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Definition of Terms
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A B C D E F G H I K L M N O P Q R S T V W Y Acceleration: The change in velocity as a function of time. Increasing velocity.Accuracy: An absolute measurement defining the difference between actual and commanded position. Actuator: Mechanical device for moving or controlling a mechanism or system. Applied Force: A surface force or body force acting on an object. External forces are sometimes called applied forces. Axial Force: A system of internal forces whose resultant is a force acting along the longitudinal axis of a structural member or assembly. Axis of Rotation: A center line about which rotation occurs. Backlash: A component of bidirectional repeatability. The lost motion or looseness (play) between the balls in the nut and the threads on the screw. In the case of ballscrews, it refers to how much the ballnut can be moved back and forth without moving along the length of the screw. Backlash Error: Error in positioning caused by the reversal of travel direction. This is caused by clearance between the elements in the drive train. Ball Screw or Ballscrew: A precision device for translating rotary motion into linear motion. A lead screw is a lower cost, lower performance device performing the same function. Unit consists of an externally threaded screw and an internally threaded ball nut. Ball Screw Lead: The linear distance a carriage or table will travel for one revolution of the ball screw (or lead screw). Ball Screw Support: Bearing Support used to hold a ball screw in position while allowing rotational movement in around a single axis. Bearing: A support mechanism allowing relative motion between two surfaces loaded against each other. This can be a rotary ball bearing, linear slide bearing, or air bearing (zero friction). Bending Moment: A system of internal forces whose resultant is a moment. This term is most commonly used to refer to internal forces in beams. Body Force: An external force acting throughout the mass of a body. Gravity is a body force. An inertial force is a body force. Brinelling: A form of mechanical damage in which metal is displaced or upset without attrition. It is a permanent deformation of the ball track. Brinelling usually results from excessive load. Brittle: A brittle structure or material exhibits low ductility, meaning that it exhibits very little inelastic deformation before complete failure. Cam Follower: Bearing with a stud incorporating ball or needle rolling elements. Designed for outer ring rotation along a cam roller guide profile surface. Cam Profile: A technique used to perform nonlinear motion that is electronically similar to the motion achieved with mechanical cams. Cam Roller Guide: Mechanical structure or device that guides a cam follower bearing along a linear or profiled path. Cantilevered Load: A load not symmetrically mounted on a slide, stage, table, or guide block. Center of Gravity: The location of the resultant of gravity forces on an object or objects: sometimes called center of mass. Centroid: Similar to the concept of center of gravity, except that it applies to a two dimensional shape rather than an object. For a given shape, the centroid location corresponds to the center of gravity for a thin flat plate of that shape, made from a homogeneous material. Closed Loop: A broad term relating to any system where the output is measured and compared to the input. Output is adjusted to reach the desired condition. CNC: Computer Numerical Control. A computer-based motion control device programmable in numerical word address format. Coefficient of Friction: Defined as the ratio of the force required to move a given load to the magnitude of that load. Cogging: Non-uniform angular/linear velocity. Cogging appears as a jerkiness or vibration, especially at low speeds. Concentrated Force: A force considered to act along a single line in space. Concentrated forces are useful mathematical idealizations, but cannot be found in the real world, where all forces are either body forces acting over a volume or surface forces acting over an area. Concentrated Load: An external force which is a concentrated force. Coordinated Motion: Coordinated motion - Multi-axis motion where the position of each axis is dependent on the other axis, such that the path and velocity of a move can be accurately controlled. Drawing a circle requires coordinated motion. Coupling: A device used to connect two shafts together at their ends for the purpose of transmitting power. Crabbing: Vibration or binding condition usually found in 2 axis system where the supports are spaced significantly apart from the driven axis. The following axis tends to move in a jerky motion. Critical Speed: The angular velocity at which the screw (or nut) develops severe vibrations. Crossed Roller or Cross Roller Bearing: A bearing that uses cylindrical shaped roller elements to allow very precise antifriction travel of one surface relative to another. Rollers nest in V-grooved raceways ground out of the guides, and crisscross at 90 degree angles. Cross Roller Slide: Slide that incorporates a cross roller bearing to allow very precise antifriction travel of one surface relative to another. Cycle: The complete forward and reverse motion of a system. When motion is repeated (move and dwell) such as repetitive back-and-forth motion. Deceleration: The change in velocity as a function of time. Decreasing velocity. Deflection: A term that is used to describe the degree to which a structural element is displaced under a load. Deformation: A change in the shape of an object or material. Degree of Freedom: A displacement quantity which defines the shape and location of an object. In the two dimensional plane, a rigid object has three degrees of freedom: two translations and one rotation. In three dimensional space, a rigid object has six degrees of freedom (three translations and three rotations). Displacement: A change in position. A displacement may be a translation a rotation or a combination of those. Distributed Load: An external force which acts over a region of length, surface, or area: essentially any external force which is not a concentrated force.
Duty Cycle: Duty cycle is the ratio of "ON" time over the "ON + OFF" time ("TOTAL" time) for any one cycle of operation. Duty cycle is usually expressed in percentage. Dynamic Equilibrium: Equilibrium which includes inertial forces.
Dwell Time: Time in a cycle at which no motion occurs. Used in the calculation of rms power.
Efficiency: Ratio of input power vs. output power. Electronic Gearing: Technique used to electrically simulate mechanical gearing. Causes one closed loop axis to be slaved to another open or closed loop axis with a variable ratio. Elastic: A material or structure is said to behave elastically if it returns to its original geometry upon unloading. Elastic Energy: The energy stored in deformed elastic material (e.g., a watch spring). Elastic energy equals where k is the stiffness, and is the associated deflection. Elastic energy is sometimes called elastic potential energy because it can be recovered when the object returns to its original shape; see potential energy. Elastic Limit: The point beyond which the deformations of a structure or material are no longer purely elastic. E-Modulus: In solid mechanics, Young's modulus, also known as the tensile modulus, is a measure of the stiffness of an isotropic elastic material. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. Encoder (Absolute): Position encoding device in which the output is a unique digital code for each distinct angle of the shaft. Encoder (Incremental): Position encoding device in which the output is a series of pulses relative to the angle of the shaft. Sometimes called relative encoder. Encoder Marker: Once-per-revolution signal provided by some incremental encoders to accurately specify a reference point within that revolution. Also known as Zero Reference Signal or Index Pulse. Encoder Resolution: Measure of the smallest positional change which can be detected by the encoder. A 1000-line encoder with a quadrature output will produce 4000 counts per revolution. Energy: A property of a body related to its ability to move a force through a distance opposite the force's direction; energy is the product of the magnitude of the force times the distance. Energy may take several forms: see kinetic energy, potential energy, and elastic energy. Equilibrium: An object is in equilibrium if the resultant of the system of forces acting on it has zero magnitude. See static equilibrium and dynamic equilibrium.
External Force: A surface force or body force acting on an object. External forces are sometimes called applied forces.
Feedback: Signal that provides process or loop information such as speed, torque, and position back to the controller to produce a closed-loop system. Flatness: Deviation from the true line of travel perpendicular to the direction of travel in the vertical plane. Fixed Support: In two dimensions, a fixed support restrains three degrees of freedom: two translations and one rotation. Flexibility: Flexibility is the inverse of stiffness. When a force is applied to a structure, there is a displacement in the direction of the force; flexibility is the ratio of the displacement divided by the force. High flexibility means that a small load produces a large displacement. Flexure: Bending deformation, i.e., deformation by increasing curvature. Force: A directed interaction between two objects that tends to change the momentum of both. Since a force has both direction and magnitude, it can be expressed as a vector. Force (Continuous): The value of force that a particular motor can produce in a continuous stall or running (as calculated by the rms values) condition. Force (Peak): The maximum value of force that a particular motor can produce. When sizing for a specific application, the peak force is usually that required during acceleration and deceleration of the move profile. The peak force is used in conjunction with the continuous force and duty cycle to calculate the rms force required by the application. Force System: One or more forces and/or moments acting simultaneously.
Friction: The resistance to motion between two surfaces in contact with each other.
Gain: Comparison or ratio of the output signal and the input signal. In general, the higher the system gain, the higher the response. Galling: A condition whereby excessive friction between high spots results in localized welding with subsequent splitting and a further roughening of rubbing surfaces of one or both of two mating parts. Gravity: An attractive force between two objects; each object accelerates at a rate equal to the attractive force divided by the object's mass. Objects near the surface of the earth tend to accelerate toward the earth's center at a rate of ; this value is often called the gravitational constant and denoted as g.
Ground Ball Screw: A component of a ball screw assembly used to transform rotary motion in linear motion. The external screw lead is very precisely ground to mate with a corresponding ball nut for high accuracy positioning.
Home: Reference position for all absolute positioning movements. Usually defined by a home limit switch and/or encoder marker. Home Switch: A sensor used to determine an accurate starting position for the home cycle. High Temperature Bearing: Bearing specially prepare for operation in high temperature (> 250 deg F) environment.
Hysteresis: A component of bidirectional repeatability. Hysteresis is the deviation between actual and commanded position and is created by the elastic forces in the drive systems.
Incremental Move: A move referenced from its starting point (relative move). Inelastic: Not surprisingly, the opposite of elastic. A deformation of a structure or material under load is described as inelastic when the deformation remains after the load is removed. The term plastic is often used with the same meaning. Inertia: The physical property of an object to resist changes in velocity when acted upon by an outside force. Inertia is dependent upon the mass and shape of an object. Inertial Force: A fictitious force used for convenience in visualizing the effects of forces on bodies in motion. For an accelerating body, the inertial force is considered as a body force whose resultant acts at the object's center of gravity in a direction opposite the acceleration. The magnitude of the force is the mass of the object times the magnitude of the acceleration.
Internal Force: Forces which hold an object together when external forces or other loads are applied. Internal forces are sometimes called resisting forces since they resist the effects of external forces.
Kinetic Energy: The energy of a moving mass; equal to . Where m is mass and v is the magnitude of the velocity.
Lead Error: The deviation of a lead screw or ball screw from its nominal pitch. Lead Screw or Leadscrew: A device for translating rotary motion into linear motion. Unit consists of an externally threaded screw and an internally threaded carriage (nut).
Life: Life of a motor is greatly affected by how it is used in the application as well as the environment in which it is used. Generally excessive friction and impact or an excessive operating temperature may result in a reduction in the life of a
motor. Limit Switch: A sensor used to determine the end of travel on a linear motion assembly. Limits: Sensors called limits that alert the control electronics that the physical end of travel is being approached and motion should stop. Line of Action: The line of action of a force is the infinite line defined by extending along the direction of the force from the point where the force acts. Linear: A structure is said to behave linearly when its the deformation response is directly proportional to the loading (i.e. doubling the load doubles the displacement response). For a material, linear means that the stress is directly proportional to the strain. Linear Actuator: Mechanical device for moving and/or controlling a mechanism or system motion in a linear fashion. Linear Bearing: A linear motion bearing or linear slide designed to provide free motion in one dimension. Linear Elastic: A force-displacement relationship which is both linear and elastic. For a structure, this means the deformation is proportional to the loading, and deformations disappear on unloading. For a material, the concept is the same except strain substitutes for deformation, and stress substitutes for load. Linear Guide: Guide along which a ball, roller, or cam roller type guide travels in a linear fashion. Linear Motor: A motor consisting of two parts, typically a moving coil and stationary magnet track. When driven with a standard servo amplifier, it creates a thrust force along the longitudinal axis of the magnet track. Linear Rail: Precision profiled rail along which a ball or roller type guide travels in a linear fashion. Linear Roller Bearing: A linear motion bearing or linear slide with antifriction rolling element designed to provide free motion in one dimension. Linear Roller Guide: Guide that provides a smooth, precision, guiding surface on which the roller elements of a linear bearing rides. Load: An external force. The term load is sometimes used to describe more general actions such as temperature differentials or movements such as foundation settlements.
Load Capability: The maximum recommended payload that does not degrade the listed specifications for a mechanical stage.
Mass: A property of an object measured by the degree that it resists acceleration. Master Slave: Type of coordinated motion control where the master axis position is used to generate one or more slave axis position commands. Magnitude: A scalar value having physical units. Miniature Ball Screw: Ball screws less than 14 or 16 mm diameter are typically classified as miniature. Miniature Linear Guide: Guides or runner blocks with two bearing rows and less than size 15 are typically classified as miniature. Modulus of Elasticity: The proportional constant between stress and strain for material with linear elastic behavior: calculated as stress divided by strain. Modulus of elasticity can be interpreted as the slope of the stress-strain graph. It is usually denoted as E, sometimes known as Young's Modulus Y, or E-Modulus. Moment: The resultant of a system of forces causing rotation without translation. A moment can be expressed as a couple. Moment of Inertia: Moment of inertia has two distinct but related meanings: 1) it is a property of a an object relating to the magnitude of the moment required to rotate the object and overcome its inertia. 2) A property of a two dimensional cross section shape with respect to an axis, usually an axis through the centroid of the shape. Motion Profile: A method of describing a process in terms of velocity, time, and position. Motor (Brush type): The conductive element in a DC brush-type motor used to transfer current to the internal windings. Motor (Brushless type): Type of direct current motor that utilizes electronic commutation rather than brushes to transfer current.
Motor (Stepping type): Specialized motor that allows discrete positioning without feedback. Used for noncritical, low power applications, since positional information is easily lost if acceleration or velocity limits are exceeded.
Non-recirculating Bearing: Bearing in which the rolling elements do not circulate throughout the travel circuit. A cross roller bearing with cage guide elements is a good example of this. Normal Strain: Strain measuring the intensity of deformation along an axis. Normal strain is usually denoted by . Average normal strain between two points is calculated as , where L is the original distance between the points, and L is the change in that distance. Normal strain is often simply called strain.
Normal Stress: Stress acting perpendicular to an imaginary plane cutting through an object. Normal stress has two senses: compression and tension. Normal stress is often simply called stress.
Open Loop: Control circuit that has an input signal only, and thus cannot make any corrections based on external influences. Optical Encoder: A linear or angular position feedback device using light fringes to develop position information. Orthogonality: The condition of a surface or axis which is perpendicular (offset 90 degrees) to a second surface or axis. Orthogonality specification refers to the error from 90 degrees from which two surfaces of axes are aligned.
Overshoot: In a servo system, referred to the amount of velocity and/or position overrun from the input command Overshoot is a result of many factors including mechanical structure, tuning gains, servo controller capability, and inertial mismatch.
Pitch (of travel): Angular motion of a carriage around an axis perpendicular to the motion direction and perpendicular to the yaw axis. Pitch Error: Positioning error resulting from a pitching motion. Positioning Stage: Stages and modules are typically linear motion assemblies that consist of linear guides, tables or carriages, and a drive mechanism (i.e., ball screw, belt, or linear motor). Positioning table: tables and/or carriages are components of a linear motion position system that support and directly attach to the payload. Potential Energy: The energy stored in a raised object (e.g. the weights in a grandfather clock). Potential energy equals mgh, where m is mass, g is the acceleration of gravity, and h is the vertical distance from a reference location. It is called potential energy because the energy can be regained when the object is lowered. This type of potential energy is sometimes called gravitational potential energy in order to distinguish it from elastic potential energy: see elastic energy. PLC: Programmable Logic Controller. A programmable device that utilizes "ladder logic" to control a number of input and output discrete devices. PWM: Pulse Width Modulation. Switch-mode technique used in amplifiers and drivers to control motor current. The output voltage is constant and switched at the bus value (160 VDC with a 115 VAC input line).
Pressure: Pressure is a similar idea to stress, the force intensity at a point, except that pressure means something acting on the surface of an object rather than within the material of the object. When discussing the pressure within a fluid, the meaning is equivalent to stress.
Quadrature: Refers to the property of position transducers that allows them to detect direction of motion using the phase relationship of two signal channels. A 1000-line encoder will yield 4000 counts via quadrature.
Racking: The distortion of a rectangular shape to a skewed parallelogram. Radial Runout: Positioning error of the rotary stage in the horizontal direction when the tabletop is oriented in the horizontal plane. Radial runout is defined as the total indicator reading on a spherical ball positioned 50 mm above the tabletop and centered on the axis of rotation. Ramp Time: Time it takes to accelerate from one velocity to another. Range: Time it takes to accelerate from one velocity to another. Raydent: Is a special surface treatment technology to form an alloyed surface that is wear and corrosion resistant. Recirculating Bearing: Bearing in which the rolling elements circulate throughout the travel circuit. Repeatability: The maximum deviation from the mean (each side) when repeatedly approaching a position. Unidirectional repeatability refers to the value established by moving toward a position in the same direction. Bidirectional repeatability refers to the value established by moving toward a position in the same or opposite direction. Resolution: The smallest change in distance that a device can measure. Reaction: A reaction is a force exerted by a support on an object: sometimes called support reaction. Using this definition, a reaction is an external force. Resisting Force: see internal force. Resultant: The resultant of a system of forces is a single force or moment whose magnitude, direction, and location make it statically equivalent to the system of forces. Rigid: An idealized concept meaning something which does not deform under loading. In fact, all objects deform under loading, but in modelling it can be useful to idealize very stiff objects as rigid. RoHS Compliant: RoHS directive prohibits the sale of equipment containing certain hazardous substances such as lead, cadmium, mercury, hexabalent chromium, PBB, and PBDE in the European Union. Being RoHS compliant ensures a product meets these standards. Roll: Rotation around an axis in the horizontal (side to side) plane parallel to the direction of travel. Rolled Ball Screw: A precision ball screw that is formed by a rolling process rather than precision machining. Roll Error: Positioning error resulting from a roll motion. Roller Guide: A block or guide that mates and moves freely in a linear fashion along a linear rail. This type of guide incorporates cylindrical rolling elements. Roller Support: Roller support: In two dimensions, a roller support restrains one translation degree of freedom. Rotation: Motion of an object where the path of every point is a circle or circular arc. A rotation is defined by a point and vector which determine the axis of rotation. The direction of the vector is the direction of the axis and the magnitude of the vector is the angle of rotation.
Runout: The deviation from the desired form of a surface during full rotation (360 degrees) about an axis. Run-out is measured as Total Indicated Reading (TIR). For a rotary stage, axis run-out refers to the deviation of the axis of rotation from the theoretical axis of rotation.
Scalar: A mathematical entity which has a numeric value but no direction (in contrast to a vector). Scraper: A non contact flat metal plate attached to each end of a guide or runner block used to prevent debris from entering the bearing. Section Modulus: A property of a cross sectional shape, which depends on shape, and orientation. Section modulus is usually denoted S, and S = I/c, where I = moment of inertia about an axis through the centroid, and c is the distance from the centroid to the extreme edge of the section. Servo System: Refers to a closed loop control system where a command is issued for a change in position and the change is then verified via a feedback system. Settling Time: Time required for a motion system to cease motion once the command for motion has ended. Shaft Radial Load: Maximum radial load that can be applied to the end of the motor shaft at maximum motor speed. Shaft Runout: Deviation from straight line travel. Shear: An system of internal forces whose resultant is a force acting perpendicular to the longitudinal axis of a structural member or assembly: sometimes called shear force. Shear Stress: Stress acting parallel to an imaginary plane cut through an object. Shear Strain: Strain measuring the intensity of racking in the material. Shear strain is measured as the change in angle of the corners of a small square of material. Shear Modulus: The ratio of shear stress divided by the corresponding shear strain in a linear elastic material. Slides: Highly accurate short stroke linear motion mechanisms that use non-recirculating linear bearings (crossed roller or ball bearing elements), have rolling elements that move on a straight track usually separated by a retainer. The bearing reaches the end of its travel when the retainer or rolling element contacts a limiting component, typically either a screw head, end cap or some other physical hard stop. Stability: Stability is best defined as the opposite of instability, which is the occurrence of large structural deformations which are not the result of material failure. Stages: Positioning stages are slides that are typically manually driven by a linear actuation device, most commonly a micrometer type mechanism. The devices will smoothly translate to a position which can be measured and then locked into position. Often ball screw driven and combined to form multi-axis systems. Static Equilibrium: Equilibrium which does not include inertial forces. Statically Determinate: A statically determinate structure is one where there is only one distribution of internal forces and reactions which satisfies equilibrium. In a statically determinate structure, internal forces and reactions can be determined by considering nothing more than equations of equilibrium. Statically Equivalent: Two force systems are statically equivalent when their resultants are equal. Physically, this means that the force systems tend to impart the same motion when applied to an object; note that the distribution of resulting internal forces in the object may be different. Straightness: Deviation from the true line of travel perpendicular to the direction of travel in the horizontal plane. Statically Indeterminate: A statically indeterminate structure is one where there is more than one distribution of internal forces and/or reactions which satisfies equilibrium. Stroke Bearing: Rotary stroke bearings consist of the cylindrical shaft and bush, which act as guiding elements, and the steel balls, which act as the rolling bearing elements. The balls are held in a brass or plastic smooth-moving tube where they roll easily. Travel is limited since the rolling elements do not re-circulate. Support Unit: A bearing housing assembly used to support a ball screw. Stiffness: This is a general term which may be applied to materials or structures. When a force is applied to a structure, there is a displacement in the direction of the force; stiffness is the ratio of the force divided by the displacement. High stiffness means that a large force produces a small displacement. When discussing the stiffness of a material, the concept is the same, except that stress substitutes for force, and strain substitutes for displacement; see modulus of elasticity. Stiction: Friction encountered when accelerating an object from a stationary position. Static friction is always greater than moving friction, and limits the smallest possible increment of movement. Straightness of Travel: Measure of the side-to-side deviation of a stage as it travels in a horizontal plane. Strain: The intensity of deformation at a point in an object. See normal strain and shear strain. Strength: A very general term that may be applied to a material or a structure. In a material, strength refers to a level of stress at which there is a significant change in the state of the material, e.g., yielding or rupture. In a structure, strength refers to a level of level of loading which produces a significant change in the state of the structure, e.g., inelastic deformations, buckling, or collapse. Stress Resultant: A system of forces which is statically equivalent to a stress distribution over an area. Stress: The intensity of internal force acting at a point in an object. Stress is measured in units of force per area. See shear stress and normal stress. Support: A support contributes to keeping a structure in place by restraining one or more degrees of freedom. In a structural model, supports represent boundary entities which are not included in the model itself, e.g., foundations, abutments, or the earth itself. For each restrained translation degree of freedom at a support, there is a corresponding reaction force; for each restrained rotation degree of freedom, there is a reaction moment. Surface Force: A force applied to the surface of an object.
System of Forces: One or more forces and/or moments acting simultaneously.
Torque: Rotary equivalent to force. Equal to the product of the force perpendicular to the radius of motion and distance from the center of rotation to the point where the force is applied. Torque (Continuous): Torque needed to drive a load over a continuous time. Torque (Peak): Maximum amount of torque a motor can deliver when the highest allowable peak currents are applied. Torque (RMS): Root Mean Square (rms) is a mathematical method to determine a steadfast or average torque for a motor. Torque (Stall): The torque which is produced by a device when the output rotational speed is zero. The maximum torque an electric motor can produce in the long term when stalled without causing damage is called the maximum continuous stall torque. Total Indicated Reading (TIR): The full indicator reading observed when a dial indicator is in contact with the part surface during one full revolution of the part about its axis of rotation.
Tuning: In a servo system, the process of optimizing loop gains (usually PID terms) to achieve the desired response from a stage or mechanism from an input command.
Vector: A mathematical entity having a magnitude and a direction in space. Velocity: A vector quantity equal to the rate that position changes with time.
Velocity (Command): Motor driver or amplifier configuration where the input signal is commanding motor velocity. Motors with analog tachometers are normally driven by this driver configuration.
Weight: The force on an object resulting from gravity.
Wobble: An irregular, non-repeatable rocking or staggering motion of the table top of a rotary stage. Wobble is defined as an angular error between the actual axis of rotation and the theoretical axis of rotation.
Yaw: Rotation around an axis in the vertical (up/down) plane perpendicular to the direction of travel. Yaw Error: Positioning error resulting from a yaw motion. Yield Stress: A material loaded beyond its yield stress, no longer exhibits linear elastic behavior. Metals, particularly mild steel, generally have a very well defined yield stress compared to other materials. Yield stress is sometimes called yield strength. Yield Strain: A material deformed beyond its yield strain, no longer exhibits linear elastic behavior. See yield stress.
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