GB2099644A - Brushless direct current motor - Google Patents

Abstract

The motor (10) has a permanent magnet rotor and a plurality of angularly spaced field windings (A, B and C) and is provided with a rotor position sensor comprising a disk containing one or more copper segments and a plurality of sensor coils which are alternately tuned and detuned as the disk rotates past the sensor coils. The voltage developed across the sensor coils is sensed by envelope detectors which provide output pulses equal in number to the sensors. These output pulses are applied to a logic circuit which generates commutating pulses twice in number to the number of sensor coils employed. These commutating pulses simulate a three phase alternating signal. The commutating pulses are then applied to power switching transistors (Q1-Q6) which control the application and direction of current through the field windings. <IMAGE>

Description

SPECIFICATION Brushless direct current motor Background of the invention This invention relates to a brushless direct current motor of the type including a permanent magnet armature, a plurality of angularly spaced field windings surrounding the armature, and means for controlling the commutation of current through the field windings in such a way that a rotating magnetic field is created to induce torque into the armature and to cause it to rotate.

There are several techniques employed to sense the angular position of the armature with respect to the field windings, including the use of inductors, photoelectric devices and magnetic sensors. A preferred technique is shown in U.S.

Patent No. 3,714,532 wherein sensing inductors, supplied with a high frequency current from a fixed frequency oscillator, are used to detect the angular position of a metallic segment mounted to rotate with the motor armature. There are also several circuit schemes employed for controlling the current through the field windings in response the information obtained from the armature position sensors.

Summary of the invention In this invention, an improved brushless direct current motor includes control circuitry for generating twice the number of commutating pulses as there are position indicating sensors, and for directing current through the field windings alternately in both directions in response to the commutating pulses to generate a rotating magnetic field to rotate the armature.

In a preferred embodiment of the invention, sensing inductors are used to detect the position of a metallic segment which is mounted to rotate with the armature. The sensing inductors are provided with high frequency current from a fixed frequency oscillator and are tuned to resonate at the frequency of the oscillator. Tuning and detuning of the sensing inductors occurs as the metallic segment rotates 'past the inductors. The voltage developed across the inductor is demodulated to provide an indication of when the edge of the metallic segment passes the inductor, and circuit means responsive to the output of the detectors generates the commutating pulses.

The present invention may be used with brush less direct current motors having any number of poles and any number of phases. Since current flows through the field windings in both directions, the torque and efficiency of such a motor are improved over prior art devices.

It is therefore an object of this invention to provide a brushless direct current motor of the type described above including a permanent magnet rotor having any number of poles, a plurality of angularly spaced field windings surrounding said rotor, means mounted for rotation with the rotor for indicating its angular position with respect to the field windings, a plurality of sensors mounted for sensing the position of the position indicating means as the armature rotates, and means responsive to the output of the sensor means for generating twice the number of commutating pulses as there are sensor means, and for directing current through the field windings alternately in both directions in response to the commutating pulses to cause said rotor to rotate.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

ricf descnption of the drawings Figure 1 is a simplified block diagram showing the basic components which comprise this invention.

Figure 2 illustrates the rotor position sensing components with Figure 2a showing a metallic segment for rotation with the armature of a three phase, four pole motor and Figure 2b showing the position of the sensor coils used with the disc of Figure 2a; Figure 2c showing a metallic disc for use with a three phase, two pole motor, and Figure 2d showing the position of the sensor coils used with the disc of Figure 2c.

Figure 3 is an electrical schematic diagram showing the oscillator for supplying high frequency energy to the sensor coils and the envelope detector.

Figure 4 is a logic circuit which receives the output of the sensor coils and develops commutating pulses.

Figure 5 is a wave form diagram showing the output from the sensor coils.

Figure 6 is a wave form diagram showing the commutating pulses which are the output of the circuit of Figure 4.

Figure 7 is a wave form diagram showing the time and polarity of the current applied to each of the field windings of a three phase motor.

Figure 8 is an electrical schematic of a driver circuit for the field windings wherein the field windings are connected in a wye configuration.

Figure 9 is an electrical schematic diagram of a driver circuit for the field windings which are connected in a delta configuration.

Description of the preferred embodiment Referring now the the drawings which show a preferred embodiment of the invention, and particularly to the block diagram of Figure 1, a brushless direct current motor 10 is provided with an armature position sensing mechanism shown generally at 20. This mechanism is shown in more detail in Figure 2 and includes a disc 25 which is coupled to the armature of the motor 10 and rotates therewith.

The disc is preferably a printed circuit board on which is placed a copper pattern, the particular configuration of the pattern being determined by the type of motor to be controlled. For example, the disc shown in Figure 2a is provided with two 900 copper segments 26 and 27, to be used with four pole motors. The disc shown in Figure 2c contains one 1800 copper segment 28 and is used with two pole motors.

The other part of the sensing mechanism 20 includes a plurality of sensor coils SC-1, SC-2 and SC-3 which are placed adjacent the disc 25 so that as the disc rotates, the inductance of each sensor coil changes according to whether the copper pattern is in front of the coil or not. Each sensor coil is an air wound solenoid having an 0.050 inch inside diameter, an 0.250 inch outside diameter, and 0.020 inch thickness wound with 100 times of 39 AWG wire. This design yields an inductance of 35.3 microhenrys.

The sensor coils in Figure 2b are placed 600 apart, and these coils, when associated with the copper pattern shown in Figure 2a, allow for the control of a four pole, three or six phase motor.

The sensor coils shown in Figure 2d are placed 1200 apart, and these coils, when associated with the disc of Figure 2c, allow for the control of a two pole, three of six phase motor. It should be mentioned that two or four phase motors could also be controlled by using sensor coils placed 450 apart and the disc arrangement shown in Figure 2a, or 900 apart and using the disc pattern shown in Figure 2c.

Referring again to Figure 1 , the sensor coils will change inductance as the copper pattern on the disc 25 rotates past the coils, and this change of inductance, when the coils are excited by signal from the sign wave generator 40 through amplifier 45, will cause the voltage across the coils to vary. This variation in voltage will be sensed by the envelope detector circuit 50. The output of this circuit, which is a plurality of square wave signals, is applied to the logic circuits 60, and this circuit produces the commutating pulses which are applied to the power transistors, shown generally at 70, for controlling the current through the field windings of the motor 10.

Referring now to Figure 3, the oscillator 40 generates a sine wave signal having a one volt peak to peak output and a frequency of 282 kHz.

This sine wave is applied to the amplifier circuit 45, the output of which are applied to the sensor coils SC-1, SC-2 and SC-3. Across the sensor coils are capacitors C1, C2 and C3 which tune the resonant frequency of the circuits to the oscillator output frequency.

An envelope detector 50 is connected to each of the coils SC-1, SC-2 and SC-3. These detectors comprise a rectifier diode D1, D2 and D3, and a filter circuit including capacitors C4, C5 and C6 and shunting resistors R1, R2 and R3. The outputs l, II and Ill of the envelope detectors are approximate square waves, and these are shown in the wave form diagram of Figure 5. This represents the output pulses which would be derived from the disc arrangement of Figure 2c and the sensor position of Figure 2d. Each output pulse is 180" in duration, and each pulse is displaced from the previous pulse by 1200.

The output pulses I, II and Ill are applied to the logic circuit arrangement 60 shown in Figure 4.

Each of the three pulses is applied first to a threshold detector, T1, T2 and T3, then to a buffer amplifier B1, B2 and B3, and then to an inverter, 11,12 and 13. The outputs of the buffers and inverters are applied to six NOR gates, N1--N6, whose outputs comprise six commutating pulses which are used for controlling the application of current to the field windings of the motor Figure 6 shows the commutating pulses generated by the circuit of Figure 4. It will be noted that each commutating pulse is 1200 in duration, and that there are twice the number of commutating pulses as there are sensor coils.

The commutating pulses are applied to the power transistor circuit 70 of either Figure 8 or Figure 9. Figure 8 represents a control circuit for field windings which are connected in a wye configuration whereas Figure 9 represents a control circuit used when the field windings are connected in a delta configuration.

In Figure 8, the commutating pulses 1-6 are applied to the gates of transistors Ql -Q6.

Transistors Q7-Q9 are in turn controlled by transistors Q1--Q3, and therefore switching transistors Q4-Q9 direct the flow of current through the field windings A, B and C.

As shown in Figure 7, commutating pulse 1 causes the current to flow in one direction through field winding A from 0 to 1200 of armature rotation, commutating pulse 2 causes current to flow in one direction through field winding B from 1200 to 2400 while commutating pulse 3 causes current to flow in one direction through field winding C from 2400 to 3600.

Current will flow in the reverse direction through field winding A from 1 800 to 3000 as a result of the commutating pulse 4; through field winding B from 3000 to 600 in response to commutating pulse 5, and through field winding C from 600 to 1800 in response to commutating pulse 6.

Thus, in the present invention, means (Figure 4) are provided to create twice the number of commutating pulses as there are sensor coils and further means (Figures 8 and 9) cause current to flow in both directions through each of the field windings, simulating a three phase, source of current, thereby improving the effectiveness and efficiency of a brushless DC motor utilizing this invention over the prior art.

The invention has been described with reference to a two pole, three phase motor; however the principles revealed herein also apply to other types of brush less direct current motors, such as two pole, four or six phase motors and four pole, three, four or six phase motors with appropriate modifications to the circuits of Figures 3, 4, 8 and 9.

While the form of apparatus herein described constitutes a preferred embodiment of this invention, it is to be understood that the invention is not limited to this precise form of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

Claims (3)

Claims

1. A brushless direct current motor including: a permanent magnet rotor, a plurality of angularly spaced field windings surrounding said rotor, means mounted for rotation with said rotor for indicating its position, a plurality of sensor means mounted for sensing the position of said position indicating means as said armature rotates, and means responsive to said sensor means for controlling the application of electrical current through said field windings for causing said rotor to rotate including, circuit means for generating twice the number of commutating pulses as there are sensor means, and means for directing current through said field windings alternately in both directions in response to said commutating pulses.

2. A three phase brushless direct current motor including: a permanent magnet rotor, a plurality of angularly spaced field windings surrounding said rotor, a segmented disk mounted for rotation with said rotor for indicating said rotor's position, a plurality of inductors mounted for sensing the position of the segments on said disk as said armature rotates, and an oscillator for supplying a fixed frequency signal to said inductor.

a capacitor connected across each of said of inductors to form a resonant circuit which will be alternately tuned and detuned by the segments in said disk.

an envelope detector for sensing the voltage across each inductor to produce a set of output pulses equal in number to said inductors, logic circuit means for generating commutating pulses twice in number as there are inductors, and switching means for directing current through said field winding alternately in both directions in response to said commutating pulses.

3. A brushless direct current motor substantially as hereinbefore described with reference to the drawings.