paper: New Control Functions - Drive System Testing

[magnets]

Toward the end of our match, we get the battery voltage down to 6.5V easily during normal driving. This will affect us.

Perhaps there could be an option to disable this "feature". I understand that it's there for a reason, but I'm comfortable cutting it a little closer.

[Arpan]

As a related question (I was never electrically closely involved) - could you regulate the 12V power being fed to the roborio?

[cgmv123]

Originally Posted by Ether

How would that be implemented with a CAN connection without affecting other non-motor CAN devices?

(I could guess, but if anyone knows where this is documented would you please post a link)

CAN Speed Controllers need an enable 'flag' to be sent out over CAN before they output anything. That enable 'flag' (or lack thereof) presumably wouldn't affect devices that it doesn't need to.

Originally Posted by Arpan

As a related question (I was never electrically closely involved) - could you regulate the 12V power being fed to the roborio?

The VRM does have enough capacity to run the RoboRIO, but that configuration will most likely not be legal for competition.

[Thad House]

Originally Posted by Alan Anderson

Consider this an opportunity to do dynamic power management. You can monitor battery voltage and motor currents, and you should be able to come up with a way to keep the voltage from sagging low enough to trigger the automatic brownout.

Is there a way to calculate how much current it would take in total to drop battery voltage to a certain level? It seems like there should be, but I don't know where to start.

[AdamHeard]

Originally Posted by Thad House

Is there a way to calculate how much current it would take in total to drop battery voltage to a certain level? It seems like there should be, but I don't know where to start.

You'd have to know the batteries current nominal voltage (no load), the batteries resistance (which will change some over the match), and the current current draw.

I imagine you could BS this enough to get a decent cutoff that is somewhat conservative.

[Ether]

Originally Posted by cgmv123

CAN Speed Controllers need an enable 'flag' to be sent out over CAN before they output anything.

so connect the dots. how does that relate to the original question.

and where is it documented.

[Mark McLeod]

Staged Brown Out when the battery voltage drops too low proceeds through the following stages:

Stage 1: 6.5-7.3v -PWM/CAN motor controllers/Relay outputs are cutoff, 6v power rail drops out
- Driver Station displays “Voltage Brownout”
- Servo power lost
- Disable pulse sent to CAN motor controllers, turning them off.
- Can result in robot stutter as motors pull the voltage low, get cut off, voltage rebounds, motors immediately pull voltage low again

Stage 2: 4.5-6.4v - , GPI outputs go High, 5v/3.3v power rails drop out
- Sensor brownout occurs, e.g., encoders!
- Stage 2 persists until battery voltage recovers to 7.5v

Stage 3:<4.5v - roboRIO drops out
- <3.5v the VRM/DLink drops out

[Arpan]

Originally Posted by Mark McLeod

That slide posted earlier from Behind The Lines was inadvertently incorrect as it was taken from an early sketch of the notes we were developing for the show.

Here are the latest notes:

Staged Brown Out when the battery voltage drops too low proceeds through the following stages:

Stage 1: 6.5-7.3v -PWM/CAN motor controllers/Relay outputs are cutoff, 6v power rail drops out
- Driver Station displays “Voltage Brownout”
- Servo power lost
- Disable pulse sent to PWM and CAN motor controllers, turning them off.
- Can result in robot stutter as motors pull the voltage low, get cut off, voltage rebounds, motors immediately pull voltage low again

Stage 2: 4.5-6.4v - , GPI outputs go High, 5v/3.3v power rails drop out
- Sensor brownout occurs, e.g., encoders!
- Stage 2 persists until battery voltage recovers to 7.5v

Stage 3:<4.5v - roboRIO drops out
- <3.5v the VRM/DLink drops out

I'm really not certain why this was done; we regularly pull our batteries below 7 volts in normal operation. Why is there no regulated supply to the roborio? Is there a technical hurdle to accomplish this?

[AustinSchuh]

Originally Posted by Ether

so connect the dots. how does that relate to the original question.

and where is it documented.

Not sure where it is documented (Q/A on the beta forums is the closest that I know of), but there is an explicit pulse/message sent out over PWM and CAN to disable all controllers as soon as the system enters the brownout state. The goal of this (whether you agree with it or not) is to save the rest of the system from browning out.

[Ether]

Originally Posted by AustinSchuh

..there is an explicit pulse/message sent out over ... CAN to disable all controllers as soon as the system enters the brownout state.

that's the opposite of what cgmv123 posted:

Originally Posted by cgmv123

CAN Speed Controllers need an enable 'flag' to be sent out over CAN before they output anything.

[AustinSchuh]

Originally Posted by Ether

that's the opposite of what cgmv123 posted:

I'm quoting what I was told by the engineer who implemented the brownout procedure when I asked for the gory details. Not sure where cgmv123 got his info. There should be some sort of heart beat on the bus in addition, but I don't have any hard information about how that works.

[RufflesRidge]

It doesn't describe the implementation to the detail Ether is asking about, but page 7 of the roboRIO user manual describes the brownout: https://decibel.ni.com/content/servl...r%20Manual.pdf

[Alan Anderson]

Originally Posted by Arpan

I'm really not certain why this was done; we regularly pull our batteries below 7 volts in normal operation. Why is there no regulated supply to the roborio? Is there a technical hurdle to accomplish this?

There is a regulated roboRIO supply. It's built in to the roboRIO. It functions properly until battery voltage drops below 4.5 volts.

[MrForbes]

The new "low battery" indicator....a jerking robot. Wonder how long it will take teams (who don't read Technical Paper threads on CD) to figure this out?

[Monochron]

Originally Posted by MrForbes

The new "low battery" indicator....a jerking robot. Wonder how long it will take teams (who don't read Technical Paper threads on CD) to figure this out?

Hopefully they will read teh RoboRio documentation and learn about it . . . but we'll see if they do. Though even that seems to not match what teams are reporting. Especially with CAN brownout as far as I can tell.