Improved brushless DC motor speed controller with digital signal processor
produced can be calculated as in (5). As shown in (5), the electromechanical torque is proportional to the DC-link current
H. Kahveci, H.I. Okumuş and M. Ekici Brushless motors are used in many applications owing to their advantages. In most of the applications, conventional control methods with a hysteresis band (HB) controller are used. An improved speed and current control scheme for brushless DC motors with a trapezoidal shape back EMF is presented. Instead of the conventional HB and proportional-integral (PI) controllers, a fuzzy logic controller that is independent of motor equations and based on expert knowledge has been employed for current and also for speed regulation; thereby, the disadvantages of the HB are eliminated and the overall performance of the controller is enhanced. Experimental studies have been carried out with a TMS320F2812 digital signal processor. The presented control scheme has been validated through comparative experiments with the fuzzy logic speed and HB current controller. The results show that the proposed control scheme operates satisfactorily and provides a constant switching frequency.
E = Ke vm
(4)
Te = 2Ke i
(5)
Control implementation: The proposed control scheme is shown in Fig. 1. Rotor position and speed are calculated with signals coming from Hall effect sensors (Ha, Hb, Hc). The speed error (es) is evaluated by the FL speed controller in the outer loop and the reference current (Idcref ) is produced. The reference current is compared with the current drawn from the source (Idc). The current error (ec) is evaluated by the FL current controller in the inner loop and the control signal is generated. The switching signals in the next step are updated according to the control signal. a b
Vdc
Introduction: Brushless motors are classified into two groups according to the structural properties and type of induced back EMF: brushless AC (BLAC) and brushless DC (BLDC) motors. Although BLDC motors (BLDCMs) have trapezoidal shape back EMF, BLACMs, also known as permanent magnet synchronous motors, have a sinusoidal shape back EMF. Advantages such as high efficiency, high air gap flux density, high acceleration and deceleration rates, high torque/inertia, low maintenance and silent operation have opened the ways for the use of BLDCMs in variable speed drives, fuel pump controls, steering wheel controls, industrial robots etc. Many control methods have been introduced in the literature about BLDCMs. To obtain rapid torque response, both speed and current feedbacks are used in conventional control methods. Although controllers such as the proportional-integral [1], fuzzy PID [2], sliding mode [3], fuzzy neural network [4] etc. are used to regulate the speed error in the outer loop, the only controller to be used to regulate the current error in the inner loop is the hysteresis band (HB) controller. The HB controller, having a simple structure, causes variable and high switching frequencies that increase the switching losses. To overcome this problem, a fuzzy logic (FL) speed and current control scheme is presented in this Letter. Mathematical model of BLDCM and control theory: Stator winding resistances (R), self inductances (L) and mutual inductances (M) are considered to be equal for a BLDCM with a surface-mounted permanent magnet and star connected windings. The electrical and mechanical equations of the motor are as in (1) and (2), if iron and hysteresis losses are omitted R 0 0 ia L−M 0 0 va L−M 0 vb = 0 R 0 ib + 0 vc ic 0 0 R 0 0 L−M ea ia (1) × p i b + e b ic ec Te = J
dv + B vm + T L dt
c gate signal generator control signal FL current controller ec
current sampling Idc
+ –
Hb
Ha
Hc
position sensing ωm speed –+ calculation
dec
1/s
load
BLDC motor
Idcref FL speed controller
–+
ωref 1/s
es des
+ –
Fig. 1 Proposed control scheme initialise peripherals (TIMER, ADC, PWM...) main program rotor position sensing and comparing and PWM outputs and waiting interrupts timer1 int. current sensing and generating control signal (CMPRx) with fuzzy logic speed and current controller
PWM1 . . . PWM6
cap3 int. rotor speed calculation with one Hall effect sensor
Fig. 2 Implementation of control scheme in TMS320F2812 digital signal processor
(2)
υx, ix and ex are the phase-neutral voltage, phase current and induced voltage for each phase, where x is a, b or c. Te, J, B and TL are the electromechanical torque, inertia, friction coefficient and load torque, respectively. In addition, Te can be represented as in (3), where ωm is the mechanical speed of the rotor Te =
ea ia + eb ib + ec ic vm
(3)
The flat top portion of the induced voltage (E) can be represented as in (4), where Ke is the voltage constant. BLDCMs have two operation modes: electronic commutation and continuous conduction mode. Only two phases conduct during the continuous conduction mode and the third one must be free. Therefore, the conducting phases have the same current, named as the DC-link current (i), in this Letter. As the induced voltages and phase currents have the same sign, the torque
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Fig. 3 Experimental setup
Experimental studies have been carried out using a high-performance (150 MHz) TMS320F2812 digital signal processor (DSP) developed for motor control applications. Inherently, the HB controller software is written into the main program which is executed at 150 MHz. In this case, the control signal is updated very quickly in different time intervals. Therefore switching signals at higher and variable frequencies are obtained. The organisation of the control scheme in the DSP is shown in Fig. 2. The FL speed and current controller software is
Doc: {IEE}El/ISSUE/50-12/Pagination/EL20140609.3d Control engineering
written into timer1 interrupt service routine. The interrupt request is sent at every 64 µs and the control signal is updated. Thus, the switching signals are generated at a frequency of 15 kHz. The experimental setup is shown in Fig. 3. 290
speed, rpm
232 174 116 58 0 0
0.5
1.0 time, s
0.5
2.0
a 290
speed, rpm
232
Conclusion: The proposed speed and current controller using the TMS320F2812 DSP for BLDCMs has been implemented. Instead of the HB controller used in conventional controllers of BLDCMs, the FL controller is used. As a result, the constant switching frequency is obtained. Moreover, the comparative experiment shows that the proposed controller provides a faster response at different speeds and loads without oscillation.
174 116 58 0 0
0.5
1.0 time, s b
0.5
Fig. 4 Experimental results a With FL speed and HB current controller b With proposed FL speed and current controller
Table 1: Parameters of BLDCM Phase res.
Experimental results: The proposed control scheme has been tested at different speeds and load conditions. It has also been compared with the FL speed and HB current controller. The results obtained from both the controllers are shown in Fig. 4. The parameters of the motor used in the experiments are given in Table 1. In the experiments, an 8 Nm load has been applied at 0.25 s via an electromagnetic brake unit, whereas the motor is running at a 180 rpm speed. After reaching the reference speed under load, the reference has been increased to 220 rpm at 1.2 s. It can be seen from Fig. 4 that the proposed speed controller has a faster response under load without oscillation and a faster unit step response without overshoot. The switching signals of a single phase with two different controllers are shown in Fig. 5. Although the motor operates at 180 rpm under the 8 Nm load, the switching signal of about 15 kHz has been obtained with the proposed controller. It is seen from the Figure that the frequency has remained constant under the 8 Nm load and the only thing changing is the duty cycle. On the other hand, a switching signal at variable and high frequencies has been obtained as in Fig. 5b with the FL speed and HB current controller under the 8 Nm load condition.
0.22 Ω
Phase induct. 0.0054 H Inertia 0.0081 kgm2 Friction coef. 0.002 Nm.s Torque const. 0.716 Nm/A
Voltage const. 74.99 V/krpm Pole number Rated speed Rated voltage Rated power
a
b
Fig. 5 Switching signals under 8 Nm load a With proposed controller b With FL speed and HB current controller
16 330 rpm 48 V 500 W
2.0
© The Institution of Engineering and Technology 2014 25 February 2014 doi: 10.1049/el.2014.0609 One or more of the Figures in this Letter are available in colour online. H. Kahveci, H.I. Okumuş and M. Ekici (Department of Electrical and Electronics Engineering, Karadeniz Technical University, Trabzon, Turkey) E-mail:
[email protected] References 1 Kumar, B.M., Ravi, G., and Chakrabarti, R.: ‘Sensorless speed control of brushless dc motor with fuzzy based estimation’, Iran. J. Electr. Comput. Eng., 2009, 8, (2), pp. 119–125 2 Reddy, C.S.R., and Kalavathi, M.S.: ‘Performance evaluation of hybrid fuzzy logic controller for brushless dc motor drive’, Int. J. Eng. Sci. Technol., 2011, 3, (6), pp. 4749–4758 3 Rath, J.Y.: ‘Sliding mode load torque observer based effective disturbance rejection for a 3-phase BLDC drive’, Int. J. Comput. Appl., 2012, 43, (16), pp. 33–40, doi: 10.5120/6190–8676 4 Lv, Y., Fun, H., Zou, Q., and Wang, J.: ‘Brushless DC motor speed control system based on fuzzy neural network control’. Int. Workshop on Information Security and Application, Qingdao, China, November 2009, pp. 173–176
