February 2011 Archives

Pneumatic circuits and control technique basics are explained based on a scenario of a factory using LCA. In the #066, an outline of Clean Air Systems was discussed. Explained in this volume are the devices placed downstream to the Clean Air System such as air control devices including directional valves and velocity control valves, as well as JIS standard symbols signifying pneumatic actuators (air cylinders, etc.).

(1) Basic Pneumatic Circuit

A typical pneumatic device system is shown in [Fig.1].

The Clean Air System explained in #066 corresponds to the Air Quality Control Devices section in [Fig.1] above. The compressed air generation devices are located upstream to the Air Quality Control section, and the flow control devices and actuators are placed downstream of the Air Quality Control section.
The air pressure generally used is 7kgf/cm² or less. The compressed air is cleaned of oil, water, and stored in the air tank, then further cleaned by the FRL unit, and supplied to the air cylinder through a regulator (pressure adjustment valve).
The system portion from the compressed air generator to the air quality control section is similar to water service supply system. The waterline pressure is equivalent to air pressure. The LCA are driven by pressured air derived from the air quality control section which is analogous to a water faucet.

(2) Symbol Description of Basic Air Circuits

The system in the [Fig.1] is described a compressed air circuit diagram using the symbols defined by "JIS0125 Graphic symbols and circuit diagrams". [Fig.2] below is a graphical symbol representation of the [Fig.1].

From the figures [Fig.1] and [Fig.2], it can be observed that easily and accurately describing compressed air device systems is possible by mastering the basics of compressed air symbols and air circuit diagram drawings. Please refer to JIS standards documentation, etc. to become familiar with the compressed air system symbols.

For multi-action cylinders composed of multiple air cylinders, there are Dual Rod Type, Multi-position Type, Dual Type, Lateral Load Resistant Type, and Long Stroke Type. Discussion here will be on force direction control, 2 and 3 step controls with a multi-action cylinder ([Fig.1]) constructed with two back-to-back connected cylinders.

(a) Multi-point Positioning Example with Force Direction Change

The dual multi-action cylinders are typically used where large forces are needed, or space is constrained in radial but not in axial directions.

(b) Positioning Methods of Other Air Cylinders

	-	Air Cylinder with a Brake Types of air cylinders that can be stopped at arbitrary positions
	-	Air Cylinder with a Positioner Types of air cylinders with position sensors that can be positioned proportionally to input signals

Electromagnetic valves control the compressed air supply that generates power to air cylinders. By controlling the electromagnetic valves with sequential programming, compressed air supply timing sequences and pressure can be controlled and the air cylinder's actuation directions, stop positions, and speeds can be controlled. In this discussion, multiple point positioning and force control of air cylinders are explained.

Position Control Example

A cylinder type with two air cylinders connected in series is called Multi-position Cylinders ([Fig.1]). These are capable of two positions as well as increased force control.

(a) Two Position Control

Positioning to two locations is possible by changing which of the three inlet ports the air is supplied to.

(b) Obtaining a Large Force

Force can be doubled by supplying air to Port B and Port A.

There are Dual Rod, Multi-position, Dual, Lateral Load Resistant types of multi-action cylinders.