Making an RC Mower -- need advice

[bbsux]

I am going to make an RC Mower - similar to the one on this site:
http://members.iinet.net.au/~tnpshow/RCLM/intro.htm

except he didn't do several things....

I would want a alternator to recharge "some" of the juice I am using.  I guess it would be easiest to just cut a hole behind the motor, weld a bearing in place, and put a shaft on it with a pulley attached to the motor shaft.

He jury-rigged the wheel chair control system by making an interface board to send it signals like the joystick is still there.  I'm not sure thats reliable.  I will be using a spektrum BR6000 & dx6.  I would design my own system with controllers but I'm having trouble finding ones that will handle the amps I will be using.


I'm also having problems finding specs on motors.  I found two that look like they would be perfect.
They are off an Invacare E690a (left motor Part # 1039759)  Invacare lists the motor but gives no specs.
any ideas?

Thanks guys

[nanob0t]

I do not understand this application.  It is neat, but why would you run a robot vs doing it yourself?  I would rather make it autonomous.  I guess it would make the work more fun.

With the alternator.  Instead of hacking the motor apart, maybe it would be possible to attach a pulley to the shaft of the motor right before the wheel to run the actuator.  Ripping the motor apart and installing all of that may be a bit risky to the robot, and may require routine maintence.

He jury-rigged the wheel chair control system by making an interface board to send it signals like the joystick is still there.  I'm not sure thats reliable.  I will be using a spektrum BR6000 & dx6.  I would design my own system with controllers but I'm having trouble finding ones that will handle the amps I will be using.


I'm also having problems finding specs on motors.  I found two that look like they would be perfect.
They are off an Invacare E690a (left motor Part # 1039759)  Invacare lists the motor but gives no specs.
any ideas?

I am unclear on the motor stuff you are talking about, a link would be nice if you could provide it.  It may be better just to purchase the motors themself.  This way you are clear on the torque, velocity and such you require and can optimize power.  Doing this, you have all of the specs right in front of you. 

Sotu also started a thread on hooking a wireless controller up to motors.  I think that would be useful to look at.  It gives a basic explaination and may sway your thinking.  Again, I am unfamiliar with the spektrum BR6000 & dx6, a link would be nice.

With a little more information, I'm sure we can help you more. 

[bbsux]

I never said I was going to hack the motor apart to install the alternator.  I said I was going to "cut a hole behind the motor, weld a bearing in place, and put a shaft on it with a pulley attached to the motor shaft."  That is obviously cutting a hole in the mower chassis NOT the motor.

Maybe you trust your "autonomous robot", but I am NOT going to trust what can be a lethal machine to a program on a chip.  I would much rather use it remote control because that has safety shutoffs in case signal is lost.

[nanob0t]

I guess I did not see what you meant with the motor.  If that works, then that will work fine.  I thought you were going to drill a hole in the motor and do... whatever 

Well, autonomous can be as safe as you make it.  If you map out your yard on it then tell it to mow, I agree, it can be lethal.  But if you put ultrasonics, or a camera or something onboard, you can develop it to avoid something like that.  You can put a wireless kill switch on it too.  It was just a suggestion.

Well, links would still be nice.  If I can get an idea what motors you are looking at, I can probably dig something up or find something better if it comes down to it. 

[bbsux]

Well, links would still be nice.  If I can get an idea what motors you are looking at, I can probably dig something up or find something better if it comes down to it. 

I gave you the model number and brand name of the motor in the first message.  If I could find out anything about them I wouldn't need to ask.

[nanob0t]

Okay, I just assumed you had a link.  Hmmm, after intense searching I found it!  Hmmm, it looks like a high-quality motor.  It better be high quality considering you're paying 580$ for it.  They also seem to be discontinued.  I don't know if I'm looking at the same site as you.  I have no doubt that these motors will be sufficient for you.  They are looking to carry a load of ~200lbs from a person sitting on top and sustain a decent speed.  You play with them and increase the velocity of them if you want due to the fact that they don't have to lug around a body.

That reciever seems plausable too.  It would be simple to interface it with the motors on your robot.  Well I found an article that if you live in California and you purchase one and are unsatisfied, you can refund it because of some crap.  Completely irrelevant.  If you buy it at the site you can get it $10 cheaper, probably still irrelevant. 

It is made for robot applications, so I see no reason why it would be bad.  You do have a fuel engine on the robot, which might cause some issues with possible circuitry.  I couldn't find any data sheets on either of the equipment, so I don't know what to say.  It has a kill switch, so if it randomly decides it wants to kill your neighbor's dog, you can stop it.  Always a good thing. 

Amperage might be a problem.  If anything, it might be worth it to get a motor controller that can handle it.  Then you input stuff into the reciever, which just pulses to the motor to move, whatever.  The motor controller will be capable of handeling the amperage too.  I have a very large robot that runs on two lawn mower batteries that I'm working on for a company and this is what it uses. 

[bbsux]

I wonder if I can run the motors at 12 volts instead of 24?  That would cut the Speed problem down some.

BTW I am getting the whole chair FREE.  The best thing....!!! LOL

I intended on isolating the electronics inside a waterproof box to minimize the sparking/fuel hazards...

[nanob0t]

Oh.  Getting a free wheelchair is always nice     I guess that eliminates everything I was rambling on about.  That's a bunch of money saved.


You can probably get the specs of the motor when you get it.  It may have it on the motor.

The electronics are fine.  A waterproof box would be good, just don't let the heat collect up within the box.  Heat + Electronics = Bad 

I take it since the motor came from a wheelchair it has the electronics to control within the motor?  I don't know...  It's late.  Gotta sleep.  I had too much fun building my robot today 

[sotu]

I wonder if I can run the motors at 12 volts instead of 24?  That would cut the Speed problem down some.

BTW I am getting the whole chair FREE.  The best thing....!!! LOL

I intended on isolating the electronics inside a waterproof box to minimize the sparking/fuel hazards...

24 to 12? Wow woulden't that change the speed extremly!?

[ed1380]

lowering the volts makes the motors weaker
using pwm keeps the torque the same but you can control speed

[Admin]

check the motor specs for torque at 12V and calculate if that is ok for you.

the motor will be much weaker, but at the same time you only need half the battery weight . . .

[Somchaya]

lowering the volts makes the motors weaker
using pwm keeps the torque the same but you can control speed

Hmm, this might be a silly question, but how does pwm for dc motors work exactly?

If you have a 3V motor, and you want it to go at half speed, you pulse 3V, 0V, 3V, 0V, etc? Wouldn't that mean that when you pulse 0V the motor has no torque?

[Admin]

Hmm, this might be a silly question, but how does pwm for dc motors work exactly?

If you have a 3V motor, and you want it to go at half speed, you pulse 3V, 0V, 3V, 0V, etc? Wouldn't that mean that when you pulse 0V the motor has no torque?

motors are just really big inductors. even though the power is a square wave from 3V to 0V only, the voltage across the motor takes a specific amount of time to transition between those states so it still outputs torque. its kinda like a charging and discharging capacitor . . .

but i dont really understand why the torque stays the same and the speed decreases . . .

[nanob0t]

Hmm, this might be a silly question, but how does pwm for dc motors work exactly?

If you have a 3V motor, and you want it to go at half speed, you pulse 3V, 0V, 3V, 0V, etc? Wouldn't that mean that when you pulse 0V the motor has no torque?

A PWM works by sending a certain width of pulse to an object, hence the name.  If you had a 3V motor (for example), and sent 3 volts to it using two AA batteries, it would be running at optimum speed.  If you took them off, it would be running with 0 volts and no speed.  A pulse width modulator is put in between the batteries and the motors and it sends a certian width of pulse to the motor.  It sends this pulse so many times during a second, that it is not noticable by a robot.  It does not come to a complete halt, because the pulses aren't allowing it stop.  Here's a visual representation:


---------------------------   This is high, the motor running at optimum speed

_|-|_|-|_|-|_|-|_|-|  Now there are pulses (very very crude) being sent.  They are going at 1/2 of the rate at which the electricity would normally run, so the motor runs at 1/2 the speed.


I believe the same amount of torque is applied, because the robot is still traveling the same amount of distance, and because it is still running high at that 3 volts, the torque remains.  If you were to cut it down to 1.5 volts or something, the torque would decrease due to the decrease of voltage to the motor.