BOE bot power issue

involute

My robot group is working on a BOE bot project that requires the bot to pick up 8 ping pong balls and dump them into a bin. To do this we are running 3 servos, 4 line following sensors and 1 6v electromagnet that controls the dumping mechanism.

We are having trouble getting power to all of these. Individually, they work, but the robot sputters and stops when we attempt to run the whole system. We've been told that the servos are drawing so much current that there is not enough available for the other systems. To overcome the power limitations , we have attempted to connect multiple 9v batteries in parallel.

This hasn't been successful. We've come up with another battery from an RC car that is 1 amp and 9.6 volts. We have some reservations about using this source. Although we know that it is within the 12 V range of the board, we haven't seen any power ratings for the BOE bot board and we're worried that we could fry the board if we ran an amp through it.

Here's how we're thinking of connecting this battery:

Instead of using the 9V port, we want to splice the new 1 amp battery pack·through the 6 9 VDC port. If we do this, I think we'll have enough current to power everything.

I want to know if anyone here is knowledgable enough of the BOE bot's power capabilities to advise us on this power issue and anything else that may come up as we attempt to fix it. We are working in a 1 week time frame to have this project completed, and although the power issue is the only thing holding us back other issues may come up from fixing the power problem.

Dgswaner

it's not about battery capacity or voltage, the board has a 1amp regulator (I think). it sounds like your drawing too much amperage through the circuit.

are you trying to power the electromagnet from the same source? they can draw a ton of amperage.

you need to look up, calculate or measure the current through each part add them up and supply the required amperage. you could easily add another Power supply go give you more amperage. adding batteries isn't the solution. how ever the more amperage your drawing at once the quicker your going to drain your batteries. hope that helps

ok I'm a little confused when I reread your message. Is the 1 amp battery pack a battery or a battery and power supply.

bottom line add another power supply as required to provide the right amperage. only connect the ground between each power supply.

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"A complex design is the sign of an inferior designer." - Jamie Hyneman, Myth Buster

DGSwaner

Post Edited (Dgswaner) : 12/7/2007 11:11:31 PM GMT

Mike Green

I wouldn't worry about the line following sensors. The servos can draw quite a bit under heavy load (over 1A) and I expect the
electromagnet is similar when activated. It's quite possible that you're getting noise spikes that are causing the Stamp to reset.
The BOE regulators are current limited and will shut themselves down if the current through them goes over roughly 1-1.5A or
if they overheat since they're dissipating the extra voltage (above 5V) times the current through them.

In any event, isolating the high current loads and feeding them from a separate power supply would be helpful. The servo ground (black wire) and control signal (white wire) still need to be connected to the BOE, but the power source (red wire) can go to a source of 5-7 volts. Do not run servos from greater than 7.5V or you will shorten their life and your electromagnet is specified for 6V. Your best
solution is an external 6V battery pack.

If you want to use NiMH cells, a 5 cell battery (like C cells - check their current capacity. A lot of rechargable C cells that are sold are only AA cells in a larger package) would be excellent to feed the servos and electromagnet. The battery's negative terminal has to be connected to the BOE's ground. The BOE itself needs to be powered from a 6V battery as well. Because of the lower drain from just the Stamp and the sensors, you could run this off 4 alkaline cells or 5 NiMH cells (like AA cells).

Franklin

You probably will not fry the stamp but you may fry the servos. (.v is too much for the servos and if you are running the other devices off the onboard regulator you might overheat and damage the voltage regulator. 7.6 is a better choice for a battery and don't cut anything on the boe just get a power plug from RS and convert the battery connection.

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- Stephen

involute

Thanks for the advice. We're going to put the power supply for the motors outside the BOE bot, and to do that we're going to need a relay to regulate the power to them from the board. Where should we look to get a relay, and what type should we use?

Mike Green

Why do you need a relay rather than a simple on-off switch? You would normally turn on the supply to the BOE, then turn on the motor supply once at the start of using the BOT, then turn off the motor, then the BOE. A relay would take some current to operate and you're running off batteries. You could even get a double pole toggle switch and use that to turn on power to both the BOE and motors at the same time and leave the BOE switch alone (in the on position).

involute

Mike Green said...
Why do you need a relay rather than a simple on-off switch? You would normally turn on the supply to the BOE, then turn on the motor supply once at the start of using the BOT, then turn off the motor, then the BOE. A relay would take some current to operate and you're running off batteries. You could even get a double pole toggle switch and use that to turn on power to both the BOE and motors at the same time and leave the BOE switch alone (in the on position).

Would an on/off switch have to be manually operated? The rules for our project specify that we can only press the button on the circuit board to start the entire assembly. The reason we chose a relay was because we could control the current flow from the circuit board.

Mike Green

Ah! It's a rules thing. Then you do need a relay. Figure out the total maximum current drawn from the motor battery ... that's the current rating of your relay contacts. You'll need an SPST (single pole single throw) relay. There are solid state relays, but they're relatively expensive. Jameco has a 5V relay with enough current capacity: Jameco #187151CB (Omron #G5LE-1DC5). You'll need a transistor switch to turn it on since the coil current is too high for a Stamp to switch by itself. There are examples of this in the Industrial Control tutorial around page 86. You'll also need a diode across the coil as show on that page.

involute

Thanks! I don't know what manual you're referring to. The only one we have is the one that came with the boe bot.

As for the relay, right now we're using a SPDT relay rated at 5V and 1 amp with a nominal current of 83.9 mA. We also have a 12V, 1 amp relay with a nominal current of 37.5 mA. I've heard that the boe bot's current output is around 22 mA. How would we wire a transistor to one of these to get it to work?