Robots achieve the ability to move or execute specific mechanical tasks with the help of various types of actuators, such as electric, hydraulic, pneumatic, etc. Often called the muscles of robots, actuators significantly impact a robot’s functional features, i.e., degrees of freedom (DOF), speed or velocity, accuracy, repeatability, load capacity (payload), etc.
Therefore, the world of actuation is always transforming with new technologies and ideas to fulfill the growing demand for faster, smaller, and more powerful actuators. Actuators are essential devices in robotics and widely common, particularly for industrial applications.
Actuation is the process of conversion of energy to mechanical form. An actuator is a hardware device that accomplishes this conversion. It converts a controller command signal into a change in a physical parameter.
Actuators come in various types and sizes, depending on the load associated with factors like force, torque, speed of operation, precision, accuracy, and power consumption. One of the prevalent types of actuators is electric motors such as servomotor, stepper motor, and direct current (DC) motors. A motor allows the robot to control a wheel, a switch, or even an arm.
Robot manufacturers usually use electric actuators since they are fast, efficient, and accurate. They are easy to control, can achieve high velocities (1000 – 10000 rpm), and have ideal torque for driving. At the same time, they are very weak or unpleasantly heavy because of their complexity.
Servo motor is a mechanism based on feedback control. It has a high maximum torque/force that allows high (de)acceleration. It is robust and has a high bandwidth that provides accurate and fast control.
Key advantages of servo motors are as follows:
- Power supply available everywhere.
- Low cost
- Large variety of products
- High power conversion efficiency
- Easy maintenance
- No pollution in the working environment
The disadvantages are:
- Overheating in static conditions
- Need special protection in flammable environments.
Stepper motors provide rotation in the form of discrete angular displacement. They can achieve precision angular rotation in both directions and are commonly employed to accommodate digital control technology. Stepper motors are, in general, heavier than servomotors for the same power. The high the voltage of electric motors, the better the power-to-weight ratio.
Then, we have electromechanical actuators that convert electrical energy into mechanical energy. Based on the basic principle of magnetism, they come in DC, AC, and stepper motors.
The third option is hydraulic and pneumatic actuators, which use fluid power and compressed air, respectively. Fluid power refers to the energy that is transmitted via a fluid under pressure. When pressure is applied to a confined chamber containing a piston, the piston will exert a force causing a motion. Pneumatic systems deliver the lowest power-to-weight ratio, while hydraulic systems have the highest power-to-weight ratio. Pneumatic actuators are mostly used for the opening and closing of grippers.
Hydraulic systems are very stiff and non-compliant, whereas pneumatic systems are easily compressed and thus are compliant. Notably, stiff systems have a more rapid response to changing loads and pressures and are more accurate. Although stiffness causes more responsive and more accurate systems, it also creates a danger if all things are not always perfect. Hydraulic actuators are very efficient, yet their cost is high.
Here are the advantages of a hydraulic actuator.
- Easy to control and accurate
- Simpler and easier to maintain
- Constant torque or force regardless of speed changes
- Easy to spot leakages of system
- Less noise
Disadvantages of the hydraulic actuator.
- Proper maintenance is required
- Leakage of the fluid creates environmental problems
- Wrong hydraulic fluid for a system can damage the components
Advantages of pneumatic actuators
- Clean, less pollution to the environment
- Safe and easy to operate
Disadvantages of pneumatic actuators
- Loud and noisy
- Lack of precision controls
- Sensitive to vibrations
The fourth type of actuators is called piezoelectric actuators, which are successfully implemented in many applications today. They use the piezoelectric effect to create motion. When electricity flows through a piezoelectric material, it creates a physical deformation proportional to the applied electric field, known as the indirect piezoelectric effect.
Piezoelectric actuators are used in loudspeakers, piezoelectric motors, acceleration sensors, vibration sensors, etc., and can be used to create either rotational or linear motion. These actuators’ main advantages are their very high dynamics (up to 40 kHz), theoretically unlimited resolution (in the field of nanometers), high force, low consumption of electrical energy, and very compact construction.
Another significant advantage is the possibility of having the actuator, force sensor, and position sensor contained in a one-piece unit. However, the main problem with implementing piezoelectric actuators is the small oscillating movements caused by its expansion and contraction.
Ideal characteristics of actuators
- Weight, power-to-weight ratio, operating pressure
- Stiffness against deformation
- Appropriate torque output.
- High torque density or the continuous output torque per mass
- High back drivability that protects the system against damage in environmental impacts, especially unexpected ones.
- High transparency and smooth energy flows between the actuator and end effector in both directions.
- High Efficiency.