900-00008 Datasheet 900-00008 | P4-P6

Calibration – "Center" the Servo

The servo has a pot entiometer access port, right above the plac e where the cable attaches to the case.

The port allows the user to adjust the servo to hold completely still when receiving a 1.5 ms pulse width.

This is the value in the "center" of the range of control pulses the servo will accept.

To center the servo, program your host device to deliver a 1.5 ms pulse, continually refreshed ever y

20 ms. S ample calibration code is giv en below f or all BASIC Stamp model s, Spin for the Propeller

™

P8X32A microcontroller, and SX/B for the SX chip. All are avail able for downl oad from th e 900-00008

product page at www.parallax.com.

Connect the servo to your microcontroller's I/O pin. The example programs below specify an I/O pin.

BASIC Stamp Calibration Code - for all BS2 models

√ Connect the servo to BASIC Stamp 1/O pin P12, or update the ToServo PIN declaration.

√ Run the program, and gently twist the potentiometer adjustment screw until the servo does not

turn or vibrate.

NOTE: Calibrating the servo may take some patience. The potentiometer is very

sensitive so a very light touch will be required.

' CenterParallaxCrServo.bs2

' {$STAMP BS2}

' {$PBASIC 2.5}

#SELECT $Stamp

#CASE BS2, BS2E, BS2PE ' PULSOUT Duration units are 2 us for these models

Center CON 750

#CASE BS2SX, BS2P, BS2PX ' PULSOUT Duration units are 0.8 us for these models

Center CON 1875

#ENDSELECT

ToServo PIN 12 ' connect servo to I/O pin P12, or change it here

PULSOUT ToServo, Center ' ToServo pin outputs 1.5 ms pulse

PAUSE 20 ' refresh pulse every 20 milliseconds

LOOP

Propeller Chip Calibration Code – for P8X32A

√ Download and unzip the Propeller code file from the 900-00008 product page.

√ Connect the servo signal pin to Propeller I/O pin P0.

√ Run the program CenterParallaxServo.spin, and gently twist the potentiometer adjustment screw

until the servo does not turn or vibrate.

NOTE: Calibrating the servo m ay take some p atience.

The potentiometer is very sensitive so a very light touch will be required.

{{ CenterParallaxServo.spin

For centering Parallax Continuous Rotation Servo

or holding Parallax Standard Servo at 90° position.

Sends a 1.5 ms pulse approx every 20 ms }}

CON

_clkmode = xtal1 + pll16x ' System clock → 80 MHz

_xinfreq = 5_000_000 ' Using 5 MHz external crystal oscillator

servoPin = 0 ' Servo signal to this I/O pin-change if needed

PUB CenterServo | tInc, tc, tHa, t

ctra[30..26] := %00100 ' Configure Counter A to NCO

ctra[8..0] := servoPin

frqa := 1

dira[servoPin]~~

' Set up cycle and high times

tInc := clkfreq/1_000_000

tC := tInc * 21_500

tHa := tInc * 1500

t := cnt ' Mark counter time

repeat ' Repeat PWM signal

phsa := -tHa ' Set up the pulse

t += tC ' Calculate next cycle repeat

waitcnt(t) ' Wait for next cycle

Communication Protocol

The Parallax Continuous Rotation Servo is controlled through pulse width modulation. Rotational speed

and direction are determi ned by the d uration of a high pulse, i n the 1.3– -1.7 ms range. In order for

smooth rotation, the servo needs a 20 ms pause between pulses. Below is a sample timing diagram for a

centered servo:

As the length of the pulse decreases from 1.5 ms, th e servo will gradually rotate faster in th e clockwise

direction, as can be seen in the figure below:

Likewise, as the length of the pulse increases from 1.5 ms, the servo will gradually rotate faster in the

counter-clockwise direction, as can be seen in the figure below:

Voltage and RPM: Maximum RPM will vary with input voltage; 50 RP M @ 5 V is typical. Using

regulated Vdd as the sup ply source will reduce fluct uations in RPM for a given pulse width that might

otherwise occur with unregulated battery supplies.

BASIC Stamp

Programming Examples

PBASIC has a PULSO UT command that sets the I/O

to an output and sends a pulse of the specified

Duration

. Since the servo needs this pulse refreshed every 20 ms for continuous operation, the PULSOUT

command is put in a coun ted FOR…NEXT loop to su stain continuous operation for the speci fied number

of cycles.

PULSOUT

Duration

Different BASIC Stamp m odules use different units for the P ULSOUT command's

Duration

argument.

When adapting BS2 code to another BASIC Stamp model, you may need to make adjustments. The table

below lists the PULSOUT ranges for each BASIC Stamp microcontroller. See the BASIC Stamp Manual or

BASIC Stamp Editor Help for more information.

BASIC Stamp Model

1.3 ms

(Full speed clockwise)

1.5 ms

(Center, no rotation)

1.7 ms

(Full speed counterclockwise)

BS1

130 150 170

BS2, BS2e, BS2pe

650 750 850

BS2sx, BS2p, BS2px

1625 1875 2125

The example shown below for a BASIC Stamp 2 causes a servo connected to BASIC Stamp 1/0 pin 12 to

first rotate full-speed counterclockwise for about 3 seconds, hold still f or about 3 seconds, and then

rotate counterclockwise for about 3 seconds.

' RotateParallaxCrServo.bs2

' {$STAMP BS2}

' {$PBASIC 2.5}

counter VAR Word

servoPin PIN 12 ' change I/O pin for servo signal here

FOR counter = 1 TO 100 ' Rotate counterclockwise for ~3 seconds

PULSOUT servoPin, 850

PAUSE 20

P1-P3 P4-P6 P7-P8

900-00008

Mfr. #:

Buy 900-00008

Manufacturer:

Parallax

Description:

AC, DC & Servo Motors ROTATION SERVO S148 FUTABA

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900-00008