Instrumentation (and Process Control)...

Instrumentation (and Process Control)

Fall 1393

Bonab University

Fundamentals

&

Characteristics

Instrumentation - Course Information

Department of Electrical EngineeringInstructor: Fariborz Rahimi

Lectures: 13-14:30pm (Sat), 8-10am (Sun*)

Prerequisites/Useful courses: Signals-Systems, Electrical measurements, Linear control systems

Evaluation:• Assignments: 10%

• Quizzes: 5%

• Midterm 20%

• Final Exam 65%

References:• “Measurement and Instrumentation. Theory and Application”, Alan S. Morris, Reza Langari, 2011

• “Instrumentation and Control Systems”, W. Bolton, Elsevier, 2004

• “Advanced Industrial Control Technology”, Peng Zhang, Elsevier, 2010

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3-instrumentation and control systems (w. bolton)--good case studies.pdf

Instrumentation - Course Information

References:

• سید علی سالمتی ,حمیدرضا تقی راد ,”مبانی اندازه گیری در سیستمهای ابزار دقیق“ [1] 1392

• ”اصول و روشهای کنترل صنعتی“ [2] -1388–محمد رضوانی –سید علی اکبر صفوی

• سید حجت سبز پوشان–1379 ,”اصول و اجزائ کنترل صنعتی“ [3]

• Introduction Video

3

Intro-Instrumentation-H-1.mp4

Instrumentation– Course Contents & Summary

• Introduction [1-1, A-1&2]

• Definitions, Components, Characteristics (Sta-Dyn), Importance, Need/application of instrumentation & control

• Measurement & Sensors• Errors & key parameters [1-1, A-2&3]

• Sensors categories (variable resistors, transformers, capacitors…) [1-1, A-2&13]

• Conditioning systems (active-passive) [1-2, A-9&10]

• Calibrating Measurement systems [A-4]

• Measurement systems:• Force/Torque, pressure [1-4, A-15 & 18]• Displacement, Velocity, acceleration [1-5, A-19]• Temperature [1-6, A-14]

• Basics of Control Technology [2-1]• Open/closed loop control systems, Components of control loops, Principles/Performance of such

systems

• Industrial/Process Control [2-2,2-5]

• Function and terminology of process control systems, Control algorithms & tuning controllers, proportional/integral/derivative (PID) control laws, PLC controllers (or Micro-controllers)

• Correction Elements (Actuators) [3-5]

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Introduction

• Instrumentation (usually in)• Process Control

• Industrial Control

• Components:

5

Automation field

Sensor /

Transducer

Signal

Conditioning

Amplifier Recorder

Data

Analysis

ControllerControl

Command

Power

Supply

Introduction

Introduction

• The aim (application):• Engineering Analyses (of a machine, system, process)

• A new product optimization

• Mathematical/ computer modeling

• Analyzing performance/safety

• Monitor

• Quality control, fault/failure detection (useful for

Parameter adjustment)

• Process Control (*)

• In a feedback loop such information is used for

automatic control

• Control in process industries refers to the regulation

of all aspects of the process (Precise control of level,

temperature, pressure and flow)

• Components of an instrumentation system: Sensor, Transducer, Transmitter, Actuator, Controller, Interface

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Introduction

Introduction

• Why process control is important?

Refining, combining, handling, manipulating inputs (raw) to profitably produce end products is:

• Precise

• Demanding

• potentially hazardous process

Small changes in a process = a large impact on the end result.

Variations in:• Proportions, temperature, flow, turbulence, and many other factors

must be carefully and consistently controlled to produce the desired end product with a minimum of raw materials/energy

Process control more precise operations =profitability,

quality & safety

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Introduction

Instrumentation Systems

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Measurement

&

Sensors

Instrumentation Systems

• Measured numerical value ≠ true value of the variable

Errors in measurement:• Unavoidable (but can be reduced)

• Error , Repeatability

Error Sources:

1. Gross errors (خطای عمده)• Mistake (wrong summation of couple values)

• Distraction (23.2 32.2)

• Misuse of instruments

2. Systematic errors (روشمند)• Observational/personal errors

• Parallax error: Angle of observing a scale ( mirror)

• Interpolation error: pointer stays between the divisions of the scale (observer has to estimate)

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Measurem

ent SystemInput:

True value of a variable

-Temperature of a liquid

-Pressure, speed, flow,…

Output:

Measured value of a variable

-The value for temperature,

Pressure, speed, flow,…

May

Measurement

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Sensors

Instrumentation Systems - Errors

2. Systematic errors• Instrumentation errors

• Due to inherent shortcomings in the instrument

• Due to positioning/levelness

• Due to loading effects of the instrument

• Environmental errors

• due to conditions external to the measuring device (effects of temperature, pressure, humidity, dust or of external electrostatic or magnetic field)

3. Random/Residual errors (تصادفی)

due to a multitude of small factors which change or fluctuate from one measurement to another. The happenings or disturbances about which we are unaware are lumped together averaging and statistical methods help

Error Categories:• Linear (reading is related to true value in a Linear way)

• Zero shift error

• Non-Linear (don’t follow a general trend, affect repeatability)

• Hysteresis, dead-zone errors

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Measurement

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Instrumentation Systems - Errors

• Error combination

• Calibration: Calibration is the process of comparing the output of a measurement system against standards of known accuracy

• Key parameters in an instrumentation system:

1. Sensitivity

2. Accuracy (= صحت)Closeness to the true value

3. Precision (= دقت )Identifyability, clearity, freedom fromRandom error

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𝑒𝑐 = 𝑒𝑡2 + 𝑒𝑠𝑐

2 + 𝑒𝑎2 + 𝑒𝑅

2

Input quantity Qi

Outp

ut

quanti

ty Q

o

QiR

Zero shift

Allowable

deviation

𝑆 = lim 𝑄𝑖 → 0

∆𝑄𝑜

∆𝑄𝑖

Measurement

&

Sensors

(Root Mean Square, RMS)

Instrument Types (Passive-Active)

• Passive: instrument output is produced entirely by the quantity being measured

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Measurement

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Sensors

Instrument Types (Passive-Active)

• Active: the quantity being measured simply modulates the magnitude of some external power source

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Measurement

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Sensors

Instrument Types (Null-Type and Deflection)

• Deflection type: the value of the quantity being measured is displayed in terms of the amount of movement of a pointer (pressure gauge)

• Null-type (dead-weight gauge): weights are put on top of the piston until the downward force balances the fluid pressure. (Pressure measurement in terms of weight) • A general rule: null-type instruments

more accurate than deflection types

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Measurement

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Sensors

Instrument Types (Analogue - Digital)

• Analogue: output varies continuously as the measured quantity changes. (output can have an infinite number of values within the range, the deflection-type of pressure gauge)

• Digital: output varies in discrete steps and so can only have a finite number of values (needed for Microprocessor/computer)

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Measurement

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Sensors

Instrument Types (Indicating - with a Signal Output)

• Indicating: merely give an audio or visual indication of the magnitude of the physical quantity measured

• all null-type instruments

• most passive ones

• analogue output (liquid-in-glass thermometer)

• digital display

• With a Signal Output: output in the form of a measurement signal whose magnitude is proportional to the measured quantity (commonly as part of automatic control systems)

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Measurement

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Sensors

Instrument Types (Smart - Non-smart)

• The advent of the microprocessor has created a new division in instruments between those that do incorporate a microprocessor (smart/intelligent) and those that don’t

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Measurement

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Instrument Characteristics: A-Static

• Accuracy - Inaccuracy (Measurement Uncertainty)• how close the output reading is to the correct value

• Inaccuracy/uncertainty (more common in practice): the extent to which a reading might be wrong. often quoted as a percentage of the full-scale (f.s.) reading

• Example: A pressure gauge, measurement range of 0–10 bar has a quoted inaccuracy of ±1.0% f.s. (±1% of full-scale reading).

• (a) What is the maximum measurement error expected for this instrument?

• (b) What is the likely measurement error expressed as a percentage of the output reading if this pressure gauge is measuring a pressure of 1 bar?

• Solution:

• (a) 1.0% of the full-scale reading: maximum likely error is 1.0% ± 10 bar = 0.1 bar

• (b) The maximum measurement error is a constant value related to the full-scale reading of the instrument. Thus, when measuring a pressure of 1 bar, the maximum possible error of 0.1 bar is 10% of the measurement value

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Measurement

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Sensors


• Precision/Repeatability/Reproducibility

• Degree of freedom from random errors

• Large number of readings (the same quantity) by a high-precision

instrument spread of readings = very small

• Often confused with accuracy

• High-precision may have low-accuracy (bias re-calibration)

• Repeatability / reproducibility mean approximately the same

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Measurement

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Sensors

robots programmed to place components

at a particular point on a table


• Tolerance • Tolerance (closely related to accuracy): the maximum error that is to be expected in some

value. (not, strictly speaking, a static characteristic of measuring instruments, but the accuracy of some instruments is sometimes quoted as a tolerance value)

• Describes the maximum deviation of a manufactured component from some specified value

Example: A packet of resistors gives the nominal resistance value as 1000 Ohm and the manufacturing tolerance as ±5%. If one resistor is chosen at random from the packet, what is the minimum and maximum resistance value that this particular resistor is likely to have?

Solution: 950 & 1050 Ohm

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Measurement

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Sensors


• Range or Span • the minimum and maximum values of a quantity that the instrument is designed to measure

• Linearity • desirable : output reading of be linearly proportional to

the quantity being measured

• Usually: draw a good fit straight line through the Xs

• least-squares method

• Nonlinearity: the maximum deviation of any of the output

readings marked X from this straight line

• Nonlinearity: usually expressed as a percentage of

full-scale reading.

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Measurement

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Sensors


• Sensitivity of Measurement• A measure of the change in instrument output that occurs when the quantity being

measured changes by a given amount

• The slope of the straight line drawn

• Example: resistance values of a platinum resistance thermometer: Determine the measurement sensitivity of the instrument in ohms/oC

Resistance (V) Temperature (oC)

307 200

314 230

321 260

328 290

Solution: 7/30 = 0.233 Ohm/oC

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Measurement

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Sensors


• Threshold• Input is increased gradually from zero, the input will have to reach a certain minimum level

before the change in the instrument output reading is of a large enough magnitude to be detectable• Manufacturer indicates it as:

• Absolute

• Percentage of full-scale readings

• Example: a car speedometer typically has a threshold of about 15 km/h (start to move)

• Resolution• When an instrument is showing a particular output reading, there is a lower limit on the

magnitude of the change in the input measured quantity that produces an observable change• Manufacturer : (Absolute, Percentage full-scale)

• A car at 90 km/h, how finely its output scale is divided into subdivisions (say, 20km/h) Resolution = 5km/h

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Measurement

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• Sensitivity to Disturbance• All calibrations and specifications of an instrument are only valid under controlled

conditions of temperature, pressure, …

• Variations occur certain static instrument characteristics change, and the sensitivity to disturbance is a measure of the magnitude of this change affect instrument:• Zero shift (bias): zero reading is modified

a constant error (bathroom scale)

• Normally removed by calibration

(a thumbwheel for the scale)

• Usually measured in Volts/oC (Volts/x)

• Sensitivity drift: amount by which an instrument’s

sensitivity of measurement varies as ambient

conditions change • Example: modulus of elasticity of a spring

• Units: (angular degree/bar)/oC

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Measurement

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• Sensitivity to Disturbance• Example:

The following table shows output measurements of a voltmeter under two sets of

conditions:

(a) Use in an environment kept at 20oC which is the temperature that it was calibrated at

(b) Use in an environment at a temperature of 50oC

Voltage readings at calibration temperature

of 20oC (assumed correct)

10.2 10.5

20.3 20.6

30.7 40.0

40.8 50.1

Determine the zero drift when it is used in the 50oC environment, assuming that the

measurement values when it was used in the 20oC environment are correct. Also calculate

the zero drift coefficient.

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Measurement

&

Sensors

Voltage readings at

temperature of 50oC


• Sensitivity to Disturbance• Solution:

Zero drift at the temperature of 50oC is the constant difference between the pairs of output readings, that is, 0.3 volts.

• The zero drift coefficient is the magnitude of drift (0.3 volts) divided by the magnitude of the temperature change causing the drift (30oC). Thus the zero drift coefficient is 0.3/30 = 0.01 volts/oC.

• Example-2:

• A spring balance is calibrated in an environment at a temperature of 20C and has the following deflection/load characteristic:

Load (kg) 0 1 2 3

Deflection (mm) 0 20 40 60

• It is then used in an environment at a temperature of 30oC, and the following deflection/

• load characteristic is measured:

Load (kg) 0 1 2 3

Deflection (mm) 5 27 49 71

• Determine the zero drift and sensitivity drift per oC change in ambient temperature

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Measurement

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Sensors


• Sensitivity to Disturbance• Solution:

At 20oC, deflection/load characteristic is a straight line. Sensitivity = 20 mm/kg.

At 30oC, deflection/load characteristic is still a straight line. Sensitivity = 22 mm/kg.

Zero drift (bias) = 5 mm (the no-load deflection)

Sensitivity drift = 2 mm/kg

Zero drift/oC = 5/10 ¼ 0.5 mm/oC

Sensitivity drift/oC = 2/10 = 0.2 (mm/kg)/oC

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Measurement

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• Hysteresis Effects1. the input measured quantity to the instrument is increased steadily from a negative value

Output: curve-A

2. the input variable is then decreased steadily

Output: curve-B

• Hysteresis = The non-coincidence between

these loading and unloading curves • Maximum output Hyst. (%f.s.d. output)

• Maximum input Hyst. (%f.s.d. intput)

• Found where: springs (passive pressure gauge),

friction (mechanical flyball), magnetic hyst. in iron

cores (LVDT-RVDT)

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Measurement

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Sensors


• Dead-space• the range of different input values over which

there is no change in output value

• System with Hyst. also dead space

• Some systems: no Hyst dead space

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Measurement

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Sensors

A typical cause of dead space: Backlash in gears

Example: converting between translational and

rotational motion

Instrument Characteristics: B-Dynamic

• Static characteristics: concerned only with the steady-state reading that the instrument settles down to (e.g. accuracy)

• Dynamic characteristics: describe instrument behavior between the time a measured quantity changes value and the time when the instrument output attains a steady value in response

• Dynamic characteristics also: under specific conditions outside these conditions: variation in Dyn. Char.

• In any linear, time-invariant measuring system:

• Limiting the measured quantity to step:

• further simplifications:

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Measurement

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• Zero-Order Instrument:• a0 ≠ 0

• K (constant) = instrument sensitivity

• a step change in the measured quantity at time t

the instrument output moves immediately to a new value

• Example: a potentiometer that measures motion

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Measurement

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• First-Order Instrument:• a0 & a1 ≠ 0

• Solve Analytically -- ----------------

• The time constant τ of the step response is time

taken for the output quantity q0 to reach 63% of final

• Example: a thermocouple plunged into boiling water

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Measurement

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• Example:

A balloon: • temperature- and altitude-measuring instruments

• radio equipment that can transmit the output readings

• initially anchored to the ground with the instrument output readings in steady state

• altitude-measuring instrument is approximately zero order

• temperature transducer is first order with a time constant of 15 seconds

• temperature on the ground, T0, is 10oC

• temperature Tx at an altitude of x meters is given by the relation: Tx = T0 - 0.01x.

(a) balloon released time = zero , upward velocity = 5 meters/second

draw a table showing the temperature and altitude measurements (intervals of 10s) 50s

Show also in the table the error in each temperature reading

(b) What temperature does the balloon report at an altitude of 5000 meters?

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Measurement

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• Solution:

a) temperature reported at general time t be Tr

x=5t

• Transient (complimentary function, Tx=0):

• Particular Integral part:

• Initial conditions: t=0, Tr=10 C= -0.75

b) 5000m, t=1000

• Could: error converges towards 0.75

• Large t lag 15s = 75m = .75o

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Measurement

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_ _ _


• 2nd -Order Instrument:• a0 & a1 & a2 ≠ 0

• Re-express: K (static sensitivity), ω (undamped natural frequency), and ξ (damping ratio)

• Analytic solution: depends on ξ (damping ratio)

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Measurement

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Standard format

2nd order instrument


• 2nd -Order Instrument:

A. No-damping = oscillation

B. Light damping = oscillatory

C. Critically damped (the most practical)

reduces osc. and overshoot

D. Damped

E. Over-damped

• A & E unsuitable for any measuring sys.

• C (ξ=0.707) is the best

• In practice step input is rare

• ξ: 0.6-0.8

• Example: accelerometer

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Measurement

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Sensors

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