Actuators are used for moving the robot’s joints (such as picking up a block and placing it somewhere else). To be more specific, they require a control signal and a source of energy to provide a mechanical output.

TYPES OF ACTUATORS

- MOTION BASED

* ROTARY

Actuators (like Motors) that convert energy into mechanical work in the form of rotational motion (along the axis of the motor) to produce torque are called Rotary Actuators.

* LINEAR

A linear actuator is an actuator that creates motion in a straight line, in contrast to the circular motion of a conventional Rotary actuator. These are generally modified rotary actuators with a Gearbox, designed to provide Translation motion from the rotational motion.

- ACTUATION BASED

* HYDRAULIC ACTUATOR

Basic principle of hydraulic actuators is Pascal’s Law and the main drawback of this kind of actuator is limited acceleration.

* PNEUMATIC ACTUATOR

They have air as the working fluid. Therefore, they need a compressed air network system for its working. These enable considerable forces to be produced from relatively small pressure changes. It can quickly respond as the power source does not need to be stored in reserve for operation. Moreover, pneumatic actuators are cheaper.

* ELECTRIC ACTUATOR

An electric actuator is a mechanical device used to convert electricity into kinetic energy.

DC MOTORS

They are used often because of the ease of controlling speed and direction. They quickly respond to changes in the controlling signal.

BRUSHED

These motors generally have low torque and high speed. They, however, offer high resistance (due to the presence of brushes), causing a lot of heat dissipation and jerky motion.

BRUSHLESS (BLDC) MOTOR

Brushless DC electric motors are also known as electronically commutated motors (ECMs, EC motors).

Primarily efficiency is the most important feature for BLDC motors. Because the rotor is the sole bearer of the magnets and it doesn’t require any power. i.e. no connections, no commutator and no brushes.

Construction of Brushless DC motor

In this motor, the permanent magnets are attached to the rotor. The current-carrying conductors or armature windings are located on the stator. They use electrical commutation to convert electrical energy into mechanical energy.

(Diagram for Inrunner Motor)

• * Outrunner – The field magnet is a drum rotor which rotates around the stator. This style is preferred for applications that require high torque and where high rpm isn’t a requirement.
• * Inrunner – The stator is a fixed drum in which the field magnet rotates. This motor is known for producing less torque than the outrunner style but is capable of spinning at very high rpm.

(Diagram for Inrunner Motor)

A trapezoidal AC waveform is used to electro-magnetize the stator coils. When one pair of stator coils is at high voltage, the other two are at low voltage. This forms a moving electromagnet. The rotor rotates, when attracted to this moving electromagnet. These motors are less jerky and more power-efficient. They are more efficient at high speeds.

Uses: In multirotor, CD Drives, & Electric Vehicles

SERVO MOTOR

Servo motors are used in closed-loop systems with a digital controller. The controller sends velocity commands to a driver amplifier, which in turn feeds the servo motor. Some form of feedback device, such as a resolver or encoder, provides information on the servo motor’s position and speed. These motors thus are capable of actuating motion with precision and have a holding torque which makes them useful in cases like aileron control in RC planes.

This is how it looks from the inside. Did I read it right? A potentiometer in a motor? Yup, that’s right, the potentiometer is connected to the final output shaft of the servo motor, this produces a variable voltage across the potentiometer which changes relative to the angle of the shaft. This reading is then compared (in the control circuit) to the angle input by the user. When the difference comes to zero, the motor stops rotation indicating it has reached the position it was supposed to. Since the potentiometer has a limited range, the standard (not modified ones) have a range of 180 degrees only.

STEPPER MOTOR

Stepper motors can operate with or without feedback, with the rotation of the motor broken up into small angular steps. This helps to control the rotation with steps precisely, but unlike servo motors, this allows complete rotational freedom for the stator. It is controlled by pulsed command signals and can stop precisely at a commanded point without the need for brakes or clutch assemblies. When power is removed, a permanent-magnet stepper motor generally remains in its last position.

Multiple stepper motors can be maintained in synchronization by driving them from a common source.

Uses: To Drive Coordinate Axes in 3d Printers

ADVANCED ACTUATOR

Actuators can also be actuated by applying thermal or magnetic energy to a solid-state material. Thermal actuators use shape-memory materials such as shape memory alloy (SMAs).

Here is how the training is done for the Thermal Actuators:

1. Fix the SMA into the desired shape (using clamps)
2. Heat it to 400°C for 8-10 mins
3. Drench it in cold water
4. Take out the SMA from the clamps
5. Test it in hot water
6. Redo steps 1 to 4 in case the SMA doesn’t retain its shape

Similarly, magnetic actuators use Magnetic shape memory alloys (MSMAs). Objects made up of these materials are trained (by applying strong magnetic fields for the MSMAs) to take up a certain shape when introduced to some specific conditions.