Biomedical Engineering Reference
In-Depth Information
FIGURE 7-15
Beam scanning of a
10-element linear
array witha4mm
spacing between
elements.
If the relative phase of each of the elements is referred to the center of the array, then the
equation for the sum can be written as
sin
N
N
+
1
2
sin
(
N
ψ/
2
)
sin
E
a
=
ω
t
+
k
ψ
−
ψ
=
sin
(ω
·
t
)
(7.5)
(ψ/
2
)
k
=
1
Substituting for
ψ
into equation (7.5) results in the classical equation that defines the
voltage output of the one-dimensional (1-D) linear phased array
sin
N
π
d
λ
sin
θ
E
a
=
sin
(ω
t
)
·
sin
π
d
λ
sin
θ
(7.6)
By introducing linearly increasing phase shifts across the array, it is possible to steer the
main lobe of the beam over a reasonably wide sector while still maintaining a reasonably
narrow beam and low sidelobes, as shown in Figure 7-15.
In reality, the beam pattern of the array is modified by the shape of the beam of the
individual elements. For that reason small elements with wide beamwidths, rather than
the large elements shown in Figure 7-12, are used. This also allows the elements to be
packed close together, typically
<λ/
2 apart; otherwise, unwanted grating lobes appear
on either side of the main lobe (Brooker, 2008).
7.5.1.6 GuideCane
This fairly recently developed device was designed at the University of Michigan to help
blind people navigate safely and quickly (Borenstein and Ulrich, 1997). It consists of
an array of ultrasonic sensors on a wheeled robotic vehicle that can navigate through a
cluttered environment. The operational principles are shown in Figure 7-16.
The GuideCane is used much like a white cane, though it is heavier and rolls on a
pair of wheels. A computer-controlled steering servo directs the wheels with respect to
the cane. An array of ultrasound sonar transducers mounted on a circular arc senses the
environment to the front and sides. In addition, a flux gate compass and wheel encoders
provide information about the direction and distance traveled. Finally, a miniature, thumb-
controlled joystick is positioned on the handle of the cane.
The joystick commands the initial direction of travel, either straight ahead or to the
left or right from the current direction; the steering servo will turn as the GuideCane is
pushed forward until it has reached that direction, after which it will follow a straight
path. The sonar array images over the 120
◦
sector ahead looking for obstacles as it moves
