Chapter 5Pneumatic System: Multi Actuator Circuit

Prepared by:Mohd Shahril Shariff

Multi Actuator Circuit

Most of the practical pneumatic circuits use multi cylinders. They are operated in specific sequence to carry out the desired task. For example, to drill a wooden component first we need to clamp and then drill. We can only unclamp the cylinder, if and only if the drill is withdrawn away from the workspace. Here sequencing of movement of clamp cylinders and cylinder which carries the drill is important. This sequencing is carried out by actuation of appropriate final control valves like directional control valves. The position of the cylinders is sensed by the sensors like limit switches, roller or cam operated valves.

Multi cylinder pneumatics circuits can be designed in various methods. There is no universal circuit design method that suits all types of circuits. Some methods are commonly used for compound circuits but would be too expensive for simple circuits. There are five common methods used by engineering and they are given below

1. Classic method2. Cascade method3. Step-counter method4. Logic Design method (with Karnaugb-Veitch Maps)5. Combinational circuit design

Multi Actuator Circuit

Classical Method

In classical method, circuit design is done by use of general knowledge of pneumatics following the sequence through intuitively. In general, steps involves Write down sequence and draw motion diagrams Draw in cylinders and control valves Complete circuits intuitively.

Coordination Motion & Sequence Control

In majority of the pneumatic applications more than one cylinder is used. The movement of these cylinders are coordinated as per the required sequence. Sensors are used for confirming the cylinder position and the resultant actuation of the final control element. Normally limit switches are used. The activation of limit switches of different cylinders will provide set or reset signal to the final control valves for further controlling the movement of various cylinders. The limit switches have to be arranged in the proper location with the help of motion diagram

Motion Diagram/Step Displacement Diagram

• In order to develop control circuitry for multi cylinder applications, it is necessary to draw the motion diagram to understand the sequence of actuation of various signal input switches-limit switches and sensors• Motion diagram represents status of cylinder position -whether extended(outstroke) or retracted(instroke) in a particular step

Example: Coordinated Motion Control for a Stamping Application

Clamping, Stamping and Ejection Application

Multi Cylinder Application with Two Cylinders A and B

Input Signals• Cylinder A - Limit switch at home position ao

- Limit switch at home position a1

• Cylinder B - Limit switch at home position bo

- Limit switch at home position b1

Out put Signals• Cylinder A advancing step is designated as A+• Cylinder A retracting step is designated as A-• Cylinder B advancing step is designated as B+• Cylinder B retracting step is designated as B+

Design of Signals

Sequential Motion of CylindersIt is possible to have the following sequence of operationwith two cylindersSequence Example of ApplicationA+, B+, A-,B- Lifting & Shifting / shifting of parts

in two directions ,A+, B+, B-,A- Clamping & Stamping/RivetingA+, A-, B+, B- Feeding and Ejection of parts

Example 1Two cylinders are used to transfer parts from a magazine onto a chute. When a push button is pressed, the first cylinder extends. Pushing the part from the magazine and positions it in preparation for transfer by the second cylinder onto the out feed chute. Once the part is transferred, the first cylinder retracts, followed by the second. Confirmation of all extended and retracted positions are required.

Solution Example 1: Step 1-2Step 1: Write the statement of the problem: Let A be the first cylinder (Pushing) and B be second cylinder (feeding) as shown in the Figure xxx. First cylinder A extends and brings under stamping station where cylinder B is located. Cylinder B then extends and stamps the job. Cylinder A can return back only cylinder B has extended fully.

Step 2: Draw the positional layout.

Solution Example 1: Step 3-4Step3: Represent the control task using notational form Cylinder A advancing step is designated as A+ Cylinder A retracting step is designated as A- Cylinder B advancing step is designated as B+ Cylinder B retracting step is designated as B- Therefore, given sequence for clamping and stamping is A+B+A-B-

Step 4 Draw the Displacement –step diagram

Solution Example 1: Step 5Step 5 Draw the Displacement –time diagram

Solution Example 1: Step 6Step 6: Analyse and Draw Pneumatic circuit. Step 6.1 Analyse input and output signals. Input Signals Cylinder A – Limit switch at home position ao Limit switch at home position a1 Cylinder B - Limit switch at home position bo Limit switch at home position b1 Output Signal Forward motion of cylinder A ( A+) Return motion of cylinder A (A-) Forward motion of cylinder B( B+) Return motion of cylinder B( B-)

Step 6.2 Using the displacement time/step diagram link input signal and output signal. Usually start signal is also required along with b0 signal for obtaining A+ motion. 1. A+ action generates sensor signal a1, which is used for B+ motion 2. B+ action generates sensor signal b1, which is used for A- motion 3. A- action generates sensor signal a0, which is used for B- motion 4. B- action generates sensor signal b0, which is used for A+ motion Above information is shown below graphically

Solution Example 1: Step 7Step 7 Draw the power circuiti) Draw the cylinders A(1.0) and B(2.0). ii) Draw the DCVs 1.1 and 2.1 in unactuated conditions iii) Mark the limit switch positions for cylinders A(1.0) and B(2.0).

Solution Example 1: Step 8Step 8 Draw the control circuit

Solution Example 1: Step 9

Step 9 Analysis of pneumatic circuit 1. When the start button is pressed, the signal appears at port 14 of valve 1.1 through limit switch signal b0. 2. Check for the presence of the signal at the other end (12) of valve 1.1. Notice that the signal is not present at port 12 of valve 1.1. (Because b1 is not pressed). There is no signal conflict and valve 1.1 is able to move. So A advances to forward position. 3. When cylinder A fully extends, it generates a limit switch signal a1, which is applied to port 14 of the valve 2.1. Cylinder B advances to forward position.

5. Check for the presence of the signal at the other end (12) of valve 2.1. Signal is not present at port 12 of valve 2.1 (because a0 is not pressed, A is already in extended position now) and hence there is no signal conflict 6. Signal applied to port 14 of the valve 2.1 causes the shifting of DCV 2.1 and cylinder B extends. 7. When cylinder B fully extends, it generates a limit switch signal b1, which is applied to port 12 of valve 1.1 . Cylinder A returns and ao is pressed. There is no signal conflict, as ao and a1 are mutually exclusive signals. 10. When the cylinder A is fully retracted, it generates a limit switch signal a0, which is applied to port 12 of the valve 2.1. Cylinder B retracts.

Example 2: Lifting and Shifting• Products are required to be transferred from lower level conveyor to higher levelconveyor using two Pneumatic Cylinders• Lifting Cylinder A lifts the product on receiving it at lower level• Shifting Cylinder B shifts the product from the platform to the higher levelConveyor•Lifting cylinder retracts•Shifting cylinder retracts

Example 2: Lifting and Shifting (continue….)

Motion Diagram Lifting and Shifting• Motion and Control Diagrams are shown for Lifting and Shifting Application:A+,B+,A-,B-• Signal 1.2 –Start Signal• Signal 1.3- Extended position limit switch for cylinder B• Signal 2.2- Extended position limit switch for cylinder A• Signal 2.3-Home position limit switch for cylinder A• NO SIGNAL OVER LAP

Example 2: Lifting and Shifting

Signal OverlapSignal Overlap can occur when simultaneously two active signals appear on both set and reset pilot ports of Final Control Valve. This is due to the required sequencing of cylinder. At the start, both signals ao and bo appear at the same time. This will not result in any change.

Illustration of Signal over lap

Example : Clamping and Riveting

Sheet metal components are to be riveted using two Pneumatic Cylinders. A Clamping cylinder (A) first advance and clamps the sheet metal parts. While the parts are clamped a second cylinder (B) advance and performs riveting operation. The riveting cylinder retracts and finally clamping cylinder retracts

• Control Diagram is drawn below the motion diagram represents the status ofvarious signals- the limit switches used to interrogate the piston position• Signals 1.4 and 1.3 correspond to home position and extended position limitswitches of cylinder A respectively• Signals 2.2 and 2.3 correspond to home position and extended position limitswitches of cylinder B respectively

Example : Clamping and Riveting (continue….)

Example : Clamping and Riveting (continue….)

Signal Over Lap at Step 1

Example : Clamping and Riveting (continue….)

Signal Over Lap at Step 3

Multi Cylinder Applications Signal Elimination

On analyzing the status of set signal and reset signal for final control valve fordifferent cylinders, it is observed that both set and reset signals could be presentsimultaneously at any instant of time, depending on the sequential operation of the cylinder. This does not permit further change in status of the valve. This situation is termed as signal over lap. To overcome this problem signal elimination techniques are used as listed below:

• Use of Idle return lever limit switches• Use of N.O Timers• Use of Cascading with the help of reversing valves• Use of Stepper Sequencer modules

Idle-Return Roller Valve

ROLLER LEVER LIMIT SWITCH IDLE -RETURN ROLLER LEVER LIMIT SWITCH

• Roller Lever type Limit Switch gives mechanical signal which can be sensed in both direction movement of piston rod cam• Idle-Return Roller Lever Limit Switch gives mechanical signal due to actuation of roller only in one direction. This is conveniently used in Signal Elimination

Idle-Return Roller Lever Valve (continue...)

An idle-return roller lever valve consists of a 3/2 DCV fitted with an idle return roller mechanism. The two designs of the idle roller is shown in

The idle return roller may be positioned in the control system so that when the cylinder extends, the piston passes over the idle – roller mechanism of the valve, thus activating the valve. (Figure a ), but also permitting the valve to be deactivated immediately when the piston moves to the extreme end position (Figure b). As a result, the valve generates a short output pulse during the forward motion of the cylinder. The idle return mechanism also allows the cylinder to retract without re-activating the valve (Figure c and Figure d). Hence, in the end position or during the return motion of the piston, the valve does not gets actuated, and no output signal is produced. For the generation of short output pulse by the idle-return roller valve during the return motion of the cylinder, this valve may be positioned in the opposite direction as compared to the case during the forward motion of the cylinder.

Idle-Return Roller Lever Valve (continue...)

Figure below shows the circuit for getting the control sequence 1A1+ 2A1+ 2A1- 1A1- using the idle –return rollers at the position 1S2 and 2S1. The roller valves at position 1S3 and 2S2 no need be replaced with the idle return rollers as these valves do not cause signal conflicts for the given sequence circuit.

The cascade method is simple to apply and results in reliable and easily understood circuits. Steps involvedStep 1:Each cylinder is given a code letter and theft sequence is determined. For example A+, B+, A-, B- etc. ‘+‘ and ‘-’ represent extension and retraction of the cylinder respectively.Step 2:The sequence is split into minimum number of groups. Care should be taken to see that no letter is repeated within any group. Next the circuit is drawing it up using the following steps.Step 3:Each group is assigned a pressure connecting line which must be pressurized only while that particular group is active. So the number of pressure connecting lines equals the number of groups.

Cascading Method Cascade definition : A series of stages in which the output of one stage is the input of the next stage

Step 4:Selecting the valves.a. The limit valves are denoted as a0, a1, b0, b1 etc. where the

suffix 0 corresponds to valves which are actuated at the end of return stroke and the suffix 1 corresponds to valves which are actuated at the end of forward stroke. Each cylinder requires two limits valves and it equals twice the number of cylinders. Each manifold line supplies air pressure to those limit valves within its particular group.

b. In order to pressurise the various connecting lines in the proper order, one or more group changing valves or cascade valves (double air piloted of 4/2 DCV or 5/2 DCV) are employed. The number of group valves always equals the number of groups minus one (=number of group – 1).

c. For each cylinder, a pilot operated direction control valve is selected. The number of cylinder acting valves equals the number of cylinders.

Cascading Method (continue….)

Step 5:The valves are connected as follows. The output of each limit valve is connected to the pilot input corresponding to the next sequence step with one exception. The limit valve corresponding to the last step of the given group is ‘not’ connected to the actuating valve of the next cylinder, but rather to the pilot line of a group changing valve so as to pressurise the connecting line of the next group. This connecting line is then connected to the pilot line corresponding to the first step of the next group.

Cascading Method (continue….)

Designation of Signals

Designation of Signals From Limit Switches

Cascading Stages

Development of Cascade Stages

Input Signals to Cascade Stages

Step 1: Write the statement of the problem: First cylinder A extends and brings under stamping station where cylinder B is located. Cylinder B then extends and stamps the job. Cylinder A can return back only cylinder B has retracted fully.

Step 2: Draw the positional layout.

Demonstration of Cascade method In order to develop control circuitry for multi cylinder applications, as done before in classic method, it is necessary to draw the motion diagram to understand the sequence of actuation of various signal input switches-limit switches and sensors. Motion diagram represents status of cylinder position -whether extended or retracted in a particular step

Cylinder A advancing step is designated as A+ Cylinder A retracting step is designated as A- Cylinder B advancing step is designated as B+ Cylinder B retracting step is designated as B- Given sequence for clamping and stamping is A+B+B-A-

Step3: Represent the control task using notational form

Step 4 Draw the Displacement –step diagram

Step 5 Draw the Displacement –time diagram

Step 6: Analyse and Draw Pneumatic circuit. Step 6.1 Analyse input and output signals.

Input Signals Cylinder A – Limit switch at home position ao Limit switch at home position a1 Cylinder B - Limit switch at home position bo Limit switch at home position b1

Output Signal Forward motion of cylinder A ( A+) Return motion of cylinder A (A-) Forward motion of cylinder B ( B+) Return motion of cylinder B( B-)

Step 6.2 Using the displacement time/step diagram link input signal and output signal. (Figure 1.31) Usually start signal is also required along with a0 signal for obtaining A+ motion. 1. A+ action generates sensor signal a1, which is used for B+ motion 2. B+ action generates sensor signal b1, which is used group changing. 3. B- action generates sensor signal b0, which is used for A- motion 4. A- action generates sensor signal a0, which is used for group changing Above information (given in 6.2) is shown below graphically

Step 7 Draw the power circuit (Figure 1.32) i) Divide the given circuits into groups. Grouping should be done such that there is no signal conflict. Do not put A+ and A- in the same group. Similarly B+ and B- should not be put in the same group. In other word A+ and A- should belong to different group to avoid signal conflict.

In our example of A+ B+ B- A- we can group as

ii) Choose the number of group changing valve = no of groups -1

In our example, we have 2 groups so we need one group changing valve Connect the group changing valve as follows. From the figure it is clear that when the control signals I and II are applied to group changing valve, the air (power) supply changes from Group 1(G1) to Group 2 (G2)

iv) Draw the power circuit