[FTC]: [FTC] Samantha and batteries

[Hagrid]

Hi All,

There has been an active discussion regarding powering Samantha from the main Tetrix/Matrix battery. The new Samantha user manual specifically advises against this with Matrix and I wanted to know why. The current season rules also strongly lean this way.

The issue is that with the Samantha connected to the primary battery, any dodginess in the earth connection to the motor controller can cause the main motor current to take an alternate route via the NXT cable to the brick and then up the USB cable to the Samantha and finally via the Samantha's power back to the battery.

After drilling down into the issue with HiTechnic/Matrix as to why this issue is specific to Matrix, they have just come back and said that it can also happen with the Tetrix controllers.

The currents involved could potentially be 10's of amps running through light gauge wires/connectors rated at a very small fraction of that current.

The result may include damaged Samantha, damaged NXT, and loss of communication events.

I am not a spokesman for FIRST FTC, Hitechnic, Matrix, or Tetrix - just a tyre-kicking mentor who doesn't like unsolved mysteries.

[Scott_4140]

The main reason its restricted to Matrix is the voltage difference. Tetrix uses a 12 VDC battery and Matrix uses a 9 VDC battery. While the Samantha module can use either voltage, the margins are lower with 9 VDC. A stall of a Matrix motor can bring the voltage down below acceptable levels for the Samantha. A stall of a Tetrix motor does not have the same effect.

The only power issues I've seen affect the Samantha module on a Tetrix system is poor battery connections. The Samantha power plug is adequate if you secure it with a zip-tie. Its typically the other end that's the problem. Using Anderson Power Poles or similar power connections alleviates this issue.

[pyroslev]

Skimmed the Wiring and Samantha docs. They only recommend the separate battery only for Matrix. According to the manual, it's also the only way it's legal to have a dedicated battery for the Samantha with Matrix per RG10a.

The Tetrix Motors can stall and lead to brown out/power cycling. Rare but in rougher matches it happens. If it browns out, then the robot is down for the match. If it power cycles on it, that'll fix it and it'll run again. It is mostly caused by internal failure of the gear box. If a team has NXT motors onboard, functioning NXT motors can be helpful in diagnosing a stall.

To me, the Samantha should be as early in the power chain as possible. They use to have the Alternate Samantha Wiring guide before the new documents went up. In it, the Samantha got it's power directly after the switch before the motor controllers. You can see this on Page 22 in the Robot Wiring Guide. I would add a APP Quick connect in there so you can remove and replace the module in a hurry.

Matrix's dedicated battery pack is the greatest perk I've seen in Matrix so far. I just wish the FCS got updated so it would learn this.

[Scott_4140]

Originally Posted by pyroslev

Matrix's dedicated battery pack is the greatest perk I've seen in Matrix so far. I just wish the FCS got updated so it would learn this.

I agree, if Tetrix robots were also allowed to run a separate battery for the Samantha.

[Hagrid]

Originally Posted by Scott_4140

I agree, if Tetrix robots were also allowed to run a separate battery for the Samantha.

This is kinda funny. From the Matrix end of the stick, we (our team at least) have been seeing the separate 9V battery as a liability as we have been obliged to use the rectangular 9V batteries that go flat very quickly and are expensive.

Based on the argument from HiTechnic which is not at all concerned with the functioning of the Samantha but on the potential to damage the NXT, it should be easy to argue that the same option (separate battery) should exist for Tetrix as well.

It has baffled me as to why the rules insist on these really heavy duty battery wires to the Samantha <R10.d.ii> when the wires supplied with battery box for our Samantha were no where near that size and the internal components of the Samantha are relatively low current. Maybe that mystery has been unraveled as well - those wires were expected to handle back-door motor currents.

[Hagrid]

Originally Posted by pyroslev

To me, the Samantha should be as early in the power chain as possible.

I would need to check, but I think this is exactly the configuration that would be of concern with respect to high ground current via NXT in the case a damaged ground connection.

Have I got this wrong?

[pyroslev]

Could you pose it in a situation on the Field, a scenario. I think I'm following you but I think better with a detailed scenario. (Mostly because it's not a week into the season and I need to retrain my FTA brain cells.)

[Hagrid]

[Sorry for the funkiness with the pictures - trying to transplant drawings from a smartphone]

Consider this arrangement...


The black lines are the ground connections between the components. The yellow approximates the current flowing in the wire. The wider the yellow, the higher the current.

If the ground connection ONLY is broken between the battery and the motor controller, you will have this...



The NXT cable and USB cable (and Samantha power cable) have to handle the return current for all the DC motors/servos powered from the battery.

If the Samantha is powered from the end of the Tetrix controller chain, it will be less of an issue as a broken connection is likely to take out the Samantha as well. But this creates the possibility for differences in the ground potential that *could* produce other problems.

With Matrix there is no power-out connection for daisy chaining: one controller, one battery.

I found the Samantha alternate wiring for Matrix document which highlighted the issue back in 2012.

[pyroslev]

If I were wiring up a Tetrix Robot, I'd still use the Samantha Alternate Method. I would put it after the Switch so it's the first item powered before any Controllers or a Power Distro Board. Instead of crimps, I would use Anderson Powerpoles for quick connect swapping. Teams using this setup have been able to drive with 4.018 volts.

I've seen many Samantha's in this configuration. The only issue I've seen related to electrical was where a First Gen Samantha's ESD protection was broken and led to power cycling. If you use the correct guage wire, proper connections and at your discretion and legality per the rules a fuse, then your Samantha should have power, be protected and be ok.

If the grounds get that broken up, then you're involved in a match were everything has gone wrong. I've only seen this happen once where a team burnt out a motor controller and two motors. Extreme ESD played a factor here. In fact, If you have not already, I would say read the document Addressing NXT Lockups. Once you combine all the information in the Wiring, Samantha and Lockup Documents, you should be able go ahead and wire up the robot.