STUDY GUIDE · GKU 7
Measurements, instrumentation and controls study guide (Mechanical Engineering)
Every control loop starts with a measurement, so this unit has two halves that depend on each other. The first is about getting a trustworthy number: how sensors turn a physical quantity into a signal, how that signal is amplified, filtered and digitised, and how much the final reading can be trusted once calibration, scatter and the propagation of uncertainty are accounted for. The second is about using that number: modelling a machine as a transfer function, predicting how it responds in time and in frequency, and choosing and tuning a controller (P, PI, PID, lead or lag, or state feedback) so that the closed loop is stable, fast and accurate.
Structured by the official ETEC subject standard for this topic: every listed topic and learning outcome, with formulas, figures and worked examples. The full sections are available to members in the app.
SKU 7.1 · Mechanical measurements
The measurement system, static characteristics and calibration
How an instrument turns a physical quantity into a reading, the vocabulary that describes how good that reading is, and why every instrument must be calibrated.
Sensors and transducers: classification and principles
How sensors are grouped (active or passive, primary or secondary, analogue or digital) and the physical principle behind each common type.
Displacement, motion and dimensional measurement
Potentiometers, LVDTs, capacitive gauges and encoders for displacement; vernier, micrometer, gauge blocks and limit gauges for dimensions; and the loading error of a potentiometer.
Strain gauges, the Wheatstone bridge and load cells
Why a stretched wire changes resistance, how a Wheatstone bridge turns a resistance change into millivolts, and how gauge placement adds signals and cancels temperature.
Pressure, flow, level and speed measurement
The working principle, range and main limitation of the common pressure gauges, flowmeters, level sensors and speed pick-ups.
Temperature measurement: RTDs, thermistors and thermocouples
Resistance thermometers, thermistors and thermocouples: how each works, how to convert its signal to temperature, the thermocouple laws, and why calibration and the reference junction matter.
Dynamic response of instruments: first- and second-order
How fast a sensor follows a changing input: time constant, amplitude ratio and phase lag, and how damping and natural frequency decide what an accelerometer or a thermometer can measure.
Signal conditioning: amplifiers, bridges and filters
Turning a weak, noisy sensor output into a clean voltage a meter or an A/D converter can use: op-amp gain stages, instrumentation amplifiers, common-mode rejection and RC filters.
Circuits, diodes and transistors for instrumentation
The electrical basics behind every sensor circuit: DC and AC circuit laws, impedance and power, semiconductors, the ideal diode and the transistor used as a switch or amplifier.
Digital instrumentation: sampling, A/D and D/A conversion, data acquisition
How a continuous sensor voltage becomes numbers in a computer: quantization and resolution, the sampling theorem and aliasing, converter types, D/A output and the layout of a computer-based data acquisition system.
Errors and statistics of repeated measurements
Gross, systematic and random errors; the mean, standard deviation and standard error of replicated readings; the normal and Student t distributions; and least-squares fitting of a calibration line with its correlation coefficient.
Uncertainty analysis, propagation and sensor selection
How sure a result is: Type A and Type B uncertainties, the root-sum-square propagation rule, combined and expanded uncertainty, significant figures, and using an uncertainty budget to choose instruments.
SKU 7.2 · Control of dynamic systems
Feedback control and block diagrams
Open and closed loops, the parts of a feedback system, what feedback buys you, and how to reduce a block diagram or signal flow graph to one transfer function.
Laplace transforms and transfer functions
How the Laplace transform turns differential equations into algebra, how a transfer function is defined, what poles and zeros mean, and how to get back to time by partial fractions.
Modelling mechanical, electrical, fluid and thermal systems
Build lumped models from inertia, resistance and capacitance elements, turn them into transfer functions, and use the analogies between mechanical, electrical, hydraulic, pneumatic and thermal systems.
Time response of first- and second-order systems
Step responses of first- and second-order systems, the transient specifications (rise time, peak time, overshoot, settling time), pole locations, and how to identify a mechanical system from a measured response.
Steady-state error and system type
How far the output ends up from the reference for step, ramp and parabolic inputs, how the number of integrators (system type) decides it, and how the error constants are used to size a gain or choose a controller.
Stability and the Routh–Hurwitz criterion
What stability means for a linear system, where the poles must lie, and how the Routh array counts right-half-plane roots and gives the range of gain for stability without solving the characteristic equation.
Proportional, integral and derivative control
What each of the three PID actions does to speed, overshoot, offset and stability, how they are combined, and how a loop is tuned.
Root locus
How the closed-loop poles move as the gain rises, the sketching rules, and how to pick the gain that meets a damping specification.
Frequency response, Bode plots and stability margins
Sinusoidal steady state, how to sketch a Bode plot from its factors, and how gain and phase margins measure the distance from instability.
State space, state feedback and compensator design
Writing a model as first-order matrix equations, placing poles with state feedback, and designing lead and lag compensators to a specification.
Mechatronic systems: motors, actuators, robots and simulation
Beyond the handout: how sensors, microcontrollers and electric motors combine into mechatronic products, with DC and stepper motor sums and simple robot kinematics.
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