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Motion Control Handbook CONSIDERATIONS

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Motion Control Handbook CONSIDERATIONS ENGINEERING Motion Control Handbook CONSIDERATIONS The following section covers a variety of technical topics In addition to the material presented in this section, more related to precision positioning and motion control. It is our extensive application notes are available, which provide goal here to provide material that will help prospective users greater depth on a variety of positioning system topics. The reach an informed decision concerning the suitability of application notes listed below are available free of charge specific positioning components for their application. We through our Sales Department and on our website– have noted the general dearth of information of this type, and www.NEAT.com. are familiar with the tendency to hype “specs” at the expense 1. Slow Down to Speed Up! of substance. Glib claims of “sub-micron accuracies” in par- A discussion on how to optimize linear motor performance ticular, only have meaning when a comprehensive error bud- through careful consideration of move profiles. get is prepared, taking an integrated approach to both the 2. Accuracy in Positioning Systems positioning components and the specifics of the application. A look at the factors limiting accuracy, with emphasis on interferometer-based systems. We take our responsibility seriously as vendors of secondary 3. Positioning Systems Overview reference standards for dimension. We also feel that our cus- A broad overview of positioning technologies, examined from the standpoint of the individual components with tomers are better served by having more information at their which positioning systems are designed. disposal, not less. Feel free to contact us should you wish to 4. Linear Motor Applications Note discuss any of the material presented here, or examine the A detailed look at linear motor systems, with several specifics of your application. We look forward to serving you. accompanying MathCAD spreadsheets that analyze system performance. Contents Metrology Considerations System Component Considerations Units of Measure Leadscrews and Ballscrews Accuracy Rotary Stepping Motors Repeatability Rotary Servo Motors Resolution Linear Servo Motors Abbé Error Interferometer Feedback Systems Thermal Expansion Limit Sensors Mapping Rotary Motor Mount Cosine Error Standard Product Pinouts Mounting Issues Stepping Motor Drives Full Coil vs. Half Coil Application Considerations Microstepping Move and Settle Time Midrange Resonance Constant Velocity Systems Servo Motor Drives Interpolated Motion Stacking Order and Eucentric Motion Environmental Considerations Torque and Force Requirements High Vacuum Positioning Tables Motion Calculations Vibration Isolation Systems Retainer Creep Low Magnetic Field Tables High Rad Tables WWW.NEAT.COM 800.227.1066 • 603.893.0588 METROLOGY CONSIDERATIONS Units of Measure While Imperial dimensions and thread standards remain pop- The SI unit of angle is the dimensionless radian, which is the ular in America, engineering calculations benefit from the plane angle whose arc length is equal to its radius. A full cir- metric system, and in particular the SI, or MKS (meter-kilo- cle has 2π radians, and common subdivisions include the mil- gram-second) system. A number of dimensional units are liradian (10-3), microradian (10-6), and nanoradian (10-9). The employed when discussing positioning systems, and some Imperial system, which is probably more familiar, divides a cir- may not be familiar to all users. The fundamental units are cle into 360 degrees; each degree into 60 minutes of arc; and those of distance, mass, time, and temperature; all other units each minute of arc into 60 seconds of arc (or more simply, arc- can be derived from these (we neglect here the equally fun- seconds). For comparison purposes, a radian is ~57.3 degrees, damental Ampere, mole and Candela). one arc-second is very nearly 5 microradians, and there are 1,296,000 arc-seconds in a full circle. The positioning commu- Time is employed uniformly in both the Imperial and SI sys- nity is fairly fond of degrees and arc-seconds, and as there is tems; we have the second, millisecond (10-3), microsecond less ground for confusion here than was the case for mass and (10-6), and nanosecond (10-9). Confusion creeps in where force, we employ both systems of angular units, as we wish. mass and force are concerned: in the SI system, the unit of mass is the kilogram, and the unit of force is the Newton. The most common positioning units are those of length, for Weight is the gravitational force on a body and is proportional which the SI unit is the meter. Common subdivisions include to its mass, W=mg. A kilogram force is the weight of 1-kg the millimeter (10-3 m), micrometer (also called the micron, at mass, and is equal to 9.81 Newtons or 2.2 pounds. The 10-6 m), and nanometer (10-9 m). Since we are now working Imperial pound, ounce, etc. are actually units of force despite with applications whose resolutions are sub-nanometer, we the fact that you can “convert” kilograms to pounds by mul- should probably include the picometer, at 10-12 m. The chart tiplying by ~2.2; the units have taken a beating. The Imperial below places these units in relation to both their Imperial unit of mass is, of course, the slug; in general, it’s better to counterpart (the inch), and recognizable objects of matching forget the Imperial units, and stick to SI. dimensions. Torque is expressed in SI units by the Newton-meter; the Length corresponding Imperial unit is either the ounce-inch or the foot-pound. The SI units for linear and torsional stiffness are Imperial Metric Newtons/meter and Newton-meters/radian, respectively; the Yard Meter Imperial equivalents are pounds/inch and ounce- inches/degree. Foot 10-1 Inch Centimeter (10-2) Millimeter(10-3) 10-4 . .......human hair (a “mil”) 0.001 inch 10-5 (a “tenth”) 0.0001 inch Micrometer, or micron (10-6) 10-7 .......semiconductor line width (micro-inch) 0.000001 inch 10-8 Nanometer (10-9) Angstrom (10-10). atomic diameter 10-11 Picometer (10-12) 10-13 10-14 10-15 . Nucleus diameter WWW.NEAT.COM 800.227.1066 • 603.893.0588 METROLOGY Accuracy CONSIDERATIONS Positioning system accuracy can be conveniently divided into these errors (angular rotation, for example, produces a transla- two categories: the accuracy of the way itself, and the linear tional error at any point other than the center of rotation), it is positioning accuracy along the way. The former describes the worthwhile to carefully examine the effects of each type of degree to which the ways (ball and rod, crossed roller, air error and its method of measurement. bearing, etc.) provide an ideal single-axis translation, while the latter is concerned with the precision of incremental Way Translation Errors motion along the axis (typically related to the leadscrew, lin- Since all useful methods of producing linear motion average ear encoder, or other feedback device). over a number of points (due to multiple balls or rollers, or the area of an air bearing), “pure” translational errors from WAY ACCURACY straight line motion (that is, without any underlying angular error) are usually minor. An exaggerated sine wave error in Any moving object has six available degrees of freedom rolling element ways could achieve a pure translational error (Figure 1). These consist of translation, or linear movement, without rotation, as would the case of each roller in a way along any of three perpendicular axes (X, Y, and Z), as well running over a contaminant particle at the same time; both of θ θ θ as rotation around any of those axes ( x, y, and z). The these cases are never encountered in practice. If a rolling ele- function of a linear positioning way is to precisely constrain ment stage has been subjected to a large impact, the ways may the movement of an object to a single translational axis only be brinelled (dented) at each ball or roller location; this can (typically described as the X axis). Any deviations from ideal result in a pure translational error that occurs periodically straight line motion along the X axis are the result of inaccu- along the travel. racy in the way assembly. Positioning tables do nonetheless, exhibit some vertical and horizontal runout (typically referred to as errors of flatness and straightness, respectively), as can be measured by plac- ing a sufficiently sensitive indicator on a table and measuring the vertical or horizontal displacement along its travel. A typ- ical high-resolution measurement technique would mount a conductively coated optical flat on the stage under test, and monitor the runout with a capacitance gauge. This will reveal errors which can be divided into three categories: 1. A potentially large component, which is roughly lin- ear with distance. This is due to a lack of parallelism between the optical flat Figure 1 – Six Degrees of Freedom and the ways. This can be eliminated by adjusting the flat There are five possible types of way inaccuracy, corresponding so as to be parallel to the ways of the stage. Note, how- to the five remaining degrees of freedom (Figure 2): translation ever, that to minimize vertical runout (flatness errors), the in the Y axis; translation in the Z axis; rotation around the X customer part must be similarly aligned parallel to the axis (roll); rotation around the Y axis (pitch); and rotation ways, which is not necessarily exactly parallel to either the around the Z axis (yaw). Since there are interrelations between base of the stage or its top. Figure 2 – Possible Way Inaccuracies WWW.NEAT.COM 800.227.1066 • 603.893.0588 METROLOGY CONSIDERATIONS Accuracy (Cont.d) 2. A low frequency component, which cannot be elimi- nated by adjustment of the optical flat. 250mm This is rarely a “pure” translational error, but is rather a 20µm consequence of the underlying angular errors (pitch, roll, and yaw) in the ways.
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