Actuators allow the robot to manipulate its physical environment. Common types of actuators are limbs, wheels, and propellers. Almost all robots are powered and controlled by electrical circuits. Hence the most common way of building an actuator is to come up with a mechanical design that translates the rotational motion of an electric motor into the desired motion of an actuator.
The brushed motor design is the oldest electric motor design. The rotor carries multiple inductive windings that are electrically isolated from each other and wound on the rotor with evenly distributed angular offsets. The stator is made of a pair of magnets with opposite polarities. While the rotor turns, each winding carries the input current for a fraction of the full turn such that a rotational force (due to magnetism) is maintained. If the current would flow through only one of the windings all the time then the rotor would be driven to an angle at which the magnetic field created by the rotor winding aligns with the magnetic field of the rotor and it would stay at that angular position. The switching of the current from one winding to to the next is facilitated by the current-carrying contacts of the rotor brushing over the contacts of the windings, which are evenly distributed over the rotor axis, as the rotor turns. Hence the name of this type of motor.
The L293D is a good choice if your motor will not require more than 600 mA.
Stepper motors are brushless motors where the winding that carries the current is selected digitally. This allows digital control over driving the rotor to a specific discrete orientation or over the rotation speed. Stepper motors are used in applications where the rotor needs to be stepped through discrete angles. That's why they are used in milling machines or 3D printers. In robotics, stepper motors are often used in places where previously a traditional brushed motor would be used. For example, to turn the wheels of a wheeled robot. The advantage using a stepper motor in these cases is that the number of rotational steps can be counted by the microcontroller and therefore eliminate the need for an odometer.
Trinamic is a manufacturer of stepper motors and stepper motor controllers known for their high quality.
Like many DC motor drivers, the L293D can also be used as a driver for stepper motors. If you need to supply more current, for stepper motors that can deliver more torque, you should look into the L298, which is typically connected to a supply voltage significantly higher than the motor's rated voltage and then controlled via the L297 chip, which makes sure that current is restricted despite the higher-than-rated voltage applied to the motor. The schematics on the datasheets and on this page show how to connect the two.
Servo motors are actually traditional DC motors with additional circuitry. A servo motor does not perform a continuous rotation motion but rotates to an angle that depends on the applied voltage. There are numerous applications for servo motors. For example, steering flaps or wheels in vehicles.
Most electric motors sold for use in toy-sized vehicles have high no-load rotation speeds. They are rated for multiple hundreds, thousands, or ten thousands of revolutions per minute (rpm) with no load attached. Depending on your application, you may want a motor that rotates slower. That's what gear boxes are for. A gear box has a transmission ratio by which the speed of the rotor is slowed down and the maximum power can be achieved at speeds suitable to the application. However, there are some disadvantages to using gear boxes:
- they are fragile,
- they incur energy losses in the form of friction,
- they reflect the rotor's inertia to the output proportional to the square of the transmission ratio.
Hydraulic And Pneumatic Actuators
If electric motors can not provide enough torque for your application you may want to look into hydraulic or pneumatic systems, like in the ATLAS robot by Boston Dynamics.