Brownout Prevention?

[Conor Ryan]

Yeah the whole back up battery thing won't fly.

If you really want to get into Brown Out Prevention

1) Load Test your batteries. I recommend sharing this with your alliance partners at competition.

2) 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/contr...ical_resources

3) 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.

[sastoller]

This doesn't 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/servl...ser+Manual.pdf

[arichman1257]

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 team's robots.

On the code side, if you're 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 won't notice the brown out symptoms.

[sastoller]

I was just browsing and found some good documentation from FIRST/WPI on current monitoring and brownout prevention. Figured I'd leave it here too.
https://wpilib.screenstepslive.com/s...24166/l/289498

[Al Skierkiewicz]

OK, spoiler alert, Al is going to talk about electrical issues again....
In most cases brownouts are preventable with a good electrical system design. To begin, let's talk about real world issues that every team experiences.
1. Wires have resistance! Resistance drops voltage when current flows. V=I*R.
2. The battery has internal resistance. Typical is 0.011 ohms. At 400 amps that resistance drops 4.4 volts. Yes that is real and it occurs with a freshly charged battery. Stall or start four CIM motors and that is what you will actually measure at the battery terminals.
3. Batteries do age, typical for our batteries is about 400 charge discharge cycles maximum. The Battery Beak will give you some indications but a true test is one that duplicates the curves published by the manufacturer. The West Mountain Radio CBA-IV is capable of performing this test and keeping the data from year to year so that you can overlay the tests and see graphically the deterioration of the battery capacity.
4. Terminals, connectors, breakers, and hardware all add resistance to the robot electrical system.
5. Smart chargers can be confused by certain battery conditions and operation. Read the charger manual and follow the instructions.

Many of us (Inspectors and CSAs) saw these issues throughout the season. They are preventable and we showed teams how to overcome these issues.
1. Make the terminations on the battery tight and non-moving. Add a #10 external tooth star washer between the battery terminal and the wire terminal to cut through all the surface crud and to prevent the terminals from moving. Use the supplied locking hardware to keep the screw and nut tight as well. These two steps will keep the termination as low resistance as possible over the possible three or fours years of the battery life. If the terminals move, any locking hardware will loosen. Loose connections are high resistance.
2. Terminate the #6 wiring so it won't pull out and it won't loosen. If you use the screw type terminals, strip the wire sufficiently to push bare copper all the way through the connector. This one step will prevent wire pullout. I don't like the type where the screw actually compresses the wire (as shown below). I prefer a clamping type where the wire is held between two moving parts. With the type shown, wire strands will be pushed to the side, reducing the wire gauge and increasing resistance.

If you use this type, attach it to the battery terminal so that the screw faces the center of the battery. This will give you less of a chance of having the terminal catch on mechanical parts in your robot when you install the battery. Follow up with good insulation. Duct tape is not insulation. Use electrical tape or heat shrink tubing.
3. I prefer the Burndy crimp type terminals. You can obtain these at AndyMark (am-0805) or many other locations. You can use a vise to crimp (see my other posts) these but the tool sold also by AndyMark (am-3326) is the perfect device. We follow up with solder to improve reliability and reduce resistance.
4. Tighten all hardware, Al tight, not freshmen tight. If you use any terminal that is 1/2" wide, be sure that your tighten the hardware on the PDP sufficiently that the terminal is in firm contact with the PDP terminals. It is easy to think you have done a nice job, but the plastic cover may be preventing you from having maximum contact with the PDP. This is a source of increased resistance.
5. Center the battery, the main breaker and the PDP in the robot to keep all wiring as short as possible. You may have to rotate the PDP to get this right. Every foot of #6 wire will drop 0.2 volts at 400 amps. All of your robot current flows through this wire. Make sure it is as short as possible.
6. Make sure the fuses at the very end of the PDP are fully inserted. Take the time to inspect this when the PDP is not in the robot. The shape of the fuse has a handle molded into the fuse to let you grab it. That handle should be almost touching the surface of the PDP when correctly inserted. The fuse feeds power to the RoboRio. If it is loose, the Rio gets upset or completely reboots.
7. The push type terminals require that wire insulation be stripped to 3/8" (0.375" or just under 1cm) . If you do not meet that minimum specification, the wire will not be retained in the terminal and the resistance will be high. Be sure to check that there are no whiskers touching the other wire.
8. Inspect your main breaker often for correct operation. Lightly tapping the red button while your robot is on, should not cause any blinking lights. If you see that, replace the main breaker. If there is any damage to the outside of the breaker (cracked or chipped case, broken handle or cover) assume the breaker has been damaged and replace. When you push the flag in, there should be one, smooth, movement. If you feel a slight resistance (as if the handle is moving over a speed bump) and then it pushes the rest of the way, the breaker is bad. This is an indication that the handle is damaged and will fail at some point.
9. It is possible that your drive system is drawing excessive current. This could be do to the type of drive you are using, the friction of the tires with the carpet or how aggressively your driver turns. Tank treads are the highest current demand in turns. Eight (or more) wheel drives with no drop center wheels are next. Knobby tires are also notorious for high current in turns. If you are experiencing problems in turns, it could be any one of these designs. Practice with your driver to not turn in place but do arcing turns to reduce the current draw. High friction drives in turns will put the drive motors into near stall conditions. A CIM motor stalls at 131 amps but typical robots will see current in excess of 100 amps per motor.
10. Finally, too many teams do not follow the instructions that came with their battery chargers. Let your battery rest for at least 30 minutes prior to charging. This will bring the internal temperature down to near ambient and allow your charger to optimize charge current. Charging a 'hot' battery may not get you full charge.

[MrRoboSteve]

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 -- 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 can't turn the robot, it's probably going to take a lot of energy to turn.

Don't bother doing either of these tests on a hard surface. The results don't replicate the actual field. Don't 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 won't be exactly the same as the field, but close enough to test it.

Here's 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 -- they had the same difficulties as the six wheel ones, but are fundamentally difficult to fix.

[Alan Anderson]

Originally Posted by Al Skierkiewicz

...I don't like the type where the screw actually compresses the wire (as shown below). I prefer a clamping type where the wire is held between two moving parts. With the type shown, wire strands will be pushed to the side, reducing the wire gauge and increasing resistance.

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 doesn't touch the wire itself.

[GeeTwo]

Originally Posted by MrRoboSteve

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.

If all four corner wheels of a drop center robot carry significant weight, you don't really have a drop center. If you want the turning advantage of drop center, you either need more drop or to stiffen your wheel motion (e.g. inflate the wheels higher). Further, many 6-wheel drop-center robots are intentionally designed with the CoG offset from the center axle. This is most commonly done to improve stability when manipulating game pieces (pickup, launch or placement), but can also be used to help cross obstacles such as the step in 2012 or the defenses in 2016.

[Chris is me]

Originally Posted by GeeTwo

If all four corner wheels of a drop center robot carry significant weight, you don't really have a drop center.

This isn't true. Even if all six wheels on a 6WD drop touch the ground due to compliance in the wheels, dropping the center wheel increases the normal force that wheel carries. For situations like roughtop treaded wheels, this makes a big difference even if all wheels don't leave the ground fully. More drop and less contact from the outer wheels does mean better turning though (to a point).

[Al Skierkiewicz]

Alan,
Many of the terminals look like the one I included except for one difference. The part that normally would be a wire clamp is actually welded or mechanically held in place such that the screw becomes the only wire retention device. Those terminals where the retention device moves and the wire has no contact with the screw work OK, if one removes enough insulation to allow at least 1/8" to 1/4" or bare wire to extend beyond the retention clamp. When the clamp screw is tightened, it should produce a compression on the captive wire and the remaining wire will actually expand a little. This expansion will keep the wire from pulling out during vibration. If no wire is exposed, the vibration will actually push the wire out of the terminal (like squeezing the end of a banana).

[Thad House]

Originally Posted by Al Skierkiewicz

3. I prefer the Burndy crimp type terminals. You can obtain these at AndyMark (am-0805) or many other locations. You can use a vise to crimp (see my other posts) these but the tool sold also by AndyMark (am-3326) is the perfect device. We follow up with solder to improve reliability and reduce resistance.

The AM crimper is great, but for about the same price you can also buy a hydraulic crimper from Harbor Freight. We've used it for years, and it crimps amazingly. Highly recommended for any large gauge wire crimping.

Originally Posted by Al Skierkiewicz

7. The push type terminals require that wire insulation be stripped to 3/8" (0.375" or just under 1cm) . If you do not meet that minimum specification, the wire will not be retained in the terminal and the resistance will be high. Be sure to check that there are no whiskers touching the other wire.

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.