Most of the practical pneumatic systems involve the use of multiple actuators (cylinders, semi-rotary actuators, etc) which when operating in specified sequences carry out the desired control tasks. Double-pilot directional control (DC) valves are used as final control elements to control the forward and return strokes of the actuators. Sensors are used for confirming the actuator positions and providing signals to the final control elements. However, it is important to remember that the double-pilot DC valves are susceptible to the problem of ‘signal conflicts’. You may remember that the signal conflict is due to the appearance of pilot signals simultaneously on both sides of a double-pilot DC valve, due to which the valve will not be able to move its position in response to a control signal.
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A pneumatic circuit has to be developed for realizing the following control task using two cylinders A (1.0) and B (2.0) as shown in the schematic diagram. The cylinder A is to extend and bring a job under the stamping cylinder B. The cylinder B is then to extend and stamp the job. The cylinder A can return back only after the cylinder B has retracted fully. The sequential control task can also be expressed in a displacement-step diagram as shown.
The displacement-step diagram for the control task is given below. The displacements of cylinders are plotted in accordance with the required sequence of the cylinder actions in equal steps. For example, the displacement of any cylinder from the retracted position (0) to the extended position (1) is shown in the diagram with a line from 0 to 1. Similarly, other displacements can also be shown.
Developing the Power Circuit
The first step in the designing of a circuit diagram for the control task is to draw the power circuit with the cylinders, final control elements, and properly designated sensors. Two 5/2-way double-pilot valves 1.1 and 2.1 act as final control elements to control the cylinders A and B respectively. For the automatic operation of the desired sequence, a pair of sensors is used per cylinder to confirm the end positions of the piston. Sensors a0 and a1 (or A0 and A1) are positioned in the retracted and extended positions respectively of the cylinder A. Similarly, sensors b0 and b1 (or B0 and B1) are positioned in the retracted and extended positions respectively of the cylinder B. The partial circuit with the cylinders, the final control elements, and the sensors are shown in the figure.
The entire sequence of the cylinder actions and the associated sensor outputs can be represented in a notational form before developing the control circuit. The developed circuit is then checked for the presence of signal conflicts.
Notational Form of the Control Task
In the notational form, the ‘+’ sign is appended to the cylinder designation (A, B, etc) to represent the forward stroke and the ‘-’ sign is appended to it to represent the return stroke. For the control task specified above, the sequence of the cylinder actions along with the details of the sensor outputs can be represented by:
As can be seen, the A+ action generates ‘a1’ signal and is used for the B+ action, the B+ action generates ‘b1’ signal and is used for the B- action, and so on. Usually, a “Start” signal is also required along with the a0 signal (the last signal of the control sequence) for obtaining the A+ action (the first cylinder action of the control sequence).
Adding the Control Circuit to the Power Circuit
Incorporate all the sensors and the “Start” pushbutton as per the required control sequence, as shown in the figure. Represent the sensors a0 and b0 initially in the actuated state.
Check for Signal Conflict
Next step is to analyze the circuit for the presence of signal conflicts. When the “Start” pushbutton is pressed, a signal appears at port 14 of the valve 1.1 through the sensor a0. It can be seen that a signal is also present at port 12 of the valve 1.1, resulting in a signal conflict. As result, the valve 1.1 is unable to switch over and the circuit will not work at all. You may check for other signal conflicts if any.
Elimination of signal conflicts
Various methods are devised to eliminate the problem of signal conflicts. Most popular methods are based on controlling the air supply to different sections of the control circuit. Any of the following methods may be used for the purpose of avoiding the signal conflicts: (1) Cascade method and (2) Shift register. This article explains only the cascade method of eliminating the signal conflict.
In this method, the sequence of operations of cylinders, that is, A+B+B-A- are divided into appropriate groups in such a way that there is no possibility of a signal conflict. That is, in case A+ and A- happen to be in the same group, there is a possibility of signals appearing simultaneously at both ends of the final control element (valve 1.1) controlling the cylinder A. Similarly, there is a possibility of signals appearing simultaneously at both ends of the final control element (valve 2.1) controlling the cylinder B, if the cylinder actions B+ and B- happen to be in the same group. Hence the sequence of operations is divided in such a way that the A+ and A- actions fall into different groups (G1 and G2), and the B+ and B- actions fall into different groups (G1 and G2), as demonstrated.
It should be remembered that the desired sequence should be maintained. In this method, every attempt should be made to keep the number of groups to a minimum so as to keep the number of valves to a minimum.
Next, divide the power supply for the control circuit into an equal number of groups in such a way that at any given point in time, only one group will have the supply with all other group(s) connected to the exhaust. By an appropriate interconnection of the 5/2-DC valves, the power supply can be divided into 2 groups, 3 groups, 4 groups etc. The different positions of the standard 2-group circuit are illustrated in the figures. It can be seen that initially, the supply is in the last group, G2 (see figure b). When the control signals are applied to inputs e1 and e2 in that sequence the supply changes to groups G1 and G2 respectively across the cascade. The group changing circuits for the three groups, four groups etc can also be drawn in a similar manner.
Circuit Design Using the Cascade Method
Add the group changing circuit for two groups just below the power circuit. The group changing circuit ensures that only one group will have the supply at any point in time with the other group connected to the exhaust. Add the control valves and sensors as specified in the notational form given above. Initially, the sensors a0 and b0 are shown in the actuated position. It may be observed that the port 14 (for A+ action) and the port 12 (for A- action) of the valve 1.1 are always connected to different supply groups. Similarly, the port 14 (for B+ action) and the port 12 (for B- action) of the valve 2.1 are always connected to different supply groups. This inhibits the signals from appearing simultaneously on both sides of each of the final control elements and hence avoids the possibility of signal conflicts. Finally, designate the valves and cylinders.
Circuit Position during the A+ Action
When the “Start” pushbutton is pressed, the air supply from the group G2 is directed to the port 14 of the reversing valve 0.1 through the “Start” pushbutton and the actuated sensor a0. The reversing valve switches over causing the group supply to change from the group G2 to G1. (Note: a0 is shown in the released position due to the subsequent extension of cylinder A). When the group is changed from the group G2 to G1 with the group G2 vented, the air supply from the group G1 is directed to port 14 of the valve 1.1. As there is no possibility of a signal conflict here, the valve 1.1 switches over causing the A+ action.
Circuit position during the B+ Action
The sensor a1 is actuated as a result of the A+ action, allowing the air supply from the group G1 to reach port 14 of the valve 2.1. As there is no possibility of a signal conflict here, the valve 2.1 switches over causing the B+ action automatically.
Circuit position during B- Action
The sensor b1 is actuated as a result of the B+ action, allowing the air supply from the group G1 to reach the port 12 of the reversing valve 0.1. As a result, the reversing valve switches over, causing the group supply to change from G1 to G2. As the group is changed from G1 to G2 with the group G1 vented, the air supply from the group G2 is directed to the port 12 of the valve 2.1. As there is no possibility of a signal conflict here, the valve 2.1 switches over causing the B- action automatically.
Circuit position during A- Action
The sensor b0 is actuated as a result of the B- action, allowing the air supply from the group G2 to reach the port 12 of the valve 1.1. As there is no possibility of a signal conflict here, the valve 2.1 switches over causing the A- action automatically.
The cascade system provides a straightforward method of designing sequential circuits. It will always give a workable circuit and only rarely will it be possible to suggest any improvements.
Authored by JOJI Parambath, Founder/Director, Fluidsys Training Centre, Bangalore
JOJI P, Pneumatic Controls, WILEY India, 2008
Note: A comprehensive account of the topic is given in the textbook on ‘Pneumatic Controls’ by Joji P
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