Instrumentation & Measurement (ME342) Chapter 7: Electrical Indicating and Test Instruments Dr. Hani Muhsen

7.1 Introduction Measurement operations are always associated with converting the measured quantity into an electrical signal. Measurement of electrical signals (i.e. voltages) can be performed by using indicating instruments that can be divided as follows: Digital meters Analogue meters Oscilloscopes All types of digital meters are modified forms of the digital voltmeter (DVM) Can contain conversion circuits that allow for measurement of voltage, current, and resistance within one instrument.

7.1 Introduction Measurement operations are always associated with converting the measured quantity into an electrical signal. They are designed to satisfy a need for higher measurement accuracies and a faster speed response to voltage changes than can be achieved with analogue instruments. Characteristics. Parallax error eliminated in digital meters. Digital meters useful for computer control applications. Digital meters have a very high input impedance (10 MΩ compared to 1-20KΩ for analogue meters) (( no loading problem)) Digital meters have the ability to measure frequencies up to 1MHz. Automatic ranging preventing overload and reverse polarity connection.

7.2 Digital Meters Some Common Types of DVM Voltage-to-Time Conversion Digital Voltmeter Potentiometric Digital Voltmeter Dual-Slope Integration Digital Voltmeter Voltage-to-Frequency Conversion Digital Voltmeter Digital Multimeter

7.2.1 Voltage-to-Time Conversion Digital Voltmeter This is the simplest form of DVM and is a ramp type of instrument. When an unknown voltage signal is applied to input terminals of the instrument, a negative slope ramp waveform is generated internally and compared with the input signal. Δt = t2 – t1 = Vin/m = nT Vin = nmT; t1 start of count t2 end of count Vin input analog voltage m slope of the ramp curve n number of clock pulses to counter T clock period

7.2.1 Voltage-to-Time Conversion Digital Voltmeter This is the simplest form of DVM and is a ramp type of instrument. When an unknown voltage signal is applied to input terminals of the instrument, a negative slope ramp waveform is generated internally and compared with the input signal. When the two are equal, a pulse is generated that opens a gate, and at a later point in time a second pulse closes the gate when the negative ramp voltage reaches zero Main drawbacks: Main advantages. Nonlinearities in the shape of the ramp waveform used. Lack of noise rejection Typical inaccuracy of ±0.05% Relatively inexpensive.

7.2.2 Potentiometric Digital Voltmeter Uses a servo principle in which the error between the unknown input and a reference voltage is applied to a servo-driven potentiometer that adjusts the reference voltage until it balances the unknown voltage. Relatively inexpensive.

7.2.3 Dual- Slope Integration Digital Voltmeter Unknown voltage is applied to an integrator for a fixed time (T1) Then a reference voltage of opposite sign is applied to the integrator, which discharges the capacitor down to a zero and the output period (T2) is measured by a counter. Main advantages. Has better noise-rejection capabilities than many other types. Gives better measurement accuracy. ((Inaccuracy as low as ±0.005%)) Disadvantages. Quite expensive. Response time! Vi = Vref(T1/T2)

7.2.4 Voltage-to-frequency conversion Digital voltmeter The unknown voltage signal is fed via a range switch and an amplifier into a converter circuit whose output is in the form of a train of voltage pulses at a frequency proportional to the magnitude of the input signal. Main advantage of this DVM is its ability to reject noise.

7.2.5 Digital Multimeter It can measure both a.c. and d.c. voltages over a number of ranges through a set of switchable amplifiers and attenuators. Used for circuit test applications. Includes protection circuits that prevent damage if high voltages are applied to the wrong range.

7.3 Analogue Meters Moving Coil Meter Moving Iron Meter Clamp-on Meters Analogue Multimeter Figure 7.2 Eltime analogue panel meter (reproduced by permission of Eltime Controls).

Moving coil meter Commonly used for its sensitivity, accuracy and linear scale. Also It only responds to d.c. signals. The signal being measured is applied to the coil, which produces a radial magnetic field. Interaction between this induced field and the field produced by the permanent magnet causes torque, which results in rotation of the coil. Amount of rotation of the coil is measured by attaching a pointer to it that moves past a scale. Figure 7.3 Mechanism of a moving coil meter.

Moving Iron meter Principle of operation Measures d.c. signals and a.c. signals at frequencies up to 125Hz Commonly used. Least expensive form of meter available. Principle of operation Signal to be measured is applied to a stationary coil. The associated field produced is often amplified by the presence of an iron structure. The moving element in the instrument consists of an iron vane suspended within the field of the fixed coil. When the fixed coil is excited, the iron vane turns in a direction that increases the flux through it. Typically measures voltage of 0-30V Resistance in series used to measure higher voltages.

Moving Iron meter Figure 7.4 Mechanisms of moving iron meters: (a) attraction type and (b) repulsion type.

Clamp on Meters Used for measuring circuit currents and voltages in a noninvasive manner that avoids having to break the circuit being measured. Figure 7.5 Schematic drawing of a clamp-on meter.

Principle of operation Clamp on jaws of the instrument act as a transformer core and the current carrying conductor acts as a primary winding. Current induced in the secondary winding is rectified and applied to a moving coil meter. It has a low sensitivity and the minimum current measurable is usually about 1 amp.

Oscilloscope Function: To draw a graph of an electrical signal. Types Analogue (low specification, low cost) Digital (used for professional work) Digital storage oscilloscopes Digital phosphor oscilloscope Digital sampling oscilloscope

Oscilloscope Measures a.c. and d.c. voltage signals Besides voltage, it measures frequency and phase of a signal. (10 MHz- 25 GHz) Has high input impedance ( typically 1 MΩ) negligible “loading” effect. Voltage Time

Oscilloscope Specifications Bandwidth: the range of frequencies over which the oscilloscope amplifier gain is within 3dB of its peak value. Rise time: the transit time between 10 and 90% levels of the response when a step input is applied to the oscilloscope . Accuracy : Inaccuracy ±1% to ±5%

Oscilloscope Specifications 7.4.1 Analogue Oscilloscope (Cathode Ray Oscilloscope) Figure 7.10 Cathode ray tube.

7.4.1 Analogue Oscilloscope (Cathode Ray Oscilloscope) Controls of Simple Oscilloscope Focus Control: adjusts the focusing mechanism Intensity control: varies the cathode heater current varies the rate of emission of electrons

7.4.1 Analogue Oscilloscope (Cathode Ray Oscilloscope) Channels electron source, focusing system, and deflector plates. Common oscilloscope configuration with 2 channels can display 2 separate signals simultaneously. Single-ended input: only allows signal voltages to be measured relative to ground (one input terminal plus a ground terminal per oscilloscope channel) Differential input: allows comparison of the potentials at 2 non-grounded points in a circuit (two input terminals plus a ground terminal per oscilloscope channel)

7.4.2 Digital Storage Oscilloscopes When first created they consisted of: conventional analogue cathode ray oscilloscope THEN Added facility that converts analogue signal to digital format and stores it in computer memory within the instrument Displays a sequence of dots rather than a continuous line. The density of dots is dependent on the sampling rate at which the analogue signal is digitized.

7.4.2 Digital Storage Oscilloscopes Can analyze measured waveform and compute signal parameters such as: Max. and min. signal levels, peak-peak values, mean values, r.m.s. values, rise time, and fall time. 7.4.3 Digital Phosphor Oscilloscope Uses parallel- processing architecture instead of series-processing architecture. Overcomes the problem of missing changes occurring between sampling instants (fast signal transients) as it has a higher waveform capture rate.

7.4.5 Personal Computer-Based Oscilloscope Consists of a hardware unit that connects to standard PC via either a USB or a parallel port. Hardware unit: signal scaling, A/D conversion, and memory functions. PC: control interface and display facilities. Advantages: low cost, large display, and portability. Disadvantages: electromagnetic noise, limited signal sampling rates.