CHAPTER 6 TESTING AND VERIFICATION
6.4 Traction System and Transformability
CAESAR has an overall weight of 56.5 kg, which is significantly in excess of the target of 25 kg. The weight at initial tests were 25 kg, but the addition of the overall payload, the protective layer and the phenolic resin to bond the protective layer added the additional 31.5 kg. Weight could be reduced with the layering and manufacturing of the composite material in a single stage, as the amount of phenolic resin used for bonding is then reduced.
The CAESAR robot was tested on different terrains to determine the feasibility of the traction. The silicone pads decreased slippage on smooth and oily surfaces while the chain-type tracks allowed grip for climbing over obstacles.
The efficiency of the traction system is required to determine the difference of the actual output compared to the theoretical output. This decrease in the output value is due to friction caused by the load of the components used by the system.
The sprocket configuration of the side tracks are shown in figure 6-7.
Figure 6-7: Sprocket / chain configuration of the main tracks
The motor is connected to sprocket A, which drives sprocket B. Sprocket C is joint to sprocket B via a shaft. The specifications of A is:
TorqueA = 12 Nm NA = 30 rpm TA = 13 teeth
Radius: 0.02 m; Diameter: 0.04 m
Sprocket B has the following specifications:
TB = 21 teeth
Radius: 0.0325 m; Diameter: 0.065 m Sprocket C has the following specifications:
TC = 13 teeth
Radius: 0.04 m; Diameter: 0.08 m
The turn rate of sprocket B is required. This is determined from:
NATA=NBTB
NB=30x13
21=18.571rpm {39}
Similarly,
TorqueATA=TorqueBTB
TorqueB=12x21
13=19.385Nm {40}
As NB = NC, the velocity of C with diameter of 0.08 m, determines the velocity of the tracks, by:
v=D n=x0.08x18.571
60 =0.078m/s {41}
Comparing this theoretical velocity to the actual velocity of the tracks of 0.074 m/s (without the weight of the robot), the efficiency can be determined as shown in equation 42.
Efficiency= velocityactual velocitytheoretical
x100=0.074
0.078 x100=95percent {42}
The loss in efficiency is due to friction of bearings and sprockets.
Similar calculations can be performed for the efficiency of the flipper arms. The flipper arms has a configuration as shown in figure 6-8.
Figure 6-8: Flipper arms gear ratio
Sprocket A has half the torque, as this is split between two arms. Sprocket A has the following specifications:
TorqueA = 6 Nm NA = 30 rpm TA = 13 teeth
Radius: 0.02 m; Diameter: 0.04 m
The known specifications for sprocket B is:
TB = 30 teeth
Radius: 0.046 m; Diameter: 0.092 m
The turn rate of sprocket B is determined by:
NATA=NBTB
NB=30x13
30=13rpm {43}
Similarly,
TorqueATA=TorqueBTB
TorqueB=6x30
13=13.846Nm {44}
As NB = NC, 0.231 revolutions will occur in a second. Therefore, one revolution will occur in 4.333 seconds.
When comparing this theoretical time to the actual time of the flipper arms to rotate, the efficiency can be determined as shown in equation 45.
Efficiency=timetheoretical
timeactual x100=4.333
5 x100=86.67 percent {45}
The loss in efficiency is due to friction on the bearings and sprockets as there is a load on the system.
The accuracy of the CAESAR robot turning 360° was tested. One hundred tests were performed on smooth concrete and the results are shown in figure 6-9.
Figure 6-9: Accuracy of 360° turns
As observed from figure 6-9, the absolute accuracy was less than 70 mm. This would indicate that a 70 mm space is required around the CAESAR robot to allow for a successful turn in an concealed area. Chauvenet's criterion statistical analysis [68] was performed on the readings to determine the validity. All readings were acceptable.
Different weights were placed onto CAESAR's chassis and the velocity was observed. The results of the tests are shown in figure 6-10.
Figure 6-10: The velocity vs weight relationship
The initial velocity was measured for the weight of the robot chassis, which is 56.5 kg. As the weight increased, the velocity decreased. Traction was still possible up to 164 kg, after which the weight prevented the chain tensioners to hold the chain under tension, and therefore slippage occurred at the sprockets. The design and construction of previous USAR robots made it difficult to carry equipment that might be required into the disaster area [3], which is possible with CAESAR.
The flipper arms were also tested with different weights on the robot. Elevation is possible with the weight of the robot chassis, but anything more than this was not possible.
It is suggested that not more than 50 kg payload be added to CAESAR's chassis, as this could cause traction failure due to the force on the tensioners. This payload was obviously only needed to move over a terrain where elevation with the flipper arms was not needed due to the restriction of the flipper arms strength. The ability for an addition of a payload is useful as it allows the rescuers to send additional equipment into the disaster scenario. Additional equipment could either assist the rescuers or the victims when located.
Transformation with the flipper arms assisted the CAESAR robot maneuver over an obstacle that had a gradient of 30° or less for long distance climbing, while 45°
slopes were climbed for shorter distances. This gradient for the respective distances was considered acceptable for the environment where such rescues would occur. A comparison of speed vs the angle of inclination is given in figure 6-11. This analysis is dependent on the terrain that CAESAR is on, as loose obstacles can cause slippage for short time periods.
0 50 100 150 200 250 300
0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080
Weight (kg)
Velocity (m/s)
Figure 6-11: Angle of inclination vs the Speed of motion
These flipper arms were also able to assist the CAESAR robot by pushing it over an obstacle by rotating the arms down under the chassis before contracting them to the homing position.
The overall dimensions of the CAESAR robot to determine the confined space it could enter, is:
• Height of composite chassis: 150 mm
• Length of robot excluding extended flipper arms: 730 mm
• Length including the extended arms: 1090 mm
• Width: 700 mm
• Height with arms at 90°: 395 mm
• Height including the side tracks: 364 mm
• Height from the ground to the top of the composite body: 250 mm
0 10 20 30 40 45 50
0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080
Angle (degrees)
Speed (m/s)