1 Sensors and Measurements Penderia & Pengukuran ENT 164 Signal Processing Elements Hema C.R. School of Mechatronics Engineering Northern Malaysia University College of Engineering Perlis, Malaysia Contact no:

2 What is a signal? Signal: A physical variable whose value varies with respect to time or space. Continuous time signal: When the value of the signal is available over a continuum of time Discrete time signal: When the value of the signal is available only at discrete instants of time Continuous time signal Discrete time signal

3 Signal Processing Output from conditioning elements are in the form of dc or ac voltage / current. Certain calculations [signal processing] are to be performed on these output signals, in order to establish the value of variable being measured Signal Processing Element Data Presentation Element Output from Signal Conditioning Elements Output Measured Value

4 Digital Signal Processing A / D CONVERSION DIGITAL SIGNAL PROCESSING D / A CONVERSION Analog Signal DATA PRESENTATION DIGITAL OR ANALOG Analog Signal Sensing Element

5 Digital Signal Processing: Advantages Digital signal processing has a number of advantages compared with analog signal processing Robustness: digital signal processing is insensitive to process variations, supply voltage change, temperature variation, interference and aging. Programmability: Digital signal processing algorithms can be changed easily by changing the coefficients or software codes. Flexibility: Some signal processing algorithms have extra degree of freedom implemented in digital such as linear phase filter. Digital comes from the French which means numeric

6 Analog to Digital Conversion [A / D] Sampling Quantisation

7 Sampling Sampling is the process of representing a continuous signal y(t) by a set of samples taken at discrete intervals of time [sampling interval] Sampling Frequency

8 Nyquist Sampling Theorem For to be an adequate representation of, should satisfy the conditions of the Nyquist sampling theorem A continuous signal can be represented by and reconstituted from a set of sample values provided that the number of samples per second is at least twice the highest frequency present in the signal

9 Aliasing Phenomenon of two different signals being constructed from a given set of samples values is referred to as aliasing If the sampling components occupy the same frequency range as the original signal and it is impossible to filter them out and reconstitute the signal

10 Quantisation In a sampled signal value of can take any value in the signal range of to. In quantisation sample voltages are rounded either up or down to one of Q quantisation values or levels where q = 0,1,2,…,Q-1. If and then there are (Q-1) spacings occupying span of The spacing width or quantisation interval is therefore:

11 Quantisation produces an error termed as quantisation error. The maximum quantisation error is therefore or expressed as percentage of span Maximum percentage quantisation Error

12 Frequency to Digital Conversion

13 Frequency to Digital Conversion A.C. voltage → Signal from sensing elements or conditioning elements. Tachogenerator Electrical oscillator Flow meters Conversion of frequency to digital output Frequency Measurement Period Measurement

14 Frequency Measurement Frequency is measured by counting number of pulses during fixed time interval. Set counters to zero, load number then counting interval Frequency Measurement Principle Frequency Measurement System

15 Clock pulses are input to clock counter which counts down to zero Signal pulses are input to signal counter, this counts until clock counter reaches zero, when count is stopped. Signal count is then Signal count is proportional to input frequency Resolution is limited to signal count. At low frequencies percentage resolution is poor

16 Period Measurement Period of the signal is measured by counting the number of clock pulses within. The number of o f positive going edges is counted giving Period measurement Principle Period measurement System

17 Digital to Analog Converters

18 Binary Weighted Resistor Network DAC give an analog output which is proportional to the 8 bit binary input signal. An OPamp is used to sum the currents which are either zero or nonzero depending on bit equal 0 or 1

19 R-2R Ladder Network Current distribution is obtained with only two values of resistance, R and 2R

20 Analog to Digital Converters

21 Dual Slope Converter Input is a continuous voltage not sampled voltage These converters are used in digital voltmeters and indicators.

22 Control logic, switches V in onto the Integrator V 1 is a positive ramp with slope proportional to V in, output of comparator is 0 At end of fixed time V 1 is proportional to V in. Control logic switches ref voltage –V ref onto integrator and resets the counter to 0. The integrator output is a negative ramp with fixed rate of decrease Time taken by V 1 to decrease to 0 is proportional to V in. During this time control logic routes clock pulses to the counter and the counter increments. When V 1 fall to 0 comparator output Vc changes to 1 and the count is stopped. Count is proportional to fall time and therefore to V in and the parallel digital output signal is proportional to the count

23 Successive Approximation ADC Method involves making successful guesses at the binary code corresponding to input voltage The trial code is converted to an analog voltage using DAC Comparator decides whether the guess is too high or too low

24 Flash or Parallel ADC In Flash ADC there are Q-1 comparators in parallel and Q-1 corresponding voltage levels V 1 to V Q-1 In each comparator q, the input sample value y i is compared with the corresponding voltage level V q. If y i is less than or equal to V q the output is zero corresponding to 0. If y i is greater than V q the output is non-zero corresponding to a 1

25 Comparator provide a Q-1 digit parallel input to a priority coder which generates an n-digit binary parallel output code corresponding to the values of q. Advantages : short conversion time Disadvantage: large number of comparators

26 Computer and Microcontroller Systems

27 General Computer System Computer System Input Devices Central Processing Unit Output Devices External Storage Devices Central Processing Unit Also known as Microprocessor Microprocessor Microcontroller

28 Microprocessor Three main parts Control Unit Controls processing of instructions by providing control and time signals Arithmetic Logic Unit Arithmetic operations and Logic operations Data register Stores Instructions Addresses Data Output Devices Input Devices Backing Storage Data Flow Memory Processor Central Processing Unit ALU & CU

29 Read Only Memory [ROM] Permanent Storage, unique address, instructions storage, read only Random Access Memory [RAM] Temporary Storage, unique address, read and write, instructions and data storage Input/Output Interface Ports to connect external devices Serial, parallel Address Bus One-way flow of address codes to memory and I/O ports Data Bus Two-way flow Data from I/P, RAM to Microprocessor Instruction from ROM to Microprocessor Results to RAM or O/P Control Bus Two-way flow Sends clock signal to co-ordinate information transfer Receives information on status of elements

30 Microprocessor Layout Processor divided into two units Executive Unit Executes instructions Bus Interface Unit [BIU] Interfaces computer buses Fetch information Read/write - data /results EU Control System Fetches instruction from instruction queue Arithmetic Logic Unit Performs arithmetic and logic operations Data Register Stores data [before and after operations] Flag register Status flag of arithmetic operations Pointer and Index Register Creates an address in BIU Bus Control Logic Fetches next instruction and places them in instruction queue Address Segment register and Instruction Pointer Creates parts of instruction address Address Summing Block Combines all addresses to create full address of next instruction to be fetched Layout of Typical Microprocessor

31 Microcontroller Single Chip Processor Memory I/O 8bit Microcontroller CPU Memory Four parallel ports Two serial ports 12 MHz Clock Single bus Two 4K EPROMS Non volatile Two 128 byte RAM Volatile Layout of a Microcontroller