Robotics: DC and Stepper Motors

Actuators

• Motors control the movement of a robot.
• Actuators come in three common types: DC motors, stepper motors, and servo motors.

DC Motors

• DC motors are the most common and cheapest type of motor.
• They are powered with two wires from a source.
• DC motors draw large amounts of current.
• These motors cannot be wired straight from a PIC(Programmable Interface Controller) microcontroller.
• DC motors do not offer accuracy or speed control.

Stepper Motors

• Stepper motors have many electromagnets.
• They are controlled by sequentially turning the magnets on and off.
• Each pulse provides a step angle.
• An example shows a step angle of 90°.
• Stepper motors provide poor control with a large angle.
• A better step angle can be achieved with the toothed disc.
• A step angle of 3.6 degrees has 100 steps per revolution.
• With 25 teeth and 4 steps per tooth, there is 100 steps for 25 teeth.
• They are controlled using output Blocks on a PIC.
• A correct sequence is essential.
• A reverse sequence will reverse the motor.

Servo Motors

• Servo motors offer the smoothest control.
• They can rotate to a specific point.
• Servo motors offer good torque and control.
• They are ideal for powering robot arms, etc.
• The degree of revolution is limited.
• Servo motors are not suitable for applications that require continuous rotation.
• They contain a motor, gearbox, driver controller, and potentiometer.
• Servo motors have three wires: 0v, 5v, and PIC signal.
• A potentiometer is connected to the gearbox, which monitors movement.
• Servo motors provide feedback, and if the position is distorted, it provides an automatic correction.
• The pulse width modulation is 0.75ms to 2.25ms.
• The pulse width takes the servo from 0 degrees to 150 degrees of rotation.
• There is a continuous stream every 20ms.
• On the programming block, pulse width and output must be set.
• Pulse width can also be expressed as a variable.

End Effectors

• The correct name for the hand attached to a robot is an "End Effector."
• End effectors are used for grasping, drilling, painting, and welding.
• Different end effectors allow a standard robot to perform numerous operations.
• There are two different types of end effectors: grippers and tools.

End Effectors - Tools

• Tools are used where a specific operation needs to be carried out, such as welding, painting, and drilling.
• The tool is attached to the mounting plate.

End Effectors - Grippers

• Grippers can be mechanical, magnetic, and pneumatic.
• Two-fingered grippers are most common, but multi-fingered ones are also available.
• Grippers apply force that causes enough friction between an object to allow it to be lifted.
• However, they are not suitable for delicate or brittle objects.

Control Theory

• Control theory finds a controller that adjusts control input to achieve a specific measured output in the presence of disturbances, given a target system and transducer.

Feedback Control System - Components

• Target system: what is being controlled
• Controller: exercises control
• Transducer: translates measured outputs

Feedback Control System - Data

• Reference input: objective
• Control input: manipulated to affect output
• Disturbance input: other factors that affect the target system
• Transduced output: result of manipulation

Control System Examples

• AC temperature control: controlled variable is temperature, control variables include fan speed and the time that the compressor is on.
• Car cruise control: controlled variable is car speed, control variable is the angle of pushing the accelerator and brake.

Control System Goals

• Reference Tracking: Ensures that the measured output follows a target, desired level.
• Disturbance Rejection: Maintains measured output at a given stable level even in the presence of disturbances.
• Optimization: Maintains the measured output and control input at optimal levels when no reference input is given
  • Minimize petrol consumption and maximize speed.
  • Minimize power consumed by CPU, and maximize performance.

Control System: Basic Working

• A control loop is executed every T time units.
  • y(k): Value of measure at time instant k
  • u(k): Value of control input at time instant k
  • r(k): Value of "reference" at time instant k
  • e(k): Error between reference and measured value
• The next value of the control input, u(k+1), is computed based on feedback, e(k).

Medical Robotics Applications

• Robots are now used in many clinical sub-domains like neurosurgery, orthopedic surgery, dental, eye, ear-nose-throat, abdominal surgery/laparoscopy, and radiosurgery.
• Medical robots have emerged into four main types:
  • Robots for Navigation: A surgical instrument is moved by a robot arm allowing precise positioning.
  • Robots for Motion Replication: Replicates the surgeon's hand movements for more accurate and minimally invasive methods.
  • Robots for Imaging: Mounts an imaging device to a robotic arm to acquire 2D or 3D images.
  • Rehabilitation and Prosthetics: Supports the recovery process of stroke patients with robotic exoskeletons controlled by brain-computer.
• Remote surgery (telesurgery).
• Medical Training.