Hardware Components for Automation Ch 6
Ch 6 Hardware Components for Automation •
Sections: 1. Sensors 2. Actuators 3. Analog-to-Digital Analog-to-Digital Conversion Conversion 4. Digital-to-Analog Conversion Conversion 5. Input/Output Devices for Discrete Discrete Data Data
Computer-Process Interface •
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The computer must collect data from and transmit signals to the production process for implementing process control, Components are required to implement the interface: –
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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
Computer Process Control System
Sensors •
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A sensor is a transducer transducer that converts a physical stimulus or variable of interest (e.g. force, temperature) from one form into a more useful form (e.g. electrical voltage) to measure the stimulus or variable Sensors are classified as: 1.
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An analog sensor: sensor: produces continuous analog signal such as electric voltage, e.g. thermocouple, strain gages, potentiometer Output signal analog ADC digital data A discrete sensor: sensor: produces an produces an output that can have only a limited number of values Two categories discrete sensor: •
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Binary sensor produces on/off signal, e.g.: limit switch, photoelectric sensor, proximity switch Digital sensor produces digital output signal, e.g. photoelectric sensor array, optical encoder, pulse counter)
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Digital transducers are common to used –
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Easy to read Compatible with digital computer system
Sensors are distinguished as –
Active sensor •
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Responds to the stimulus without any external power e.g. thermocouple thermocouple
Passive sensor •
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Requires an external power source in order to operate e.g. thermistor
Transfer Function •
The relationship between the value of the physical stimulus and stimulus and the value of the signal produced by the sensor in response to the stimulus : S = f(s) where S = output signal, s = stimulus (input), and f(s) is the functional relationship between them
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Transfer function is I/O relationship Calibration of the device is needed to needed to determine transfer transfer function
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Limit switch or binary sensor •
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S = 1 if s S = 0 if s
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Ideal function form for an analog measuring is a simple proportional relationship: S = C + ms C is output value at stimulus value of zero zero (sensor sensitivity) m is proportional constant between s and S
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Common measuring devices used in automation (Table 6.2) Desirable features for selecting sensors (table 6.3)
Actuators •
Hardware devices that convert convert a a controller command signal into into a a change in a physical parameter
The change is usually mechanical (e.g., position of a worktable or velocity of a motor)
An actuator is a transducer because it changes one type of physical quantity into some alternative form
An actuator is is usually usually activated by a low level command signal signal,, so an amplifier may be required to provide sufficient power to drive the actuator
Types of actuators 1. Ele Electr ctrical ical ac actua tuators ors
Electric motors
DC motors AC motors Stepper motors Linear motors
2. Hydrau drauli licc actu actua ators ors
Use hydraulic fluid to amplify the controller command signal
3. Pne Pneuma umatic tic actu actua ators ors
Use compressed air as the driving force
DC motor •
is powered by a constant constant current and voltage
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allows higher speed operation
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Widely used since convenience of using direct current as current as the power source and torque-speed relationship are relationship are attractive in many application DC servomotors servomotors are are a common type used in mechanized and automated because possible to regulate speed
A rotating electric motor
Torque-Speed Curve of a DC Servomotor and load torque torque plot
AC motor •
Is widely used in many industrial applications
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Is classified into: –
Synchronous motors
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Induction motors •
Relative Relative simple construction and low manufacturing cost
Stepper motor •
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Provide rotation in the form of discrete angular displacement Each angular step is actuated by a discrete electrical pulse The rotational speed is controlled controlled by the frequency of the pulses Two operating mode: –
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locked step mode each pulse received by motor causes a discrete angular step, the motor starts and stops, direction of rotation can be reserved Slewing mode higher speed, does not allow for stopping or reversing with each subsequent step
Typical Torque-Speed Curve of a Stepper Motor
Mechanisms to convert rotary motion into linear motion
Linear motor •
Provides a linear motion directly
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Does not require a rotary to linear conversion
Three styles Linear motor
Other types of actuator actuator •
Electrical actuator –
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Linear Solenoid is often used to open and close valves in fluid flow system
Rotary solenoid is provide rotary motion
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Hydraulic actuator
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Pneumatic actuator
Solenoid
Cylinder and piston: piston: (a) Single-acting and (b) Double-acting
Hydraulic vs Pneumatic system
Analog-to-Digital Conversion •
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An ADC converts a continuous analog signal from transducer into digital code for use by computer Hardware device for ADC – –
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Sensor and transducer transducer measuring device Signal and conditioning rendering analog signal to remove and conversion from one signal form to another Multiplexer switching device Amplifier to scale the incoming signal up or down to be compatible range ADC to convert analog signal into digital
Hardware Devices in Analog-to-Digital Conversion
Analog Signal Converted into a Series of Discrete Data by A-to-D Converter
Step for ADC •
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Sampling – converts the continuous signal into a series of discrete analog signals at periodic intervals Quantization – each discrete analog is converted converted into one of a finite number of converted converted (previously defined) discret d iscrete e amplitude levels Encoding – discrete value of amplitude levels are converted into digital code
Relevant factors factors of an ADC
Sampling rate rate – rate at which continuous analog signal is polled Conversion time – how long it takes to convert the sampled signal to digital code Resolution – depends on number of quantization levels Conversion method – means by which analog signal is encoded into digital equivalent, e.g. successive approximation method
Successive Approximation Method
A series of trial voltages are successively compared to the input signal whose value is unknown Number of trial voltages = number of bits used to encode the signal First trial voltage is 1/2 the full scale range of the ADC If the remainder of the input voltage exceeds exceeds the trial voltage, voltage, then a bit value of 1 is entered, if less than trial voltage then a bit value of zero is entered The successive bit values, multiplied by their respective trial voltages and added, becomes the encoded value of the input signal
Successive Approximation Method
Digital-to-Analog Conversion Converts the digital output of the computer into a continuous analog signal to drive an analog actuator(or other analog device) DAC consists of two steps: 1. Decoding – digital output of computer is converted
into a series of analog values at discrete moments in time 2. Data holding – each successive value is changed into a continuous signal that lasts until the next sampling interval
Data Holding Step in DAC: (a) ZeroOrder Hold and (b) First-Order Hold
Input/Output Devices for Discrete Data •
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Two types of contact interface: input and output Input and an d output are relative to the computer
Contact Input/Output Interfaces Contact input interface –series of contacts that are open or closed to indicate the status status of individual i ndividual binary devices such as limit switches and valves
The computer periodically scans the contacts to update values in memory Can also be used for discrete data other than binary (e.g., a photoelectric photoelectric sensor array) array)
Contact output interface –communicates on/off signals from the computer to the process
Values are maintained until changed by the computer
Contact input interface can be used to 1. Binary data:
Contact input interface – input data to computer (a device are read into the computer from some external source, e.g. e. g. a process: closed-open, closed- open, contactnon contact, on-off) Contact output interface –output data from computer
2. Disc Discrrete ete data data othe otherr tha than n bin binar ary: y: Contact input interface –input data to computer (e.g.
photoelectric sensor array) Contact output interface –output data from computer
Contact input interface can be used to 3. Pulse Counters and Generators Pulse counter –converts a series of pulses (pulse train)into a digital value
Digital value is then entered into the computer through its input channel Most common – counting electrical pulses Used for both counting and measurement applications
Pulse generator –a device that produces a series of electrical signals
The number of pulses or frequency of the pulse train is specified by the computer computer
