor 22 is detected in step 101, microprocessor 20 proceeds to step 104 and provides at least one width modulated current pulse of pulses to motor 22. If instead a switch is detected in step 101 the microprocessor increments a counter N1 in step 102 and sets a detection flag FLAG1. Then in step 103 the microprocessor 20 checks if the counter N1 reached a predetermined value M. If N1 did not reach the predetermined value M, the microprocessor 20 goes to step 104 and provides at least one current pulse

ES 2 362 008 T3 pulse width modulated to motor 22. If N1 reached or exceeded the predetermined value M, the microprocessor proceeds to step 105 to verify if a commutation has been detected for the second motor 24.

Essentially, steps 105-108 correspond to steps 101-104. Thus, if no commutation of motor 24 is detected in step 105, the microprocessor proceeds to step 108 and provides at least one width modulated current pulse. of pulses to motor 24. If instead a switch is detected in step 105 the microprocessor increments a counter N2 in step 106 and sets a detection flag FLAG2. Then in step 107 the microprocessor 20 checks if the counter N2 reached a predetermined value M. If N2 did not reach the predetermined value M, the microprocessor 20 goes to step 108 and provides at least one pulse width modulated current to the motor. 24. If N2 reached or exceeded the predetermined value M, the microprocessor proceeds to step 109 to verify whether both N1 and N2 reached or exceeded the predetermined value M, N1 and N2 are set to zero, and the flags FLAG1 and FLAG2 are removed in step 110, and the microprocessor returns to step 101 to repeat the process.

The reason for using the FLAG1 and FLAG2 markers is that after the detection of the predetermined number of switches, the current supply to the motor will be stopped by the microprocessor, which will not allow direct detection in steps 101 or 105.

It should be mentioned that although the preceding description refers to a method for controlling two motors, one skilled in the art will understand that it is possible to control more motors using the same principle, that is, adding appropriate variables n3 to Nn and FLAG3 markers to FLAGn, and duplicating steps 101 to 104 for each of the other motors.

The default value M will depend on the specific application, in which the synchronized operation of two or more motors is desired. In particular, M can have the value 1, which effectively makes each of the DC motors 22, 24 a stepper motor. This would be the case for a single engine, which would not need any timing.

As will be understood, the method according to the invention operates each of the motors 22, 24 in steps such that the fastest of them stops after a predetermined angle and waits for the rest of the motors to reach this predetermined angle. This angle can correspond to one commutation as indicated above, but in principle any multiplicity of commutation can be used, for example corresponding to one or more complete revolutions of the engine.

Regarding timing, it should be noted that the microprocessor 20 can also adjust the pulse width modulation to increase the current provided to a motor if this specific motor runs consistently slower than the first one to reach the predetermined angle. Obviously, as an alternative, the current to the faster motor can be reduced.

Obviously each of the motors 22, 24 can be controlled by its own individual microprocessor 20. However, the use of a single microprocessor 20 for controlling several motors 22, 24 is preferred, since the processing speed of the microprocessor 20 will be sufficient for this.

Figure 2 illustrates a microprocessor controlled DC power supply suitable for executing the method according to the invention.

The power source is preferably a DC / DC power source. The power supply receives DC, such as from a rectified network, at the positive terminal 1 and the negative terminal 2. Between the positive terminal 1 and the negative terminal 2 a voltage divider is provided 3. In the preferred embodiment, the divider voltage 3 comprises two capacitors 4, 5 and two resistors 6, 7. Preferably, the two capacitors 4, 5 have the same capacity and the two resistors 6, 7 have the same resistance. Therefore, in the central tab 8 of the voltage divider 3, there will be an intermediate potential. For illustrative purposes, the intermediate potential is marked at 0, but evidently the intermediate potential fluctuates and is subject to variations according to the current drawn by the voltage divider 3, unless it is stabilized by an external connection to a fixed reference potential.

The current is taken from the voltage divider 3 intermittently by a transformer having a main winding 9 with a first end 10 and a second end 11. The transformer has two secondary windings 12, 13 with a common terminal 14, and a first end terminal 15 and a second end terminal 16. The two secondary windings 12, 13 are preferably identical in number of turns and direction. The primary-secondary ratio is preferably 20: 4/4 for the applications described but may obviously differ from that ratio, depending on the particular application. For the sake of clarity, it should be noted that although reference is made to a transformer, it is not operated as a traditional AC transformer but merely as an inductive energy storage medium.

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To allow the transformer to draw current from the voltage divider 3 intermittently, the DC / DC power supply comprises two controlled switches 18, 19. The first controlled switch 18 is connected between the first end 10 of the main winding 9 of the transformer and the positive terminal 1. The second controlled switch 19 is connected between the first end 10 of the main winding 9 of the transformer and the negative terminal 2. Preferably, the controlled switches 18, 19 are semiconductor switches controlled by a microprocessor 20. If galvanic separation is desired between the main winding 9 and the secondary winding 12 of the transformer, the control lines of the controlled switches 18, 19 may include optocouplers (not shown).

Both controlled switches 18, 19 are operated intermittently by the microprocessor 20, for example, according to a pulse width modulation scheme, as mentioned above.

The microprocessor 20 ensures that the controlled switches 18, 19 are not active at the same time, but otherwise the microprocessor operates the controlled switches 18, 19 independently of each other.

When said first controlled switch is active, the current flows from the positive terminal 1 through the main winding 9 to the central tab 8 of the voltage divider 3, giving rise to a flow in a first direction in the transformer, which in turn gives rise to a current in the secondary winding. Current flows in the first secondary winding, through a first diode 21 connected to the first end of terminal 15 to a first motor 22 and returns to common terminal 14. A second diode 23 prevents current from passing to the second secondary winding 13.

When said second controlled switch is active, current flows from negative terminal 2 through main winding 9 to central tab 8 of voltage divider 3, giving rise to a flow with a second direction in the transformer, which in turn gives rise to a current in the secondary winding. In this case, the current flows in the second secondary winding 13, through the second diode 23 connected to the second end terminal 16 to a second motor 24 and returns to the common terminal 14. Here, the first diode 21 prevents any current from passing to the 1st secondary winding 12.

Since the controlled switches 18, 19 can be controlled independently of each other, it is possible to control the supply to the two motors 22, 24 independently of each other using a single transformer. In particular, with the appropriate choice of modulation and duty cycle, the two controlled switches 18, 19 can be controlled alternately, with the first controlled switch 18 being in the active part of its duty cycle, while the second controlled switch 19 it is in its idle part of the duty cycle, and vice versa. However, it is also possible for the first controlled switch 18 to perform a series of duty cycles without the second controlled switch 19 being active, and vice versa.

According to the invention, the power supply is specially adapted for the control of mechanical commutated DC electric motors 22 and 24, respectively.

Since in many applications, such as the table tops mentioned above, it is necessary to reverse the direction of rotation of the motor, selector switches 22, 24 have been inserted in the supply lines to the motors 22, 24 to reverse the polarity in the supply lines to the motors 22, 24. The selector switches are preferably solid state switches controlled by the microprocessor 20. The microprocessor 20 is preferably adapted to operate the selector switches 25, 26 in conditions where there is no current flow only to avoid unnecessary efforts. The selector switches 25, 26 therefore do not need to have isolating capability. Furthermore, the microprocessor prevents the two selector switches 25, 26 from assuming conditions where the secondary windings 12, 13 are short-circuited.

Taking into account the context of the invention, namely, the upper part of adjustable height tables where two motors are used and their operation must be synchronized to maintain the level of the table top, even when they are subjected to different loads. , the power supply also comprises two inductors and / or resistors 27, 28 located in series with one of said first motor 22 and respective second motor 24. The inventors of the present invention noted that the back electromotive force of the motors can be detected and therefore used as a means of determining the angular position of the DC motor armature, provided the number of commutations per revolution of the motor 22 is known. , 24. Switches are detected by microprocessor 20 by a measured voltage across inductors and / or resistors 27, 28, possibly using appropriate additional circuitry, not shown. In this sense, it should be noted that currently it is preferred to use an inductor rather than a resistor. However, since ideal inductors without a resistance component, for example cable, are more or less theoretical, the use of an inductor will usually involve a combination with some kind of resistance.

If a motor is subjected to a smaller load than the other motor, and therefore operates faster, the microprocessor 20 will stop the supply of power to that motor at each and every switch, or after a predetermined number of switches. commutations. The microprocessor 20 will wait for the motor to switch,

ES 2 362 008 T3 before supplying power to the first motor again. Therefore, the first motor to switch can simply be left to wait until the other motor switches, and the first motor 22 and the second motor 24 will effectively run synchronously, provided the general angle of rotation is taken into account.

As indicated above, steps 105 to 108 of the methods essentially correspond to steps 101 5 to 104. One skilled in the art will understand that the determination of commutation by detecting the change in back electromotive force is not limited to the operation of two or more mechanical commutation electric motors. Instead, this way of determining a commutation can also be used to control any individual mechanical commutation electric motor to allow it to operate as a stepper motor.

As mentioned, the fact that the motors are operated in steps, does not exclude that the duty cycle ratio 10 for the motors is reduced or increased according to a predetermined scheme, which depends on the application, and is not part of the present invention. Duty cycle ratio refers to the ratio between the time that the switching means are active and the sum of one active-inactive cycle.