Research Paper - Pneumatic Control Systems Design

ME 1000 Sequencing of pneumatic cylinders

INTRODUCTION

Automation is the process of technological development, which is aimed at reducing

human involvement in performing a task. This process is carried out through the use of

electric circuitry, pneumatics and hydraulics. In many industrial applications, such as

clamping the work piece and then machining it, it is necessary to ensure that the

operation is done in definite order or sequence.

Our group was asked to research on sequencing of pneumatic cylinders and with the

relevant knowledge carry out the following experiment:

The sequence of operation: A+, B+, C+, A-, C-, B-, D+, D-, is to be initiated on the

laboratory setup. The operation is to be initiated by a manual push button. The electrical

circuit should be designed such that once it is started, the whole operation is carried on

automatically. The next working cycle should be automatically initiated after a time delay

of 5 seconds upon the completion of the earlier cycle. The automated operation can only

be stopped by pressing an END push-button that stops the recycling at the end of a cycle,

and for safety reason, by an Emergency Stop push-button that halts the whole operation

instantaneously.

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SEQUENCING CONTROL

The laboratory setup was as follows:

4 solenoid-actuated, spring offset, 2 position valves

Push button switches

Limit Switches

Control Relays

Timer Relay

In order for the group to wire up the experiment, we had to draw a schematic diagram of

the electrical circuit, also known as a ladder diagram.

There are a few ways in which we can go about designing the circuit to suit our needs.

They are: The Shift Register Technique

The CASCADE method

The Grouping method

The Shift Register Technique

The essential features of The Shift Register Technique is to allow only 1 group of the

control circuit to turn “ON” at any one time and to enable each group to be turned “ON”

in a predetermined sequence. This technique eliminates the Signal Overlap problem in a

multi-cylinder control circuit.

To ensure shift register chain to function properly, each group of control circuit in the

shift register chain must perform the following 3 basic tasks when it is being activated via

a push button or a limit switch:

Memory - to turn on and latch the CR of its group

Set - to prepare the next group

Reset - to reset the previous group

Each Group is built from a memory CR. There are 2 signals to turn on the memory CR :

the essential feedback signal, confirming the completion of the previous sequence step

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and the preparatory signal from the previous group. When the memory CR is turned on, it

is supposed to accomplish 3 functions: it provides solenoid valves with a signal command

to extend or retract an actuator; it resets the memory CR of the previous group; and it

provides the next group CR with a preparatory signal. Using these steps, we can come out

with a ladder diagram to aid us in the experimental setup.

The CASCADE method

This is a convenient approach for complex problems. It is systematic and is

recommended when using programmable controllers. The sequence is divided into

groups following this rule: A new group must be started the moment it becomes

necessary to shut off any output signal actuated during the presently active group.

To put it simply, no letter of the sequence is to be repeated in a single group. Each group

in the sequence is allocated 1 relay. The relay is connected as a RS flip-flop. At any one

moment, only the flip-flop corresponding to the currently active group is set, while the

other groups are reset. As one passes from 1 group to the next, the next flip-flop is set and

its first task is to reset the previous flip-flop. Resetting the previous flip-flop

automatically shuts off all output signals that were on during the previous group.

Ladder-Diagram Design

Due to the set-up in the laboratory, our group has to design an industrial automation

system of the type: Single Path Sequencing Systems with Sustained Outputs.

The cylinder-actuating valve has only one solenoid and a return spring. In order to

activate such a valve into its “+” state, we need to provide a sustained solenoid voltage.

Sequence:

START / A+, B+, C+ / A-, C-, B-, D+ / D-,

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The sequence is divided into groups such that no letter is repeated within any group. Each

group in the sequence is allocated one relay. Since there is no A-, B-, C-, D- solenoids,

these motions will be executed by cutting off the respective “+” solenoids.

Once a new group is activated, all outputs of the previous group are automatically cut off.

For outputs to be sustained into the next group, they need to be reactivated in that group.

Referring to the diagram above, the left vertical line represents the high electrical

potential while the right vertical line represents the low electrical potential.

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The first rung is normally used to set, start and stop the cycle. Once the normally open

START button (line 1) is pushed down at the beginning of the sequence, it provides the

SET signal for relay Y4. The y4 contact connected in line 2 parallel to START provides

the memory that converts relay Y4 into a flip-flop.

Once Y4 is set, line 3 is closed as y4 contact is latched, thus relay Y1 is set. Likewise, the

y1 contact will be set and this will actuate the A+ solenoid in line 10, i.e. piston in

cylinder A will extended out.

When piston extends to touch limit switch a2, line 11 will be closed and this will actuate

the B+ solenoid. Likewise, as the piston in cylinder B touches the limit switch b2, the C+

solenoid is actuated since line 13 is now closed. Hence the actions in Group I is

completed and relay Y2 (line 5) will be activated immediately since limit switch c2 is

touched.

Once Y2 is set, the contact y2 in line 3 will unlatch and relay Y1 will be de-energized or

reset. The result is that line 10 will be open now and hence the A+ solenoid is cut off,

thereby piston in cylinder A will retract. Limit switch a1 is then touched and this will cut

off the C+ solenoid as in line 14. Following that, piston in cylinder B will retract and

touch limit switch b1.

Line 15 is closed and finally the D+ solenoid is actuated, touching limit switch d2 and

retracting straightaway. This is possible since the relay Y3 is activated, in which Y2 will

reset (line 5) resulting the D+ solenoid to be cut off, as contact y2 unlatches in line 15.

Finally, the timer in line 11 will be actuated and once the delay of 5 seconds is achieved,

the relay Y3 (line 7) will be reset, thereby relay Y1 (line 3) will be set again. Hence the

cycle is repeated all over again.

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The EMERGENCY button will be connected to the left vertical line to cut off the main

supply to stop the process immediately without the completing the rest of the

uncompleted cycle.

The Grouping Method

The grouping method was another method examined by the group to see if such a method

was suitable for our needs. This method involved using a relay to control each individual

action, e.g. 1 relay for A+. 1 relay for B+ etc….

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Method Used

The group decided that the CASCADE method was best suited for our needs.

The Shift Register method was used when double solenoid valves were involved. Only

the CASCADE and Grouping method could be used when dealing with single solenoid

valves, as in our case. The Grouping method although usable, could not be carried out on

the equipment available. As we can see from the sequence, (A+, B+, C+, A-, C-, B-, D+,

D-,), we would need 8 relays to carry out the job. However, there are not so many relays

in the laboratory setup. With the CASCADE method, it was spilt into 3 groups, as

explained earlier. Thus we were able to carry out the sequence with relays available.

Moreover, in an industrial situation, lesser relays means that the system is more efficient

and less resources are needed for each cycle of the system. The resulting ladder diagram

drawn using the CASCADE method was implemented and the experiment was set up

accordingly.

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SAFETY CONSIDERATIONS

In an industrial situation, safety is a very important factor. We have to ensure that the

safety of the operators and the physical well being of the machines are not compromised

and put in any danger of harm or damage. Safety considerations are needed in the setting

up of such automated systems. We have to be able to control the stopping of machines to

enable us to prevent further damage and harm in the event of an accident.

Researching into this area leads us to two different ways in which we can enhance the

safety of the automated system.

Interlocking

We can consider arranging the operation of any particular mechanisms such that they

function in a desired sequence. The ultimate objective here is to design some form of

a “two hand safety circuit”. This is also known as interlocking. This is used for the

actuation of dangerous equipment such as the punch presses or metal sheets clamps. In

prevention of any accident on the operator when processing sheet metal work, we can

mount two push buttons sufficiently apart so that both cannot be reached with one hand.

At the same instance, both push buttons must be pressed simultaneously to actuate the

press and clamp. This serves to ensure that the operator withdraw both his hands away

from the dangerous area before operation begins.

At the same instance, it is essential to devise circuits of a certain degree of sophistication.

This is to prevent the situation when the operator, wanting to make things easier, might

disable the safety feature by permanently tying down one of the push buttons with a wire

or with chewing gum. Our final rationale here is thus to achieve a two hand “no tie

down” safety circuit whereby to operate the press and clamp, both push buttons must first

be released to reset the system and then pressed together.

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The method shown above ensures that the cycle can only be restarted with both hands off

the 2 start buttons before every attempt.

Emergency Stop

It is sometimes necessary to interrupt a sequence because of an emergency condition,

such as a jammed machine, a missing or misaligned part, or some other malfunction. This

is carried out by pressing an Emergency stop button. This button is usually large, red and

easily accessible. This is especially important in cases where human lives are endangered.

There are various stop modes that can be chosen, depending mainly on the situation and

safety considerations.

They are: Stop-Restart circuits

“No Change” Stop modes

“No Motion” Stop modes

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Stop-Restart Circuits

In this type of circuits, after any emergency stop, and any remedies made, a Restart

button must be activated to resume operation. This button resumes the operation where it

had been stopped previously. Different needs are satisfied by designing this circuit to suit

them.

“No Change” Stop modes

In this mode, any cylinder at rest when the STOP button is pressed must remain at rest.

Any cylinder in motion must complete its stroke and then come back to rest. This mode is

easily achieved by using a single 3/2 valve connected in the line supplying air to the

various limit valves.

“No Motion” Stop modes

In this mode, any cylinder at rest will remain at rest and any cylinder in motion will cease

moving, freezing in the position that it was at the moment the STOP button was pressed.

This is especially useful in cases where allowing the full motion of the piston will result

in the work piece being dropped or crushed. Human lives can be saved in the instance

where a person accidentally gets in the way of a piston.

Other stop modes such as “Lock Piston” Stop mode and “Safety Position” Stop modes

and various combinations of the various mentioned stop modes can be utilized as well.

However we will not go into it as they cannot be applied to our experiment.

In our experimental setup, our Emergency Stop button shuts off the circuit by cutting off

the power supply to the circuit. However, as we are using single solenoid valve with

return spring and a double acting cylinder, when we push the Emergency Stop button, the

cylinder completes its stroke before coming to rest, thus creating a “No Change” Stop

mode. This is however not ideal in many cases, where we want the cylinder to freeze in

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its current position as in the “No Motion” Stop mode. This is not possible in our setup

due to the equipment being used. We can only achieve the “No Motion” Stop mode if we

were to change part of the circuit and to use additional apparatus.

Fig A

The above diagram shows one possible configuration, where a 3-position piston valve is

used in which the spring centers the mid position. The emergency valve would be

arranged to remove the pilot signals so that the 5/3 way valve would center and freeze the

cylinder with fluid trapped on both sides of the piston.

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IMPROVING THE SETUP

We can further improve the circuit we have come up with by adding a counter. A counter

is used to track the number of cycles a cylinder is oscillating and the oscillation circuit

will stop after a pre-selected count number is reached.

A normally closed counter contact has been added to the circuit. The STOP pushbutton or

Counter count-up signal will be able to reset the Y1 memory circuit for the automatic

cycle. Each time limit switch d2 is actuated, it not only causes the cylinder to retract, it

also sends a count to the counter. When the pre-selected count is reached, the set coil of

the counter energizes and the normally closed 1CT|R contact in line 1 opens, breaking the

latch and stopping the operation. The counter can be reset by pressing the Reset CTR

button.

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ALTERNATIVE SEQUENCE

Our group decided to explore other sequences and attempted to make a new setup.

The sequence above: A+, B+, C+, C-, A-, B-, D+, D-

We decided to experiment with a different sequence to ensure that we do indeed

understand fully what we have learned in the course of the experiment. By designing a

new circuit to carry out the sequence, successful implementation would validate our

understanding of the CASCADE method and the resulting ladder diagrams. We now

further appreciate ladder diagrams as they are more schematic as compared to

conventional wiring diagrams. This systematic approach to designing the circuit makes it

more convenient when it comes to making a new sequence of controls.

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CONCLUSION

The objectives the group set out to accomplish were done and we have a deeper

understanding of sequencing of pneumatic cylinders. CASCADE method was used

because of the use of lesser relays. We also explored the various safety measures that we

can implement as this is necessary in an industrial setting. We also looked into how we

could improve the set up by introducing a counter.

Researching into sequencing has introduced us to the field of industrial automation. From

the various references and text, we not only learnt about sequencing but industrial

automation in general. We understand the complexity involved in relating our theoretical

work into practicality. The sequence given to us is simple compared to what is usually

found in the industry. The field of industrial automation is vast and there is more to learn

about it. By exposing ourselves to this field, we have a greater interest in pursuing for

more knowledge about industrial automation. As mechanical engineers, we have a

possibility of specializing in this area. Learning more about it has definitely help us

understand the job of a mechanical engineer.

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BILBOGRAPHY

1. Bollinger, J.G & Hawson H.L “ Int. to Automatic Controls”, Appendix G, pp.

426-443

2. Pessen, David W. “Industrial Automation” John Wiley and Sons , 1989

3. Fawcett J.R, “Pneumatic circuits and low cost automation” Tech. Press Ltd,

England, 1968

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Research Paper - Pneumatic Control Systems Design