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Saturday, December 08, 2012

Robotic Parts Explained

Robotic Sensors
Sensors are divided into two main categories: physical and logical. Physical sensors handle the raw processing of data. Logical sensors then extract data for modules to interpret.
  • Physical Sensors
  • Logical Sensors
Sensor data may be gathered from several sensor devices in parallel and processed in a multitude of ways. A complex hierarchy of processing is possible. Varying characteristics of sensors include accuracy and repeatability.
Robots utilized sensors in one of three ways:
  • Proprioceptive Sensors
  • Exteroceptive Sensors
  • Exproprioceptive Sensors

Robot Controllers
The controller is the processing interface between the robot and its environment. It provides intelligence and feedback to the robot through the compilation of the sensor measurements. It also acts as a storage device, retaining the sensory information and inputted programs.
The computation engine is generally able to process many tasks at one time and seamlessly changes calculations to account for sensory information. The controller includes the hardware that interfaces with the outside world. It stores the user interface, which is used to input various commands.

Robot Actuators - Different Types
An actuator is a mechanism for activating process control equipment by use of pneumatic, hydraulic, or electronic signals. There are several types of actuators in robotic arms.
  • Synchronous - The motor contains a rotor that rotates in synchrony with the oscillating field or current. 
    • Brushless DC Servo - This synchronous electric motor features permanent magnet poles on the rotor, which are attracted to the rotating poles of the opposite magnetic polarity in the stator creating torque. It is powered by a DC current that has an electronically controlled commutation system instead of a system based on brushes. Current, torque, voltage, and rpm are linearly related. The advantages of a brushless motor include higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and an overall reduction of electromagnetic interference (EMI).
      • Stepper - A type of brushless servo motor, this motor is generally electric and moves or rotates in small discrete steps. Stepper motors offer many advantages, such as dual compatibility with both analog and digital feedback signals. They can be used to easily accelerate a load because the maximum dynamic torque occurs at low pulse rates. Drawbacks of their use include low efficiency; much of the input energy is dissipated as heat and the inputs must be matched to the motor and load. The load should be carefully analyzed for optimal performance. Damping may be required when load inertia is exceptionally high to prevent oscillation.
    • Brushed DC Servo - The classic DC motor generates an oscillating current in a rotor with a split ring commutator, and either a wound or permanent magnet stator. A coil is wound around the rotor, which is then powered by a battery. The rotational speed is proportional to the voltage applied to it and the torque is proportional to the current. Speed control can be achieved by applying tape to the battery, varying the supply voltage, resistors, or electronic controls. The advantage to using a brushed motor over a brushless is cost. The brushless motor requires more complex electronic speed controls; however a brushed DC motor can be regulated by a simple variable resistor, such as a potentiometer or rheostat. This is not efficient, but proves satisfactory for cost-sensitive applications.
  • Asynchronous - This motor is designed to slip in order to generate torque.
    • Traction Motor - A type of electric motor that is used to power the driving wheels of a vehicle. The availability of high-powered semiconductors has now made practical the use of much simpler, higher-reliability AC induction motors known as asynchronous traction motors.
  • AC Servo Motors - Used in applications that require a rapid and accurate response, these motors are basically two-phase, reversible induction motors that are modified for servo operation. AC Servo motors have a small diameter and high resistance rotors. This design provides low inertia for fast starts, stops, and reversals. AC Servo Motors can also be classified as asynchronous or synchronous.
  • Pneumatic - Powered by the conversion of compressed air, these actuators are used to control processes that require a quick and accurate response, but not a large amount of force. These compact and lightweight actuators are less energy efficient than electric motors.
  • Hydraulic - With the ability to convert hydraulic pressure and flow into torque and rotation, these actuators can be used when a large amount of force is needed. The most common example is a piston. This motor uses hydraulic fluid under pressure to drive machinery. The energy comes from the flow and pressure, not the kinetic energy of the flow.

Robot Parts: Mechanisms and Kinematics
Robot Manipulator Styles
  • Articulated (Jointed-Arm) - This rotary jointed body can range from simple two-jointed structures to systems with ten or more interacting joints. The advantage of articulated robot arms is that the configuration enables the robot to reach any part in the working envelope.
  • SCARA - A horizontal articulated arm robot with three axes.
  • Cartesian - Rectangular arm that moves on an X, Y, Z coordinate system.
  • Cylindrical - Three degrees of freedom, but moves linearly along the Y and Z axes. It rotates around the base for the third degree of freedom.
  • Polar - Also known as a spherical arm, this manipulator has one sliding motion and two rotational.