International Research Journal of Applied and Basic Sciences www.irjabs.com

International Research Journal of Applied and Basic Sciences
© 2013 Available online at www.irjabs.com
ISSN 2251-838X / Vol, 5 (9): 1108-1114
Science Explorer Publications
The Most Efficient Sensor less Control Technique
Selection for Switched Reluctance Motor
Maryam, Bahramgiri1, Alireza, Siadatan1, Mehran, Rafiee1, Rana, Moeini1
1. Department of Electrical Engineering, West Tehran Branch, Islamic Azad University, Tehran, IRAN
Corresponding Author email: r, [email protected]
ABSTRACT: Rotor position detection is required for Switched Reluctance Motor (SRM) control
which has to be done in all controlling methods .This paper presents the comparison of three different sensorless control strategies for SRM in order to select the best method. All of these methods
are efficient techniques which are tested at laboratory. One of them is based on Amplitude Modulation (AM) technique which uses DSP processor as the controller chip. Another one which includes
FPGA processor presents a new circuit structure in which a capacitor is connected in series with the
phase winding and the rotor position is detected from the capacitor voltage rise time. The last one
which has an AVR as the controller and uses different driver circuit structure detects the position of
the rotor using the voltage phase shifting information. The strategies and their operation are studied
in different sections at first. Then they are compared with each other based on their quality, reliability,
cost, simplicity, etc and at the end the best method is selected.
INTRODUCTION
SRM controlling requires the information of the location and the angle of the rotor pole to the stator
pole. In order to detect this information some direct and indirect techniques have been presented so far (Christopher, 2010). In direct methods, position detection sensors are used. Using position sensor decreases the
driver reliability and on the other hand, increases the cost and wiring as well (Hedlund and Lundberg, 1991),
(Lyons et al., 1991). Indirect methods which are also called sensorless strategies are more favorable in which
the rotor position detecting is done by using the SRM parameters like flux, inductance, current, etc.
A large number of sensorless methods have been presented (Lyons et al., 1992; Moraveji et al., 1992).
In this paper three efficient sensorless strategies are studied and compare with each other in order to select the
best. The paper is organized as follows: Section one, two and three introduce and study the presented methods
and their specifications. Section four, presents the comparison and the conclusion is presented in section five.
AM strategy using DSP processor
In this section an affordable sensorless method is presented which uses AM technique in order to detect the rotor position (Zeng et al., 2009). A powerful and high speed DSP is used as a micro processor in this
method. The TMS320C 6414 32 bit DSP processor is able to operate in 1 GHz clock frequency which is so
high.
The driver circuit is presented in Fig. 1 where an IRF4905 P-channel and an IRF540 N-channel
MOSFET are used as power switches in each SRM phase. The TSC426 and TSC427 MOSFET drivers are
used in order to drive the transistors. Two MBR745 schottky diodes are placed in order to discharge the energy
of the phase winding when the phase is turned off. Without them, the transistors are burnt because of the energy saved in the phase that shows itself as high spikes in the transistor drains.
Intl. Res. J. Appl. Basic. Sci. Vol., 5 (9), 1108-1114, 2013
Figure1. driver circuit schematic
In order to separate the digital and power sections, some isolation devices are required to protect the
digital section from the dangerous fluctuations of the power electronic section.
The control algorithm in this method is presented in Fig.2. At first, a low voltage high frequency sinusoidal signal is generated and transmitted to the un-energized phase winding which causes a flux is generated.
The flux value is varies with rotor position variation. The phase winding flux has its maximum value in the fully
aligned position of the rotor and stator pole. It has the minimum value in fully unaligned position. The created
flux moves in a close loop of the stator pole, rotor pole, shaft and the motor yoke. This produces voltage in the
motor phase winding which is then sampled and used in order to rotor position detection process which uses
AM technique. After detecting the rotor position, affordable firing commands are produced and transmitted to
the motor driver where MOSFETs are used in order to excite the phase windings. The desired phase is then
turned on and the motor rotates. This process is repeated again.
Figure2. control algorithm
The generated 3 KHz sinusoidal signal amplitude is about 4 volt which is shown in Fig.4. The signal
current should be first amplified.
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Figure3. the generated sinusoidal signal
Then the signal is applied to the phase winding. The analog signal which includes rotor position information is sampled with 48 KHz sample frequency.
SRM generates a lot of noises in phases during operation. This causes the induction voltage varies
steadily. So, before using the sampled signal, first it has to be filtered and demodulated to be a free of any
noises signal. After that it is converted to digital information using an accurate 16 bit A/D. the sampled signal is
illustrated in Fig.4.a before filtering, in Fig.4.b after filtering, in Fig.4.c after AM demodulation.
Figure4. the sampled signal (a): before filtering, (b): after filtering and (c): after AM demodulation
The sampled signal is then analyzed by DSP in order to detect the rotor position. For a proper rotor rotating, the phase winding should be turned on in about the region where the rotor starts alignment and turned
off before the fully aligned region.
CAPACITOR COUPLED METHOD USING FPGA PROCESSOR
This section presents a novel method in which a FPGA is used as a controller of a new circuit configuration.
In the circuit shown in Fig.5, two switches are used per phase. A small capacitor is connected in series
with the phase winding of the motor and its voltage is sampled in order to rotor position detection process.
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Figure5. capacitor coupled circuit
The phase winding inductance varies with rotor position variation, so for different value of inductance,
different rise-time of the capacitor voltage is resulted which can be used to detect the rotor position.
As it’s illustrated in Fig.6.a, at first the switch T1 is turned on and T2 is turned off. The current flows
through Vcc, T1, phase winding and then enters the capacitor. Depending on the value of the phase winding
inductance, the capacitor charging time differs. The voltage equation of the capacitor can be written with equation (1):
t
(t) = V [1 − cos (
V
)]
(1)
√L(θ)C
Where L(θ) is the phase winding inductance value in Henry and C is the capacitor value in Farad.
After this process switch T2 is turned on and switch T1 is turned off so, the capacitor energy is discharged to
the GND shown in Fig.6.b.
Figure6. capacitor coupled circuit operation
The generated voltage of the capacitor which is shown in Fig.7.a is then compared with a pre-set value
voltage in a comparator which produces a pulse with different duty cycle for different voltages of the capacitor.
This depicts in Fig.7.b. So the duty cycle of the produced pulse is proportional to the phase winding inductance
and in other word, to rotor position.
(a)
(b)
Figure7. the capacitor voltage (a) and the comparator generated pulse (b) for different rotor position
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Intl. Res. J. Appl. Basic. Sci. Vol., 5 (9), 1108-1114, 2013
The cycle shown in Fig.8 is performed for all the motor phase and the produced pulses are then applied
to a FPGA which analyses them and by using a digital pulse width modulation (DPWM), detects the rotor position and generates the command firing pulse.
Figure8. Capacitor coupled method block diagram.
Voltage phase shifting method using AVR
This method is based on using the equivalent circuit of the phase winding shown in Fig.9 where Rph
and Lph are the resistance and inductance of the phase winding. Cout and Rout are external capacitor and resistance. A sinusoidal signal is applied to the phase with the same resonant frequency of the circuit. By sampling the voltage of Rout, the current flowing through the winding is obtained. By rotor rotating and changing the
value of Lph the frequency of the sampled signal doesn’t change but the phase of the signal is shifted which
can be used as a parameter in order to detect the rotor position.
Figure9. Equivalent circuit of the phase winding.
The output voltage equation is:
R
V (S) =
V (S)
(2)
Z(S) + R
Where Z(S) is:
1
Z(S) = R + L (S) +
(3)
C (S)
For a 6 by 4 SRM, using the unaligned inductance of the motor and a 100nf capacitor, the resonant frequency is obtained 10 KHz. So a sinusoidal voltage with 10 KHz frequency is applied to the phase. Fig.10
shows the Vin and Vout signal at unaligned position. As it’s illustrated in unaligned position Vout phase is same
as the Vin phase and it is obtained from the equation (4) because the circuit is in the resonate state:
R
V (S) =
V (S)
(4)
R +R
Figure10. Vin and Vout signal at unaligned position.
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By rotor rotating, the value of the phase inductance varies and as a result the circuit exits from the resonant state. The Vin signal and Vout signal is depict in Fig.11 in fully aligned position. As it’s illustrated there is
a phase shift between two signals.
Figure11. Vin and Vout signal at fully aligned position.
Zero cross detector is used to specified the value of the phase shifting. The output of the zero cross detector of the both input and output signals are applied to a XOR gate and the gate output is applied to an ATmega32 AVR microcontroller which analyses the information and generates adequate fire commands which are
then transmitted to the convertor.
CONCLUSION
This paper presented the study of three novel sensorless methods which all were efficient strategies.
The strategies and their circuit configurations were explained at first and at the end they were compared to
each other. The quality, reliability, cost, simplicity were the parameters that were used in this comparison. In all
of them, some method was performed in order to rotor position estimation. They all needed additional parts
which increased their cost.
The three presented strategies are all efficient sensorless methods. Each has some advantages and
disadvantages. The AM technique, in which DSP processor is used, is a high accurate and high speed strategy.
It can be used over a large range of speed and because of the ultra frequency of the DSP, there is no problem
in signal sampling, rotor position estimation or command pulses generating. But the circuit need robust isolation
sections and filters otherwise the noise don’t allow the controller adequately operate. AM technique implementation is rather hard. The DSP chip high cost in addition to the cost of the isolator devices make this strategy so
expensive.
The capacitor coupled circuit is adequate for standstill and low speed because in higher speed, there is
not enough time in order to charge the capacitor, sample the voltage and estimate the rotor position. But this
method has simple logic which makes its implementation so easy. It seems that cheaper controller could be
used because the algorithm doesn’t need ultra frequency. Comparators are additional parts which increase the
cost.
Voltage phase shifting technique has simple logic which is an advantage. Using AVR microcontroller decreases
the circuit cost but requiring sinusoidal signal means additional sinusoidal signal generator is needed
since ATmega32 doesn’t able to generate analog signal. Requiring zero cross detectors and XOR gates increases the cost as well. At high speed the rotor position detection become so hard which causes the strategy
is adequate only in standstill and low speed.
AM method which was a modulation method was an accurate and high speed technique. Although it
was so complex and rather hard, it seems that its reliability, quality and accuracy were much more than two
others. It was adequately able to operate in large range of speed.
REFERENCES
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