Modifications to conventional components have moved pneumatic systems into the world of highly accurate motion control.
Humphrey Products Co.
Although pneumatics are known for their high speed, the compressibility of air has, until recently, impaired the accuracy of pneumatics for control applications. As a result, pneumatic systems were previously limited to on-off functions in motion applications. Proportional control was the domain of electronics, especially in automated assembly processes where accuracy and repeatability are paramount. Electromechanical control systems, however, have become increasingly complex to set up and maintain, even to the extent of making the cost of controlling the assembly process a factor in product design.
Today, however, improved pneumatics hardware has greatly improved the accuracy and versatility of pneumatic control systems. Manifolds, valves and sensors now can be linked directly to a PC to create electropneumatic systems with positioning accuracy comparable to electromechanical systems. Proportional-control valves, together with rodless cylinders, can produce accurate, repeatable motion.
Pneumatic systems offer low maintenance, long service life, and proven reliability. They also withstand dust, shock, vibration, and extreme temperatures. Pneumatic circuits are easy to build and standard modular components produce complex motions. Adjustments and troubleshooting require little technical expertise. All of these benefits mean lower initial and ongoing expenses.
Electropneumatic systems also are relatively simple. Input signals from a PC generate output from a proportional-control valve, producing accurate, repeatable motion. Flow changes can be made by changing the voltage input to the proportional controller. Set-point changes are also easy to make. Just move the sensor or magnet. To increase a system’s versatility, install multiple sensors at all anticipated positions, then activate or deactivate them from the PC.
Pneumatics produce inherently smooth movements at fast speeds, and systems can be designed to precisely accelerate and decelerate loads. In addition, pneumatic systems, unlike their electromechanical counterparts, are safe to use near explosive vapors. They also are ideal for sterile environments, unlike hydraulics.
THE ELEMENTS OF A PNEUMATIC SYSTEM
Components of pneumatic systems include computers, controllers, valves, actuators, sensors, manifolds, serial relays, and tubing. In a typical electropneumatic circuit, a computer sends a signal to a controller which sends electrical current to a solenoid to initiate movement of an actuator. The solenoid may be an integral part of the actuator, part of a separate valve that supplies the actuator, or part of a pilot valve that controls another valve. Most pneumatic circuits also have positioning sensors that send an input to the PC when a system component or workpiece reaches a set point. The output from the PC then actuates a component such as a valve, cylinder, rotary actuator, gripper or vacuum cup.
Valves are fundamental components of any pneumatic system. A valve has an internal element, such as a spool, poppet, or plate, that selects which outlet ports will be pressurized, exhausted, or blocked. These valve elements, called operators move mechanically, electrically, or pneumatically.
Manual operators, controlled by humans, include hand levers, push buttons and pedals. Electrically operated valves use a solenoid to convert electrical current into mechanical operation. Direct-operated valves have the solenoid coupled directly to the valve operator. These valves have several advantages. For example, direct-operated valve actuation is independent of system pressure. Supply or exhaust restrictions do not cause valve malfunctions. And directoperated valves with balanced spools remain in position even when pressure fluctuates.
Two-position valves with single-solenoid operators use air pressure or a spring to return the operator to its normal position. Three-position, direct-operated solenoid valves use solenoid forces to shift the operator from the center position to either extreme. Spring forces or pressure imbalances return the operator to the center.
Pilot-operated valves, on the other hand, use air pressure to move the operator. In these designs, pneumatic pressure is ported to the spool ends or to separate pilot pistons. Pilot-operated valves are used with air logic controls, in remote locations, or when explosives are present. Pilot-operated valves can generate high forces to shift the operator.
Internally piloted valves use pneumatic energy from the valve’s pressure port to move the operator. These valves, however, have minimum and maximum pressure limitations. Externally piloted valves allow finetuning of the pressure input so that the valve can be moved at a selected speed.
When high valve pressure shifts the operator, correspondingly high velocities and impact forces can shorten valve component life. But large valves often require high shifting forces. While direct-operated solenoid valves are an option, their high forces require large solenoids which, in turn, demand high currents. A better choice is a solenoidcontrolled pilot valve. These valves provide high pressure and use small solenoids. They draw low currents and reduce the mechanical pounding that causes component failure.
Until recently, the compressibility of air limited the accuracy and repeatability of pneumatic systems. The high supply pressures that produced the speeds necessary for efficiency also rapidly compressed air inside the actuator, causing a springback effect when actuators reached their stopping point.
This drawback is overcome by proportional control valves, which gradually reduce actuator speeds as they approach their stopping points. The valves accept an electronic input from a PC and emit a proportional pressure output. When a moving object passes a set point, a sensor sends a feedback signal to the PC, which then lowers the voltage input to the proportional controller. This produces a proportional air pressure reduction to the actuator. Pneumatic systems with proportional control valves maintain repeatable accuracy because the actuator’s reduced approach speed minimizes air compression.
Another component in any pneumatic system is the actuator, which provides linear motion, rotary motion, or a combination of both. Cylinders, rotary actuators, and grippers are examples of pneumatic actuators. When the resulting motion is linear, the actuator is called a cylinder.
Cylinders are either single acting or double acting. In single-acting cylinders, air pressure produces force in only one direction, either extension or retraction. An internal spring returns the piston to its normal position. In double-acting cylinders, air pressure produces rod movement in either direction.
One drawback of rod cylinders is their lack of accuracy. Variable air pressure on each side of the piston produces motion, while equal air pressure on both sides holds the piston in the desired position. Most pistons in rod cylinders have a flat face on one side and the rod connected to their other face. Air pressure is a function of the face area of each side of the piston. The unequal areas of each piston face make it difficult to balance the pressure on each side of the piston, preventing accurate motion.
Rodless cylinders, on the other hand, have a slider containing a magnet that moves on a stationary cylinder, which has an internal piston also containing a magnet. The two magnets cause the slider and piston to move together so that when air pressure moves the piston, the slider moves with it. Rodless cylinders occupy less space than conventional cylinders and can stop at an infinite number of intermediate positions. They also are accurate, unlike rod cylinders, because each piston face has an equal surface area.
Rotary actuators have an internal piston with a rack that meshes against a pinion fitted on the output rod. Air pressure moves the piston, turning the rod.
Grippers are another type of actuator. They have angular or parallel mechanical jaws that hold workpieces. Devices, such as magnets, or pneumatic screwdrivers are mounted to gripper jaws. When combined with cylinders, grippers accurately perform complex, multiaxis, robotic functions.
A vacuum cup is another device that is frequently used in industry. Vacuum cups are a simple, low-cost and reliable technology for lifting, holding and moving objects weighing between a few ounces and several hundred pounds. The vacuum cup adheres to a surface when the surrounding atmospheric pressure is higher than the pressure between the cup and the surface. The low pressure is created by connecting the cup to a vacuum pump. The cups are easy to install and maintain, and they quickly attach and detach without damaging the workpiece. One of their few limitations is requiring a smooth surface on the workpiece.
A sensor is another important component in a pneumatic system. Sensors provide a feedback signal to a controller, which then adjusts the flow to the device being monitored. Most sensors use a magnet to activate a circuit when the device reaches a predetermined set point. For example, a rodless cylinder might reach set points at the left end, right end, and middle of its stroke. The output signal generated at each set point could then become an input for another actuator, such as a gripper. Sensor positioning is often as simple as loosening a nut and repositioning the sensor.
Pneumatic systems with several valves and actuators often use manifolds to simplify the arrangement. Manifolds are completely assembled valve modules. When systems contain several manifolds, an electrical device called a serial relay can link them all to a computer. A serial relay provides a direct connection between a PC and up to 32 manifolds. One cable from the PC to the serial relay routes signals to individual valves. The cable can control up to 512 valves, and individual valves can be changed in seconds without disturbing other connections. All processes can be controlled from as far as 10,000 ft.
DESIGNING A SYSTEM
The first step in designing a system is in determining how the workpiece is to move. Motion should be defined based on process, distance, velocity, force or torque, required positioning accuracy, and beginning and end object orientation. Use these parameters to calculate the system requirements, including load, velocity, inertia and stroke length. You can then calculate the required flow based on these values. Finally, size the pneumatic components to achieve the desired force, use electronic packages to control response times, and select sequencing to turn air on and off or redirect the air path. Remember that higher speeds hinder precise positioning due to the compressibility of air, while lower speeds allow precise positioning.
The first component to select is the actuator. The type of motion — linear, rotary, or a combination — determines which actuator to use, commonly a cylinder. Cylinder selection depends on the desired output force and the air supply pressure. First determine the load that must be moved. When the object is moving vertically, the load is simply the weight of the object. When the object is moving across a surface, multiply its weight by the coefficient of friction to determine the load. A cylinder that develops 100 lb of force, however, can balance a 100-lb load, but cannot move it. An air cylinder must be oversized to move a load. A rule of thumb is to oversize by 25% to move the load at low speeds, and oversize by at least 100% for high speeds. Low and highspeed classifications are general values that depend on the type of application. A low speed in one industry may be excessively high in another.
When precise calculations are necessary, a theoretical value called a load ratio is used to oversize the cylinder. A load ratio of 0.70 is typical for medium cylinder rod speeds. When the rod needs to move faster, use a smaller load ratio. Increase the load ratio for lower rod speeds. First, calculate an adjusted output force as:
where Fa = adjusted output force, lb; F = balanced output force, lb; and R = load ratio. The adjusted output force is used to calculate the bore diameter. First the piston’s face area is calculated from the equation:
where A = piston face area, in.2; and P = input pressure, psi.
where D = bore diameter, in.
Using this value, select the proper cylinder from the manufacturer’s data. When this bore diameter doesn’t match a stock size, choose the next greater size.
With rotary actuators, torque, angular velocity, and stopping capacity are the design parameters. Total required torque depends on working torque and angular acceleration.
To determine the required torque, use the equation:
T = Jm × a
where T = torque, lb-ft; Jm = polar moment of inertia, in.-lb-sec2, and a = angular acceleration, rad/sec2.
The system must also stop the load once it is moving. The stopping capacity of the actuator must balance the kinetic energy of the load, which is calculated as:
K = 0.5Jmo × ω2
where K = kinetic energy of the load, in.-lb; Jmo = moment of inertia of the load about the fixed axis, o, lb-in.-sec2; and ω = angular velocity, rad/sec.
Finally, select a method for decelerating the load. Common options are an internal air cushion, a shock absorber, or a proportional control valve. Never use the internal stop of an actuator to decelerate the load because damage will result.
With grippers, the maximum load capacity depends on the size of the object being lifted, texture of the object, velocity of the gripper, working air pressure, and shape of the gripper fingers. Heavier loads require larger grippers and should move at lower speeds. Use smaller grippers for lighter loads or when the part moves fast. Try increasing air pressure when grippers cannot lift the load at the required speed. When the gripper has internal springs, check the direction of their force, and include the spring force in the total grip force.
Airflow must be regulated to control the actuator speed. Flow adjustment is necessary to maintain positional accuracy since operating conditions vary. These conditions change due to internal fluctuations, such as variable supply pressure, and external changes, such as variable clamping forces for different workpieces.
Manual control valves have a knob, allowing a human operator to make adjustments to the system. For basic circuits, a pneumatic flow-control valve provides sufficient regulation. Other flow-control devices use pressure input and output valves to provide the required control. Proportional flow-control valves should be used when systems require higher accuracy. The actuator volume and the desired output force determine the valve size. The type of actuator and the combination of actions determines the valve type.
Tube selection is also important when designing a pneumatic system. Properly sized, unrestricted supply lines help air travel unhindered. Excessively small supply lines, on the other hand, restrict airflow and often cause the pressure in components such as pilot valves to drop below their minimum operating values.