Bypass Disable Switch

[John Gutmann]

Originally Posted by JBotAlan

On a side note, how do you plan to power this power-sled? I thought about doing this, but when I thought it through I realized that our Exide match battery lasts about 2 minutes...then it's pretty dead. That isn't a very useful lifetime. And if you put the batteries in series, 6 of them would deliver 12 minutes. Still not very useful.

.
If you wire them in series you will just have more voltage. IF you wire them in parallel though, it will last longer.

[Al Skierkiewicz]

Originally Posted by JBotAlan

On a side note, how do you plan to power this power-sled? I thought about doing this, but when I thought it through I realized that our Exide match battery lasts about 2 minutes...then it's pretty dead. That isn't a very useful lifetime. And if you put the batteries in series, 6 of them would deliver 12 minutes. Still not very useful.

Good luck,
JBot

I hate to pile on here but six batteries in series will output 72 volts. Can you say "WOW that was a huge fireball! Where did the top of the RC go?" You will not get any greater current out of the batteries in this configuration. You can get longer run time with robot batteries in parallel (as above) but these need some steering diodes to keep from self-discharging via the other batteries. Dual battery diodes can be purchased at RV centers but are not cheap. A diode in series with the positive lead of each battery can then be tied in parallel. A simpler solution is to install a larger battery. A car battery or boat battery would work great. If you keep the same breakers, you should have no problem running for a much longer time. Be careful, car batteries can only be used in the upright position!

Back to the death in a two minute match, a well designed robot should be able to run several matches without killing the battery. When teams test their designs, they should monitor individual currents in order to determine whether the design is efficient. Often times, tank style steering with sticky wheels produce the greatest demand on the electrical system. I have experienced near stall currents on robots I have been called to look at. On a four Chalupa drive, that is well over 400 amps. A simple calculation will show that 400 amps flowing through the .011 ohms of internal resistance on the battery (full charge) will drop the battery terminal voltage to 7.6 volts. That is 0.4 volts below the point at which the RC drops out and goes to backup battery. If the drop lasts long enough, the RC will stop controlling your robot. When that occurs, motor current stops, the terminal voltage rises to normal and the robot continues. Watch some matches and when you see a robot hesitate or pause, it is often going into protect mode and disabling the PWM outputs.

[JBotAlan]

"WOW that was a huge fireball! Where did the top of the RC go?"

Oopsie.
I really need to put a disclaimer in my sig...

I don't understand why you need extra circuitry when using them in parallel. Why would they self-discharge if you tie the positive of each battery together and the negative and draw from that? Time for me to learn something...it's a good reminder that I do not know everything.

"Often times, tank style steering with sticky wheels produce the greatest demand on the electrical system."

Yes, that's out 'bot. I know it won't last more than one match, but then again, I know we are really heavy and do a lot of pushing. I guess it totally depends on how efficient whatever it is you are powering is. A great, heavy pusher = lots of drain on the battery (right? I was pretty confident 'series' was the right word, and now I don't know how much I really know...).

Thanks,
JBot

[Robert Flanagan]

Originally Posted by jakep

We are making a robot over the summer using the 2004 RC. However, this is a fully automous robot, and doesn't need to be hooked up via radio (but it can be). We want to be able to let the robot run, with full control of the motors without any OI turned on or anything. However, if you do decide to turn on the OI, the robot should still accept input from it.

My question: Is there anyway to let the RC output to the motors even without an OI plugged in?

Thanks!

This is a theory but should work. Go into main.c. You should see something like this.

if(autonomous_mode)
{
User_Autonomous_Code;
}

It seems that if u delete this if statement to just read out

User_Autonomous_Code;

It should just run Autonomous entirely.

[Al Skierkiewicz]

Originally Posted by JBotAlan

"WOW that was a huge fireball! Where did the top of the RC go?"

Oopsie.
I really need to put a disclaimer in my sig...

I don't understand why you need extra circuitry when using them in parallel. Why would they self-discharge if you tie the positive of each battery together and the negative and draw from that? Time for me to learn something...it's a good reminder that I do not know everything.

"Often times, tank style steering with sticky wheels produce the greatest demand on the electrical system."

Yes, that's out 'bot. I know it won't last more than one match, but then again, I know we are really heavy and do a lot of pushing. I guess it totally depends on how efficient whatever it is you are powering is. A great, heavy pusher = lots of drain on the battery (right? I was pretty confident 'series' was the right word, and now I don't know how much I really know...).

Thanks,
JBot

Batteries in parallel will naturally not all be at the same terminal voltage. Variables in production will make the output voltage vary due to the internal resistance, the absolute surface area of individual plates, the concentrations of the acid within each cell, even the resistance of the bars that connect each cell internally. That being said, some of the batteries will have slightly higher or lower values. The batteries with the higher terminal voltage will attempt to pass current to the lower battery to keep it at the same voltage. After it has done that for while, it's terminal voltage will drop and the other battery will try to pass current to it. Think domino effect and eventually, the batteries run themselves down to zero volts. All of the current having gone into heat along the way. By placing a diode in series with each battery, it can only supply current it cannot accept current. All current will only flow out of each battery and into the common (all cathodes tied together) of all the diodes. Regular power diodes will exhibit a voltage drop of at least 0.6 volts and increasing as more current is drawn out of the battery pack. Although that works, it is not ideal when you are already taking the terminal voltage down with a high current demand. The diodes at the RV shops are Schottky ( or should be) and they have a much lower voltage drop.
Heavy pushers need not be high current robots. However, a motor in stall is still a motor in stall is still a motor that is not moving. Many designers will take this into account and choose wheels that will eventually break the friction with the floor so as not to stall the drivetrain. In tank steering, a turn will produce the same high currents unless there is modification. Omni wheels are an ideal solution.
Note to all: When a designer sees a such a need in this competition to go into manufacture to make parts that teams need, that is a red flag. Take a look at the Andymark website or the IFI website and see parts designed to solve the particular problems we face in robot design. These people are not going to design and market items that no one will buy. Like selling refrigerators to Eskimos. Native people in Alaska and Canada do not need refrigeration when the ground outside their door is permanently frozen.

Keep asking questions. A person who recognizes they do not know it all will continue to progress and be successful.

[JBotAlan]

Originally Posted by Robert Flanagan

This is a theory but should work. Go into main.c. You should see something like this.

if(autonomous_mode)
{
User_Autonomous_Code;
}

It seems that if u delete this if statement to just read out

User_Autonomous_Code;

It should just run Autonomous entirely.

This I do know...

Your program won't run at all at first without an OI--that loop doesn't even execute when you power on the RC with no OI--and no outputs are generated even after an OI is connected and then disconnected, even though this loop is running. Follow the instructions of the person who posted earlier on making the RC run without an OI to make it run all the time, and Robert's modification will make the RC run whatever is in your User_Autonomous_Code function.

JBot

EDIT: Just out of curiosity, how did you learn how to do these calculations--the life of the battery? I really don't know any of this. I don't imagine it's that hard, but I just never learned it and I recognize this is some of the stuff I should know being that I'm dealing with electrical this coming season.

Thanks

[Al Skierkiewicz]

Originally Posted by JBotAlan

EDIT: Just out of curiosity, how did you learn how to do these calculations--the life of the battery? I really don't know any of this. I don't imagine it's that hard, but I just never learned it and I recognize this is some of the stuff I should know being that I'm dealing with electrical this coming season.
Thanks

A lot of this is not calculable. It comes from experience. There is so many variables from match to match and robot to robot it is almost impossible to take everything into account. A couple of rules of thumb help to analyze what is taking place.
1. Tank style steering will put the drive motors into stall when ever they turn unless something is done to reduce the drag (friction) of the side motion of the wheels. More turns in a match will draw stall currents more often, reducing terminal voltage, available battery power and overall battery life.
2. Drive trains that are optimized for pushing only will draw high currents when running flat out at their highest speeds. Making some trade offs in speed vs. pushing will help in current drain. Drivetrains that are designed for high speed (greater than 12 fps) will likely draw extreme currents when starting out from a dead stop and when pushing, bumping into field objects or driving up ramps. Most mechanical experts design for 8-12 fps.
3. Software modification of drive signals will help in current draw. Ramping up to full speed for instance will smooth out the current demands. Dual speed transmissions will also help by giving the designer two separate optimizations for the drive motors.
4. Six motor drives generally will be less effective electrically due to the additional friction of the extra gears, balancing of the loads, etc. Although they will be the greatest pushers if the wheels can couple to the floor, it has been rare that a robot would only push and do nothing else. If you are in stall every time you start a motor, then you are adding two more loads to the battery. Stall currents are listed in the motor spec sheet. 6 x 129 amps for Chalupa drives exceed the max current spec on the battery. Repetitive current demand near the max reduces battery life. I have seen six motor drives that were not able to move the robot if minor damage to the robot caused some additional friction in the drivetrain.
5. One only needs to observe robot operation during practice to recognize problems. An aggressive practice where the robot stalls, falters or halts for a few seconds, is drawing too much current. A tank style robot that "hops" when turning has too much side friction and can be expected to be at max current every time the robot hops up or down. Any odd problems in robot operation such as software failures, "no data radio", intermittent IO tally LED operation or other apparent failures can be attributed to electrical system failures due to low battery.
6. During practice, a robot driving without opponents ought to last at least 6-8 minutes and preferably 10-15 before the battery runs down. Any less, and your design is inefficient and you are reducing the life of the battery. Our batteries have about a 400 charge/discharge cycle life if used in the normal operating range. A robot that eats a battery in 2 minutes has likely cut that life to 100 cycles or less and no amount of charging is going to fix that. It is also going to fail without warning. Check your design by using a current probe or amp clamp to check overall currents while the robot is practicing and check each individual motor current. Keep a list of these currents. If during a competition something seems wrong, a quick check of motor current can identify bent mechanical systems or wheels that have been pushed out of alignment. Currents should be nearly identical from side to side. If they are not, you may have an electrical problem, bent parts or a bad motor.
Hope this helps.