6 CIM robo-rio brown out

[GeeTwo]

Originally Posted by Basel A

Does that match teams' real world observations?

We have gotten better performance on the individual snap actions than advertised. We didn't do a proper analysis, but it seems we've gone about 30-50% beyond those specs (in time) without tripping. We have never really tested the main breaker numbers, because it has never been worth any significant chance of losing EVERYTHING for the rest of the match. As far as I am aware, we've only tripped a main breaker one time in 4-1/2 years. In that one case, we found a possibility of a dead short due to missing insulation, so we fixed that, moved on, and never had it again.

[philso]

Originally Posted by Joe Ross

1) Verify all electrical connections are tight. Loose electrical connections will increase current draw.
...

3) Reduce long wire runs. Wire electrical resistance is proportional with length, and reducing wire length will reduce current draw.

Are you sure of this?

[GeeTwo]

Originally Posted by philso

Are you sure of this?

Yes, if viewed from the standpoint of accomplishing a given task, which is how we usually consider the robot design. That is, with loose connections or long runs, you will need to draw more Amp-seconds from the battery to (for example) lift a stack of four totes by fourteen inches.

[BoilerMentor]

Someone asked for real world testing with a 6 cim drive. I have some things to report.

The robot tested uses the cRio based control system, but we logged voltage to look from situations where we saw voltage drop below 7.5V. We consider this to be an adequate safety factor to ensure we would be in the clear.

Testing completed on slightly lower pile carpet than normal field carpet, but similar in construction.

The drive train is configured as follows:

Mechanical

2 VexPro 3 Cim Ball Shifters at 18.75:1 and 7.08:1 ratios.
6 VexPro traction wheels 6" diameter 2" width with blue nitrile rough top tread
1/8" center drop
Center wheel direct driven.
#35 roller chain to drive front and rear wheel on each side
Outer axles are bolt-through-tube style dead shafts with bearings in the wheel/sprocket
No tensioners used, but chain run has worn in and is not putting un-due load on bearings/gear boxes.
Plate and standoff style construction bolted to a box channel core frame
****Robot weight: Roughly 100 lbs****

Electrical

6 Standard Cim motors, all new at the beginning of 2014 competition season
10 awg wire between PD Board -- motor controller -- motors
Power pole connector between PD Board and each speed controller as well as between each speed controller and motor
6 Black Jaguar speed controllers on CanBus through a 2Can driving motors
Battery was changed 2 times during testing.

Programming

Smoothing on inputs was implemented in code for this drive train, but still responds sharply and operates precisely
Automated shifting used in competition.
I need to verify the operational mode used, but I believe 1 speed controller on each side was implemented in speed control mode and the other 2 controllers were slaved to that controller based on current
In testing condition shifters were set manually to high or low gear and shifting code was disabled.

Results

High gear, No compressor, from standing, maximum achievable acceleration
-- Would have browned out; voltage dropped significantly below 7.5V

Low gear, No compressor, from standing, maximum achievable acceleration
-- Would not have browned out; voltage did not drop below 7.5V

High gear, Compressor running, from standing, maximum achievable acceleration
-- Would have browned out; As expected based on first indicated result

Low gear, Compressor running, from standing, maximum achievable acceleration
--Not conclusive; Voltage drop below 7.5V, but not below stated brown out condition voltages

High gear, no compressor, from standing against wall, ramp to traction limit
--Would have browned out; Motor stall achieved critical voltage drop

Low gear, no compressor, from standing against wall, ramp to traction limit
--Would not have browned out; traction broke before 7.5V limit was reached
****driver modulation to stall condition could force brown out.

As much as it pains me to say it, I don't have actual data to share. This was a quick and dirty test and I thought our experience might be useful.
I'm convinced automated shifting is of critical importance with the consideration of brown out. I recall a number of teams losing 120amp breakers with 6 cim shifting drives at competition in 2014, which is where this drive base originated, and we never had that problem.

Please let me know if you have questions. I'm going to try and set this up and collect actual data early in build season if it appears it might be useful.

[GreyingJay]

Originally Posted by GeeTwo

In our experience, you will definitely notice. This is what happens:

1. The current draw causes the RIO to "brown out" the motors.
2. The voltage recovers.
3. The system restores the motors.
4. The motors draw inrush current.
5. Return to step 1.

This results in a very distinctive "stutter" at about 15-20 jitters per second that gets the robot nowhere fast.

My team last year definitely noticed this in our practice sessions, especially when trying to turn (scrub friction increasing the load on the motors). It happened after a few minutes of driving as the battery wore down, and of course it was worse when the robot was trying to move a stack of totes because of the added weight load. Our forklift design caused much of the weight of the totes to press down on the front two wheels.

We would start with a fresh battery, get in a few minutes of driving and stacking, then at some point try to turn the robot while carrying a stack and experience the CLACK-CLACK-CLACK-CLACK stutter which told us it was time to change the battery.

With a fresh battery for each match in competition we didn't notice it as much. We also did a MacGyver fix by applying duct tape to the treads of the two front wheels to reduce the scrub friction against the carpet.

[philso]

Originally Posted by GeeTwo

Yes, if viewed from the standpoint of accomplishing a given task, which is how we usually consider the robot design. That is, with loose connections or long runs, you will need to draw more Amp-seconds from the battery to (for example) lift a stack of four totes by fourteen inches.

The effect of the instantaneous current draw (in Amps) is different from the effect of the state of charge of the battery (in Amp Seconds).

Brown-out and under-voltage phenomena occur when the instantaneous current draw is high enough to cause the battery output voltage to drop sufficiently low. The state of charge of the battery will determine the amount of margin that one has before brown-out and under-voltage phenomena occur. A high enough draw will cause a fully/freshly charged battery to drop low enough to cause a problem. A lower instantaneous current will cause a depleted battery to drop to the same voltage level.

The recommendations made by Joe are correct. I was questioning the reasoning given for the recommendations.

Loose connections will introduce extra series resistance leading to brown-out and under-voltage phenomena occurring at lower instantaneous current than if the connection is good. If the connection is really loose, one will get intermittent operation where the voltage drops to zero.

Reducing wire length will reduce the circuit loop resistance leading to higher maximum currents.

[GeeTwo]

Originally Posted by philso

The effect of the instantaneous current draw (in Amps) is different from the effect of the state of charge of the battery (in Amp Seconds).

Reducing wire length will reduce the circuit loop resistance leading to higher maximum currents.

The maximum currents aren't the killer as much as the minimum voltage. If you draw extra amp-seconds to do your tasks, you will have a more drained battery that will be more likely to drop below brownout threshold when you make that big effort at the end (such as a climb, which has a favorite FIRST endgame, including this year's FTC). It is quite unlikely that inefficiency will contribute to the solution more than to the problem.

[philso]

Originally Posted by GeeTwo

The maximum currents aren't the killer as much as the minimum voltage. If you draw extra amp-seconds to do your tasks, you will have a more drained battery that will be more likely to drop below brownout threshold when you make that big effort at the end (such as a climb, which has a favorite FIRST endgame, including this year's FTC). It is quite unlikely that inefficiency will contribute to the solution more than to the problem.

According to Kirchoff's "Voltage Law", both the instantaneous current (in Amps) and the cumulative drain on the battery (in Amp seconds) are significant when looking at brown-out and under-voltage conditions but in different ways.

It is not clear what inefficiency you are referring to in your last sentence.