Activity Centering The Servo

Before we can use the Parallax Continuous Rotation Servo, it must be "centered." This servo motor is designed to hold a steady position when receiving 1.5 ms pulses, and to rotate when it receives pulses less than or greater than this "center" value. Our Sun Tracker program design makes use of this characteristic, but the servo must first be manually calibrated to make sure it does hold steady when receiving the 1.5 ms pulse.

Parts Required

(1) Parallax Continuous Rotation servo Parallax Screwdriver

If you are using a Board of Education Rev B or A, you will also need: (1) 3-pin header (3) jumper wires

V Disconnect power to your Board of Education.

V If you are using the Board of Education Rev C, refer to Figure 4-10. Set the jumper between the servo ports to Vdd, then plug the servo into the P12 Servo Port with the red and black leads lined up with the labels on the board.

P12 O

P12 O

Figure 4-10

Board of Education Rev C Connection for Centering the Servo

Figure 4-10

Board of Education Rev C Connection for Centering the Servo

V If you are using the Board of Education Rev B or A refer to Figure 4-11. Build the servo connection on the breadboard, using the 3-pin header to connect the servo to the board. Make sure that the servo's black lead connects to Vss, the red lead connects to Vdd, and the white lead connects to P12. Note: do not use the servo ports on the Board of Education Rev B for this activity.

Figure 4-11

Board of Education Rev B or Rev A Connection for Centering the Servo

Note: Do not use the Board of Education Rev B servo ports for this activity.

V Double-check your servo connection, and jumper if necessary. If you plug the servo in backwards, or connect it to Vin, and reconnect the power, you will damage your servo.

V When you are sure your connections are correct, reconnect power to your board. (If you are using a Board of Education Rev C, the 3-position switch must be in position-2 to power the servo ports.)

V Enter and run the program CenteringServo.bs2.

If your servo is new and has not been centered before, it is probably rotating. If it is not moving, it is probably already centered. Either way, follow the steps below.

V Very carefully, insert the tip of the Parallax screwdriver into the potentiometer access hole in the servo case, as shown in Figure 4-12. Don't apply too much pressure or you will damage the servo.

V Gently rotate the screwdriver a little bit, first one way, then the other. You should see that the servo speeds up when you turn the screwdriver one way, and slows down or even reverses the other way.

V Gently twist the screwdriver in tiny increments until the servo holds still, indicating that it is exactly centered.

V Disconnect power to your board.

V Unplug the servo from the Board of Education before continuing on.

Figure 4-12

Centering the Servo

Gently insert the screwdriver tip into the access hole, and slowly twist it until the servo stops rotating. Do not apply too much pressure to prevent damage to the servo.

' Experiments with Renewable Energy v1.0 - CenteringServo.bs2 ' Delivers a 1.5 ms pulse to P12 so servo can be centered. ' {$STAMP BS2} ' {$PBASIC 2.5}

DEBUG "Sending 1.5 ms pulse to servo on P12" DO

PULSOUT 12, 750



Learn more about the PULSOUT command and Parallax Continuous Rotation servo control

* in Robotics with the Boe-Bot, another Stamps in Class title from Parallax, Inc. This text can

* be downloaded free from the Educational Downloads page on the Education menu at

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