Automated Controls Flashcards
Where are automated controls most common?
commercial and industrial applications
List examples of an automated control system
- Cruise Control
- HVAC
- Food Production
- PF correction
- Autopilot for ships and aircrafts
List benefits of automated controls
- Consistent
- Reduced labour (initial labour is usually much higher)
- Improved safety
- Lower operating cost
What are the two categories for AC?
Open Loop and Closed Loop
Open loop characteristics and examples of applications?
- Requires an operator
- not self correcting
- no feedback
- Bathroom heat lamp
- Heat and a/c for most cars
Closed loop characteristics and examples?
- no operator required
- self correcting
- has feedback
- Bathroom baseboard heat
- climate control in some cars
advantages and disadvantages of open loop?
Advantages:
- Low initial cost
- easy to install
- simple to troubleshoot
Disadvantages:
- ongoing labour
- error
Closed loop advantages and disadvantages?
Advantages:
- self correcting
- reduced ongoing labour costs
- reduced error
- higher performance
Disadvantages:
- High initial cost
- expensive to install
- more complex to troubleshoot
What is the error signal?
the difference between the set point and sensor signals
Examples of process variables?
- temperature
- level
- pressure
- speed
- light
- flow
- weight
What are the two modes of control for each category?
- Discontinuous
2. Continuous
What do discontinuous controls use and what are the characteristics? How is the process variable corrected?
- Use actuators which are either on or off
- corrected by varying the time intervals in which the actuator is on or off
In discontinuous control, the process variable error can be substantial if the ___________ is too wide
Dead Band
In continuous control, the actuator output is ________________________________________?
variable and proportional to the difference between the set point and the sensor.
Are the sensors in continuous control analog or digital? Why?
Analog, because their output can vary continuously
What is the process variable error dependant on?
- RESOLUTION of the sensors
- PERFORMANCE of the actuators
- COMPLIANCE of the process variable
what does the “PID” stand for in PID control?
- Proportional
- Integral
- Derivative
What are P & ID Drawings, what do they include and which block diagram is it?
- Piping and Instrumentation Drawings
- Pipes, pumps, tanks, sensors, actuators, controllers, etc…
- Process control block diagrams
What does the first and then remaining letters stand for in a P & ID balloon
First letter = Process Variable
Next letters = instrument
List the methods and what they mean in PID Control
Proportional = "How BIG is the error?" Integral = "How Long has the error been present?" Derivative = "How FAST is the error changing?"
What are sensors also known as?
Transducers
What does a transducer do?
Converts one form of energy to another
What do active transducers do? list an example
- Generate output without an external supply
- a thermo couple which generates a small voltage when heated up
What do passive transducers do? list an example
- Do not generate an output
- only physically or electrically change
- a resistive thermal device changes resistance with temperature
What are the two elements within sensors?
Primary and secondary
What does the primary element do in a sensor
convert the process variable into a format suitable for measurement
what does the secondary element of a sensor do?
convert the primary element into an electric change
What are the 3 pressure/vacuum sensors and their properties?
Bellows = LOW pressure
Diaphragms = MORE pressure
Bourdon Tubes = MOST pressure
What are strain gauges?
Sensors of FORCE
Characteristics of a bonded wire strain gauge?
R=Kl/CMA
- when stretched R increases
- when compressed R decreases
- Low gauge factor (less sensitive)
Characteristics of Piezoresistive strain gauges?
- Crystal changes R with force
- When stretched R increases
- When compressed R decreases
- High gauge factor (more sensistive)
List examples of Flow sensors
- Flow Switches
- Manometer (atmospheric Pressure gauge)
- Turbine Flow Meter
- Magnetic Flow Meter
list examples of level sensors
- Floats
- Switches
- Conduction
- Ultrasonic
As a temperature sensor, bimetallic strips are used in what systems?
Discontinuous
What is a thermocouple and its characteristics?
- Temperature sensor
- generates a small voltage
- continuous or discontinuous
- active sensor
What are several thermocouples connected in series called?
Thermopile
An enclosed thermopile with radiation focused on it is called?
A radiation pyrometer
Resistive thermal device characteristics?
- positive temperature coefficient
- considered linear
Thermistor characteristics?
- negative temperature coefficient
- non linear
Integrated circuit temperature sensor characteristics? standard output control signals?
-Uses either a RTD or Thermistor and corrects any non linearity
-requires a power supply
4-20mA, 10-50mA, 1-5 Volts, 1-10 Volts, 3-15 PSI
Give 5 examples of sensors
- Thermostat
- Thermocouple
- Float
- Limit switch
- Photocell
What is the first step in troubleshooting?
Verify the Problem
what does a phototube (photo-emissive cell) have within it?
a photosensitive cathode that emits electrons when exposed to light
in a photo tube (photo-emissive cell), what happens when a potential is placed across it?
the electrons emitted by the the cathode will be attracted to the anode and a micro current will flow
Characteristics of a Light Dependant Resistor (LDR) ?
- Resistance changes with light intensity
- usually made with cadmium sulphide
- Light releases free electrons
What is the comparison of resistance between the light and dark in a LDR?
- 100M ohms in darkness
- less than 100 ohms in sunlight
What is a Photovoltaic Cell (solar cell) and what does it do?
- PN Junction with a LARGE light sensitive area
- converts light energy to electrical energy with no external power supply
What can a single solar cell produce?
about .5 volts and 4 amps
how are the output voltage and current increased in Photovoltaic Cells (solar cells) ?
- Output voltage is increased by adding cells in series
- Output current is increased by adding cells in parallel
How does a Photo Diode work?
- Reverse leakage current increases with temperature
- if light is directed on PN Junction, current will be proportional to the amount of light
How are Phototransistors made and what are they sensitive to?
- Made like a conventional BJT with a lens to focus light on ther base-collector junction
- sensitive to heat/light
How are Light Activated SCR’s made, how do they work and what is the max wattage they can control?
- Similar to a conventional SCR but has a lens to focus light on the GATE
- Light will trigger SCR on
- 500W max
How do LEDS create energy?
Light energy (photons) striking a PN Junction
How are photons produced in a LED?
When electron/hole pairs recombine
What is a LEDs approx PIV?
about 6 Volts
What do LEDS require?
current limiting resistor
What is the max amps on an LED?
about 10 to 50 mA
What do Opto-Couplers do? What do they eliminate? What are they used for?
- isolate systems electrically
- eliminates propagation of EMI and RF noise
- Used for solid state relays
