Electronic Speed Variator for a Brushless DC Motor

Transcription

1 Electronic Speed Variator for a Brushless DC Motor Jorge M. Jaimes Ponce, Jesús U. Liceaga C., Irma I. Siller A. and Enrique Arévalo Zamudio Abstract In this paper the development of an electronic speed variator for a Brushless DC (BLDC) motor is presented. The speed variator is comprised of several blocks which include a power amplifier, an angular position sensing based on Hall Effect sensors, a voltage regulator and digital processing and data generation. The main characteristic of this work is that it is not based on the use of predesigned commercial components giving the basis for the development of new ideas. Also, it is possible to extend this work for broader range of BLCD motors and applications such as medicine, remote operating vehicles (ROVs) and general industrial uses. Keywords Speed variator, brushless DC motor. I. INTRODUCTION HE idea behind the design of the speed variator is to T design, construct and control a ROV quadcopter which obviously requires to regulate the speed of the propellers brushless DC motors. The trajectory control of unmanned aerial vehicles (UAV) has received great attention in the past years. In particular, the trajectory control of quadcopters, []. In order to achieve this objective, it is necessary to design and construct appropriate actuators to manipulate the propellers speed and consequently the forces and torques that control the trajectory or flight path of the quadcopter. The speed variator must have appropriate time responses such that it can be inserted as an actuator- in a control loop. From the academic perspective a second objective of this project is to generate the required knowledge to extend the design of speed variators for higher power BLDC motors. J. M. Jaimes-Ponce is with the Electronic Department of the UAM- jjp@correo.azc.uam.mx). J. U. Liceaga-Castro is with the Electronic Department of the UAM- Azcapotzalco, Av. San Pablo 180 C.P , México (phone: ; julc@correo.azc.uam.mx). I. I. Siller-Alcalá is with the Electronic Department of the UAM- sai@correo.azc.uam.mx). E. Arévalo-Zamudio is with the Electronic Department of the UAM- enarza@hotmail.com). II. GENERAL DESCRIPTION The system capable to regulate the speed of each of the 4 motors of a quadcopter is based on three blocks with different tasks: the first, dedicated to data processing; the second to signal conditioning, and the third a power driver. That is, the system is a three-phase driver powered by a DC voltage source which generate three PWM voltage signals required to operate a three poles BLDC motor, as shown in Figure 1, [1, 2]. It should be noted that the rotors speed is proportional to the commutation frequency of the three-phase inverter. Fig. 1 Simplified Block Diagram The 7.4 voltage source is a rechargeable LiPo battery with 2 cells is series. The microcontroller and the Hall Effect sensors are powered by a linear regulator L4931ABD33 from National Instrument. This regulator has an output of 3.3 volts and is powered by the 7.4 volts source. The three-phase inverter requires a source of 12 volts to feed the signal conditioning circuits and the power transistors. The 12 volts source is based on the commuted regulator LT1372 from Linear Technology. It was configured to generate 12.5 volts and is also powered by the LiPo battery. To sense the rotors position 3 Hall Effect sensors US1881KUA from Melexis are allocated around the rotor to detect the magnetic field of its permanents magnets, Figure 2. These are open-drain sensors; therefore, its 3.3 volts polarization is by means of pull-up resistances. Each of the three signals is then led to the microcontroller ISBN:

2 dspic33fj12mc202 [3, 4] from Microchip which is constantly monitoring these signals. Fig. 4 EPS periods Fig. 2 Hall sensors positioning The DC motor is the ZS BLDC [5] out-runner three-phase from Hyperion, shown in Figure 5. When the motor is powered by the inverter its rotor spins producing changes in the Hall Effect sensors signals which subsequently close the feedback loop associated to the operation of the speed variator system. Also, the microcontroller generates [3, 4], at the same time, the 6 signals that through the three-phase inverter control the high power transistors. In the three-phase inverter these signal are conditioned to comply with the level of current and voltage required by the high power transistors. The three-phase inverter generate in its output nodes Phase A, Phase B and Phase C (Figure 3) which are the signals for the Energized Phase Sequence (EPS), Figure 4. Fig. 5 ZS BLCD Motor A. BLDC Motor operation III. GENERAL DEVELOPMENT Fig. 3 Three-Phase inverter scheme. The Brushless DC motors (BLDC) are permanent magnets synchronous motors (PMSM). The magnets, normally refer as poles, are allocated in the rotor meanwhile the windings are in the stator, Figures 6 and 7. This kind of motors does not have commutators or mechanical switchers; therefore, they require an inverter or an electronic commuter to switch the DC power [1, 2] to the stator windings. ISBN:

3 Figura 6. Motors stator windings Table 1 Sensors signals for the 6 RSPS steps Each of the two possible polarities detected by the Hall Effect sensors corresponds to a discrete voltage signal at its outputs. Figure 8 shows a graphic representing these signals. Figura 7. Motors magnetic poles As mentioned above, to activate a BLCD motor is necessary to power, electronically with a specific sequence [5], the 3 phases of the motor with a DC voltage. The specific sequence is in fact the action effectuated by the mechanical switch in the case of motors with brushes. Because the BLCD motor is also a PMSM the rotors speed is proportional to the frequency of commutation. Moreover, the magnetic force by which the stator windings attracts the rotors poles is proportional to the average of the input voltage; hence, during the period of time in which each of the phases is activated they are not connected directly to the DC source B. Rotor s position sensing To detect the actual angular position of the rotor 3 sensors with Hall Effect are allocated around the rotor with a separation of 60 (Figure 2). Each sensor must be positioned between 2 stator windings of any of the 3 windings sets (Figure 2). Each spin step of the Rotor Spin Partial Sequence (RSPS) generates changes in the output signals of some of the Hall Effect sensors. Therefore, for each step there is a unique combination of these 3 signals as shown in Table 1. Fig. 8 RSPS periods graphic Detecting the rotors angular position is necessary to determine the exact instant in which the EPS must be incremented in order to maintain the rotor spinning continuously. C. EPS Electric Signals To revolve the rotor at a constant angular velocity it is necessary to constantly repeat the EPS and each of its 6 steps (Table 1) must last enough time in order to induce a rotors angular displacement from an actual position to the next position. Hence, the RSPS period is equal to 6T RSP, where T RSP is the time of one step of the ESPS. This time is equal to total time duration of the EPS. Therefore, the total time of one spin of the rotor is then: T = ( T )(6)(7) sec (1) R RSP Therefore, the average angular velocity is: ISBN:

4 ω = 1 1 rad / sec T = (2) ( T )(6)(7) R RS That is, to complete a rotation of 360 it is necessary to fulfil 7 RSPS. It must be noted that the real T RSP is calculated based on the Hall Effect sensor information. The PWM duty cycle (DuC) depends on the average voltage required in the phase during the fraction of time it is active. The average voltage is defined according to a desired constant velocity. This applies only for steady state conditions. The RSPS signals are generated by the three phase inverter. This receives logic input signals from a microcontroller and generates similar output signals but upgraded in voltage and current. These signals are referred as Power-Signals. Fig. 11 Experiment Set Up IV. EXPERIMENTAL RESULTS The speed variator electronic circuits and the experimental set up are shown in Figures 9, 10 and 11, respectively. To show the experimental results and the effectiveness of the speed variator, the Power-Signal where displayed by pairs in a two channel oscilloscope. The 9 pairs of signals analyzed were: 1. LA and HA 2. LA and HB 3. LA and HC 4. LB and HA 5. LB and HB 6. LB and HC 7. LC and HA 8. LC and HB 9. LC and HC After the interconnection of the sensors, motor, main board and oscilloscope, the system was turned on and the microcontroller run its program as expected: Fig. 9 Upper side of the speed variator circuit a. During a period of one sec. the motor stand still. This is the programmed safety delay. b. After the safety delay, the rotor is aligned to the stator to the position 1 of the RSPS (Figure 8) c. Once the rotor is aligned to the stator it slowly starts to spin up to a constant velocity In Figure 12, the pair of signals LA and Ha capture in the oscilloscope is shown. Fig. 10 Lower side of the speed variator circuit ISBN:

5 V. CONCLUSION In this paper the design, construction and experimental tests of an electronic speed driver are presented. One of the most important aspects of design is that it is not based on the use of predesigned commercial components. It was found that a precise location of the Hall Effect sensors around the rotor is crucial to achieving high speeds. Due to its compact design it was possible to implement four of these drivers in a quadcopter without a significant payload increment. Fig. 12 LA and HA signals In figure 13, signals LA and HA are display overlapping a visual guide to clearly distinguish each of the 6 step of the inverter sequence (IS) REFERENCES [1] Rashid M. H., "Electrónica de Potencia, Circuitos, Dispositivos y Aplicaciones", Prentice Hall, 3ra ed., México, [2] Hart D. W., "Electrónica de Potencia", Prentice Hall, 1ra ed., México, [3] Angulo Usategui, Garcia Zapirain, Angulo Martinez, Vincente Saez, Microcontroladores avanzados dspic, homson-paraninfo, 2006, ISBN , [4] Tim Wilmshurst Designing Embedded Systems with PIC Microcontrollers, Principles and applications, ELSEVIER-NEWNES. ISBN: [5] Freescale Semiconductor, Application Note, AN1916, Rev. 2.0, 11/2005. Fig. 13 LA and HA signals together with the inverter sequence The microcontroller program responds to 3 user s commands: a. Speed increment. b. Speed decrement. c. Stop These buttons are allocated in the protoboard shown in Figure11 ISBN: