Introduction to Industrial Automation

possibility to save the stopping commands.

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Logical Design of Automation Circuits 

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155

those included in the automation circuit of Figure 4.41, in order to save the three stopping signals 

p

i

 (i=1, 2, 3) and also to replace the “p

i

” contacts with “d

i

” corresponding ones. The new automa-

tion circuit, without the above-mentioned disadvantage, is shown in Figure 4.42.

Problems

  4.1.  In the reciprocating lead screw set-up described in Section 4.4.1, the working table motion 

stops by pressing the button b

0

 only if it is moved right at a low speed or, in other words, it 

is in state S

1

. In any other state of the automation system, pressing the button b

0

 does not 

cause any action. Design an automation circuit which will permit the working table to stop 

by pressing the same button b

0

, also from state S

3

 when it moves left at a low speed.

  4.2. Based on the description of Problem 4.1, design an automation circuit which will permit the 

working table to stop from any state (S

1

, S

2

, S

3

, or S

4

) of operation.

  4.3. Based on the description of Problem 4.1, design an automation circuit which will permit the 

working table to stop, by pressing the button b

0

, at the end w when it is moving left or at the 

end z when it is moving right, independently of time instant when the button is pressed.

  4.4. Explain why the state diagram of Figure 4.20 does not include, as a stop condition, the case 

where the working table is moving right and the limit switch z is pressed while the limit 

switch w has been previously energized and remains energized due to a fault.

  4.5.  For the application described in Section 4.4.3, design an improved automation circuit which 

will save the instantaneous signal Stop of the last operated machine and will cause its stop-

ping with delay T after the sensor is energized.

 4.6. In a processing station, the motor pushes, via the worm screw and the sliding platform, 

objects from location 1 to locations 2, 3, and 4 in order for these to accept the corresponding 

treatment. Then the sliding platform returns back to initial place 1. Design an automation 

circuit only for the movement of objects according to the following specifications:

 

a.  The platform slides one space at each press of button b.

 

b.  The sliding is sequential, cyclic, and non-reversible from an intermediate location.

 

c.  Hence, starting from the initial place 1,

•  with the first pressing of b, the platform slides to location 2

•  with second pressing of b, the platform slides to location 3

•  with third pressing of b, the platform slides to location 4

•  with the fourth pressing of b, the platform returns back to location 1

•  with fifth pressing of b, the platform slides to location 2 and so on.

 

d.  The detection of the platform’s successive positions is achieved via the corresponding 

proximity sensors.

b

Proximity

switch

1

2

3

4

Platform

Motor

(

)

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  4.7.  From three locations A, B, and C of initial production, objects are sent via the central con-

veyor D to the assembly table. The conveyors A, B, and C operate continuously and therefore 

are not considered in this automation system. Each of the three workers always puts the 

same object on the conveyor, for which he is responsible at a random time but only under the 

condition that the corresponding light H

1

, H

2

, or H

3

 is on. When an object reaches the con-

veyor D, it is detected by the corresponding proximity switch PS

i

. The workers must put two 

objects A, two objects B, and one object C, and then the corresponding light H

i

 turns off, 

which means “do not put another object on the conveyor”. For example, two activations of 

PS

2

 cause the turnoff of the light H

2

. There is no specific order for the placement of the five 

objects from the workers. When the last object reaches conveyor D, that means there are five 

objects on the central conveyor D, then the light H

4

 turns on and the central conveyor D 

can operate if a technician in the assembly table presses the button b

0

. After pressing the 

button, conveyor D operates for 45 s. At the end of this period the conveyor D stops, the 

lights H

1

, H

2

, and H

3

 turn on while the light H

4

 turns off. Design the required automation 

circuit using the state diagram method.

A

B

C

PS

1

D

H

1

H

2

H

3

H

4

b

0

PS

2

PS

3

As

sembly

table

  4.8. A processed object in a production line undergoes three different treatments on a complex 

machine tool. The three treatments, which are performed via the operation of three cor-

responding motors, start simultaneously but have different durations. Design an automa-

tion circuit so that an instant signal Start to cause the simultaneous operation of the three 

motors, while an instant signal Stop to stop one motor immediately, the second one after 

20 s, and the third one after 30 s. 

  4.9.  N machines start to operate or stop by hand via an equal number of button pairs Start-Stop. 

For the Stop operation of any machine, there are no specific requirements. For the Start 

operation, however, it is desired to respond immediately to any of the N machines except for 

the first one, which will start-up with delay T. It is not predefined which of the N machines 

will start first, it may be any of the N machines at random. Design an automation circuit for 

N = 3 in an extendable form. 

 4.10. In a complex machine, there are two motors, M

1

 and M

2

, both with direct start-up. After 

an instant Start signal from a button, the motor M

1

 starts immediately and with time 

delay T starts the motor M

2

. After an instant Stop signal from another button, the motor 

M

2

 stops immediately and with time delay T stops the motor M

1

. The Stop signal acts 

only if both motors are in operation. Design the required automation circuit using the 

state diagram method.

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Logical Design of Automation Circuits 

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157

 4.11. Three machines, M

1

, M

2

 and M

3

 (motors with direct on line starting), start to operate after 

a common instant Start signal and with null-time latch sequence. This means that if the 

machine M

1

 has been started, only then the machine M

2

 starts immediately; and if M

2 

oper-

ates, then M

3

 immediately starts to operate. If any of the three machines stop during opera-

tion for any reason (e.g., the thermal relay is energized) then all the machines must stop. 

Also, the three machines will stop after a common instant Stop signal. Design the required 

automation circuit.

Note:

 If a machine M

i

 starts, and then a machine M

i+1

 operates after passing a time period, we 

refer to this functionality as a “delay time latch sequence”.

 4.12. Automate the chemical mixer of the following figure by designing an automation circuit 

satisfying the following specifications:

 

a.  The mixing process starts with an instant signal from a button (mixing cycle).

 

b.  If one of the tanks A, B, and C do not contain the minimum required amount of liquid 

(dashed lines), the mixing process can’t begin.

 

c.  The filling process includes (in the following respective order) the filling of a central tank 

T with liquid A up to level L

A

, then with liquid B up to level L

B

, and finally with liquid 

C up to level L

C

, by opening the corresponding valves V

i

, (i=1, 2, 3).

 

d.  After the end of the filling process, a time period of 10 minutes follows for a chemical 

reaction to occur.

 

e.  At the end of this period, the pump M starts to operate until the tank T is empty. This 

expresses the end of the mixing cycle.

 

f.  For the detection of various liquid levels, use electronic level switches with immersed 

electrodes, whose location must be defined in all tanks.

V

1

V

2

A

B

C

V

3

L

C

M

C

L

A

L

B

T

 4.13. Centrifugal separators are machines designed especially for liquid-based applications. Using 

centrifugal force, they separate substances and solids from liquids. They are equally as effec-

tive at separating liquid mixtures at the same time as removing solids. Because of the large 

inertia of the rotated mechanical parts and the high rotation speed, these machines must not 

vibrate during their operation. Design an automation circuit of a centrifugal separator with 

two speeds of rotation according to the following specifications:

 

a.  The separator starts at low speed by pressing the Start button.

 

b.  After 30 s, the separator goes over the high speed automatically.

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c.  If the vibration sensor (SPDT output) is activated during the operation at high speed, 

then the separator must stop immediately, and cannot reoperate except if a reset button 

is pressed (lock of operation due to vibrations). The operator of the machine is informed 

about the lock of operation via a light indicator.

 

d.  With an instant signal Stop from a corresponding button, the separator stops to operate 

independently of the rotation speed.

Note:

 If the vibration sensor is activated during the rotation at a low speed, which is entirely 

possible, then this fact should not have any effect on the operation of the separator. 

 4.14. The automation system of the assembly station described in Problem 4.7 can be redesigned 

in order to make better utilization of processing time and also to include the remaining parts 

of the set-up that have deliberately not been encountered. Specifically, the control and opera-

tion of the three conveyors A, B, and C (Figure 4.7) must be incorporated in the automation 

circuit in order to permit the placement of the second two objects even if the corresponding 

light is turned off. This is reasonable when a worker or object production delays the forma-

tion of the objects’ triple on conveyor D. Then, the other two workers can put second objects 

on their corresponding conveyors, but these objects must stop just before conveyor D, other-

wise there will be duplicate objects on conveyor D. In order to achieve this, additive sensors 

are required to be placed on conveyors A, B, and C and possibly additive signaling on initial 

production places. It is clear that this problem becomes complex and should be faced as a 

project of an automation system study where you can improvise choosing additional equip-

ment and not as a simple tutorial problem. Therefore, design the automation of the assembly 

station so that its operation is as efficient and flexible as possible.

 4.15. Design an automation circuit for the manual correction of the power factor (cos

φ

) in an 

industrial electric AC power station.  The correction is performed by inserting successively 

five capacitors in the power circuit until the desired factor value is achieved. The insertion of 

capacitors is achieved by energizing an equal number of relays via pressing the same button 

b

in

 several times. In a similar way, the subtraction of the capacitors is achieved by pressing 

the button b

out

 successively as many times as needed.

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