Controlling the Outside World

1
Controlling the Outside World

• Servo Motor

2
Servo motor

• A feedback controlled motor
• The Parallax standard servo motor has built in
  feedback
• Range of motion is limited
• For those of you that have used the Parallax
  BoeBot kits, youre used to continuous servos
• This is not a continuous servo!
• That is, it will not go 360º
• It will hit stops at either direction around

3
The servo motor
4
Wiring the servo motor

• You can wire up the servo on the proto board as
  shown in the schematic
• but, theres an easier way

5
Wiring the servo motor
6
Wiring the servo motor

• The X4 and X5 connectors are designed to control
  the servo motors
• But, the power source must be set via the jumper
  connector

7
Power

• Check your power source!
• If you still have one of my DC power blocks, it
  may be 9V or it may be 6V
• Set the Vdd/Vin jumper as specified
• Vdd if using a 9V source (or 9V battery)
• Vin if using a 6V source
• Power switch must be in position 2 to send power
  to the X4/X5 (servo) connectors

8
How it works

• To light up an LED we send a continuous, level
  signal (DC voltage)

5v
HIGH pin
voltage
0v
time
9
Controlling the servo

• To move the servo horn we send a pulse train

1.5ms
1.5ms
5v
voltage
20ms
0v
Time (in milli-seconds)
10
Controlling the servo

• The pulse width will control the location of the
  servo
• The time between pulses should be 20
  milli-seconds

11
From PBASIC

• PULSOUT command
• PULSOUT pin, duration
• pin is the pin number to which your servo is
  wired (12, 13, 14, or 15 if you are using the
  servo ports on the RevC board)
• duration is the pulse width

12
PULSOUT command details

• The duration parameter is in units of 2µS (2
  micro-seconds)
• The servo prefers values in units of
  milli-seconds
• The time between pulses is under software control
  (PAUSE command units of milli-seconds)

13
Activity in class

• Test the servo
• Test all 4 servo ports
• Find the values of the duration parameter (in the
  PULSOUT command) that cause the servo to hit the
  stops in both the clockwise and counter-clockwise
  directions

14
Homework

• Write a program that continually moves the servo
  from one stop to the other
• That is, it goes from full counter-clockwise to
  full clockwise and back again, continuously
• Add two buttons to the board (B1 and B2)
• B1 is the emergency stop (EStop)
• When it is pressed all servo motion stops
• B2 is the reset button
• When it is pressed the servo motion begins again