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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ME 1000 Sequencing of pneumatic cylinders

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

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to turn on and latch the CR of its group

 Set

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to prepare the next group

 Reset

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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 2

ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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+, DWe 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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ME 1000 Sequencing of pneumatic cylinders

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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ME 1000 Sequencing of pneumatic cylinders

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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