Applied Control Systems

Robotics&

Robotic Control

DC Motors

• Most common and cheapest

• Powered with two wires from source

• Draws large amounts of current

• Cannot be wired straight from a PIC

• Does not offer accuracy or speed control

Stepper Motors

• Stepper has many electromagnets

• Stepper controlled by sequential turning on and off of magnets

• Each pulse moves another step, providing a step angle

• Example shows a step angle of 90°

•Poor control with a large angle

•Better step angle achieved with the toothed disc

Stepper motor operation

Step1

Step 2

Stepper motor operation

Stepper motor operation

Step 3

Stepper motor operation

Step 4

Stepper Motors

• 3.6 degree step angle => 100 steps per revolution

• 25 teeth, 4 step= 1 tooth => 100 steps for 25teeth• Controlled using output Blocks on a PIC• Correct sequence essential• Reverse sequence - reverse motor

Servo motors

• Servo offers smoothest control

• Rotate to a specific point

• Offer good torque and control

• Ideal for powering robot arms etc.

However:• Degree of revolution is limited

• Not suitable for applications which require

continuous rotation

Servo motors

• Contain motor, gearbox, driver controller and potentiometer

• Three wires - 0v, 5v and PIC signal

• Potentiometer connected to gearbox - monitors movement

• Provides feedback

• If position is distorted - automatic correction

+ 5V

Servo motors Operation

• Pulse Width Modulation (0.75ms to 2.25ms)• Pulse Width takes servo from 0° to 150° rotation• Continuous stream every 20ms• On programming block, pulse width and output pin must be set.• Pulse width can also be expressed as a variable

Open and Closed Loop Control All control systems contain three elements:

(i) The control

(ii) Current Amplifiers

(iii) Servo Motors

• The control is the Brain - reads instruction• Current amplifier receives orders from brain and sends required signal to the motor• Signal sent depends on the whether Open or Closed loop control is used.

Open Loop Control

For Open Loop Control:

• The controller is told where the output device needs to be

• Once the controller sends the signal to motor it does not

receive feedback to known if it has reached desired position

•Open loop much cheaper than closed loop but less accurate

Open Loop Control

Closed Loop Control

• Provided feedback to the control unit telling it the actual

position of the motor.

• This actual position is found using an encoder.

• The actual position is compared to the desired.

• Position is changed if necessary

The Encoder

• Encoders give the control unit information as to the actual

position of the motor.

• Light shines through a slotted disc, the light sensor counts

the speed and number of breaks in the light.

• Allows for the calculation of speed, direction and distance

travelled.

Closed Loop Control

• The desired value is compared to the actual value.

• Comparator subtracts actual from desired.

• The difference is the error which is fed to the controller

which generates a control action to eliminate the error.