Futaba servos

Andy B

hi all
I·am a newbie at the robotic scene.
What i am trying to find out is how i control a servo with the pulsout command

I:E Pulsout 15,750
What i am trying to do is make a servo SCAN ( FULL LEFT THEN FULL RIGHT )
if someone could show me the code to do this i will understand it.
Then i can work out the same for UP AND DOWN

I am trying to make a simple pan and tilt using the two servos X,Y

many thanks

Andy

Jon Williams

Have a look at our "Whats A Microcontroller?" and "Robotics" text books (available online as PDF) -- they will explain everything in detail.

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA

Andy B

many thanx.

i tried this from the book:-

' Test the servo at three different position signals.
' {$STAMP BS2sx}
' {$PBASIC 2.5}
counter VAR Word
DEBUG "Center 12 o'clock", CR
FOR counter = 1 TO 150
PULSOUT 14, -100

PAUSE 20
NEXT

DEBUG "Counterclockwise 10 o'clock", CR
FOR counter = 1 TO 150
PULSOUT 14, 1000
PAUSE 20
NEXT
DEBUG "Clockwise 2 o'clock", CR
FOR counter = 1 TO 150
PULSOUT 14, 500
PAUSE 20
NEXT
DEBUG "All done."
END


My Servos seem to not centre at all

Are servo's different in the code they need?
i am using FUTABA S3003

Andy B

please ignore the PULSOUT 14, -100
i tried - 100 also
but didnt seem to make any difference

andy

Ken Gracey

Andy:

Try this assuming it is a standard (not continuous rotation) servo.

' {$STAMP BS2sx}
' {$PBASIC 2.5}

Counter VAR Word

RotateCCW:
FOR Counter = 500 to 1000 Step 10
'DEBUG DEC4 Counter, CR
PULSOUT 12, Counter
PAUSE 20
NEXT

RotateCW:
FOR Counter = 1000 to 500 Step 10
'DEBUG DEC4 Counter, CR
PULSOUT 12, Counter
PAUSE 20
NEXT

Reduce or increase the Step value as you experiment. Add a GOTO RotateCCW to the end of the code. Add in your DEBUG commands after the label. Download the What's Microcontroller? text, too, please. This book offers superior explanation to what you could receive on the forums. And it's not difficult to read, not to mention you can jump to the relevant section you need to read for servo control.

Ken Gracey
Parallax, Inc.

Andy B

many thanx KEN

that worked wonderful, one last question.
i had to set my servo's go get full CCW and full CW as this:

' {$STAMP BS2sx}
' {$PBASIC 2.5}

Counter VAR Word

RotateCCW:
FOR Counter = 499 to 3000 Step 10
'DEBUG DEC4 Counter, CR
PULSOUT 12, Counter
PAUSE 20
NEXT

RotateCW:
FOR Counter = 3000 to 499 Step 10
'DEBUG DEC4 Counter, CR
PULSOUT 12, Counter
PAUSE 20
NEXT

that worked perfect !
Question is:- i added another part for Example:

scancentre:
FOR Counter = 1500 to 499 Step 10
'DEBUG DEC4 Counter, CR
PULSOUT 12, Counter
PAUSE 20
NEXT

this again scanned CCW and then CW then centred itself, But !
when it started the CCW scan again it spins fast (about STEP 50) to the far CW possition then does its normal slow scan

infact what i am trying to do and i have read from the book also is get the servo's to scan CCW and the CW
and return to centre position.

any ideas?

many thanxs
Andy B

Ryan Clarke

First of all be sure that you are not 'straining' your servos. The exact numbers you use will vary from Servo to servo. Spinning really fast usually is the result of trying to send a position or motion from a starting position that the servo isn't actually at when the command was sent- so with no ramping at all it tries to jump to where it is told to start. From what you describe it sounds as if you scanned CCW, then CC, then centered. The code then went to the CCW scan, which did not start from 'right side' but from center (where you last left it) - so it jumped to the side because of where the CCW assumes you are positioned when you start.

i.e. counter is 499 when you end CC, but counter STARTS at 1500 in the centering part. This will cause a fast 'jump' as you describe it.

Also you should read up on ramping, it's easy to do.

Ryan

Post Edited (LostboY) : 3/17/2005 8:29:51 AM GMT

Jon Williams

Servos usually want a pulse betwee 1000 (CCW) and 2000 (CW) microseconds. With the BS2 we use 500 and 1000 and the PULSOUT values as the resolution for PULSOUT is 2 uS. For the BS2sx you need to change this the values to:

1000 uS / 0.8 uS/unit = 1250
2000 uS / 0.8 us/unit = 2500

Give those values a try and see if thing start behaving.

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Jon Williams
Applications Engineer, Parallax
Dallas, TX· USA

Andy B

Thank - you so much guys.

i will give that a go tonight,i'm currently at work.

andy b