Pneumatic System in CNC

What is a pneumatic system?

A pneumatic system is a collection of components that includes a pneumatic pressure line, a control system, and actuators. These elements work together to form a complete pneumatic system. In a pneumatic system, pressurized air is used to drive various mechanical actions, such as moving a cylinder piston, rotating a motor, or operating other pneumatic actuators. These systems find applications in different industries, including construction, mining, automotive, CNC machines, and more. The block diagram below illustrates the components of a pneumatic system and how they function.

The pressure inside a gaseous substance increases when it is compressed and this compressed air can be used to perform different mechanical operations. It can be stored in a reservoir for later use. Equipment called an 'Air Compressor' is employed to generate pneumatic pressure, which is then stored in a container known as a compact air reservoir. The compressed air is then supplied to machinery through a pipeline and discharged into the atmosphere once it has been fully utilized. Sometimes, a large compressed air plant is located far away from a machine plant, and air pressure is provided to the machinery through pipelines.

The pneumatic pressure that comes out of an air compressor unit is typically maintained between 7 to 8 kg, while CNC machines typically require 5.5 to 6 kg of pneumatic pressure. Each machine is equipped with a pressure regulator unit in the incoming line to receive the necessary pressure. In a CNC machine, pneumatic pressure is used to perform light mechanical work such as opening doors and unclamping tools. This is accomplished using solenoid-controlled valves and cylinders. There are several advantages and disadvantages of using pneumatic pressure.

 Advantages of using pneumatic pressure:

• Air is readily available almost everywhere. 
• Pressurized air can be easily stored. 
• Air pressure lines can be routed over long distances. 
• Pneumatic systems operate quickly. 
• No return line is required. 
• There is no risk of explosion. 
• Generation cost is very low. 
• Pneumatic systems are clean and pollution-free. 
• They work within a wide temperature range.

  Disadvantages of using pneumatic pressure:

• Special care is required to remove dirt and moisture from the pneumatic pressure. 
• Cylinder piston movement does not provide uniform and constant speed. 
• Pneumatic pressure is only suitable for applications where modest force is required. 
• The exhaust sound of compressed air is very loud. 
• Silencers must be used with all valve exhaust ports. 
• Air, having low fluid resistance, tends to escape from any gaps or clearances.

 

Elements of a pneumatic system

A complete pneumatic system consists of the following four fundamental mechanisms:

 1. Air compressor unit 

 2. Air service unit

 3. Pneumatic control elements

 4. Pneumatic actuators

 

1. Air Compressor Unit

The air compressor unit is the most important component of a pneumatic system and is usually powered by a three-phase induction motor. The compressor intakes air from the atmosphere through a filter unit, decreases its volume, and compresses it to a higher pressure. Since a pneumatic system requires a continuous air supply, the compressed air is stored in an 'Air receiver unit or Reservoir' at a certain pressure level. The reservoir unit also helps in cooling the air and condensing any moisture present. The energy stored in the compressed air is crucial for the system's operation. The compressor unit stops automatically when it reaches a specific air pressure inside the reservoir and starts again when it falls to a certain level. A pneumatic pressure switch and a control mechanism are integrated with the compressor motor power to manage this system, turning the induction motor on and off. Different types of compressors are used in CNC machines, including reciprocating piston compressors, sliding vane rotary compressors, screw compressors, diaphragm compressors, etc. The following picture illustrates how a reciprocating piston compressor works.

Air Compressor Unit: A complete air compressor unit typically consists of the following components: 

• Motor and Regulation
• Air Filter
• Air Cooler
• Air dryer
• Reservoir tank

Motor and Regulation - The compressor unit is usually driven by a three-phase induction motor, with the size or rating depending on the compressor's capacity. Compressed air consumption fluctuates in a pneumatic system, so the compressor must be able to regulate its operation automatically. Pneumatic compressors employ various regulation systems, including exhaust regulation, shut-off regulation, and on-off regulation.

Exhaust Regulation - This is the simplest regulation system, where a compressor unit operates against a pressure relief valve. When specific air pressure is reached inside the receiver, a pressure relief valve opens, allowing excess pressurized air to discharge into the atmosphere. A check valve is also used to protect the receiver from emptying.

Shut-off Regulation – This system closes the suction side of the compressor, preventing it from extracting air from the atmosphere. The compressor runs in a vacuum, generating no air pressure inside the receiver unit.

On-off Regulation – In this system, the compressor unit is controlled by a control unit, starting and stopping based on the consumption of air from the receiver unit. Pneumatic pressure switches are used to ensure the pressure inside the reservoir remains at a certain level. There are two pressure level adjustment switches, P-min and P-max. An input signal from the P-min pressure switch activates a control circuit when the air pressure drops below the minimum level. It starts the induction motor connected to the compressor unit, which continues running until the P-max pressure is reached. The pressure switches allow adjustment of the minimum and maximum pressure inside the reservoir, typically maintained between 7 to 8 kg.

 

Air filter - The air filter unit prevents dust particles from entering the compressor by filtering the air from the atmosphere. It separates contaminants from the air and is fitted with the intake pipeline of the compressor.

Air cooler - As air is compressed, its temperature increases. If hot compressed air is transferred to pneumatic circuits, it can damage the components. Therefore, a cooling mechanism is always present in an air compressor unit to cool the compressed air. Small compressors use cooling fans, while larger compressors may have separate chilling units.

Air dryer - Moisture in compressed air is detrimental to pneumatic devices, as it leads to corrosion and compromises the lubrication of pneumatic components. To remove moisture, a compressor unit always employs an air dryer system.

Reservoir tank - A continuous air supply is required in a pneumatic system, but compressed air is generated gradually by the compressor unit. Therefore, it is stored in a reservoir tank. The tank helps to smooth the pulsating air flow from the compressor, cools the hot compressed air, and condenses any moisture present. In small air compressor units, all accessories such as the motor, compressor, cooling unit, and filters are arranged over the reservoir tank. For larger air compressor plants with multiple compressors, a separate reservoir tank known as the 'Air receiver' unit is installed outside the compressor units. Compressed air from different compressors is sent to the air receiver unit and distributed to the machines through pipelines. The picture below shows a reservoir tank and its components.

 

2. Air service unit

The air service unit is connected to the pneumatic pressure line and provides clean, moisture-free pneumatic pressure. It consists of an air filter, air regulator, and air lubricator units. The air service unit, also known as the FRL (Filter, Regulator & Lubricator) unit, is commonly used with CNC machines and other pneumatically operated machinery to supply moisture-free air to the pneumatic circuits.

Air filter - The air filter unit removes all contamination from the compressed air passing through it. Compressed air enters the filter bowl (usually made of transparent polycarbonate material) through a pipeline, and moisture and dirt particles are separated by the centrifugal force of the air. The accumulated condensate mixture (water and dirt) can be drained out using a drain cock release system located at the bottom of the bowl.

Air regulator - The outgoing pressure line from the filter unit passes through the regulator unit, which has a rotating knob for control. The regulator maintains a constant operating pressure for the pneumatic system, regardless of fluctuations in incoming air pressure and air consumption.

Air lubricator - The outgoing pressure line from the regulator unit further passes through the lubricator bowl. The air lubricator delivers a small and metered quantity of oil-mist through the entrance port of the air distribution system, ensuring that pneumatic components remain free from corrosion.

3. Pneumatic Control Elements

Various control elements or valves are used in pneumatic systems in CNC machines. The main function of these pneumatic valves or elements is to precisely control the pneumatic pressure lines to ensure the proper functioning of different actuators. These control valves regulate, manage, and monitor the direction of flow and pressure. Sometimes, they are also used as interlock and safety devices between pneumatic pressure lines. Pneumatic valves can be categorized into the following types based on their design, functioning, or the type of work they perform:

• Direction control valve
• Non-return valve
• Shuttle valve
• Flow control valve
• Pressure control valve

Direction Control Valve - Similar to hydraulic systems, a direction control valve controls the movement of pneumatic pressure flow in a specific channel. It opens, closes, or diverts the compressed airflow to ensure the proper functioning of the pneumatic actuator. Pneumatic directional control valves can be classified into groups based on the following criteria:

Number of ports: 2, 3, or 5-way
Number of positions: 2 or 3
Method of actuation: Manually or electrically operated
Method of retrieval: Spring or air return

One main difference between pneumatic valves and hydraulically operated directional control valves is that pneumatic valves do not have a tank line. Instead, the return air pressure is exhausted directly to the atmosphere through a silencer. Pneumatic valves are usually lighter than hydraulic valves as they are made with aluminum alloy, while hydraulic valves are made with iron. The following picture shows an electrically operated five-port and two-position (5/2 directional control valve) sliding spool valve and its inner parts. These valves are commonly found in CNC machines, where they function with double-acting cylinders.

 

    

Non-return valve - A non-return valve is used to allow pneumatic pressure flow in one direction while blocking it in the opposite direction. Different types of non-return valves are used in pneumatic circuits, such as check valves, shuttle valves, and quick exhaust valves. Typically, a check valve is used in pneumatic circuits of a CNC machine. A check valve effectively blocks the pneumatic pressure from one side and allows it to pass freely from the other. Pneumatic check valves come in different structural configurations, including plug type, ball type, plate type, and diaphragm type. The following picture shows a typical ball-type pneumatic check valve and its inner structure, commonly found in CNC machines' pneumatic circuits. 

Shuttle valve - A shuttle valve is a three-port valve with two incoming pressure ports and one output port. The shuttle valve switches the pressure entering either of the input ports. When pressure is present in both input ports, the higher pressure takes preference and passes out through the output port. The following picture shows a pneumatic shuttle valve, where the air pressure at 'Input-1' is higher than at 'Input-2'. Hence, the output pressure will be equal to the air pressure at Input-1.  

Flow control valve - A flow control valve regulates the compressed air flow through a passage to control the actuation speed of an actuator. This type of valve is also sometimes referred to as a speed controller. Flow control valves are used in pneumatic circuits to manage the speed of an actuator, such as a cylinder stroke. Sometimes, a flow control valve is combined with a check valve to make it unidirectional, allowing unrestricted flow in one direction only. Based on their structure, flow control valves can be of two types: variable restrictor or manifold type, and variable restriction or inline type. The following picture shows an 'inline type' simple pneumatic flow control valve, where the airflow is controlled by a movable poppet that rotates with a control knob.

Pressure control valve -  The pneumatic system pressure often needs to be regulated in different ways using various pneumatic pressure control valves. These valves can be categorized as pressure-limiting valves, pressure sequence valves, and pressure-regulating or pressure-reducing valves, depending on the type of work they perform.

Pressure limiting valve – It allows the pneumatic pressure to be fixed at a specific level and prevents it from exceeding that level. A pressure limiting valve is typically used with the compressor unit to ensure the receiver pressure stays within safe limits. It has an automatic safety release function that mechanically opens and releases excess pneumatic system pressure into the atmosphere. After releasing the excess pressure, the valve is plugged again by spring tension.

Pressure sequence valve  – The principal function of a pressure sequence valve is similar to a pressure limiting valve, but it is used for different purposes within a pneumatic circuit. The output pressure line remains closed until the incoming pressure reaches a specific level. Once the incoming pressure to the valve reaches a pre-defined level, it opens the output port, and the air pressure becomes available in the output line. A pressure sequence valve is applied when precise pressure control is required for the pneumatic actuator. The actuator stops working if there is insufficient pressure in the incoming line.  

Pressure-regulating/reducing valve – This valve ensures a constant and precise output pressure in a pneumatic circuit. The pressure available at the output port of the valve does not fluctuate with minor changes in incoming air pressure. The valve includes elements like integral loading, sensing, actuating, and control, which work harmoniously to provide a constant pressure in the output line. Pressure-regulating valves are usually classified into three types: general-purpose, special-purpose, and precision valves. General-purpose pressure regulating valves are used in CNC machines, while special-purpose and precision valves are employed for accurate or precise pressure control systems only.

4. Pneumatic actuator

A pneumatic actuator converts the energy of compressed air into mechanical motion. When the volume of gaseous elements, such as air, is reduced through compression, the pressure inside the substance increases, allowing it to perform various mechanical work. Compressed air can be accumulated in a reservoir for later use. In CNC machines, pneumatic actuators are used for various functions, such as automatically opening and closing the machine door or changing the arm movement of a cutting tool. Since pneumatic pressure typically ranges from 5 to 7 kg, pneumatic actuators are generally suitable for overcoming light loads. For heavier weights, a large-diameter cylinder piston is required. Pneumatic cylinder bodies are usually made of aluminum or its alloy, making them lighter than hydraulic cylinders. Pneumatic actuators can be either linear or rotary types, similar to hydraulic actuators.

Pneumatic linear actuator -  Pneumatic linear actuators refer to a range of pneumatic cylinders that have different categories depending on their construction and type of operation. The mechanical energy available with a pneumatic cylinder is lower than that of a hydraulic cylinder, as the compressed air pressure is lower than the hydraulic pressure. Therefore, the structure of a pneumatic cylinder is always more lightweight compared to a hydraulic cylinder. If the same amount of work is performed using a pneumatic cylinder instead of a hydraulic one, the barrel diameter of the pneumatic cylinder needs to be larger. Using pneumatic cylinders is generally more convenient in mechatronics systems. However, for applications requiring a steady force and operating with a fluctuating load, a hydraulic cylinder is often preferable over a pneumatic cylinder. The following picture shows a pneumatic cylinder and its interior, commonly used in mechatronics systems.

    

The response time of a pneumatic cylinder is instantaneous, but using it also has some disadvantages. The piston thrusts considerably at the end covers when the cylinder completes its stroke, which can lead to cylinder damage. To overcome this problem, almost every pneumatic cylinder includes a cushioning system. This system reduces the piston's movement when it reaches the cylinder's edge. Pneumatic cylinders have two types of cushioning systems: fixed and adjustable. The fixed cushioning system is employed in lower-diameter pneumatic cylinders, while the adjustable cushioning system is used when the cylinder piston speed is relatively high. Single-acting, double-acting, and rodless pneumatic cylinders are commonly used in different mechatronics systems.

 

        Single-acting pneumatic cylinder - The operation of a single-acting pneumatic cylinder is similar to that of a hydraulic cylinder. It consists of a piston seated inside a cylindrical barrel, and a rod attached to the piston moves with the piston's movement. The piston displacement is achieved by air pressure, and the retraction is accomplished through the compression or expansion of spring tension. These cylinders also have fixed or adjustable cushioning systems. Like hydraulic cylinders, single-acting pneumatic cylinders are available in two varieties: push-type and pull-type. The following picture shows a push-type single-acting pneumatic cylinder.  

        Double-Acting Pneumatic Cylinder - This type of cylinder also functions similarly to a hydraulic cylinder. Double-acting pneumatic cylinders come in two types: with a piston rod on one side and with a piston rod on both sides. Both types of cylinders are equipped with a cushioning system. The following pictures display both types of double-acting pneumatic cylinders. 

     Rodless pneumatic cylinder - This type of cylinder operates differently from the basic cylinder operation. It does not have a piston rod connected inside the cylinder piston, but rather it is coupled with an outer load-carrying cartridge through a magnetic or mechanical coupling. Rodless pneumatic cylinders are available in three types: cable, slotted, and magnetically coupled cylinders. A cable attached to the piston and pulleys on each end connect the carrier to a cable cylinder. The slotted cylinder has a slot in the cylinder's length with an inner and outer metal band to prevent leakage and provide a direct mechanical linkage to the carrier. A magnetically coupled cylinder has a magnetic link between the piston and the carrier, making it a leak-free cylinder. The following picture shows a magnetically coupled rodless pneumatic cylinder and its interior. 

Pneumatic rotary actuator - A pneumatic rotary actuator utilizes compressed air to generate operating energy and provide a turning or angular movement, allowing for stroke in an oscillating motion over a defined angle. Pneumatic rotary actuators can be found in two types: continuous rotary movement and limited rotary movement. Continuous rotary motion actuators are sometimes referred to as air motors, as they provide a constant rotary motion along with a steady pneumatic pressure line. There are several types of air motors, with the most commonly used being piston motors, sliding vane motors, and turbine motors. The following picture shows a sliding vane pneumatic motor. 

Limited movement pneumatic rotary actuators allow for greater torque, and the standard rotations of these actuators are usually at 90°, 180°, and 270° angles. There are three types of limited movement pneumatic rotary actuators: vane, rack and pinion, and helix spine. The following pictures show a rack and pinion type limited movement rotary actuator.