Servo Direction

Your cyber:bot servos need to receive a high signal pulse every 20 milliseconds to keep rotating smoothly.  The speed and direction of rotation are determined by how long that high pulse lasts. Take a look at the timing diagrams below.

Servo Direction

Full speed clockwise requires 1.3 ms high pulses.  To do  this, use servo_speed(-75).

Full speed counter-clockwise requires 1.7 ms high pulses. To do  this, use servo_speed(75).

Full speed typically falls in the 50 to 60 RPM range.

To center the servos, we used servo_speed(0). This generates high signals that last 1.5 ms — halfway in between — that not only make the servo stay still, also but resist being twisted.  This is useful for stopping on slopes, or pausing in the middle of a series of maneuvers.

To stop sending signals to the servos entirely, use servo_speed(None). This will also make the servos stay still, but they will not resist being twisted.

What’s RPM?  Revolutions Per Minute—the number of full rotations turned in one minute.

What’s a pulse train?  Just as a railroad train is a series of cars, a pulse train is a series of pulses (brief high signals).

Example Script: left_servo_clockwise.py

  • Put the cyber:bot board power switch in Position 1.
  • Make sure your micro:bit has the cyberbot.py module loaded.
  • Enter, save, and flash left_servo_clockwise.py.
  • Move the power switch to position 2.
  • Verify that the left servo is rotating between 50 and 60 RPM clockwise.

 

# left_servo_clockwise.py

from cyberbot import *

bot(18).servo_speed(-75)        # 1.3 ms full speed clockwise

 

Try This: Left Servo Counterclockwise

Now, try turning the left servo the other direction.

  • Save left_servo_clockwise.py as left_servo_counterclockwise.py.
  • In the servo_speed function, change (-75) to (75).
  • Save the modified script and flash it to the micro:bit.
  • Verify that the servo connected to pin 18 now rotates the other direction, which should be counterclockwise, at about 50 to 60 RPM.

 

Your turn: Left Servo Stay Still

Now, try making the left servo stop, two different ways.

  • Save left_servo_clockwise.py as left_servo_stay_still.py.
  • In the servo_speed function,  change (-75) to (0).
  • Save the modified script and flash it to the micro:bit.
  • Verify that the left servo is not rotating in any direction, but gently resists being twisted.
  • Change (0) to (None) and re-flash the code. Verify that the left servo neither turns nor resists being turned.

 

Example: right_servo_clockwise.py

  • Save left_servo_clockwise.py as right_servo_clockwise.py.
  • Replace bot(18)with bot(19).
  • Run the script and verify that the right servo is rotating between 50 and 60 RPM clockwise.

 

# right_servo_clockwise.py

from cyberbot import *

bot(19).servo_speed(-75)        # 1.3 ms full speed clockwise

 

Try This: Right Servo Counterclockwise

  • Save right_servo_clockwise.py as right_servo_counterclockwise.py.
  • In bot(19).servo_speed(-75) change (-75) to (75).
  • Save the script and flash it to the micro:bit.
  • Verify that the pin 19 servo turns full-speed counterclockwise, about 50 to 60 RPM.

 

Your Turn: Right Servo Stay Still

  • In the servo_speed function,  change (-75) to (0).
  • Save the modified script and flash it to the micro:bit.
  • Verify that the right servo is not rotating in any direction, but gently resists being twisted.
  • Change (0) to (None) and re-flash the code. Verify that the right servo neither turns nor resists being turned.