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Mechanics and Construction => Mechanics and Construction => Topic started by: ROBOT420 on March 22, 2013, 12:19:55 PM

Title: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 22, 2013, 12:19:55 PM
Hello, I am glad to have finally found this forum! I am doing your basic remote controlled power wheelchair base, and I was wanting to make sure how to go about keeping the speed control as well as the charger plug. I have a Jazzy powerchair, a Sabertooth 60a motor controller, and a transmitter and receiver. Any tips or advice that will save me some head scratching would be greatly appreciated! Thanks, Steve
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 22, 2013, 12:46:00 PM
Steve,

I just finished interfacing to a working wheelchair, but I kept the wheelchair electronics.

Am I correct in assuming (since you mention the Sabertooth) that you're bypassing or removing the onboard wheelchair controller? If so, what do you mean by keeping the speed control?

Am I also correct that you plan on using your wheelchair motors at 24v, and that's why you want to keep the charger wiring?

John

PS. two VERY IMPORTANT TIPS (from experience) : 1. ALWAYS test your chair up on blocks, wheels off the floor. If your controller or your code screws up, wheelchair motors are very powerful. You can get injured or some serious damage can be done to your workshop/testing area. Put it on blocks every time you change code or hardware, and test it there first. And 2. Install a kill switch somewhere on the top of it. In case it comes off the blocks, or say, gets interference from the remote while using it, you or those around you need a quick way to kill the power to the motors. A big red switch (illuminated if possible) is the most intuitive solution for this.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 22, 2013, 12:57:18 PM
Thanks John! Two great points, shopping for a big red button now!
 I do plan on using the chair batteries as they have been replaced recently.
I am using the Sabertooth for ease of use in wiring up my receiver, once I have this one under my belt I will try other ways no doubt.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 22, 2013, 01:18:36 PM
What model Jazzy is it?  Do a google search for "Jazzy xxxxxx service manual". You can find most of them online, and many of them will give you the pinouts for the connectors like the charging port, etc.

Also, don't forget (if you're using the Sabertooth), wheelchair motors have electronic brakes in them that must be released before energizing the motor circuit. The wheelchair controller usually handles this. I mention it because if you wish to use the brakes, you'll have to control them before and after you energize the motor. If you're not using the brakes, you can remove them from the rear of the motors. Either way, you need to be aware that they are there. The motor won't spin until you release the brakes. The brakes are engaged by default, and must be energized before they release.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 22, 2013, 01:50:30 PM
Great point! Thanks again, the Sabertooth has regenerative breaking and reverse (so it charges the battery's when you stop somehow?) I will see about the breaks though.
The "speed control" that is on the stock controller lets you adjust the speed at full stick. If I just use the Sabertooth will it only supply what the motor(s) need(s)? I want full speed (such as it is) but I do not want to melt anything. Thanks again, anything else you can think of?
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 22, 2013, 02:04:09 PM
Nothing more until you get further into what you want to do, or get started.

If you're using the Sabertooth, you'll control your speed through software (somewhere). lol. The Sabertooth will work just fine...it will basically switch the power coming from your batteries into your motors. I'm not sure how it handles speed control. You'll have to look that up. I put a microcontroller into the joystick of my chair, and by varying the voltage I can change my speed. Other more experienced forum members can help you on the Sabertooth side of things.

You'll have to choose between using the Sabertooth and the stock electronics on the chair. I'm afraid it's an all or nothing choice when it comes to controlling the chair. Both have advantages and disadvantages. If you're planning remote control and already have the Sabertooth, that's the best/easiest way to go. How you deal with your brakes is going to be a choice you make.

If you think you might want to play with the stock chair electronics later, just remove them or disconnect them after taking pictures or making a drawing of the connections. Remove them without damaging them, and you can reuse them later. Or if you don't want them, sell them on Ebay to help finance your project.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 22, 2013, 05:26:59 PM
If you can walk me through how to do it with the stock controller then I would be fine with that and will just use the Sabertooth on something else. I don't know enough about it to know WHAT choices I have. Thanks again!
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 22, 2013, 05:49:29 PM
Using the chair's controller is a lot more work, and a lot touchier, due to all the safety features built in for the wheelchair user. At a minimum, you'd have to use a microcontroller hacked into your joystick for proportional speed control, or on some controllers, there's an external 9pin connector you can use for simple on/off in each direction, but no speed control. But a simple variation in any of 3 simultaneous voltages can throw the chair into error mode and stop it dead.

Since you already have it, I'd really recommend starting with your Sabertooth, while leaving the original electronics untouched. Then re-use them later when you get bored, or sell them. You'll have better motor control from the Sabertooth, too.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 22, 2013, 06:40:11 PM
Sounds good. What was the verdict on speed with the sabertooth? will it have them at wide open (would be fine)? I guess I need to ask....How does the chair controller vary the speed? Is it PWM or voltage, orrrr? Thanks again
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 22, 2013, 07:07:49 PM
Somebody else here will have to answer for the Sabertooth speed control. I've not worked with them. I like what I read about them, and they get great reviews, plus they feature an RC mode, so you should have no problems there. Hopefully someone here can chime in on how the speed control for them works.

As for the chair controller...it depends on your chair. I hacked into my joystick. At neutral, it throws 2.5 volts on both axis wires. I measured full forward, left, right, and reverse, and voltage was up as high as 4V and as low as .7V depending on position. I send the voltage I need across 2 wires depending on what direction and speed I want to go. One problem with this is that I need 2 wires at 2.5V for chair-neutral. If I shut off my controller card BEFORE I shut off the motors on the chair, the voltage drops to 0V and the chair starts to move. (See why I mentioned a kill switch? LOL). Some joysticks use 5 volts like mine. Some use 12. Some use 24.  :o Just make good friends with your multimeter.
 
Internally, wheelchair controllers have their own network protocol. Rnet is just one of them. Others use single wire serial communication, or just SP1 internally. It's a confusing mess, and most wheelchair manufactures don't play nice when it comes to sharing technical information.

Seriously though, if you just want to have fun, use the Sabertooth. Bypass the chair electronics completely. It will be much easier, and the speed control shouldn't be an issue. I just don't know how it's done on the RC circuit, but I'm sure someone here can help you with that. What took me a couple weeks of research to learn, and a couple of days to implement and test using the wheelchair controller, you can accomplish in a couple of hours by using the Sabertooth.

Good luck! And keep us posted!
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 23, 2013, 09:13:34 AM
Well here goes nothin'! About to (gently) dismantle this chair ad see what secrets it wants to share with me. If possible I plan to do it WITHOUT shocking the piss out of myself somehow! I seem to have a real knack for doing that :-(  .... I will post pics later.....
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 25, 2013, 09:26:17 AM
Hey on my jazzy the stock motor controller seems to also have the battery charger built in to it. Anyway to separate the two or do I need to design a new charging system? Thanks, Steve
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 25, 2013, 10:55:41 AM
You've got to be more specific, Steve. Do you mean the plug is on the joystick housing? Do you mean you plug the controller into a normal wall socket? Do you have a seperate charger that you plug into the chair somewhere, then into the wall? (Pics might help)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Tommy on March 25, 2013, 08:50:35 PM
Quote
seems to also have the battery charger built in
the stock jazzy on-board 7Amp charger is separate from the motor controller and can
be used without the stock joystick controller, just got to keep it wired to the batteries.

I believe it's worth the bs of using the stock joystick controller because of all the safety's
built into it. if anything goes wrong it goes into a fault(very handy).
wont let you burn it up, or start up without the joystick centered(Sweet!)


for last three years I'v been using the stock motor controller in my project, but I'm testing
the sabertooth 2x60 right now to see if it can be used if my current motor drive ever fails.
but I had to add a micro to the system to mimic the safety features of the stock controller.

best to keep the brakes if possible, your going to need them.

Tommy     
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 26, 2013, 07:03:28 AM
Thanks for jumping in, Tommy. Your Mule was a big inspiration to me  8)

I'm curious though, what software did you use to get the dashboard that shows in one of your videos? Pm me if you don't mind. I don't want to hijack Steve's thread.

Thanks.

John
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 26, 2013, 12:36:48 PM
Hijack away! I will take all the related info I can get! :-) Well I have the breaks off, and the whole chair stripped sown to the frame/battery tray. I am now starting to sand it some and spray it with Purple metal flake paint from Roth. At least this way, even if it NEVER works, it will still LOOK cool! lol!
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 26, 2013, 03:52:00 PM
lol. We're going to want pictures ;)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 28, 2013, 10:58:35 AM
I now have both motors under remote control! ;D

Why is it so hard to even add ONE pic on here? I got my Avitar changed to my new Ohm/Watts law tat (just in time for finals! 8)) jut no dice posting pics to the forum. What am I doing wrong? Thanks, Steve
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on March 28, 2013, 11:42:07 AM
Great job on the motor control!!  ;D

When I post pics, I host them on my own website then put the url between the "img" tags. I think you have to host your pictures elsewhere, like photobucket, your own website, etc etc then just use the link here.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on March 28, 2013, 11:53:55 AM
Thanks! I will try to get some pictures of the project up soon.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on April 23, 2013, 11:57:09 AM
Here is my project, and a video of how the paint turned out. looks crappy in the video but it came out real decent.

http://s1362.photobucket.com/user/420photo420/media/20130327_190829_zps55588f8b.mp4.html?sort=3&o=1 (http://s1362.photobucket.com/user/420photo420/media/20130327_190829_zps55588f8b.mp4.html?sort=3&o=1)

http://s1362.photobucket.com/user/420photo420/media/20130405_140416_zps02c15b01.mp4.html?sort=3&o=3 (http://s1362.photobucket.com/user/420photo420/media/20130405_140416_zps02c15b01.mp4.html?sort=3&o=3)

Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on April 23, 2013, 12:15:12 PM
More project pics...

http://s1362.photobucket.com/user/420photo420/media/20130326_145746_zps366e62f7.jpg.html?sort=3&o=12 (http://s1362.photobucket.com/user/420photo420/media/20130326_145746_zps366e62f7.jpg.html?sort=3&o=12)

http://s1362.photobucket.com/user/420photo420/media/20130412_153629_zps50ba4641.jpg.html?sort=3&o=9 (http://s1362.photobucket.com/user/420photo420/media/20130412_153629_zps50ba4641.jpg.html?sort=3&o=9)

http://s1362.photobucket.com/user/420photo420/media/20130324_201233_zps5375c004.jpg.html?sort=3&o=5 (http://s1362.photobucket.com/user/420photo420/media/20130324_201233_zps5375c004.jpg.html?sort=3&o=5)

http://s1362.photobucket.com/user/420photo420/media/20130404_155442_zpsbeb23cbf.jpg.html?sort=3&o=7 (http://s1362.photobucket.com/user/420photo420/media/20130404_155442_zpsbeb23cbf.jpg.html?sort=3&o=7)

http://s1362.photobucket.com/user/420photo420/media/20130404_161054_zpsea2c0cdd.jpg.html?sort=3&o=8 (http://s1362.photobucket.com/user/420photo420/media/20130404_161054_zpsea2c0cdd.jpg.html?sort=3&o=8)

http://s1362.photobucket.com/user/420photo420/media/20130405_123536_zpsa1c1c81c.jpg.html?sort=3&o=10 (http://s1362.photobucket.com/user/420photo420/media/20130405_123536_zpsa1c1c81c.jpg.html?sort=3&o=10)

http://s1362.photobucket.com/user/420photo420/media/20130324_202202_zps97435923.jpg.html?sort=3&o=4 (http://s1362.photobucket.com/user/420photo420/media/20130324_202202_zps97435923.jpg.html?sort=3&o=4)

http://s1362.photobucket.com/user/420photo420/media/20130323_115245_zps9144120c.jpg.html?sort=3&o=0 (http://s1362.photobucket.com/user/420photo420/media/20130323_115245_zps9144120c.jpg.html?sort=3&o=0)

Now for final assembly! :-)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on April 30, 2013, 11:31:55 AM
Phase one complete!

http://s1362.photobucket.com/user/420photo420/media/20130429_123146_zpsae19c0c5.mp4.html (http://s1362.photobucket.com/user/420photo420/media/20130429_123146_zpsae19c0c5.mp4.html)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Duane Degn on April 30, 2013, 01:38:22 PM
Wow! That thing is cool!

It looks like that has lots of power to spare.

So what's phase two?
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on April 30, 2013, 01:59:58 PM
Dude...that looks awesome! You should be proud. Really cool  8)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on April 30, 2013, 07:54:16 PM
Thanks guys! ;D You should see the 5 coats of Purple flake and 3 coats of clear in the direct sunlight! Phase two is going to include me putting a third (accessory) battery on that rack I welded on the back. This will make it ride wheelies further (I have wheelie bars planned out in case I need them), as well as power the duel horns, LED's, these little speakers I have with a voice module that records and plays back 10 seconds of sound, and if I have time (only 4 weeks left until final presentation for school) I might mount this robotic claw I have one there but with a small water cannon on it (not too sure about the whole water and electronics thing though)...and then.....TRY TO TAKE OVER THE WORLD! MUUUHAHAHAHA :o! Or, get an A on my final project...either one will due.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on April 30, 2013, 08:22:20 PM
Thanks John! Two great points, shopping for a big red button now!
 I do plan on using the chair batteries as they have been replaced recently.
I am using the Sabertooth for ease of use in wiring up my receiver, once I have this one under my belt I will try other ways no doubt.


SO, first of all I would like to second the motion on the whole "test it up on blocks" thing. My dumb ass thought it was neat that the programmable transmitter allowed me to drive the motors ABOVE 100%, so I set it on 110% and just figured that I would just BARELY move the stick. What I failed to realize is that these motors are WAY restricted by the factory motor controller, so when I bypassed that with the Sabertooth it was about 3 times faster then stock even before I set it to 110%! To make things worse, I crossed 2 signal wires during final assembly making left and right into up and down and vice versa. Long story short, unless you want your robot to damn near run your cat over, and then almost tear your own leg off, on the way to smashing a dent in the wall, then use the blocks!

Also, any clue where one might get a big red emergency shutoff button that will not melt with 30A-60A running through it?
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on May 01, 2013, 05:56:59 AM
lmao. I'm not too worried about my wife's cats....I was worried about my old dog who doesn't move as fast as she used to.

I'm still looking for a BIG RED BUTTON, too. I can kill my motors by grounding one wire (thanks to the stock controller). I've got a small toggle switch on it now, and showed the wife where it is, but I'd still like to get something more intuitive.  Let me know if you finally find one.

And yep. I read about using blocks here, and started out that way, but I'll be honest...my first few tests off the blocks resulted in the chair trying to climb my leg and also pushing my workbench around. lol. I got lucky. Only a few bruises and a chance to sweep the dirt out from under my bench before I pushed it back into place. lol

John

PS I really like how you "tricked it out". The lights, etc. Your bot looks cool. Really nice job.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 01, 2013, 07:34:07 AM
Thanks J. I will keep ya posted on the off button, right now I just use two "quick connect" plugs to open the motor circuit via the ground wire to the sabertooth. I will post an update when I get more bells and whistles going. Steve

P.S. My department chair ruled that the Ohms/Watts law tattoo that is pictured on my avitar DOES NOT count as cheating! :-)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 01, 2013, 09:48:18 AM
Wow! That thing is cool!

It looks like that has lots of power to spare.

So what's phase two?

Thanks!
LOTS of power to spare! ;D
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jwatte on May 01, 2013, 02:20:05 PM
Quote
Also, any clue where one might get a big red emergency shutoff button that will not melt with 30A-60A running through it?

http://www.trossenrobotics.com/store/p/6362-Huge-Red-Panic-Fire-Button.aspx (http://www.trossenrobotics.com/store/p/6362-Huge-Red-Panic-Fire-Button.aspx)

This button does not do 60A; you'll need a relay for that. I *think* it's spdt, so you can wire it to break current; use one button to energize the relay coil; let the relay latch itself; use this button to break that latch.

(https://www.circuitlab.com/circuit/kk99u4/screenshot/540x405/) (https://www.circuitlab.com/circuit/kk99u4/power-relay-on_off/)
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Duane Degn on May 02, 2013, 12:12:25 AM
The big red button jwatte linked to was the first button that came to my mind when you mentioned you wanted a big red button. I have a couple of those big red buttons and they are cool by they might be a bit too big for your application. The red dome is also prone to scratches so you'd want to be careful how you used it.

I do think it's one of those parts just crying out to be used.

Have you seen MPJA's buttons?

http://www.mpja.com/Push-Button-Switches/products/63/ (http://www.mpja.com/Push-Button-Switches/products/63/)

I think they have some fun buttons and pretty reasonably priced.

I'm looking forward to your future progress on this robot.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 02, 2013, 09:05:49 AM
Thanks for the links and info fellas! Thant BIG red button is a bit big for the space still available in my design, but, IT IS pretty damn cool so I will use on on something at some point.
Here is a diagram of the run/charge circuit on my bot. Right now I manually disconnecting the ground wire from the battery to the motor controller to turn it off when not in use. To charge, I unplug both quick connects going from the battery's to the motor controller and plug them in to the charger's quick connects.

http://s1362.photobucket.com/user/420photo420/media/20130502_103217_zps0add7ea2.jpg.html?sort=3&o=0 (http://s1362.photobucket.com/user/420photo420/media/20130502_103217_zps0add7ea2.jpg.html?sort=3&o=0)

Where can I find a 60A DPDT (a DPDT is what I need for this correct? :-[)switch that I could wire all 6 leads (+&- from 24V battery pack, +&- from the 24V charger, +&- from the motor controller) to, and simply switch between "Run" and "Charge" mode? If we can figure this out it would double as an "off" switch by just switching it to charge mode weather the charger is plugged in or not. Thanks again for all the help! ;D Steve

Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jwatte on May 02, 2013, 12:01:04 PM
You actually only need a SPDT, as you can keep the grounds connected at all times (charger, battery, and bot, can all share a ground.)
Breaking the ground may be advantageous in some cases (if you get ground loops, say,) and may actually be a disadvantage at other times (if you use a big fuse, you may lose that protection, for example.)
60A is a lot. You need a "contactor" instead of a "relay" at that point. However, are you really drawing 60A continuous? If the 60A number is just transient spikes, and average draw is, say, 20A, then a 30A relay will serve you just as well.

You can also build a 60A breaker using a P-channel MOSFET. It might actually be cheaper than going with contactors. Just beware to specify the P-channels based on your highest load-dump voltage -- for a 24V system, 60V minimum, and ideally higher than that. (A TVS across it might help protect it, too.)

SPST isn't so bad, though. Keeping the charging port connected to the battery at all times is convenient when you have the bot on blocks and are testing.

Here are some parts from Digi-key you might want to look at:
http://www.digikey.com/product-detail/en/1393276-7/PB1319-ND/3318138 (http://www.digikey.com/product-detail/en/1393276-7/PB1319-ND/3318138)
http://www.digikey.com/product-detail/en/ADQM16024/255-2597-ND/2125665 (http://www.digikey.com/product-detail/en/ADQM16024/255-2597-ND/2125665)
http://www.digikey.com/product-detail/en/LEV100A5ANG/A101164-ND/2362833 (http://www.digikey.com/product-detail/en/LEV100A5ANG/A101164-ND/2362833)

Note the lack of insulation on the first (cheap) part! Also note that it's just barely within your specification. I'd go with the second.

Or, if you want MOSFETs: http://www.digikey.com/product-detail/en/IXTP96P085T/IXTP96P085T-ND/1995412 (http://www.digikey.com/product-detail/en/IXTP96P085T/IXTP96P085T-ND/1995412)
Just make sure to have a sufficient heat sink! (I might also put two in parallel, just to make sure there's sufficient overkill.)

Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Duane Degn on May 02, 2013, 05:06:57 PM
Those relays sure seem expensive.

I think this one would work just fine. Edit: As long as it meets your current requirements.

https://www.sparkfun.com/products/10924 (https://www.sparkfun.com/products/10924)

Since the current rating is when used with 220VAC I believe it could take even more current at lower voltages (I'm not at all sure about this). I've used them to control kitchen ovens.

I believe the above relay has a 5V coil (which you'll likely have handy). You don't wont to power the relay directly from a microcontroller pin. You'll want to use a transistor to power the relay. Never mind. You want to control the relay with a button.

Here's a similar relay with a 12V coil.

http://www.mpja.com/12VDC-SPST-NO-30A-Omron-Relay/productinfo/19059%20RL/ (http://www.mpja.com/12VDC-SPST-NO-30A-Omron-Relay/productinfo/19059%20RL/)

I'm confused why it's rated at 30A @ 250VAC but only at 15A @ 125VAC. Does that mean it would be only good for 3A @ 25VAC? I'd think it would be the other way around with the relay being able to handle higher currents at lower voltages?

MJPA has lots of other relays as well.

http://www.mpja.com/Mechanical-Relays/products/134/ (http://www.mpja.com/Mechanical-Relays/products/134/)

Solid state relays tend to last longer than mechanical relays in my experience. I doubt you'd need one but MPJA also has some SSR.

http://www.mpja.com/Solid-State-Relays/products/133/ (http://www.mpja.com/Solid-State-Relays/products/133/)

Be aware with SSR, you need to make sure you get the right kind for your needs. AC and DC SSR are not interchangeable.

Edit: I had lots of bad advice in this post. Make sure and read jwatte's post below.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jwatte on May 02, 2013, 08:48:16 PM
Since the current rating is when used with 220VAC I believe it could take even more current at lower voltages (I'm not at all sure about this). I've used them to control kitchen ovens.

That is wrong, and dangerous advice! The reason the high-amperage relays are expensive, is that the contactors have to be a lot higher quality. The reason for this is Ohm's law. The voltage switched does NOT MATTER in the current rating. The reason for this is, again, derived from Ohm's law.

U = I * R
P = U * I
-> P = I * R * I
-> P = I-squared-R

So, the heat generated by the contacts is proportional to the SQUARE of the current. For a given level of heat, the resistance (electrical or thermal) has to be four times lower for twice the current. Comparing 20A relays to 60A relays is like comparing an RC car to a Tesla Model S :-)

This is why I asked whether the 60A rating was continuous, or just occasional spikes. If the occasional spikes are less than a second, and the average is well below 20A, then a 20A rated relay may do just fine, and an auto relay or other cheap sealed relay is usable.

If the current will actually be 60A for some significant amount of time, do not use a 20A relay. The contactors will weld, and you won't be able to break contact, and the next emergency you have, the robot will run away from you, cause a traffic accident, and then have its battery burst into flames, burning down the nearby daycare center. If you still do this, make sure to have a camera at the ready; perhaps the YouTube advertising gains will help defray the costs of settling the lawsuit :-)

The voltage rating has to do with the arcing that happens as the relay makes and breaks contact. A higher voltage rated relay is "better" at making/breaking contacts, and might be expected to mechanically last longer if used below its rated voltage. I don't know the exact physics of this, though, so if you're really interested, go to an Omron seminar or something ;-)

Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Duane Degn on May 02, 2013, 10:37:14 PM
That is wrong, and dangerous advice!

Yes, you are absolutely right. Thank you very much for catching my error. (At least I expressed some reservations about the advice.)

I was thinking about the problem wrong. I was thinking the voltage of the system somehow influenced the power in the P = I*V equation so a lower voltage system would increase the current carrying ability of the relay. But the voltage I should have used in any power calculation was the voltage drop across the relay (current times resistance of the relay) so you're right about the current being the important factor.

Thanks again for minimizing the damage my dumb advice could have caused.

Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 03, 2013, 10:27:11 AM
Thanks for the links Duane :)

J, as for the REAL Amps I am not too sure. The stock power-chair had a 30A re-settable in line fuse, so I was worried that the 25A Sabertooth motor controller might not cut it so I went to the next bigger I found and that was the 60A Sabertooth. I found out that "full throttle" on the power-chair was WAY slower (restricted for safety reasons no doubt) then what the motors are actually capable of, so I did away with the 30A fuse (bad move, I know). How can I measure peak amp draw when the only way I can get a meter on it is with it up on blocks and I am sure that the weight of the bot will increase the actual amp draw. Like I said not to sure....I suppose this is where I need to start so that I can put the fuse (or a bigger one) back in line somewhere and also so I can choose the right switch. 20ft leads on my amp meter maybe? ;) Thanks again!
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jwatte on May 03, 2013, 11:51:19 AM
Quote
How can I measure peak amp draw

Get a bench dyno? :-)

Clamp the motor output shaft with steel clamps. Get a really beefy power supply. Put the multimeter across the terminals, measuring resistance (when not powered.) Then measure voltage (when powered, and stalled.) Your current draw is resistance times voltage voltage divided by resistance.
Note that stalling the motor for any large amount of time is likely to overheat it / burn it out.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 08, 2013, 09:40:13 AM
Hello all, this is the switch I went with.
http://s1362.photobucket.com/user/420photo420/media/20130508_102303_zps71f5209d.jpg.html (http://s1362.photobucket.com/user/420photo420/media/20130508_102303_zps71f5209d.jpg.html)
http://s1362.photobucket.com/user/420photo420/media/20130508_102208_zps363a2032.jpg.html?sort=3&o=1 (http://s1362.photobucket.com/user/420photo420/media/20130508_102208_zps363a2032.jpg.html?sort=3&o=1)
It was a bit pricey from ZORO.com but it is VERY high quality and HEAVY duty construction, it works great!

Here is how I wired it up so now I have "run" mode as well as "charge/off" mode.
http://s1362.photobucket.com/user/420photo420/media/20130508_102241_zps49f8d4ab.jpg.html (http://s1362.photobucket.com/user/420photo420/media/20130508_102241_zps49f8d4ab.jpg.html)
http://s1362.photobucket.com/user/420photo420/media/20130508_102326_zps76b50f2f.jpg.html?sort=3&o=5 (http://s1362.photobucket.com/user/420photo420/media/20130508_102326_zps76b50f2f.jpg.html?sort=3&o=5)
I used my adjustable power supply (such as it is) and my Volt meter to put 5V on it and test it to get a feel for how it worked.
http://s1362.photobucket.com/user/420photo420/media/20130508_113243_zpsf356fd52.jpg.html?sort=3&o=0 (http://s1362.photobucket.com/user/420photo420/media/20130508_113243_zpsf356fd52.jpg.html?sort=3&o=0)

Next I cut the whole bird nest of wires that you cann see in the "phase one" video I posted off and re-wired it all so that it is more smooth looking and easier to work on.
http://s1362.photobucket.com/user/420photo420/media/20130508_102737_zps5239506d.jpg.html?sort=3&o=11 (http://s1362.photobucket.com/user/420photo420/media/20130508_102737_zps5239506d.jpg.html?sort=3&o=11)
http://s1362.photobucket.com/user/420photo420/media/20130508_102719_zps3437b044.jpg.html?sort=3&o=10 (http://s1362.photobucket.com/user/420photo420/media/20130508_102719_zps3437b044.jpg.html?sort=3&o=10)
http://s1362.photobucket.com/user/420photo420/media/20130508_102418_zpsf460593c.jpg.html?sort=3&o=9 (http://s1362.photobucket.com/user/420photo420/media/20130508_102418_zpsf460593c.jpg.html?sort=3&o=9)
Looks much better this way.

I also got the mount for my mini water pump working from my transmitter, this will give me a "up down" control, and I will use the bot to move left and right.
http://s1362.photobucket.com/user/420photo420/media/20130508_102336_zps124b1624.jpg.html?sort=3&o=6 (http://s1362.photobucket.com/user/420photo420/media/20130508_102336_zps124b1624.jpg.html?sort=3&o=6)
http://s1362.photobucket.com/user/420photo420/media/20130508_102355_zps298d0a5f.jpg.html?sort=3&o=7 (http://s1362.photobucket.com/user/420photo420/media/20130508_102355_zps298d0a5f.jpg.html?sort=3&o=7)
http://s1362.photobucket.com/user/420photo420/media/20130508_102403_zps9fc52714.jpg.html?sort=3&o=8 (http://s1362.photobucket.com/user/420photo420/media/20130508_102403_zps9fc52714.jpg.html?sort=3&o=8)
I will post a video of it once it is mounted and working.

 
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 10, 2013, 09:19:43 AM
Here are the 2 switches (glowing green) and 2 volt meters that I wired in.
http://s1362.photobucket.com/user/420photo420/media/20130510_105930_zps63565ccf.jpg.html?sort=3&o=0 (http://s1362.photobucket.com/user/420photo420/media/20130510_105930_zps63565ccf.jpg.html?sort=3&o=0)
http://s1362.photobucket.com/user/420photo420/media/20130510_105937_zps26f3f01b.jpg.html?sort=3&o=1 (http://s1362.photobucket.com/user/420photo420/media/20130510_105937_zps26f3f01b.jpg.html?sort=3&o=1)
http://s1362.photobucket.com/user/420photo420/media/20130510_105945_zps3682544f.jpg.html?sort=3&o=2 (http://s1362.photobucket.com/user/420photo420/media/20130510_105945_zps3682544f.jpg.html?sort=3&o=2)
One turns on the volt meter to the main batteries and the other turns on the headlights and taillights as well as the accessories battery volt meter.

I am now mounting the water tank (a rabbit water bottle) under neath my bot (now I need a wheelie bar to protect it from scraping when I wheelie) I am now sorting out the plumbing, and getting the water pump relay to work via my RC remote.

Any suggestions for other cool stuff that might fit on there? Thanks, Steve.
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on May 18, 2013, 07:46:26 AM
My bot made a friend!

http://s1362.photobucket.com/user/420photo420/media/20130513_171434_zps533a5650.mp4.html (http://s1362.photobucket.com/user/420photo420/media/20130513_171434_zps533a5650.mp4.html)

I think the next one (not even quite done with this one yet) I build will be a RC lawn mower. Yes it has been done before but I would love to cut grass from the shade!
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: jlizotte on May 18, 2013, 07:32:25 PM
lol. made a friend. that's the perfect caption for that video. lol.

J
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: Sisco on June 17, 2013, 08:52:40 PM
Does your bot have any sensors or is it just R/C?

Rick
Title: Re: First attempt at a RC bot, PLEASE help! :-)
Post by: ROBOT420 on June 20, 2013, 06:49:55 PM
Does your bot have any sensors or is it just R/C?

Rick
[/quote

Just RC for now. I would like to build a quad rotor that just hovers about 4 feet above it though.... LOTS of room left on there for expansion.