Module 4.2-4.3 Printed circuit boards and servomechanisms

Knowledge levels

Item	B1 / B3	B2 / B2L
4.2 Printed circuit boards	1	2
4.3(a) Servo principles: open/closed loop, feedback, follow-up, null, overshoot, damping, deadband, hunting, proximity switches, transducers, synchros, tachometers, encoders	1	2
4.3(b) Synchro components, servomechanism and PID controller construction and use, fault-finding	—	2

4.2 Printed circuit boards

• A PCB is an insulating board (usually glass-fibre reinforced epoxy, e.g. FR-4) carrying copper tracks that connect the components, replacing point-to-point wiring.
• Types: single-sided, double-sided (tracks both sides, joined by plated-through holes/vias), multilayer (several copper layers laminated inside).
• Component mounting: through-hole (leads through holes, soldered on the opposite side) and surface-mount technology (SMT) (components soldered to pads on the surface; smaller, allows both sides to be populated).
• Protective conformal coating guards against moisture, contamination and vibration; solder mask prevents solder bridging.
• Advantages: compact, light, repeatable, reliable, easy to replace as a line replaceable unit (LRU/shop replaceable unit).

Maintenance and handling

• PCBs often carry ESD-sensitive devices: handle by the edges, use wrist straps and conductive bags (Module 5.12).
• Typical defects: cracked tracks, lifted pads, dry/cracked solder joints, burnt components, corrosion, delamination.
• Repairs are done in approved workshops according to the component maintenance manual; line staff normally replace the whole unit.

4.3 Servomechanisms - principles

• A servomechanism is an automatic control system that makes an output (position, speed) follow a command input, using power amplification and feedback.
• Open-loop system: no feedback; the output is not measured (e.g. a simple timer-controlled motor). Errors are not corrected.
• Closed-loop system: the output is measured and compared with the input; the difference (error signal) drives the system until the error is zero.

Term	Meaning
Feedback	signal representing the actual output, returned to the comparator
Follow-up	feedback signal that "follows" the output movement to cancel the command
Null	the condition of zero error - the system stops
Overshoot	output moves past the commanded position
Damping	reduces overshoot and oscillation (viscous, velocity/rate feedback)
Deadband (dead zone)	small range of error to which the system does not respond - prevents constant hunting
Hunting	continuous oscillation about the null position (too little damping or too small a deadband)

Key fact: Under-damped systems overshoot and may hunt; over-damped systems are sluggish; critically damped systems reach the commanded position fastest without overshoot.

Transducers and sensors

• Analogue transducers: potentiometers (position), LVDT/RVDT (linear/rotary variable differential transformers), strain gauges, pressure transducers.
• Proximity switches: inductive or magnetic sensors that detect a metal target without contact - landing gear, door and flap positions.
• Inductance and capacitance transmitters: position or quantity measured by changing inductance (moving core) or capacitance (fuel probes).
• Digital tachometers and encoders: produce pulses whose frequency or code represents speed or position (optical or magnetic shaft encoders; absolute encoders give a unique code for each position, incremental encoders count pulses).