ENT356 Instrumentation System

1. ENT356Instrumentation System Lecture 4 – Signal Characteristic

2. SIGNAL ???

3. The speed of rotation of an engine Data presentation True speed Measured speed Digital display Tacho generator Sensing Frequency Counter Schmit trigger Computer Frequency Signal conditioning Signal processing

4. Measurement system (1) The speed of rotation of an engine is sensed by an electromagnetic tachogenerator which gives an AC output signal with frequency proportional to speed. (2) The Schmit trigger convert the sine wave into sharp-edge pulses which are then counted over a fixed time interval. (3)The digital count is transferred to a computer which calculates frequency and speed, and the speed is presented on a digital display

5. Signal Characteristics School of Mechatronics Engineering • Definitions specific to subfields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. • In a communication system, a transmitter encodes a message into a signal, which is carried to a receiver by the communications channel. • For example, the words “HELLO" might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; and at the receiver it is reconverted into sounds.

6. Signal Characteristics School of Mechatronics Engineering • Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. • Discrete-time signals are often referred to as time series in other fields. • Continuous-time signals are often referred to as continuous signals even when the signal functions are not continuous; an example is a square-wave signal. • A second important distinction is between discrete-valued and continuous-valued. Digital signals are discrete-valued, but are often derived from an underlying continuous-valued physical process.

7. Signal Characteristics School of Mechatronics Engineering • Discrete-time and continuous-time signals • If for a signal, the quantities are defined only on a discrete set of times, we call it a discrete-time signal. In other words, a discrete-time real (or complex) signal can be seen as a function from the set of integers to the set of real (or complex) numbers. • A continuous-time real (or complex) signal is any real-valued (or complex-valued) function which is defined for all time t in an interval, most commonly an infinite interval. • Less formally than the theoretical distinctions mentioned above, two main types of signals encountered in practice are analog and digital. • In short, the difference between them is that digital signals are discrete and quantized, while analog signals possess neither property.

8. Signal Characteristics School of Mechatronics Engineering • Discrete signal • One of the fundamental distinctions between different types of signals is between continuous and discrete time- set of real numbers (or some interval thereof), whereas the domain of a discrete-time (DT)- set of integers (or some interval). • DT signals often arise via sampling/quantizing of CT signals. • For instance, sensors output data continuously, but since a continuous stream may be difficult to record, a discrete-time signal is often used as an approximation.

9. Signal Characteristics School of Mechatronics Engineering • Quantization • Quantization is the process of approximating a continuous range of values by a relatively small set of discrete symbols or integer values. • For example, rounding a real number in the interval [0,100] to an integer [0, 1, 2, …. 100.] • If a signal is to be represented as a sequence of numbers, it is impossible to maintain arbitrarily high precision - each number in the sequence must have a finite number of digits. • As a result, the values of such a signal are restricted to belong to a finite set; in other words, it is quantized

10. Signal Characteristics School of Mechatronics Engineering

11. Signal Characteristics School of Mechatronics Engineering Examples of signals Motion. The domain of a motion signal is one-dimensional (time), and the range is generally three-dimensional. Position is thus a 3-vector signal; position and orientation is a 6-vector signal. Sound. A microphone converts sound pressure at some place to just a function of time, using a voltage signal as an analog of the sound signal. Compact discs (CDs). CDs contain discrete signals representing sound, recorded at 44,100 samples per second. Each sample contains data for a left and right channel, which may be considered to be a 2-vector (since CDs are recorded in stereo). Pictures. A picture assigns a color value to each of a set of points. Since the points lie on a plane, the domain is two-dimensional. If the picture is a physical object, such as a painting, it's a continuous signal. If the picture a digital image, it's a discrete signal. Videos. A video signal is a sequence of images. A point in a video is identified by its position (two-dimensional) and by the time at which it occurs, so a video signal has a three-dimensional domain. Analog video has one continuous domain dimension (across a scan line) and two discrete dimensions (frame and line).

12. Signal Characteristics School of Mechatronics Engineering • Advantages of Digital Signals Compared to Analog Signals: • Increasing the number (and decreasing the size) of the time divisions in a digital signal can make the digital signal nearly as smooth as an analog signal.  • Digital signals are much easier to store than analog signals and are much less prone to degradation.  • By definition, each piece of information in a digital signal is a number, easily distinguished from other numbers.  • One analogy for a digital signal could be a table of numbers andcomparable analogy for an analog signal would be a graph.  • It is much easier to accurately copy a table than a graph.  And once they have been copied several times, the table has a good chance of staying unchanged while the graph will probably look quite different.

13. Signal Characteristics School of Mechatronics Engineering • Signal Modulation • An information signal, without modification, to be sent somewhere, may not be able for transmission. • This is because the signal (in electronics and communication) are usually a voltage that varies with time. • To avoid large signal lost, the information signal must be used to vary carrier signal that is more technically appropriate, has the frequencies, spectrum, and power needed for the application. • The process is called modulation. • Real-world example: car driver pressing the gas pedal.

14. Signal Characteristics School of Mechatronics Engineering • Signal Modulation • Signal Modulation definition: the process of encoding a baseband source signal (information signal) onto a carrier signal. • The carrier waveform is varied in a manner directly related to the baseband signal. • The carrier can be a sinusoidal signal or a pulse signal. • The result of modulating the carrier signal is called the modulated signal.

15. Signal Characteristics School of Mechatronics Engineering • Signal Modulation • A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). • A device that can do both operations is a modem (modulator–demodulator).

16. Signal Characteristics School of Mechatronics Engineering

17. Signal Characteristics School of Mechatronics Engineering

18. Signal Characteristics School of Mechatronics Engineering • Signal Modulation • Amplitude Modulation (AM), or Amplitude Shift Keying (ASK): “The amplitude A” of the carrier signal changes in direct proportion to the baseband signal. • Phase Modulation (PM), or Phase Shift Keying (PSK): “The phase ” of the carrier signal changes in direct proportion to the baseband signal. • C.Frequency Modulation (FM), or Frequency Shift Keying (FSK): “The frequency f ” of the carrier changes in direct proportion to the baseband signal.

19. Signal Characteristics School of Mechatronics Engineering

20. Signal Characteristics School of Mechatronics Engineering Summary • Signal: • Can be in Discrete of Continuous form, • Widely known as Digital and Analog signal, • Convey information, • Can be transmitted and received, • Can be in any space, • Can be processed., i.e modulated, • and importantly, can be MEASURED.