20-Bio-B7 Ergonomics · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2016 — 04-Bio-B7, 3 hours, closed book (one aid sheet allowed, written on both sides; approved Casio/Sharp calculator only). Six questions are printed; the first five as they appear in the answer book constitute a complete exam paper (all six are answered here as a complete study resource). Each question is of equal value; some require an essay-format answer.
Reference texts: M. P. Groover, R. Weiss, R. N. Nagel & N. G. Odrey, Industrial Robotics: Technology, Programming, and Applications (2nd ed. — robot configurations, end-effectors/grippers, machine vision, sensors and transducers); M. P. Groover, Automation, Production Systems, and Computer-Integrated Manufacturing (5th ed. — Geneva mechanisms/dial indexing, PLC ladder logic, production-rate and line-efficiency analysis).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(a) Variables (Groover's convention: X = input, Y = output, C = internal control relay).
| Symbol | Type | Meaning |
|---|---|---|
| X1 | Input | Start pushbutton (momentary, normally open) |
| X2 | Input | Bottom limit switch (closed when the drill reaches preset depth) |
| X3 | Input | Top limit switch (closed when the drill is fully retracted — i.e. already closed at rest) |
| C1 | Internal relay | Cycle running |
| C2 | Internal relay | Depth reached (remembers that X2 was made, until the next start) |
| Y1 | Output | Spindle motor |
| Y2 | Output | Coolant pump/valve |
| Y3 | Output | Feed-down solenoid (advances the drill) |
| Y4 | Output | Retract solenoid (withdraws the drill) |
| Y5 | Output | “Part done” indicator light |
(b) Ladder logic diagram. Each scan the PLC solves every rung from the present state of its inputs, so the sequence needs memory: seal-in contacts hold C1 (cycle running) and C2 (depth reached) between scans. The top limit switch cannot be used on its own as the stop condition, because the drill rests on it — X3 is already closed when the start button is pressed — so it is qualified by C2 (“top reached after depth was reached”).
Operation, rung by rung. The worker clamps the stock and presses X1. In rung 1 the normally closed X1 contact clears C2, which turns off the done light left over from the last part. In rung 2, X1 energizes C1, and C1 seals itself in through its own parallel contact; the second branch (NOT C2 or NOT X3) is closed because C2 is now off. Rungs 3 and 4 run the spindle (Y1) and coolant (Y2) whenever C1 is on, and rung 5 (C1 and NOT C2) energizes the feed-down solenoid Y3. When the drill reaches the preset depth the bottom limit switch X2 closes: rung 1 energizes C2 and seals it in, so rung 5 drops the feed and rung 6 (C1 and C2) energizes the retract solenoid Y4. X2 opens again as the drill rises, but C2 holds. When the drill reaches the top, X3 closes; in rung 2 both normally closed contacts (C2 and X3) are now open, so C1 drops out, and the spindle, coolant and retract all stop on the same scan. Rung 7 (C2 and NOT C1) then lights Y5, which stays on while the worker unloads the part, until X1 is pressed for the next part. At power-up C1 and C2 are both off, so nothing runs and the light is off even though X3 is closed. In operation, feed and retract never overlap, the spindle and coolant are on whenever the drill moves, and the light comes on only after a completed cycle.
