ME 360 Test #3 – Spring ’00 5/6/00

Closed book, closed notes portion of test.

1. Mark the following true (T) or false (F) questions

______ a) Pressure relief valves are used to limit pneumatic system pressure to a preset value.

______ b) Photoelectric proximity sensors are used in the thru-beam, reflective, and cross-beam configurations.

______ c) A pressure regulator attempts to maintain upstream pressure regardless of downstream pressure.

______ d) Position, way, and port are three terms used to describe features of pneumatic valves that use sliding spools (or poppets) to direct and control fluid flow.

______ e) Hydraulic systems can usually use much cheaper flexible plastic tubing and fittings than pneumatic systems.

______ f) Pneumatic systems typically operate at low pressures, 50 to 200 psig (with 80-100 psig shop" air the most common).

______ g) With a PLC controlled system, it is generally preferable to place at least one emergency stop button outside the ladder logic, such that PLC program malfunctions cannot prevent system shutdown.

______ h) According to the rules we use in ME 360, an individual output device can appear on a ladder logic diagram as many times as needed.

______ i) SPDT switches have three connectors labeled common (COM), normally open (NO), and normally closed (NC).

______ j) Inductive proximity sensors are able to detect opaque objects made of any material.

2. The wiring diagram on the left is provided by the manufacturer of a proximity sensor. You have a +12 volt power supply, a 10 kW resistor, and a DMM. Show how you would wire the proximity sensor and how you would measure the resulting output voltage with the DMM.

3. Identify each component shown in the pneumatic system schematic shown below.

ME 360 Test #3 – Spring ’00 5/6/00

Closed book, one open note page (student-prepared) portion of test.

4. A PLC controlled pneumatic system is given below. The PLC ladder logic works correctly. Give a complete description of the operation of this system.
5. 

6. In the system shown the pneumatic cylinders are used to push parts into the “short” and “tall” part hoppers. The position of each cylinder is detected by the externally mounted proximity sensors X1 to X4 as shown. The “short part" photo sensor (X5) activates when a “short” part breaks the beam (dotted line). A part is in position to be ejected into the hopper when the front edge breaks the beam. The “tall part” photo sensor (X6) works similarly. The motor moves the conveyor belt (when Y0 is activated ) to the left after either part is ejected and the cylinders retract. Parts are automatically fed to the conveyor belt by another automated system. Two push-buttons are provided, X10 and X11.

7. 

There are exactly 4 mistakes in this PLC ladder logic diagram.

Identify the mistakes.

Suggest “simple” changes that will correct the mistakes.

6. A local manufacturer has a need for a pneumatic system controlled by a PLC. The system employs

· one double acting, single ended cylinder A controlled by a 2 position, 5 port, solenoid actuated (Y2), spring return, directional control valve, and

· one single acting (“push”), spring return cylinder B controlled by a 2 position, 2 way, normally closed, solenoid actuated (Y3), spring return, directional control valve

The retracted and extended positions of cylinder A are indicated by limit switches wired normally open to outputs X0 and X1 respectively.

The retracted and extended positions of cylinder B are indicated by limit switches wired normally open to X2 and X3 respectively.

The desired operation is:

1) operator presses both PB-1 (wired normally open) and PB-2 (wired normally closed) to start system in operation,

2) cylinder A completely extends, waits 5 seconds, then fully retracts,

3) once cylinder A is fully retracted, cylinder B then completely extends and immediately retracts 6 times in sequence,

4) if the “stop” button PB-3 (wired normally open) is pressed at any time, the cylinders both retract and the circuit returns to the start-up configuration.

Your problem:

a) Sketch a fluid power circuit (including speed controls for all cylinders),

b) Draw a PLC wiring diagram for the limit switches and solenoids as indicated above,

c) Design a PLC ladder logic diagram to control the system. Be sure to provide a brief description beside each rung of the ladder to describe what you are trying to accomplish.