Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 17
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Accuracies of 0.05% can be obtained froman instrument-quality precision multiturn potentiometer, and resolutionscan exceed 0.005º if the output signal is converted with a 16-bit ADC.Precision multiturn potentiometers have wirewound or hybrid resistive elements. Hybrid elements are wirewound elements coated withresistive plastic to improve their resolution. To obtain an output from apotentiometer, a conductive wiper must be in contact with the resistiveelement. During its service life wear on the resistive element caused bythe wiper can degrade the precision of the precision potentiometer.SOLENOIDS AND THEIR APPLICATIONSSolenoids: An Economical Choice for Linear orRotary MotionA solenoid is an electromechanical device that converts electrical energyinto linear or rotary mechanical motion.
All solenoids include a coil forconducting current and generating a magnetic field, an iron or steel shellor case to complete the magnetic circuit, and a plunger or armature fortranslating motion. Solenoids can be actuated by either direct current(DC) or rectified alternating current (AC).Solenoids are built with conductive paths that transmit maximummagnetic flux density with minimum electrical energy input. Themechanical action performed by the solenoid depends on the design ofthe plunger in a linear solenoid or the armature in a rotary solenoid.Linear solenoid plungers are either spring-loaded or use external methods to restrain axial movement caused by the magnetic flux when thecoil is energized and restore it to its initial position when the current isswitched off.Cutaway drawing Figure 1-50 illustrates how pull-in and push-outactions are performed by a linear solenoid.
When the coil is energized,the plunger pulls in against the spring, and this motion can be translatedinto either a “pull-in” or a “push-out” response. All solenoids are basically pull-in-type actuators, but the location of the plunger extensionwith respect to the coil and spring determines its function. For example,the plunger extension on the left end (end A) provides “push-out” motionagainst the load, while a plunger extension on the right end terminatedby a clevis (end B) provides “pull-in” motion. Commercial solenoidsperform only one of these functions. Figure 1-51 is a cross-sectionalview of a typical pull-in commercial linear solenoid.Chapter 1Motor and Motion Control Systems61Figure 1-50 The pull-in andpush-out functions of a solenoidare shown.
End A of the plungerpushes out when the solenoid isenergized while the clevis-end Bpulls in.Rotary solenoids operate on the same principle as linear solenoidsexcept that the axial movement of the armature is converted into rotarymovement by various mechanical devices. One of these is the use ofinternal lands or ball bearings and slots or races that convert a pull-instroke to rotary or twisting motion.Motion control and process automation systems use many differentkinds of solenoids to provide motions ranging from simply turning anevent on or off to the performance of extremely complex sequencing.When there are requirements for linear or rotary motion, solenoidsshould be considered because of their relatively small size and low costwhen compared with alternatives such as motors or actuators.
Solenoidsare easy to install and use, and they are both versatile and reliable.Figure 1-51 Cross-section viewof a commercial linear pull-typesolenoid with a clevis. The conicalend of the plunger increases itsefficiency. The solenoid ismounted with its threadedbushing and nut.62Chapter 1Motor and Motion Control SystemsTechnical ConsiderationsImportant factors to consider when selecting solenoids are their ratedtorque/force, duty cycles, estimated working lives, performance curves,ambient temperature range, and temperature rise. The solenoid musthave a magnetic return path capable of transmitting the maximumamount of magnetic flux density with minimum energy input.
Magneticflux lines are transmitted to the plunger or armature through the bobbinand air gap back through the iron or steel shell. A ferrous metal path ismore efficient than air, but the air gap is needed to permit plunger orarmature movement. The force or torque of a solenoid is inversely proportional to the square of the distance between pole faces. By optimizingthe ferrous path area, the shape of the plunger or armature, and the magnetic circuit material, the output torque/force can be increased.The torque/force characteristic is an important solenoid specification.In most applications the force can be a minimum at the start of theplunger or armature stroke but must increase at a rapid rate to reach themaximum value before the plunger or armature reaches the backstop.The magnetizing force of the solenoid is proportional to the numberof copper wire turns in its coil, the magnitude of the current, and the permeance of the magnetic circuit.
The pull force required by the load mustnot be greater than the force developed by the solenoid during any portion of its required stroke, or the plunger or armature will not pull incompletely. As a result, the load will not be moved the required distance.Heat buildup in a solenoid is a function of power and the length oftime the power is applied. The permissible temperature rise limits themagnitude of the input power. If constant voltage is applied, heatbuildup can degrade the efficiency of the coil by effectively reducing itsnumber of ampere turns. This, in turn, reduces flux density andtorque/force output.
If the temperature of the coil is permitted to riseabove the temperature rating of its insulation, performance will sufferand the solenoid could fail prematurely. Ambient temperature in excessof the specified limits will limit the solenoid cooling expected by convection and conduction.Heat can be dissipated by cooling the solenoid with forced air from afan or blower, mounting the solenoid on a heat sink, or circulating a liquid coolant through a heat sink.
Alternatively, a larger solenoid than theone actually needed could be used.The heating of the solenoid is affected by the duty cycle, which isspecified from 10 to 100%, and is directly proportional to solenoid ontime. The highest starting and ending torque are obtained with the lowestduty cycle and on time. Duty cycle is defined as the ratio of on time toChapter 1Motor and Motion Control Systemsthe sum of on time and off time.
For example, if a solenoid is energizedfor 30 s and then turned off for 90 s, its duty cycle is 30⁄120 = 1⁄4, or 25%.The amount of work performed by a solenoid is directly related to itssize. A large solenoid can develop more force at a given stroke than asmall one with the same coil current because it has more turns of wire inits coil.Open-Frame SolenoidsOpen-frame solenoids are the simplest and least expensive models. Theyhave open steel frames, exposed coils, and movable plungers centered intheir coils. Their simple design permits them to be made inexpensively inhigh-volume production runs so that they can be sold at low cost.
Thetwo forms of open-frame solenoid are the C-frame solenoid and the boxframe solenoid. They are usually specified for applications where verylong life and precise positioning are not critical requirements.C-Frame SolenoidsC-frame solenoids are low-cost commercial solenoids intended for lightduty applications. The frames are typically laminated steel formed in theshape of the letter C to complete the magnetic circuit through the core,but they leave the coil windings without a complete protective cover. Theplungers are typically made as laminated steel bars.
However, the coilsare usually potted to resist airborne and liquid contaminants. These solenoids can be found in appliances, printers, coin dispensers, security doorlocks, cameras, and vending machines. They can be powered with eitherAC or DC current. Nevertheless, C-frame solenoids can have operationallives of millions of cycles, and some standard catalog models are capableof strokes up to 0.5 in. (13 mm).Box-Frame SolenoidsBox-frame solenoids have steel frames that enclose their coils on twosides, improving their mechanical strength.
The coils are wound on phenolic bobbins, and the plungers are typically made from solid bar stock.The frames of some box-type solenoids are made from stacks of thininsulated sheets of steel to control eddy currents as well as keep stray circulating currents confined in solenoids powered by AC.
Box-frame sole-6364Chapter 1Motor and Motion Control Systemsnoids are specified for higher-end applications such as tape decks, industrial controls, tape recorders, and business machines because they offermechanical and electrical performance that is superior to those of Cframe solenoids. Standard catalog commercial box-frame solenoids canbe powered by AC or DC current, and can have strokes that exceed 0.5in. (13 mm).Tubular SolenoidsThe coils of tubular solenoids have coils that are completely enclosed incylindrical metal cases that provide improved magnetic circuit return andbetter protection against accidental damage or liquid spillage.
These DCsolenoids offer the highest volumetric efficiency of any commercial solenoids, and they are specified for industrial and military/aerospace equipment where the space permitted for their installation is restricted. Thesesolenoids are specified for printers, computer disk-and tape drives, andmilitary weapons systems; both pull-in and push-out styles are available.Some commercial tubular linear solenoids in this class have strokes up to1.5 in. (38 mm), and some can provide 30 lbf (14 kgf) from a unit lessthan 2.25 in (57 mm) long. Linear solenoids find applications in vendingmachines, photocopy machines, door locks, pumps, coin-changingmechanisms, and film processors.Rotary SolenoidsRotary solenoid operation is based on the same electromagnetic principles as linear solenoids except that their input electrical energy is converted to rotary or twisting rather than linear motion.
Rotary actuatorsshould be considered if controlled speed is a requirement in a rotarystroke application. One style of rotary solenoid is shown in the explodedview Figure 1-52. It includes an armature-plate assembly that rotateswhen it is pulled into the housing by magnetic flux from the coil. Axialstroke is the linear distance that the armature travels to the center of thecoil as the solenoid is energized. The three ball bearings travel to thelower ends of the races in which they are positioned.The operation of this rotary solenoid is shown in Figure 1-53. Therotary solenoid armature is supported by three ball bearings that travelaround and down the three inclined ball races. The de-energized state isshown in (a). When power is applied, a linear electromagnetic force pullsin the armature and twists the armature plate, as shown in (b).
RotationChapter 1Motor and Motion Control Systems65Figure 1-52 Exploded view of arotary solenoid showing its principal components.continues until the balls have traveled to the deep ends of the races, completing the conversion of linear to rotary motion.This type of rotary solenoid has a steel case that surrounds and protects the coil, and the coil is wound so that the maximum amount of copper wire is located in the allowed space. The steel housing provides thehigh permeability path and low residual flux needed for the efficient conversion of electrical energy to mechanical motion.Rotary solenoids can provide well over 100 lb-in.
(115 kgf-cm) oftorque from a unit less than 2.25 in. (57 mm) long. Rotary solenoids areFigure 1-53 Cutaway views of arotary solenoid de-energized (a)and energized (b). When energized, the solenoid armature pullsin, causing the three ball bearingsto roll into the deeper ends of thelateral slots on the faceplate,translating linear to rotarymotion.66Chapter 1Motor and Motion Control Systemsfound in counters, circuit breakers, electronic component pick-and-placemachines, ATM machines, machine tools, ticket-dispensing machines,and photocopiers.Rotary ActuatorsThe rotary actuator shown in Figure 1-54 operates on the principle ofattraction and repulsion of opposite and like magnetic poles as a motor.In this case the electromagnetic flux from the actuator’s solenoid interacts with the permanent magnetic field of a neodymium–iron disk magnet attached to the armature but free to rotate.The patented Ultimag rotary actuator from the Ledex product groupof TRW, Vandalia, Ohio, was developed to meet the need for a bidirectional actuator with a limited working stroke of less than 360º but capable of offering higher speed and torque than a rotary solenoid.
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