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Unit 4 Sensors and Actuators Sections: 1. Sensors 2. Actuators 3. Analog-to-Digital Conversion 4. Digital-to-Analog Conversion 5. Input/Output Devices for Discrete Data
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Computer-Process Interface To implement process control, the computer must collect data from and transmit signals to the production process Components required to implement the interface: Sensors to measure continuous and discrete process variables Actuators to drive continuous and discrete process parameters Devices for ADC and DAC I/O devices for discrete data
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Computer Process Control System Transformation Process Continuous and Discrete Continuous and Discrete Parameters Variables ActuatorsActuators SensorsSensors DACDAC ADCADC ComputerComputer ControllerController Output Devices Input Devices
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Analog and Digital DIGITAL ANALOG Rời rạc theo thời gian, gần Liên tục theo thời gian, gần Tín hiệu gũi với máy tính và thiết bị gũi với tự nhiên hơn. tính toán. Thường dùng cho đo lường, Dùng trong tính toán, truyền Công dụng làm việc với các thiết bị thông dữ liệu số. chuyên về analog. Lưu dưới dạng sóng, trên Lưu dưới dạng bit, trên các các thiết bị từ (băng từ, đĩa thiệt bị nhớ đắt tiền (fash, Khả năng lưu trữ từ,...), chứa nhiều thông tin rom, ...), chứa ít thông tin hơn. hơn. Lượng dữ liệu truyền đi trong Lượng dữ liệu truyền đi Truyền thông 1 khoản thời gian rất ít và dễ nhiều hơn và ít bị lỗi. bị nhiễu.
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Sensors Stimulus (s) Signal (S) PhysicalPhysical MediumMedium SensingSensing ConditioningConditioning TargetTarget ElementElement HandlingHandling Temperature Resistance Voltage Information Transducers Micro-sensors 10-6m
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Transfer Function S f (s) where S = output signal; s = stimulus; and f(s) = functional relationship For binary sensors: S = 1 if s > 0 and S = 0 if s < 0. The ideal functional form for an analogue measuring device is a simple proportional relationship, such as: S C ms where C = output value at a stimulus value of zero and m = constant of proportionality (sensitivity)
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Example The output voltage of a particular thermocouple sensor is registered to be 42.3 mV at temperature 105C. It had previously been set to emit a zero voltage at 0C. Since an output/input relationship exists between the two temperatures, determine (1) the transfer function of the thermocouple, and (2) the temperature corresponding to a voltage output of 15.8 mV.
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Solution S C ms 42.3 mV = 0 + m(105C) = m(105C) or m = 0.4028571429 S = 0.4 (s) 15.8 mV = 0.4 (s) 15.8 / 0.4 = s s = 39.22C
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Sensors A sensor is a transducer that converts a physical stimulus from one form into a more useful form to measure the stimulus Two basic categories: 1. Analog Ultrasonic 2. Discrete (distance) Binary Digital (e.g., pulse counter) Light (light intensity) Sound Touch (db pressure)
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Other Sensors Temperature RFID Barcode Proximity Vision Gyroscope Compass Tilt/Acceleration Etc.
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Actuators Hardware devices that convert a controller command signal into a change in a physical parameter The change is usually mechanical (e.g., position or velocity) An actuator is also a transducer because it changes one type of physical quantity into some alternative form An actuator is usually activated by a low-level command signal, so an amplifier may be required to provide sufficient power to drive the actuator
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Actuators Logical MechanismMechanism Signal Signal Processing Signal Processing ElectricElectric HydraulicHydraulic & Amplification& Amplification Pneumatic Final ActuationFinal Actuation Pneumatic Element Element ActuatorActuator SensorSensor
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Types of Actuators 1. Electrical actuators Electric motors DC servomotors AC motors Stepper motors Solenoids 2. Hydraulic actuators Use hydraulic fluid to amplify the controller command signal 3. Pneumatic actuators Use compressed air as the driving force
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Stepper motor and Servomotor Đi u khi n có ph n hề ể ả ồi và không ph n hả ồi T o chuy n đạ ể ộng cho m t trộ ục của máy CNC
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Torque-Speed Curve of a DC Servomotor and Load Torque Plot Torque, T Load Stepper AC Servo DC Servo Operating Points Speed, ω
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Motor Controllers The POSYS® 3004 (Designed & Made in Germany) is a PC/104 form factor board dedicated to high performance motion control applications with extensive interpolation functionality. The POSYS® 3004 is designed to control up to 4 axes of servo and stepper motors and provides hardware linear, circular, Bit Pattern and continuous interpolation which allow to perform the most complex motion profiles. Update rates per axis do not exist as each axis runs in absolute real-time mode simultaneously which makes these boards to one of the best performing motion controllers for up to 4 axes in the market.
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Stepper Motors 360 Step angle is given by: : n s where n is the number of steps for the stepper motor (integer) s A n Total angle through which the motor rotates (A ) is given by: m p m where n = number of pulses received by the motor. p 2f p Angular velocity is given by: n where f = pulse frequency p s 60 f N p Speed of rotation is given by: n s
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Example A stepper motor has a step angle = 3.6. (1) How many pulses are required for the motor to rotate through ten complete revolutions? (2) What pulse frequency is required for the motor to rotate at a speed of 100 rev/min?
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