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

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.