Let's Make Robots!

Robot gutless and slow....

jair2k4's picture

Ok, so I finally got him put together, and it turns out that he doesnt move worth a damn.... I cant figure out what I did wrong. The tamiya twin motor is set for low range, but it still shouldnt be THIS slow... turning is almost impossible for him... Even on smooth flooring.... I shot a quick video for you all to see and hopefully help me figure out what is going on...


This is the H bridge I am using... SN754410   from sparkfun. The data sheet said i could save pins by hooking the enable pins up to a 5v source, so I did that..... VCC1 is connected to the 5v on the arduino, Vcc2 is hooked up to the same power source as the motors/IC, with common ground. My arduino has its own power source. Ignore the wire coloring, I only had 2 colors and the ones i used here were for my reference, not to indicate hot and ground.

 Power for the board is 4xAA alkaline batteries. Power for arduino is 1 9v battery. 



I have no idea what to do here....
Can anyone help? Thanks in advance.

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robologist's picture

A wider stance allows an easier turn than a narrow stance for tracks. Think of the track contact in a turn. If they are close together, more of the "push" force vector of the track is wasted in scrubbing across the surface, perpendicular to the track. If the tracks are further apart, the radius of turn allows more of the track force to be used to move the robot rather than be wasted in skid.

The tightness of the tracks is something to definitely lessen, but not so much that the tracks are easily thrown. 

Most motors will spin faster when presented with no load (ie : off the ground) so this isn't really a good test. If spinning the same, it does sound possible that the tracks are too tight.  

Ro-Bot-X's picture
Yes, you're right about the stance... Mea culpa, I was thinking actually about the length of the track... Oh well, seems like I should refrain to post when I'm tired... Thanks for correcting me.
jair2k4's picture

I have one more 16 pin ic holder laying around somewhere... maybe i should rebuild my h-bridge and see what happens?


Its the only thing i havent thought of doing.... aside from upping the voltage beyond 5v. 5 AAA's would be around 6v for yours to run.... right?

 I've just finished charging 5 NIMH rechargables. I am going to go put my AA battery holder back on instead of my 5v psp battery pack i was using and see what happens. I tried this with normal AA's before though and it didnt help at all. I've rebuilt his gearing to the higher ratio and re-did his tracks already.... it really is starting to seem to me like there is something wrong with the H bridge.....



jair2k4's picture
Ok, I ordered the motors from pololu that have the higher rated voltage. Think this will solve my problem? I should be able to use a 7.2v battery pack with no issues, but will using the current 5v rechargeable battery pack i have work as well? I'd really like to get this thing moving fast with the motors i have right now.... even if it burns them out. any thoughts?
ezekiel181's picture
This robot used the standard tamiya motors with 5 AAA rechargables and an L293NE (no diodes on chip) and ran fine. Your problem is not the motors.
robologist's picture

The flyback diodes suggested have absolutely nothing to do with the performance of the motors with the current power source. Additionally, the 754410 does have diodes on the outputs, which are described as ESD diodes, but no one has been able to describe to me what the difference is between a diode that conducts ESD and a diode that conducts flyback spikes. 

The main problem has somewhat been guessed at, a combination of batteries with low current capabilities, as well as the FA130 motors being somewhat current hogs and meant to run at 3 volts.  Pololu has some documentation on the quality of the FA130 motors, as well as some better quality replacement motors that run reasonably well at 6 volts.

There could be some additional problems with crossed wiring, but it really appears that rechargeable batteries should be used, either NiCd or NiMh, that actually have better current characteristic than alkalines. Note that if increasing voltage it will correspondingly increase current (Ohms law) and have the possibility of popping the h-bridge and/or burning out the current motors.   

jair2k4's picture

I think i may have got it figured out.... these motors just suck. I need to push more voltage to these things. I will have to find some sort of improved battery to use. I've got a rechargeable 5v battery from a psp accessory that I have crammed into an altoids can. With full charge, he seems to do fairly well, but still not enough. I'm going to up the ante and get a 7.2 v pack somewhere and see if that works. I've also adjusted the treads a bit so there is not too much slack. i will be ordering the motors from pololu as soon as i get paid.

Any more suggestions for me guys? 

Aniss1001's picture

It isn't just a matter of increasing the VOLTAGE. It could also be that your batteries isn't capable of supplying enough CURRENT. So you may wanna check out the (max) current draw of your motors as well as how much the battery is capable of supplying.

jair2k4's picture
So let me get this right here.... each output pin is going to have 3 connections: 1 to common ground, 1 to the motor itself, and 1 to the same 5v source that ive tied into the enable pins? I'm thinking just buying a true l293 might be the better option here..... ive already got WAY too much wiring, and WAY too much soldering into this part, and i'd rather take the easy route....
ignoblegnome's picture

Just to be clear, the connections from each output are:


  1. To the motor
  2. Through a diode pointed to your motor's power supply (5V)
  3. Through a diode pointed to common ground
I'm not sure if this has anything to do with your problem, but the flyback diodes will protect your motor controller.