Static Performance Characteristics Flashcards
Range or span
Minimum and maximum values of a quantity that the instrument is designed to measure.
What is instrumentation?
Application of instruments for monitoring,sensing and measurement
Static performance characteristics
Part of the data specification for a particular instrument
Repeatability
Display the same reading on a number of occasions when input is applied at short intervals of time
Stability
Display the same reading on a number of occasions when input is applied over a long period of time
Accuracy and precision
Accuracy is stated in terms of errors introduced
Precision is a measure of how close together the results lie
Resolution
Measure of the smallest detectable change in the instrument output
Non linearity
Measure of the maximum deviation from a linear input/output relationship
Tolerance
Measure of the maximum allowable error
Threshold
Measure of the minimum level of input required to produce an observable change in the instrument output
Zero drift
Where the zero reading is altered by a change in ambient conditions.
Normally removable be re-calibration
Sensitivity equation
Change in output/change in input
Measuring system
Primary quantity – transducer -(transducer signal)-signal conditioner -(conditioned signal)-display or recorder
Transducer
Is an energy converter which receives the physical quantity being measured and converts it into some other physical variable
Signal conditioner
Rearranged the signal into a form which can be readily recorded or monitored
Main features of an open loop system
1-no comparison between actual and set values
2-each input setting determines a fixed operation condition
Main features of a closed loop system
1-output is fed back so that comparisons can be made with the set point
2- closed loop systems have a self regulating property since any external disturbance will change the output , resulting in an error signal being generated by the comparison element
Open loop system basic elements diagram
Input Control element Actuator Process or plant Output
Control element
Determines action to be taken with reference to input to the control system
Actuator
Responds to input from control element and initiates action to change the variable to the required value
Process or Plant
Is the system of which a variable is being controlled
Closed loop control system basic element
Input Control element Actuator Process or plant Measurement element
Comparison element
Compares reference and actual output variable elements and produces error signal
Control element
Decides on action when receives error signal
Actuator
Produces change in process to remove error
Process or plant
Is the system in which a variable is being controlled
Basic forms of mechanical system building blocks
Spring - associated with stiffness of the system
Damper- exhibits force which opposes motion
Mass- offers inertia / resistance to acceleration.
What are the specific tasks a mass,Spring and damper perform in a mechanical system
Spring - energy storage device
Mass- energy storage device
Damper - energy dissipation device
What is a hydraulic and pneumatic fluid system
Hydraulic - liquid is deemed incompressible
Pneumatic - gas which is compressible and exhibited a change in density
Hydraulic resistance
Due to resistance of flow through valves or change in pipe diameter
Hydraulic capacitance
Associated with energy storage of a liquid in the form of PE
Hydraulic inertia
Equivalent to a Spring in a mechanical system
Transient response
Occurs as a result of change in the system input and dies away after a short interval of time
Steady state response
Remains after all transient responses have died down completely
Draw the graph for the typical input signals to a control system Step Impulse Ramp input Sinusoidal
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Sin wave
Draw the graph for a first order system subject to a step input
Slow increase in cure which levels out
Draw the graphs for the response of second order systems to a step input Step input No damping Some damping High camping
Look at notes
What is rise time (performance measures for second order systems )
The time taken for the response @0 to rise from 0 to the the steady state value and is a measure of how fast a system responds to an input.
What is peak time (performance measures for second order systems )
Is the time taken for the response to rise from 0 to the first peak value
Time taken for the oscillatory response to complete a half cycle
What is overshoot (performance measures for second order systems )
The maximum amount by which the response overshoot @ss
What is Settling time (performance measures for second order systems )
A measure of time taken for the oscillations to die away
Open loop (control strategies)
On-off switching or timed switching sequence or action
Closed loop (control strategies)
Often a two step mode control, proportional control or proportional combined with other features
Two step mode control
The error input signal to the control element results in on/off output which is used to switch the correct element on or off
Proportional control
Output from the control element is a signal which is proportional to the error
Derivative control
The output is proportional to the rate of change of the error signal
Integral control
The output at time t is proportional to the integral of the error signal
Three term controller
A proportional plus integral plus derivative
