Discrete Control Logic

1. Pneumatic circuits - Low forces - Discrete, fixed travel distances - Rotational or reciprocating motion

Main components: compressor, valves, cylinders

•

double -acting

spring-return

air supply

vent to atmosphere (air discharge

Pneumatic components: cylinders

tepping away from this mat, the person has to manually switch the robot back ON

Pneumatic components: valves

a 3/2 valve a 5/2 valve

return spring pneumatic

push buttonfoot pedal

solenoid (electrical) roller (mechanical)

valve actuation

•

•

Start

• VA

A+A-

A- +

Simple Pneumatic control

START, A+, A-

Pure Pneumatic control design: Cascade method

Example: Punch Press Operation: (i) part is clamped in position (ii) press punches the part (iii) the clamp is released (iv) part is removed from the table

START, A+, B+, B-, A-, C+, C-

Functions of A, B, C ?

How to design pneumatic system?

Pure Pneumatic control design: Cascade method

(1) Write cylinder action sequence

(2) Partition sequence into minimum no. of groups (no letter repeated in group)

(3) Merge last group, first group (if possible)

(4) Each cylinder is double-acting

(5) Each cylinder is controlled by 5/2 valve (both pilot lines: pneumatic)

(6) Each + and – position of cylinder: limit valves

(7) Each group => manifold line.

The manifold line connects to the limit valves associated with the cylinders.

(8) The air pressure in the manifolds is controlled by 5/2 valves called group valves.

no. of group valves = (no of groups – 1)

Cascade method: forming groups

Break it down into groups: START, A+, B+ / B-, A-, C+ / C- GRP 1 GRP 2 GRP 3

START, A+, B+, B-, A-, C+, C-

Merge Group 3 with Group 1 ? START, A+, B+ / B-, A-, C+ / C- GRP 1 GRP 2 GRP 1

Cascade method: draw cylinders, manifolds, valves

- Draw the cylinders - For each cylinder, draw the limit valve (3/2 way) - For each cylinder, draw the control valve - Draw manifold lines - Limit valve connections: a2, b2 and c1 get their air supply from manifold 1 a1, b1 and c2 get their air supply from manifold 2 - Group valve connections: air supply: initially to GRP 1 (manifold 1), when pilot line 1 is active. line 1: activated by c2 (transition from GRP 2 GRP 1) line 2: activated by b2 - Connect air supply of each cylinders valve, and supply + and – ports of each cylinder - Connect the logic lines according to sequence: START A+ B+ B- A- C+ C-

Cascade method: forming groups

Start

- + - + - +

•••

•

A

a1 a2

B

b1 b2

C

c1 c2

- +VC+- VB- +VA

1

2

21

Cascade circuit for: START, A+, B+, B-, A-, C+, C-

Pure Pneumatic Controls

- For more complex logic, difficult to debug - Less versatile than electronic control (e.g. no counters, poor timer control)

- pneumatics timer control: delay valves.

Programmable Logic Controllers

History: avoiding complex/large relay boards - Why are relay boards required?

PLC Basics: computer + relays

A/C

220Vinput 0

input 1input 2

input 3input 4

input 5input 6

Power supply wire

O u t p u t s

Box with Computer (controller)

data communication wire

PLC: example 1

Pressure_Switch is ON Warning_Light ON

PLC: example 1

STEP 1: Write this logic into a PROGRAM STEP 2: Load program into PLC STEP 3: Connect the sensor output to External Input terminal. STEP 4: Connect the PLC External Output Terminal to Warning Light STEP 5: EXECUTE the logic program on the PLC.

PLC: example 1

Programming language: LADDER LOGIC

PLC: example 1

Programming language: LADDER LOGIC

IF THEN

PLC: example 2

Outer mat ON warning light ON

Inner mat ON warning light ON AND Robot OFF

Stepping away from inner mat Manually switch robot ON

PLC: example 2

Two actuators: Warning light, Robot master switch

LOGIC for Warning light

External Input 1: outer mat

External Input 2: inner mat External output: light

PLC: example 2

LOGIC for Robot

LOGIC for Warning Light

PROBLEM ?

PLC: example 2

LOGIC for Robot

Robot must STAY OFF until manual reset to ON

Solution: LATCH

External Input 2: inner mat Internal (logical) relay

latch

External output: robot

PLC: example 2

LOGIC for Robot

Robot must STAY OFF until manual reset to ON

Ladder Logic Programs

Switch (Relay) naming conventions

Lecture notes (Rockwell™ Automation PLC): External inputs: I:0/1, I:0/2, …, I:1/1, I:1/2, … I:n/m External outputs: O:0/1, O:0/2, …, O:1/1, O:1/2, … O:n/m Internal Relays: B0, B1, … etc.

Lab (SMC™ PLC): External inputs: X0, X1, … External outputs: Y0, Y1, …, Internal Relays: R0, R1, … etc.

PLC Example: XOR Logic

A xor B: (A is ON AND B is OFF) OR (A is OFF AND B is ON)

Ladder Logic: Timers

Solenoid actuated door-lock

Solenoid ON for 5 sec, then OFF

Solenoid ON Door unlocked

Solenoid actuated when: (i) ON signal from number-pad outside door (ii) ON signal from door-open switch inside door

While O:0/1 remains ON, Timer COUNTS DOWN from PRESET COUNT DOWN = 0 ( T4:1) set to ON

Ladder Logic: Timers

Solenoid actuated door-lock

Solenoid ON for 5 sec, then OFF

Solenoid ON Door unlocked

Solenoid actuated when: (i) ON signal from number-pad outside door (ii) ON signal from door-open switch inside door

Ladder Logic: Timers -- reset

Solenoid ON for 5 sec, then OFF

Solenoid actuated when: (i) ON signal from number pad outside door (ii) ON signal from door-open switch inside door

During ON, if button is pressed, Timer resets to PRESET

During ON, light indicator is ON

LEGEND: I:0/1 door-open I:O/2 card-reader O:0/1 solenoid O:0/2 light indicator

Ladder Logic: counters

Count the number of occurrences of an event

Pallet loading in factory After 10 parts arrive on conveyor, worker comes to load pallet

Examples:

Pneumatic press hammer Hit the part 20 times, then wait for part to be unloaded

Rice cooker alarm Beep 5 times when rice is cooked

EVENT: switch goes from OFF ON

Ladder Logic: counters

Pallet loading in factory After 4 parts arrive on conveyor: STOP conveyor belt turn ON the indicator light

Ladder Logic: car wash

Car arrives limit switch ON Limit switch ON Washer ON Washer ON: (i) Soapy water SPRAY ON (30 secs) (ii) Rinse: clean water SPRAY ON (30 secs) (iii) Automatic scrubber brushes car (15 secs) (iv) After washing 50 cars, the scrubber brush Auto-change

I:0/1 I:0/2 B1

O:0/0B1 T4:0 T4:1 T4:2

O:0/1B1

O:0/2B1

O:0/3

B1

B1

B1

I:0/1 : System On I:0/2 : Emergency Stop I:0/3 : Limit Switch

O:0/0 : Soap Water On O:0/1 : Rinse On O:0/2 : Scrubber On O:0/3 : Activate Scrubber Change

Notice how B1 and the timer outputs are used to control the logic according to the required timing.

O:0/2

Preset:Accum:

T4: 1

60000

Base: 0.01

Preset:Accum:

T4: 2

75000

Base: 0.01

Preset:Accum:

T4: 0

30000

Base: 0.01

DNT4:0

DNT4:0

DN

DNT4:1

DNT4:1

DN

DNT4:2

DNT4:2

DN

Preset:Accum:

C5: 0

500

CTU

I:0/3 T4:0RES

I:0/3 T4:1RES

I:0/3 T4:2RES

O:0/3 C5:0RES

C5: 0

DN

EN

EN

EN

EN Car arrives limit switch ON Limit switch ON Washer ON Washer ON: (i) Soapy water SPRAY ON (30 secs) (ii) Rinse: clean water SPRAY ON (30 secs) (iii) Automatic scrubber brushes car (15 secs) (iv) After washing 50 cars, the scrubber brush Auto-change

Programming a PLC

(1) Hand held console (direct feed of program into PLC)

(2) Computer-interface: (i) Complete the program on a computer (ii) Test the program on PC (iii) Upload the program to the PLC processor memory (persistent) (iv) Connect external Inputs and Outputs (v) Run the program on PLC

Operation cycle of PLC

Phase 1Phase 2

Phase 3

Program Memory

Processor

Accumulator

output register

output

30

37

30

31

32

33

34

35

36

37

input input register

00

01

02

03

04

05

06

07

08

09

10

11

00

11