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| Solenoids Used in Short-stroke linear motion |
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Short-stroke linear solenoid actuators are the most widely used devices for applications that require electromechanically based linear motion. These simple actuators are cost-effective, can be designed in a variety of linear configurations, are easily driven by simple electronic packages, and often have long life - to 100 million or more actuations in certain conditions.
Types:
The linear solenoid is a deceptively simple device. It consists of a coil to carry current and generate ampere turns, an iron shell or case to provide a magnetic circuit, and a movable plunger or pole to act as the working element and provide linear motion.
There are four main types of short-stroke linear actuators, each defined by a number of key characteristics. These four types are further categorized as push or pull devices.
Low profile: Offering the highest force, longest life, and largest sizes, low-profile actuators make up the high end of the actuator product range; they are also the most expensive.
Forces for these actuators range from 5 lb for a _-in.-OD unit to 350 lb for a 3 3/8-in.-OD unit. Millions of cycles are possible with this type, with a potential top end of 25 million actuations. Maximum strokes are 0.140 to 0.700 in. At least eight common sizes are available, each with various options related to stroke.
"Flat face" versions of the low-profile actuator are typically used for strokes below approximately 0.060 in. The flat-face design allows the full component of force to be used, because the force vector is parallel to pole motion.
Conical versions are used for applications that require higher forces and strokes over 0.060 in. The conical design has a greater pole surface area an a shorter air gap, which causes higher flux density and provides more output for longer strokes.
An interesting variation of the low-profile actuator is a variable positioning unit that combines high starting force (up to 25 lb) and quiet operation. One version is a push or pull axial device that can move at a slow, controlled rate against either a spring or gravity. Unlike the exponential force curves of typical linear actuators, it has a horizontal force curve, in which force is independent of position, and the actuator can be controlled via voltage modulation with a pulsing circuit. The circuit rapidly turns the coil on and off to achieve an average current that is 100% of what current would be if left on continuously. Further, the pulse-width modulation can be provided by an electronic controller or microprocessor. This low-profile design can be a low-cost alternative to a linear step motor with a leadscrew with up to a 0.420-in. stroke.
Tubular: These are typically available in 1/2, 3/4, and 1-in. OD, although much larger sizes have been made for special applications. Forces for standard sizes are 20 oz to 8 lb. Life expectancy can exceed 25 million actuations. Strokes to 0.7 in. are feasible.
Within the three common OD sizes of one manufacturer, three standard cone angles or plunger configurations are often used, which can affect force-and-stroke performance.
Open frame: Open-frame actuators are the least expensive short-stroke linear actuators, with a typical life expectancy of 50,000 cycles. Much longer-life units are available with some modification. Sizes range from 0.5 to 4 in., with forces from 4 to 7 lb. Strokes range from 0.1 to 1 ". More than 30 sizes are commonly available, each with different plunger configurations.
Magnetically latching: These actuators incorporate an integral permanent magnet. When a momentary pulse is applied to the solenoid, the plunger is pulled to the energized position, where the magnet holds it. A secondary pulse of opposite polarity releases the plunger.
This product suits applications where the actuator will be used only intermittently and where the energized position must be maintained. A good example is a door lock. If a door is to be held open by a lock which includes a magnetic latching actuator, the actuator will energize to its full stroke and be held there by the magnet. This will keep the door open with no further power consumption. Only when the door is to be closed does power have to be applied to release the locking mechanism.
Push or pull: All linear actuators can perform either Push or Pull functions because the actuator coil causes the unit's two poles to have opposite polarity. In a magnetic circuit, opposites attract, and there is a force attracting one pole to the other. Because one pole (the stator) is held stationary, the force pulls the plunger toward the stator pole giving it a Pull function. The same magnetic attraction occurs with the push version. The only difference is that a rod is positioned through a hole in the stator pole. As the plunger is pulled in, it forces the rod out in such a way that it pushes away from the stator.
Pull actuators are designed with most of the plunger outside the actuator in the de-energized state. When the actuator is energized, the magnetic field pulls the plunger into a seated position fully inside the actuator body. Push actuators are designed with a push rod extending from the end of the plunger. The plunger is pulled into a hole in the end of the actuator body. In the pull version, the motion which the actuator causes occurs opposite to the plunge cone; in the push version, motion occurs at the same end of the actuator as the plunger cone.
Modifications: As mentioned previously, short-stroke linear actuators are often modified to meet specific requirements.
The most common alterations to the basic low-profile, tubular, and open-frame types are:
· Plunger modifications to optimize force output.
· Plunger modifications to use specific linkages.
· Terminations modifications to allow specific lead length, connectors, terminals, or harness assemblies.
· Coil modifications to meet special environmental needs, such as high temperature, or specific insulation-system requirements.
Many linear-actuator manufacturers have adopted computerized techniques to greatly cut prototype development time. Thus, custom prototypes can often be delivered in 2 weeks or less.
Selection factors: There are a number of factors involved in choosing the right linear actuator for the job:
Magnetic flux - The number of magnetic flux lines is directly related to the number of turns in the coil, the current, and the area and permeability of the medium through which the flux lines travel. These flux lines are transmitted through the iron shell and the air gap between shell and plunger. (An iron path is much more efficient than air, but the air gap is needed to permit movement of the plunger or armature.)
The force of a given actuator is theoretically inversely proportional to the square of the length of the air gap; thus, the strongest force is generated when the air gap is smallest. Based on these principles of operation, two of the major objectives in the design of an actuator are to:
· Provide an iron path capable of transmitting magnetic flux density with minimum energy input.
· Get the best relationship between the variable ampere turns and the working flux density in the air gap.
Force - Force requirements can be affected by a number of external factors, including actual load, frictional load due to side loading or misalignment, return spring, temperature rise, duty cycle, and spatial orientation.
Life - Actuator life is determined by the unit's bearing system and shaft surface finish, and by load balance and side loading. Closely matching mechanical load requirements to the actuator's output helps provide optimum life.
Ampere turns - This is the absolute value of magnetostrictive force (analogous to voltage or water pressure). It is the product of the turns of wire in the actuator coil and the current passing through the coil. The force output capability of a given actuator is proportional to the square of the number of ampere turns. In constant-voltage applications, however, an increase in temperature reduces the effective ampere turns, thereby reducing the torque or force output. Force output ratings for actuators are generally stated at 20 C ambient temperature.
Saturation - Saturation is the magnetic limitation of the iron plunger, core, or pole pieces to an increase in magnetic flux density. Once a unit's magnetic flux capacity is saturated, further power input results in added heat, with little increase in force. Duty cycles above 20% often cause saturation.
Stroke - It is best to keep the stroke as short as possible, thus minimizing the size and power consumption of the linear actuator. With the proper linkages, an actuator with a very short stroke can provide over a foot of linear travel.
Duty cycle - Duty cycle is the percentage of time during which the actuator is energized. For example, if an actuator is energized for 1 out of every 4 sec, the duty cycle is 25%. The duty cycle determines the permissible power input and the subsequent amount of force and heat. For example, if 10-W input power causes a heat rise of 20 C in 10 sec, approximately the same temperature rise will result if a power of 100 W is applied for 1 sec. In terms of duty cycle, an actuator designed for continuous duty can dissipate 10 times input power at 10% duty.
Maximum On time - Actuators have a maximum On time for a given duty cycle and power input. For example, if an actuator is energized for 1 sec out of 4 (25% duty cycle) its On time of 1 sec will cause no damage. But if you tried to energize the solenoid for 10 min out of every 40 min at the 25% duty cycle wattage, one pulse this long could cause thermal degradation of the actuator.
Operating speed - The energizing time for an actuator to complete a given stroke is measured from the beginning of the initial pulse to the seated or energized position. This time is dependent on the load, duty cycle, input power, stroke, and ambient temperature.
Heat - Heat can be dissipated by controlling the air flow, by mounting the actuator on a surface large enough to dissipate the energy (for example, a heat sink), or by some other method. When space permits, a simple solution is to use a larger actuator. Heat in an actuator is a function of power and the time during which it is applied. For continuous duty, hold-in circuits are commonly used to provide higher starting forces than are obtainable at continuous duty rating. Stock-model actuators are usually designed to operate in ambient temperatures of -55 to 80 C.
Environment - Actuator performance can be greatly affected by the operating environment. Temperature, sand and dust, humidity, shock and vibration, and the presence of foreign materials such as certain chemicals can affect performance.
Coil (arc) suppression - The very nature of actuator design, based on conversion of electrical energy into magnetic energy, often requires that an electronic protection device be installed to reduce the arc caused by interruption of the current through the mechanism.
AC vs DC - AC actuators are commonly used in household appliances. They are often specified when the cost to rectify AC to DC is prohibitive. Because AC actuators require as much as twice the power of an equivalent DC unit, there is a growing trend toward DC-powered actuators.
A few cautions:
Although short-stroke linear actuators can be used in literally thousands of applications, there are some cases where they are not the best choice.
For example, where force must be provided in steps rather than all at once - or slowly, as with a variable positioning actuator - a step motor is clearly the better choice.
Very harsh environments, those which include abrasive chemicals and other fluids, are also inappropriate for most actuators, because most actuators are not of sealed construction.
Finally, as their name implies, actuators generally are best suited to short strokes, if only to minimize their size and power consumption. In any case, these devices are almost never used with strokes beyond 3 in.
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