Actuators in Robotics: Electric, Hydraulic, and Pneumatic Systems, Slides of Introduction to Robotics

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Actuators in Robotics

Introduction to Actuators

• (^) Actuators enable the movement of robot joints and enable the robot arm and grippers to move and perform tasks. Like muscles, they give the power to the joints to make proper movements as per the design.
• The commonly used actuators are electric actuators, hydraulic actuators, and pneumatic actuators based on the source from which power is obtained, that is, electricity, compressed fluid, and pressurized air, respectively.
• (^) In electric actuators, electric energy is the source of power for the actuation. In hydraulic actuators, the hydraulic energy stored in a reservoir is converted into mechanical energy by making use of valves and pumps. In pneumatic actuators, the energy stored in the compressed air is converted into mechanical energy.
• The various links of a robot manipulator are connected through joints, and the energy needed for moving the links is provided by the actuators provided at the joints.
• (^) The required characteristics for a good actuator are as follows:
  • (^) It needs to have a low inertia.
  • Its power-to-weight ratio should be high.
  • (^) It should be able to develop high accelerations and wide range of velocities.
  • (^) It should have higher accuracy in positioning.
  • (^) It should be able for trajectory tracking.

Components of a Hydraulic system

Components of a Hydraulic system

• (^) Hydraulic actuators convert the fluid power into mechanical power to do useful work.
• (^) Hydraulic oil is used as working fluid in for hydraulic systems. It is stored in a reservoir.
• (^) An hydraulic pump used to supply the oil to the system, which converts the mechanical energy into hydraulic energy by forcing fluid from the reservoir into the system.
• (^) Fluid lines will help to transport the fluid from the pump through out the hydraulic system.
• (^) Valves are used to control pressure, direction and flow rate of the hydraulic fluid to do the required work.
• (^) Actuator converts hydraulic energy into mechanical energy to do work.

• (^) Pneumatic actuators convert the fluid power into mechanical power to do useful work.
• (^) Compressors are used to compress the fresh air drawn from the atmosphere.
• (^) Storage reservoirs are used to store a given volume of compressed air.
• (^) Valves are used to control the direction, flow rate and pressure of compressed air. External power supply (motor) is used to drive the compressor.
• (^) The piping system carries the pressurized air from one location to another.
• (^) Air is drawn from the atmosphere through the air filter and raised to the required pressure by an air compressor.
• (^) As the pressure rises, the temperature also rises and hence, air cooler is provided to cool the air with some preliminary treatment to remove the moisture.
• Then the treated pressurized air needs to get stored in the reservoir to maintain the pressure. In the storage reservoir, a pressure switch is fitted to start and stop the electric motor when pressure falls and reaches the required level, respectively.
• (^) The cylinder movement is controlled by a pneumatic valve. One side of the pneumatic valve is connected to the compressed air and the other side is connected to port A and port B of the cylinder

Electric Motors as actuators

• (^) Electric motors can easily be controlled by a computer and

provide more than pneumatic actuators.

• (^) Also, they do not create contamination of the environment

unlike hydraulic actuators.

• (^) Electric motors are fast and accurate in their operation,

easy to operate, and can be managed with advanced

control systems.

Stepper Motors

• (^) Unlike normal motors that make continuous movements, stepper motors move in fixed angular movements or angular steps.
• (^) They move through a fixed angle when a pulse is received at the driver of motor.
• (^) This train of pulses synchronize the step wise rotation of the stepper motor.
• (^) Stepper motor is also an electromagnetic actuator, just like normal motors that are continuously driven. All these convert the electromagnetic energy to mechanical energy and perform work.
• (^) Stepper motors are widely used in industrial applications as well as in small and medium robots mainly because of their advantage that they do not require any feedback system.
• (^) They can be controlled digitally, because of which they are also called digital motors or actuators. These motors do not require the Digital to-Analog Converter (DAC) when they are connected to a computer-controlled system.

Stepper Motors

• (^) A stepper motor consists of a stator, which is multi-pole with multi-phase winding and a rotor structure, which is normally a permanent magnet or variable reluctance type.
• (^) A train of electric pulses is applied at the excitation windings of the stepper motor, which is converted into step-by-step rotation of the mechanical shaft.
• (^) For each discrete pulse, the shaft of the motor rotates through a fixed angle.
• (^) It moves through one step corresponding to a single pulse input.
• (^) The angle through which the stepper motor shaft turns for each pulse is referred to as the step angle.
• (^) If rotor makes n movements to complete a rotation, then step angle = 360°/n.
• (^) The resolution of the stepper motor is given by, Resolution = Step angle / 360°

Stepper Motors

• (^) The stator has two phases P1, and P2 (two sets of windings) placed at 90°, with four salient poles in the stator geometrically separated by an angle of 90° with the adjacent one.
• (^) The rotor consists of a two-pole permanent magnet.
• (^) Each phase can be in one of the three states: 0 if not energized, 1 and - 1 if the phase is energized based on the direction of current flow to control the direction of motor rotation.
• (^) There are two methods of coil excitation: full-step excitation and half- step excitation.
• (^) In full-step excitation, only one phase is energized at a time. For a half- stepping sequence, both phases are energized simultaneously in alternate steps.

Stepper Motors

• (^) When one pair of coils of the stator is energized, the rotor (permanent magnet or soft iron in case of variable reluctance type motors) will rotate so as to align itself with the magnetic field developed by the stator.
• (^) The rotor will stay at this position unless the field changes. When the power to these coils is discontinued and when the other set of coil is energized, the rotor will again rotate to align itself with the new field developed by the new excitation pattern in the stator.
• (^) In the third step, the first coil will once again be energized, but this time with opposite polarity, while the second coil pair is turned off. This will cause the rotor to rotate one more step in the same direction. In the last step, the first coil is turned OFF and the second coil is turned ON with opposite polarity.

Servo Motors