Differential drive - Fwd Caster versus Aft Caster

[pomprocker]

I noticed that admin builds most of his diff drive robots with a caster in the front.

I built my $50 robot prototype chassis with a rear caster.


What are the advantages and disadvantages of both?

[Steve Joblin]

Interesting question... I would assume that depends on how centered the differential drive wheels are compared to the rest of the bot and where the center of gravity of the robot is.

[Ro-Bot-X]

A trailer caster follows the driving wheels easier than a leading caster is pushed by them. This means that if the caster gets in a small bump, if it is in the back of the robot it doesn't influence the direction of the robot as much as if it is in the front. The center of mass (weight) should be between the wheels at all times, but the more on the driving wheels the more traction the robot has. If the robot has a gripper and grabs objects, the weight of the object will move the center of mass so it may be too much over the driving wheels and the robot will tip over when stopping, if the caster is in the back.

There is the option of having both front and rear casters, one of them being mounted on a spring.

And of course there is the balancing robot with no casters at all. This is the best choice, but the hardest to do. A dynamically balanced robot is more stable than a statically balanced one. If something moves the center of mass, the robot adjusts it's position accordingly, re-stabilizing itself. Heck, it doesn't even need bumper sensors, if it bumps into something it pushes itself away from the obstacle to re-stabilize itself.

[pomprocker]

I think I will opt to leave my caster in the back then, as I can mount the IR sensors in the rear so that the close distance it can't see is obsolete. also I could mount any robot arms in the rear too so that it has extra leverage to help it not tip forward. Then I could mount the battery, electronics, and drive servos up front with the drive wheels.

Unless Admin pipes in with some magical answer...

[Ro-Bot-X]

Mount the bateries as low as possible to lower the center of mass. Depending on the arm size and weight, I think it is better to have it centered (on top of the center of mass) if it is more than 2 DOF. Think about a Johny 5 design but without the threads. If your arm is one or 2 DOF, you can mount it in the front of the robot, since the weight of the lifted objects would not be so heavy to matter. Think about a Boe-Bot with the attached arm.

[Admin]

This is been fully discussed in another post before, if you look around a bit you might be able to find it.

The problem with a rear caster is when the robot turns, the robot 'butt' swings around and hits things.

If the caster was in the front, the sensors located on the front would detect objects before it would swing into them during turns.

As for mounting robot arms on your robot, two wheels and a caster won't work - I tried, but its too unstable. You will need to have an additional caster and a suspension system to keep balance. Also, mounting the arms on the rear means your arms need to be longer to reach the front - requiring bigger arm motors.

[jman571]

This topic has got me questioning my design, which is always a good thing as I right now have a front caster in mind for my R/C bot, but perhaps a rear caster would be more ideal, as some people said above: If the powered wheels are on the front, they will be able to get over any obstacle a small caster cannot (in my robot I'm using a tamiya ball caster). However, I think I may get more torque if the motors 'push' the robot, instead of 'pulling' it. So I may just leave it behind the robot.

[Ro-Bot-X]

... If the powered wheels are on the front, they will be able to get over any obstacle a small caster cannot...

It all depends on the diameter of the caster, however, the caster is not powered, so it can't grip to the object it tries to climb onto (get over the carpet's edge). A caster in the back design doesn't suffer from this flaw.

The problem with a rear caster is when the robot turns, the robot 'butt' swings around and hits things.

If the caster was in the front, the sensors located on the front would detect objects before it would swing into them during turns.

Sensors may be mounted to counter this problem, but if the robot looks like your ERP with such large arms, it will need those sensors anyways...

[airman00]

http://www.societyofrobots.com/robotforum/index.php?topic=2224.0

[ryan]

Good to know