Robo-Cross B

[gh]

A 2 channel 25 amp speed controller from Dimension Engineering that I choose for its regenerative braking and built in current protection and protection for Lipo's http://www.dimensionengineering.com/Sabertooth2X25.htm

I used this motor controller at regionals yesterday without fully testing out the robot at first, and I must disagree with WSM's praise for the device. Its cost, while quite low for its featureset ($125 + shipping), is still quite high and is not justified by its ability.

• Its lithium protection mode is far too overzealous, cutting back power at around 3.2 to 3.1 volts per cell, which caused problems with my LiFePO4 batteries, which are nominally 3.3V per cell, despite the fact that the datasheet states a 3.0V cut-off. Accelerating too quickly would result in enough of a drop in battery voltage that the controller would simply switch off.
• Its servo pulse input mode, especially with the RC radio auto-centering and auto-ranging, is far too flakey. When operating under normal, stable conditions, the controller would randomly become uncalibrated, reverse directions, or often even reset.
• While its current capacity is very high, it refuses to operate at its stated peak and sustained current ratings. It simply decides to cut off, though I was using it on only two 550-size motors, geared down 51:1, driving a 5 pound device. It would often lock up as I accelerated (even with the lithium protection off), even though I know for a fact that the robot's current draw is under 5 amps as it drives and that its peak current (when accelerating) doesn't come close to the 50A peak stated in the data sheet.

All of the above forced me, the operator, to operate my sumo bot at under half power, with slow and smooth control stick movements to prevent the controller from locking up. Also, I didn't find out lithium protection was problematic until this morning, when I unpacked my robot and tested it.

I replaced the motor controller with Dimension Engineering's second in line model, the 2 channel 10A controller, and it now functions perfectly, unless I bump the control stick from 0% to 100% instantaneously. I simply cannot understand how a nearly identical model of the controller, with worse specs, is so much better than the top of the line model made by the same company.

Basically, don't get the 2x25A controller. The 2x10 is fantastic, and is a better deal than the 2x5.

[ddudealex1]

I probably should get the VEXplorer, but Id have to build it myself.

[Pleiades]

That's the point of the event You cant just buy a pre-built robot, anyone could do that.

[theguitarmaster101]

Well, i have seen some other schools in past years and for this year. We always build our own bot, but I have seen about 10 schools do some really last minute, rushed, pre bought kits. Here's what i've seen:

1. About 5 out of the 10 schools bought a remote control car, bought a claw kit, and taped the claw to the top of the remote controlled car. One teammate drove the car, the other controlled the claw.

2. I have seen 5+ people buy this thing called the "vexplorer" which is a vex robot kit with a really good claw and treds, however they simply followed the included instructions and didn't do anything themselves really...

Shouldn't there be a rule against this

[bearasauras]

At our regional and state, we had DQ'ed some robots where it is obvious that the students just bought a kit and used it without modifying it. While this is true, we also see many teams using the same kit and modifying it the original design so that the performance is improved. Of course, these teams were not DQ'ed.

[robotman]

so it has to have mods?
it cant be stright out of the box?

[ddudealex1]

No it cant.

[robotman]

ok it doesnt matter anyway

[Pleiades]

I think that kits should be allowed, which they ARE the last time i checked. I mean, look at events like wright stuff. Kit's are allowed in that event and it is not hard to built an entry level plane. Robo-cross, is pretty hard to do compared to that event. You still learn stuff from building from the kit and obvioulsy if you make your own robot you'll have an advantage so it's not like it's unfair for some teams to use a kit.

[ddudealex1]

Fnnly buit robot... can pick up everything but blocks.