An actuator is a mechanism for activating process control equipment by the use of pneumatic, hydraulic, or electronic signals. There are several types of actuators in robotic arms.
Synchronous Actuator - 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 Actuator - 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.