How do teams accomplish this?
My idea was to have secondary batteries in-between our main battery and our component, so if we lose power from our battery at any point, we continue on extra power from the extra battery until either the match ends or power is restored. Unfortunately, this is illegal on many levels.
So my question: How do we prevent brownouts and power-cycling?
We anticipated having brownouts, but never saw them. As a preventative measure, the code would check if there was a brownout (using DriverStation.isBrownedOut()) and then throttle down on the drivetrain if so. The arm would also measure current and stop if it detected a stall.
Yeah the whole back up battery thing wonât fly.
If you really want to get into Brown Out Prevention
Load Test your batteries. I recommend sharing this with your alliance partners at competition.
If you want to get more advanced: Have your code monitor the power usage on the PDP and reduce power to motors accordingly: http://www.ctr-electronics.com/control-system/pdp.html#product_tabs_technical_resources
You can prevent it by design if you do some analysis for your power consumption and set a power budget. There has been discussion on various threads here in the past about that.
Depending on your design I would add the ability to control power on your motors on the driver station and allow the drivers to override/control power as necessary.
This doesnât really fall under the category of a brownout, but it seems like the most common way that teams lost power on the field this year was due to power plugs coming loose. It happened to even the best teams this year. Make sure you tape in or secure power connectors for the radio and roboRIO. Make sure that battery lugs are tightened at the battery terminals, at the main breaker and PDP, and that your SB50 is plugged in solidly. Any high-resistance connection can cause excessive voltage drop when you pull a lot of current.
Your roboRIO will automatically shut off PWM outputs and disable CAN based motor controllers if it detects a brownout condition. This happens at 6.3V. If you want to avoid this, then you will need to decide on a way to monitor voltage, and scale back motor controller output (reduce current) under certain conditions.
Page 7 of this document describes the roboRIO brownout bahavior in detail.
https://decibel.ni.com/content/servlet/JiveServlet/download/30419-60-90614/roboRIO+User+Manual.pdf
There are plenty of good ways to prevent brown out.
On the electrical side, make sure the battery wire terminals are tight on the battery terminals. Use lock or star washers. The type of battery that come in the KOP have them included. Make sure that the screws holding the wire on the PDP are also tight. Any ability to turn them is bad. Also make sure the 6 gauge wire is crimped well. These loose connections will cause higher resistance in the whole circuit. I solved that problem on our robot and on other teamâs robots.
On the code side, if youâre using TalonSRXs then you can use current limiting. The motors for drive will pull lots of current. So if you have the code write the motor output to 0 when the current is too high, the problem will go away and you wonât notice the brown out symptoms.
I was just browsing and found some good documentation from FIRST/WPI on current monitoring and brownout prevention. Figured Iâd leave it here too.
https://wpilib.screenstepslive.com/s/4485/m/24166/l/289498
OK, spoiler alert, Al is going to talk about electrical issues againâŠ
In most cases brownouts are preventable with a good electrical system design. To begin, letâs talk about real world issues that every team experiences.
Many of us (Inspectors and CSAs) saw these issues throughout the season. They are preventable and we showed teams how to overcome these issues.
Al did a great job of covering electrical sources of brownouts, which were the most common reason for brownout I saw. There was one non-electrical source of brownouts â drivetrain designs. Specifically, six-wheel drop-center (i.e., KOP chassis) drivetrains with AndyMark AM-0970 pneumatic wheels, or similar.
Teams needed to pay particular attention to robot CG and tire pressure to keep the turning current low.
One test I use is to put the robot, with battery and bumpers installed, on carpet. Put a sheet of paper underneath each of the corner tires, and pull it out. If the force needed to pull the sheet out varies dramatically, you need to balance the robot.
Another way to check is to put the robot, with battery and bumpers installed, on carpet, and try to turn the robot by pushing on the corner of the robot with your finger. If you canât turn the robot, itâs probably going to take a lot of energy to turn.
Donât bother doing either of these tests on a hard surface. The results donât replicate the actual field. Donât have a piece of real field carpet? Ask the Regional Planning Committee about getting some leftovers after the event. Or go to the carpet store and buy a remnant of a low pile carpet made of the same material as the field carpet. It wonât be exactly the same as the field, but close enough to test it.
Hereâs what we did. First, we used a similar tire, but with a smooth profile like this. Second, we paid close attention to robot balance, ensuring that the CG of the robot was near the center. Third, we ran the center tires on each side at 30 PSI and the outer tires at 10PSI. This caused the center tire to be bigger, and helped with turning energy.
Our drivers could tell instantly whether the pit crew had forgotten to set the tire pressure.
I also saw a couple four wheel pneumatic tired robots â they had the same difficulties as the six wheel ones, but are fundamentally difficult to fix.
Did you post the wrong image, or am I seeing it wrong? What I see looks to me like the kind where the wire goes on the other side of the tab from the screw, to get sandwiched between the bottom of the tab and the inside of the rectangular cage. The screw doesnât touch the wire itself.
The AM crimper is great, but for about the same price you can also buy a hydraulic crimper from Harbor Freight. Weâve used it for years, and it crimps amazingly. Highly recommended for any large gauge wire crimping.
This year, we actually crimped ferrules onto all of these wires, and they worked amazingly all season. Only time we lost a wire was when our shooter came down on one of the connectors and literally snapped the crimp. Otherwise they worked amazingly. There are lots of places to purchase these.
NOTE: This solution does not fix the problem of brownouts, rather it provides a way to eliminate the brownout from affecting the compressor.
This was a problem my team faced during the end of the build season, right down to the final hours. This year we used a six wheel drivetrain with a dropped center and pneumatic wheels. Our problem was caused by a rapid changes in direction from a CIM motor. This would cause a brownout to the PCM, cutting off power to the compressor for about 3 seconds. Normally this wouldnât bother us, but this year we relied on pneumatic to deploy our boulder intake, and fire our catapult.
Instead of running the compressor off of the PCM as is normal, run it instead off of a spike using 20 Amp breaker. Connect the spike directly to the power distribution board with a 20 Amp breaker, then connect the spikeâs PWM to one of the RoboRIOâs relay pins. Connect the compressor to the spike, and with a little coding, the compressor will be unaffected by any brownouts. This is also completely legal. While this may not fix your problem, hopefully it mitigates the effects.
I should point out that you also MUST wire the pressure switch into the roboRIO as well.
You canât simply hook up the spike and say in your code:
compressorRelay.Set(Forward);
It has to turn off when the pressure switch trips.
See R88: (Insert remark that 2016 rules donât apply to 2017 or offseason projects).
But yes, this is a valid way to do so. Also doing it this way allows you to turn the compressor OFF when you are in a high-current draw situation, where every last bit of power helps.
Brownouts turn off all actuator outputs from the roboRIO, including relay outputs. The compressor will still shut down when the battery voltage drops into the brownout zone.
You can turn off the compressor at will using the PCM connections too.
I was not aware of that, thanks!
The talon SRX if used with CAN buss control Has the ability to ramp. A slight ramp can do wonders to control current spikes and brown out. We did this this year.
We took the Nâth degree approach this year, and it turned out relatively well:
-Monitor battery condition over time. Generate an adaptive model of the battery to determine how much current can be drawn from it before system voltage drops too low.
-Proactively estimate how much current draw a given driver command will pull from the battery. Involves knowledge of the physical parameters of the motor, and its present speed (measured via encoders)
-If the estimated current from the driver commands is sufficient to cause the system voltage to go too low, scale back driver commands.
The algorithm we used this year is still in trial - it kicked in plenty during competition, and we never browned out. However, we donât have good enough data yet to support that it was 100% necessary. We are working on a whitepaper (there are a few drafts of it floating around the internet), and will hopefully by end of summer have more data on the limits of what we came up with.
We actually came across this exact problem earlier this season. We tried a purely software approach by writing a âpower controllerâ that essentially monitored all the current draw by all the systems (RIO, drivetrain, intake, outtake, compressor, etc), then the controller would dynamically modify the current flow to or from these systems depending on priority. The priority changes when the particular subsystem is being used, and takes into account the battery voltage.
The numbers we used were based on a lot of testing (we generated a linear regression model for current - speed of the drive train motors, for example, so weâd know how much current each motor would pull depending on speed) and didnât really seem to match up perfectly all the time to what the WPILib tells you about your max current. ie we would draw up to 200 amps sometimes for a second before the brownout sequence kicked in.
Obviously, itâs not /perfect/ since the power methods in the WPILib are pretty noisy, so we had a switch on the driver station to turn the power controller on or off. We had a match at champs where it was pretty clear that the power controller was modifying currents â and we never had a brownout during a match our whole season!
We had a very extensive flowchart for the logic and passed out flyers to people explaining how awesome it was :^)
Yes this is legal but, the Spike has been discontinued from what I have been told.
+1 Everything Al said.
In every case of brownouts I investigated at champs, 100% of the time there was at least one of (A) loose battery cable, (B) loose connection at breaker, or © loose connection into the power distribution board.
If you can move the battery cable up and down, the bolt isnât tight.
If you can move the cable back and forth at the breaker, the bolt isnât tight.
If you can move the cable back and forth at the PDP, the bolt isnât tight.
The PDP connector is harder to reach because the covers.
Was anyone brave enough to run a 6-CIM drive this year? If so, how frequent were brownouts? Also, what were the gear ratios and wheels used?
