experimental and model results. Note that, in Figure 9, there is close correlation between
the experimental and model performance for the load resistance values of 500, 1000,
and 200Q.
800
,
700
•
-
,
,
'"
I
R, = 200 Ohm Exp
R, = 200 Ohm Mod
600
7ii'
R,
•
R, = 50 Ohm Exp
Q.
I
I
1
I
I
Ohm Exp
R, = 100 Ohm Mod
_
.E500
= 100
o
I
I
. -, , . i '"
R, = 50 Ohm Mod
~400
c
co
-g300
a.
E
-200
-
100
Fig. 9.
2.4.
Impedance of transmission line transformer with terminating resistor
System Performance
Having verified the TLT model, the next step is to evaluate the system performance
when a number of nodes are connected to the CPS line through the Guanella TLT (See
Figure 10). Note that the Guanella TLT is arranged in symmetric form for the sender and
the receivers. Specifically, we are interested in broadcasting data to many receiver nodes,
as illustrated in the figure. Gain and impedance must be evaluated as the number of nodes
n becomes large. The impedance seen from the sender-side terminals is given by:
ZOw) = Vin~W)
linOw)
(2)
The frequency transfer function from the input voltage of the sender to the output
voltage of a receiver is given by:
21
the two are not directly linked. Therefore, the gain dip frequency and the peak impedance
frequency can be separated.
2500
:.:._.~~:
~-_.--g:.:.. ~:=...
0.8
I
-
L=5<-7 1
L=5e-O
:
.
(A)
Fig. 14.
2000 -
1= ~:~ I
.-
L-5e-5
.
1500----------1000 ------------------- --
(B)
Characteristic gain dip and impedance peaks for system seen from sender side
Thus, tuning the TLT components also gives us the ability to separate the impedance
peak and gain dip, and as a result, allows us to use the high-fanout design protocol. This
unique contribution, based on the use of the TLT, allows us to broadcast information to
multiple receivers.
28
Central Control
Unit
Fig. 15.
Prototype
DC powerlinc communication
system with 30 nodes
Figure 16 shows the modem used to interface each node with the system.
D/A Converter
PIC16F84
Transmission Line
Transformer
(A)
Fig. 16.
AM Modulator
Schmitt-Trigger
Amplifier
(B)
Top and bottom views of prototype
modem
It consists of a transmission line transformer, a coupling capacitor, a microprocessor
(PIC 16F84), a signal modulation chipset, and a DC/DC converter. A DC servomotor
equipped with a 12V PWM amplifier was connected to each node. The microprocessor is
30
-30
-35
muu
Freq [Hz]
Fig. 18.
Noise spectrum of operating motor and noise induced by randomly turning the
motors on and off
The desired transmission frequency depends not only on the line impedance and noise
characteristics described above but also on the required transmission rate and the gain
and peak impedance conditions discussed in the previous sections. Sending command
information at and above 100 kbps determines the carrier frequency for modulation.
Frequency shift keying (FSK) modulation chipsets using carrier frequencies between 3
and 10 MHz meet this requirement. The required amplitude modulation (AM) carrier
frequency is on the order of mega-Hertz2. The exact carrier frequency was determined
based on the modem's coupling circuit design.
3.2.2.
Selection of Coupling Circuit Component
Values
We design the coupling circuitry based on the analysis described in the preViOUS
section. In turn, experimental data is recorded and compared to the model to verify the
design procedure. In addition to the bandwidth restrictions imposed by the power supply
2 Carrier frequency values of AM chip sets have much more variability than their FSK
counterparts. We chose to use AM in order to evaluate the transmission characteristics for a range
of carrier frequencies from 2 - 10 MHz.
33
600
500
-------i----t-_.LrtlliI~:I-t---,--!-i-i[r!-----,
100
--.m..~----LtU.UU. ..------l---.tLl-1~j-lmm--.~m.J..-i~~.~~:
. :.. :.. •:. :.. .•: •.•:::
.. .. .. .. .. .. ...
....- ,
.....
..
'
""
••
•
"
•••
•
•
••••••••••••
:
•
••••••
o
••••••••••••••••••••••••
I
•
•
,
•••••••••••••••
•
•
•
•
-
•
:::.
~----[---i--i--!-!-!i!---------i----!-----i-1-i-!!
••••••••••
::::
::::
• • ••••
I
..
•
•••••••••••••
.•.•
••••
"'
I
I
••••••
•
,
10
1.1
: :::
• • . o.
,
",..;
1 111111
.
. ....
.
:
"'
,
,"'"
•••
6
Freq [Hz]
impedance viewed from sender for n = 1
Signal transmission
Fig. 20.
3000
2500 uum+ ........
--- -+ --+ --~- +-H
...
•
:
.. .. . ....
~
•
•
•
,
•
,
•
H, --------or ----+ ---r n1-+-1-~~-1- u _m r n;. _u~- -1-+-~-~
+ ~---------r_m -r n -;--H
,
•••
""
••••
•
.
••••
•••
l:::::::
I
::::
.: ::::
.
•••••••••••••
""
::::
-;-r :.
. . ...
: :::
f:::l]:~ftjw~mItI:IJllli ltj::IlIIJIII::::I::::I:I!Irll
:
:
1
:
:
:
:
: : : :::
: : : :::
i 1 ill
111
::::
::::
j
i
j
i
: : :: : :
: ::::::
~
j j j 1 ~)
:
:
:
:
j
j
:
:
i
: : : :::
: : : :::
::::
::::
j
j
i j ill
: : ::
: :::
j
j j
i j ij
50: mu-l:--t--tHl[tlu---I--turlJ~iu\::tUl-ttt!u-u!uLlJliJl
104
105
106
107
108
Freq [Hz]
Fig. 21.
Signal transmission impedance viewed from sender for n = 30
The question may arise as to how well our model predicts the limiting case as n
becomes large. To address this issue, it is helpful to look at the model and experimental
results as a function of n. Specifically we can look at the magnitude of the impedance
peak, and the frequency at which it occurs. We showed earlier that the peak magnitude
and frequency reach a maximum and minimum value (respectively) well before n reaches
infinity. We now try to calculate the error in the predicted and actual steady-state values.
37
Figure 22(A) shows how the peak magnitude varies as a function of n according to the
model and experimental results. There is 'final' error of
soon
(20%). For the frequency
plot given in Figure 22(B), there is an error of 50 kHz (5%). Thus, while the limiting
value of impedance magnitude shows significant error, the frequency at which it occurs
can be predicted quite closely_ In our design procedure, this frequency is critical because
it is used to help determine the parameter values of Table 1. However, the magnitude is
not as important. Thus, the utility of the model is verified once again in terms of its
ability to predict the limiting behavior as a function of n.
2
------ Experimental
Model
i
1_5
\ ..:
N
I.
I
~
--r-----------j------- ------j---- --- ----1- __
-r---t---j- -1 -m_
g
1 ---
~
I
~
!
0.5
--t
10
15
# Receivers
(A)
Fig. 22.
20
n
25
30
i
I
I
I
!
!
!
I
I
I
II
I
I'
I
5
I
!
I
I
1
15
# Receivers
20
25
30
n
(B)
Resonant behavior; peak magnitude and peak frequency as a function of the
number of receivers
38
Fig. 23.
Mobility aids for disabled children and elderly persons
The mechanized walker and standing aid shown here both use some standard servosystem design. With the apparatus presented in this work, such machines can be
simplified. The central controller and power supply (a battery, in this case) can be kept to
a small size. Actuators can then be placed directly at the wheels, leaving the rest of the
apparatus
aesthetically
appealing
and
easily
maneuverable.
A simple
schematic
illustrating this idea is pictured in Figure 24.
40