Automated Controls Flashcards
Closed Loop
A signal path which includes a forward path, a feedback path, and a summing point, and forms a closed circuit.
Process Variable
The parameter that is desired to be controlled. ex. temperature, flow rate, pressure, liquid level
Controller
A device which operates automatically to regulate a controlled variable.
Derivative (rate) control action
how fast the error is changing
Error Signal
this is the signal derived in a closed loop control system from the difference between the set-point signal and the feed-back signal.
Integral (reset) control action
how long the error has been present
Open loop
a signal path without feedback
Process (controlled) variable
in a control loop, it is the variable which is sensed to originate a feed-back signal.
Proportional control action
how big the error is
Set-point
an input variable which determines the desired value of the controlled variable
Span
the algebraic difference between the upper and lower range values, if the range where -20 to 200, the span would be 220.
Transducer
A device or element which converts energy from one type to another
Dis-continuous Control
uses actuators which are either on or off
ex. baseboard heater, water pump.
Continuous Control
the actuator output is variable and is proportional to the difference between the sensor and the setpoint.
sensors are analog
What does P & ID stand for?
Piping and Instrumentation Drawings
What does PID stand for?
Proportional
Integral
Derivative
What is a P&ID drawing?
process control block diagrams, they are like schematics but include pipes, pumps, tanks, sensors, actuators, controllers etc.
In a P&ID balloon the first letter =
Process variable
Ex. T - Temperature, L - Level, F - Flow, P - Pressure
P&ID next 1,2 or 3 letters =
Instruments
ex. I - indicator, R - recorder, C - controller, V - valve, A - alarm
Proportional control is how ____ the error is?
BIG
Integral control is how ____ the error has been present
LONG
Derivative control is how _____ the error is changing
FAST
Active Transducer
generates output without and external power supply
ex. thermocouple
Passive Transducer
do not generate and output, only physically or electrically change, an external supply is required.
ex. RTD
Sensor primary element
converts the process variable into a format suitable for measurement
ex. pressure to motion
Sensor secondary element
converts the primary element into an electric change
ex. motion to a change in resistance, permeability, etc.
Low pressure vacuum/pressure sensor?
Bellow
More pressure vacuum/pressure sensor?
Diaphragm
Most pressure vacuum/pressure sensor?
Bourdon tube
Bonded wire strain gauge characteristics
R=Kl/CMA
When stretched R increases
When compressed R decreases
Low "gauge factor"
Less sensitivePiezoresistive Strain Gauge characteristics
Crystal changes R with force
When stretched R increases
When compressed R decreases
Higher "gauge factor"
More sensitiveThermocouple characteristics
self powered simple rugged inexpensive wide variety wide temp range non-linear low voltage least stable least sensitive
RTD characteristics
most stable most accurate more linear than thermocouple positive temp coefficient expensive current source required
Thermistor characteristics
high output fast two-wire ohms measurement non-linear current source required negative temperature coefficient
I.C. Sensor characteristics
most linear
highest output
inexpensive
Common output standard control signals
4-20mA 10-50mA 1-5Volts 1-10Volts 3-15 PSI
Op-Amp open loop Gain =
200,000 w/o feedback
Another name for an actuator?
Servomechanism
3 types of actuators
hydraulic
pneumatic
electrical
Types of electrical actuators
heaters
coolers
lights
electromechanical
types of electromechanical actuators
solenoids magnetic clutches stepper motors linear actuators eddy current couplings valves
Describe a solenoid
magnetic coil moving a permeable core AC or DC available Intermittent or Continuous duty Single acting spring return Double acting energized return
Magnetic Clutch characteristics
Generally fully on or off
friction disks or drums are electrically applied/released
some mag clutches use magnetic particles to engage
What is an actuator?
a device that converts a signal input into a mechanical motion
Accurate positioning control is commonly achieved in applications like robotics and computer disk drives by using unique motors called ?
Stepper Motors
What is a stepper motor?
an increment, brushless DC motor that moves in discrete angular steps in accordance with set sequences of current pulses that are supplied to the motor.
A stepper motor converts
digital data into an angular position, or pulses into speed of rotation
Name three stepper motors
Variable reluctance stepper motor
Permanent magnet stepper motor
Hybrid stepper motor
Which is the most widely used stepper motor?
Hybrid
what is an encoder?
a device that is fastened to a rotating shaft to produce a series of pulses that indicate shaft position
What is the most popular encoder?
incremental optical shaft encoder
What does the discriminator circuit do when connected to a LVDT?
it sends out an output the polarizes the output of the LVDT when the output of the LVDT rises above a predetermined threshold
How does a mag meter work to measure liquid flow?
liquid with conductive properties passes through a magnetic field induces a voltage proportional to the flow rate
what is the principle operation of an ultrasonic level sensor?
sound waves are emitted and bounce back from the control variable, the time it takes for this signal to return can be equated to a level
RTDs are typically made of?
Platinum and nickel
what is the function of a bifilar winding?
eliminate inductance
RTDs and thermistors both indicate a change in ______ for a change in _____
Resistance
Temperature
Bellows and diaphragms are both used to convert pressure into ?
Motion
What is negative feedback?
compares the set point signal to the process variable (the error)
What is positive feedback?
adds the process variable and the set point (not good)
An enclosed thermopile with radiation focused on it is called ?
Radiation pyrometer
