VAULT
Pneumatic Logic—Present and Future
Fluidic logic had found a home in specialized areas, such as sensing applications. But a specialist in the field looked ahead to electropneumatics when this article was first published in March 1985.
Written by Jay R. Haertel, Air Logic, Racine, Wis.
THE BASIC PNEUMATIC CIRCUIT consists of a control circuit, power supply, input to the control circuit, and finally, an output from the control circuit. The power supply is the fuel to be used by the control circuit and the input devices are the sensors that provide the information to the control circuit. The control circuit converts the information from the sensors to the output device which completes a specific operation.
The control circuit is the intelligence of a pneumatic logic circuit. It consists of a logic board for controlling a sequence of operations and timing.
Power is supplied by an air compressor or reservoir for fluids or gas. A dryer and filter prepare the air for its use. Regulators control the air supply and pressure gauges monitor the air.
Inputs to the control circuit can be an array of sensors: back pressure sensor, proximity sensor, interruptible jet sensor, and contact sensors. Other input devices are air-operated pushbuttons and solenoid valves.
Output from the control circuit is the end process. These interfacing devices usually consist of pressure to electric switches, air cylinders, actuators, and visual indicators.
There are two basic types of control circuits: fluidics and moving-part logic.
FLUIDICS
Fluidics had their birth in 1959 at the Harry Diamond Laboratories. The word fluidics is derived from fluid logic.
Fluidic controls have no moving parts, an advantage which extends the life of the controls. Fluidics rely on changing the direction of air streams to achieve an ON or OFF position. They provide a very reliable method of sensing that is simple to use and inexpensive for simple control circuits. In the mid-60s, fluidic logic seemed to be the answer to all control problems. Unfortunately, the technology was ahead of its time and fluidic logic was oversold by its proponents who failed to properly educate its users. The most important requirement of a fluidic circuit is clean air, which was almost unheard of at that time. Clean air was found only in laboratory environments.
During the 70s electronics invaded the market and their low cost helped fluidics to fade away. Although fluidic logic has not disappeared, it has found a home in specialized areas. It is best used in sensing applications. Presently fluidic logic is being used in the large medical and instrumentation equipment markets. It is used in bottle-filling equipment and in explosive environments, and ironically, fluidic logic is used on assembly equipment for electronics.
Fluidic logic is also popular in Europe, because electronics have been slow to develop there and fluidics have had a chance to gain a strong foothold.
MOVING-PART LOGIC
The second type of control circuit is moving-part logic. Moving-part logic was developed in 1964. While fluidics were making the headlines, it took until 1970 for moving-part logic to gain a foothold. Moving-part logic is available in several variations: a miniature spool-type air valve, packless spool valves, and diaphragm pilot-operated valves. These valves respond to jets of air shifting the spool that does the controlling.
The advantages of moving-part logic are that it can be used in hazardous locations or wet, dirty environments and is highly reliable without failures. Today it can be used on any air-operated machine, especially automated material-handling equipment and medical and dental equipment, and it is also used in the textile and electronic industries.
ELECTROPNEUMATICS
The future of pneumatic logic lies in the area of electropneumatics—linking the best part of electronics to the best part of pneumatics. Several manufacturers have already begun this process by mounting the pneumatics on an electronic printed circuit board. This is only the beginning.
Electronic systems can handle complex control circuits with simplicity and can handle long time delays and multiple time delays with accuracy. Pneumatic supply and output devices are superior to electronic ones and the pneumatic devices last longer. A simple pneumatic control circuit is less expensive to use than an electronic one and with short time delays pneumatic devices are also cost effective. Pneumatics work better in sensing with air jets.
Electropneumatics will play an important role in the area of input an output devices, where the electronic and pneumatics will be linked together to form one part. Some of these devices such as pressure to electric switches, solenoid valves, voltage control regulators, voltage control needle valves, and digital readout pressure and flow gauges, are no being manufactured.
The future will bring smaller devices with reliable electronics and lower cost. Vacuum will play a very important role creating a new area—electrovacuum—there is a very limited choice of vacuum devices in the market today. Electropneumatics and electrovacuum will bring a totally new concept to automated industry.
Jay R. Haertel of Racine, Wis., was part of a group of employees who later purchased Air Logic in 1993.
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