1 CHAPTER 3: ACTUATORS As summarized in the previous sections, a mechatronics system can be partitioned into function blocks illustrated in below Fig. CHAPTER 3: ACTUATORS The major factors in selecting an actuator for mechatronic applications are • Precision • Accuracy and resolution • Power required for actuation • Cost of the actuation device The most popular actuators in mechatronic systems are direct current (DC) motors. DC motors are electromechanical devices that provide precise and continuous control of speed over a wide range of operations by varying the voltage applied to the motor. The DC motor is the earliest form of electric motor CHAPTER 3: ACTUATORS Therefore, an actuator can be viewed as having the structure depicted in below Fig. Typically, actuators are considered only as energy conversion devices. However, with the proliferation of power electronics, we will take a more inclusive view of actuators that also includes power amplification Actuator functional diagram CHAPTER 3: ACTUATORS An electrohydraulic linear actuator can also be similarly classified, where the spool valve is the power amplification/modulation block with spool position as the control signal and the hydraulic pressure/flow is the energy source. The hydraulic cylinder acts as the energy conversion device that converts fluidic energy to mechanical energy Electrohydraulic and electromechanical actuators CHAPTER 3: ACTUATORS For a typical electromechanical actuator, such as a DC motor the power amplification block is the motor driver that amplifies signal level (low current/power) control signal to the higher power (large current) signal that is used to convert electrical energy to mechanical energy through the electromagnetic principle Electrohydraulic and electromechanical actuators CHAPTER 3: ACTUATORS Mechatronic systems employ actuators or drives that are part of the physical process being monitored and controlled. Actuation is the result of a direct physical action upon the process, such as removing a workpiece from a conveyor system or the application of a force. It has a direct effect upon the process. Actuators take low power signals transmitted from the computer and produce high power signals which are applied as input to the process. There are many types of actuating devices, some of the most common ones include solenoids, electrohydraulic actuators, DC or AC motors, stepper motors, piezoelectric motors, and pneumatic devices. Electrical actuators convert electrical command signals into mechanical motions. In this chapter, emphasis is placed on DC motors, stepper motors, and fluid power devices (electrohydraulic) because of their popularity in mechatronics. Although the main focus in this chapter is on DC motors, it should be noted that AC motors are also widely used for servomechanism. 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors The desirable features of DC motors are their high torque, speed control ability over a wide range, speed-torque characteristics, and usefulness in various types of control applications. DC motors are well suited for many applications, including manufacturing equipment, computer numerically controlled systems, servo valve actuators, tape transport mechanisms, and industrial robots. The DC motor converts direct-current electrical energy into rotational mechanical energy. It makes use of the principle that a wire carrying a current in a magnetic field experiences a force. The windings wrapped around a rotating armature carries current. The armature is the rotating member (rotor), and the field winding is the stationary winding (stator). The rotor has many closely spaced slots on its periphery. These slots carry the rotor windings. The rotor windings (armature windings) are powered by the supply voltage. An arrangement of commutation segments and brushes ensures the transfer of DC current to the rotating winding. A schematic of a DC motor is shown in Figure 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors 3.1 DC Motors the response of the system for a constant voltage of 10 V DC. As seen from the figure, if 10 V is constantly applied to the motor, the motor shaft will move by 2130 rad in 10 sec (i.e., the mass will move by 0.022 x 2130 46.86 m). As expected, the result shows that, with a constant voltage given to the motor, it will continue to rotate. However, to lift the mass to a specified height, we would need a controller that would monitor the angular displacement of the motor shaft and develop a controlled input voltage to the motor that would take the mass to the specified height 3.1 DC Motors A permanent magnet DC (PMDC) motor is a motor whose poles are made out of permanent magnets to produce the stator field. Advantages: Since no external field circuit is needed, there are no field circuit copper losses. Since no field windings are needed, these motors can be considerable smaller. Widely used in low power application. Field winding is replaced by a permanent magnet (simple construction and less space). No requirement on external excitation. Disadvantages: Since permanent magnets produces weaker flux densities then externally supported shunt fields, such motors have lower induced torque. There is always a risk of demagnetization from extensive heating or from armature reaction effects (Some PMDC motors have windings built into them to prevent this from happening). 3.2 AC Motors CHAPTER 3: ACTUATORS CHAPTER 3: ACTUATORS CHAPTER 3: ACTUATORS In recent years, the stepper motor has emerged as a cost-effective alternative to DC motors in motion-control applications. The stepper motor is an actuator which translates electrical pulses into precise, equally spaced, angular movements of the rotor in the form of steps. The rotor is positioned by magnetically aligning the rotor and stator teeth, which occur when the air gap between the two sets of teeth is minimized and aligned. Stepper motors are categorized according to their type. Two basic types of motors are 1. Variable reluctance (VR) stepper motors. 2. Permanent magnet (PM) stepper motors. In VR motors, the stator windings are excited in a sequence that will cause the rotor to align to a position that minimizes magnetic reluctance between the stator and rotor. In PM motors, the excitation pattern is provided by the permanent magnets. Permanent magnet motors have a smaller step than variable reluctance motors—typical values being 1.8° versus 15°, which makes them more suitable for accurate positioning applications; however, the torque per unit volume of the PM motor is considerably lower than that of the VR moto CHAPTER 3: ACTUATORS CHAPTER 3: ACTUATORS The system consists of three components; the drive, the stepper motor, and the load. The drive circuit has two command inputs, a step per second command, sps*, and a direction command, dir*. It produces four voltage outputs, one for each phase of the motor. CHAPTER 3: ACTUATORS CHAPTER 3: ACTUATORS The complete block diagram model for the four-phase PM motor is presented in Figure. The contents of the phase B, C, and D blocks are identical to that of the phase A model. World of Eclectrical Motors http://www.electrical-knowhow.com/2012/05/classification-of-electric-motors.html Tịnh tiến ↔ Quay Servo motor h. Servo Motor The motor power The motor torque The operating speed of the motor The torque–speed characteristic 1.0- Thùng dầu thủy lực; 2.0- Động cơ điện; 3.0- Bơm dầu thủy lực; 4.0- Đồng hồ đo áp suất; 5.0- Van an toàn thủy lực; 6.0- Van phân phối thủy lực; 7.0- Van tiết lưu một chiều; 8.0- Xi lanh thủy lực; 9.0- Lọc dầu đường hút Basic Hydraulic Schematic Example of pneumatic system pneumatic cylinder Force F= p.A
