Sensor Fundamentals and Safe Work Practices

What Is a Sensor?

A sensor is a device that detects a physical condition—position, temperature, pressure, light, or flow—and converts that condition into an electrical signal that a controller such as a PLC can read. Sensors are the eyes and ears of automated machinery. Selecting the right sensor and installing it safely are equally important skills for a mechatronics technician.

Sensor Taxonomy: Four Classification Axes

1. Discrete vs. Analog Output

Discrete (digital) sensors produce an ON/OFF output—either a voltage is present or it is not. A proximity switch that turns ON when a metal part arrives at a conveyor station is a discrete sensor. PLC input cards read these as a logic 1 or 0.

Analog sensors produce a continuously variable output proportional to the measured quantity. A pressure transducer that outputs 4–20 mA across a 0–100 psi range is an analog sensor. The PLC analog input card converts that current to a numeric value the program can use.

2. Active vs. Passive

Active sensors require an external power supply to operate. Inductive proximity sensors, photoelectric sensors, and ultrasonic sensors are all active; they typically need 10–30 V DC.

Passive sensors generate their own signal from the measured phenomenon without needing supply power. A thermocouple generates a small millivolt signal from the temperature difference at its junction. A simple reed switch mechanically closes a circuit using only the magnetic field acting on it.

3. Contact vs. Non-Contact

Contact sensors must physically touch the target to operate. Mechanical limit switches and tactile bumper switches are contact types. They are rugged but subject to mechanical wear.

Non-contact sensors detect targets without touching them, which reduces wear and allows detection of fragile or fast-moving parts. Inductive, capacitive, photoelectric, and ultrasonic sensors all operate without physical contact.

4. Common Sensor Operating Principles

| Sensor Type | Operating Principle | Typical Target |

|---|---|---|

| Inductive proximity | Eddy-current damping of an oscillating electromagnetic field | Metal only |

| Capacitive proximity | Change in capacitance of an oscillating electric field | Metal, plastic, liquid, bulk material |

| Photoelectric | Light beam interrupted or reflected | Most opaque/reflective objects |

| Ultrasonic | Echo time of a sound pulse | Most objects; works in some dusty environments |

| Mechanical limit switch | Physical actuation of a plunger or roller lever | Solid targets |

| Thermocouple / RTD | Seebeck effect (TC) or resistance change (RTD) | Temperature |

| Pressure transducer | Deflection of a diaphragm sensed by a strain gauge | Liquids, gases |

Safe Work Practices: The Three Standards You Must Follow

Before touching any sensor wiring or installation on industrial equipment, you must address three sets of requirements. Violating any one of them has caused serious injuries and fatalities.

OSHA 29 CFR 1910.147 — Control of Hazardous Energy (Lockout/Tagout)

The energy-control program requires these steps every time:

1. Notify affected employees that equipment will be shut down.

2. Identify all energy sources (electrical, pneumatic, hydraulic, gravity).

3. Shut down the equipment using the normal stopping procedure.

4. Isolate each energy source using its disconnect, valve, or block.

5. Apply lockout/tagout devices—each authorized employee applies their own lock.

6. Release or restrain stored energy: bleed pneumatic lines, block suspended loads, discharge capacitors.

7. Verify zero energy before work begins. Use a calibrated meter to confirm voltage is absent; try the start button to confirm the machine will not restart.

8. Restore in reverse order when work is complete, ensuring all workers are clear.

Sensor bypass—jumping out a sensor signal so a machine runs without it—eliminates a safety interlock. This practice violates 1910.147 and has caused fatalities when machines cycled unexpectedly.

NFPA 70E — Electrical Safety in the Workplace

NFPA 70E governs shock and arc-flash hazards. Key points for sensor work:

• Shock boundaries: the Limited Approach Boundary and Restricted Approach Boundary define how close an unqualified or qualified worker may come to exposed energized parts. Know your qualification level before approaching.
• Arc-flash boundary: the distance at which an arc-flash incident energy equals 1.2 cal/cm². Anyone inside this boundary must wear arc-rated PPE.
• Energized work permit: if work must be performed on energized circuits, a written permit and qualified supervision are required. For most sensor wiring tasks on 24 V DC control circuits the arc-flash hazard is low, but the policy of your facility and an incident-energy analysis determine PPE requirements.
• Verify absence of voltage with a properly rated and tested meter—never assume a circuit is de-energized.
• Minimum PPE for low-voltage sensor wiring tasks typically includes safety glasses, insulated gloves rated for the voltage, and no loose jewelry or clothing.

OSHA 1910.212 — Machine Guarding

Sensors are often mounted near pinch points, rotating parts, and moving tooling. Machine guarding requirements state that guards must be in place before the machine is restarted. Never restore power with a guard removed to "just check" a sensor. If a guard must be removed during troubleshooting, LOTO must be applied first, and the guard must be reinstalled and confirmed secure before LOTO is removed.

Why All Three Standards Work Together

LOTO (1910.147) removes the energy before you touch wiring. NFPA 70E tells you what PPE to wear and when energized work requires a permit. Machine guarding (1910.212) ensures mechanical hazards are controlled before the machine runs again. Skipping any one standard leaves a gap that can result in injury.