PRACTICAL AUTOMATION: Building cost-effective pneumatic circuits

The development and marketing of electronic technology has made a tremendous impact, not only on our personal lifestyles, but also on the manufacturing methods that are used in industry. Nowhere is this more predominant than in the use of computers and PLCs to control industrial machinery. Some of the smaller, dedicated sequential controls, used in many pneumatic cylinder applications, can however, still be more economically applied by using simple pneumatic logic.

Pneumatic logic circuits are primarily based on air-piloted valves costing a fraction of solenoid-operated equivalents–and the entire control system uses only one operating medium, compressed air. As we develop a few of these simple pneumatic circuits, I am sure you will identify a number of other benefits of value to you.

A number of applications require an air cylinder to cycle back and forth between two set points, sometimes rather slowly–such as in a wire guide system for winding spools of wire. The action can be accomplished with the use of limit valves to detect the two ends of the stroke.

Sometimes limit valves can be replaced by two normally open 3-way valves, pilot-operated, with a spring return. A standard 4-way, double-piloted directional valve is used to power the cylinder with two flow controls mounted as close to the cylinder ports as possible to control the exhausting air. The pilots of the two normally open valves are “tee’d” into each of the output lines to the cylinder, between the flow controls and the cylinder. The output from each of these valves goes, of course, to the respective control pilots on the power valve.

The secret to the circuit is to realize that the only time that the pressure falls to zero in the line between the flow control and cylinder port is when the piston stalls and the exhausting air escapes past the flow controls. This piston motion is usually stalled by contacting a positive stop block or bottoming out against the end of the cylinder. At this point, the particular 3-way valve springs to the open position, shifting the power valve in the opposite direction (Fig. 1).

This circuit is commonly used to operate paint stirring paddles or other liquid agitating devices. It can be modified to indicate when an air clamp is in position and holding with full power. An added benefit is its compatibility with explosive atmospheres.

Three-input “and” circuits

Many control systems require the assurance that a number of conditions are met before another action takes place. A typical example might be ensuring that each operator on a three-operator machine is standing far enough from the machine to allow its safe operation. The use of a three-input “and” circuit will give an output only when pushbuttons A and B and C are depressed. Remove any of the inputs and the output will disappear (Fig. 2).

Priority lockout signal

Conveyor systems are notorious for jam-ups when two feeder conveyors converge into one line. If outputs A and B control brakes or directional flappers on feed lines A and B respectively, the priority circuit below will prevent jam-ups. A study of the circuit will show that there is at least one line operating at any time (Fig. 3).

The binary redirect circuit

We have all encountered, I’m sure, an electric light socket with a pull chain. Pull the chain once and the light goes on. Pull it again and it goes off. This is a simple device doing a job that is slightly more complicated to do with pneumatic circuitry–it’s known as the binary redirect circuit or single-input flip-flop.

Fig. 4 shows a relatively simple way of achieving the single-input control of a double-acting air cylinder.

The first output from valve E provides an air signal through valve B to pilot A1 and through shuttle valve C to hold B1 activated as long as E is maintained. Power valve A shifts to cause the cylinder to extend and provide a signal through shuttle valve D to B2. B2 will be unable to shift valve B until the signal of B1–caused by pushbutton E–disappears.

When you remove your finger from pushbutton E, B1 and A1 are de-energized, allowing B2 to shift valve B (now with no supply) to the other position, ready for the next signal. The circuit is now stabilized with the cylinder extended.

When pushbutton E is depressed again, a similar series of events will occur to cause the cylinder to retract.

These four circuits need not be used only by themselves. Quite often they are used as integral parts of larger circuits to control complete dedicated machines. We will develop these concepts further in the next column.

Ted Grove is corporate training manager for Wainbee Limited, Mississauga, Ont., and an widely experienced fluid power trainer. His email address is tgrove@wainbee.com Practical Automation is a regular column in Machinery & Equipment MRO. Previous columns can be viewed on our website at www.mro-esource.com.

POWER TRANSMISSION INDUSTRY TO LEARN ABOUT TOOLS FOR SUCCESS AT OCTOBER CONVENTION

The Power Transmission Distributors Association’s (PTDA) upcoming annual convention–a networking and educational event for the power transmission/motion control industry–will be held Oct. 4-6, 2001, at the Atlanta Marriott Marquis in Atlanta, Georgia.

The PTDA 2001 Convention will offer a unique mix of educational sessions, opportunities to meet with potential business partners and peer networking. Note: The convention is open only to PTDA members and qualified prospective members.

This year’s theme, Focus on Growth: Trends and Tools for Success, is designed to provide companies with tactics and strategies to help grow their companies.

Sessions will include in depth presentations on potential growth markets, investing in tools to effectively enhance business operations and how distributors and manufacturers can team up to deliver value to end users. Other highlights include the popular manufacturer-distributor idea exchange (MD-IDEX), an independent distributor forum on “Finding the Money to Grow,” scheduled time for private business meetings and an optional golf outing.

The event has been organized this year to minimize time out of the office. For further details, contact Beth Silas at 312-876-9461, or visit PTDA’s website at www.ptda.org.