Servo Direction

Your cyber:bot servos need to receive a high (5 volt) 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).

Hardware Setup

• Set the cyber:bot board’s power (PWR) switch to Position 0.
• Make sure the battery holder is loaded with 5 AA batteries.
• Make sure the battery holder’s barrel plug is firmly plugged into the cyber:bot board’s barrel jack. 
• Connect your micro:bit module to your computer with a USB cable.

Example Script: left_servo_clockwise

• In a Google Chrome or Microsoft Edge browser, go to python.microbit.org to open the micro:bit Python Editor.
• Make sure the cyberbot.py module is added to the Project Files.
  (See Add modules to your micro:bit).
• Set the project’s name to left_servo_clockwise, enter the script below, and then click Save.
  (See Save & Edit Scripts and Flash Scripts with Python Editor.)
• Click Send to micro:bit.  
  (See Flash Scripts with Python Editor.)
• Set the power (PWR) switch to position 2.
• Verify that the left servo (connected to P18) is rotating between 50 and 60 RPM clockwise.
• Set the PWR switch to 0.

# left_servo_clockwise

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.

• Change the project name from left_servo_clockwise to left_servo_counterclockwise.
• In the servo_speed function call, change (-75) to (75).
• Save your modified script.
• Flash the script with the Send to micro:bit button.
• Set the PWR switch to 2.
• Verify that the servo connected to pin 18 now rotates the other direction, which should be counterclockwise, at about 50 to 60 RPM.
• Set the PWR switch to 0.

Your turn: Left Servo Stay Still

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

• Change the project name from left_servo_clockwise to left_servo_stay_still.
• In the servo_speed function,  change (-75) to (0).
• Save the modified script and flash it with the Send to micro:bit button.
• Set the PWR switch to 2.
• Verify that the left servo is not rotating in any direction, but gently resists being twisted.
• Change (0) to (None) and Save your modified script.
• Flash the script with the Send to micro:bit button. 
• Verify that the left servo neither turns nor resists being turned.
• Set the PWR switch to 0.

Example script: right_servo_clockwise

• Open the script left_servo_clockwise.
• Rename the project to right_servo_clockwise, and don’t forget to update the name in the comment!
• Replace bot(18)with bot(19).
• Flash the script into the micro:bit with the Send to micro:bit button.
• Set PWR to 2.
• Verify that the right servo (connected to P19) is rotating between 50 and 60 RPM clockwise.
• Set PWR to 0.

# right_servo_clockwise

from cyberbot import *

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

Try This: Right Servo Counterclockwise

• Change the project name from right_servo_clockwise to right_servo_counterclockwise.
• In bot(19).servo_speed(-75) change (-75) to (75).
• Update the code comments!
• Save the script and flash it with the Send to micro:bit button.
• Set PWR to 2.
• Verify that the servo connected to P19 turns full-speed counterclockwise, about 50 to 60 RPM.
• Set PWR to 0.

Your Turn: Right Servo Stay Still

• Change the project name from right_servo_clockwise to right_servo_stay_still.
• In the servo_speed function, change (-75) to (0).
• Save the modified script and flash it with the Send to micro:bit button.
• Set PWR to 2.
• Verify that the right servo is not rotating in any direction, but gently resists being twisted.
• Change (0) to (None)
• Save the modified script and flash it with the Send to micro:bit button.
• Verify that the right servo neither turns nor resists being turned.
• Set PWR to 0.