Servos

1
Servos

• The material presented is taken from a variety of
  sources including http//www.seattlerobotics.org/
  guide/servos.html, http//www.baldor.com/pdf/manua
  ls/1205-394.pdf, and Parallax educational
  materials at http//learn.parallax.com/

2
Overview

• Servo motors are used for angular positioning.
  They typically have a movement range of 180 deg
  but can go up to 210 deg.
• The output shaft of a servo does not rotate
  freely. It seeks a particular angular position
  under electronic control.
• Servos are typically rated by torque and speed. A
  servo rated 40 ounce-in/.21 means that at 1 inch
  from the hub, the servo can exert 40 ounces of
  force and move 60 deg in 0.21 sec.

3
What makes a Servo

• Servo motors and are constructed out of basic DC
  motors, by adding
• some gear reduction
• a position sensor for the motor shaft
• an electronic circuit that controls the motor's
  operation

• The basic hobby servo has a 1801 gear ratio.
  The motor is typically small.
• Typically, a potentiometer (variable resistor)
  measures the position of the output shaft at all
  times.
• An informative link

4
Feed-back loop

• open-loop

closed-loop
5
Control

• An external controller (such as the Arduino)
  tells the servo where to go with a signal know as
  pulse proportional modulation (PPM) or pulse code
  modulation (which is often confused with pulse
  width modulation, PWM).
• PPM uses 1 to 2ms out of a 20ms time period to
  encode information.

6
PPM

• A control wire communicates the angular movement.
  The angle is determined by the duration of the
  pulse on the control wire.
• The servo expects a pulse every 20 millisec (.02
  seconds). The length of the pulse determines how
  far the motor turns.
• For a 1.5 millisecond pulse, the motor turns to
  the 90 degree position (i.e., the neutral
  position).
• If the pulse is shorter than 1.5 ms, the motor
  turns the shaft to closer to 0 degrees. If the
  pulse is longer than 1.5ms, the shaft turns
  closer to 180 degrees.

7
PPM

• Each pulse is from 1300 to 1700 microsec (µs) in
  duration
• The pulses repeat about 50 times each
  second---once every 20 millisec

8
PPM

• The amount of power applied to the motor is
  proportional to the distance to be traveled.
• If the shaft needs to turn a large distance, the
  motor runs at full speed.
• If it needs to turn a small amount, the motor
  runs at a slower speed.

9
Modified Servos

• Servo motors can also be retrofitted to provide
  continuous rotation
• Remove mechanical limit (revert back to DC motor
  shaft).
• Remove pot position sensor (no need to know
  position) and replace it with 2 equal-valued
  resistors with a combined resistance equivalent
  to that of the pot. Makes the servo think it
  is in the 90 deg position.

Not always necessary
10
Modified Servos

• The idea is to make the servo think that the
  output shaft is always at the 90 degree mark.
• This is done by removing the feedback sensor, and
  replacing it with an equivalent circuit that
  creates the same readings as the sensor at 90
  degrees.
• Then, the control signal for 0 degrees causes the
  motor to turn full speed in one direction. The
  signal for 180 degrees causes the motor to turn
  full speed the other direction.
• Since the feedback from the output shaft is
  disconnected, the servo continues in the
  specified direction as long as the signal
  remains.

11
Parallax Servos

• The parallax servos are modified servos with the
  potentiometer intact.
• The potentiometer (a.k.a., pot) should be
  adjusted to make the servo think that it is at
  the 90 degree mark.

12
Parallax Servo Connections
Servo Connector Black ground Red
power White signal
13
Connecting to the Arduino
14
Calibration Program

• include ltServo.hgt
• Servo myServo
• void setup()
• myServo.attach(9) myServo.writeMicroseconds(15
  00) // Stop
• void loop()

15
Demonstration Program

• include ltServo.hgt
• Servo myServo // Create Servo object
• void setup()
• myServo.attach(9) // Servo is connected to
  digital pin 9
• void loop()
• myServo.writeMicroseconds(1700) // Counter
  clockwise
• delay(2000)
  // Wait 2 seconds myServo.writeMicroseconds(1300)
  // Clockwise
• delay(2000)
• myServo.writeMicroseconds(1500) // Stop
• delay(2000)