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View Full Version : Boe - Bot Servo speeds cannot be matched.



TASMAN57
03-06-2012, 09:55 PM
Symptom:

Cannot get the two servos to run in speed terms within an 1/8 rotation increment of each other. This happens whether on 5.14 V wall receptacle or on board battery pack with bo-boost. Not a power issue.......

What does this mean......The Boe Bot will not track in a straight line and as it runs longer, the skew or differential of wheel speeds adds up. There is absolutely no repeatability. Not very impressed with the investment so far.......

The instructor indicated maybe trying to use other servos, so he gave me a couple. No change, no difference.

OK, servo on servo position 12 and set it at 650. Take servo on position 13 and set it at 650. OR use 850 and work down. Run them both and one always runs faster. So you adjust the duration setting up, of the faster motor to slow it down. When you get close to the others speed, you are either 1/8 rotation faster or if you go to the next duration setting up then the faster servo then runs 1/8 rotation slower than the other which is now the faster one........

I assume that each servo has it's own clocking pulse. If that is correct, then the clocks must be pulsing at different rates and not matched, correct.

If someone from Parallax has seen this before, I would appreciate an educated answer.

TASMAN57

erco
03-06-2012, 10:14 PM
You are correct, sir. Without encoders, the best you can do is "close". I agree it can be frustrating if you are trying to navigate accurately.

They offer this kit to help sync the wheels: http://www.parallax.com/StoreSearchResults/tabid/768/List/0/SortField/4/ProductID/80/Default.aspx?txtSearch=boe+bot+encoders

Duane Degn
03-06-2012, 11:06 PM
I assume that each servo has it's own clocking pulse. If that is correct, then the clocks must be pulsing at different rates and not matched, correct. Not exactly. Each servo will get a pulse of power to it motors but motors all vary a little as do the gears attached to the motors. This just the nature of servos.


If someone from Parallax has seen this before, I would appreciate an educated answer.

TASMAN57

I bet everyone at Parallax who has used a Boe-Bot has seen this.

This is one reason the new Scribbler has encoders.

I just got the encoders working on one of my latest robots (http://forums.parallax.com/showthread.php?130797-Mecanum-Wheeled-Robot-with-Machine-Vision&p=1078740&viewfull=1#post1078740). Boy do they make a difference.

If you want to build robots, you're going to need to learn about encoders.