ITI Technician Mechatronics (AITT Theory) key terms

A safety procedure in which an energy-isolating device is switched off, locked and tagged so machinery cannot be re-energised while a person works on it. Each worker applies a personal lock and holds the only key.

The condition in which every stored or residual energy source in a machine - electrical, pneumatic, hydraulic, gravitational, thermal or spring - has been released, restrained or verified absent before maintenance begins.

A written authorisation issued before high-risk maintenance, stating the task, the isolation applied, the precautions required and the period of validity. It is signed by the issuer and the receiving technician.

The ranked order of risk-reduction measures: eliminate the hazard, substitute it, engineer controls, then administrative controls, with personal protective equipment last. Higher-ranked controls are more reliable because they do not depend on human behaviour.

A structured examination of a task that identifies hazards, estimates the likelihood and severity of harm, and records the controls needed to reduce risk to an acceptable level before work starts.

Wearable or handheld protection such as safety glasses, gloves, hearing protection and arc-rated clothing that reduces exposure to a hazard. It protects only the wearer and is the last line of defence.

A standardised document supplied with a hazardous chemical that lists its hazards, safe handling and storage, first-aid measures, spill response and disposal requirements. Technicians consult it before using an unfamiliar product.

Separating incompatible wastes - for example used oils, solvents, batteries and electronic scrap - into labelled containers so they cannot react, contaminate each other or enter the general waste stream.

A sudden release of intense radiant heat and pressure caused when an electric arc forms across an air gap. It can ignite clothing and cause severe burns, so live work requires arc-rated protection and an assessed boundary.

A drawing method that shows a three-dimensional object as separate two-dimensional views, each projecting the object's features onto a plane at right angles to the viewing direction, with no perspective distortion.

A projection convention in which the object sits between the observer and the projection plane, so the view seen from the front is placed below the top view and the left-hand view appears on the right of the front view.

A drawing view in which material is imagined cut away by a cutting plane to reveal internal features. The exposed cut surface is shown with hatching, and hidden detail lines are normally omitted for clarity.

A symbolic language on drawings that specifies permitted variation in form, orientation, location and runout using feature control frames, datums and tolerance zones, rather than coordinate limits alone.

A theoretically exact reference plane, axis or point on a part from which GD&T tolerances are measured. Datum features are identified on the drawing with labelled triangular symbols and establish consistent inspection alignment.

The total permissible variation in a dimension, equal to the difference between the upper and lower limits. It defines how much a manufactured feature may deviate while the part still functions and assembles.

The relationship between the limits of a shaft and a hole that assemble together, classified as clearance, interference or transition according to whether the shaft is always smaller, always larger or may be either.

The positive gap between the mating surfaces of a shaft and a bore, equal to hole size minus shaft size. Clearance allows relative motion and is essential for sliding or rotating joints.

An assembly in which the shaft is larger than the hole, so parts must be pressed or shrunk together. The resulting elastic deformation transmits torque and resists relative movement without a key.

A precision measuring instrument using a calibrated screw and thimble to measure external dimensions, typically to 0.01 mm. The reading combines the barrel scale with the thimble graduation and must be zeroed before use.

A measuring tool with a fixed main scale and a sliding vernier scale that reads internal, external and depth dimensions. The vernier graduation that aligns with a main-scale mark gives the fractional reading.

Comparing an instrument's readings against a traceable reference standard and recording any deviation, so measurement uncertainty is known. Instruments are recalibrated at defined intervals or after damage or repair.

The relationship stating that current through a resistive element equals the voltage across it divided by its resistance (I = V/R). It underpins circuit analysis and fault-finding by voltage and resistance measurement.

Two basic connection arrangements. In series, the same current flows through each element and voltages add; in parallel, the same voltage appears across each branch and currents add. Combined circuits mix both.

A semiconductor device that conducts current in one direction only, from anode to cathode. It is used for rectification, freewheeling across inductive loads and reverse-polarity protection in control circuits.

A circuit of one or more diodes that converts alternating current into pulsating direct current. A bridge rectifier uses four diodes to use both halves of the AC waveform.

An electromagnetically operated switch for switching motor and other power circuits, with main power contacts and auxiliary contacts for control logic. It is designed for frequent operation and is held closed by its coil.

A protective device that senses sustained overcurrent in a motor circuit, typically through thermal or electronic sensing, and opens the control circuit to stop the contactor before the motor overheats.

An electronic power converter that varies the frequency and voltage supplied to an AC induction motor, controlling its speed and torque. It also provides soft starting, braking options and programmable protection.

An AC motor in which a rotating magnetic field induced by stator windings induces currents in the rotor, producing torque. The rotor turns slightly slower than the field, and that slip determines torque.

A switching technique that varies the proportion of on-time in a fixed-frequency signal to control average output voltage or current. It is used in drives, servo amplifiers and proportional valve controllers.

The ratio of the number of teeth on a driven gear to that on the driving gear, which sets the change in speed and torque. A larger ratio reduces output speed and multiplies torque.

Lost motion between mating mechanical elements caused by clearance between teeth, threads or splines. Excessive backlash introduces positioning error and shock loading when direction reverses.

An intentional axial or radial load applied to a bearing assembly to remove internal clearance, increasing stiffness and reducing vibration. Excessive preload, however, raises friction and shortens bearing life.

A pneumatic or hydraulic valve that routes fluid flow between ports to start, stop or reverse an actuator. It is described by the number of ports and positions, such as a 5/2 valve.

A valve that regulates the rate of fluid flow to or from an actuator, thereby setting its speed. Meter-in and meter-out arrangements control speed differently, especially under overrunning loads.

Trapped fluid pressure remaining in a circuit after isolation because of check valves, accumulators or closed centre valves. It must be released and verified at a gauge before any component is opened.

A device that converts one form of energy into another, such as converting pressure, force or temperature into an electrical signal suitable for a controller input. Sensors are transducers used for detection.

An interface circuit that samples a continuous voltage from a sensor and converts it into a discrete binary number for the PLC. Its resolution and sampling rate limit how faithfully the signal is represented.

An industrial computer that reads inputs, executes a stored user program cyclically and updates outputs to control machinery. It is built for harsh environments and deterministic, repeatable scan behaviour.

The repeating sequence in which a PLC reads all inputs, executes the program, writes all outputs and performs housekeeping. Cycle time affects how quickly the controller responds to a change.

A PLC programming language drawn as rungs between two rails, with contacts and coils resembling a relay circuit. It is widely used because it mirrors the hard-wired control diagrams technicians already read.

An articulated industrial manipulator with six independently driven joints, giving six degrees of freedom so a tool can reach a position and orientation within its working envelope.

A handheld control unit used to jog a robot, record positions and step through programs. It usually includes an enabling switch and emergency stop that must be held or pressed for safe operation.

How closely a robot or actuator returns to the same commanded position over repeated cycles, expressed as a spread. It differs from accuracy, which is closeness to the true target position.

An optical sensor that detects objects by a transmitted light beam, operating in through-beam, retro-reflective or diffuse mode. Output changes when the beam is interrupted or reflected back to the receiver.

A non-contact sensor that detects a nearby target, using an inductive field for metal or a capacitive field for most materials. It gives a discrete switching output at a set sensing distance.

The average operating time between successive failures of a repairable system, calculated from recorded failure history. It guides spare-parts policy and preventive maintenance intervals rather than predicting a single unit's life.

Scheduled servicing and inspection carried out before failure occurs, based on time or usage, to preserve reliability. Tasks include lubrication, cleaning, tightening, calibration checks and wear-part replacement.

A structured investigation, using tools such as the five-whys or fishbone diagram, that traces a failure back to its underlying cause rather than the visible symptom, so recurrence is prevented.

A simple root-cause technique in which the question 'why' is asked repeatedly, each answer forming the basis of the next question, until the fundamental cause of a problem is exposed.