Industrial Automation and Control Unit 4 Notes – AKTU B.Tech 5th Semester

Industrial Automation and Control Unit 4 Notes cover the sensors and signal interfaces used to provide feedback to an automatic control system. The unit, Industrial Sensors and Applications, explains optical, inductive and capacitive sensing, PNP/NPN outputs, PLC interfacing, 4–20 mA current loops and conventional wired HART communication.

These notes are for AKTU B.Tech 5th Semester EE and EN students studying BEE053. Each topic connects the sensing principle with its wiring, application and PLC interpretation. Worked examples, comparison tables and examination questions support revision.

Topics Covered

4.1 Introduction to Industrial Sensors

An industrial sensor responds to a physical condition and supplies information that a control system can use. It may detect presence, position, level, pressure, temperature or another process quantity.

A PLC depends on this feedback to interpret what is happening in the machine. The sensor detects the condition; the program decides the response; an output interface and actuator perform the action.

Sensor, Transducer and Transmitter

A proximity switch normally reports presence or absence. A measuring transmitter reports a variable quantity, such as pressure over 0–10 bar. A sensor with a digital switching output is not automatically a digital communication device.

Three Questions for an Installation

1. What is detected? Identify the target or process variable.
2. How is it detected? Choose the suitable sensing principle.
3. How is it connected? Match the output with the PLC interface.

Industrial applications include part counting, motion confirmation, product positioning, level supervision and process measurement.

4.2 Sensor Characteristics and Selection

A proximity sensor’s sensing distance describes target detection under stated conditions. A measuring sensor’s measurement range describes its specified variable interval. These are different quantities.

Hysteresis

An approaching target can operate a proximity switch at one distance. As the target withdraws, the switch releases at a slightly greater distance. This prevents repeated switching when the target vibrates near the threshold.

For the illustrated model:

Hysteresis = Release distance − Operate distance

Operate distance = 5.0 mm; release distance = 5.5 mm:

Hysteresis = 5.5 − 5.0 = 0.5 mm

Relative to the operate distance:

Hysteresis percentage = (0.5 / 5.0) × 100 = 10%

Use the manufacturer’s definition for a particular sensor.

Selection Steps

1. Identify target material, size and motion.
2. Choose the sensing principle.
3. Determine operating distance and speed.
4. Check mounting, surroundings and contamination.
5. Match supply and output to the PLC.
6. Test actual targets under normal and fault conditions.

A sufficient nominal distance alone does not establish suitability. Response speed, electrical output and target compatibility also matter.

4.3 Optical or Photoelectric Sensors

A photoelectric sensor detects a change in received light. Its arrangement determines whether the target interrupts a beam, blocks a reflected return or supplies the reflected light itself.

Construction

Typical components include:

• Light emitter.
• Lens system.
• Photodetector or receiver.
• Signal-evaluation electronics.
• Output stage.
• Protective housing.

Visible or infrared light may be used. Many sensors modulate the emitter and evaluate the corresponding received signal to reduce interference from unrelated light.

Operating Sequence

1. The emitter sends light.
2. The target changes the optical path.
3. The receiver converts received light into an electrical signal.
4. Evaluation circuitry checks the switching criterion.
5. The configured output changes state.

Light-On and Dark-On Modes

This table assumes a simple binary light threshold. Through-beam sensing often uses dark-on to indicate an opaque target; diffuse sensing often uses light-on because the target returns light.

Light-on/dark-on describes the optical decision. PNP/NPN describes the electrical output arrangement. Confirm both separately.

4.4 Through-Beam Optical Sensing

Arrangement and Principle

The emitter and receiver are in separate housings facing each other. An opaque target sufficiently interrupting the beam reduces the received light and changes the output.

Advantages and Limitations

Conveyor Counting

Install the beam across the travel path and define PartPresent = 1 when a target interrupts it. A rising PartPresent event increments the counter once. A part remaining in the beam for several scans should still generate one event.

Worked Speed Check

A 20 mm target moves at 1 m/s:

Occupied time = Target length / Speed

Occupied time = 0.020 / 1 = 0.020 s = 20 ms

With a 20 mm gap, the full target-and-gap cycle lasts 40 ms:

Event frequency = 1 / 0.040 = 25 Hz

The sensor, input filter and PLC sampling must resolve both occupied and clear intervals. Effective beam width also affects pulse duration.

4.5 Retroreflective and Diffuse Sensing

Retroreflective Arrangement

The emitter and receiver share a housing. A separate reflector returns light toward the sensor. A target is detected when it sufficiently interrupts this return path.

Only the sensor side normally needs electrical wiring, but the reflector still requires mounting and alignment. Suitable polarized arrangements help reject misleading shiny-target reflections. Transparent objects may require dedicated clear-object sensors and setup.

Diffuse-Reflective Arrangement

The emitter and receiver share a housing, and the target itself returns light. No separate reflector is used.

Diffuse sensing is convenient with access from one side. Performance depends on target colour, reflectivity, size, orientation and distance. A bright nearby background can produce an unwanted response.

Background Suppression

A background-suppression sensor uses a suitable distance-sensitive optical method to distinguish the intended region from more distant objects. It still has specified limits and installation requirements.

Retroreflective and Diffuse Comparison

4.6 Optical Applications and Maintenance

Photoelectric sensors detect cartons, bottles, sheet edges, packages and product position. Select the arrangement for the actual product and environment.

Through-Beam and Diffuse Comparison

Commissioning Steps

1. Secure the mounts and align the optical path.
2. Verify the clear-path condition.
3. Test the smallest, darkest, shiniest or most transparent expected target as applicable.
4. Set sensitivity and output mode.
5. Run at operating speed.
6. Confirm PLC interpretation and counting behaviour.

Troubleshooting

Clean lenses and reflectors appropriately. Investigate weak signal margin before repeatedly increasing sensitivity.

4.7 Inductive Proximity Sensors

An inductive proximity sensor detects a suitable metallic target through its effect on an alternating magnetic field. It provides non-contact metal-position feedback.

Construction

A typical sensor contains an oscillator, coil, ferrite structure, evaluation circuit and switching output. The active face directs the field toward the target.

Operating Principle

1. The oscillator excites the coil.
2. An alternating magnetic field forms near the active face.
3. A metal target enters the field.
4. Eddy currents induced in the target load the sensing circuit.
5. Electronics detect the oscillator change and switch the output.

Target Material and Distance

Detection depends on target material, size, thickness, orientation and sensor design. A rated reference distance may use a specified steel target. Other metals can produce shorter ranges, while some specialized sensors provide approximately equal response for selected metals.

Worked correction example: Reference distance = 8 mm; applicable material factor = 0.5:

Estimated distance = 8 × 0.5 = 4 mm

This factor is illustrative, not universal for every aluminium target or sensor.

Inductive sensing avoids optical alignment and contact. Ordinary inductive sensors generally cannot directly detect non-metallic plastic, paper or liquid. Nearby metal can influence operation.

4.8 Inductive Mounting and Applications

Flush and Non-Flush Mounting

A flush or shielded sensor is designed for specified flush installation in surrounding metal. A non-flush or unshielded sensor normally requires free space around its active region.

Use the actual mounting drawing. Adjacent sensor separation also depends on construction and operating frequency.

Applications

• Metal slide or actuator end-position confirmation.
• Gear-tooth or slot detection.
• Metal workpiece confirmation before machining.
• Metal-part counting.
• Tool-fixture presence detection.

Gear Pulse-Rate Example

A gear has 12 detectable teeth and rotates at 300 rpm:

Revolutions per second = 300 / 60 = 5

Event rate = 12 × 5 = 60 Hz

Check the actual tooth and gap durations, target runout, vibration, sensor gap, adjacent fields, cable strain and mechanical protection. A sensor’s maximum switching-frequency rating alone does not verify PLC acquisition.

4.9 Capacitive Proximity Sensors

A capacitive proximity sensor detects a change in effective capacitance caused by nearby material. Suitable targets include metals, non-metals, liquids, powders and solids.

Construction and Principle

An active electrode and return or reference arrangement create an electric field. A target changes the capacitance seen by the sensing circuit.

1. Establish the sensing field.
2. Introduce the target.
3. Detect the capacitance change.
4. Compare it with the switching threshold.
5. Change the configured output state.

Capacitance Relationship

For an ideal parallel-plate capacitor:

C = ε₀ εᵣ A / d

This illustrates the influence of material and geometry. A proximity sensor uses a fringing field, so the ideal two-plate equation is not a complete sensor-distance model.

Advantages and Limitations

Capacitive sensing can detect a broader material range than ordinary inductive sensing. Suitable equipment can detect contents through a non-metallic wall.

Response can also change with humidity, deposits, moisture, composition, wall thickness and nearby objects. Test the intended material against the empty container and expected surrounding conditions.

4.10 Capacitive Level Detection

External Point-Level Sensing

A suitable capacitive sensor can be mounted outside a non-metallic tank at a defined height. Its field passes through the wall sufficiently to detect the intended contents.

This is point-level sensing: covered or uncovered at one position. It does not automatically provide continuous percentage level.

Empty/Full Setup

1. Mount at the required height.
2. Check the empty or below-level state.
3. Introduce the actual material to the sensing height.
4. Adjust or teach the switching criterion.
5. Remove the material and confirm release.
6. Repeat under expected temperature, deposit and material variations.

Two-Point Filling Control

Define:

• LowWet = 1: Lower sensor covered.
• HighWet = 1: Upper sensor covered.
• FillPrevious: Previously maintained filling state.

FillNext = Enable AND NOT HighWet AND (NOT LowWet OR FillPrevious)

When the lower point is uncovered, filling starts. Covering the lower point does not immediately stop it because the stored filling state maintains the command. Covering the upper point clears the command.

Add separately required equipment and fault permissions.

A metal wall generally shields the external electric field. Ordinary external capacitive sensing cannot simply detect through it. Deposits near the sensing face can also keep an output active after the main contents fall.

4.11 Comparing Sensing Principles

Inductive and Capacitive Sensors

Optical and Inductive Sensors

Selection by Task

Verify speed, repeatability, access, cleaning, mounting and signal compatibility. A suitable principle with incorrect PLC wiring still produces an unusable installation.

4.12 PNP and NPN Sensor Outputs

Many three-wire DC sensors contain an electronic output switch. PNP normally sources current, while NPN normally sinks current when active.

Three-Wire Connections

Verify the device diagram. Colours and connector pin functions are not universal assumptions.

PNP Output

An active PNP output supplies current from the positive side through the load or PLC input. The connected circuit needs a suitable return to 0 V.

NPN Output

An active NPN output returns load current toward 0 V. Current passes from the positive supply through the load and into the sensor’s return switch.

PNP/NPN and NO/NC Are Separate

For an illustrative NO presence output, target absent means inactive and target present means active. An NC presence output reverses those states.

4.13 Interfacing Digital Sensors with a PLC

Match Source to Sink

A typical PNP sensor connects to a sinking input. The sensor provides current and the input completes its return.

A typical NPN sensor connects to a sourcing input. The input supplies current and the sensor returns it.

Interface Checks

• Supply range and polarity.
• Active voltage and current.
• OFF-state leakage and switching drop.
• Input threshold and common reference.
• Isolation and grouped-common rules.

Some inputs share a common terminal. Do not assume that individual points can use conflicting polarity arrangements independently.

A sensor LED being ON does not prove that the PLC receives a valid input. Sensor outputs normally serve inputs or suitably rated small loads, not large motors directly. The machine needs the appropriate rated output interface, contactor or drive.

4.14 Sensor Interpretation and PLC Programming

Define the TRUE meaning of each input before writing the ladder logic.

Example Permission Logic

Run = RunRequest AND PositionOK AND NOT HighWet

The program tests stored bits. Electrical output polarity and physical sensing behaviour establish those bits.

One Count per Object

Pulse = PartPresent AND NOT PreviousPartPresent

Evaluate Pulse, use it to count and then update the previous-state memory. Define startup handling for an object already present, so it is not automatically mistaken for a new arrival.

Input filters can remove disturbances and short valid pulses alike. Where ordinary sampling is insufficient, use suitable high-speed input or capture facilities.

Commissioning Steps

1. Verify supply and common.
2. Observe the sensor output with actual target states.
3. Confirm the input indication and stored bit.
4. Test the program’s interpretation.
5. Test motion, reset, signal loss and restoration.

An inactive input may mean either no target or a disconnected cable. Use suitable supervision where these states must be distinguished.

4.15 The 4–20 mA Current Loop

A 4–20 mA loop communicates a variable measurement or command as controlled current. For a linear measurement, 4 mA represents the lower range value and 20 mA represents the upper range value.

Basic Elements

• DC power source.
• Transmitter.
• Receiver, such as a PLC current-input module.
• Complete current return path.

A two-wire transmitter obtains power from the same loop carrying its signal.

Live Zero

The 4 mA lower-end signal is called a live zero. It supports loop-powered transmitter operation and helps distinguish the lowest valid measurement from loss of current.

0 mA is not a normal 0% measurement on a configured 4–20 mA range. It suggests a supply, wiring or device problem. Diagnostic bands and failure direction depend on the instrument configuration.

Benefits and Limits

Within the permitted voltage and load limits, series loop elements carry the same current. Cable resistance creates voltage drop rather than directly dividing the measurement as in a simple voltage-divider circuit.

This does not allow unlimited cable resistance or remove every noise problem. Excess drop can prevent the transmitter from maintaining its current. Suitable wiring, grounding and isolation remain necessary.

An ordinary analog input reads current; it does not automatically decode HART communication.

4.16 Current-Loop Interfacing with a PLC

Active and Passive Inputs

An active current input commonly supplies loop power. A passive current input receives current and requires a suitable external powered arrangement. Confirm the manufacturer’s terminal definitions.

Receiver Burden

A suitable resistor converts current into voltage:

V = I × R

For R = 250 Ω:

A current-input module may already contain its own sense element. Adding another resistor can create excessive burden or an incorrect connection. A voltage receiver needs the correct range, precision resistance and reference or isolation.

Resistor Dissipation Example

At 20 mA through 250 Ω:

P = I²R = (0.020)² × 250 = 0.10 W

This is nominal dissipation. Select an actual rating with appropriate margin, temperature and tolerance considerations.

4.17 Scaling, Span and Worked Examples

Let LRV be the lower range value, URV the upper range value and I the current in mA.

Fraction of span = (I − 4) / 16

Measurement = LRV + [(I − 4) / 16] × (URV − LRV)

For the inverse conversion:

I = 4 + 16 × (Measurement − LRV) / (URV − LRV)

Pressure Example

For a 0–10 bar transmitter at 12 mA:

Fraction = (12 − 4) / 16 = 0.5

Pressure = 0 + 0.5 × 10 = 5 bar

At 16 mA:

Pressure = [(16 − 4) / 16] × 10 = 7.5 bar

Nonzero Lower Range Example

Temperature range = −20 to 80°C; current = 8 mA:

Fraction = (8 − 4) / 16 = 0.25

Temperature = −20 + 0.25 × 100 = 5°C

Map 4 mA to the actual lower range value, which is not necessarily zero.

Raw-Count Scaling

If Raw4 and Raw20 are the configured counts at 4 and 20 mA:

Measurement = LRV + [(Raw − Raw4) / (Raw20 − Raw4)] × (URV − LRV)

Use the module’s documented convention. A 4–20 mA range and a 0–20 mA range may use different raw representations.

Validate signal quality before using the result. Define responses to invalid current, stale data and device faults. Silently treating every fault as 0% can hide a failed measurement.

4.18 Loop Voltage Budget and Fault Tracing

Compliance Voltage

The supply must cover the transmitter’s minimum terminal voltage, resistive drops and other series drops.

Vs ≥ Vt + Imax × RL + Vd

RL,max = (Vs − Vt − Vd) / Imax

Worked Voltage Budget

Given:

• Supply = 24 V.
• Minimum transmitter voltage = 12 V.
• Receiver resistance = 250 Ω.
• Total cable resistance = 100 Ω.
• Maximum current = 20 mA.
• No other specified series drop.

Resistive drop = 0.020 × (250 + 100) = 7 V

Transmitter voltage = 24 − 7 = 17 V

Margin above minimum = 17 − 12 = 5 V

Maximum total resistance = (24 − 12) / 0.020 = 600 Ω

After allocating 250 Ω to the receiver:

Remaining simplified resistance allowance = 600 − 250 = 350 Ω

Account for actual supply tolerance, other drops and design margin in a real selection.

Loop Fault Diagnosis

An ammeter is inserted in series; a voltage measurement is taken across the intended element. Do not place a low-resistance current-measuring connection across the supply.

4.19 HART Protocol Fundamentals

HART stands for Highway Addressable Remote Transducer. Conventional wired HART combines a 4–20 mA analog process signal with bidirectional digital communication on the same pair.

The analog path carries a configured primary measurement. Digital communication can provide measurements, units, identification, configuration and diagnostic information through supported commands.

Classic Wired HART Signalling

Classic wired HART uses frequency-shift keying, or FSK.

The alternating component has essentially zero average. A suitable analog input can recover the underlying current, while a compatible modem decodes the digital messages.

Tone frequency and bit rate are different. The 2200 Hz tone is not another data rate.

Analog and Digital Paths

A HART transmitter does not automatically turn an ordinary PLC analog input into a HART modem.

4.20 HART Point-to-Point Configuration

A point-to-point loop contains one intended field device connected to the host interface. Its analog current can track the process variable while digital messages supply additional information.

Primary and Secondary Hosts

The control-system interface commonly serves as the primary host. A compatible handheld communicator can serve as a secondary host. Host coordination must prevent conflicting operations or configuration changes.

Loop Impedance and Communicator Access

HART needs suitable loop impedance to develop a detectable digital signal. A 250 Ω burden is common, but equipment specifies the actual resistance and capacitance limits.

The input may already provide sufficient burden. Do not automatically add another resistor. Any barrier, isolator or conditioner in the path must pass HART signalling where communication is required.

A communicator normally connects in parallel according to its instructions, rather than as a series ammeter.

Typical Uses

• Read device identity and measurement units.
• Examine status and diagnostics.
• Configure the measurement range.
• Perform a supported fixed-current loop test.
• Compare analog interpretation with the digital measurement.

Changing the transmitter range changes what its current represents. Update PLC scaling accordingly.

4.21 HART Multidrop Configuration

A conventional multidrop arrangement connects multiple compatible addressed devices to a shared communication pair. Individual measurements are obtained digitally.

Fixed-Current Operation

In the usual multidrop configuration, loop-current process tracking is disabled and each device draws a configured fixed current, commonly 4 mA.

Four devices drawing 4 mA each require approximately:

Total device current = 4 × 4 = 16 mA

This combined current is not an independent analog measurement for each device.

Addressing and Revision

• Older HART 5 multidrop installations commonly use addresses 1–15.
• HART revision 6 and later support polling addresses 0–63.
• Actual supported device count depends on the host and electrical limits.
• Later implementations may configure loop-current mode separately from the address.

Do not assume that changing the address alone changes analog-output behaviour on every device. Configure unique addresses and current mode according to the equipment instructions.

Point-to-Point and Multidrop Comparison

4.22 HART Communication Modes

Point-to-point/multidrop describe configuration. Request–response/burst describe message behaviour. These are separate distinctions.

Request–Response

Classic wired HART uses half-duplex communication: one direction transmits at a time on the shared channel.

1. The host sends a command to an addressed device.
2. The device returns the defined response.
3. The response can contain measurements or supported status and configuration information.

Burst Operation

A compatible device can be configured to repeatedly transmit a selected supported message under burst rules. This reduces the need for a new request before every repeated update.

Burst support, scheduling and coexistence depend on device and host capabilities. Burst is not a separate voltage signal and does not provide unlimited update speed; the classic 1200 bit/s channel still limits throughput.

Mode Comparison

Command Groups

• Universal commands: Common supported information.
• Common-practice commands: Broadly used functions with device-dependent support.
• Device-specific commands: Functions particular to a device family.

For an examination answer, explain who initiates the message, what information it carries and how the mode differs from the network configuration.

4.23 Integrated Applications and Commissioning

Packaging-Station Example

Different tasks can require different sensing principles:

The PLC assigns explicit bit meanings, evaluates permissions and commands rated equipment interfaces.

Pressure-Loop Example

A transmitter has a 0–10 bar range and a 4–20 mA output. At 12 mA, the expected PLC value is 5 bar. A HART-capable tool can separately inspect the digital measurement, configured range and device status.

If analog interpretation is wrong, distinguish transmitter configuration from PLC scaling and loop electrical faults. Disagreement can involve range settings, sensor trim, output trim or acquisition error. Investigate before changing calibration.

Commissioning Procedure

1. Inspect installation and the actual target or process.
2. Verify supply, polarity, common and interface type.
3. Test digital states or analog endpoints.
4. Confirm PLC tags and engineering units.
5. Verify communication and address settings where used.
6. Test fault response and recovery.

HART Fault Checks

One normal LED or plausible current value does not verify the whole system. Test the path from physical condition through sensor, wiring, input and program to the controlled response.

4.24 Quick Revision and Examination Practice

Essential Revision Points

• Optical sensors evaluate received light.
• Through-beam uses separate emitter and receiver housings.
• Retroreflective sensing uses a separate reflector.
• Diffuse sensing uses light returned by the target.
• Inductive sensors detect suitable metals through alternating-field interaction.
• Capacitive sensors detect material-induced capacitance changes.
• PNP outputs normally match sinking inputs; NPN outputs normally match sourcing inputs.
• NO/NC behaviour is separate from PNP/NPN wiring.
• A 4–20 mA loop uses live zero and needs sufficient voltage for every series drop.
• Current, raw count and engineering value are different representations.
• Classic wired HART adds FSK communication to the analog loop.
• Network configuration and communication mode must be distinguished.

Important Formulas

Numerical Practice with Answers

Long-Answer Questions

1. Draw and explain through-beam, retroreflective and diffuse sensing.
2. Explain inductive construction, eddy-current operation and mounting.
3. Explain capacitive detection and external point-level sensing through a suitable wall.
4. Compare inductive and capacitive sensors in separate columns.
5. Draw PNP/sinking-input and NPN/sourcing-input current paths.
6. Explain a two-wire 4–20 mA loop, live zero and PLC scaling.
7. Derive a loop voltage budget and solve a numerical example.
8. Explain HART signalling and compare point-to-point with multidrop.
9. Compare request–response and burst communication.

Short-Answer Questions

1. Why does PNP not mean normally open?
2. Why can dark and bright targets produce different diffuse-sensor responses?
3. What is the purpose of sensor hysteresis?
4. Why is 0 mA not a valid zero measurement on a 4–20 mA range?
5. Why does an ordinary analog input not automatically read HART data?
6. Why do multidrop devices need unique addresses?

Frequently Asked Questions

What does Industrial Automation and Control Unit 4 cover?

Unit 4 covers industrial sensors and their applications, including optical, inductive and capacitive detection, PNP/NPN outputs, PLC connections, current loops and HART.

What is the main difference between inductive and capacitive sensors?

Inductive sensing detects suitable metals through their effect on an alternating magnetic field. Capacitive sensing detects changes in effective capacitance and can respond to suitable metallic and non-metallic materials.

What does 4 mA represent?

It represents the configured lower range value in a linear 4–20 mA measurement. That value may be zero, a positive number or a negative number.

Can an ordinary PLC analog input read HART messages?

It can measure compatible analog current. Digital HART messages require a compatible modem, interface or communicator.

How should a sensor application be studied?

Identify the target, sensing principle, output, PLC connection and bit meaning. For an analog loop, also check range, voltage budget, scaling and quality. For HART, distinguish configuration from message mode.

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