electrical system design3 · a linear encoder is a sensor, transducer or readhead paired with a...

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Electrical System Design

UNIT 2

Measurement System for Electric

Drives

Rotational Displacement Measurement

1. Potentiometer:

The operation of the rotational potentiometer is similar to the linear one.

The figure shows a typical rotational potentiometer

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2. Optical encoders INCREMENTAL:

Optical encoders provide digital output as a result of linear / angular

displacement. These are widely used in the Servo motors to measure the

rotation of shafts. Figure shows the construction of an optical encoder. It

comprises of a disc with three concentric tracks of equally spaced holes.



Three light sensors are employed to detect the light passing thru the

holes. These sensors produce electric pulses which give the angular

displacement of the mechanical element e.g. shaft on which the Optical

encoder is mounted. The inner track has just one hole which is used

locate the ‘home' position of the disc. The holes on the middle track

offset from the holes of the outer track by one-half of the width of the

hole. This arrangement provides the direction of rotation to be

determined. When the disc rotates in clockwise direction, the pulses in

the outer track lead those in the inner; in counter clockwise direction

they lag behind. The resolution can be determined by the number of

holes on disc. With 100 holes in one revolution, the resolution would be,

360°/100=3.6°.

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2. Optical encoders:



A slit disk is fixed to a rotating shaft of a rotary encoder, which has

equally-spaced lattice scale. Opposite to the slit disk, another slit disk

with equally-spaced lattice scale is fixed in a main unit of rotary encoder.

This two slits are sandwiched in between light-emitting diode and

phototransistor. Light from the light-emitting diode is interrupted by

rotating shaft every at 1 slit pitch so that this light-dark change is

repeated the number of rotations proportional to the rotational amount.

The output of a rotary encoder is an electric signal after waveform

shaped, which is converted from light-dark change via receiving

element. In generally, this output signal is a 2-phase signal adjusted to

have 1/4-pitch phase difference each other. By using these signals in

combination with a reversible counter having a direction discriminating

circuit, it is possible to add and subtract the amount of rotations.

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2. Optical encoders ABSOLUTE:

Absolute encoders output the absolute value of rotation angles. The

encoders are used for position control of servo motors mounted on

machine tools or robots. As shown in the Figure 2, rotation slits are lined

from the center on concentric circles. Slits indicates binary code strings

of 2 pulses/rev from the center. Multi-turn absolute encoders memorize

the rotation quantity data over one rotation.

Linear Displacement Measurement1- Potentiometer:

- An electrically conductive wiper that slides against a fixed resistive

element.

.

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Linear Displacement Measurement1- Potentiometer:

- To measure displacement, A potentiometer is typically wired in a

voltage divider configuration.

- A known voltage is applied to the resistor ends. The contact is attached

to the moving object of interest

- The output voltage at the contact is proportional to the displacement.

VA = I RA

As we know that R = ρL /A where ρ is

electrical resistivity, L is length of resistor

and A is area of cross section

Linear Displacement Measurement2- Linear Optical Encoders:

A linear encoder is a sensor, transducer or readhead paired with a scale

that encodes position. The sensor reads the scale in order to convert the

encoded position into an analog or digital signal, which can then be

decoded into position by a digital readout (DRO) or motion controller.

The encoder can be either incremental or absolute. Motion can be

determined by change in position over time. Linear encoder technologies

include optical, magnetic, inductive, capacitive and eddy current. Optical

technologies include shadow, self imaging and interferometric. Linear

encoders are used in metrology instruments, motion systems and high

precision machining tools ranging from digital calipers and coordinate

measuring machines to stages, CNC Mills, manufacturing gantry tables

and semiconductor steppers.

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Incremental

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Incremental


Absolute

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Rotational Speed Measurement

1. Optical Encoder:


1. Optical Encoder:

Two methods are available for determining velocities using an

incremental encoder:

– pulse-counting method

– pulse-timing method

Pulse-Counting Method

The pulse count over the sampling period of the digital processor is

measured and is used to calculate the angular velocity. For a given

sampling period, there is a lower speed limit below which this method is

not very accurate.

To compute the angular velocity ω, suppose that the count during a

sample period T is n pulses. Hence, the average time for one pulse is T/n.

If there are N windows on the disk, the average time for one revolution

is NT/n. Hence ω (rad/s) = 2πn/NT.

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1. Optical Encoder:

Pulse-Timing Method

– The time for one encoder cycle is measured using a high-frequency

clock signal. This method is particularly suitable for measuring low

speeds accurately.

– Suppose that the clock frequency is f Hz. If m cycles of the clock

signal are counted during an encoder period (interval between two

adjacent windows), the time for that encoder cycle (i.e., the time to rotate

through one encoder pitch) is given by m/f.

– With a total of N windows on the track, the average time for one

revolution of the disk is Nm/f. Hence ω = 2πf/Nm


2. Magnetic Encoder (Hall Effects):

For industrial environments, magnetic encoders are resistant to dust,

moisture, shock, vibration and other contaminants. The main

components of a rotary magnetic encoder are magnetized rotor and

sensor circuitry. The sensor circuitry is either magneto-resistive using

resistors sensitive to magnetic field change or Hall-effect to detect the

voltage change. Speed and direction of the rotor is determined by the

signal conditioning circuit.

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In order to measure the output of a rotary encoder that is in the form of

pulses, we need a counter. A typical counter will provide an output count

of the number of edges, i.e. low to high transitions. Counters have three

inputs including source, up/down, and gate. The registered events in the

input source are counted by the counter. The count increments or

decrements depending on the state of up/down input.


3. Tachogenerator:

An electromechanical generator is a device capable of producing

electrical power from mechanical energy, usually the turning of a shaft.

When not connected to a load resistance, generators will generate

voltage roughly proportional to shaft speed. With precise construction

and design, generators can be built to produce very precise voltages for

certain ranges of shaft speeds, thus making them well-suited as

measurement devices for shaft speed in mechanical equipment. A

generator specially designed and constructed for this use is called a

tachometer or tachogenerator. Often, the word “tach” (pronounced

“tack”) is used rather than the whole word.

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3. Tachogenerator:


3. Tachogenerator:

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Current Measurement

1. Current Sensor:

A current sensor is a device that detects electric current (AC or DC) in a

wire, and generates a signal proportional to it. The generated signal

could be analog voltage or current or even digital output. It can be then

utilized to display the measured current in an ammeter or can be stored

for further analysis in a data acquisition system or can be utilized for

control purpose.

Current Measurement

1. Current Sensor:

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Current Measurement

1. Current Sensor:

Torque Measurement

1. Torque Sensor:

A Torque Sensor is a transducer that converts a torsional mechanical

input into an electrical output signal. There are two types of Torque

Sensors a reaction that measures static torque, and rotary that measures

dynamic torque.

Commonly, torque sensors or torque transducers use strain gauges

applied to a rotating shaft or axle. With this method, a means to power

the strain gauge bridge is necessary, as well as a means to receive the

signal from the rotating shaft. This can be accomplished using slip rings,

wireless telemetry, or rotary transformers. Newer types of torque

transducers add conditioning electronics and an A/D converter to the

rotating shaft. Stator electronics then read the digital signals and convert

those signals to a high-level analog output signal, such as +/-10VDC.

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Torque MeasurementStrain Gauges:

The strain in an element is a ratio of change in length in the direction of

applied load to the original length of an element. The strain changes the

resistance R of the element. Therefore, we can say,

where G is the constant

of proportionality and is

called as gauge factor. In

general, the value of G

is considered in

between 2 to 4 and the

resistances are taken of

the order of 100 Ω.

Strain Gauges:

This change in resistance can be detected by a using a Wheatstone’s

resistance bridge as shown in Figure 2.2.4.In the balanced bridge we can

have a relation

where Rx is resistance of strain gauge

element, R2 is balancing/adjustable

resistor, R1 and R3 are known

constant value resistors. The measured

deformation or displacement by the

stain gauge is calibrated against

change in resistance of adjustable

resistor R2 which makes the voltage

across nodes A and B equal to zero.

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Signal conditioning elements convert the output of sensing elements into

a form suitable for further processing. This form is usually a voltage.

Therefore, if the variation of the measured variable is reflected in a

variation in resistor, capacitor, inductor, current or variable frequency

signal we need a mechanism to convert this variation into voltage. The

following techniques are used for conversion.

1- Deflection Bridges

2- Current Transformers

3- Frequency to Voltage Converters

4- Amplifiers

5- Filters

6- Isolators

7- Analog to Digital Converters

Signal Conditioning

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Deflection BridgesDeflection bridges are used to convert the output of resistive, capacitive

and inductive sensors into a voltage signal.

For the most accurate measurement of resistance, the Wheatstone Bridge

circuit is used. This circuit avoids most of the difficulties of the ammeter-

voltmeter method. This is a null method, in which no meter reading

needs be taken except for a judgment of when the deflection of a

galvanometer has been reduced to zero.

Wheatstone Bridge

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Thévenin equivalent circuit for a deflection bridge


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In a resistive or Wheatstone bridge all four impedances Z1 to Z4 are

pure resistances R1 to R4.

We first consider the case when only one of the resistances is a

sensing element. Here R1 depends on the input measured variable I,

i.e. R1 = RL, and R2, R3 and R4 are fixed resistors. This gives


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The Current Transformer ( C.T. ), is a type of “instrument

transformer” that is designed to produce an alternating current in its

secondary winding which is proportional to the current being

measured in its primary.

Current transformers reduce high voltage currents to a much lower

value and provide a convenient way of safely monitoring the actual

electrical current flowing in an AC transmission line using a standard

ammeter. The principal of operation of a current transformer is no

different from that of an ordinary transformer.

Unlike the voltage or Power Transformer, the current transformer

consists of only one or very few turns as its primary winding. This

primary winding can be of either a single flat turn, a coil of heavy

duty wire wrapped around the core or just a conductor or bus bar

placed through a central hole as shown.

Current Transformer

Due to this type of arrangement, the current transformer is often

referred too as a “series transformer” as the primary winding, which

never has more than a very few turns, is in series with the current

carrying conductor.

There are three basic types of current transformers: “wound”,

“toroidal” and “bar”.

Current Transformer

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Generally current transformers and ammeters are used together as a

matched pair in which the design of the current transformer is such

as to provide a maximum secondary current corresponding to a full-

scale deflection on the ammeter. In most current transformers an

approximate inverse turns ratio exists between the two currents in

the primary and secondary windings. This is why calibration of the CT

is generally for a specific type of ammeter.

Current Transformer

Most current transformers have a the standard secondary rating of 5

amps with the primary and secondary currents being expressed as a

ratio such as 100/5. This means that the primary current is 100 times

greater than the secondary current so when 100 amps is flowing in

the primary conductor it will result in 5 amps flowing in the

secondary winding, or one of 500/5 will produce 5 amps in the

secondary for 500 amps in the primary conductor, etc.

By increasing the number of secondary windings, N2, the secondary

current can be made much smaller than the current in the primary

circuit being measured because as N2 increases, I2 goes down by a

proportional amount. In other words, the number of turns and the

current in the primary and secondary windings are related by an

inverse proportion.

We know from our tutorial on double wound voltage transformers

that its turns ratio is equal to:

Current Transformer

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As the primary usually consists of one or two turns whilst the

secondary can have several hundred turns, the ratio between the

primary and secondary can be quite large. For example, assume that

the current rating of the primary winding is 100A. The secondary

winding has the standard rating of 5A. Then the ratio between the

primary and the secondary currents is 100A-to-5A, or 20:1. In other

words, the primary current is 20 times greater than the secondary

current.

Current Transformer

It should be noted however, that a current transformer rated as

100/5 is not the same as one rated as 20/1 or subdivisions of 100/5.

This is because the ratio of 100/5 expresses the “input/output

current rating” and not the actual ratio of the primary to the

secondary currents. Also note that the number of turns and the

current in the primary and secondary windings are related by an

inverse proportion.

But relatively large changes in a current transformers turns ratio can

be achieved by modifying the primary turns through the CT’s window

where one primary turn is equal to one pass and more than one pass

through the window results in the electrical ratio being modified.

Current Transformer

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So for example, a current transformer with a relationship of say,

300/5A can be converted to another of 150/5A or even 100/5A by

passing the main primary conductor through its interior window two

or three times as shown. This allows a higher value current

transformer to provide the maximum output current for the

ammeter when used on smaller primary current lines.

Current Transformer

A bar-type current transformer which has 1 turn on its primary and

160 turns on its secondary is to be used with a standard range of

ammeters that have an internal resistance of 0.2Ω’s. The ammeter is

required to give a full scale deflection when the primary current is

800 Amps. Calculate the maximum secondary current and secondary

voltage across the ammeter.

Secondary Current:

Voltage across Ammeter:

Current Transformer

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So for example, assume our current transformer from above is used

on a 480 volt three-phase power line. Therefore:

This 76.8kV is why a current transformer should never be left open-

circuited or operated with no-load attached when the main primary

current is flowing through it. If the ammeter is to be removed, a

short-circuit should be placed across the secondary terminals first to

eliminate the risk of shock.

Current Transformer

In several cases the output signal from primary sensing or signal

conditioning elements is an a.c. voltage with a frequency which

depends on the measured variable.

There are two main methods of converting a variable frequency

sinusoidal signal into a parallel digital output signal. The sine wave

must first be converted into a square wave signal with sharp edges

using a Schmitt trigger circuit.

In the first method the frequency fS of the signal is measured by

counting the number of pulses during a fixed time interval T. The

principle is shown in the Figure. The number NS of positive-going

edges during T is counted, giving:

Frequency to Voltage Converter

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Schmitt trigger

Instrumentation Amplifiers

An instrumentation amplifier is a high-performance differential

amplifier system consisting of several closed-loop operational

amplifiers. An ideal instrumentation amplifier gives an output

voltage which depends only on the difference of two input voltages

V1 and V2, i.e.

where the gain K is precisely known and can be varied over a wide

range. A practical instrumentation amplifier should have a gain which

can be set by a single external resistor and should combine the

following:

• High input impedance

• High common mode rejection ratio

• Low input offset voltage

• Low temperature coefficient of offset voltage.

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Filters

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Filters

Filters

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Filters

Filters

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Analog to Digital Converters

An electronic integrated circuit which transforms a signal from

analog (continuous) to digital (discrete) form.

Analog to Digital Converters

Holding signal benefits the

accuracy of the A/D

conversion

Minimum sampling rate

should be at least twice the

highest data frequency of

the analog signal

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Quantizing and Encoding


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There are two ways to best improve the accuracy of A/D conversion:

increasing the resolution which improves the accuracy in measuring

the amplitude of the analog signal.

increasing the sampling rate which increases the maximum

frequency that can be measured.

Types of A/D Converters

Dual Slope A/D Converter

Successive Approximation A/D Converter

Flash A/D Converter

Delta-Sigma A/D Converter

Other

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Fundamental components

Integrator, Electronically Controlled Switches, Counter, Clock, Control

Logic, Comparator


Fundamental components

Integrator

Electronically Controlled Switches

Counter

Clock

Control Logic

Comparator

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A dual-slope ADC (DS-ADC) integrates an unknown input voltage (VIN)

for a fixed amount of time (TINT), then "de-integrates" (TDEINT) using a

known reference voltage (VREF) for a variable amount of time.

The key advantage of this architecture over the single-slope is that the

final conversion result is insensitive to errors in the component values.

That is, any error introduced by a component value during the

integrate cycle will be cancelled out during the de-integrate phase.


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Flash A/D Converter

Fundamental Components (For N bit Flash A/D)

2N-1 Comparators, 2N Resistors, Control Logic

Flash A/D Converter

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SIGMA-DELTA A/D Converter

electrical system design3 · a linear encoder is a sensor, transducer or readhead paired with a...

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