Author Topic: Arduino and external power source  (Read 1279 times)

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Offline itmth19Topic starter

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Arduino and external power source
« on: May 31, 2012, 08:09:45 AM »
I have a sketch of H-bridge controlling 2 DC motor. 2 have two questions:

- The motor is Mabuchi FA 130 DC motor, I got 2 motors with external power of 3V( 1.5 V * 2 instead of 9V *1 as in the tutorial). When i raise power from LOW to HIGH to pins to control the two motors, I saw them rotate at the speed which is slower than the speed when I have one motor connected directly to one 1.5V battery. My question is WHY? Where else did the power(current) go?

- Last week, I use a microchip using Cortex 3M (Beauto Rover kits - Japan), which also controls those two motors with those power source of 3V ( 1.5V*2). However, the car robot could drive on the floor without no problem. However, when I change the gear type for better speed and but lower tor..(I don't know ow to spell it), replace the micro with arduino with H-bridge, , the wheel could turns faster but the car could not drive on the floor? Why was THAT?

- When designing robot, If I choose not to use gears, just motors with wheel and axle, is it better or more worse?

I'm in kinds of learning now :)) robot is interesting.

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Quote
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1231328526/0#0
Is it because of Power Supply??
« Last Edit: May 31, 2012, 08:21:56 AM by itmth19 »

Offline mstacho

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Re: Arduino and external power source
« Reply #1 on: May 31, 2012, 09:42:30 AM »
Howdy:

By increasing the speed, you decrease the torque.  What that means is that, if the motor has no load on it at all, it will spin faster.  Now, let's consider a motor with a load of, say, 1 pound.  The motor with the lower torque MIGHT spin that load slower than the motor with the higher torque (it uh...depends on lots of stuff, but in your case, this is why the motor can't move the car).

Torque is like strength.  If you are stronger, you can lift more.  But speed is like the ability to sprint.  You might be a great sprinter, but you can't lift a lot, and a 300 pound weight lifter might not be able to run that fast.

I THINK that the motor is slowing down with the battery because the current draw of both motors is too large for the batteries to handle.  Maybe...are they in parrallel or in series?  If they are in parrallel, you should only get a small decrease in speed, accounting for losses and things.  If they are in series, then you'll get a big decrease in speed.

MIKE
Current project: tactile sensing systems for multifingered robot hands

Offline jkerns

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Re: Arduino and external power source
« Reply #2 on: May 31, 2012, 10:12:56 AM »
When you use an H bridge to control the motor, the current flows from the battery, through some kind of switching device (Bipolar transistor, Mosfet, whatever), through the motor, then through another switching device, then back to the battery.

The transistors used for switching are not perfect switches, there is some voltage drop across them - typically a few tenths of a volt - depends on the exact device. As a result the motor doesn't see the full battery voltage. Now, if you are using a 12 volt power supply, loosing a few tenths of a volt across each switch is no big deal. But you are using only a 3 volt supply. If you lost, for example, .3 volts across each switch, then the motor would see 2.6 volts instead of 3 volts - a 20% drop. So it runs slower.

You might want to look carefully at the data sheet for the components you are using to see what kind of voltage drop you are getting (or measure it with a voltmeter). Then you could increase the voltage of your power supply to compensate. 4.5 volts (3 cells) may be pretty close to what you need to get the full 3 volts at the motor.
I get paid to play with robots - can't beat that with a stick.

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Offline itmth19Topic starter

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Re: Arduino and external power source
« Reply #3 on: May 31, 2012, 06:36:39 PM »
Quote
If they are in parrallel, you should only get a small decrease in speed, accounting for losses and things.  If they are in series, then you'll get a big decrease in speed.

Could you explain more about this? I thought that batteries in parallel schematic has U = sum(Ui) and in series has Ui maintains the same?

@jkerns: The information of the DC motor
Quote
Operating voltage 1.5VDC to 3VDC. At 1.5VDC no load output is 9100RPM at 200mA, stall torque is 26g-cm (0.36oz-in) at 2.2A and maximum efficiency output is 6990RPM delivering 6g-cm (0.08oz-in) torque while drawing 660mA current.
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- Could I have a external battery of 9V for the 2 motors at the same time? I don't know exactly how the two motors share the external battery power, because I saw in the H-bridge schematic at
Quote
http://itp.nyu.edu/physcomp/Labs/DCMotorControl
, it has one Motor external power supply, and if I use, let's say 4.5V external battery? How many volts does it motor get?

- In the H-bridge, I understand that we have to use the diode to have the current flow only from one direction in order for the current not to flow reverse into the board when the DC motor begins to stop. However, is it the problem at the 3V, 4.5V or 9V supply that gives damage to the Arduino board or the problem is from the "reverse current" that goes from the other side into the Arduino board?

Offline mstacho

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Re: Arduino and external power source
« Reply #4 on: June 01, 2012, 07:23:11 AM »
Sorry, I meant if the MOTORS are in series or parrallel.  Although now that I think of it, I can't imagine a time when you'd ever put motors in series :-P -- the voltage drop across the first motor would kill the voltage of the second, so you get a wildly unpredictable voltage divider.  Although come to think of it, it would be cool to watch...

MIKE
Current project: tactile sensing systems for multifingered robot hands

Offline jkerns

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Re: Arduino and external power source
« Reply #5 on: June 01, 2012, 10:49:33 AM »

@jkerns: The information of the DC motor
Quote
Operating voltage 1.5VDC to 3VDC. At 1.5VDC no load output is 9100RPM at 200mA, stall torque is 26g-cm (0.36oz-in) at 2.2A and maximum efficiency output is 6990RPM delivering 6g-cm (0.08oz-in) torque while drawing 660mA current.
.
- Could I have a external battery of 9V for the 2 motors at the same time? I don't know exactly how the two motors share the external battery power, because I saw in the H-bridge schematic at
Quote
http://itp.nyu.edu/physcomp/Labs/DCMotorControl
, it has one Motor external power supply, and if I use, let's say 4.5V external battery? How many volts does it motor get?

- In the H-bridge, I understand that we have to use the diode to have the current flow only from one direction in order for the current not to flow reverse into the board when the DC motor begins to stop. However, is it the problem at the 3V, 4.5V or 9V supply that gives damage to the Arduino board or the problem is from the "reverse current" that goes from the other side into the Arduino board?


9 volts seems to be a bit much.

Each motor should get power from the battery independently and get the same voltage.  I would suggest looking at the H-Bridge tutorial on this site: http://www.societyofrobots.com/schematics_h-bridgedes.shtml and sliding down about a screen or so to "ghetto visual graphic of the H-bridge logic chart"

The top drawing (with the red typing)  shows how the current flows during operation - it comes from the battery, through a semiconductor device (in this case a MOSFET), through the motor, through another semiconductor then back to the battery through ground.  You get a voltage drop through each of these elements that the current passes through - the exact voltage drop would depend on the particular semiconductor device that you are using - I would look at the data sheet to see what you can find.

I'm not sure that I understand the last question. If the current / voltage induced as the magnetic field of the motor collapses when you turn it off results in damage it would seem most likely be to the switching devices in your driver circuit.  Or it may just create "noise" that causes problems like bad A/D readings.
« Last Edit: June 01, 2012, 10:51:19 AM by jkerns »
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

Offline itmth19Topic starter

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Re: Arduino and external power source
« Reply #6 on: June 01, 2012, 08:00:38 PM »
@mstacho: no it's ok. Now I got your idea. I'll try some day ;D

@jkerns: 3*1.5V battery in series would be good:-? I understand the case of filtering noise for A/D inputs, thanks you a lost.

In the case of H-Bridge SN754410NE
Quote
http://www.ti.com/lit/ds/symlink/sn754410.pdf
, I have some questions. (May I start another topic, because I saw it does not relate to external power source anymore). Anyway, My thought is that because SN754410NE is a H-bridge, so it has internal diode (4 diodes for preventing back currents) and of course, 4 transistors right? I just wonder how could that have one small H-bridge with lots of stuffs like that:))

I saw the schematic you request
Quote
http://www.societyofrobots.com/schematics_h-bridgedes.shtml
uses MOFSET for example. I wonder If I use one MOFSET to control 1 DC motor, I could not control reverse rotate but just motor speed right? In that case, I would have a capacitor parallel with the DC motor to filer noise and another transistor also parallel to prevent back current?

 


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