fpga based indirect position detection and speed control of pmbldc--Journals456.pdf

International Journal of Communications and Engineering

Volume 03– No.3, Issue: 01 March2012

Page 45

FPGA BASED INDIRECT POSITION DETECTION

AND SPEED CONTROL OF PMBLDC MOTOR

USING VIRUTAL INSTRUMENTATION

ABSTRACT

Recently, the brushless dc (BLDC) motor is becoming popular in various applications because of its

high efficiency, high power factor, high torque, simple control, and lower maintenance. Moreover,

reducing of the drive components is more attractive for low cost applications. This paper proposes a

speed response of the FPGA-based sensorless control of brushless dc(BLDC) motor drives. The

sensorless position detection proposed in this paper is based on the detection of zero crossing points of the

line voltage difference measured at the terminals of the motor using Virtual Instrumentation.The speed

control of the motor is obtained by pulse width modulation of the pulses given to the MOSFET inverter

circuit using FPGA. The effectiveness of the proposed method is demonstrated through simulation and

experimental results

Index Terms: Brushless machines, Inverters, Zero-crossing Detection, Virtual Instrumentation, Labview,

Field programmable gate array (FPGA), Sensorless control.

I.INTRODUCTION

Permanent magnet brushless dc (BLDC)

motors have used wide application due to their

power density and ease of control. Moreover, the

machines have high efficiency over a wide speed

range. Therefore, it is suitable for variable speed

applications and results in energy saving

[machines and Stepper motors.

The control of BLDC motors can be done in

sensor or sensorless mode, but to reduce overall

cost of actuating devices, sensorless control

techniques are normally used. The advantage of

sensorless BLDC motor control is that the

sensing part can be omitted, and thus overall

costs can be considerably reduced. Sensorless

control is highly advantageous when the motor

is operated in dusty or oily environment, where

cleaning and maintaining of Hall Sensors is

required for proper sensing of rotor position.

Sensorless method is preferred when the

motor is in less accessible location.

Accommodation of position sensor in motor

used in compact unit such as computer hard disk

may not be possible. Novel direct back emf

detection for sensorless BLDC motor is given in

[7]. The BLDC motor without position and

speed sensors has attracted wide attention and

many papers have reported work on this. These



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methods are based on using back-EMF of the

motor, detection of the conducting state of

freewheeling diode in the unexcited phase ,

back-EMF integration method, detection of

stator third harmonic voltage components.

Back-EMF estimation methods typically on

the zero crossing detection of the EMF

waveform. In the method proposed in this paper

the line voltage difference is fed to the virtual

instrument developed in Lab VIEW through

serial communication and the zero crossing

points are detected. Using thisdetails pulses for

the inverter circuit are generated. The generated

pulses are given to the FPGA (field

programmable array) through serial port and the

pulse width modulation is done according to the

set speed. The method is simple, reliable and

does not involve any integration. Further, since

line voltages are used, the requirement of neutral

potential has been eliminated. The Zero crossing

instants are done using virtual circuits developed

in Labview. This also eliminates the common

mode noise. Device drops and their variations

would also not play a part since line voltages are

used.

Section II describes the proposed back-EMF

zero crossing estimation method. Section III

hardware implementation of the proposed

method. Section IV presents the virtual

instrument and results that validate the proposal

and section V presents the conclusion.

II. THE PROPOSED BACK-EMF ZERO

CROSSING ESTIMATION METHOD

Typically, a BLDC motor is wound in a

three-phase wye configuration. This

configuration connects one end of each phase

together to make a center point of a “Y” or the

motor neutral point. This is then driven by a

three-phase inverter with what is called six-step

commutation. At any step, only two of the three

phases are conducting current where current

flows into one phase and then out another. For

example, when phase A and phase B conduct

current, phase C is floating. A transition from

one step to another step is called commutation.

So, totally, there are six steps in one cycle. As

shown in Fig. 1, the first step is AB (phase A and

B conducting current), then to AC, to BC, to BA,

to CA, to CB and then the pattern is repeated. In

order to produce maximum torque, the inverter

should be commutated every 60 electrical

degrees so that current is in phase with the back

EMF. The conducting interval for each phase is

120 electrical degrees, or two steps. The

commutation timing is determined by the rotor

position, which can be determined every 60

electrical degrees by detecting when the back

EMF on the floating phase crosses the zero

potential point, or “zero crossing.” Fig. 1 shows

the typical inverter configuration and current

commutation sequence.

As mentioned above, since only two phases

are conducting current at any step, one phase is

available to measure back EMF. To measure the

back EMF across a phase the conventional

method (shown in Fig. 2) requires monitoring

the phase terminal and the motor neutral point.

The zero crossing of the back EMF can be

obtained by comparing the terminal voltage to

the neutral point. In most cases, the motor

neutral point is not available. The most

commonly used method is to build a virtual

neutral point that will, in theory, be at the same

potential as the neutral point of the wye-wound

motor. This conventional detection scheme is

quite simple and has been in use for some time

[1]. However, this scheme has its drawbacks.

When using PWM to regulate motor speed or

torque/current for instance, the virtual neutral

point fluctuates at the PWM frequency. As a

result there is a very high common-mode voltage

and high-frequency noise.



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Then the phase A terminal voltage with

respect to the star point of the stator Van, is

Van = Raia + La (dia/dt) +ean - (1)

Similar equations for the other two phases are,

Vbn = Rbib+ Lb (dib/dt) +ebn - (2)

Vcn = Rcic + Lc (dic/dt) +ecn - (3)

From equation (1),(2),(3) the line voltage Vab

and Vbc may be determined

Vab = Van - Vbn

= R (ia- ib) + L (d (ia- ib ) /dt) + ean-ebn

Vbc = Vbn – Vcn

= R (ib- ic) + L (d (ib- ic) /dt) +ebn- ecn

.

Fig 1: PMBLDC drive and Zero crossing

points of the back-EMF and phase current

commutation points.

In the interval when phases A and C are

conducting and phase B is open, phase A

winding is connected to the positive of the DC

supply, phase C to the negative of the DC

supply and phase B is open. Therefore ia = -ic

and ib = 0. Therefore in this interval the equation

is simplified as,

Vabbc = Vab - Vbc = -2 ebn

The line voltage difference waveform is thus

an inverted representation of the back-EMF

waveform. The error between the line voltage

difference and back EMF ,also shown in Fig. 2

is negligible at the zero crossing instant.

Therefore the operation Vab-Vbc (Vabbc) enables

detection of the zero crossing of the phase B

EMF. Similarly the line voltage difference

Vbcca enables the detection of zero crossing of

phase C back-EMF. The line voltage difference

Vcaab waveform gives the zero crossing of phase

A back-EMF. Therefore the zero crossing

instants of the back-EMF waveforms may be

estimated indirectly from the line voltage

differences.

The line voltage difference waveform is fed in

to the virtual instrument developed in LabVIEW

using RS232 serial communication. In the VI the

zero crossing of the back e.m.f is determined

and the switching signals are generated.

The switching signals are further fed to the

FPGA. In Field programmable array the pulse

width modulation of the particular signals are

performed and fed to the MOSFET inverter

circuit. The set speed is given by the user in the

VI developed in LabVIEW and its fed to the

FPGA using serial communication.



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Fig 2: Line voltage difference and back-EMF

III. DESCRIPTION OF EXPERIMENTAL

SETUP

The block diagram of the experimental setup of

the proposed sensorless BLDC motor drive is

shown in fig.3. The experimental setup consists

of a 24v, 8000rpm 4pole PMBLDC drive. In a

three-phase brushless motor and a 120-

electrical-degreetype inverter, transistor

commutation occurs at intervals of 60 electrical

degrees. This causes important transient in the

motor terminal voltages, so some techniques are

necessary for the direct use of these voltages for

sensing rotor position. Three phase bridge

inverter fabricated using n – channel MOSFET

is operated in 120 degree mode to provide

square wave current excitation to the stator

windings. MOSFETs are used to control the

speed of the motor by varying the supply voltage

to the motor. It is switched with very high speed

with the help of PWM waves. The PWM waves

are generated by the FPGA.

Inverter switches are triggered in a sequence

provided by the high performance digital

controller IC MC 33035. The IC contains all the

functions required to implement a full featured

open loop control. The output of MC 33035 is

ideally suited for driving power MOSFETs of

low ratings. To drive MOSFETs of higher

ratings the outputs of MC 33035 are given to

driver circuits through opto-couplers. MC 33035

is capable of controlling the speed of the motor

in open loop mode only. For closed loop

operation, it requires an input voltage

proportional to the motor speed. This can be

achieved using MC 33039.

Fig 3: Experimental setup

The isolation circuit is mainly used to isolate

the high voltage and low voltage circuit, SCR

and Triac circuit, mechanical relay and pulse

transformer. Here the 4N35 opto coupler

constructs the isolation circuit. The opto coupler

consists of photo LED and phototransistor. The

phototransistor conducts only when light rays

falls on the base of the transistor. The signal to

be isolated is given to base of BC547 switching

transistor.

An FPGA platform used for controlling the

BLDC machine is Spartan 3 family, from Xilinx.

Reference speed value was set digitally, and a

speed loop was used to compare the actual speed

and the reference speed and based on error to

determine the duty cycle for the next period. The

actual speed was easily calculated as a time

between line voltages. Furthermore, this scheme

can be effectively implemented on an FPGA as

opposed to DSPs that are used for the

implementation of complex control schemes.



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This makes the present technique significantly

cost effective. The FPGA operates at a clock

frequency of 12 MHz.

IV.VIRTUAL INSTRUMENTATION AND

ITS EXPERIMENTAL RESULTS

LabVIEW is a graphical programming language

that uses icons instead of lines of text to create

applications. LabVIEW programs are called

virtual instruments, or VIs, because their

appearance and operation imitate physical

instruments, such as oscilloscopes and

multimeters LabVIEW program facilitates

virtual instrumentation (VI), which imitates the

appearance and operation of physical

instruments: VI is defined as a process of

combining hardware and software with industry

standard computer technology to create a user-

defined instrumentation solution. Several other

add-on toolsets can be incorporated for

developing the specialised applications. Labview

done the zero crossing detection and speed ,

current and voltage measurements.

Fig 4: Timing simulation of the trigger for

estimated commutations

Table B shows how much logic resource of

FPGA is used to implement the whole system,

and as shown in the table every item is below

40%. It means one can select a smaller and Low

cost FPGA S350, to further reduce the cost, or

one can also build up a microcontroller

Intellectual Properties (IP)into FPGA to

implement more sophisticated control algorithm.

The Generation of pwm is implemented in

Xilinx FPGA. Fig 4 shows the timing simulation

of the trigger for estimated commutations

Fig 5: Experimental set-up for the proposed

method

The experimental results for the speed

response of the FPGA-based sensorless control

of brushless dc motor are discussed for the

desired speed. Figure 5 shows the experimental

set-up for the proposed method. Figure 6 shows

the measured back emf waveform for the

proposed method. Figure 7 shows the results for

the speed control of brushless dc motor using

Labview. The set speed and the actual speed of

the motor is also displayed in front panel of

labview. The actual speed value for the different

set speed is given in Table1.



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Fig 6: Line voltage Difference

Table 1 Speed Obtained for different Set

speed in this proposed method.

Fig 7: Results for the proposed method

V. CONCLUSION

This paper has presented a novel FPGA-based

sensorless control scheme for six-switch three-

phase brushless dc motor drives. In the scheme,

the speed response of the FPGA-based

sensorless control of brushless dc(BLDC) motor

drives is proposed. The position information is

estimated from the crossings of voltage

waveforms in floating phases, and a low cost

FPGA is utilized to implement the algorithm.

Because the stator current waveforms of the

inverter using this novel voltage PWM scheme

are rectangular, the motor will operate smoothly

and the torque ripple will be reduced. Compared

with the conventional back EMF zero crossing

sensorless control, the proposed new sensorless

control methods for brushless DC technique is

more robust, easier to implement, and cost

Effective because of virtual instrumentation and

FPGA. Theoretical analysis and experimental

results verified that satisfactory performance is

achieved with the motors with the proposed

method.

APPENDIX

A. PMBLDC Motor Specifications

Number of poles 4 Poles

Line to line resistance 0.2 ohms

Lineto line inductance 0.45 mH

Nominal voltage 24 V

Rated current 3A

Rated Speed 8000RPM

B.Device utilizations summary of Fpga:

S.No Set Speed

RPM

Actual Speed

RPM

1 2500 2560

2 4500 4560

3 5000 5120

4 6200 6260

5 7500 7520



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fpga based indirect position detection and speed control of pmbldc--Journals456.pdf