ITI Technician Mechatronics (AITT Theory) common mistakes
ITI Technician Mechatronics (AITT Theory) · Understand the traps and how to avoid them.
When reading a drawing, the candidate treats a dimension as measured from the nearest visible edge rather than from the stated datum, and adds or subtracts chain dimensions to 'reconstruct' a missing size instead of using the baseline dimensions given.
Trainees learn dimensioning as 'put the number on the line' and rarely practise datum-referenced (baseline) versus chain dimensioning. Under time pressure the visually nearest feature feels like the natural origin, and missing dimensions tempt arithmetic guesses rather than recognition that a datum-based value is already supplied elsewhere on the sheet.
What to do instead
Before answering, identify the datum symbol and the surfaces/axes it references, then trace each dimension to its own origin. Classify the scheme as chain, baseline, or combined, and note that in chain dimensioning tolerance accumulates while baseline does not. Practise redrawing small parts with an explicit datum and re-dimensioning them both ways so the distinction becomes automatic.
The candidate computes a shaft/hole fit by comparing only nominal sizes, or by subtracting diameters and calling the result the clearance, without computing maximum and minimum clearance from the actual limit deviations.
Nominal-size thinking is reinforced by everyday work where '20 mm shaft in a 20 mm hole' is described loosely. Candidates also confuse the sign convention of upper and lower deviation (especially for holes, where lower deviation is often zero) and forget that a transition fit can be either clearance or interference depending on the actual sizes produced.
What to do instead
Adopt a fixed tabular routine: write hole limits (upper and lower), shaft limits, then max clearance = hole max − shaft min and min clearance = hole min − shaft max; a negative minimum clearance means interference. Drill on H7/g6, H7/k6, H7/p6 and similar pairings until the sign logic is reflexive, and always state the fit type only after both extremes are computed.
On a pneumatic or hydraulic circuit question, the candidate reads the valve symbol by its box shape or by the actuator it feeds rather than by the arrow and port positions, and therefore misidentifies the centre condition of a directional control valve or the number of positions.
ISO 1219 symbols are learned as pictures rather than as a positional grammar: number of boxes equals number of positions, arrows show flow paths in the actuated state, and the centre box governs the de-energised condition. Candidates also forget that the symbol shows the unactuated state and that a spring return is drawn on one side only.
What to do instead
Apply a fixed decoding sequence: count the boxes (positions), count the ports per box, locate the actuating symbols (solenoid, pilot, lever, spring), then read the centre box for the neutral condition. Practise describing 5/2, 5/3 closed-centre and 4/2 valves aloud from symbol to behaviour, and sketch the corresponding circuit before selecting an answer.
The candidate calculates total resistance or current in a mixed series-parallel network by adding all resistor values, or by applying the product-over-sum rule to branches that are not actually in parallel.
Candidates pattern-match on the layout of the drawing rather than on node connectivity, and the two-terminal rule for parallel branches is often memorised as a formula without the underlying condition that both ends share the same nodes. Bridge-like or re-drawn circuits defeat visual grouping.
What to do instead
Redraw the circuit with nodes labelled A, B, C and mark which components share the same pair of nodes; only those are parallel. Reduce the network in explicit stages, writing the equivalent value at each stage, and check plausibility: an equivalent resistance must lie between the largest single resistor and its value divided by the number of parallel branches.
When selecting or wiring a sensor, the candidate ignores the output type (PNP vs NPN, sourcing vs sinking, analogue 4–20 mA vs 0–10 V) and the load it must drive, then answers a wiring or interfacing question as if any sensor could connect directly to any PLC input.
Training rigs often use one sensor family throughout, so the sourcing/sinking distinction never becomes a decision point. Candidates also conflate the sensor's supply requirement with its signal convention, and forget that a 4–20 mA loop is powered in series while a 0–10 V signal is measured in parallel with a high-impedance input.
What to do instead
For every sensor question, first write down: supply voltage, output configuration, signal range, and load. Then check compatibility with the PLC input card (sourcing input needs a sinking sensor, and vice versa) and confirm the input's impedance suits the signal. Memorise that 4–20 mA gives live-zero fault detection while 0–10 V does not, and practise converting a 4–20 mA reading to a percentage.
The candidate answers PLC questions by tracing the ladder rung left to right as if it were a wired circuit, confusing normally-open/normally-closed instruction state with the physical contact state, and mis-predicting the output when a latch, timer, or internal memory bit is involved.
Ladder logic visually mimics relay circuits, which invites reading it as wiring rather than as a repeated scan of Boolean expressions. Candidates also forget scan order, that a timer's done bit is evaluated on the next scan, and that a physically closed contact may be wired to an instruction shown as normally open.
What to do instead
Evaluate rungs as Boolean equations with explicit bit states, and step through at least two complete scans when timers, counters or latches appear. Keep a table of bit states per scan. For field-device questions, separate the physical contact condition from the programmed instruction state and state both explicitly before concluding.
The candidate sizes or specifies a motor or drive from the motor's mechanical output power alone, ignoring efficiency, starting current, duty cycle and the driven load's torque-speed characteristic, and so picks an undersized contactor, overload setting or drive rating.
Nameplate power is the most visible number, and candidates treat it as the input power drawn from the supply. Starting current multipliers, service factor, and the difference between breakaway torque and running torque are covered once and rarely revisited, so they are not applied under exam pressure.
What to do instead
Use a fixed checklist: mechanical output power, efficiency to get electrical input, full-load and starting current, starting method (DOL, star-delta, soft starter, VFD), duty and ambient derating. Set overloads from the actual full-load current, not the nominal power, and confirm the drive's current rating exceeds the motor's full-load current continuously.
The candidate treats a robot's degrees of freedom, work envelope, and end-effector position as interchangeable concepts, and answers kinematics or automation-layout questions by counting joints rather than by identifying independent motions and reach.
Degrees of freedom are often taught as 'one per joint', which fails for parallel mechanisms and ignores that some joints are constrained or redundant. Work envelope and payload are learned as catalogue figures rather than as consequences of link geometry and axis limits.
What to do instead
Define degrees of freedom as the number of independent parameters needed to specify pose, and check it against joint count only for simple serial arms. For envelope questions, sketch the reach at axis extremes rather than recalling a number. Practise classifying common industrial configurations (articulated, SCARA, delta, Cartesian) by their axes, typical payload and envelope shape.
In fault diagnosis, the candidate jumps to the component they suspect and proposes replacement, rather than following a structured sequence of observation, verification against documentation, isolation by test, and confirmation of root cause.
Workshop habits reward speed and part-swapping, and candidates transfer that to written questions. They also skip the cheap first checks (supply present, emergency stop latched, guard interlock open, fuse intact) and misread a symptom as a cause, such as blaming a sensor when the wiring or the PLC input card is faulty.
What to do instead
Use a repeatable loop: gather symptoms, compare with normal operation and documentation, form a hypothesis, test it with the least invasive measurement, then confirm the fault is cleared. Always state the first three safety and supply checks before component-level diagnosis, and distinguish symptom, fault, and root cause explicitly in the answer.
The candidate answers safety and lockout questions by describing the general principle ('isolate the machine') without the specific sequence: identify all energy sources including stored pneumatic, hydraulic and gravitational energy, isolate, lock and tag, verify zero energy, then restore in a controlled order.
Safety content is studied as attitudes rather than as procedures, so the answer sounds correct but omits verifiable steps. Stored energy in accumulators, suspended loads and capacitors is easy to overlook because the electrical supply is treated as the only source.
What to do instead
Memorise and rehearse the isolation sequence as numbered steps, and for each machine type list every energy source before answering. Include verification of zero energy by test, dissipation or blocking of stored energy, and a controlled re-energisation with checks that guards and personnel are clear. Practise on a pneumatic press and a VFD-driven conveyor to cover both stored fluid and stored electrical energy.