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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?

follower_1_1 follower_1 follower_2

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    $\begingroup$ 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 $\endgroup$ – N. Staub Nov 30 '17 at 10:56
  • $\begingroup$ Ugh makes sense, I automatically assumed that encoders were already present in the motors, seems that's not the case here. $\endgroup$ – Magnuss Nov 30 '17 at 11:37
  • $\begingroup$ The boards are mega packed so it's hard to see for sure, but it seems to me that there are only 2 wires per motor, so not enough for position feedback $\endgroup$ – N. Staub Nov 30 '17 at 11:44
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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.

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  • $\begingroup$ FYI: In this particular competition robots memorize the track on the first run, so that the robot knows when to expect sharp turns on the second run, thus precise odometry is definitely required. For some reason this solution is better than placing an encoder on one of the four motors. $\endgroup$ – Magnuss Nov 30 '17 at 12:11
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    $\begingroup$ This solution is better because you avoid slippage and you have measurements in the center of the robot, so theoretically on the track, so it is easier to exploit than measurement a bit on the side, eg on a single motor. $\endgroup$ – N. Staub Nov 30 '17 at 12:47
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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.

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  • $\begingroup$ Why exactly isn't the ackerman steering a good fit for a 90 degree turn? $\endgroup$ – Magnuss Dec 19 '17 at 6:54
  • $\begingroup$ What I meant were 90 degree turns with 0 radius where 4-wheeled robots with steering have problems because they cannot turn in place. Look at a fast run from a differential drive robot at this year's Robotex competition where there are multiple 90 degree turns with 0 radius: youtube.com/watch?v=d8eesFMPTeE $\endgroup$ – supermedo Dec 19 '17 at 7:24
  • $\begingroup$ Yes, I saw it live, I took part in Robotex with a differential drive robot as well. I'm still considering trying to make a 4-wheel robot. The 0 radius doesn't seem like such a huge problem - with an array of sensors extended in front, I'm sure it would still manage to take a corner. In any case - I'm anxious to try something else, rather than yet-another-differential-drive robot. :) $\endgroup$ – Magnuss Dec 19 '17 at 11:00
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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

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