Controlling Systems Using IT (Level 3)

Lecture 11

0900 – 1030 Thursday 23/04/2015Boston College (Rochford Campus)

Recap What are the different ways in

which analogue signals are conditioned/manipulated?

Learning Outcomes

Exercise 1 (20 mins)

Working collaboratively in groups of 2-3, research the design and working of an operational amplifier. Upload your findings to the Moodle

glossary titled ‘Op-Amp’.

The Operational Amplifier (Op-Amp)

Essentially an Op-Amp can be considered to be an electronic system with an input and output and a voltage gain (i.e. amplification).

The voltage gain is the ratio of the output and input voltages when each is measured relative to the earth.

The Op-Amp

It is a high-gain d.c. amplifier that is supplied as an integrated circuit on a silicon chip, the gain typically being of the order of 100 000 or more.

The Op-Amp forms the basis of many signal conditioning modules.

Signal Conditioning Modules using the Op-Amp Inverting amplifier – input is inverted

i.e. output is 1800 out of phase, with respect to the input.

Summing amplifier – sum of currents entering the inverting connection equal that leaving the amplifier.

Integrating amplifier – circuit contains capacitor as well as resistor. The output is proportional to the integral of the input voltage, i.e. the area under a graph of input voltage with time.

Signal Conditioning Modules using the Op-Amp

Differentiation amplifier - results when the capacitor and resistor are interchanged in the circuit of the integrating amplifier. The output is proportional to the rate of change of input voltage.

Difference amplifier, logarithmic amplifier, comparator amplifier circuits.

Question

Where can the summing, integrating and differentiation amplifiers used?(Hint: We studied them recently)

Filtering Process of

hindering a certain band (stop band) of frequencies from a signal and permitting others (pass band) to be transmitted.

Cut-off frequency – boundary between stopping and passing.

Filtering Low-pass filters very commonly used as part of

signal conditioning because most of the useful information transmitted is low frequency. Noise tends to occur at high frequencies.

Digital Signals

Digital signals can be considered to be a sequence of on/off signals, which represents the value of the variable. E.g. the following three-digit signals may represent the voltages on the right:

no pulse, no pulse, no pulse

0V

no pulse, no pulse, pulse 1V

…  

pulse, pulse, pulse 7V

Digital Control Systems

Inputs and outputs of control systems are usually analogue e.g. temperature measurement (input), heating element (output) etc.

Therefore digital control systems necessitate ADC (for inputs) and DAC (for outputs) conversions.

Exercise

Modify the general analogue closed-loop control system block diagram, replacing the controller with a digital one and including ADC and DAC conversion units where appropriate. You may draw the block diagram by

hand or on computer.

Digital Closed Loop Control System

Correction Unit

Digital Control

Unit

Process

Measuring Device

Reference Value

Measured Value

Comparison Element

Error Signal

Process Variable (PV)

Disturbances

Setpoint (SP)

DAC

ADC

Digital Control Systems Despite the additional elements in

the digital control system, there are some advantages: Digital operations can be controlled

by a program Information storage is easier Accuracy can be greater Digital circuits are less affected by

noise Generally, easier to design