Motors slow down using shield after a few minutes

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Ai_mentor
 
Posts: 3
Joined: Tue Apr 10, 2012 9:42 pm

Motors slow down using shield after a few minutes

Post by Ai_mentor »

Hi,

I am using an Arduino Mega with an Adafruit motor shield. I’ve got 4 Tamiya high power gearboxes connected to the motor shield.

The motors say they draw .15A with no load .64 A at maximum efficiency and 2.7A if stalled and will work with voltages from 1.5 to 4.5 but are best at 3V. I know the motor shield is rated at .6A per channel and 1.2A peak, so I am using a speed setting of 115 which is about 45% of the maximum speed. My thinking is that at maximum efficiency I’ll draw about .28A and 1.22A when stalled because that’s about 45% of the original specs. I am using a 7.2V NiMh battery to power the Arduino and motor shield.

All starts great when I run it, but after a couple of minutes the motors seem to slow down and after 3 or 4 minutes of continuous running the robot comes to almost a complete stop. I’ve got the robot running a square pattern. Weird thing is that if I take the robot off the table that’s covered with felt and put it on the floor which is smoother, it seems to run fine. I don’t even turn it off when moving between surfaces. I’ve even gone the other way from floor to felt without turning the robot off and then for a while it seems to do ok on the felt but eventually slows down to nothing, but works again when placed on the floor.

The L293’s get quite hot about 55 degrees celsius, and I’ve put clip-on heatsinks on each. I think without the sinks and at a speed setting of 255 the motor shield would die. Here’s a code snippet:

const int NSMaxTime = 1500, EWMaxTime=1000;

void loop()
{
goForward();
delay(NSMaxTime);
goRight();
delay(EWMaxTime);
goBackward();
delay(NSMaxTime);
goLeft();
delay(EWMaxTime);
}

Any thoughts/help would be greatly appreciated.

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adafruit_support_bill
 
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Re: Motors slow down using shield after a few minutes

Post by adafruit_support_bill »

Have you measured the actual current draw? There is probably a lot more friction on the felt surface, so you are going to be pulling more current and draining the battery faster than on the smooth floor. What is the capacity of the battery you are using? It sounds like you are either draining the battery, or pushing the L293D's to the edge of thermal shutdown.

If you are pushing the limits of the L293D, you might consider going to a piggy-backed H-bridge:
http://forums.adafruit.com/viewtopic.php?f=31&t=26873
Image

Ai_mentor
 
Posts: 3
Joined: Tue Apr 10, 2012 9:42 pm

Re: Motors slow down using shield after a few minutes

Post by Ai_mentor »

Thanks for the quick reply. I tried some new L293D chips made by ST and put a heat sink on each and they worked great. For the record, I am using a 1600mAh 7.2V battery.

Weird thing is that the original chips are L293DNE by Texas Instruments and they don’t work even with the heat sink. Is the quality or temperature shutoff different between the two makers or models?

Thanks again for the help, I’ll try the piggyback and piggyback with heat sink ideas with my other robots.

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adafruit_support_bill
 
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Joined: Sat Feb 07, 2009 10:11 am

Re: Motors slow down using shield after a few minutes

Post by adafruit_support_bill »

I have not noticed any significant difference in performance between the various suppliers of the L293D chips. But I tend to the conservative side when specifying motor controllers. IME, chips from major manufacturers generally meet or exceed the advertised specs. Unfortunately, many motors exceed advertised specs also.

Ai_mentor
 
Posts: 3
Joined: Tue Apr 10, 2012 9:42 pm

Re: Motors slow down using shield after a few minutes

Post by Ai_mentor »

Thanks again for the help.

I just tried to piggyback two of the chips from Ti with a heat sink, and I was able to run the robot for 30 minutes before the batteries started to die. It's an omni wheel robot so I had all 4 motors running continuously on the high friction felt surface. I was happily surprised to see the 1600 mAh batteries hold out so long and the shield not overheat.

I am just posting this hoping anyone else using Tamiya motors can benefit from this. I had a lot of trouble with these motors and couldn’t find replacements for the ones in the high power gearboxes. They’re not like the 130 size motors in the other Tamiya gearboxes.

Oh, and I found the datasheets on the Ti and ST L293DNE and L293D chips. It looks like the Ti chips are good till 70 degrees and the ST are good till 80 degrees, if I've read the specs correctly.

Thanks!

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