A small business buys cooking oil in bulk and bottles it locally for resale. The bulk delivery vehicle pumps oil directly into a main reservoir.

As an intermediate step of the filling process, a smaller, buffer reservoir is used. This is the fill- tank, from which the bottling plant draws.

The main reservoir feeds into the fill-tank, by means of a 5 kW 3-phase 400 V AC pump. This fill pump, controlled by a PLC, can be started either by a manual FILL pushbutton or by a Oil Level Low switch on the fill tank or by a momentary action remote control relay, RL1 that can be activated remotely by radio. The Oil Level Low level switch is fitted at a liquid height of roughly 33% on the fill tank.

Once started, the PLC will continue to run the fill pump, topping up the fill-tank from the reservoir, until a Oil Level High level switch, fitted

at 100% level, is activated, at which time the fill pump will be shut down. (The fill tank allows 110% of its rated volume before spillage occurs.)

On the 24V DC contactor that controls the filling pump, an auxiliary contact, called FILL_ACK, acknowledges that the fill contactor has actually activated. Once the filling pump contactor has actually locked, this fact shall be transmitted by the PLC via Telem1 radio input.

A second 3 kW 3-phase 400 V AC centrifugal pump draws from the fill-tank and pressurises the bottling plant. This pump is not allowed to run if the fill-tank level has dropped below 33% as airlocks may ensue.

The bottling plant is fully self-contained as far as automation is concerned and does not form part of the automation described here, apart from status lines. The status lines consist of a +24V output to the bottling plant indicating that pressurised product is available and a N.C. return dry output from the bottling plant, indicating that an error occurred and feed should be stopped.

The bottling feed pump is started/stopped manually by Bott_START and Bott_STOP pushbuttons or by a momentary-action remote control relay RL2. On the 24V DC contactor that controls the bottling feed pump, an auxiliary contact, called Bott_ACK, acknowledges that the bottling feed pump contactor has actually locked. Once the bottling feed pump contactor has actually locked, it shall be transmitted by the PLC via Telem2.

Master of Engineering (Industrial Automation) 8

Due to limited capacity of the three-phase connection to the local power utility, the two pumps are not allowed to run simultaneously and the fill pump has precedence. You do not need to worry about the motors star-delta starters and switch-on trips as the starters have been replaced with automatic soft-starters already.

Near to the two pumps, a single mushroom-shaped red emergency pushbutton has two sets of Normally Closed contacts fitted. The one contact is used to cut supply feed (+24 V DC) to all contactors in case of emergency, whilst the other contact is monitored by the PLC. Once activated, this emergency pushbutton needs to be manually reset. When an emergency shutdown has occurred, a 400 V 3-phase siren should sound non-resettable for 5 minutes as well as be transmitted via the PLC through Telem3.

+24 V DC is reticulated in common to all sensors, relays, switches, and contactors and the return wires are connected to the PLC. The PLC is wired to 3 telemetry inputs (Telem1 .. 3) on a telemetry transmitter. These are active low inputs on the transmitter.

Various analog and digital I/O Modules are available for this particular PLC in 1, 2, 4 or 8 I/Os per module. For digital modules, you need to choose current sinking or current sourcing digital units or as special-order dry relay modules.

Questions:

a) Does the scenario describe a digital- or analog control? Explain.

b) List and count all inputs to the PLC that you identify from the above text.

c) List and count all outputs from the PLC that you identify from the above text.

d) Fully specify suitable input module(s) for the PLC.

e) Fully specify suitable output module(s) for the PLC.

f) Is the safety arrangement adequate? Explain fully.