Are the Servos Centered?

The cyber:bot robot's drive motors are Parallax Continuous Rotation Servos.  These servos need to be centered before moving on to navigation activities.

  • If you just followed the directions in the Build your cyber:bot tutorial, you will have already centered your servos, and you can skip to the next page.
  • If you were just given an assembled cyber:bot to use, the servos might need to be re-centered. Use the test below to find out.

 

Checking the Servos for Centering

Tool Required

You’ll need a Parallax screwdriver, or a different Phillips #1 point screwdriver with a 1/8″ (3.18 mm) or smaller shaft.

Sending the Center Signals

If a servo has not yet been centered, it may turn, vibrate, or make a humming noise when it receives the “stay-still” signal.

  • Make sure the servos are plugged into the servo ports, the battery pack is plugged into the board, and the jumper is set for the type of batteries you are using:

  • Set the 3-position switch on the cyber:bot board to Position 1.
  • Make sure the cyberbot.py module is loaded on your micro:bit (see Add modules to your micro:bit).
  • Enter and save left_servo_stay_still.py.
  • Make sure to include the line from cyberbot import *.
# left_servo_stay_still.py

from cyberbot import *
 
bot(18).servo_speed(0)
  • Flash left_servo_stay_still.py to your micro:bit.
  • Set the cyber:bot’s power switch to 2, to provide power to the servos.  The servo should not turn.
  • Test the right servo with the the script right_servo_stay_still.py. The servo should not turn.
# right_servo_stay_still.py
 
from cyberbot import *
bot(19).servo_speed(0)

If either of the servos turned during these tests, you will need to center it. On the side of the servo is a small access hole. Inside the hole is what appears to be a small screw. This is a plastic potentiometer adjustment knob.

To center a servo, run its script above, and gently turn the potentiometer knob until the servo stops turning, vibrating, or humming. You can either remove the servos first and then re-mount them, or adjust them in place if you have a very long, thin screwdriver. This video will help:

 


Did you know?

A potentiometer is an electrical device that provides variable electrical resistance. Some potentiometers have a knob for adjusting the resistance, like these servos, while others may have a sliding bar. Either way, the schematic symbol for a potentiometer is the same.