A mid-level course for electricians, industrial machinery mechanics and electrical engineers who work on automated equipment. You will read start/stop seal-in circuits and interlocks on ladder diagrams; match proximity, photoelectric and analog sensors to applications; explain how a variable frequency drive controls motor speed; describe PLC hardware, I/O and the scan cycle; simulate seal-in, interlock, timer and counter logic in small browser exercises; explain the roles of HMI and SCADA systems; isolate an I/O fault with a systematic method; explain machine guarding, emergency stops and safety relays; and read vibration and thermal data for predictive maintenance. Prerequisites are Electrical Circuits and Workplace Safety & Tools. The code activities are fill-in-the-blank: the program already runs and you write one rung or formula, so no programming course is needed. Practice uses simulators and paper only; never work on live control panels without employer training, supervision and the required qualifications, and for electricians a registered apprenticeship and licence.

Read relay ladder diagrams, match sensors and actuators, understand VFDs, program PLC logic with timers and counters, and troubleshoot, guard and maintain automated machines.

Industrial Controls & PLCs

## Why it matters

Before PLCs, machines were controlled by panels full of relays wired together, and electricians drew those circuits as **ladder diagrams**. PLC programs still use the same picture, so the start/stop circuit you learn here appears on motor starters, conveyors and pumps everywhere. If you can read it, you can follow the logic of nearly any machine.

## The ladder

A ladder diagram has two vertical **rails** and horizontal **rungs**.

- The left rail is the control power supply (for example L1 of a 120 V AC control transformer, or +24 V DC).
- The right rail is the return (L2, neutral or 0 V).
- Each rung is one circuit: **conditions** on the left (switches and contacts) and one **output** on the right (a coil, lamp or solenoid).
- Read each rung left to right: if there is a complete path of closed contacts, the output is energized.
- Rungs are numbered down the left side, and contacts carry cross-references to the rung where their coil lives.

## Contacts and coils

- A **normally open (NO)** contact, drawn `-| |-`, is open when its device is at rest and closes when it is actuated.
- A **normally closed (NC)** contact, drawn `-|/|-`, is closed at rest and opens when actuated.
- "Normal" means the device's shelf state, not the machine's usual running state.
- A **coil** `-( )-` is a relay or contactor. When it energizes, every contact with the same label changes state: its NO contacts close and its NC contacts open.

## The three-wire start/stop circuit

This is the most important rung in industrial control:

```text
   L1                                                     L2
    |                                                     |
1   |--[ STOP (NC) ]--+--[ START (NO) ]--+--[ OL (NC) ]--( M )--|
    |                 |                  |                    |
    |                 +----[ M (NO) ]----+                    |
    |                                                         |
2   |--[ M (NO) ]------------------------------------( RUN LAMP )--|
```

How it works:

1. At rest, STOP is closed (NC), START is open, the M auxiliary contact is open. M is off.
2. Press **START**: current flows through STOP, START and the overload contact, so coil **M** energizes. The contactor pulls in and the motor runs.
3. M's auxiliary NO contact, wired **in parallel** with START, closes. This is the **seal-in** (or holding, or latching) contact.
4. Release START: current now flows through the M contact instead, so M **stays on**.
5. Press **STOP**: the path breaks, M drops out, and its seal-in contact opens. Releasing STOP does not restart the motor.
6. If the motor overloads, the **OL** contact opens and drops M out the same way.

Rung 2 shows a run lamp driven by another M contact.

## Why stop buttons are normally closed

A broken wire on a normally closed stop circuit has the same effect as pressing stop: the machine stops. If the stop button were normally open and its wire broke, pressing it would do nothing. This **fail-safe** principle is why stops, overloads and safety devices use NC contacts.

The three-wire circuit also gives **low-voltage protection**: after a power cut, M drops out and the motor will not restart by itself when power returns. A **two-wire** control (a maintained switch or float switch with no seal-in) restarts automatically, which is fine for a sump pump but dangerous for a saw.

## Interlocks

An interlock stops two things from happening at once. A forward/reverse motor starter has two contactors, F and R. Energizing both would short two phases together, so each rung includes an NC contact of the other contactor:

```text
3   |--[ STOP ]--+--[ FWD PB ]--+--[ R (NC) ]--[ OL ]--( F )--|
    |            +----[ F ]-----+                            |
4   |--[ STOP ]--+--[ REV PB ]--+--[ F (NC) ]--[ OL ]--( R )--|
    |            +----[ R ]-----+                            |
```

When F is on, its NC contact in rung 4 is open, so R cannot energize. Real reversing starters also use a **mechanical interlock** between the contactors.

## Worked example: reading a fault

A conveyor starts when START is pressed but stops as soon as the button is released. Which part of rung 1 is suspect? The seal-in branch: the M auxiliary contact is not closing (a failed auxiliary block or a broken wire on that branch). The rest of the rung clearly works, because the motor runs while START is held.

## Common mistakes

- Confusing "normally closed" with "closed while running".
- Forgetting that every contact with the same label as a coil changes when that coil energizes.
- Drawing the seal-in contact in series with START instead of in parallel.
- Putting more than one output coil in series on a rung.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Rails are supply and return; each rung is conditions on the left, one output on the right.
- Seal-in: an NO contact of the coil in parallel with START keeps it on.
- Stops and overloads are NC so a broken wire fails safe.
- Interlocks use NC contacts of the opposing output.

## Try it

Open the **Do** tab and take the quiz on reading start/stop and interlock rungs.


Read a ladder diagram, including a three-wire start/stop seal-in circuit with overload protection and an interlock.

Ladder diagrams

What is Ladder Logic?

Motor Start/Stop Seal-In Circuit Explained | Ladder Logic Basics

## Why it matters

A controller is only as good as what it can sense and what it can move. Most automation faults are not in the PLC program: they are a dirty photo-eye, a proximity sensor knocked out of range, a cut cable or a sticking valve. Knowing which sensor suits which job, and how it is wired, is what lets you fix a machine quickly and replace a part with the right one.

## Discrete (on/off) sensors

**Limit switches** are mechanical: a lever, plunger or roller is pushed by the moving part, which changes NO and NC contacts. Rugged and easy to understand, but they wear and need contact with the target.

**Inductive proximity sensors** detect metal without contact, using an electromagnetic field at the sensing face. Range is short, typically a few millimeters up to a few tens of millimeters, and depends on the metal: steel is detected at the rated distance, aluminum and brass at shorter range. Uses: detecting a cylinder rod end, gear teeth for speed, a metal pallet in position.

**Capacitive proximity sensors** detect almost anything, including plastic, wood, powder and liquid, often through a non-metallic tank wall. They usually have a sensitivity adjustment. Uses: level in a plastic hopper, detecting cardboard boxes.

**Photoelectric sensors** use a light beam:

- **Through-beam**: separate emitter and receiver facing each other. Longest range and most reliable; the object breaks the beam.
- **Retroreflective**: emitter and receiver in one housing, aimed at a reflector. Easier to install; shiny objects can fool them unless the sensor is polarized.
- **Diffuse**: the light bounces off the object itself. Simplest to mount but shortest range, and affected by the object's color.

**Magnetic (reed or Hall-effect) switches** clip onto pneumatic cylinders and detect a magnet in the piston to report extended or retracted.

**Pressure, flow, level and temperature switches** change state at a set point, for example low oil pressure.

## Sinking and sourcing (NPN and PNP)

Three-wire DC sensors have power (brown, +24 V), common (blue, 0 V) and output (black).

- A **PNP (sourcing)** sensor switches +24 V onto the output wire when it detects. It needs a **sinking** PLC input.
- An **NPN (sinking)** sensor connects the output to 0 V when it detects. It needs a **sourcing** input.

Mixing them up gives an input that never turns on. Check the sensor's part number and the input card's wiring diagram before replacing one. Most sensors also have an indicator LED showing when they detect, which is the first thing to check when troubleshooting.

## Analog sensors

Analog sensors give a value, not just on/off:

- **Pressure transmitters**, **level transmitters**, **flow meters**.
- **Temperature**: thermocouples (wide range, small voltage) and RTDs (accurate resistance change, often Pt100).
- **4 to 20 mA** is the standard signal. A live zero of 4 mA means a reading of 0 mA tells you the wire is broken.

**Worked example.** A pressure transmitter is ranged 0 to 150 psi on 4 to 20 mA. The PLC reads 13.6 mA.

```text
pressure = (13.6 − 4) / (20 − 4) × 150 = 9.6 / 16 × 150 = 90 psi
```

**Encoders** count pulses as a shaft turns, giving position and speed. Incremental encoders count from a reference; absolute encoders know their position at power-up.

## Actuators

Outputs move the machine:

- **Contactors and motor starters** switch motors; **relays** switch smaller loads.
- **Solenoid valves** shift pneumatic and hydraulic valves.
- **Pilot lights, horns and stack lights** tell people what is happening.
- **Variable frequency drives** control motor speed (next lesson).
- **Servo drives and stepper drives** control precise position.

PLC output cards come as relay outputs (switch AC or DC, slower, contacts wear), transistor outputs (DC, fast) and triac outputs (AC).

## Matching sensors to applications

```text
Application                                     Good choice
Detect a steel bracket 5 mm away                Inductive proximity
Detect clear plastic bottles on a conveyor      Through-beam or a clear-object photo sensor
Detect level of grain through a plastic bin     Capacitive proximity
Know if an air cylinder is retracted            Magnetic reed switch on the cylinder
Measure tank level continuously                 4-20 mA level transmitter
Detect a door is closed (not for safety)        Limit switch or inductive sensor
Measure shaft speed                             Inductive sensor on gear teeth, or an encoder
```

Safety functions such as guard doors use special **safety-rated** devices, covered in the machine safety lesson. An ordinary sensor is not a safety device.

## Common mistakes

- Replacing a PNP sensor with an NPN one.
- Expecting an inductive sensor to see aluminum at the steel rating.
- Diffuse photo-eyes aimed at a shiny background that reflects light back.
- Reading 0 mA as "zero pressure" instead of a broken loop.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Limit switches touch; inductive sensors see metal; capacitive sensors see most materials; photo sensors use light (through-beam is most reliable).
- PNP sources +24 V, NPN sinks to 0 V; match the input card.
- 4 to 20 mA analog signals: value = (mA − 4) / 16 × span.

## Try it

Open the **Do** tab and take the quiz on matching sensors to applications.


Match limit switches, proximity, photoelectric and analog sensors to applications, and name the actuators that controls drive.

Sensors and actuators

Ep 1: What Are Industrial Sensors? The Basics Explained

What are Proximity Sensors and How Do They Work? - A Galco TV Tech Tip | Galco

## Why it matters

Most industrial motors are three-phase induction motors. Connected straight to the supply, they run at one speed. A **variable frequency drive (VFD)** lets a pump, fan or conveyor run at exactly the speed the process needs, with soft starts and big energy savings. VFDs are everywhere, and they have their own faults, settings and hazards.

## Motor speed and frequency

An induction motor's magnetic field rotates at the **synchronous speed**:

```text
synchronous rpm = 120 × frequency (Hz) / number of poles
```

A 4-pole motor on 60 Hz: 120 × 60 / 4 = **1,800 rpm**. The rotor runs a little slower because of **slip**, so the nameplate might say 1,750 rpm. On 50 Hz supplies the same motor would turn at 1,500 rpm synchronous.

Change the frequency, and you change the speed. At 30 Hz the same motor runs at about 900 rpm.

## Inside a VFD

A typical drive has three stages:

1. **Rectifier**: diodes convert the incoming three-phase AC into DC.
2. **DC bus**: large capacitors smooth the DC. On a 480 V drive the bus sits around 650 V DC.
3. **Inverter**: fast transistors (IGBTs) switch the DC bus on and off thousands of times a second. By varying the width of the pulses (**pulse width modulation, PWM**), the drive builds an output that acts like a sine wave at whatever frequency and voltage it chooses.

The drive keeps the **volts-per-hertz** ratio roughly constant so the motor's magnetic flux, and therefore its torque capability, stays steady. A 460 V, 60 Hz motor gets about 230 V at 30 Hz. More advanced drives use **sensorless vector** or **closed-loop vector** control for better torque at low speed.

## Why plants use VFDs

- **Energy**: for centrifugal pumps and fans, power falls with the cube of speed (the affinity laws). Running a fan at 80 percent speed uses roughly 0.8³ ≈ 51 percent of full-speed power, compared with throttling a damper.
- **Soft starting**: the drive ramps up, avoiding the large inrush current and mechanical shock of across-the-line starting.
- **Process control**: speed follows a setpoint from a PLC or a PID loop, for example holding pipe pressure constant.
- **Ramps and braking**: controlled acceleration and deceleration, and braking resistors for high-inertia loads.

## Control and common parameters

A VFD gets **start/stop** and **speed reference** from its keypad, hard-wired terminals (digital inputs and a 0 to 10 V or 4 to 20 mA analog input) or a network (EtherNet/IP, PROFINET, Modbus). Common parameters:

- Motor nameplate data: voltage, current, frequency, rpm, power.
- **Minimum and maximum frequency**.
- **Acceleration and deceleration times**.
- **Current limit** and motor overload settings.
- Stop mode: ramp to stop or coast.

**Worked example.** A conveyor should run between 20 and 60 Hz, with speed set by a 4 to 20 mA signal. At 12 mA the reference is halfway, so the drive runs at 20 + 0.5 × (60 − 20) = 40 Hz. On a 4-pole motor, synchronous speed is 120 × 40 / 4 = 1,200 rpm.

## Faults you will see

- **Overcurrent**: acceleration too fast, a jammed load, or a short in the motor cable.
- **Overvoltage on the DC bus**: deceleration too fast with a high-inertia load (the motor acts as a generator). Lengthen decel time or add a braking resistor.
- **Undervoltage**: supply dips.
- **Overtemperature**: blocked cooling fans or dirty heat sinks.
- **Ground fault**: damaged motor insulation or cable.

Drives keep a **fault log**: read it before resetting. Repeated resets hide a real problem.

## Side effects

The fast PWM switching causes **electrical noise**, voltage spikes on long motor cables and **shaft currents** that can damage bearings (the fluting you saw in the bearings lesson). Fixes include proper shielded cable, grounding, output reactors and shaft grounding rings. Motors on VFDs should be inverter-duty rated. Self-cooled motors at low speed also cool poorly, because their fans slow down too.

## Common mistakes

- Setting a very short deceleration time and getting overvoltage trips.
- Running a standard motor slowly at full torque for long periods, so it overheats.
- Putting a contactor between the drive and motor and opening it while running.
- Resetting faults without reading the log.

## Safety

The DC bus capacitors stay charged after the drive is switched off. Drives carry a label stating how long to wait (often several minutes) before opening the cover, and the bus voltage must be verified as zero with a properly rated meter before touching anything. A drive can also restart a motor automatically after a fault reset or power return. Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Synchronous rpm = 120 × Hz / poles; a VFD varies frequency to vary speed.
- Rectifier, DC bus, IGBT inverter with PWM; volts per hertz stays roughly constant.
- Fans and pumps save energy with the cube of speed.
- Read the fault log; respect stored energy in the DC bus.

## Try it

Open the **Do** tab and take the quiz on how VFDs control speed.


Explain how a variable frequency drive turns fixed AC into variable frequency to control an induction motor's speed.

Motor drives (VFDs)

Variable Frequency Drives Explained - VFD Basics IGBT inverter

VFD 101 Basics

Ladder diagrams, start/stop seal-in circuits and interlocks, the sensors and actuators that connect controls to a machine, and variable frequency drives.

Relay Logic

## Why it matters

A **programmable logic controller (PLC)** is an industrial computer built to run machines reliably for years in heat, dust and electrical noise. It replaced walls of relays with software that can be changed in minutes. To troubleshoot or program one you need a clear picture of its hardware, its addressing and, most of all, the **scan cycle**, which explains behavior that otherwise looks strange.

## Hardware

- **Power supply**: usually 24 V DC for the CPU and I/O.
- **CPU (processor)**: stores and runs the program, holds data, and talks to other devices. It has modes such as **RUN**, **PROGRAM** (stopped, outputs off) and sometimes **REMOTE**.
- **Input modules**: take signals from field devices (push buttons, sensors, switches) and convert them to bits the CPU can read. Each input has a status LED.
- **Output modules**: switch field devices (contactors, solenoids, lamps) according to the program. Relay, transistor and triac types exist.
- **Analog modules**: read 4 to 20 mA or 0 to 10 V signals as numbers, and write analog outputs to drives and valves.
- **Communication**: Ethernet ports for programming software, HMIs and networks such as EtherNet/IP, PROFINET or Modbus TCP; remote I/O racks connect over the network.
- **Chassis or rack** (modular PLCs) or everything in one unit (compact PLCs).

Major vendors include Rockwell Automation (Allen-Bradley), Siemens, Mitsubishi, Omron, Schneider Electric and AutomationDirect. OpenPLC is a free, open-source option for learning (https://autonomylogic.com/). Checked on 2026-10-23.

## Addressing

Every input and output has an address, and the format depends on the vendor:

- Siemens style: `I0.0` (input byte 0, bit 0), `Q0.1` (output byte 0, bit 1).
- Older Allen-Bradley style: `I:1/0` (input slot 1, bit 0), `O:2/3`.
- Modern Allen-Bradley (Logix) and many others use **tags**: named variables such as `Conveyor_Start_PB` mapped to physical I/O.

Electrical drawings show which wire goes to which terminal and address. Good tag names and comments make a program readable.

## The scan cycle

A PLC does not watch its inputs continuously while running logic. It repeats a fixed loop called the **scan**:

1. **Read inputs**: copy the state of every physical input into memory (the **input image table**).
2. **Execute the program**: solve the logic rung by rung, top to bottom, left to right, using the input image and internal memory. Results go into the **output image table**.
3. **Write outputs**: copy the output image to the physical outputs.
4. **Housekeeping**: communications, diagnostics, then start again.

A scan usually takes a few milliseconds up to a few tens of milliseconds, depending on program size and CPU. A **watchdog timer** faults the CPU if a scan takes too long.

## What the scan explains

- **Inputs are frozen during the scan.** If a button changes halfway through, the program sees the new value on the next scan.
- **Rung order matters.** A bit set on rung 10 is already set when rung 11 runs in the same scan, but rung 9 only sees it next scan.
- **The last write wins.** If two rungs both write the same output coil, the lower rung decides the output. This "double coil" problem is a classic bug.
- **Very short pulses can be missed.** A sensor pulse shorter than one scan may fall between input reads. Fast events need high-speed counter inputs or interrupt routines.

**Worked example.** A program has a 20 ms scan. A photo-eye on a fast line produces a pulse 5 ms long for each part. Some parts will be missed, because the pulse can start and end between two input reads. The fix is a high-speed counter input, a pulse stretcher or a sensor with an off-delay setting, not a faster guess.

## Programming languages

The IEC 61131-3 standard defines PLC languages:

- **Ladder Diagram (LD)**: relay-style rungs, most common in North America and easiest for electricians to troubleshoot.
- **Function Block Diagram (FBD)**: blocks wired together, common in process control.
- **Structured Text (ST)**: a text language similar to Pascal, good for math and data handling.
- **Sequential Function Chart (SFC)**: steps and transitions for sequences.
- **Instruction List (IL)**: an older assembly-like language, now deprecated in the standard.

## Online monitoring and forcing

Programming software can go **online** with a running PLC and show which contacts are true (often highlighted green). This is the main troubleshooting tool. Software can also **force** an input or output on or off regardless of the logic. Forcing can make a machine move unexpectedly, so sites restrict it to authorized people with the area cleared, and forces must be removed and logged.

## Common mistakes

- Writing the same output coil in two places.
- Expecting a mid-scan input change to be seen immediately.
- Leaving forces installed.
- Editing a running program without a backup or a management-of-change process.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- PLC = power supply, CPU, input and output modules, communications.
- Scan: read inputs, solve logic top to bottom, write outputs, housekeeping, repeat.
- Rung order matters; the last write to an output wins; short pulses can be missed.
- Online monitoring is the main tool; forcing is restricted and dangerous.

## Try it

Open the **Do** tab and take the quiz on PLC hardware and the scan cycle.


Explain the parts of a PLC, how inputs and outputs are addressed, and how the scan cycle reads, solves and writes.

PLC hardware and scan cycle

PLC Basics | Programmable Logic Controller

Programable Logic Controller Basics Explained - automation engineering

## Why it matters

In the relay lesson you read circuits made of wires and contacts. In a PLC, the same logic is software: contacts and coils are instructions that read and write bits in memory. Writing it yourself, and predicting what happens scan by scan, is the step from reading programs to changing them safely.

## The three basic instructions

- **XIC, examine if closed** (`-| |-`): true when the referenced bit is 1. On an input, that means the input has voltage.
- **XIO, examine if open** (`-|/|-`): true when the bit is 0.
- **OTE, output energize** (`-( )-`): writes 1 to its bit when the rung to its left is true, and 0 when it is false.

Siemens calls these NO contact, NC contact and coil; the idea is the same. Series instructions are **AND**, parallel branches are **OR**, and an XIO is **NOT**.

## The key mental shift: field contact vs instruction

In a relay circuit the STOP button is a normally closed contact. In a PLC, the NC stop button is wired to an **input**, so that input is **on (1) when the button is not pressed**. In the program you therefore use **XIC** on the stop input, not XIO.

```text
Field device           Input bit when at rest    Instruction to use for "OK to run"
NC STOP button         1                         XIC (examine if closed)
NC overload contact    1                         XIC
NO START button        0                         XIC (true while pressed)
```

This trips up beginners more than anything else: the instruction is about the **bit**, not the button.

## Seal-in in ladder logic

```text
      STOP_PB   START_PB   OL_OK          MOTOR
 1 ---] [----+---] [----+---] [--------------( )---
             |          |
             +---] [----+
                MOTOR
```

As Boolean logic:

```text
MOTOR = (START_PB OR MOTOR) AND STOP_PB AND OL_OK
```

The MOTOR bit in the branch is the PLC's version of the auxiliary seal-in contact. On the scan where START is pressed, MOTOR becomes 1. On the next scan, START is 0, but the MOTOR branch is true, so MOTOR stays 1. When STOP is pressed, STOP_PB becomes 0, the rung goes false and MOTOR is written 0.

## Adding an interlock

For forward and reverse, each rung gets an XIO of the opposite output:

```text
FWD = (FWD_PB OR FWD) AND STOP_PB AND OL_OK AND NOT REV
REV = (REV_PB OR REV) AND STOP_PB AND OL_OK AND NOT FWD
```

Because of the scan, FWD is solved first. If both buttons are pressed on the same scan from a stop, FWD becomes 1, and then the REV rung sees FWD already 1 and stays 0. Rung order decides the tie, which is one more reason to think in scans. Real reversing starters keep a hard-wired electrical and mechanical interlock as well, because software alone must not be trusted to prevent a phase-to-phase short.

## Worked example: tracing scans

Start with the motor stopped. The table shows inputs and outputs at the end of each scan.

```text
Scan  STOP_PB  FWD_PB  REV_PB  OL_OK  ->  FWD  REV
1     1        1       0       1          1    0     forward pressed
2     1        0       0       1          1    0     seal-in holds
3     1        0       1       1          1    0     interlock blocks reverse
4     0        0       0       1          0    0     stop pressed
5     1        0       1       1          0    1     reverse starts
```

## Latch and unlatch

Many PLCs also have **OTL/OTU** (set/reset) instructions that latch a bit on until another rung unlatches it. Latched bits survive a power cycle on some platforms, which can make a machine restart unexpectedly. For motor control, the seal-in rung is preferred because it drops out on power loss, just like a relay circuit.

## Practise in a simulator

Free tools let you practise without hardware: OpenPLC Editor includes a simulator (https://autonomylogic.com/), and many vendors offer free or trial software with simulation. The Do tab has a small in-browser simulation of the forward/reverse rungs. Checked on 2026-10-23.

## Common mistakes

- Using XIO for an NC stop button, so the motor runs only while STOP is pressed.
- Forgetting the seal-in branch, so the motor stops when START is released.
- Writing the same output in two rungs.
- Relying on a software interlock alone where a hard-wired one is required.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- XIC is true when the bit is 1, XIO when it is 0; OTE writes the rung result.
- An NC stop button gives a 1 at rest, so use XIC on it.
- Seal-in: OUTPUT = (START OR OUTPUT) AND STOP AND OL.
- Interlock: AND NOT the opposite output; keep hard-wired interlocks too.

## Try it

Open the **Do** tab and write the forward rung of a reversing starter.


Write PLC ladder logic for a seal-in with an interlock and simulate it scan by scan.

Ladder logic programming

Ladder Logic for Absolute Beginner

PLC Programming Tutorial - Ladder Logic #1: Binary & Boolean Operations

## Why it matters

Almost every machine needs to wait or count: run a horn for 3 seconds before a conveyor starts, hold a clamp for 2 seconds, count 12 parts into a box, stop after 500 cycles for a tool change. Timers and counters are the PLC instructions that do this, and their accumulated values are among the first things you check when a sequence stalls.

## Timer basics

A PLC timer has:

- a **preset (PRE)**: the target time, for example 3,000 ms;
- an **accumulator (ACC)**: the time counted so far;
- status bits: **EN** (enabled, the rung is true), **TT** (timer timing) and **DN** (done).

Timers count in the background, scan by scan: each scan the PLC adds the time since the last scan to ACC. That is why timer accuracy is limited by the scan time.

### TON: on-delay

While the rung is true, ACC counts up. When ACC reaches PRE, **DN turns on**. If the rung goes false at any time, ACC **resets to 0** and DN turns off.

```text
Rung true:   ____|‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾|____
TON DN:      __________________|‾‾‾‾‾‾‾|____
                  <--- PRE --->
```

Uses: a start-up warning horn, debouncing a sensor that flickers, an alarm only if low pressure lasts more than 5 seconds.

### TOF: off-delay

DN turns on as soon as the rung is true and stays on for PRE **after** the rung goes false. Uses: keep a cooling fan running for 2 minutes after a motor stops.

### RTO: retentive timer

Like TON, but ACC **holds its value** when the rung goes false. It needs a separate **reset (RES)** instruction. Uses: total run hours on a pump before a PM is due.

## Counter basics

A counter has a preset, an accumulator and a done bit too, but it counts **events**, not time.

- **CTU (count up)**: adds 1 each time its rung goes from false to true (a **rising edge**). Holding the input on does not keep counting.
- **CTD (count down)**: subtracts 1 on each rising edge.
- **RES**: resets the accumulator to 0.

DN turns on when ACC is at or above PRE. Counters usually keep counting past the preset, so the logic must decide what happens next.

## Worked example 1: warning horn

When START is pressed, a horn sounds for 3 seconds, then the conveyor runs.

```text
1  START_REQ seal-in (from START, STOP)               -> START_REQ
2  START_REQ ---------------------------------------- TON T1 PRE 3000
3  START_REQ AND NOT T1.DN -------------------------- -> HORN
4  START_REQ AND T1.DN ------------------------------ -> CONVEYOR
```

Pressing STOP drops START_REQ, which resets T1 and stops everything.

## Worked example 2: counting into a case

A photo-eye produces one pulse per bottle. After 6 bottles the conveyor stops so the operator can swap the case; 2 seconds later the counter resets and the conveyor restarts.

```text
1  PHOTO_EYE ---------------------------------------- CTU C1 PRE 6
   T2.DN ---------------------------------------------- RES C1
2  C1.DN ---------------------------------------------- TON T2 PRE 2000
3  NOT C1.DN ------------------------------------------ -> CONVEYOR
```

Trace it: each bottle adds 1 to C1.ACC. On the 6th, C1.DN turns on, rung 3 stops the conveyor and rung 2 starts T2. When T2.DN turns on, C1 resets; C1.DN goes off, which disables T2 (resetting it) and restarts the conveyor. This is the station you will simulate in the Do tab.

## Timing and the scan

In the simulation, each scan adds a fixed time step, which is how a real PLC behaves: if the scan is 10 ms, ACC goes up in roughly 10 ms steps. A 2,000 ms timer in a 10 ms scan finishes on the 200th scan after it is enabled. If a scan happened to take 25 ms near the end, ACC would still stop at the preset rather than overshoot, because the timer **clamps** ACC at PRE.

## Troubleshooting with timers and counters

When a sequence stops, go online and look at the timer and counter values:

- A TON with ACC stuck at 0 means its rung is false: find the missing condition.
- A TON that keeps resetting near the end means a flickering input: consider a debounce timer.
- A counter that counts double may be seeing a bouncing sensor, or two rungs may be counting the same event.
- A counter that never reaches preset may be missing fast pulses (see the scan cycle lesson).

## Common mistakes

- Expecting a TON to remember its time when the rung goes false (that is an RTO).
- Forgetting to reset a counter or retentive timer.
- Using a TON where a TOF is needed, or the reverse.
- Units: some platforms use milliseconds, others a time base of 0.01 s or 1 s. Check before typing 3000.

## Safety

Timers must never be the only protection on a safety function such as a guard door: those use safety-rated devices, covered later in this course. Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- TON: DN on after PRE of continuous true; resets when false. TOF: delay after false. RTO: keeps ACC until RES.
- CTU counts rising edges; DN at ACC ≥ PRE; RES clears it.
- ACC goes up in scan-time steps and clamps at PRE.

## Try it

Open the **Do** tab and finish the TON instruction that drives the case-packing station.


Use on-delay timers (TON) and up-counters (CTU) in PLC logic, and simulate a counting station with a timed delay.

Timers and counters

PLC Timer Programming for Beginners

PLC Counter Programming for Beginners

PLC hardware, I/O and the scan cycle, then ladder logic programming with seal-ins, interlocks, timers and counters in browser simulations.

PLC Programming

## Why it matters

Operators rarely look at ladder logic. They run machines from touch screens at the machine and from control rooms that watch whole plants, water systems or pipelines. Technicians need to know how those screens get their data, how alarms should behave, and why these systems are a target for cyber attacks.

## HMI: human-machine interface

An **HMI** is the operator's screen for a machine or a process area, typically an industrial touch panel or a PC.

- It **reads** PLC tags to show status: motor running, tank level, temperatures, counts, alarms.
- It **writes** tags when the operator acts: start and stop buttons, setpoints, recipe selection, mode changes.
- It shows **alarms** and their history.
- It often has **user levels**: an operator can start a line, a technician can change a setpoint, an engineer can change configuration.

An HMI button is a request, not a command. Pressing START on the screen sets a bit; the PLC logic still decides whether the motor may run, using all its interlocks. That is why the PLC, not the HMI, is the right place for permissives and interlocks, and why an HMI button must never be the only emergency stop: hard-wired E-stops are required.

## SCADA: supervisory control and data acquisition

**SCADA** systems supervise many controllers spread over a large area: a water utility with pump stations miles apart, an electric grid, a pipeline, or a large factory.

Typical parts:

- **Field controllers**: PLCs and **RTUs** (remote terminal units) at each site.
- **Communications**: plant Ethernet, fiber, radio, cellular or leased lines, using protocols such as Modbus, DNP3, EtherNet/IP, PROFINET or OPC UA.
- **SCADA servers** that poll the field devices, store data and raise alarms.
- A **historian** that stores time-stamped values for trends and reports.
- **Operator workstations** in a control room.

The rule of thumb: the HMI controls one machine or area up close; SCADA supervises many, collects data and lets a small team manage a large system. A related term, **DCS** (distributed control system), is used in continuous process plants such as refineries, where control and supervision come from one vendor's integrated system.

## The automation pyramid

Plants often describe their systems in levels (the ISA-95 model):

```text
Level 4  Business systems (ERP: orders, inventory)
Level 3  Manufacturing operations (MES: scheduling, quality, genealogy)
Level 2  Supervision (HMI, SCADA, historian)
Level 1  Control (PLCs, drives, RTUs)
Level 0  Field devices (sensors, valves, motors)
```

Data flows up for reporting; orders and setpoints flow down.

## Good screen design

Modern guidance (ISA-101) favors **high-performance HMI** design:

- Mostly gray screens, so color is reserved for abnormal conditions and alarms stand out.
- Trends and analog indicators with normal ranges, not just numbers.
- A clear hierarchy: plant overview, area, unit, detail.
- Consistent navigation and button placement.

## Alarm management

An alarm should demand an operator action. Bad alarm systems flood operators with hundreds of nuisance alarms, so real ones get missed, a factor in several major industrial accidents. The ISA-18.2 standard describes the alarm life cycle. Good practice:

- Each alarm has a defined cause, consequence and response.
- Priorities (for example, high, medium, low) used sparingly.
- **Deadbands and delays** to stop chattering alarms (a short TON in the PLC often does this).
- Regular review of the most frequent alarms.

## Worked example: following a value

A tank level shows 72 percent on the HMI. Where does it come from? A level transmitter (Level 0) sends 15.5 mA to an analog input (Level 1). The PLC scales it: (15.5 − 4) / 16 × 100 ≈ 72 percent and stores it in a tag. The HMI (Level 2) polls that tag over Ethernet and displays it; the historian logs it every few seconds for trends. If the HMI shows a stale value with a communication error, the problem is the network or HMI, not the transmitter.

## OT security

Industrial control systems were designed for isolated networks and many old protocols have no authentication. Today they are connected for remote access and data, so security matters:

- **Segment** networks: keep control networks separate from office IT, with firewalls and a demilitarized zone between them.
- Control **remote access** tightly, with multi-factor authentication and no shared accounts.
- **Patch** carefully and keep backups of PLC programs and HMI projects.
- Never plug unknown USB drives into HMIs or engineering laptops.

The ISA/IEC 62443 series is the main OT security standard family (https://www.isa.org/standards-and-publications/isa-standards/isa-iec-62443-series-of-standards), and CISA publishes free ICS guidance and advisories (https://www.cisa.gov/topics/industrial-control-systems). Checked on 2026-10-23.

## Common mistakes

- Putting interlocks only in the HMI.
- Using an HMI button as the emergency stop.
- Bright, colorful screens where alarms do not stand out.
- Leaving default passwords on HMIs and PLCs.

## Recap

- HMI: one machine or area, reads and writes PLC tags; the PLC still decides.
- SCADA: supervises many sites, with RTUs, communications, servers and a historian.
- Good HMIs are calm; good alarms demand an action.
- Segment, control access, back up: OT security is part of the job.

## Try it

Open the **Do** tab and take the quiz on HMI and SCADA roles.


Explain what HMIs and SCADA systems do, how they connect to PLCs, and why alarm design and OT security matter.

HMI and SCADA

What is the difference between SCADA and HMI?

What is SCADA?

Draw the input signal path as a chain of boxes: target box → photo-eye PE-12 → cable → terminal block → input module point I:3/5 → tag PE12_Box_Present → program rung.

Mark each box as OK, NOT OK or UNKNOWN using the observations. Where does the signal stop?

Explain in one sentence why the PLC program and the CPU are very unlikely to be the cause.

List the possible causes between the sensor LED and the module LED (for example, broken output wire, loose terminal, wrong sensor type: PNP vs NPN, blown field fuse for that sensor group).

Ask: was PE-12 replaced recently? Write how a PNP/NPN mismatch would produce exactly these symptoms.

Write the order of checks a qualified technician would make, starting with ones that need no tools (look at the terminal, compare part numbers, check the drawing), then voltage checks under the site's procedure.

Optional: in a PLC simulator (for example OpenPLC Editor) or on paper, add a 2-second TON alarm rung that raises an HMI alarm when the pusher is waiting and PE12_Box_Present stays false.

Write the work order closing note: symptom, cause found (pick the most likely), fix, and how it was tested.

## Why it matters

When an automated line stops, everyone looks at the technician. Guessing and swapping parts wastes hours and money. A systematic method finds the fault quickly, and the PLC gives you tools a relay panel never had: status LEDs on every point, online monitoring and diagnostic buffers. Most faults turn out to be in the field devices and wiring, not the PLC.

## A systematic method

1. **Gather information.** What exactly happened? What does the HMI alarm say? When did it start? What changed recently (a new part, a program change, a cleaning shift)?
2. **Observe the machine** safely from outside the guards. Which step of the sequence is it stuck on? Which actuators moved?
3. **Check the PLC status.** Is the CPU in RUN with no fault lights? Are I/O modules healthy? Check the diagnostic buffer or fault log.
4. **Go online with the program** and find the output that should be on but is not. Trace the rung back to the condition that is false.
5. **Half-split the signal path**: decide whether the fault is on the input side, in the logic, or on the output side, then keep dividing.
6. **Fix the root cause**, test, and **document** it in the work order.

## Inputs: is the signal reaching the PLC?

The path for an input runs: field device → wiring → terminal block → input module → input image bit → program.

Checks, from least to most invasive:

- Does the **sensor's own LED** come on when the target is present? If not, the sensor, its target, its alignment or its power is the problem.
- Does the **input module LED** for that point come on? If the sensor LED is on but the module LED is off, the fault is in the wiring between them (or a PNP/NPN mismatch).
- Does the **bit in the program** go true when the module LED is on? If not, check the address or tag mapping, or whether the module is faulted.

## Outputs: is the PLC command reaching the device?

The path for an output runs: program → output image bit → output module → fuse → wiring → interposing relay or contactor → field device.

- Is the **output bit true** in the program? If not, the problem is a missing condition in the logic: trace the rung.
- Is the **output module LED** on? If the bit is true but the LED is off, suspect a faulted module or a forced-off point.
- Is the **output fuse** intact and is there **field power** to the module?
- Does the **interposing relay or contactor** pull in? Does the **device** (solenoid, motor) respond? A coil can be open; a valve can be stuck mechanically.

## Half-splitting

Rather than checking every point in order, test in the **middle** of the path. If the signal is good there, the fault is downstream; if not, it is upstream. Each test halves the search. On a long chain of series contacts or a long cable run, this saves a lot of time.

**Worked example.** A cylinder will not extend. The HMI shows the machine waiting at step 4, "extend pusher". Online, the output bit `Pusher_Extend_Sol` is true. The output module LED is on. That splits the problem: logic and module are fine, so the fault is in the field side. The valve's solenoid LED connector is dark, so voltage is not arriving at the valve. Under the site's lockout and test procedure, a qualified technician checks the field wiring and finds a cable crushed in a cable track. Fix, test, record. Total time: 15 minutes instead of an hour spent swapping the valve and the output card.

## Using the program as a troubleshooting tool

- **Cross-reference** a tag to see every rung that reads or writes it.
- **Trends** in the HMI or programming software show when a signal flickers.
- **Sequence step numbers** tell you where the machine is waiting; the transition condition for the next step tells you what it is waiting for.
- Compare the running program with the **backup** to see whether anyone changed it.

## What not to do

- Do not **force** I/O to "see what happens" on a real machine. Forcing overrides the logic and safety interlocks in the program, and can move equipment with people nearby. Only authorized people may force, with the area cleared, and all forces must be removed afterwards.
- Do not jumper out sensors or safety devices to get production running.
- Do not download program changes without authorization and a backup.
- Do not replace the PLC first: CPUs and modules are rarely the cause.

## Practise safely

You can practise these skills without a live machine: paper exercises with I/O tables like the one in the Do tab, PLC simulators such as OpenPLC Editor's simulation mode (https://autonomylogic.com/), and the ladder logic simulations earlier in this course. Checked on 2026-10-23.

## Safety

Voltage testing in control panels is energized electrical work: it needs the right meter, PPE, an energized work justification under NFPA 70E, and employer training and authorization. Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Gather information, observe, check PLC status, go online, half-split, fix the root cause, document.
- Inputs: sensor LED → module LED → program bit.
- Outputs: program bit → module LED → fuse and field power → relay or device.
- Never force or jumper on a live machine without authorization.

## Try it

Open the **Do** tab and isolate an I/O fault on paper.


Isolate an input or output fault on an automated machine with a systematic, half-splitting method.

Troubleshooting automated systems

PLC Troubleshooting 101:   Basic Tips and Tricks

Troubleshooting a PLC Input

## Why it matters

Automated machines move fast and do not notice people. Amputations and crush injuries from unguarded or bypassed machinery are among the most common serious injuries in manufacturing. Controls technicians design, install, test and maintain the safety systems that prevent them, and they are often asked to bypass them. Knowing how they work, and why they must not be defeated, is part of the job.

## Risk assessment first

Machine safety starts with a **risk assessment** (ISO 12100 describes the method): identify the hazards in every task, including setup, cleaning and maintenance, estimate the risk, and reduce it using the same hierarchy as in workplace safety:

1. **Inherently safe design**: remove the hazard (lower force, rounded edges, no pinch point).
2. **Safeguarding**: guards and protective devices.
3. **Information and training**: warnings, procedures, PPE.

Standards such as ISO 13849-1 then set how reliable each safety function must be, expressed as a **performance level** (PLa to PLe) or, under IEC 62061, a **SIL**. In the US, OSHA requires machine guarding (29 CFR 1910.212) and NFPA 79 covers electrical equipment of industrial machinery (https://www.nfpa.org/codes-and-standards/nfpa-79-standard-development/79). Checked on 2026-10-23.

## Guards

- **Fixed guards**: bolted or welded, need a tool to remove. Best where access is rarely needed.
- **Interlocked guards**: doors or covers with a safety switch; opening the guard stops the hazardous motion, and the machine cannot start while it is open. **Guard locking** keeps the door locked until motion has stopped, for machines that coast.
- **Adjustable and self-adjusting guards**: common on saws and similar tools.
- Guards must prevent reaching around, under, over or through to the hazard. Openings and distances are set by standards tables.

## Protective devices

- **Light curtains**: rows of infrared beams; breaking any beam stops the machine. They must be mounted at a **safety distance** that allows the machine to stop before a hand reaches the hazard:

```text
safety distance = hand speed × total stopping time + intrusion allowance
```

With a typical hand speed of 63 in/s (1.6 m/s) and a stopping time of 0.25 s, the distance is 15.75 in plus the allowance for the curtain's resolution. A machine whose brakes wear and stop more slowly makes that distance unsafe, which is why stopping time is tested.

- **Safety mats and laser scanners** detect presence in an area.
- **Two-hand controls** keep both hands on buttons during the hazardous motion.
- **Enabling switches** (three-position) for teaching robots.

## Emergency stops

An **E-stop** is a red mushroom-head button on a yellow background that stops hazardous motion. Key points:

- It is a **complementary measure**, not a substitute for guarding.
- It must latch in until deliberately reset, and resetting it must not restart the machine by itself.
- **Stop categories** (NFPA 79 / IEC 60204-1): **Category 0** removes power immediately (uncontrolled stop); **Category 1** brakes under power, then removes it (controlled stop); **Category 2** is a controlled stop with power kept on, not allowed for E-stops.
- Every E-stop must be hard-wired through safety-rated devices, never only through an HMI button or ordinary PLC logic.

## Safety relays and safety PLCs

An ordinary relay can weld its contacts closed and nobody would know. A **safety relay** (safety module) is designed so a single fault does not cause loss of the safety function:

- **Dual-channel** inputs: the E-stop or guard switch has two contacts wired separately; the relay checks they agree.
- **Redundant, monitored outputs**: two force-guided contactors in series with the motor, and their NC auxiliary contacts fed back to the relay (the **feedback loop** or EDM).
- **Cross-fault detection**: a short between the channels is detected.
- A **monitored manual reset**: the machine only resets when the button is pressed and released and everything checks out.

Larger machines use **safety PLCs** (safety controllers), which run certified safety logic separately from the standard program.

## Safety functions vs lockout

Guards, light curtains and E-stops protect people during **normal production**. They do **not** isolate energy. For maintenance, repair, clearing jams inside the machine or servicing, workers use **lockout/tagout** (OSHA 29 CFR 1910.147): isolate every energy source, lock it, release stored energy and verify zero energy. An E-stop is not a lockout device. See https://www.osha.gov/control-hazardous-energy and https://www.osha.gov/machine-guarding. Checked on 2026-10-23.

## Worked example: a guard door circuit

A packaging machine has an interlocked door with a coded safety switch (two channels) wired to a safety relay. The relay drives two contactors in series feeding the motor, with their NC auxiliary contacts in the relay's feedback loop. One contactor welds shut. When the door opens, the second contactor still stops the motor (redundancy). At the next reset, the relay sees the welded contactor's feedback contact open and **refuses to reset** (monitoring). The fault is found before the machine can run again with only one working channel.

## Never defeat a safety device

Taped-down interlock switches, light curtains muted with jumpers and guards left off are common causes of serious injuries. If a safety device stops production too often, the fix is a design change through the proper process, not a bypass.

## Safety

Control panels contain voltages that can kill, stored energy in drives and capacitors, and machines that can start unexpectedly. Everything in this course is practised on paper or in simulators. Working in real panels requires employer training, supervision by a qualified person, lockout/tagout and the arc-flash practices in NFPA 70E (https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e), and for electricians a registered apprenticeship and state or local licence. Checked on 2026-10-23.

## Recap

- Risk assessment, then design out hazards, then guard, then inform.
- Guards: fixed, interlocked (with locking if motion coasts), adjustable.
- Light curtain safety distance depends on stopping time.
- E-stops: latching, hard-wired, Category 0 or 1, complementary to guarding.
- Safety relays: dual channel, redundant monitored outputs, feedback loop.
- Lockout, not E-stop, for maintenance.

## Try it

Open the **Do** tab and take the quiz on guarding and safety relays.


Explain machine guarding, emergency stops, safety relays and safety-rated devices, and how they differ from lockout.

Machine safety

Machine Guard Safety Training from SafetyVideos.com

Understanding Industrial Safety Circuits | Safety Relays, E-Stops, and Best Practices Explained

## Why it matters

Preventive maintenance changes parts on a schedule whether they need it or not. **Predictive maintenance (PdM)** measures the machine's condition and acts when the data shows a problem developing. With sensors wired to PLCs and historians, controls technicians increasingly set up, read and act on this data. Catching a failing bearing weeks early turns a breakdown into a planned 2-hour job.

## The P-F curve

Most failures do not happen suddenly. A defect starts at some point, and the machine's condition then declines:

```text
Condition
  |‾‾‾‾‾‾‾‾‾‾‾‾\
  |             P   (potential failure: first detectable)
  |               \
  |                 \   vibration rises, then noise, then heat
  |                    \
  |                       F  (functional failure)
  +---------------------------------> Time
```

The **P-F interval** is the time between when you could first detect a problem and when it fails. Vibration analysis usually detects bearing defects earliest, then oil analysis, then audible noise, then heat. Inspections must be more frequent than the P-F interval, or you will miss the window.

## Vibration basics

Vibration sensors (accelerometers) are mounted on bearing housings. Readings are taken on routes with a portable data collector, or continuously with wired or wireless sensors feeding a PLC or monitoring system.

- **Overall level**: a single number summarizing the vibration, usually velocity in mm/s or in/s **RMS**. Simple and good for trending.
- **RMS (root mean square)**: square every sample, average them, take the square root. For a pure sine wave, RMS = peak / √2 ≈ 0.707 × peak.
- **Crest factor** = peak / RMS. About 1.41 for a smooth sine; higher values point to impacts, such as early bearing defects.
- **Spectrum (FFT)**: breaks the signal into frequencies, which is how analysts identify the cause: **1× running speed** usually means imbalance; **2×** often misalignment or looseness; bearing defect frequencies are non-whole multiples of speed.

**Worked example.** A velocity signal is a sine wave with a 4.2 mm/s peak. RMS = 4.2 / √2 ≈ 2.97 mm/s. Using example limits of 2.8 (alert) and 7.1 (danger), the reading is in **alert**. Severity limits depend on machine size, mounting and type; ISO 20816 (which replaced ISO 10816) gives guidance, and many sites set their own limits from baselines.

## Trends beat single readings

A single reading of 2.9 mm/s may be fine for one machine and alarming for another. What matters most is the **change** from that machine's own baseline. Weekly readings of 1.8, 1.9, 1.9, 2.1, 2.4, 2.9, 3.6 mm/s show an accelerating rise: plan a bearing inspection now. A least-squares slope gives the average rate of change, and a rising slope is a strong signal even below the alert limit.

## Thermal data

Infrared thermography finds heat that should not be there:

- **Electrical**: loose or corroded connections, overloaded or unbalanced phases, failing breakers.
- **Mechanical**: hot bearings, misaligned couplings, rubbing belts, blocked cooling.

Compare temperatures with **similar components under similar load** (phase to phase, bearing to bearing) and with **ambient**. **Worked example.** A motor's terminal connections read L1 41 °C, L2 43 °C, L3 68 °C. L3 is about 27 °C hotter than the coolest phase, a strong sign of a loose or corroded connection on that phase. Sites use severity tables (for example from NETA standards or their insurer) to decide how urgently to repair.

Thermal scans of electrical equipment are usually taken with covers open on energized gear, or through IR windows. That is energized work requiring a qualified person, PPE and an energized work procedure under NFPA 70E. Do not open panels to practise.

## Other PdM techniques

- **Oil analysis**: wear metals, water, viscosity and particle counts.
- **Ultrasound**: compressed air leaks, electrical discharge, bearing lubrication condition.
- **Motor circuit analysis and current signature**: winding and rotor problems.
- **Process data** from the PLC: rising motor current, longer cycle times, pump pressure falling over weeks.

## Reliability metrics

From the maintenance lesson: **MTBF** = operating time / failures, **MTTR** = repair time / repairs. **Availability** ≈ MTBF / (MTBF + MTTR). With MTBF 500 h and MTTR 4 h, availability ≈ 500 / 504 = 99.2 percent. PdM aims to raise MTBF and turn unplanned repairs into short planned ones.

## Common mistakes

- Reacting to one reading without a baseline.
- Comparing vibration in different units (peak vs RMS, mm/s vs in/s) as if they were the same.
- Taking readings in different places on the housing each time, so the trend is noise.
- Collecting data that nobody reviews or acts on.

## Beyond the platform

Vibration analysts commonly certify through ISO 18436-2 based programs, for example via the Vibration Institute (https://www.vibinst.org/); reliability professionals often pursue SMRP's CMRP or CMRT (https://smrp.org/). Thermographers certify through similar programs. Checked on 2026-10-23.

## Recap

- Act inside the P-F interval: measure more often than failures develop.
- RMS = √(mean of squares); sine RMS ≈ 0.707 × peak; crest factor = peak / RMS.
- Trends and baselines matter more than single readings.
- Thermal: compare similar components and ambient; energized scans need qualified people.

## Try it

Open the **Do** tab and finish a condition monitoring calculator by writing the RMS formula.


Read vibration and thermal data to spot developing faults, and calculate RMS, crest factor and trends.

Reliability and predictive maintenance

Predictive Maintenance Explained

Vibration Analysis for beginners 1 (Predictive Maintenance and vibration explanation. How it works?)

HMI and SCADA, systematic troubleshooting of automated machines, machine safety and safety relays, and reliability with vibration and thermal data.

Industrial Controls & PLCs

About this course

Course content

Relay Logic

PLC Programming

Automation Systems

Where this course leads

Careers that use it