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Ch60-I044963.fm Page 300 Thursday, July 27, 2006 9:00 AM
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1 1
0.8 0.8
0.6 0.6
0.4 0.4
) ) s
s / 0.2 / 0.2
m m
( (

y 0 y 0
t i t i
c c
o o
l e l
e -0.2
V -0.2 V
-0.4 -0.4
-0.6 -0.6
-0.8 -0.8
-1 -1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (s) Time (s)
Figure 4: A movement of 0.2 m using the wired controller and the wireless controller on the DSP
The bit rate of the transceiver was configured to 250 kbit/s. During the measurements the packet loss
from the hydraulic unit to the controller unit was monitored. The loss altered between 10% and 20%.
Most of these losses are single packets. With current compensation methods the losses increase the
effective sampling and control interval temporarily from 2 ms to 4 ms. Erroneous packets were seldom
received, approximately once in ten minutes. For future work some detection for errors will be added.
The distance between the transceivers was about 2.5 m. The packet loss stayed below 20% as the
distance was increased to 5-6 m. Due to the laboratory environment, a longer distance could not be
experimented. The reinforced concrete walls of the laboratory attenuate the signal strongly, which
makes control applications practically impossible. The RF power of 1 mW is too low for ranges above
10 m in open space. There are also devices that use high transmission power at the same 2.4 GHz
frequency band. A lot of wireless network activity might even totally block the nRF communications.
On the other hand, the same transceiver circuit is used in wireless PC equipment such as wireless game
controllers, so there should be at least some compatibility with wireless LANs.
CONCLUSIONS
The data transfer of a closed-loop control system can be done using wireless transceivers. A state
controller can be implemented but it will perform a little worse than a wired controller. With
proportional controller there is no difference at all. Some compensation method for lost packets is
required especially with the state controller. The nRF2401 transceiver circuit suits well for short range
applications at least when neither interfering signals nor obstacles are present. Improvement could be
achieved by employing a frequency hopping algorithm or by increasing transmitter power. 802.11b
network adapters could be adequate as well if there were no need for sampling intervals below 4 ms.

REFERENCES
Horjel, A. (2001). Bluetooth in Control. M.Sc. thesis, Lund Institute of Technology.

Kawka P. and Alleyne A. (2004). Wireless servo control for electro-hydraulic positioning.
Proceedings of Bath Workshop on Power Transmission and Motion Control, 159-172.

Nordic Semiconductor. (2004). Single chip 2.4 GHz transceiver nRF2401. Product specification.
Ploplys, N. (2003). Wireless Feedback Control of Mechanical Systems. M.Sc. thesis, University of
Illinois.
Ploplys N. and Alleyne A. (2003). UDP Network Communications for Distributed Wireless Control.
Proceedings of the American Control Conference, 3335-3340.