900 00008-continuous-rotation-servo-documentation-v2.2

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Copyright © Parallax Inc. Parallax Continuous Rotation Servo (#900-00008) v2.2 10/24/2011 Page 1 of 8
Parallax Continuous Rotation Servo (#900-00008)
The Parallax Standard Servo is ideal for adding bidirectional continuous rotation to your robotics projects.
Features
 Bidirectional continuous rotation
 0 to 50 RPM, with a linear response to PWM for easy
ramping
 Accepts four mounting screws
 Easy to interface with any Parallax microcontroller or
PWM-capable device
 Very easy to control with the PULSOUT command in
PBASIC or SX/B
 Weighs only 1.50 oz (42.5 g)
 38 oz-in torque @ 6 V
Key Specifications
 Power requirements: 4 to 6 VDC; Maximum current draw
140 +/- 50 mA at 6 VDC when operating in no load conditions, 15 mA when in static state
 Communication: pulse-width modulation
 Dimensions: approx 2.2 x 0.8 x 1.6 in (5.58x 1.9 x 4.06 cm) excluding servo horn
 Operating temperature range: 14 to 122 °F (-10 to +50 °C)
Quick-Start Circuit
Vµ = microcontroller voltage supply
Vservo = 4 to 6 VDC, regulated or battery
I/O = PWM TTL or CMOS output signal, 3.3 to 5 V; < Vservo + 0.2 V
Continuous Rotation ServoMicrocontroller
GND
Vservo
White
Vss
I/O
Red
Black
Vdd
Vµ

Device Information
The Parallax continuous rotation servo relies on pulse width modulation to control the rotation speed and
direction of the serv o shaft. Before u sing the servo in a project, it is important to c alibrate the center
position of the servo in order to defi ne the pul se width value at which the servo holds still (see the
section Calibration – "Center" the Servo on page 4).
Specifications
Pin Name Description Minimum Typical Maximum Units
1 (White) Signal Input; TTL or CMOS 3.3 5.0 Vservo + 0.2 V
2 (Red) Vservo Power Supply 4.0 5.0 6.0* V
3 (Black) Vss Ground 0 V
*See Board of Education Servo Header Connection Diagram.
Power Precautions
 Do not use this servo with an unregulated wall-mount supply. Such powe supplies may deliver
variable voltage far above the stated voltage.
 Do not power this servo through the BASIC Stamp®
Module's Vin pin, this can deliver voltages
above the stated voltage. See the Board of Education Connection Diagram below for jumper
settings.
 Servo current draw can spike while under peak load; be sure your application's regulator is
prepared to supply adequate current for all servos used in combination.
Board of Education Servo Header Connection Diagram
When connecting the servo to the Board of Education®
servo header, be sure the jumper is set t o Vdd
(regulated 5 VDC for this board) as shown in the figure below. Failure to place the jumper at this setting
can cause damage your servo! (Note: see the Board of Education product documentation for instructions
regarding earlier board revisions that do not have a servo header with a jumper.)
Black
Red
X4 X5
15 14 13 12Vdd
Vin
Using a Separate Power Supply on a HomeWork Board
The BASIC Stamp HomeWork Board uses a 9 V battery for a power supply. A servo can drain a fresh 9 V
battery in under 20 minutes! Foll ow these directions to build two servo p orts on the breadboard, and
power them with a separate battery pack.
Hardware Required
(1) BASIC Stamp HomeWork Board (serial #555-28158 or USB #555-28188)
(1) Battery pack with tinned leads (Parallax #753-00001)
(2) Parallax continuous rotation servos
(2) 3-pin male-male headers (Parallax #451-00303)
(4) Jumper wires (10-pack: Parallax #800-00016
(4) 1.5 V AA batteries

√ Disconnect the 9 V battery from the board, and do not put the AA batteries in their holder yet.
√ Build the servo ports shown by the schematic and wiring diagram below.
√ Double-check to make sure the black wire with the white stripe is connected to Vbp, the solid
black wire is connected to Vss, and that all the connections for P13, Vbp, Vss, Vbp (another one),
and P12 all exactly match the wiring diagram.
√ Connect the servo plugs to the male headers on the right side of the wiring diagram.
√ Connect the 9 V battery, and insert the AA batteries into their holder.
Vbp
Vss
P12 White
Red
Black
Vbp
Vss
P13 White
Red
Black
P15
P14
P11
P10
P9
P8
P13
P12
X3
Vdd VssVin
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Rev B
P15
P14
P11
P10
P9
P8
P13
P12
X3
Vdd VssVin
(916) 624-8333
www.parallaxinc.com
www.stampsinclass.com
Rev B
Port connections Servo connections by wire color
 P13
 Vbp
 Vss
 Vbp
 P12
Battery pack
solid black
wire = ground
Battery
pack black
wire with
white stripe
= Vbp
Vbp stands for Voltage battery pack.
It refers to the 6 VDC supplied by the four 1.5 V
batteries. This is brought directly to the breadboard to
power the servos for Boe-Bots built with the HomeWork
Board. Your BASIC Stamp is still powered by the 9 V
battery.
 White
 Red
 Black
 Red
 White
HomeWork Board with
servo ports built on the
breadboard, with a separate
battery pack power supply
for the servos.

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 every
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 available for download from the 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
DO
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, in 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 fluctuations in RPM for a given pulse width that might
otherwise occur with unregulated battery supplies.
BASIC Stamp®
Programming Examples
PBASIC has a PULSOUT command that sets the I/O Pin 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 counted FOR…NEXT loop to su stain continuous operation for the specified number
of cycles.
PULSOUT Pin, 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
NEXT

FOR counter = 1 TO 100 ' Hold still for ~3 seconds
PAUSE 20
NEXT
FOR counter = 1 TO 100 ' Rotate clockwise for ~3 seconds
PAUSE 20
NEXT
END
For more examples with the BASIC Stamp 2, incl uding 2-wheeled robot maneuvers and ramping, see
Robotics with the Boe -Bot Chapter 4 , available for free downl oad from the 28132 product page at
www.parallax.com.
Propeller™
P8X32A Application
The program below uses counter modules to rotate the servo first clockwise at full speed for 2 seconds,
then rests for 2 seconds, and rotates counterclockwise at full speed for another 2 seconds. This code can
also be downloaded from the 900-00008 product page.
{{ ServoContinuousRotation.spin
Turn Parallax Continuous Rotation Servo clockwise full speed for 21 sec.
hold still 2 sec, and then counterclockwise full speed for 2 sec. }}
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, tCtr, tCw, tCcw, t
ctra[30..26] := %00100 ' Configure Counter A to NCO
ctra[8..0] := servoPin
frqa := 1
dira[servoPin]~~
tInc := clkfreq/1_000_000 ' 1 μs increment
tC := tInc * 21_500 ' Low pulse
tCtr := tInc * 1500 ' Center pulse = 1.5 ms
tCw := tInc * 1300 ' Clockwise fast = 1.3 ms
tCcw := tInc * 1700 ' Counter-Clockwise fast = 1.7 ms
t := cnt ' Mark counter time
repeat 100 ' Repeat PWM signal 100x
phsa := -tCw ' Set up clockwise fast pulse
t += tC ' Calculate next cycle repeat
waitcnt(t) ' Wait for next cycle (20 ms)
phsa := -tCtr ' Set up the center pulse
t += (tC + 200) ' Calculate next cycle repeat

phsa := -tCcw ' Set up counter-clockwise fast pulse
t += (tC - 200) ' Calculate next cycle repeat
Revision History
Version 2.1: corrected values in BASIC Stamp Model PULSOUT table; updated example programs to use a
constant for the servo pin.
Version 2.2: added Voltage and RPM note on p age 6. Added Usin g a Separate Power Supply on a
HomeWork Board section beginning on page 2. Updated specifications.

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