Pneumatic Logic Sensors and Actuators

Pneumatic Logic Sensors and Actuators

Pneumatic control system is frequently used in building automatic assembly machines. Pneumatic sensors and actuators are used in such control system. Electric current flows between logic sensors and actuators during machine control sequence. In other hands, pressure airline flows between pneumatic sensors and actuators during machine control sequence. In most cases, both control systems are used to build the machine control sequence. Although, hydraulic control system can also be used, this will not be considered in current Chapter.

• Pneumatic limit, push-button and emergency stop valves 

The main structure of the limit valves, push button and emergency stop valves are the same. Pneumatic limit valves are the equivalent of mechanical limit switches. Furthermore, the pneumatic push button and emergency stop valves are equivalent of mechanical push button and emergency stop mechanical switches. The main difference between the pneumatic limit valves and the mechanical switches, in the latter, the actuating arm or plunger shifts a so-called directional-control valve rather than electric contacts. Same thing is applied for push button and emergency valves.

For example, consider Fig. 6.1, which shows a so-called 3/2 limit valve (meaning 3 connections and 2 positions valve). The valve shown as normally closed, so that the outlet line is exhausted to atmosphere when the valve is not actuated. When the valve is actuated, the roller is depressed against the return spring at the bottom, so that the valve opens and passes supply pressure to the outlet line.

 

 

 

Fig. 6.2 illustrates another pneumatic limit valve called 5/2 limit valve (meaning 5 connections and 2 positions valve), which has two independent outlet lines, one is normally open and is the second normally closed. Such a valve is analogous to limit mechanical switch with changeover contacts (NO and NC contacts switch).
For both cases, the limit valve can be used as push-button valve and do the same stated functions, as shown in Fig. 6.2.

 

• Pneumatic directional pilot control valve

Most directional-control valves are of the spool and sleeve type. Fig. 6.3, shows a 5/2 spool valve (schematically). When the spool position or pilot line has been pressurized by p1 line. Flow line between A & D ports, and B & E posts are passed, while C is blocked. If pilot line p2 is pressurized, the spool shifted to the lift with corresponding change in flow paths as illustrated in Fig 6.3. Also, Fig. 6.3 shows also the ISO or ANSI standard symbol of 5/2 way directional valve. This type of directional valve can be used as SET-REST mechanical memory in pneumatic circuit. 
 

• Solenoid actuation directional control valves

The main difference between the solenoid actuation directional control valve and pilot control valve, is the way of displacement of the valve spool. Spool valve displacement is carried out using two pressurized air p1 and p2 in case of the pilot control valve. While solenoid and spring mechanisms are used to displace the spool valve in case of the solenoid actuation directional valve (or simply solenoid directional valve) as shown in Fig. 6.4. Similarly, solenoid actuation can also be used with 3/2 ways directional valve.
 

 

1. Pneumatic directional control valve symbols

These pneumatic valves that discussed so far called directional control valves and used as ON/OFF valves to control the airflow to the actuators. Most directional-control valves have two or three discrete ports or positions. Fig. 6.5, shows the international ISO or United States ANSI standard symbols for three connections two position valve, 3/2 valve. The two rectangles represent the two discrete positions of the valve. Since the connection tubes are drawn in this case attached to the left rectangle, this means that the valve is presently in the left position.
In Fig. 6.5, there are three connections, or valve ports, labeled as A, B and C, respectively. As long as the valve remains in the left position, left rectangle, there is airflow from port A to C, while port B is blocked. When this valve is shifted to its right position, right rectangle, port A blocked, while port B and C are connected. The arrowheads shown on the symbol are optional, they indicating the airflow directions.
 

                                   

 

Fig. 6.5, shows the symbol of a five connections two position (5/2) valve. In the left position, port A and D are connected, and likewise port B and E, while port C is blocked. In the right position, port A is blocked, while port B is connected to D, and port C to E.

In addition to the rectangles showing the valve positions, the symbol must also indicate how the valve is shifted or actuated. The symbols for common actuation methods shown in Fig. 6.6. As illustrated in Fig 6.6, the actuation symbols are for only one valve position. Actually, the opposite actuation must also exist, because otherwise the valve could never shifted back to its original position. various combination are available. For example, Fig( 6.7a) shows a valve with pilot-pressure actuation and return spring. In Fig 6.7(b, a) solenoid is substituted for the pilot line. Fig 6.7(c & d), no return spring, instead the valve is shifted back by activating the opposite pilot line or solenoid, respectively.

 

 

 
Fig 6.7(e) shows additional position to the 5/2 directional valve. The mid-position blocks all the ports. This type of valves is called 5/3 directional valves (5 connections, 3 positions). The original position (or normal position) of the valve at the middle (blocked ports position). 

 

 Fig. 6.8, shows some additional pneumatic symbols, and most should be self-explanatory. For example, air supply symbol used as common (all points connected to air supply). Check valve symbol represents a one-direction air flow valve. The shuttle valve has free-float ball or disk able to seal either of two seats, if either input A or B is pressurized, the ball seals the opposite seat, hence output C is high (this type of valve used as OR gate, where the C output is high only when one of the input A or B high).
 

• Cylinder (cylinders) actuation circuit
• Single acting cylinder actuation

 

 

Operation :

• When push button pressed, pilot line of the 3/2 cylinder actuating valve is pressurized.
• Air pass the left cylinder side, results in piston moved forward.
• When push button valve release, the 3/2 valve return to it’s original position and the piston return using spring force to it’s original position(shown position).
• The circuit is drawn in it’s initial position.

 

• Double acting cylinder actuation using pilot valve actuation

 

 

Operation :

• At the beginning, all elements positioned as shown in Fig. 6.10.
• Push button valve pressed, pilot line A+ is pressurized, shifting the 5/2 pilot valve to it’s right rectangle symbol position. This will pressurized the A+  of the cylinder room, causing the piston moves forwards.
• When push button released and pilot line A+ is vented, the 5/2 cylinder valve remains in it’s new position (setting mechanical memory).
• At the end of the cylinder stroke, piston rod actuating limit valve 3/2 ( a+ ), to which pressurized pilot line A-, causing the cylinder return to backward position automatically (original position), hence, resetting the mechanical memory.

 

PROBLEMS

•  “Two-hand safety circuit”, is a pneumatic circuit used for actuating dangerous equipment such as punch press, it has two push button valves mounted sufficiently apart so that both cannot be reached with one hand. Both push buttons must be pressed simultaneously to actuate the press (or double-acting cylinder). Hence, this will forces the operator to withdraw both hands from the dangerous area before the press descends. Draw pneumatic circuit to actuates double acting cylinder using three pneumatic valves; the first and the second valves are two 3/2 push button valves, and the third valve VA is 5/2 directional valve with one pilot line and return spring ?

 

 

 

 

 

 

 

 

• Draw the ISO or ANSI symbol of 3/2 limit valve with normally open design structure?

 

 

 

 

 

 

 

• Draw a pneumatic circuit to drive two double acting cylinders ( A and B ) using two 5/2 pilot type valves, push button 3/2 valve and two 3/2 limit valves using the following control sequence :

Ans)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• Develop the pneumatic network for R/S Flip-Flop using Pilot 5/2 valve and 2 push-button valves 3/2 type with spring return, draw the pneumatic network circuit ?
• Develop the pneumatic network circuit using for the following pneumatic machine sequence:

• Develop a pneumatic network to drive a single acting cylinder using one or both two START push buttons, 3/2 type?
• Develop a pneumatic network circuit to run the following pneumatic machine sequence:

• Develop pneumatic network circuit for AND, OR and NOT gates?

Laboratory 6.1 : Develop the pneumatic logic network for question 6.3 using the pneumatic tools given lab.