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Characteristics of Digital and Analogue Control Systems

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Andrew Friel

on 2 September 2014

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Transcript of Characteristics of Digital and Analogue Control Systems

P2, M1, M2
The need for Signal Conversion
As we already understand, signals in the real world are analogue: light, sound, etc, and real world signals must be converted into digital data if we are to have technology control such signals and devices. To do this, a circuit can be used called an ADC (Analogue to Digital Converter) that will convert the real world signals into something that digital equipment can recognise and understand, process and hopefully offer a desired output.
Analogue
vs
Digital
Data can be Analogue or Digital
Analogue
and
Digital
Control Systems
Although the microprocessor in a computer is digital because it uses binary 0’s and 1’s…
…most Control Systems are BOTH analogue and digital (usually dependent on the type of sensor)
Characteristics of
Analogue
and
Digital
Control Systems

http://electronics.howstuffworks.com/cd.htm
In Summary
Top Tip! Remember, for this part of the unit, you only need to describe and understand the workings of anologue and digital systems so a basic understanding along the lines of what is covered in this presentation. If you go further, you're getting into the realms of Degree level although if you wish to dig deeper then come spend some time with me in extra revision session on any of the items covered in this unit :-)
Characteristics of Anologue Control Systems
Comparison of Analogue and Digital signals
Digital data have discrete (distinct, separate) states and take discrete values.
a) Give an example of an analogue device and a digital device

b) What is the difference between an analogue and a digital signal?

c) Which does a microprocessor use?

d) How is the speed of a microprocessor measured?

e) What are the problems with a very fast processor? How could this be an issue with a very small control system which needs to be situated in a home environment?
So if analogue data is continuous, analogue signals can have an infinite number of values in a range
Digital signals can only have a limited number of values
Analogue
vs
Digital
Signals can be Analogue or Digital
Each type has different characteristics...
signal conditioning
Computer-based measurement systems are used in a wide variety of applications measuring voltage signals in the form of light, sound, infra red line breaks etc. However, many real-world sensors and transducers require
signal conditioning
before a computer-based measurement system can effectively and accurately acquire the signal. The front-end
signal conditioning
system can include functions such as signal amplification, attenuation, filtering, electrical isolation, simultaneous sampling, and multiplexing.

So, signal conditioning means manipulating an analog signal in such a way that it meets the requirements of the next stage for further processing. Most common use is in analog-to-digital converters. For example like our image, the amplification of sound that is noisy may need to be
conditioned
so only the finer parts that are needed are filtered through.
Signal conditioning
is an important component of any complete measurement system. No matter which sensor you are using,
signal conditioning
can improve the accuracy, effectiveness, and safety of your measurements because of capabilities such amplifications, isolation, and filtering.
noise filtering
Signal Processing and Conversion
In order to overcome many challenges in today’s high precision applications, we need to collect real world media as signals / data (usually in the form of electrical impulses) and modify their structure before converting them to digital so they can be handled far more easily and, take up less memory and time. Therefore hardware and software has been designed at the front end of analogue collection before sending it to an ADC

Real world media such as vibration, temperature, pressure, and light are represented as Analogue signals when we use instruments to signify them - for example, the
needle
in a barometer measuring air pressure.
What is a barometer? http://www.wisegeek.org/what-is-a-barometer.htm

Once we can represent this media as a signal (the needle in our barometer) we can measure it by using mechanical instruments for example (the face on the barometer with numbered measuring points).

We can take these instruments one step further by converting their signals to digital. We would do this to measure more accurately these signals, record or even display them in another form such as a graph. To convert to digital, a series of processes is applied, the first being an electric pulse. This is done by a
transducer
which converts one form of energy (mechanical) into another (electrical) and uses different voltages of electricity to represent different measurements.

The next two processors are: 1. accurate signal conditioning (this just means amplifying the signal so it's more easily read and measured) and 2. signal conversion (converting from analogue to digital or vice versa) before further data processing in a digital computer can be done.
matching to sensors
Analogue to Digital
Conversion (ADC)
Noise is all electric and electronic factors which interrupt a system. In signal processing, a filter is a device or process that removes from a signal some unwanted component or feature. Filtering is a class of signal processing, this means removing some frequencies and not others in order to suppress interfering signals and reduce background noise so you can capture the signals you need.
Connecting sensors to a computer is not as easy as it seems. By their very nature sensors will produce a finite output. This is not usually of a form which can be directly input into a computer. The signal may go through many stages before it matches a required range. Plus matching is a simple case of collecting the correct frequencies for a given converter i.e. so a correct sensor must be utilised in order to help collect the frequency of choice and matched to it's given converter to accurately receive and interpret the signal / data.
Unfortunately analogue signals are normally in a continuous form but, the processors of control systems work digitally using binary zeros and ones. Some form of conversion is needed.

Analogue devices use one property of a material or event to measure a different property of the material or event. A non-computer example is the simple mercury thermometer. It is said that one property (in this case length) is analogous (the word means related or proportional) to another property (in this case temperature). This is why these devices are known as analogue devices.

Where this analogous property is an electrical property (voltage, frequency, current, etc) then it is possible to harness this for continuous monitoring, and ultimately control of a particular device.

A control system requires finite values. It cannot cope with infinite continuous values as there is not enough memory to store that amount for one and two the system wouldn't know where the data signal starts and ends hence the need for conversion into digital for the control system to process the signal. So the system has to be given a value it can cope with. Effectively this is the job of an analogue to digital converter (ADC) and it uses pulse-code modulation (PCM) to do this.

PCM is a procedure that samples an analogue signal, normally at a rate of 8,000 samples per second, and converts those sample values to a series of digital values. The complexities of PCM do not form a part of this unit, but learners should be aware of the term and the existence of the process.
Characteristics of
Digital
Control Systems
speed
memory
Digital to Analogue Conversion DAC
In order for digital information to be passed on automatically, it requires data such as instructions to be stored into memory. Some types of digital control systems require instructions that are executed automatically rather than requiring a person to execute them instead. This is because control systems require these instructions to do something. The amount of memory in bytes required for storing digital information depends on the complexity of the job.

In most digital control systems, there is a memory controller which is a digital circuit that manages the flow of data going to and from a computer's main memory for example. It has the capacity to read and write 1's and 0's too and from memory on behalf of the control system. It can be a separate chip or integrated into another chip, such as on a microprocessor (CPU). This is known as a memory chip controller (MCC).
Not every output is digital and an ADC is very handy when converting real world analogue signals into digital so we can control the input, store it and essentially do something with it. But what if the digital data needs to be converted back into analogue signal?

EXPERIMENT!

Try plugging a microphone directly into a speaker, what happens? This experiment will be carried out in lectures.

A similar process as the ADC has to occur if the output device requires an analogue signal such as the rotation speed of a motor. The system will produce a series of values representing the speed at various intervals producing actual speeds or in the case of our sound system, Analogue voice through the microphone to digital data that represents the sound back to analogue representation of the original sound.
Has anyone ever said to you, you sound different over the phone than in real life or over a microphone - can you explain why?
I/O ports
Digital systems are used more commonly in electrical devices and they use on or off values to represent data. This means that digital systems do not represent data in a continuous form unlike analogue systems. Digital systems only vary in two states such as ON or OFF, (TRUE or FALSE / 1 or 0) therefore storing 1's and 0's as data, is a stop start technology and if performed slowly, the output of the split data will also be slow. Nowadays, a fundamental concept of converting Analogue to Digital is so that the output is near as continuous and fluid as the analogue signal itself which in turn indicates that the speed of conversion (how fast the data of 1's and 0's is sent) is very important. The speed to send these signals / data, is determined and modulated by the electricity range (voltage amount used for each piece of data) to send the signal, the medium used (type of wire for example) and it's insulation (to prevent interference).

EXPERIMENT!
Take out your phone, turn on your camera and hold it up to your computer screen - why do you see constant on or off flickering of lines on your camera phone? See me in the next lecture to share some of your answers you've researched for this and compare to mine.

So, digital systems are used in computer hardware components such as the processor, in order for the processor to transfer digital information to other components, with a certain amount of speed. For example, the clock speed on a processor would allow it to processes digital graphical information through a video card to a monitor or TV screen so that we can see a fluid moving image and the higher the clock speed, the faster the digital information is passed on.
One reason is to avoid degradation and corruption of the signal. This is one reason why CDs (which are digital) are preferred over vinyl records (which are analog). On a vinyl record, the pressure of the needle eventually wears away the groove and degrades/corrupts the music on the album.
Why doesn't this happen on a CD?
Another reason is a lack of noise. On a vinyl record or a cassette tape (which are analog), there is some amount of hiss that corrupts the analog signal -
Why?
The need for Signal Conversion
A third reason is compression. With digital data there are a variety of compression algorithms that can be used to shrink the signal. An MP3 file
Others include: to capture pictures – scanners are converters by converting light, shadows and colour from pictures to images
To capture sound – your voice in a telephone is converted to digital for the sound to be captured and sent over the telephone line digitally.
level shifting
Digital logic levels (1's and 0's signals / data) are based on voltage levels that represent high and low signals. Depending on the standard used, a signal could be misread or worse, cause damage to the circuit. Shifting the voltage level of the logic signal through the use of level shifting circuitry is a great way to maintain communication between digital systems that use different voltages.

Decades ago, most logic was 5 volt logic and the I/O and digital communication voltages usually matched. As 3.3 volt devices started to emerge they were generally tolerant of being fed 5 volt logic, but may not have been capable of outputting 5 volt compatible logic. Today common logic voltages include 5v, 3.3v, 2.5v, 1.8v, and 1.5v and low voltage devices are generally not tolerance of higher logic voltages. In order to communicate reliably between them, the logic voltage levels, level shifting techniques are often the best choice, so once a signal is sent it's voltage will be shifted for the receiver.

So Analogue input devices like microphones produce finite signals at a specific voltage range which has to be adjusted if necessary to meet the required voltage range. Using amplification to adjust these voltage ranges does not always work which means that voltage ranges have to be increased or decreased to meet the voltage requirements. The main reason why ranges need to be adjusted is that ADC devices can only work with certain signal ranges and any signal that isn’t in that specific range will not be recognised.
No control system would operate without both an input and output (I/O) otherwise what's the point in trying to communicate something between the real and virtual world. So these are integral to for bidirectional communication. I/O is the communication between an information processing system and the outside world, possibly a human or another information processing system. Inputs are the signals or data received by the system and outputs are the signals or data sent from it.
The differences in requirements between a home or office computer and a control system often lead to considerable differences in the type of device required. A home or office computer is a multipurpose system that is likely to undertake a wide variety of tasks.
A control system may only need to perform one or a few simple tasks such as the thermostat in a boiler and can be, therefore, much less sophisticated systems. It is important to note however, that the underlying theory is still true for those more sophisticated systems, that they all follow the same principles of circuitry, converters, inputs and outputs to help us measure, record and control real world signals.
What do you already know and things to consider?
f) What is an ADC? Why might a control system need to use this (ensure you give an example in your answer)

g) What is a DAC?

h) ADC uses PCM – what is PCM?

i) What is signal conditioning?

j) What is noise filtering?

k) What is level-shifting
Analogue signals can have an infinite number of values in a range.
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