Mobile robots directing system
MOBILE ROBOTS DIRECTING SYSTEM
Constantin Bucsan, Mihai Avram
"Politehnica" University of Bucharest
313 Spl. Independentei, Bucharest
[email protected]
Abstract - In hostile environments (with explosion or fire risk) remote directing of mobile robots is used.
In many cases it is very important to know the exact position of the robot in every moment. The paper
presents a method to direct a mobile robot based on a sensing system placed outside the perimeter with
potential risk. This is an electro-optical system that follows a retro-reflective device placed on the robot.
Keywords: Opto-electronic directing system, laser ranging, mobile robot, retro-reflector.
1. Introduction
An important aspect of a mobile robot design is to
provide the system with the necessary devices to allow
the robot navigation, especially ranging devices. There
are many different types of ranging techniques. The
most used for mobile robots navigation are
triangulation, time of flight and phase-shift
measurement.
Time-of-flight ranging systems measure the roundtrip time required for a pulse of emitted energy to travel
to a reflecting object, then echo back to a receiver.
The phase-shift measurement (or phase-detection)
ranging technique involves continuous-wave (cw)
transmission as opposed to the short-duration pulsed
outputs used in the time-of-flight systems. In practice, a
beam of amplitude-modulated laser, RF, or acoustical
energy is directed towards the target. A small portion of
this wave is reflected by the object surface back to the
detector. Improved measurement accuracy and
increased range can be achieved when cooperative
targets are attached to the objects of interest to increase
the power density of the reflected signal. The returned
energy is compared to a simultaneously generated
reference that has been split off from the original signal,
and the relative phase shift between the two is measured
to ascertain the round-trip distance the wave has
traveled [1].
The paper deals with a novel sensing system for
space position measurement of mobile robots, that helps
directing the robot, especially in hostile environments.
2. The measuring principle
The sensing system for determining the space
position of the mobile robot works on the following
principles (figure 1):
- the sensing system is not placed on the robot, but in
a fixed position outside its working space;
- the sensing system consists of a sensor for spacedirection and distance measuring, which is placed in
20
-
the fixed point H, on the z axis of the working space
normal coordinates; this sensor automatically and
permanently follow a retro-reflective device M
fastened onto the mobile robot;
the space position finding of the robot is carried out
by computing the coordinates of the point M using
the measured values of the angles αM, βM and of
the distance D between the sensor and the reflecting
device.
z
H
αM
zM
O
D
yM
βM
y
M
xM
M’
x
Figure 1: The general arrangement of the sensing
system
The sensing system configuration
The principal component of the sensing system is
the space-direction and distance measuring sensor,
whose configuration is shown in figure 2.
The sensor is provided with an angle measuring
device AMD and a distance measuring device DMD.
The angle measuring device is an electro-optical
viewing device consisting of the objective Ob and the
quadrate photosensor FS. The distance measuring
device is an industrial laser distance measuring module.
The laser beam produced by the distance measuring
module is reflected by the retro-reflective device
attached to the robot and is captured by both the
distance measuring module and the angle measuring
device.
The Romanian Review Precision Mechanics, Optics & Mechatronics, 2013, No. 44
Mobile robots directing system
M
IT2
AMD
(+) 
Ob
The block diagram of the sensing system is shown in
figure 3.
C2
(-)
FS
MD1
M1
WG1
IT1
DMD
O
WG2
MD2
M2
WG2
IT2
Angle
measuring
device
AMD

M2
(-)
(+)
NI DAQ
PC
Space
position
Distance
measuring
device
DMD
Application
software
WG1
M1
Figure 3: The block diagram of the sensing system
C1
IT1
For the distance measuring device an industrial
device type LMC-J-0040-1 from SENTEK [2] was
chosen. Figure 4 shows a view of this device.
Figure 2: The space-direction and distance measuring
sensor configuration
When the axis of the angle measuring device
coincides with the direction of the point M, the four
signals produced by the photoconductive zones are
equal and at this moment the values of the angles αM,
βM and of the distance D can be measured.
The angle measuring device movement in order to
follow the moving point M is produced by the DC
motors M1 and M2 by means of the worm-gears WG1
and WG2.
The measurement of the rotation angle α in the
horizontal plane, against the vertical axis, is carried out
by the rotational incremental transducer IT1; the
measurement of the rotation angle β in the vertical
plane, against the horizontal axis, is carried out by the
rotational incremental transducer IT2.
The worm-gears are provided with backlashinfluence eliminating devices; the incremental
transducers are directly connected to the rotating shafts
by means of the special couplings C1 and C2, in order
to minimize the measuring errors.
The electrical signals produced by the four
photoconductive zones of the photosensor give
information on the sign of the deviation, and the motors
are controlled by the drivers MD1 and MD2 in order to
minimize this deviation. When the four signals are
equal, meaning that the angle measuring device axis
coincides with the direction of the point M, the data
from the incremental transducers and from the distance
measuring module are transferred to the PC and the
values of the angles αM, βM and of the distance D are
computed. If the point M is moving, the sensor
permanently follows it and gives information on its
direction and distance.
Figure 4: The LMC-J-0040-1distance measuring
module
The LMC-J-0040-1 is an opto-electronic distance
measuring module for industrial applications. The
module operates on the basis of non-contact
comparative phase measurement with amplitude
modulation. The laser diode (cw operation) has a
divergence of 0.6 mrad for measurement with pinpoint
accuracy.
The main characteristics of the distance measuring
module are:
- Measuring range: 0.2 to 30 m on most any natural
surface. Up to 150 m possible with optional target
board
- Measuring accuracy: ± 2 mm under defined
measuring conditions
- Measuring resolution: 0.1 mm
- Max target speed: .8 m/s
- Repeatability: - 0.5 mm
- Interface: RS 232/RS 422 (switchable), baud rate
9600
- Operating temperature: -10 to 50°C.
The retro-reflective device
The retro-reflective device placed on the mobile
robot must assure the retro-reflection of the laser beam
independent of the orientation of the robot and of the
distance between the robot and the sensing system in the
working range.
The Romanian Review Precision Mechanics, Optics & Mechatronics, 2013, No. 44
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Mobile robots directing system
Theoretical analysis and experimental results have
demonstrated that the corner cube retro-reflector
measuring accuracy decreases as the incident angle
increases, and the measuring error is largest when the
maximum incident angle is reached.
The maximum incident angle at which laser tracking
systems can still work well is ±35°. The measuring
accuracy of a corner cube retro-reflector can maintain
the manufacturer specifications of laser tracking
systems only when the incident angle is within ±20° [3].
An octagonal prism 1 as shown in figure 5 assures
the conditions for the maximum incident angle of the
beam coming from the laser 2 in the working range.
M
Dmax
M’
The Space-Position Determining
For the actual position of the retro-reflective device
M fastened onto the mobile robot, the sensor give the
values of the angles αM, βM and distance D (figure 1).
Using simple trigonometric relations we get the
coordinates of the point M as a function of αM, βM and
D given by the space position measuring system, as
following:
 x M  D sin  M cos  M

 y M  D sin  M sin  M
 z  H  D cos 
M
 M
(1)
3. Conclusions
Dmin
2
1
The research results show that the sensing system
can be used as an automatic system for following the
trajectory an autonomous robot, in order to determine
the deviations from the programmed trajectory.
The sensing system as described shows an
important advantage in not being placed on the robot,
but outside its working space, this being useful
especially in hostile environments.
Further research goal will be the development of
the working software for the directing system.
Figure 5: Retro-reflective device working scheme
4. References
A corner cube retro-reflective tape from EATON [4]
covers the active faces of the prism. This tape (figure 6)
provides the highest signal return to the sensor, typically
2000 to 3000 times the reflectivity of white paper.
Figure 6: The retro-reflective tape
Thousands of corner cube shapes are molded into a
rugged plastic vinyl tape material.
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[1] Everett, H.R.: Sensors for Mobile Robots. Theory
and Application, A. K. Peters, Ltd. Wellesley,
Massachusetts, 1995.
[2] www.sentekeurope.com.
[3] Ouyang, J., Liu, W., Qu, X., Yan, Y.: The effect of
beam incident angles on cube corner retro-reflector
measuring accuracy, Optical Design and Testing III,
68342J, November 28, 2007.
[4] www.eaton.com.
The Romanian Review Precision Mechanics, Optics & Mechatronics, 2013, No. 44