Lecture 2

Question 1

Give the key components of a block diagram

Answer 1

signals, systems, feedback, inputs, outputs

Question 2

What are signals?

Answer 2

variables that are a function of time, normally have a physical unit and dimensions

Question 3

What is a system?

Answer 3

elements that are interconnected with signals,

it is something that responds to signals

Question 4

What is a disturbance signal?

Answer 4

a signal we cant control

an unwanted input that affects the systems output signal

Question 5

What is an input signal?

Answer 5

a signal we can control

Question 6

How do you decide what the system is in a block diagram?

Answer 6

the system will be something we are interested in

Question 7

How is a signal represented on a block diagram?

Answer 7

Always as an arrow, with the direction representing the input and output flow of the signal to systems

Question 8

What is a process?

Answer 8

A special name for a system that we are trying to control

Question 9

How do we mathematically connect input and output signals?

Answer 9

Output signal = f(input signals) where the function f defines the sytem
ie the output signal is a function of the input signals

Question 10

Give the different types of signal

Answer 10

Electrical signal - value of the voltage in a cable relative to ground
Sensor signals - typically an electric signal representing a physical measurement
physical quantities - eg the height of something

Question 11

When setting up a block diagram what is often the easiest way to start?

Answer 11

Consider how manual control might work (ie via an operator)

Question 12

Give an overview of manual control of a tank liquid height

Answer 12

Operator observes fluid height (sensor)
Operator mentally compares fluid height to desired position and decides how to adjust flow (controller)
Operator adjusts input flow accordingly (actuator)

Question 13

What is a summing/difference block and how is it represented?

Answer 13

Drawn as a circle Used to compare a desired signal with an output signal

Question 14

What is an error signal?

Answer 14

desired signal - output signal

Question 15

What is a negative feedback system?

Answer 15

a negative feedback system is one in which we are taking away the output signal of a system from the desired signal to give an error signal

Question 16

Why is automated control better than manual control?

Answer 16

Operator is bored, unreliable and expensive, often the systems that a manual operator might to control could be fast, complex, dangerous and inaccessible

Question 17

What is the general progression of system blocks?

Answer 17

desired output response and measurement output -> comparison, controller, actuator, process -> actual output and feedback

Question 18

What is a multi input multi output system?

Answer 18

a system with multiple inputs and outputs that are monitored and controlled all in one loop

Question 19

What is a multiloop system?

Answer 19

a system with multiple loops - inner loop and outerloop

to control a number of signals

Question 20

In an electric motor, what is assumed about the speed and input signal? (voltage)

Answer 20

Motor speed is proportional to the input signal

motor speed = G x input signal

Question 21

Draw a simple block diagram from motor speed control

Answer 21

See lecture 2

Question 22

What is proportional control

Answer 22

Proportional control is where an error signal e is multiplied by a constant K to give the control signal c
ie c = K x e

Question 23

In proportional control what is controller gain?

Answer 23

Controller gain is the constant used to multiply the error signal by to give the control signal,
it must be chosen by the engineer

Question 24

Derive the equation for the control signal for motor speed model given
desired speed = sd
motor speed = sa = Gc = const G * input signal c
measured motor speed = sm
error value = e
controller gain = K

Answer 24

sa = sd * GK/ (1 + GK)
We want GK/ (1 + GK) -> 1
Therefore K must be as large as possible

Question 25

Give the issues with the motor speed control system described in these flashcards and their result

Answer 25

-No dynamics
-Assumed instantaneous change in motor speed
-Infinite angular acceleration
-Infinite torque
These assumption cause transient behaviour and instability