5.5 Discrete Devices, Motor Control, Motion Control & Mechatronics

Discrete Devices, Motor Control, Motion Control & Mechatronics

Process plants are dominated by analog loops, but most automated facilities also contain machines: conveyors, packaging lines, palletizers, robots and indexing tables. The CAP outline reflects this. Domain 1 lists machine control theories and mechatronics (electro-mechanical), Domain 2 lists drive systems and machine control among the basic control elements, and Domain 3 lists motor and drive selection and sizing and selecting and sizing input/output signal devices. ISA's exam-topic list adds discrete I/O devices, general manufacturing measurements, motor and drive control, and motion control.

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Discrete Sensing Devices

Sinking, Sourcing and Two-Wire Devices

Three-wire DC sensors switch either the positive supply (PNP, sourcing) or the 0 V return (NPN, sinking). A PNP sensor must feed a sinking input module, and an NPN sensor needs a sourcing input module; a mismatch means the input never turns on. Two-wire sensors draw a small leakage current even when "off," so the input module's off-state current threshold must be checked or the input may never turn off.

Encoders

An incremental encoder produces A and B pulse trains 90° apart (quadrature) plus an index pulse once per revolution. Direction comes from which channel leads, and ×4 decoding counts every edge, so a 1,000 pulse-per-revolution encoder gives 4,000 counts per revolution. On a 5 mm-lead ball screw, that is $\frac{5\text{ mm}}{4000} = 0.00125\text{ mm}$ per count. An incremental encoder loses its position at power-off and must be homed; an absolute encoder reports a unique position code (single-turn or multi-turn) and does not.

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Motor Starting, Protection and Sizing

• Across-the-line (direct-on-line) starters use a contactor and an overload relay. Starting current is several times full-load current, commonly around six to eight times.
• Wye-delta (star-delta) starting reduces starting current and starting torque to about one third of their across-the-line values, so it suits loads that start lightly.
• Soft starters ramp the voltage with SCRs to limit inrush and mechanical shock; VFDs (Section 5.3) add continuous speed control.
• Overload relays are rated by trip class: Class 10, 20 and 30 relays trip within 10, 20 and 30 seconds at 600% of rated current. Use a higher class only for high-inertia loads that take longer to accelerate.
• Motor selection considers the NEMA design letter (Design B is the general-purpose torque characteristic), service factor (for example, 1.15), enclosure (such as TEFC or explosion-proof), insulation class, duty cycle and efficiency class.

Motor power follows from torque and speed:

$P_{\text{kW}} = \frac{T_{\text{N}\cdot\text{m}} \times n_{\text{rpm}}}{9549}$

In US units, horsepower equals torque in lb-ft times speed in rpm divided by 5,252. A conveyor that needs 150 N·m at 1,450 rpm requires $\frac{150 \times 1450}{9549} \approx 22.8\text{ kW}$, so the next standard size above it (30 kW in the IEC range) would be selected once service and starting margins are checked.

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Stepper and Servo Motion Control

• Stepper motors move in fixed steps (a 1.8° motor has 200 full steps per revolution; microstepping divides each step further). They are positioned open loop by counting steps, which is inexpensive but can lose steps silently under overload unless an encoder is added.
• Servo systems close the loop with encoder or resolver feedback. The drive runs nested loops—current (torque) innermost, then velocity, then position—and, as with any cascade, the inner loops are tuned first.
• Inertia ratio: the load inertia reflected through a gearbox is $J_{\text{reflected}} = \frac{J_{\text{load}}}{N^2}$, where $N$ is the gear ratio. Many vendors recommend keeping the load-to-motor inertia ratio within roughly 10:1 for responsive, stable tuning.
• Motion profiles: a trapezoidal profile accelerates, cruises and decelerates; an S-curve profile limits jerk (the rate of change of acceleration) to reduce vibration and wear. For a symmetric trapezoidal move with equal acceleration and deceleration time $t_a$, the move time is

$t_{\text{move}} = \frac{D}{v_{\max}} + t_a$

so a 500 mm move at 250 mm/s with 0.4 s ramps takes $\frac{500}{250} + 0.4 = 2.4\text{ s}$.

• Electronic gearing and camming: a slave axis follows a master axis at a fixed ratio (gearing) or along a stored position profile (camming), replacing mechanical line shafts and cams. Deterministic motion networks such as EtherCAT, PROFINET IRT (Section 7.2), SERCOS III and CIP Motion carry the synchronized setpoints.

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Machine Safety

Machine safeguarding starts with a risk assessment (ISO 12100). Safety functions are then designed to ISO 13849-1 (Performance Levels PL a to PL e) or IEC 62061 (SIL-based). Common measures include interlocked guards (ISO 14119), safety light curtains (IEC 61496), safe torque off (STO) in drives, and robot safety per ISO 10218. IEC 60204-1 defines stop categories: Category 0 removes power immediately, Category 1 performs a controlled stop and then removes power, and Category 2 stops under control with power maintained.

A light curtain must be mounted far enough from the hazard that the machine stops before a hand can reach it. ISO 13855 gives the minimum safety distance as

$S = (K \times T) + C, \quad C = 8(d - 14)$

where $K$ is the approach speed (2,000 mm/s for distances up to 500 mm), $T$ is the total stopping time of the machine and the curtain in seconds, and $d$ is the curtain's detection capability in millimetres (the formula for $C$ applies to resolutions up to 40 mm). A 14 mm curtain on a machine that stops in 0.20 s needs $S = 2000 \times 0.20 + 0 = 400\text{ mm}$.