14-05-2012, 05:16 PM
Sinusoidal Brushless Drive with Low-Cost Linear Hall Effect Position Sensors
Sinusoidal Brushless Drive with Low-Cost Linear Hall Effect Position Sensors.pdf (Size: 442.89 KB / Downloads: 31)
Abstract
This paper presents a low-cost ac brushless drive,
which is obtained by the employment of linear Hall effect ICs for
the generation of current references in the three-phase
stationary reference frame. In this way, the traditionally
adopted position sensor (i.e. absolute or incremental encoder,
resolver, etc.) can be eliminated, thus obtaining a considerable
gain in terms of costs and reliability. Moreover, if the back
e.m.f. presents some harmonic content, the same drive allows to
increment the output power with respect to a conventional
sinusoidal drive. Finally, it can be directly employed also in dc
brushless drives, without any modification and maximising
power output.
I. INTRODUCTION
As it is well known, the critical item in brushless drives is
represented by the position sensor. In fact, in order to obtain
good waveforms and low torque ripple, an high resolution
sensor is needed, which is costly and usually needs a special
construction for the machine (for example a second shaft end
to couple the sensor). On the other hand, low resolution
sensors are undoubtedly cheaper, but do not guarantee, in
many cases, the required performances.
With respect to the position sensor that is adopted by
commercially available drives, the following classification
can be made:
- Drives with Hall effect sensors;
- Drives with incremental encoder; - Drives with absolute position sensors, which can be
obtained or with the combination of Hall effect sensors and
incremental encoder, or with an absolute encoder or with a
resolver;
- Sensorless drives.
Hall effect sensors exhibit very low cost (typically in the
order of l$), but also very poor resolution. Commonly, these
sensors are employed in groups of three, placed 120 electrical
degrees apart, and they output a logic signal that is high or
low, whether they are subjected to a negative or positive
magnetic induction field. This disposition yield to a transition
of one of the three sensors every 60 electrical degrees, which
is also the maximum resolution achievable. Hall effect
sensors are particularly suitable for dc brushless drives, in
which position information is used to switch a constant
amplitude current between two phases, but are not adopted in
0-7803-7067-8/01/$10.00 02001 IEEE
F. Crescimbini, L. Solero
Dept. of Mechanical & Industrial Engineering
University of “ROM TRE”
V. della Vasca Navale, 79
1-00146 Rome (ITALY)
ac brushless drives, where high resolution is required in order
to obtain good sinusoidal current waveforms.
Incremental encoders offer as high resolution as desired,
but they require high costs (100 $ and above) and a shaft end
to be coupled to. The more, they have the great disadvantage
that absolute position is unknown at start-up (which can lead
to wrong direction starting of the motor). A widely used
solution for this last problem, is the adoption of both Hall
effect sensors and incremental encoder. In this case, in fact,
the added cost is very low and absolute position at start-up is
available from Hall effect sensors, whose resolution of 60
electrical degrees guarantees that the applied torque moves
the machine in the desired sense.
Nonetheless, the problem of the cost of such drives is not
solved, since the incremental encoder is still employed. This
is why in the last years, great efforts have been made in the
field of sensorless drives, in which the goal is to calculate the
position of the rotor from the acquisition of phase currents
and voltages. Great developments have been made, thanks to
the increased computational power that is nowadays
available, but still some problems limit industrial
development of these algorithms, like the need for exact
knowledge of motor parameters and their variations with
working conditions, or like the inaccuracies of the algorithms
at low speed.
In this paper a low-cost drive for ac brushless is presented,
which makes use of linear Hall effect ICs for generating
current references in the three-phase stationary reference
frame. It is also shown how the same drive allows to
increment the output power with respect to a conventional
sinusoidal drive, if the back e.m.f. presents some harmonic
content.
11. LOW COST DRIVE
Control law for ac brushless drives can be developed both
in synchronous and in three-phase reference frame. Though
the first frame guarantees higher performances and is often
adopted in servo drives, where accurate speed and position
tracking are needed, the second one is usually adopted in low
cost drives, where a certain reduction in control accuracy can
be tolerated. A conventional drive with control of phase
currents in the three-phase stationary reference frame usually
follows the block diagram of Fig. 1.
799
INVERTER PMSMMOTOR +ENCODER
Fig. 1 - Conventional stationary reference frame control scheme for AC brushless dnve
In this scheme, the information of rotor position is used to
generate three sinusoidal current references, in phase with
motor back emf (unless flux weakening region is reached)
and whose amplitude depends on the torque reference.
In the proposed low-cost drive, sinusoidal waveforms
generation in phase with motor back e.m.f., is accomplished
by means of linear Hall effect ICs whose cost is less than 5$.
This sensor, in fact, produces an output which is proportional
to the intensity of the induction field in which it is immersed,
so that when conventional Hall effect sensors are replaced by
this linear sensor, the field produced by the motor permanent
magnets can generate three sinusoidal signals, displaced 120
electrical degrees fom each other, and exactly in phase with
motor back e.m.f.
The output of each signal can be regarded as an unit
amplitude analog reference for the corresponding phase
current; thus it needs only to be multiplied by the desired
current amplitude to obtain the phase current reference. From
this point on, the control scheme follows exactly the
traditional one in the stationary three-phase reference frame,
with the current reference compared with measured signal and
the difference elaborated by a current regulator in order to
obtain the phase voltage to be applied by proper modulation
technique.
Current loop can be implemented by means of an analog
board, in which the same structure is replicated for the three
phases: the signal coming from the linear se?sor and the one
which represents the amplitude reference (I ) are fed to an
analog mult$lier, whose output is therefore the phase current
reference (i, ).
Phase current is measured and then compared with its
reference; the error is processed by a PI regulator, obtaining a
desired value for the phase voltage (va*) which in turn is fed
to an analog PWM modulator to close the loop.
In the case that motor star point connection is not available,
the two requirements that
i, + i b +i, = O (1)
* * *
and
i, + ib + i, = 0 (2)
lead to a semplification of the scheme. In fact the current
loop on the third phase can be closed comparing a reference
current, calculated from the other two references, with a
measured current, calculated as well from the other two
measurements. In this way, further cost reduction is achieved
eliminating the need for a current sensor and for an analog
multiplier.
The proposed scheme can be implemented basically with
two discrete components available in the market, one is an
analog multiplier and the other is a PWM regulator chip
which integrates also the PI regulator. In order to further
decrease the costs of the proposed scheme, these two
components should be hopefully integrated, thus obtaining a
single chip for each phase current loop.
Low cost digital processors can also be added to the drive
to provide, for example, the desired current amplitude signal.
In this way it is possible to implement advanced features, like
for example closing an outer speed loop, or commanding a
multi-drive system with a supervising unit which provides the
current references for each drive.