AC Adaptor?

[Jake M]

Has anyone ever considered using an AC adaptor to power the robot, rather than the gigantic batteries. Does an adaptor even exist for our power needs? (not that we could actually use it at competition) We're working on a test electronics board we can use whenever we need to test a motor or a sensor, and having to drag around a battery kinda ruins the portability, although it doesn't really matter.

[MrForbes]

The controller itself doesn't need a real big battery (the backup battery is about right for the job), but running the robot drive motors does.

[Jake M]

Well, the idea is that the board will be an entire electronics system, with an RC and the whole power distribution system, and victors and spikes, etc. and theoretically capable of running an entire robot by just plugging things in, so we can test-drive the first-week chassis, and other things, without having to mess with the new electronics equipment until we have a better idea of all the parts we need. This year's electronics mounting was beautiful, but we built it a bit too soon and had to later add a victor and two spikes randomly to the frame, and had to run extra wires accordingly.

So, yeah, we need the same kind of power you get from the big battery.

[MrForbes]

You could run the whole system with a small 12v battery, but don't expect to be able to run any motors that are under much physical load. Look at the motor specs in the Tips and Good Practices manual, they give the no-load current draw. Figure out how much current it will take to power those motors, and read up on how battery specifications work. You should be able to figure out about how long you can run some motors with different size batteries (assuming the motors are not mechanically connected to anything, they just spin freely). Perhaps a 12v nicad battery pack would do what you want, with less weight/inconvenience. Just make sure to use the appropriate charger for it.

[Jake M]

Meh. Convenience isn't really an issue. I'd use the normal batteries before I'd go buy another battery. I just kinda thought being able to plug into a wall would be ain interesting feature.

[NOV8R]

Here's a link to a video of our "little black box" which enables us to drive any chassis with just the motors and gearboxes installed. http://www.youtube.com/watch?v=Bw7-EVB-XZU In the video we're using it to test drive a rookie team's chassis two weeks before they were able to get all the electronics installed. This box has become one of our most important development tools.

[MrForbes]

I just realized I did not answer the question you asked....which is whether or not you could use an AC adapter to power the robot.

You might want to ask IFI about this, as there may be some specifications for the power supply that would limit what you would want to do. Typical "wall wart" power supplies are rated to put out a specific voltage at some specific current load. If you have less load, they put out more voltage, which could possibly damage the controller. That is why think that using a battery might be wise.

A lab type regulated DC power supply would probably be fine, but it will be even more cumbersome to use than the regular robot battery!

[Jake M]

Well, I knew there had to be a reason no one's tried it before. I just kinda wanted to know what it was.

And that black box is very nice. Could that battery fit any more perfectly?

We had something similar to that as our actual electronics system two years ago. It was actually a large tupperware box that had the Breaker panel and all the Victors bolted in it, with the RC, Radio, and backup battery suspended from the lid. With the held of quick connectors, it could be completely removed from the robot in under a minute. Won us an award too. But then we tried it the following year with a Craftsman toolbox and that was just plain horrible. I'd have to cut metal off of the robot just to get the box out anymore.

Once I get the new Autodesk downloaded to my computer (2.16GB download on Dial-up!) I'll cad up our ideas and put up a picture.

[Al Skierkiewicz]

Jake,
Because the motors are in stall when they are not moving, the initial start current on a typical robot may be in the 200-300 amp range. An AC supply of that size would be enormous. I know that my 25 amp ham radio power supplies get bogged down just trying to start one motor. The supply is a linear supply and weighs over 20 pounds.

[sciguy125]

If you're just after a bench setup to test electronics, why not use a power supply and get small motors. I've noticed our programmers dragging around a big board with a battery on it to test sensors. A lot of the time, they could have gotten away with a smaller setup to run their experiments. I have yet to convince them of this though...

[Tom Line]

I won't say it can't be done, but I can tell you that you're talking about a HUGE AC/DC converter, and it's going to be very expensive.

For instance, if you want a DC power source of 10 amps, you'll pay ~$60 at radio shack. 20 amps is ~$100.

If you're running two motors for an arm at 15 amps each, 4 sim motors at 20 amps each, a compressor at (I don't know how many)....

You're talking hundreds of amps.

[MrForbes]

If you're running two motors for an arm at 15 amps each, 4 sim motors at 20 amps each, a compressor at (I don't know how many)....

You're talking hundreds of amps.

Not really, for testing out the electronics you would not run all these motors at once, and you would not be running the drivetrain under load, etc. 10 amps or less should be sufficient. The only thing that I can see causing problems could be the compressor, if you try to start it when there is pressure in the system it will draw a big load on startup.

[Jake M]

For instance, if you want a DC power source of 10 amps, you'll pay ~$60 at radio shack. 20 amps is ~$100.


That's about what I thought. Again, it's not a big deal.

[Mike AA]

Why not just use a car battery charger? The higher quality electronic will put out around 100 amps at almost a perfect 13 volts, which should be within tolerance of the whole system.

-Mike

[meatmanek]

You might check out the battery chargers themselves. I can't remember correctly, but I believe they're rated to 6 or 8 amps. The thing you may have to worry about is voltage - I think they're 12v output, but they might be higher. Time to break out the DMM.

[whytheheckme]

Yeah.... This whole thing doesn't sound to promising...

Sure, the battery might be rated at supplying so many amps, but when a motor turns on, it spikes way more than this. That's why the 'motorcycle' batteries that we use are so convenient; They can handle the high current spikes. I just don't think a AC-DC converter can handle this... Now maybe a system where its an AC-DC converter with a battery to handle the high current spikes... Kinda like the IFI 7.2v charging circuit, where the input voltage and the battery are used (but perhaps a better designed circuit... If we could only put in the one diode to prevent the battery from backcharging the robot... [/OffTopicRant]...)

Just some off-the-top-of-the-head thoughts... I could be completely off-base.

Jacob

[ay2b]

We have two wall-wart adapters which we use for testing the electronics. One provides 9v at 1/2 amp for the backup battery, and the other provides 12v at 1.5 amps for the RC. This works fine for testing the electronics, camera, sensors, etc, but will NOT run any motors.

Think about this -- the battery on the robot goes through a 120 amp breaker. That's a good indicator that to run it (with full motors) off of wall current, you'd need a power supply capable of providing 120 amps!

[MrForbes]

Al's reply got me curious...so I connected a lab power supply to an RS540 mabuchi motor (the BaneBots motor) and switched it on and turned up the voltage. With the current limiting set to 2.0 amps, the ammeter indicated a spike to about 1.5 amps, and running currrent of about 1.0 amps with the voltage adjusted to anywhere from a couple volts to 10 or so.

I think my estimate of 10 amps needed to test the system, having the motors without mechanical load on them, is probably reasonable, asuming you don't try to run more than two motors at once.

[MrForbes]

Might be able to use an old computer switching power supply, they have decent regulation, and should handle 5-10 amps on the 12v circuit.

[Al Skierkiewicz]

Guys,
Battery chargers rarely put out a pure DC voltage. Never use them for power supplies. Often they put out a pulsed DC that averages about 13 volts but peaks much higher. Popular 50 amp chargers just connect a few diodes to a big transformer and add a few lights and an ammeter and call it a day. Remember that a wet or gelled cell battery will charge (current flows) if the input voltage exceeds the battery voltage. That is why 50 amp chargers are not allowed for our batteries as the high current is capable of destroying the battery. (read that catastrophically) The charger supplied in the kit is a smart charger and won't even turn on unless a valid battery is connected and in need of charging. The internal electronics test the device connected before supplying full output current.
As for the compressor, start current is about 25 amps while run current is about 10 amps. Again, start current on the Chalupas or the Fisher Price is over 95 amps. With many of the modern power supplies, the start current (of even one motor) will force the supply into current foldback or just simply trip the breaker. The supplies that go into current foldback will not go back to normal output until the load is removed or the power is reset.
And you don't want to turn on a power supply at a low voltage and run it up with the electronics. For starts the RC is disabled until the input voltage is above 8 volts.
Although the breaker is rated at 120 amps, if you look at the curve you will see that it can handle a 600% overload for a few seconds and can handle almost 200% for 40 seconds.

[MrForbes]

Again, start current on the Chalupas or the Fisher Price is over 95 amps. With many of the modern power supplies, the start current (of even one motor) will force the supply into current foldback or just simply trip the breaker. The supplies that go into current foldback will not go back to normal output until the load is removed or the power is reset.

Interesting....so just for fun I connected that RS540 motor to a switch, then to the 12v (at 8 A) output of an old 200W computer switching power supply. I turned on the supply, and the motor jumped and started turning, and kept turning.

This motor is rated at 0.95 A no load current.

Would the current spike at startup of the Chalupa (CIM) motors be any less than 95 A if they were under control of a Victor, as in real life? (in the situation of no mechanical load)

[Al Skierkiewicz]

Squirrel,
The stall current for any motor is the start current since a motor that is not moving is by definition in stall. With no load the current spike is rather short and for this motor, the stall current is only 42 amps even though the no load is 1 amp. (from the Banebot website) Even a motor with a transmission that is not connected to anything still has some loading so it is likely that the computer power supply would not be able to start a motor coupled to a transmission. The caps in the output of your supply are the part of the power supply that is delivering current during that critical startup. If the current demand was higher or the value of the output caps were lower, the result would not be the same. What would be interesting is looking at the power output on a scope when you start the motor. This should show a significant drop in the output during the start time of the motor. The Chalupa motors are much higher current having 129 and 133 amps stalled respectively, as I remember. Run current, no load, is a little higher but the big difference is the amount of mass at start compared to the Banebot RS540. The armature of the larger motors may weigh more than the whole Banebot motor.

[MrForbes]

That sounds about like how I thought it worked.

Can the stall current be estimated by applying Ohm's law, using the resistance of the motor, and the nominal voltage?

And could one add a large capacitor to the power supply output and be able to get a CIM turning?

as for the torque to turn a transmission, if you wanted to test the electronics, there really is no need to have the motor installed....my understanding is that the original question is how to test just the electronics without the hassle of having to deal with a battery. If the robot is assembled, the battery is not a hassle to deal with, as it is mounted in the robot, and easy to connect.

[MrForbes]

I put the setup on a scope, and as you predicted, the power supply voltage does drop significantly, to about 6 volts as the motor is switched on. Also the filtering at the switching frequency gets pretty bad.

I don't have a storage scope, so I can't offer any pictures.

[Al Skierkiewicz]

The series resistance of the motor is also complicated at startup with the inductance of the windings and the back EMF developed in the armature. The tough part about brush type motors is that generally, more than one winding is contacted by the brush assy. Due to this switching, the series resistance and inductance changes as the armature turns. I believe for testing the factory takes several readings and then takes the highest one for stall current. Series resistance is also affected by the shape and wear of the brushes and a variety of other factors. To measure the series resistance takes a device that will measure down into the fraction of an ohm range. A typical Fluke will not get you close to the actual resistance. At 100 amps at 12 volts, for example, the series resistance would be 0.12 ohms. (The contact resistance of the leads could affect a reading that low.)
It is possible to add capacitance to the output but there is a down side to that. The inrush current for the power supply goes up as you add to the output. That could pop the fuse or cause the supply to clamp for over current when you turn it on.
The best method of testing is a battery. You can breadboard a test setup using all of the electronics, breakers and the motors(sufficiently secured to the board) and check everything that way.

[Rapt0r9]

Well, if you were to create a rectifying circuit to invert the AC to DC, you would need quite a bit of power from the AC line if you are going to run the entire bot off the line.

If you can find components rated for the current that you are going to draw, you can build a simple inverter using a transformer, rectifier (Diode Bridge), capacitor, and voltage regulator. The last two aren't absolutly nessecary, but they help to even the output voltage of the rectifier.

Personally, we use a car battery with a set of jumper cables to run motors before we wire the whole thing. This is convienient because we can just raise or lower the screw mech with out needing to install victors, an rc, breakers, and wire the it all.

[FourPenguins]

To run our lights this year one of our mentors helped us rig up a system that had the battery on the charger as it delivered power. It served as an AC power supply for the light, but the battery protected the system from inconsistencies in the AC supply from the charger.

[Al Skierkiewicz]

Ed,
That is a still a chancy situation. In most cases the battery provides enough stabilization to keep the charger from producing higher than normal voltages. Keep in mind, though, that chargers still must output a voltage higher than the battery in order for charge current to flow to the battery.

Raptor9,
Please be advised that a simple rectifier fed from the line voltage still has the ability to place the supply at 110 VAC above ground depending on the way the electrical power is wired. Transformers of some type are needed to isolate the power line from circuitry that humans can contact.

[Mr-Shutter]

A DC battery charger will NOT run your robot. It uses low current to charge the battery and is not advised to try and run you bot off of it.
Unless you have superior knowledge of AC-DC PSU construction, don't make your own.
Cough up the cash and buy a nice PSU.

For test boards, with little equipment

I mean, for a test board, running like 1-2 motors, use this! ( I doubt that the large motors use 40 amps)
http://myrcsupply.net/index.php?main_page=product_info&cPath=22_229&products_id=13383
or for 2-4 use this
http://myrcsupply.net/index.php?main_page=product_info&cPath=22_229&products_id=13008
They are equivalent to a car battery

Don't worry about blowing a fuse on the PSU's...

For the WHOLE robot

I like this one http://www.v-infinity.com/adtemplate_child.asp?c=1002843&p=1002840&catky=764537&subcatky1=46887&subcatky2=320934
It has what I believe to have sufficient current for the bot, and a lot of safety features built in, preventing damage top you, the bot, and the PSU.
Hook up a current meter to the bot and see how much current you draw with everything on. You’re going to want a PSU that can handle a little more then that amperage.