CONTROL SYSTEMSInstructor: Irfan Azhar

Open Vs Closed Loop system

Which one is more accurate? Less sensitive to disturbance noise and

variations in environment Transient response and steady state error

can be controlled more conveniently However, these systems are more complex

and expensive --- why? Toaster example Systems that perform the previously

described measurement and correction are called feed back systems.

Computer controlled systems

Computer as controller Control becomes easy since changes should be made

in the software Many loops can be controlled In space shuttle, main engine controller consists of 2

computers that control the entire engine functions. Engine sensors provide P,T,flow rate,turbobump

speed, valve position and engine servo valve actuator positions

Further, the control of thrust and propellant mixture ratio , sensor excitation, valve actuation, spark igniters as well as other functions are controlled by closed loop control.

Analysis and design objectives

Analysis: Process by which system’s performance is determined

E.g. we evaluate transient response and steady state error to determine whether they meet the spec or not.

Design: Process by which system’s performance is created or changed.

E.g. if s.s. error and transient response don’t meet the specs, we change parameters or add additional components to meet the specs.

Transient response

For elevator, a slow transient response makes passengers impatient while rapid transient response makes them uncomfortable.

Too fast transient response can cause permanent damage to elevator!

Steady state response

Resembles input Remains after transients have decayed to

0 Elevator stopping near the destination! Accuracy of s.s is important

Stablity

Total response is a sum of natural and forced response

Natural response describes how systems acquires or dissipates energy

The form or nature of this response depends on system , not the input.

Force response is dependent on input. Total response = Natural + Forced response For a system to be stable, the natural response

must 1 eventually reach 0, leaving only the forced response2 Oscillate

Unstablity!

If natural response grows without bound instead of decaying to 0, then the system is unstable.

Eventually, the natural response is so much greater than the forced response that the system cannot be controlled!!!

Could be self destructive…… Elevator crashes through the floor…… Antenna command to point to a target would rotate , line

up with the target but then begin to oscillate about the target with growing oscillations and increasing velocity until the motor or amplifiers reach their output limits or antenna was damaged structurally.

Control systems must be designed to be stable. If time reaches infinity, natural response should reach 0 or

oscillate i.e. transients should dieout.

Other considerations

Motor/ component selection Cost Robust design : system should not be

sensitive to parameter changes.