Both of these 4 wheel drive line following robots have a fifth wheel in the rear. My first thought is, that it is used as a braking mechanism. Would that be the case? If so, could someone explain - how does that mechanism work on the first robot?
I m not familiar with those robots, but I will guess it's more used for odometry, in order to know the speed and the traveled distance.
The design are packed so it is hard to see for sure, but it seems to me that there are only 2 wires per motor, so not enough for position/velocity feedback. So the best to get instantaneous speed would be a free wheel for odometr, but this is a guess as I am not familiar with these competitions.
Also it seems that the last model has an IMU, so it might use some kind of sensor fusion to get even better estimate.
The first robot is RS-100 and it won this year's all-Japan fine following competition. The fifth wheel is an encoder. The robots are allowed to remember the track - this is specific to this competition as the turns are marked by white markers outside of the line. Look at the winning run of the robot and you will see that it speeds up and down in fast runs (i.e. after the first run) https://www.youtube.com/watch?v=960e5Q_PhWg
Additional trivia: This year was the first in a long time that an Ackermann steering robot won (what is considered MCU-Rally style, a very popular competition in Japan). It looks like this is a good design for competitions in which there are no 90 degree turns. It beat the previous year's winner, Cartis, which is interesting in its own because it uses 4 propellers for downforce http://anikinonikki.cocolog-nifty.com/.shared/image.html?/photos/uncategorized/2016/11/18/cartis45d.jpg They banned using ducted fans (EDFs) for downforce in 2016.
The fifth wheel is a odometric sensor as suggested by N staub. It is not for braking. Mobile robots do not use brakes as they cause the slipping on terrain often induce errors in odometirc position estimation using technique of dead reckoning which is most primitive and well applied.
It has IR sensor attached to it which counts ticks i.e No of times
the IR sensor is interrupted due to turning of wheel.(Wheel has small holes along its rim).
Then distance traveled is estimated as follows:
distance travelled = no of ticks * ((2*pi*radius of the wheel)/angle between 2 holes )
for the estimation of speed count no of ticks in a unit time. But not higly useful info as fifth wheel does not slip.
It uses Ackermann steering mechanism. back motors drive the robot as in a car.front motors achieve turning by rotating steering motor in middle and giving more speed to motor in opposite direction of turn i.e to turn left right motor will be powered more than left motor.
Follow this link to know more:https://en.wikipedia.org/wiki/Ackermann_steering_geometry
hope this helps