PDA

View Full Version : Maximum number of servos the BS2 can control?



Renny
09-06-2008, 10:17 AM
Is there a maximum number of servos that the BS2(on the rev c board of education) can control? It seems to work ok to control 4 servos but as soon as the·5th one is added it acts erratically and doesn't run the program.· Just doing a simple program like this will even cause problems:··
· PULSOUT 11, 750
· PULSOUT 3, 750
· PULSOUT 4, 750
· PULSOUT 15, 750
· PULSOUT 8, 750

I've tried putting several servos on one pin to see what happens and it acts the same way with the fifth servo throwing everything out of whack....http://forums.parallax.com/images/smilies/rolleyes.gif

thanks for any info!

GICU812
09-06-2008, 10:35 AM
Can your power source supply enough power for the servo's?

Renny
09-06-2008, 10:41 AM
All the servos rotate but they don't stop when the program says, they keep turning.· I'm using the battery pack that comes with the board.

Beau Schwabe (Parallax)
09-06-2008, 10:56 AM
Renny,

You need a PAUSE in there somewhere. The servo's require a 15ms to 25ms pause in-between each PULSOUT it receives.

So try something like this...

MainLoop:
PULSOUT 11, 750
PULSOUT 3, 750
PULSOUT 4, 750
PULSOUT 15, 750
PULSOUT 8, 750
PAUSE 13 ' <--- 5 servos at 1.5ms = 7.5ms ... 20ms - 7.5ms = 12.5 or 13
GOTO MianLoop

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Mike Green
09-06-2008, 11:01 AM
Servos can draw up to 1 Amp each when under heavy mechanical load or stalled. They draw roughly 250mA when under light load. 5 servos draw in excess of 1 Amp (roughly 1.25 Amp) under light load. If you're running the servos directly off a 4 x AA Alkaline battery pack, it can supply over 1 Amp for a while. If you're trying to supply over 1 Amp of regulated +5V, the regulator will eventually shut down due to overcurrent or overheating.

The number of servos that a BS2 can control depends on timing. Control pulses are at most 2-2.5ms in width and have to be repeated about every 20ms. That means that a BS2 (or any Stamp) can control at most 8 servos. Other microcontrollers (like the SX or Propeller) are able to control more than that by controlling several simultaneously (overlapping).

Renny
09-09-2008, 06:18 AM
Thanks everyone.· I'll give those suggestions a try.·