Fixed (hard) automation is most suitable for:

very high production volume of a single product design

a small batch of many different products

products whose design changes every week

The order of operations in one PLC scan cycle is:

read inputs, execute the program, update outputs

execute the program, read inputs, update outputs

update outputs, execute the program, read inputs

read and update each input and output as the program reaches it

A motor is controlled by the ladder rung Motor = (Start + Motor) · Stop, where Start is a normally-open push-button and Stop a normally-closed push-button that reads true until pressed. After Start is pressed and released, the motor:

keeps running until Stop is pressed

stops as soon as Start is released

runs only while Stop is pressed

toggles each time Start is pressed

An advantage of RFID over bar codes for automatic identification is that RFID:

does not need a line of sight between the tag and the reader

In CNC part programming, the preparatory word G02 commands:

circular interpolation, counter-clockwise

linear interpolation at the feed rate

In CNC programming, G91 specifies that the coordinates which follow are:

incremental, measured from the current position

absolute, measured from the program zero

in inches rather than millimetres

Which statements about concurrent design and manufacturing planning are true?

In concurrent engineering, product design and process planning overlap in time

Design for assembly aims to reduce the number of parts

Generative CAPP synthesises a process plan from the part’s features and decision logic

In concurrent engineering, product design and process planning overlap in time

Design for assembly aims to reduce the number of parts

Concurrent engineering typically lengthens the time to market

Generative CAPP synthesises a process plan from the part’s features and decision logic

Which statements about positioning systems are true?

An open-loop stepper-driven axis has no measurement of the actual table position

A closed-loop axis uses an encoder or similar feedback device

Contouring requires the simultaneous, coordinated control of more than one axis

Increasing the gear reduction between motor and lead screw makes the basic length unit larger

An open-loop stepper-driven axis has no measurement of the actual table position

A closed-loop axis uses an encoder or similar feedback device

Contouring requires the simultaneous, coordinated control of more than one axis

Computer Integrated Manufacturing: PLCs, Material Handling, AS/RS, Automatic Identification, CNC and Positioning Systems

The second paragraph of Section A.3 is Computer Integrated Manufacturing: automation in manufacturing; programmable logic controllers; automated material handling systems; automated storage and retrieval systems; automated identification, detection and capture systems; computer numerical control; the basics of single- and multi-axis positioning systems; and the basics of concurrent design and manufacturing planning for automation. This is the factory a robot works in. The chapter treats each as GATE asks it — the kind of automation a volume and variety call for, a PLC’s scan and ladder logic, a storage machine’s cycle time, the resolution of a lead-screw axis — and closes with the planning that decides what gets automated.

1. Automation in manufacturing

Three kinds of automation
Kind	Suits	Examples
Fixed (hard)	Very high volume, one design	Transfer lines, dedicated assembly machines
Programmable	Medium volume in batches, changeover between batches	CNC machines, robots reprogrammed per batch
Flexible	Medium volume, a mix of parts with no lost time between them	Flexible manufacturing systems, cells

A system’s output is judged by its cycle time T_c and production rate R_p = 1/T_c, and by its availability, the fraction of time it can run: A = MTBF/(MTBF + MTTR) — 90/(90 + 10) = 0.9 for a machine that fails every 90 hours on average and takes 10 to repair. Automation pays where volume is high enough to spread its cost, where the work is dangerous or monotonous, or where consistency matters more than flexibility; it is justified on these, not on labour alone.

2. Programmable logic controllers

A PLC is an industrial computer built for the shop floor: a CPU with memory, input modules that condition field signals (switches, sensors) and output modules that drive loads (relays, valves, contactors), a power supply, and a programming device. It runs a repeating scan cycle: read all inputs into an input image, execute the program on that image, update all outputs, then communications and self-checks. Because an input can change just after it has been read, the worst-case response to an input change is about two scan times (plus the module delays) — 20 ms for a 10 ms scan.

The usual language is ladder logic, a relay diagram drawn between two rails: normally-open (examine-if-closed) and normally-closed (examine-if-open) contacts in series for AND and in parallel for OR, driving output coils. The seal-in (latch) circuit is the basic pattern: Motor = (Start + Motor) · Stop, where Start is a momentary push-button and Stop a normally-closed one that reads true until pressed — releasing Start leaves the motor running through its own contact, pressing Stop drops it. Interlocks put one coil’s normally-closed contact in the other’s rung, so forward and reverse can never be on together. An on-delay timer (TON) turns its output on only after its input has been continuously true for the preset time, and resets if the input drops; counters (CTU) count input transitions to a preset. IEC 61131-3 defines ladder diagram, function block diagram, structured text, instruction list and sequential function chart.

3. Material handling, AS/RS, and automatic identification and detection

Automated material handling moves parts between operations without an operator: conveyors (belt, roller, chain, overhead) for fixed high-volume routes; automated guided vehicles for variable routes, guided by embedded wire, magnetic tape, or laser and natural-feature navigation; cranes and hoists for heavy loads; and industrial robots for transfer at a station. An automated storage and retrieval system is a high-rise rack served by a computer-controlled storage/retrieval (S/R) machine — unit-load systems for pallets, mini-load systems for bins, carousels that bring the bin to the picker. Because the S/R machine moves horizontally and vertically at the same time, its travel time to a bin is the larger of the two axis times, not their sum.

Worked: a bin 30 m along the aisle and 10 m up, with horizontal speed 2 m/s and vertical speed 0.5 m/s, takes max(15 s, 20 s) = 20 s each way. A single-command cycle (store one load, or retrieve one) is out and back plus a pick-up and a deposit: with 10 s each, 40 + 20 = 60 s, 60 cycles an hour. A dual-command cycle stores one load and retrieves another in one trip, which is why it raises throughput.

Automatic identification and data capture reads an item’s identity without keying it in: bar codes (linear, or 2D such as QR and Data Matrix, read optically with line of sight); RFID, read by radio with no line of sight needed, many tags at once, passive tags powered by the reader’s field and active ones by a battery; machine vision and optical character recognition; magnetic stripes. Detection sensors tell a controller that something is there: mechanical limit switches; inductive proximity sensors, which detect metal only; capacitive proximity sensors, which also detect non-metals; and photoelectric sensors in through-beam, retro-reflective and diffuse forms.

4. Computer numerical control

CNC runs a machine tool from a stored part program on a dedicated computer; DNC connects many machines to a central computer. The axes follow the right-hand rule, with Z along the spindle and positive away from the work. Coordinates are absolute (G90), from the program zero, or incremental (G91), from the current point. The motion words are G00 rapid positioning, G01 linear interpolation at a feed rate, G02 circular interpolation clockwise and G03 counter-clockwise; among the miscellaneous words, M03 starts the spindle clockwise, M05 stops it and M30 ends the program. Point-to-point machines (drilling) need only positions; contouring machines (milling, turning a profile) need the axes interpolated together.

5. Single- and multi-axis positioning, and concurrent design

A single-axis positioning system is typically a motor driving a table through a lead screw (ball screw) of pitch p, often through a gear reduction r_g. Open-loop, a stepper of n_s steps per revolution moves the table one basic length unit per pulse, BLU = p/(n_s r_g): a 5 mm pitch, 200 steps and 2:1 reduction give 0.0125 mm. Table speed is BLU × pulse rate — at 400 pulses/s, 5 mm/s or 300 mm/min — and the motor turns at 400/200 = 2 rev/s, 120 rpm. Closed-loop, a servo motor with an encoder (on the motor or directly on the table) measures what the axis actually did and corrects it. More reduction makes the BLU smaller and the axis slower.

Multi-axis systems stack such axes (x–y tables, gantries, machine tools, robots) and either position them one after another (point-to-point) or coordinate them (linear and circular interpolation) for contouring. Their precision is described by control resolution (the BLU or the encoder’s step), repeatability and accuracy, as for robots.

Concurrent engineering develops the product and its manufacturing process together, with design, manufacturing, quality and suppliers working in parallel rather than passing the design "over the wall" in sequence; it shortens time to market and cuts late changes. Design for manufacture and assembly reduces part count, uses standard parts, designs parts to be self-locating and symmetric, and avoids fasteners where snap fits will do — and a part easy for a person to assemble is usually easy for a robot. Manufacturing planning for automation includes process planning — variant CAPP, which retrieves and edits the plan of a similar part family (group technology), and generative CAPP, which synthesises a plan from the part’s features and decision logic — and scheduling the resulting operations.

Key takeaways

Fixed automation for high volume and one design, programmable for batches, flexible for a mix without changeover; A = MTBF/(MTBF + MTTR).
A PLC scans: read inputs, execute, write outputs; worst-case response is about two scans; seal-in is (Start + M) · Stop; TON resets when its input drops.
An S/R machine moves both axes at once, so travel time is the larger of the two; RFID needs no line of sight; inductive proximity sensors see metal only.
G90 absolute, G91 incremental, G00 rapid, G01 linear, G02 clockwise and G03 counter-clockwise arcs; Z is the spindle axis.
Open-loop BLU = p/(n_s r_g) and speed = BLU × pulse rate; concurrent engineering overlaps design and process planning; generative CAPP builds plans from features.

Practice questions (15)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Fixed (hard) automation is most suitable for:
1. very high production volume of a single product design
2. a small batch of many different products
3. one-off prototypes
4. products whose design changes every week
Show answer
Answer: A — very high production volume of a single product design
A transfer line or dedicated machine is expensive to build and hard to change, so it pays only when a large, stable volume of one design spreads its cost. Variety and frequent change call for programmable or flexible automation.
A machine has a mean time between failures of 90 hours and a mean time to repair of 10 hours. Its availability is ____.
Numerical answer — type the value.
Show answer
Answer: 0.9
A = MTBF/(MTBF + MTTR) = 90/100 = 0.9: of every 100 hours, 90 are spent running. MTTR/MTBF ≈ 0.11 is the ratio of repair to running time, not the availability.
The order of operations in one PLC scan cycle is:
1. read inputs, execute the program, update outputs
2. execute the program, read inputs, update outputs
3. update outputs, execute the program, read inputs
4. read and update each input and output as the program reaches it
Show answer
Answer: A — read inputs, execute the program, update outputs
The PLC freezes the inputs into an image table, solves the whole program against that consistent image, then writes all outputs at once; communications and diagnostics follow. Reading and writing as the program goes would let one rung see a different input from the next within a scan.
A motor is controlled by the ladder rung Motor = (Start + Motor) · Stop, where Start is a normally-open push-button and Stop a normally-closed push-button that reads true until pressed. After Start is pressed and released, the motor:
1. keeps running until Stop is pressed
2. stops as soon as Start is released
3. runs only while Stop is pressed
4. toggles each time Start is pressed
Show answer
Answer: A — keeps running until Stop is pressed
Once Motor is true, its own contact in parallel with Start keeps the rung true after Start opens — the seal-in. Pressing Stop makes the series term false and the seal drops out. Without the parallel Motor contact the motor would run only while Start is held.
A PLC has a scan time of 10 ms. Neglecting input-filter and output-module delays, the worst-case time between an input changing and the corresponding output changing is ____ ms.
Numerical answer — type the value.
Show answer
Answer: 20
If the input changes just after the input image was read, it is missed for the rest of that scan (almost 10 ms); it is read at the start of the next scan and the output is written at its end (10 ms more): about two scans, 20 ms. One scan is the best case.
An on-delay timer (TON) has a preset of 5 s. Its input turns on at t = 0, turns off at t = 3 s, and turns on again at t = 4 s and stays on. The timer output turns on at t = ____ s.
Numerical answer — type the value.
Show answer
Answer: 9
A TON times only while its input is continuously true and resets when the input drops, so the 3 s accumulated before t = 3 s is lost. Timing restarts at t = 4 s and completes at 4 + 5 = 9 s. A retentive timer would keep the 3 s and finish at 6 s.
An advantage of RFID over bar codes for automatic identification is that RFID:
1. does not need a line of sight between the tag and the reader
2. needs no reader at all
3. can only store a single digit
4. works only on metal objects
Show answer
Answer: A — does not need a line of sight between the tag and the reader
RFID tags are read by radio, so they can be read inside packaging, at odd orientations and many at once; a bar code must be visible to the scanner. RFID still needs a reader, stores far more than one digit, and metal nearby actually degrades it.
An inductive proximity sensor on a conveyor will detect:
1. metallic parts only
2. any part, metallic or not
3. transparent glass bottles only
4. parts by their colour
Show answer
Answer: A — metallic parts only
It senses the eddy currents a conductive target induces in its oscillating field, so non-metals are invisible to it. Capacitive sensors detect non-metals, and photoelectric sensors can be set up for transparent objects or colour marks.
The S/R machine of an AS/RS moves horizontally at 2 m/s and vertically at 0.5 m/s, both at the same time. A storage location is 30 m along the aisle and 10 m above the pick-and-deposit station, and each pick-up or deposit takes 10 s. The single-command cycle time for this location is ____ s.
Numerical answer — type the value.
Show answer
Answer: 60
Horizontal travel takes 30/2 = 15 s and vertical 10/0.5 = 20 s; moving together, each one-way trip takes the larger, 20 s. Out and back is 40 s, and one pick-up plus one deposit adds 20 s: 60 s. Adding the axis times (35 s each way) gives 90 s.
In CNC part programming, the preparatory word G02 commands:
1. circular interpolation, clockwise
2. circular interpolation, counter-clockwise
3. rapid positioning
4. linear interpolation at the feed rate
Show answer
Answer: A — circular interpolation, clockwise
G02 is a clockwise arc and G03 a counter-clockwise one; G00 is rapid traverse and G01 a straight cut at the programmed feed. Direction is judged looking along the negative of the axis normal to the plane of the arc.
In CNC programming, G91 specifies that the coordinates which follow are:
1. incremental, measured from the current position
2. absolute, measured from the program zero
3. in inches rather than millimetres
4. polar coordinates
Show answer
Answer: A — incremental, measured from the current position
G91 selects incremental dimensions and G90 absolute ones. With G90, X50 means the point x = 50; with G91 it means move 50 in x from where the tool is.
An open-loop positioning table is driven by a stepper motor of 200 steps per revolution through a 2:1 speed reduction onto a lead screw of 5 mm pitch. Its basic length unit is ____ mm.
Numerical answer — type the value.
Show answer
Answer: 0.0125
One motor step turns the screw 1/(200 × 2) of a revolution, moving the table 5/400 = 0.0125 mm. Ignoring the gear reduction gives 0.025 mm.
A stepper motor of 200 steps per revolution is driven at 400 pulses per second, one step per pulse, and turns a lead screw through a 2:1 speed reduction. The speed of the motor is ____ rpm.
Numerical answer — type the value.
Show answer
Answer: 120
400 pulses/s ÷ 200 steps per revolution = 2 rev/s = 120 rpm at the motor; the screw turns at 60 rpm and the table moves 400 × 0.0125 = 5 mm/s. Dividing by the gear ratio at the motor gives 60 rpm, which is the screw’s speed.
Which statements about concurrent design and manufacturing planning are true?
1. In concurrent engineering, product design and process planning overlap in time
2. Design for assembly aims to reduce the number of parts
3. Concurrent engineering typically lengthens the time to market
4. Generative CAPP synthesises a process plan from the part’s features and decision logic
Show answer
Answer: A — In concurrent engineering, product design and process planning overlap in time; B — Design for assembly aims to reduce the number of parts; D — Generative CAPP synthesises a process plan from the part’s features and decision logic
(A) The defining feature, against the sequential "over the wall" handover. (B) Fewer parts means fewer operations, fixtures and assembly motions. (C) False: finding manufacturing problems during design shortens development. (D) Variant CAPP edits a stored family plan; generative CAPP builds a new one.
Which statements about positioning systems are true?
1. Increasing the gear reduction between motor and lead screw makes the basic length unit larger
2. An open-loop stepper-driven axis has no measurement of the actual table position
3. A closed-loop axis uses an encoder or similar feedback device
4. Contouring requires the simultaneous, coordinated control of more than one axis
Show answer
Answer: B — An open-loop stepper-driven axis has no measurement of the actual table position; C — A closed-loop axis uses an encoder or similar feedback device; D — Contouring requires the simultaneous, coordinated control of more than one axis
(A) False: BLU = p/(n_s r_g), so more reduction makes it smaller — finer, and slower. (B) The controller assumes each pulse moved the table one BLU. (C) The feedback closes the loop. (D) A curve or slanted line needs the axes interpolated together.