|Publication number||US7334854 B1|
|Application number||US 11/533,372|
|Publication date||26 Feb 2008|
|Filing date||20 Sep 2006|
|Priority date||20 Sep 2006|
|Also published as||US20080067961|
|Publication number||11533372, 533372, US 7334854 B1, US 7334854B1, US-B1-7334854, US7334854 B1, US7334854B1|
|Inventors||Yen-Chuan Chang, Ying-Yu Tzou|
|Original Assignee||Aimtron Technology Corp.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (7), Non-Patent Citations (1), Referenced by (15), Classifications (7), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention relates to a sensorless start-up method for driving a brushless DC motor.
2. Description of the Related Art
Along with the advancement of technology, motors play a more and more important role in everybody's daily life, such as spindle motors of disc players, drive motors of image scanners, motors in toys, and motors of windscreen wipers. Due to the lack of commutating brushes, permanent magnet brushless DC motors is easier to be taken care of and has a smaller size, a higher efficiency, and other advantages, all of which render the brushless DC motors widely applied in many areas.
A conventional method for driving the permanent magnet brushless DC motor is to detect the position and rotating rate of a rotor through the use of Hall sensors in order to effectively perform the control on the position and rotating rate. However, the Hall sensors tend to be influenced by the operational environment, which adversely reduces the accuracy of detection. For this reason, a sensorless driving method is introduced into the mainstream of technological development. With respect to the sensorless driving method, the position of the rotor is detected on the basis of BEMF (back electromotive force). However, BEMF becomes almost zero when the rotor is static or at a very low rate of rotation, resulting in the impossibility of detecting the rotor's position. Therefore, one of the most important issues is how to accurately and stably start driving the motor by the sensorless method.
Among the proposed start-up methods, the open-loop control is the most widely employed. Three tri-phase sinusoidal waves, each of which has an increasing frequency, are directly applied to three phase coils of the motor so as to raise the rate of rotation of the motor. However, the start-up process may fail since the largest torque may not be generated upon the start-up due to the lack of knowledge about the rotor's position. In order to avoid such failure of start-up, the stator's coils may at first be energized for aligning the rotor in a predetermined direction, and then the open-loop control is executed. Nonetheless, the energization of the stator's coils may cause the rotor to reversely rotate, causing limitations to the applicable range. Moreover, the third method is to firstly detect the initial position of the rotor, and then starts driving the motor by the open-loop control in accordance with the rotor's initial position, thereby avoiding the failure of the start-up and reverse rotation.
Many techniques have been proposed to detect the initial position of the rotor in accordance with a functional relationship between the coil inductance and the rotor's position. Unfortunately, these techniques require either complicated algorithms or the feedback of tri-phase voltages and currents. In U.S. Pat. No. 6,946,808, entitled “Motor Drive Control Circuit And Motor Drive Apparatus,” the rotor's initial position is detected by way of firstly applying a set of detection signals and then determining the rotor's initial position in accordance with responses of a common-terminal voltage of the stator's coils. Such a technique has an advantage of being easily carried out by using analog circuitry. However, it is difficult to retrieve the common-terminal voltage because the common terminal of the stator's coils is not qualified as a standard input/output interface of the motor structure.
Regarding with the open-loop control for driving the motor, it is unnecessary to 100%-precisely locate the initial position of the rotor, and a range covering several tens of degrees of electrical angle should be accurate enough for the open-loop start-up control. Therefore, the present invention provides a sensorless start-up method for driving a brushless motor, which has advantages of using simple steps with a low cost by detecting the rotor's initial position in accordance with peak values of motor currents, without the requirement of positional sensors and complicated algorithms. In one preferred embodiment, the resolution of detecting the rotor's initial position may achieve 30 degrees of electrical angle. The method according to the present invention detects the rotor's position before starting of the motor, i.e., the rotor is prevented from rotating during the detection period. The method according to the present invention overcomes the failure of start-up and reverse rotation occurred in the conventional methods.
According to one aspect of the present invention, a sensorless start-up method is provided for driving a brushless motor, including an initial position detection phase, an open-loop control phase, and a close-loop control phase. The initial position detection phase is used for detecting an initial position of a rotor. The rotor is kept at a rate of rotation of zero during the initial position detection phase. The open-loop control phase is used for driving the rotor in accordance with the initial position of the rotor such that the rate of rotation of the rotor gradually increases to reach a predetermined threshold rate. The close-loop control phase is used for driving the rotor such that the rate of rotation of the rotor increases from the predetermined threshold rate to reach a target rate.
The initial position detection phase includes three steps. The first step is performing a plurality of detection modes in sequence, such that with respect to each of the detection modes a composite magnetic field generated by a motor current flowing through motor coils points in a different direction. Each mode is performed to last for a corresponding detection time such that the motor current gradually increases to a peak value at an end of the corresponding detection time. The second step is comparing each of the peak values of the motor currents generated by the detection modes with respect to one another so as to select, from the plurality of detection modes, one detection mode which generates a largest one of the peak values of the motor currents. The third step is determining an initial position of a rotor on a basis of a direction of a composite magnetic field generated by the detection mode which generates the largest one of the peak values of the motor currents.
The above-mentioned and other objects, features, and advantages of the present invention will become apparent with reference to the following descriptions and accompanying drawings, wherein:
The preferred embodiments according to the present invention will be described in detail with reference to the drawings.
In a permanent magnet brushless DC motor, a rotor is made of a permanent magnet and a stator is made of coils. Subjected to the influence of magnetic poles of the permanent magnet, the inductance of the stator's coil is not fixed, but a function of an electrical angle of the rotor.
As soon as a current flows through the coil, a magnetic field is induced by the coil. In this case, the functional relationship between the coil inductance and the rotor's electrical angle is affected due to the magnetic interaction between the coil's magnetic field and the permanent magnet's field.
The motor coil may be considered equivalent to a combination of a resistor, an inductor, and a BEMF, which are coupled together in series. When the motor is static, the BEMF is zero. If a DC voltage is applied to the motor coil, a gradually-increasing coil current is generated. Different coil inductances cause different increasing rates of coil current. In other words, a smaller coil inductance results in a larger increasing rate of coil current.
In the embodiment shown in
The switch control circuit 43 applies a switch control signal SC to the driving circuit 42. In response to the switch control signal SC, the driving circuit 42 generates six switching signals S1 to S6 for controlling the high-side switches Q1, Q3, and Q5 and the low-side switches Q2, Q4, and Q6, respectively.
The initial position detecting circuit 45 applies in sequence twelve sets of initial position detecting signals PD to the switch control circuit 43. Each set of the initial position detecting signals PD determines a specific operation mode defined by a combination of ON/OFF states of the high-side switches Q1, Q3, and Q5 and the low-side switches Q2, Q4, and Q6, for controlling whether the input terminals A, B, and C of the three phase coils U, V, and W are coupled to the supply voltage Vsup, the ground potential, or the floating potential, i.e., being floated. Therefore, the twelve sets of the initial position detecting signals PD determine twelve initial position detection modes. As for each mode, a composite magnetic field with a different direction is generated because the motor current Im flows through the three phase coils in a different pattern of current paths.
If the composite magnetic field generated by the first initial position detection mode shown in
For each of the initial position detection modes, the different direction of the composite magnetic field causes a different equivalent inductance of the motor coils, as described above with reference to
Referring back to
Hereinafter is described in detail a sensorless start-up method for driving a brushless DC motor according to the present invention with reference to
As described above, the sensorless start-up method for driving the brushless DC motor according to the present invention includes a step of determining an initial position of a rotor by comparing the peak values of the motor currents generated by the twelve detection modes with respect to one another so as to select, from the twelve detection modes, one detection mode which generates a largest one of the peak values of the motor currents. In order to expand the relative difference between the peaks of the motor currents Im generated by the detection modes so as to enhance the accuracy of the initial position detection, it seems better for each mode to last for a longer corresponding detection time. As for any two first-order RL systems, if two system time constants are similar, then the current responses of the two systems generate the largest difference under a condition that the period for applying the DC voltage signal is set equal to the system time constant. But if the system time constant is too large such that the period for applying the DC voltage signal becomes too long, the motor current reaches a high enough level for causing the motor to start rotating. For this reason, the corresponding detection time of each position detection mode should be limited within a range that prevents the rotor from rotating since the rotor is required to remain static during the initial position detection phase according to the present invention.
The twelve initial position detection modes may be categorized into two groups. The first group consists of the detection modes shown in
In order to eliminate the unfairness due to the difference between the equivalent inductances and resistances of the first and second groups, the detection time TG1 applied in the first group and the detection time TG2 applied in the second group must satisfy Equation (3):
i G1(T G1)=i G2(T G2) (3)
From Equation (3) is derived Equation (4):
Therefore, after the detection time TG1 applied in the first group is chosen, the detection time TG2 applied in the second group is determined in accordance with Equation (4).
While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications.
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|U.S. Classification||318/400.11, 318/400.04, 318/722, 318/430|
|20 Sep 2006||AS||Assignment|
Owner name: AIMTRON TECHNOLOGY CORP., TAIWAN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:CHANG, YEN-CHUAN;TZOU, YING-YU;REEL/FRAME:018276/0076
Effective date: 20060906
|20 Nov 2008||AS||Assignment|
Owner name: GLOBAL MIXED-MODE TECHNOLOGY INC., TAIWAN
Free format text: MERGER;ASSIGNOR:AIMTRON TECHNOLOGY CORP.;REEL/FRAME:021861/0083
Effective date: 20080229
Owner name: GLOBAL MIXED-MODE TECHNOLOGY INC.,TAIWAN
Free format text: MERGER;ASSIGNOR:AIMTRON TECHNOLOGY CORP.;REEL/FRAME:021861/0083
Effective date: 20080229
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