Motorshield & motorpower

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eclipse
 
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Motorshield & motorpower

Post by eclipse »

Hey,

I'm building a robot out of an old rc car.
i don't know the specs of the motor (because there from the(old) rc car) but i want to drive them with the adafruit motorshield.
what i do know is that the battery that was enclosed with the rc car is a 6V NiCd.
So my question was:can i hookup the battery directly to the motorshield ? would it also be possible to connect a 7,2 V LiPoly battery to the motorshield to drive the (expected 6V) motors ? (a.ka. will the higher voltage automatically reduced by the shield ? )

and another question about the motorshield:
on the photo with the notes there are two "power related" connections:
- External motor power (screw)
- dc adapter or ext. power
which one do i connect the 6v (or 7,2V) battery to ? (i'm guessing the External motor power duh), but why then is there the other ext. power component ? the shield is connected to the arduino so there is no need for extra power for the logics on the motorshield right ?

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

7.2v is not likely to kill a 6v motor unless you go full throttle all the time. But I'd measure the current draw of your motor. Some of those RC cars need much more current than even a souped-up motor shield can reasonably deliver.

There are two power related things on the shield. The "EXT PWR" screw terminals are where you connect the battery (be sure to get the polarity right or you will fry something!). The 2 pins labeled "PWR" are for a jumper. Remove the jumper to run the motors on external power.

eclipse
 
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Re: Motorshield & motorpower

Post by eclipse »

thanks arduwino,

despite the fact i payed attention in physics class :oops: : how can i measure the current draw of the motor ?

ehm... the battery i was planning to use is actually 7,4V : http://dealextreme.com/details.dx/sku.7621 and i wasn't planning on going full throttle, so this might work right ?

now i see the answer to the "power related" things question, i see my question was quite stupid :P and i don't know why i didn't saw it rightaway.

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

The easiest and best way to measure the power is with a multi-meter. If you don't already have one, it is an essential tool for stuff like this.

You can tweak the motor shield to handle close to 2A for intermittant use. Beyond that, you should be looking at some of the heavy duty motor drivers from places like Pololu.

eclipse
 
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Re: Motorshield & motorpower

Post by eclipse »

Hey thanks again.
Well, i'm a noob (in electronics, not in programming ), but i have a multimeter :) .... but how to measure the current that the motor draws ?
it's not as simple as just power up the motor and then with a multimeter to + and - and whoop! or is it ?!

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

First make sure you plug the leads into the proper places. Most multimeters have a dedicated jack for current measurement. You can damage things if you use the wrong jack.

The best reference would be the manual for your meter. But here is a generic multimeter tutorial:
http://mechatronics.mech.northwestern.e ... meter.html

mmmalmberg
 
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Re: Motorshield & motorpower

Post by mmmalmberg »

I don't mean to hijack the thread, but I was about to post a very closely related question. How high a voltage can I plug into the external power terminals?

I have, unfortunately, a mix of motors I need to power. One brushed DC motor needs 12V, one stepper needs 12V ('though not set on this motor) and one hobby servo wants maybe 7 volts. Does the external power input feed directly through to the hobby servo power, as well as the other motor power feeds?

Thanks and I hope the answer is useful to the original poster as well!

-Mark M.

mmmalmberg
 
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Re: Motorshield & motorpower

Post by mmmalmberg »

Also what is the max current available on the hobby servo ports?

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

The 2 sets of servo pins on the motor shield draw power from the arduino regulated supply - regardless of any external power you might be using for other motors.

Servo power is limited to what your Arduino regulator can supply (minus whatever your arduino and other connected components need). Servo current demands are not steady state and peak loads can cause your arduino to reset. One or two standard servos are OK, as long as you don't drive them too hard. Large or heavy duty servos can cause reset problems, as can cheap BANNED-brand servos.

mmmalmberg
 
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Re: Motorshield & motorpower

Post by mmmalmberg »

So what I'm thinking is I'll make a separate power supply for the servo (big heavy duty thing that wants 6-9VDC) and use just the control line from the motor shield's servo out pins with separate power to the servo power feeds. That would work, yes?

(and beyond that I'll step out of this thread and start a new one if needed...)

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

That would be ideal.

eclipse
 
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Re: Motorshield & motorpower

Post by eclipse »

i haven't been able to test things yet (because i'm sick), but i'm sure i'll come up with another question. :wink:

eclipse
 
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Re: Motorshield & motorpower

Post by eclipse »

Hey,

As expected i've come up with new info and questions:

i've connected the motors to a trafo and measured:
300mA current.
start working at around 1,7V (very slowly) and about 6,5 was the highest i dared to go (but the motors weren't heating up or anything) but i'm guessing BANNED will to work till around 8 V.
(the original battery for the rc-car motors is 6V NiCd).

Question: what battery should i buy ? i've found a 3,7V Lipo and a 7,4V lipo... (i know the motor shield is 'rated' for minimal 6V motors, but my robot doesn't need to go very fast).

then a question about pwm: i'm guessing that the method setSpeed(...) is actually a implemented pwm method ? so that is the only way to control the speed (and no more (pwm)methods are available)? or am i completely wrong ?

Grtz

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adafruit_support_bill
 
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Re: Motorshield & motorpower

Post by adafruit_support_bill »

There are plenty of 6v NiMH packs available. But I think you'd be fine with the 7.4v pack.

SetSpeed() controls the pwm duty cycle. The pwm frequency is set when you declare the AF_DCMotor:

Code: Select all

AF_DCMotor motor(1, MOTOR12_64KHZ); // create motor #2, 64KHz pwm

eclipse
 
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Re: Motorshield & motorpower

Post by eclipse »

i've been testing with my trafo & battery's

i used a 9V battery (although i know it is absolutely not recommended) to drive the arduino & motors.
it seems the battery can drive the car forward and backward but it's not strong enough to make it turn (left wheels forward, right wheels backward).
the motors get the wheels spinning allright, but that's when i hold it in my hand. when i put in on the ground it only runs slowly back and forward and not turning.
one of my thoughts was that the motors weren't strong enough but that would be strange because they are the original motors in the original rc car.

i also tried driving the motors with the orininal 6V NiCd battery and the arduino with the 9V battery, but that wouldn't make it turn either.
The last time i tested it with my trafo i didn't put it on the ground because the short powercable doesn't allow it to drive around.
the motors (connected on 1 & 2 and pwm freq. 64 kHz) start running earlier but are only able to drive the car when i set speed to about 200.

i don't know what to try next. is it possible that the motors need more Voltage than orinigal in the rc car because now they are controlled via pwm ? anyone has an idea about the motors or battery and especially about getting them to work ? thanks.

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