01-15mec-314-metrology-unit-3-signal+conditioning.pdf

15 MEC 314 METROLOGY & MEASUREMENTS Signal Representation – Signal conditioners, filters COURSE FACULTY : A.SHANMUGASUNDARAM ASSISTANT PROFESSOR

Generalized measurement system  The basic function of an instrument is to sense the variable, process it and present it to human observer. Any measurement system is made up of three basic units.

Generalized measurement system

Conditioning Element

Sensing Element

Display Element

Liquid bulb thermometer

Generalized measurement system Electrical instrument • The instrument is made up of electrical elements like coils, windings and works based on electrical principles like Faraday’s law, Kirchaff’s law. It is often coupled with mechanical parts. • e.g.: Volt meter, Ammeter, Watt meter, electrical type tachometer, current transducer.

Generalized measurement system Electronic instrument • The main components in this type are electronic components and uses electronic principles for its functioning. e.g.: encoders, electronic comparator, electronic weighing machine. • It has low weight and power consumption. The response is faster and sensitivity and reliability are high.

Generalized measurement system Optical instrument • It makes use of light as major source of measurement. • It can directly use light as in case of profile projector, tool maker’s microscope etc., or the light can be converted in to an electrical signal using photo-cell, photo-transistor etc. and can be made use of. • This type makes extensive use of electronics. Noncontact type of measurement is also possible as in the case of optical tachometer.

Generalized measurement system Analog instruments • The pointer in the display unit moves continuously on the calibrated dial and can take infinite possible position on dial. • It is preferred in control panel where the parameter of interest is to be monitored manually within a range and accurate value is not so important. • The limitation of analog type of instrument is that the accurate reading is difficult and prone to observation errors. • e.g.: Analog type volt meter, bourdon gauge etc.

Generalized measurement system Digital instruments • It uses a seven-segment display or LCD (Liquid Crystal Display) or a CRT (Cathode Ray Tube) for its display. • It has resolution problem, for example, if display has 3-digit capability, it can display say, ‘238’ and ‘239’. Not the intermediate value. • It is preferred in measuring instruments where reading the exact value is important. e.g.: Optical tachometer, mechanical type energy meter.

Generalized measurement system What is Signal ? • “Signal is one which carries information of interest”. • It can take any physical form such as electrical, mechanical, hydraulic, pneumatic etc. • Red light in traffic control unit is a signal. Because it carries information “Stop”, it is a signal. Red light by itself is not a signal. In decorative lighting, we could see lights with red, blue, orange and many more colors. There, red light does not carry any information. So, it is not a signal

Generalized measurement system Why do we need to study about signals? • The nature or condition of physical parameter varies in different application and demands special instruments, methods, procedures, and result interpretations for the purpose of measurement. • By knowing the nature of the signals, we will be able to select the appropriate instrument, interpretation techniques, and signal conditioning units. So, one should have sound knowledge about signals.

Generalized measurement system Classification of signals SIGNAL L

DETERMINISTIC

STATIC

NON-DETERMINISTIC

DYNAMIC

PERIODIC

SIMPLE

COMPLEX

NON-PERIODIC

STEP

RAMP

PULSE

Generalized measurement system Deterministic signal  A deterministic signal is one, which has a pattern of repetition and behaves in a predictable manner.  It does not have any uncertainty.  Examples of this type signal will include sine wave, ramp function, step function.

Non-Deterministic signal  This type of time varying signal has uncertainty and cannot be predicted before its occurrence. Non – deterministic signals are described by some approximate series or statistical measures.  Ex: the cutting force variation in machine tool due to hot spots in work piece.

Generalized measurement system Static signal  The signal, which does not vary with respect to time, is called “Static signal”.

 Atmosphere pressure at a location, length of a rod, and diameter of a shaft. note that these parameters may vary due to temperature and other factors. But the change with respect to time is negligibly small. For all practical purpose, they can be treated as static signal.

Generalized measurement system Dynamic signal  If there is a considerable variation in the signal with respect to time, then it is called dynamic signal

Periodic signal

 A signal is said to be periodic when the variation in the magnitude of the signal repeats at regular intervals in time  E.g.: Displacement due to cam profile

Simple Periodic signal

 Its is a periodic signal with only one frequency  E.g.: frequency of single-phase current.

Generalized measurement system Complex Periodic signal  If the periodic signal has more than one frequency super imposed, one over the other, then the signal is called “complex periodic signal”.  Let us take the example of vibration signal from a motor

Non - Periodic signal

 It is a deterministic, dynamic signal, which does not repeat at regular intervals. Types are follows. (1) Impulse signal (2) Step signal (3) RAMP signal

Generalized measurement system Non - Periodic signal (1) Impulse signal It is a momentary signal for a short period of time. (2) Step signal Step signal has two states low level and high level. If the level of the signal changes suddenly and remains so, then the signal is called step signal.

Impulse signal

Step signal

Generalized measurement system (3) Ramp signal  This signal also has a low level state and a high level state. The change of state varies linearly with time

Ramp signal

Time Domain signal  Time domain is a record of what happened to a parameter of the system versus time.

Generalized measurement system Frequency Domain  When we look the signals in the direction of time axis, we get amplitude Vs time, it is called frequency domain.  In frequency domain, every sine wave we separated from real world signal appears as a vertical line.  Its height represents amplitude and its position represents its frequency.  The frequency domain representation of signals is called the “spectrum of the signal”

Generalized measurement system

GENERAL SIGNAL-CONDITIONING SCHEME

 Most measurements begin with a transducer, a device that converts a measurable physical quality, such as temperature, strain, or acceleration, to an electrical signal.  Transducers are available for a wide range of measurements, and come in a variety of shapes, sizes, and specifications.

GENERAL SIGNAL-CONDITIONING SCHEME

The transducer is connected to the input of the signal conditioning electronics. The output of the signal conditioning is connected to an ADC input. The ADC converts the analog voltage to a digital signal, which is transferred to the computer for processing, graphing, and storage.

GENERAL SIGNAL-CONDITIONING SCHEME 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. In control engineering applications, it is common to have a sensing stage (which consists of a sensor), a signal conditioning stage (where usually amplification of the signal is done) and a processing stage (normally carried out by an ADC and a micro-controller). Operational amplifiers (op-amps) are commonly employed to carry out the amplification of the signal in the signal conditioning stage.

GENERAL SIGNAL-CONDITIONING SCHEME Signal inputs accepted by signal conditioners include DC voltage and current, AC voltage and current, frequency and electric charge. Sensor inputs can be accelerometer, thermocouple, thermistor, resistance thermometer, strain gauge or bridge, and LVDT or RVDT. Specialized inputs include encoder, counter or tachometer, timer or clock, relay or switch, and other specialized inputs. Outputs for signal conditioning equipment can be voltage, current, frequency, timer or counter, relay, resistance or potentiometer, and other specialized outputs.

GENERAL SIGNAL-CONDITIONING SCHEME  Signal conditioning converts a transducer’s signal so that an analog to - digital converter (ADC) can measure the signal.  Signal conditioning can include the following functions.  Amplification, Filtering  Differential applications , Isolation  Simultaneous sample and hold (SS&H)  Current-to-Voltage conversion  Voltage-to-Frequency conversion  Linearization.  Signal conditioning also includes excitation or bias for transducers that require it.

ADC – ANALOGUE TO DIGITAL CONVERSION  An ADC converts an analog voltage to a digital number.  The digital number represents the input voltage in discrete steps with finite resolution.  ADC resolution is determined by the number of bits that represent the digital number.  An n-bit ADC has a resolution of 1 part in 2n. For example, a 12-bit ADC has a resolution of 1 part in 4096 (212=4,096).  Twelve-bit ADC resolution corresponds to 2.44 mV for a 10V range.  Similarly, a 16-bit ADC’s resolution is 1 part in 65,536 (216=65,536), which corresponds to 0.153 mV for a 10V range.

ADC – ANALOGUE TO DIGITAL CONVERSION

 Many different types of analog-to-digital converters are available.  Differing ADC types offer varying resolution, accuracy, and speed specifications.  The most popular ADC types are as follows  The parallel (flash) converter  The successive approximation ADC  The voltage to - frequency ADC  The integrating ADC.

ADC – ANALOGUE TO DIGITAL CONVERSION

Parallel (Flash) Converter

2-bit parallel converter

ADC – ANALOGUE TO DIGITAL CONVERSION

Parallel (Flash) Converter  The parallel converter is the simplest ADC implementation. It uses a reference voltage at the full scale of the input range and a voltage divider composed of 2n + 1 resistors in series, where n is the ADC resolution in bits.  The value of the input voltage is determined by using a comparator at each of the 2n reference voltages created in the voltage divider.

ADC – ANALOGUE TO DIGITAL CONVERSION

Parallel (Flash) Converter  Flash converters are very fast (up to 500 MHz) because the bits are determined in parallel.  This method requires a large number of comparators, thereby limiting the resolution of most parallel converters to 8 bits (256 comparators).  Flash converters are commonly found in transient digitizers and digital oscilloscopes.

ADC – ANALOGUE TO DIGITAL CONVERSION

Successive Approximation ADC

A successive approximation ADC employs a digital-to-analog converter (DAC) and a single comparator.

ADC – ANALOGUE TO DIGITAL CONVERSION

Successive Approximation ADC  It effectively makes a bisection or binomial search by beginning with an output of zero.  It provisionally sets each bit of the DAC, beginning with the most significant bit.  The search compares the output of the DAC to the voltage being measured.  If setting a bit to one causes the DAC output to rise above the input voltage, that bit is set to zero.

ADC – ANALOGUE TO DIGITAL CONVERSION

Successive Approximation ADC  Successive approximation is slower than flash conversion because the comparisons must be performed in a series, and the ADC must pause at each step to set the DAC and wait for it to settle.  However, conversion rates over 200 kHz are common.  Successive approximation is relatively inexpensive to implement for 12- and 16-bit resolution. Consequently, they are the most commonly used ADCs, and can be found in many PC-based data acquisition products.

ADC – ANALOGUE TO DIGITAL CONVERSION

Voltage-to-Frequency ADC

Voltage-to-frequency ADCs convert an input voltage to an output pulse train with a frequency proportional to the input voltage.

ADC – ANALOGUE TO DIGITAL CONVERSION

Voltage-to-Frequency ADC  Output frequency is determined by counting pulses over a fixed time interval, and the voltage is inferred from the known relationship.  Voltage-to-frequency conversion has a high degree of noise rejection, because the input signal is effectively integrated over the counting interval.  Voltage-tofrequency conversion is commonly used to convert slow and often noisy signals.

ADC – ANALOGUE TO DIGITAL CONVERSION

Voltage-to-Frequency ADC  It is also useful for remote sensing applications in noisy environments.  The input voltage is converted to a frequency at the remote location, and the digital pulse train is transmitted over a pair of wires to the counter.  This eliminates the noise that can be introduced in the transmission of an analog signal over a long distance.

ADC – ANALOGUE TO DIGITAL CONVERSION

Integrating ADC

A number of ADCs use integrating techniques, which measure the time to charge or discharge a capacitor to determine input voltage.

ADC – ANALOGUE TO DIGITAL CONVERSION

Integrating ADC  The previous slide shows “Dual-slope” integration, a common integration technique.  Using a current that is proportional to the input voltage, a capacitor is charged for a fixed time period.  The average input voltage is determined by measuring the time required to discharge the capacitor using a constant current.

ADC – ANALOGUE TO DIGITAL CONVERSION

Integrating ADC  Integrating the ADC input over an interval reduces the effect of noise pickup at the AC line frequency if the integration time is matched to a multiple of the AC period.  For this reason, it is commonly used in precision digital multimeters and panel meters. Twentybit accuracy is not uncommon.  The disadvantage is a relatively slow conversion rate (60 Hz maximum, slower for ADCs that integrate over multiple line cycles).

ADC – ANALOGUE TO DIGITAL CONVERSION

Summary of ADC types

ADC – ANALOGUE TO DIGITAL CONVERSION In practice, an ADC is usually in form of an integrated circuit (IC). ADC0808 and ADC0809 are two typical examples of 8-bit ADC with 8-channel multiplexer using successive approximation method for its conversion. ADC0809 National Semiconductor

For more information, http://www.national.com/ads-cgi/viewer.pl/ds/AD/ADC0808.pdf

ADC – ANALOGUE TO DIGITAL CONVERSION

Block Diagram

Start

Clock

8-bit ADC End of Conversion

Control & Timing

8 Analog Inputs

8 Channels Multiplexing Switches

S.A.R. Comparator Output Latch Buffer

Switch Tree 3-bit Address Address Latch Enable

Address Latch and Decoder 256R Resistor Ladder

VCC GND +Vref

-Vref

Output Enable

8-bit Output

ADC – ANALOGUE TO DIGITAL CONVERSION When this ADC is connected to a computer, the sequence of operation is listed below:

1. The computer reads the EOC (End of Conversion) to check the ADC is busy or not. 2. If the ADC is not busy when the computer selects the input channel and send out the “Start” signal. Otherwise, step (1) is repeated. 3. The computer monitors the EOC. 4. When the EOC is activated, the computer reads the digital output. 5. When there is more than one ADCs being linked to the Computer, they can be connected in parallel. Using the ‘output enable’ can do the selection of ADC output.

ADC – ANALOGUE TO DIGITAL CONVERSION

Selection of ADC

• Error/Accuracy: Quantising error represents the difference between an actual analog value and its digital representation. • Ideally, the quantising error should not be greater than ± ½ LSB. • Resolution: DV to cause 1 bit change in output • Output Voltage Range  Input Voltage Range • Output Settling Time  Conversion Time • Output Coding (usually binary)

ADC – ANALOGUE TO DIGITAL CONVERSION

To measure an AC voltage at a particular instant in time, it is necessary to sample the waveform with a ‘sample and hold’ (S/H) circuit. Hold   

  

Sample Input

Output to ADC

ADC – ANALOGUE TO DIGITAL CONVERSION

Accuracy  Accuracy is an important consideration when selecting an ADC for use in test and measurement applications.  The following are the important factors to be considered related to accuracy.  Resolution  Calibration  Linearity  Missing codes  Noise.

ADC – ANALOGUE TO DIGITAL CONVERSION

Accuracy  The accuracy of a measurement is influenced by a variety of factors. If each independent error is i, the total error is

 This calculation includes errors resulting from the transducer, noise pickup, ADC quantization, gain, offset, and other factors.

ADC – ANALOGUE TO DIGITAL CONVERSION Calibration  There are several common methods for calibrating an ADC. In hardware calibration, the offset and gain of the instrumentation amplifier that serves as the ADC front end is adjusted with trim pots. (The gain of the ADC can also be adjusted by changing the reference voltage.)  In hardware/software calibration, digital-to-analog converters that null the offset and set the full scale voltages are programmed via software.  In software calibration, there is no hardware adjustment. Calibration correction factors are stored in the nonvolatile memory of the data acquisition system or in the computer and used to convert the reading from the ADC.

ADC – ANALOGUE TO DIGITAL CONVERSION

Calibration  Even if an ADC is calibrated at the factory, it will need to be calibrated again after a period of time (typically six months to a year, but possibly more frequently for ADCs of greater resolution than 16 bits).  Variations in the operating temperature can also affect instrument calibration.  Calibration procedures vary but usually require either a known reference source or a meter of greater accuracy than the device being calibrated.  Typically, offset is set via a 0V input, and gain is set via a full-scale input.

ADC – ANALOGUE TO DIGITAL CONVERSION

Calibration  In many measurements, the voltage is not the physical quantity under test.  Consequently, it may be preferable to calibrate the complete measurement system rather than its individual parts.  For example, consider a load cell for which the manufacturer specifies the output for a given load and excitation voltage.  One could calibrate the ADC and combine this with the manufacturer’s specification and a measurement of the excitation voltage; however, this technique is open to error.

ADC – ANALOGUE TO DIGITAL CONVERSION

Calibration  Specifically, three distinct error sources are possible in this technique:  Error in the ADC calibration,  Error in the manufacturer’s specifications,  Error in the measurement of the excitation voltage.  To circumvent these error sources, one can calibrate the measurement system using known loads and obtain a direct relationship between load and ADC output.

ADC – ANALOGUE TO DIGITAL CONVERSION

LINEARITY ERROR

The straight line is ideal output from an ADC with infinite-bit resolution. The step function shows the indicated error for a 3-bit ADC.

ADC – ANALOGUE TO DIGITAL CONVERSION

LINEARITY ERROR  If input voltage and ADC output deviate from the diagonal lines more than the ideal step function, the result is ADC error that is nearly impossible to eliminate by calibration.  This type of ADC error is referred to as nonlinearity error.  If nonlinearity is present in a calibrated ADC, the error is often largest near the middle of the input range, as shown in the figure.

ADC – ANALOGUE TO DIGITAL CONVERSION

MISSING CODES

The straight line is ideal output from an ADC with infinite-bit resolution. The step function shows the indicated error for a 3-bit ADC.

ADC – ANALOGUE TO DIGITAL CONVERSION

MISSING CODES  Some ADCs have missing codes. In the above figure, the ADC does not provide an output of four for any input voltage.  This error can result in a significant loss in resolution and accuracy. A quality ADC should have no missing codes.  No Missing Codes: An ADC has no missing codes if it produces all possible digital codes in response to a ramp signal applied to the analog input.

ADC – ANALOGUE TO DIGITAL CONVERSION

GAIN ERROR

The gain error of an ADC or DAC indicates how well the slope of an actual transfer function matches the slope of the ideal transfer function. Gain error is usually expressed in LSB (Least Significant bit) or as a percent of full-scale range (%FSR), and it can be calibrated out with hardware or in software. Gain error is the full-scale error minus the offset error.

ADC – ANALOGUE TO DIGITAL CONVERSION

OFFSET ERROR

Offset error, often called 'zero-scale' error, indicates how well the actual transfer function matches the ideal transfer function at a single point. For an ideal data converter, the first transition occurs at 0.5LSB above zero. For an ADC, the zero-scale voltage is applied to the analog input and is increased until the first transition occurs. For a DAC, offset error is the analog output response to an input code of all zeros

ADC – ANALOGUE TO DIGITAL CONVERSION

NOISE  Many users are surprised by noise encountered when measuring millivolt signals or attempting accurate measurements on larger signals.  Investing in an accurate ADC is only the first step in accurately measuring analog input signals.  Controlling noise is imperative.  Many ADCs reside on cards that plug into a PC expansion bus, where electrical noise can present serious problems.  Expansion bus noise often far exceeds the ADC’s sensitivity resulting in significant loss of measurement accuracy.  Placing the ADC outside the PC is often a better solution.

ADC – ANALOGUE TO DIGITAL CONVERSION

NOISE  An ADC in an external enclosure can communicate with the computer over an IEEE 488 bus, serial port, or parallel port.  If an application requires placement of the ADC within the computer, the noise level should be tested by connecting the ADC input to signal common and observing deviations in ADC output.  Connecting the ADC input to signal common isolates the cause of the noise to the circuit card.  More careful diagnostics are necessary when using an external voltage source because noise can arise from the external source and from the input leads.

ADC – ANALOGUE TO DIGITAL CONVERSION

Noise Reduction and Measurement Accuracy  One technique for reducing noise and ensuring measurement accuracy is with isolation, which also eliminates ground loops.  Ground loops occur when two or more devices in a system, such as a measurement instrument and a transducer, are connected to ground at different physical locations.  Slight differences in the actual potential of each ground results in a current flow from one device to the other.  This current, which often flows through the low lead of a pair of measurement wires, generates a voltage drop which can directly lead to measurement inaccuracies and noise.  If at least one device is isolated, such as the measurement device, then there is no path for the current flow, and thereby no contribution to noise or inaccuracy.

DAC – DIGITAL TO ANALOGUE CONVERSION In an electronic circuit, a combination of high voltage (+5V) and low voltage (0V) is usually used to represent a binary number. For example, a binary number 1010 is represented by Weighting

23

22

21

20

Binary Digit

1

0

1

0

State

+5V

0V

+5V

0V

DACs are electronic circuits that convert digital, (usually binary) signals (for example, 1000100) to analog electrical quantities (usually voltage) directly related to the digitally encoded input number.

DAC – DIGITAL TO ANALOGUE CONVERSION DACs are used in many other applications, such as voice synthesizers, automatic test system, and process control actuator. In addition, they allow computers to communicate with the real (analog) world. Input Binary Number

Register

Analog Voltage Output

Voltage Switch

Resistive Summing Network

Amplifier

DAC – DIGITAL TO ANALOGUE CONVERSION

Register: Use to store the digital input (let it remain a constant value) during the conversion period. Voltage: Similar to an ON/OFF switch. It is ‘closed’ when the input is ‘1’. It is ‘opened’ when the input is ‘0’. Resistive Summing Network: Summation of the voltages according to different weighting. Amplifier: Amplification of the analog according to a predetermined output voltage range. For example, an operation amplifier

Signal Conditioning Functions • Amplification – Increase the level of input signal to better suit the DAQ. – Improve the sensitivity and resolution of the measurement. • Filtering – Reject useless noise within certain frequency range. – Prevent signal aliasing and distortion. • Attenuation – Contrary to amplification.

Signal Conditioning Functions

• Isolation – Solve improper grounding problem of the system. • Multiplexing – Sequentially transmit a number of signals into single digitiser. • Simultaneous Sampling – Issue of measuring more than one signals at the same time. • Digital Signal Conditioning

DC Signal Conditioning System

• DC bridge can be Wheatstone’s Bridge which can be balanced by a potentiometer or can be calibrated for unbalanced conditions. • Amplifier should be thermally good and stable for a long term. • Low pass filter for eliminating high frequencies components or noise. • Main disadvantages – problem of drift. (Offset-error drift is the variation in offset error due a change in ambient temperature, typically expressed in ppm/°C.)

AC Signal Conditioning System

• AC system is to overcome the problem of DC system. • Transducer can be variable resistance or variable inductance types. • Bridge circuit for modulating the amplitude of the output from the transducer stage. • The signal is then amplified and demodulated before pass through the low pass filter.

Amplifiers • Signal amplification performs two important functions: increases the resolution of the inputed signal, and increases its signal-to-noise ratio. • Required in the system to improve the signal strength which is typically in the low level range of less than a few mV. • In some cases, amplifiers is necessary in providing impedance matching and isolation. • Basic characteristics involved in designing amplifiers are: • Input impedance • Output impedance • Gain and frequency response • Noise

Type of amplifier circuits • Several amplifier circuits can be constructed using the operational amplifier (such as µA741). These are: – Non-Inverting Amplifier – Inverting Amplifier – Differential Amplifier – Instrumentation Amplifier

Filters • Filter is the network used to attenuate certain frequencies but allow others without attenuation. • Consist at least one pass band, which is a band of frequencies that the output is approximately equal to input and attenuation band that the output is equal to zero. • Cut-off frequencies is the frequencies that separate the various pass and attenuation bands. • Important characteristic of filter networks is its construction make use of purely reactive elements. • Filtering is the most common signal conditioning function, as usually not all the signal frequency spectrum contains valid data. The common example are 60Hz AC power lines, present in most environments, which will produce noise if amplified.

Filters • Filtering is the most common signal conditioning function, as usually not all the signal frequency spectrum contains valid data. The common example are 60Hz AC power lines, present in most environments, which will produce noise if amplified. • Filters play a vital role in data acquisition systems to remove selected frequencies from an incoming signal and minimize artifacts

Types of Filters • Passive filters only use passive circuit component such as resistors, capacitors and inductors. • Active filters use active elements like operational amplifiers in addition to passive elements like resistance, capacitance and inductance. • Both of passive and active filters can be classified as follows: – Low Pass Filter – High Pass Filter – Band Pass Filter – Band Stop Filter – All Pass Filter

Low pass and High pass filter • A high-pass filter allows frequencies higher than the cut-off frequency to pass and removes any steady direct current (DC) component or slow fluctuations from the signal