Basic Pneumatics. Module 7: Time delay valve and sequence control systems. Academic Services PREPARED BY. January 2013

Transcription

1 Basic Pneumatics Module 7: Time delay valve and sequence control systems PREPARED BY Academic Services January 2013 Applied Technology High Schools, 2013

2 Module 7: Time delay valve and sequence control systems Module Objectives After the completion of this module, the student will be able to: Demonstrate an understanding of the pneumatic time element. Demonstrate an understanding of the different types of time delay valves. Identify the symbol, and design of normally closed time delay valve. Explain the function and principle of operation of normally closed time delay valve. Identify the main parts of the time delay valve. Draw a pneumatic circuit diagram that consists of a time delay valve, normally closed. Simulate pneumatic circuit that consists of a time delay valve using Fluid SIM software. Build a pneumatic circuit that consists of a time delay valve as per specific requirements. Explain the the main types of displacement diagrams. Draw a displacement step diagram for a given sequence of operation. Draw a circuit diagram from a given sequence of operation. Module Contents Topic Page No. 1 Function and symbol 3 2 Design 4 3 Principle of operation 5 4 Practical task 1 (Pressing device) 6 5 Introduction to sequence control 8 6 Sequence control groups 8 7 Control of multiple actuators 8 8 Displacement-step diagram 9 2 Module 7: Time delay valve and sequence control systems

3 1. Time Delay Valve Pneumatic time elements can be formed very simply from combinations of three valves: directional control valve, one way flow control valve and reservoirs. 1.1 Function Time delay valve is used to delay the output signal. The time delay valve is actuated by a pneumatic signal through the tank after a preset time delay has elapsed. The returned to the normal position via return spring when the signal is terminated. Fig. (7.1) shows the picture of the time delay valve, normally closed. N/C time delay valve Fig. 7.1: Time delay valve 1.2 Symbols Fig. (7.2.a) shows the ISO symbol of normally closed time delay valve. (a) ISO symbol of N/C time delay valve (fluidsim) Fig. (7.2.b) shows the ISO symbol of normally opened time delay valve. (b) ISO symbol of N/O time delay valve(fluidsim) Fig. 7.2:Time delay valve symbol Module 7: Time delay valve and sequence control systems 3

4 2. Design The time delay valve is a combination of a 3/2 way valve, one way flow control valve and air reservoir. The 3/2 way valve can be normally open or normally closed. The delay time is generally ranging between (0-300) seconds for both types of valves (normally opened and normally closed). By using additional reservoirs, the delaying time can be extended. Fig. 7.3: parts of the time delay valve Fig. (7.3) shows the parts of the time delay valve, normally closed. The type of the time delay valve whether normally opened or normally closed is determined according to the type of the 3/2 way valve if it is N/O or N/C. Fig. 7.4.a 3/2 way valve (N/C) Fig. (7.4) shows the two types of 3/2 way valve. (N/C) and (N/O). Fig. 7.4.a 3/2 way valve (N/O) Fig. 7.4: 3/2 way valve 4 Module 7: Time delay valve and sequence control systems

5 3. Principle of operation The following operational principle applies for a time delay valve in normally closed position as shown in Fig. (7.5.a). The compressed air is supplied to the valve at connection 1. The control air flows into the valve at 12 through a one way flow control valve and depending on the setting of the throttling screw, a greater or lesser amount of air flows per unit of time into the air reservoir, when the necessary control pressure has built up in the air reservoir, the pilot control of the 3/2 way valve is moved downwards. This blocks the passage from 2 to 3. The valve disc is lifted from its seat and thus air can flow from1 to 2 as shown in Fig. (7.5.b). The time required for pressure to build up in the air reservoir is equal to the control time of the valve. (a) (b) Un-actuated position Actuated position Fig. 7.5: Time delay valve principle of operation Module 7: Time delay valve and sequence control systems 5

6 4. Practical Task 1: Pressing device with time control 4.1 Learning objectives: Upon the completion of this task, the student will be Familiar with the set-up and mode of operation of a time delay valve normally closed. Able to recognize and sketch the time delay valve. Able to choose different pneumatic components according to the given conditions. 4.2 Problem Description: A double acting cylinder is used to press glued components together. Upon operation of a push button, the clamping cylinder extends as shown in Fig. 7.6 below. Once the fully advanced position is reached, the cylinder is to remain for a time of T = 6 seconds and then retract to the initial position automatically. The cylinder retraction is to be adjustable. A new start cycle is only possible after the cylinder has fully retracted. Fig. 7.6 Postional schetch 6 Module 7: Time delay valve and sequence control systems

7 4.3 Requirements: 1 Draw pneumatic circuit diagram using the space provided below. 2 Simulate the circuit using FluidSIM software. 3 Assemble the circuit practically and check its operation. 4 Write down your observation below. Draw the pneumatic circuit here. Module 7: Time delay valve and sequence control systems 7

8 5 Introduction to sequence control system A control system with compulsory stepped operation where switching from one step to the next in the program is dependent upon certain conditions being satisfied. In particular, the programming of jumps, loops, branching, etc. is possible. 6 Sequence control is divided into two subgroups: Time-dependent sequence control system A sequence control where the switching conditions are dependent only on time. Step enabling conditions are generated via timers, or cam-shaft controllers with constant speed. Process dependent A sequence control system where the switching conditions are dependent only on signals from the system being controlled. Stepdiagram control is a form of process-dependent sequence control; the step enabling conditions depend purely on the stroke-dependent signals of the controlled system. 7 Control of multiple actuators In the case of multiple cylinder circuits, a clear definition of the problem is important. The representation of the desired motion of all actuators is described using the displacement-step diagram. The special conditions for the start of the sequence must also be defined. If the motion diagram and auxiliary conditions have been clearly defined, drawing of the circuit diagram can commence. The circuit diagram should be designed according to the guidelines, in order for a circuit to operate, it is essential to avoid overlapping signals. By an overlapping signal, we understand signals applied simultaneously at the two control ports of a double pilot valve. The following valves can be used to eliminate signal overlap: roller lever valves with idle return or toggle lever valves, time delay valves, reversing valves or sequencers. 8 Module 7: Time delay valve and sequence control systems

9 8 Displacement-step diagram The displacement step diagram and the displacement-time diagram represents the operating sequence of the actuators, the displacement is recorded in relation to the sequence step. If a control system incorporates a number of actuators, they are shown in the same way and are drawn one below the other. Their interrelation can be seen by comparing the steps as shown in (Fig.7.7b). There are two cylinders 1A and 2A. In step 1 cylinder 1A extends and then cylinder 2A extends in step 2. In step 3 cylinder 1A retracts and in step 4 cylinder 2A retracts. Step number 5 is equivalent to step 1. (a) (b) Positional sketch Displacement step diagram In the case of a displacement-time diagram, the displacement is plotted in relation to the time as shown in (Fig.7.7c) (c) Displacement time diagram Fig. 7.7: Displacement step and time program Module 7: Time delay valve and sequence control systems 9

10 Example (1) Find the sequence of operation for the given displacement step diagram Solution: A+A-B+B-C+C- Example (2) Draw the displacement step diagram for the following sequence of operation A+B+B-A-C+C- Solution: 10 Module 7: Time delay valve and sequence control systems

11 9. For further reading, you can use the following links: Supplementary recourses 1. Pneumatics video from Festo. 2. FluidSIM software. 11. References 1- Festo manuals and workbook TP Festo manuals and textbook TP101 Module 7: Time delay valve and sequence control systems 11

12 Student s notes 12 Module 7: Time delay valve and sequence control systems

13 Worksheet 1 Draw the symbol of time delay valve, normally closed Draw the symbol of 3/2 way, roller lever valve Draw the symbol of 5/2 way, double pilot valve What is the function of time delay valve? What are the three parts consisting the time delay valve? Module 7: Time delay valve and sequence control systems 13

14 Work sheet 2 A rod of a double acting cylinder is advanced by pressing a push button valve, after reaching the end position; it stays 5 seconds and then returns automatically to the initial position. Draw the required circuit diagram. 14 Module 7: Time delay valve and sequence control systems