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UEMH 4413Sensors and InstrumentationJAN 20151TOPIC 3 (Part 1)

Measurement Sensors and Instrument2Electrical Test Instruments3The magnitude of voltage signals can be measured by various electrical indicating and test instruments, such asDigital metersAnalogue metersCathode ray oscilloscopesDigital storage oscilloscopes

Digital Meters4All digital meters are based on the digital voltmeter (DVM).Digital meters for quantities other than voltage are in fact DVMs containing circuits to convert current/resistance signals into voltage signals.Digital Multimeters (DMM) contain several conversion circuits allowing measurement of voltage, current, and resistance in one instrumentDigital Meters5Digital meters are technically superior to analogue meters in almost every respect:Faster speed of response to voltage changes.The binary output reading can be readily applied to a display that is in the form of discrete numerals.Eliminates parallax error and reduces misreading the meter output.Higher measurement accuracy (quoted inaccuracy are between 0.005% and 2%).

Digital Meters6High input impedance (10 M compared with 120 k for analogue meters).Can measure high frequency signals (up to 1 MHz).Extra features such as automatic ranging which prevents overload and reverse polarity connection etc.Greater manufacturing cost compared with analogue meters.Important part of DVM is the circuitry for converting analogue voltage to a digital quantity.DVMs measure DC in its basic mode.AC-DC conversion is necessary to measure AC signals.Digital Meters7Voltage value is normally displayed using LCDs or transmitted to a computing device this form of display enables measurements to be recorded with much greater accuracy than that obtainable by reading an analogue meter scale.DVMs mainly differ in the technique used for analogue-to-digital conversion (ADC) expensive/complicated methods generally are faster.Digital Meters8Voltage-to-time conversion DVMSimplest form of DVM and is a ramp type of instrument.Negative-slope ramp waveform is generated internally and compared with the input signal.When two signals equal, a gate-opening pulse is generated.When ramp voltage is zero, gate-closing pulse is generated.Length of time is monitored by an electronic counter, directly gives voltage measurement.Cheap but ramp waveform is non-linear and lack of noise rejection - cause inaccuracy of 0.05%.Digital Meters9Potentiometric DVMUses a servo principle.Error between input voltage and a reference voltage is applied to the servo-driven potentiometer which adjusts the reference voltage till it balances the input voltage.Digital display also driven by the potentiometer.Cheap with good performance.Digital Meters10Dual-slope integration DVMSimple DVM, better noise-rejection and better measurement accuracy (inaccuracy 0.005%) but quite expensive.Input voltage, Vi is applied to an integrator for a fixed time T1.After that, a reference voltage of opposite signis applied to the integratortill it reaches zero in an interval T2.Vi = Vref(T1/T2)

Digital Meters11Voltage-to-frequency conversion DVMRange switch and amplifier used to feed input voltage into a converter circuit.Output is a train of voltage pulses at a frequency proportional to the magnitude of the input voltage.Able to reject AC noise.Digital Meters12Digital MultimeterExtension of DVM.Can measure both AC and DC voltages over a number of ranges.A set of switchable amplifiers and attenuators provides the selection of function and range.Normally includes protection circuits against damage from high voltage in the wrong range.Analogue Meters13Simple and inexpensive.Suffer less from noise and isolation problems.Passive instruments that do not need power supply.Analogue meters are electromechanical devices that drive a pointer against a scale.Prone to error from inaccurate scale markings, bearing friction, bent pointers, ambient temperature variations.Human errors are introduced such as parallax errors and mistakes in interpolating between scale markings.Inaccuracy figures are between 0.1% and 3%.Analogue Meters14Moving-coil metersCommonly used because of its sensitivity, accuracy and linear scale.It only responds to DC signals.Consists of a rectangular coil woundaround a soft iron core in a permanent magnetic field.Input signal induces a radial magnetic field.

Analogue Meters15Interaction between induced field and the field produced by the permanent magnet causes a torque which rotates the coil.Amount of rotation is measured using attached pointer on a graduated scale.Theoretical torque is T = BIhwNwhere B = flux density of the radial field I = current flowing in the coil h = height of the coilw = width of the coil N = number of turns in the coilAnalogue Meters16If the iron core is cylindrical and the air gap is uniform, then flux density, B is constant, and equation can be rewritten as T = KITorque proportional to coil current (linear instrument scale).Operates at low current levels (1 mA) and only suitable for measuring voltages of about 2V.Shunt resistor (in series with the coil) can be used for increased voltage range.Special magnets or core shapes can be used for increased deflection range or non-linear (e.g. logarithmic) scales.Analogue Meters17Moving-iron metersMeasures both AC (up to 125 Hz) and DC signals.Cheapest form of meter available - commonly used for measuring voltage signals.Signal applied to stationary coil, field produced can be amplified by an iron structure associated with the fixed coil.Iron vane that is suspended within the field of the fixed coil turns in the direction that increases flux through it.Moving-iron instruments can be attraction type, repulsion type (majority) or combination type (few).Analogue Meters18Attraction typeRepulsion type

Analogue Meters19Analogue Meters20The instrument has a square-law response where deflection is proportional to square of the signal (i.e. output is RMS).Range is 0-30 V, can be increased with shunt resistor.A series resistance in AC signal measurements can reduce the total resistance/inductance ratio, and hence measurement accuracy is improved.A switchable series resistance is often provided within the casing of the instrument to facilitate range extension.When voltage measured exceeds about 300 volts, external resistance is used instead due to heat-dissipation problems.Analogue Meters21Electrodynamic metersElectrodynamic meters (or dynamometers) can measure bothDC and AC (up to 2kHz).The instrument has a moving circular coil in a magnetic fieldproduced by 2 separately wound,series-connected circular stator coils.

Analogue Meters22Analogue Meters23For AC current, meter is unable to alternate torque values and instead displays mean value of current2.By suitable drawing of the scale, the position of the pointer shows the squared root of this value, i.e. the RMS current.Typically expensive but more accurate than moving coil/iron instruments.V, I, and P can all be measured with appropriate connection of fixed/moving coils.Measure voltages in the range of 0-30 V. Can be modified to measure higher voltages with the use of shunt resistance.Analogue Meters24Clamp-on metersMeasures current/voltage in a non-invasive manner which avoids breaking the circuit being measured.Meter clamps onto a current-carryingconductor, output reading is obtained bytransformer action.

Analogue Meters25Clamp-on jaws act as a transformer core and the current-carrying conductor acts as a primary winding.Current induced in secondary winding isrectified and applied to moving-coil meter.Very convenient but low sensitivity,minimum current measurable is about 1 A.

Analogue Meters26Analogue MultimeterMulti-function instrument that can measure current, resistance, AC and DC voltage signals.Consists of a moving-coil meter with a switchable bridge rectifier to allow it to measure AC signals.Rotary switches allow selection of various series/shunt resistors allowing voltage/current measurement over a number of ranges.Internal power source allows resistance measurement.Accuracy not as good as single-purpose instruments.Analogue Meters27

Analogue Meters28Measuring high-frequency signalsOne major limitation of analogue meters is that the maximum AC frequency measurable is low, 2 kHz for electrodynamic meter and 100Hz for moving-iron meter.Partial solution is to rectify the voltage signal and thenapply it to a moving-coil meter.

Analogue Meters29This extends the upper measurable frequency limit to 20 kHz.Inclusion of the bridge rectifier makes the measurement system sensitive to environmental temperaturechanges, and non-linearitiessignificantly affect measurement accuracy for voltages that are small relative to the full-scale value.

Analogue Meters30Thermocouple meterCan measure very high-frequency voltage up to 50 MHz.AC voltage signal heats up a small element.Temperature rise is measured by thermocouple.DC voltage generated is applied to a moving-coil meter.Output is RMS voltage.

Analogue Meters31Electronic analogue voltmetersActive rather than passive, high input impedance that avoids the circuit loading problems and amplification capability that enables to measure small signal levels.The standard electronic voltmeter for DC measurements consists of a simple direct-coupled amplifier and a moving-coil meter.

Analogue Meters32For measurement of very low-level voltages of a few microvolts, chopper amplifier is used.DC input is passed through 250 Hz chopper followed by capacitor, op-amp and capacitor again before applied to a moving-coil meter.

Analogue Meters33Electronic voltmeter for measuring a.c. signals:1.Average responding type - A direct-coupled DC electronic voltmeter with an additional rectifying stage at the input.The output is the average value of the measured voltage waveform.2.Peak-responding type- Consists of a half-wave rectifier at the input followed by a capacitor, an amplifier and moving-coil meter.Analogue Meters34Capacitor is charged to the peak value of the input signal, therefore gives reading of the peak voltage in the input waveform.3.RMS-responding type- A thermocouple meter with an amplification stage at the input.- Gives output reading in terms of RMS value of the input waveform.Analogue Meters35Example 1Calculate the reading that would be observed on a moving-coil ammeter when it is measuring the current in the circuit shown in Figure.

Analogue Meters36Analogue Meters37Example 2Calculate the reading that would be observed on a moving-iron ammeterwhen it is measuring the current in the circuit shown in Figure.

Analogue Meters38Analogue Meters39Example 3A dynamometer ammeter is connected in series with a 500 resistor, a rectifying device and a 240 V RMS alternating sinusoidal power supply. The rectifier behaves as a resistance of 200 to current flowing in one direction and as a resistance of 2 k to current in the opposite direction. Calculate the reading on the meter.Analogue Meters40Analogue Meters41Cathode Ray Oscilloscopes42Probably the most versatile and useful instrument available for signal measurement.Traditionally analogue, able to measure a wide range of AC and DC voltage signal levels, frequency, and phase of a signal.Measure from 20 MHz (cheapest model) to 500MHz (most expensive model).Negligible loading effect due to high input impedance, typically 1 M.

Cathode Ray Oscilloscopes43Rotary switches to alter timebase and internal circuitry protects from high voltage damage.Not particularly accurate (inaccuracy 1% for best instruments and 10% for cheapest instruments).Fragile (being built around a cathode ray tube) and costly.Most important aspects are bandwidth, rise time, and accuracy.Bandwidth is the range of frequencies over which the oscilloscope amplifier gain is within 3dB of its peak value (i.e. -3dB is when gain is 0.707 times max value).Cathode Ray Oscilloscopes44The difference between voltage levels V1 and V2 is expressed in decibels (dB) as 20 log10(V1/V2) = 20 log10(0.7071) = -3 dB.

Cathode Ray Oscilloscopes45Cathode Ray Oscilloscopes46Oscilloscope is a relatively complicated instrument that is constructed from a number of subsystems.

Cathode ray tubeThe cathode consists of a barium and strontium oxide coated, thin, heated filament which emits a stream of electrons.Electrons are focused on a fluorescent screen by an electrostatic focusing system consisting of a series of metal discs/cylinders charged at various potentials.Adjustments done by controls on front panel of oscilloscope.Cathode Ray Oscilloscopes47Intensity control varies cathode heater current rate of emission of electrons.

Cathode Ray Oscilloscopes48Application of potentials to two sets of right-angled deflector plates (horizontal andvertical) provide for deflection of the streamof electrons in 2 dimensions.Magnitude of any signal (potential) on deflector plates can be measured usingthe crossed-wire graticule etched on screen.Most commonly, time-base connected to x-axis and signal to y-axis.

Cathode Ray Oscilloscopes49ChannelOne channel describes the basic subsystem of an electron source, focusing system, and deflector plates.This subsystem is often duplicated to provide capability of displaying two or more signals at the same time.Common oscilloscope with 2-channel configuration can display two separate signals simultaneously on the same screen.Cathode Ray Oscilloscopes50Single-ended inputThis type of input only has one input terminal (plus a ground terminal).Only allows signal voltage measurement relative to ground.Normally only used in simple oscilloscopes.Cathode Ray Oscilloscopes51Differential inputThis type of input is provided on more expensive oscilloscopes.Two input terminals plus a ground terminal provided for each channel.Allows potentials of two non-grounded points in circuit to be compared.Can also work in single-ended mode by using just one of the input terminals plus ground.Cathode Ray Oscilloscopes52Timebase circuitUsed to apply a voltage to the horizontal deflector plates such that the horizontal position of the spot is proportional to time.This voltage, in the form of a ramp known as a sweep waveform, is applied repetitively such that the motion of the spot across the screen appears as a straight line when a DC level is applied to input.This timebase voltage is synchronized with the input signal in the general case of a time-varying signal such that a steady picture is obtained on the oscilloscope screen.Cathode Ray Oscilloscopes53Length of time taken for the spot to traverse the screen is controlled by a time/div switch.Each cycle of the sweep waveform is initiated by a pulse from a pulse generator.The input to the pulse generator is a sinusoidal signal known as a triggering signal. A pulse is generated every time the triggering signal crosses a preselected slope/voltage level condition.This condition is defined by the trigger level and trigger slope switches.Cathode Ray Oscilloscopes54The former selects the voltage level on the trigger signal, commonly zero, at which a pulse is generated, whilst the latter selects whether pulsing occurs on a positive- or negative-going part of the triggering waveform.Synchronization of the sweep waveform with the measured signal is most easily achieved by using measured signal as trigger signal known as internal triggering.If the amplitude of the measured signal is too small, external triggering is necessary.Cathode Ray Oscilloscopes55External triggering can be applied if the frequencies of the triggering signal and measured signals are related by an integer constant such that the display is stationary.External triggering is also used in measurement of the phase difference between two sinusoidal signals of the same frequency.50 Hz line voltage is used for external triggering when measuring signals at mains frequency (line triggering).Cathode Ray Oscilloscopes56Vertical sensitivity controlConsists of a series of attenuators and pre-amplifiers at the input to the oscilloscope.Enables the instrument to measure a very wide range of signal magnitudes by conditioning it.Selection is done using a volts/div control.Defines the magnitude of the input signal which will cause a deflection of one division on the screen.Cathode Ray Oscilloscopes57Display position controlAllows position at which signal is displayed on the screen to be controlled in two ways.Horizontal position is adjusted by horizontal position knob.Vertical position is adjusted by vertical position knob.Adjusts position by biasing the measured signal with a DC voltage level.Digital Storage Oscilloscopes58Digital storage oscilloscopes consist of a conventional analogue cathode ray oscilloscope with the additional facility of converting the measured signal to digital format and storing in computer memory within the instrument.Can be used to refresh display, allowing non-fading display of the signal on the screen.Digital oscilloscope displays consist of a sequence of individual dots.High density dots gives appearance of continuous traces.Digital Storage Oscilloscopes59Density of the dots depends upon the sampling rate at which the analogue signal is digitized.Most expensive instruments give the best performance in terms of dot density and the accuracy with which the analogue signal is recorded and represented.Some digital oscilloscopes can also compute signal parameters such as peak values, mean values and RMS values.Ideally suited for capturing (freezing) transient signals.Digital Storage Oscilloscopes60Very careful synchronization is necessary to capture such signals on an analogue oscilloscope.Often have facilities to output analogue signals and digital signals to other devices.