Actuators are used to move tooling on a machine, usually for the purpose of controlling the movement or positioning of a workpiece or sensor. They may be of a linear or rotating nature or a combination of both. Linear actuators are often used to generate a rotary movement by pushing a rotary pinned on an axis, or rotating devices such as motors can be used to generate linear motion via a belt or ball screw.
Applications of these actuators are further discussed in sub-section 7.
A word on the nomenclature used in actuation. The words home, advanced or extended, returned or retracted are often used to describe the position of an actuator or its tooling. Great care must be taken to identify whether the designer is speaking of the tooling or the actuator itself. These positions can be the opposite of each other and cause physical rework and software changes if misinterpreted. It is preferable to refer to the position of the tooling generally since it is the most easily identified by maintenance or operators.
Descriptions such as "Tooling Raised" or "Pallet Stop Extended" can help reduce the ambiguity of generic movement labels for both electrical and mechanical designers.
5.1 Pneumatic and Hydraulic Actuators and Valves
Collectively the use of pneumatic and hydraulic energy is known as fluid power. The operation of actuators in fluid power applications is similar in the flow of liquids or gases through the systems; however, pneumatic systems use easily compressible air (or other inert gases) while hydraulic power is generated by the flow of much less compressible fluids, usually oil.
Pneumatic and hydraulic actuators may be linear or rotary in nature. Air cylinders generate a linear motion by injecting air through a port on one side or the other of a rounded piston surface inside a tubular housing. As air is injected through a valve into one end of the cylinder, the same valve releases air from the other side. A diagram of the internal configuration of an air cylinder is shown in FIG. 39. The end of the piston rod is threaded for attachment to various tooling pieces, such as a clevis or ball end.
Single acting cylinders use the force provided by air to move in one direction (usually out or "advanced") and a spring to return to the "home" or retracted position. Double acting cylinders use the air to move in both extend and retract directions. They have two ports to allow air in: one for the outward stroke and one for the return stroke.
For a typical cylinder, the round piston face is attached to a rod extending through the end of the cylinder body. Some cylinders have a rod attached to both faces of the piston face and extending out through both ends of the body. These are sometimes called double ended or reciprocating cylinders.
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FIG. 39 Pneumatic
cylinder diagram. Retract "B" Port
Piston Seals Extend "A" Port Piston Rod; Rod Seal, Cushion
Adjustment Screws
FIG. 40 Guided air cylinder.
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Air cylinders are specified by their bore, or piston diameter, and their stroke, or how far the end of the shaft moves. Other specifications such as cushions to slow the last portion of motion, port sizes, and mounting method are also usually included in the part number. Sizes may be specified in both metric and standard measurements. Since stroke is specified in increments, stroke distances are sometimes limited by using shaft collars or limiting the movement of the tooling with stops. When this is done, the cushion may no longer be useful as it is at the farthest reach of the cylinder's stroke.
FIG. 40 shows a very long stroke guided cylinder. These have bearings in a guide block that take the side load off the piston rod and ensure that force is applied linearly. Guide blocks can be ordered as a separate unit to mount a cylinder into.
Rodless air cylinders have no piston rod. They are actuators that use a mechanical or magnetic coupling to impart force, typically to a table or other body that moves along the length of the cylinder body but does not extend beyond it. These are also often called band cylinders. This is shown in FIG. 41.
Air cylinders are available in a variety of sizes and range from a small 2.5 mm diameter air cylinder, which might be used for picking up a small electronic component, to 400 mm diameter air cylinders, which would impart enough force to lift a car. Some pneumatic cylinders reach 1000 mm in diameter and are used in place of hydraulic cylinders for special circumstances where leaking hydraulic oil might impose a hazard.
Pneumatic valves operate by using an electrically operated solenoid that shifts a spool inside the valve. This spool allows air to pass from an input port to an output port, also allowing air to escape from the exhaust side of the cylinder through the valve. The valves may be arranged in a variety of different ways, depending on the requirements of the application. Pneumatic valves are generally described by the number of ports in the valve body and the number of positions the spool may have. Like electrical circuits, they are also often specified as NC and NO, referring to their deenergized state.
Examples of these are 2/2 and 3/2 valves. Most automated systems tend to use banks of 5/2 and 5/3 valves with open or blocked centers, depending on whether it is desirable to be able to move the actuator by hand in the deenergized state or not.
In addition to valves, fittings and devices like flow controls, pressure regulators, filters and a wide range of tubing and hoses are necessary to complete a pneumatic or hydraulic system. Accumulators and pressure intensifiers are also components sometimes used in pneumatic circuits. TABLE 1 shows pneumatic symbols for some of these valves and devices.
FIG. 41 Rodless cylinder. (Courtesy of SMC.)
Hydraulic cylinders and actuators operate in a similar way to pneumatics except that they must be able to withstand much higher pressures and forces. More care must be taken to prevent the escape of fluids from the actuator also. Because of this, hydraulic actuators are more ruggedly built than typical pneumatic cylinders. External rods are often threaded into the end caps to help withstand the greater force exerted within the cylinder. Hydraulic cylinders are used in applications requiring great force, such as presses.
Unlike pneumatic systems, which are often supplied from a plantwide system, hydraulic systems have dedicated pumps. When oil is compressed, it generates heat, so the hydraulic fluid must also usually be cooled. Because of these extra components, hydraulic systems are much more expensive than pneumatic ones. Hybrid devices like air over oil actuators can sometimes help reduce the cost and complexity of hydraulic systems.
TABLE 1 Pneumatic Symbols
5.2 Electric Actuators
Electrically driven actuators are often used where air is not available or precision location is required. Though typically more costly than an air cylinder, they are less expensive and complex than a hydraulic system. Electric actuators are often servomotor driven and ball screw or belt based. They can be found in many of the same packaged configurations as air cylinders.
Small magnetic solenoids may also be used to extend a rod a short distance; these consist of a coil of wire wrapped around a bushing with a metal rod inside. A well-known example of this type of actuator is those used in pinball machines for the flippers and bumpers. The spool in solenoid valves uses this same principle.
5.3 Motion Control
Motion control is often considered an entire subchapter within the field of automation. Motion control differs from standard discrete controls such as pneumatic cylinders, conveyors, and the like because the positions and velocities are both controlled by analog or digitally converted analog methods. This is accomplished by the use of hydraulic or pneumatic proportional valves, linear actuators, or electric motors, usually servos. Stepper motors are also a common component in small motion control systems, especially when feedback may not be economical. Motion control is used extensively in packaging, printing, textile, semiconductor, and assembly industries.
It also forms the basis of robotics and CNC machine tools.
The basic architecture of a motion control system consists of:
- A motion controller to generate the desired output or motion profile. Movement is based on programmed set points and closing a position or velocity feedback loop.
- A drive or amplifier to transform the control signal from the motion controller into a higher-power electrical control current or voltage. This is what is applied to the actuator and actually makes it move.
- An actuator such as a hydraulic or air cylinder, linear actuator, or electric motor for output motion.
- One or more feedback sensors, such as optical encoders, resolvers, or Hall effect devices. These return the position or velocity of the actuator to the motion controller in order to close the position or velocity control loops. Newer "intelligent" drives can close the position and velocity loops internally, resulting in more accurate control.
- Mechanical components to transform the movement of the actuator into the desired motion. Examples are gears, shafting, ball screws, belts, linkages, and linear and rotational bearings.
FIG. 42 shows the physical arrangement of a motion control system.
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FIG. 42 Motion control system.
Axis Drives Controller and Power Supply Feedback Cable Motor Cable; Overtravel Switch; Home Switch; Overtravel Switch; Actuator; Coupling; Motor; Encoder
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A stand-alone motion control axis is common when positioning; however, there are times when motions must be coordinated closely.
This requires tight synchronization between axes. Robotics is an example of a coordinated motion system working together. Prior to the development of fast open communication interfaces in the early 1990s, the only open method of coordinated motion was analog control being brought back to the controller in the form of encoder, resolver, and other analog methods such as 4 to 20 mA and 0 to 10 V signals. The first open digital automation bus to satisfy the requirements of coordinated motion control was Sercos (sercos.com). This is an international standard that closes the servo feedback loop in the drive rather than in the motion controller. This arrangement reduces the computational load on the controller, allowing more axes to be controlled at once. Since the development of Sercos, other interfaces have been developed for this purpose, including ProfiNet IRT, CANopen, EtherNet PowerLink, and EtherCAT.
Besides the common control functions of velocity and position control, there are several other functions that may be considered.
Since torque feedback can be determined from the current and velocity of the servo, pressure or force control is another function of a servo actuator. Electronic gearing can be used to link two or more axes together in a master/slave relationship. Cam profiling where one axis follows the motion of a master axis is an example of this.
More detailed profiles, such as trapezoidal moves or S-curves, can also be computed by a motion controller enhancing standard positional moves. This helps to eliminate acceleration or deceleration impacts such as "jerk." One of the best online resources for motion control theory and components is Motion Control Resource (motioncontrolresource.com). This site is amazingly free of advertisements and contains links to many major motion control component manufacturers and distributors.
