Electrical Rotary Actuators - Essay Example

Actuators are devices used for driving various types of objects to obtain a desired amount of displacement. This displacement may be of a linear or rotary nature. The driving force of an actuator may be generated by either an electrical, hydraulic or pneumatic input. The rotary actuators discussed here, are concerned with applications where a rotary displacement with a desired angle of rotation is required, and use electrical energy to generate the driving force. Construction: The basic components of an electric rotary actuator are a motor and a reduction gearbox mechanism.

Electric rotary actuators drive components rotationally via electromagnetic power from a motor. The reduction gear translates the speed and torque of the motor to the required scale. Suitable damping and spring return mechanisms are also built in. The rotational element can either be a shaft or a table. Circular shafts often include keyways, while tables provide a bolting pattern to mount other components. They usually provide control and indexing capabilities to allow multiple position stops along strokes.

Other auxiliary components include return springs, seals, limit switches for both ends of rotation span, etc. A simple rotary actuator model using a DC motor is considered for the study and analysis of the device. The DC motor directly provides a rotary motion. For practical application, this rotary displacement is transmitted into suitable transitional motion by attaching an output coupling mechanism, consisting of wheels or drums and cables. The figure 1 shows the electrical circuit of the armature, and the free body diagram of the rotor of the DC motor.

Electrical circuit of Armature Free body diagram of rotor Figure 1.In the electrical circuit of the armature, the resistance of the armature coil is represented by R in ohms, and the inductance is presented by L in henrys. The input source is shown as a voltage V. The back EMF produced, is shown as a voltage source, which opposes the input source. For the rotor free body diagram, the torque developed is T and rotation angle is .

The effect of the moment of inertia of the rotor and damping method employed is also taken into the account. The damping is provided to avoid large overshoots of the rotational displacement. For the purpose of sample study, the following values of other parameters are initially assumed as:1. Moment of Inertia of the rotor, ( J ) = 3.2284E-6 kg m^2 / s^2.2. Damping ratio of the mechanical system, ( b ) = 3.5077E-6 Nms3.