Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 8
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Because load position is not continuously sampled by a feedbacksensor (as in a closed-loop servosystem), load positioning accuracy islower and position errors (commonly called step errors) accumulate overtime. For these reasons open-loop systems are most often specified inapplications where the load remains constant, load motion is simple, andlow positioning speed is acceptable.Kinds of Controlled MotionThere are five different kinds of motion control: point-to-point, sequencing, speed, torque, and incremental.• In point-to-point motion control the load is moved between asequence of numerically defined positions where it is stopped beforeit is moved to the next position.
This is done at a constant speed, withboth velocity and distance monitored by the motion controller. Pointto-point positioning can be performed in single-axis or multiaxis systems with servomotors in closed loops or stepping motors in open910Chapter 1Motor and Motion Control Systems••••loops. X-Y tables and milling machines position their loads by multiaxis point-to-point control.Sequencing control is the control of such functions as opening andclosing valves in a preset sequence or starting and stopping a conveyor belt at specified stations in a specific order.Speed control is the control of the velocity of the motor or actuator ina system.Torque control is the control of motor or actuator current so thattorque remains constant despite load changes.Incremental motion control is the simultaneous control of two ormore variables such as load location, motor speed, or torque.Motion InterpolationWhen a load under control must follow a specific path to get from itsstarting point to its stopping point, the movements of the axes must becoordinated or interpolated.
There are three kinds of interpolation: linear, circular, and contouring.Linear interpolation is the ability of a motion control system havingtwo or more axes to move the load from one point to another in a straightline. The motion controller must determine the speed of each axis so thatit can coordinate their movements. True linear interpolation requires thatthe motion controller modify axis acceleration, but some controllersapproximate true linear interpolation with programmed acceleration profiles. The path can lie in one plane or be three dimensional.Circular interpolation is the ability of a motion control system havingtwo or more axes to move the load around a circular trajectory.
Itrequires that the motion controller modify load acceleration while it is intransit. Again the circle can lie in one plane or be three dimensional.Contouring is the path followed by the load, tool, or end- effectorunder the coordinated control of two or more axes. It requires that themotion controller change the speeds on different axes so that their trajectories pass through a set of predefined points.
Load speed is determinedalong the trajectory, and it can be constant except during starting andstopping.Computer-Aided EmulationSeveral important types of programmed computer-aided motion controlcan emulate mechanical motion and eliminate the need for actual gearsChapter 1Motor and Motion Control Systems11or cams. Electronic gearing is the control by software of one or moreaxes to impart motion to a load, tool, or end effector that simulates thespeed changes that can be performed by actual gears.
Electronic camming is the control by software of one or more axes to impart a motion toa load, tool, or end effector that simulates the motion changes that aretypically performed by actual cams.Mechanical ComponentsThe mechanical components in a motion control system can be moreinfluential in the design of the system than the electronic circuitry usedto control it.
Product flow and throughput, human operator requirements,and maintenance issues help to determine the mechanics, which in turninfluence the motion controller and software requirements.Mechanical actuators convert a motor’s rotary motion into linearmotion. Mechanical methods for accomplishing this include the use ofleadscrews, shown in Figure 1-10, ballscrews, shown in Figure 1-11,worm-drive gearing, shown in Figure 1-12, and belt, cable, or chaindrives. Method selection is based on the relative costs of the alternativesand consideration for the possible effects of backlash.
All actuators havefinite levels of torsional and axial stiffness that can affect the system’sfrequency response characteristics.Figure 1-10 Leadscrew drive: Asthe leadscrew rotates, the load istranslated in the axial direction ofthe screw.12Chapter 1Motor and Motion Control SystemsFigure 1-11 Ballscrew drive: Ballscrews use recirculatingballs to reduce friction and gain higher efficiency than conventional leadscrews.Figure 1-12 Worm-drive systems can provide high speedand high torque.Linear guides or stages constrain a translating load to a single degreeof freedom. The linear stage supports the mass of the load to be actuatedand assures smooth, straight-line motion while minimizing friction.
Acommon example of a linear stage is a ballscrew-driven single-axisstage, illustrated in Figure 1-13. The motor turns the ballscrew, and itsrotary motion is translated into the linear motion that moves the carriageand load by the stage’s bolt nut. The bearing ways act as linear guides.As shown in Figure 1-7, these stages can be equipped with sensors suchas a rotary or linear encoder or a laser interferometer for feedback.A ballscrew-driven single-axis stage with a rotary encoder coupled tothe motor shaft provides an indirect measurement. This method ignoresFigure 1-13 Ballscrew-drivensingle-axis slide mechanism translates rotary motion into linearmotion.Chapter 1Motor and Motion Control Systems13Figure 1-14 This single-axis linear guide for load positioning issupported by air bearings as itmoves along a granite base.the tolerance, wear, and compliance in the mechanical componentsbetween the carriage and the position encoder that can cause deviationsbetween the desired and true positions.
Consequently, this feedbackmethod limits position accuracy to ballscrew accuracy, typically ±5 to 10µm per 300 mm.Other kinds of single-axis stages include those containing antifrictionrolling elements such as recirculating and nonrecirculating balls orrollers, sliding (friction contact) units, air-bearing units, hydrostatic units,and magnetic levitation (Maglev) units.A single-axis air-bearing guide or stage is shown in Figure 1-14. Somemodels being offered are 3.9 ft (1.2 m) long and include a carriage formounting loads. When driven by a linear servomotors the loads can reachvelocities of 9.8 ft/s (3 m/s). As shown in Figure 1-7, these stages can beequipped with feedback devices such as cost-effective linear encoders orultra-high-resolution laser interferometers.
The resolution of this type ofstage with a noncontact linear encoder can be as fine as 20 nm and accuracy can be ±1 µm. However, these values can be increased to 0.3 nm resolution and submicron accuracy if a laser interferometer is installed.The pitch, roll, and yaw of air-bearing stages can affect their resolution and accuracy. Some manufacturers claim ±1 arc-s per 100 mm as thelimits for each of these characteristics. Large air-bearing surfaces provide excellent stiffness and permit large load-carrying capability.The important attributes of all these stages are their dynamic andstatic friction, rigidity, stiffness, straightness, flatness, smoothness, andload capacity.
Also considered is the amount of work needed to preparethe host machine’s mounting surface for their installation.14Chapter 1Motor and Motion Control SystemsFigure 1-15 Flexible shaft couplings adjust for and accommodate parallel misalignment (a)and angular misalignmentbetween rotating shafts (b).Figure 1-16 Bellows couplings(a) are acceptable for light-dutyapplications. Misalignments canbe 9º angular or 1⁄4 in. parallel.Helical couplings (b) preventbacklash and can operate at constant velocity with misalignmentand be run at high speed.The structure on which the motion control system is mounted directlyaffects the system’s performance. A properly designed base or hostmachine will be highly damped and act as a compliant barrier to isolatethe motion system from its environment and minimize the impact ofexternal disturbances. The structure must be stiff enough and sufficientlydamped to avoid resonance problems.
A high static mass to reciprocatingmass ratio can also prevent the motion control system from exciting itshost structure to harmful resonance.Any components that move will affect a system’s response by changing the amount of inertia, damping, friction, stiffness, or resonance. Forexample, a flexible shaft coupling, as shown in Figure 1-15, will compensate for minor parallel (a) and angular (b) misalignment betweenrotating shafts. Flexible couplings are available in other configurationssuch as bellows and helixes, as shown in Figure 1-16. The bellows configuration (a) is acceptable for light-duty applications where misalign-Chapter 1Motor and Motion Control Systemsments can be as great as 9º angular or 1⁄4 in. parallel.
By contrast, helicalcouplings (b) prevent backlash at constant velocity with some misalignment, and they can also be run at high speed.Other moving mechanical components include cable carriers thatretain moving cables, end stops that restrict travel, shock absorbers todissipate energy during a collision, and way covers to keep out dustand dirt.Electronic System ComponentsThe motion controller is the “brain” of the motion control system andperforms all of the required computations for motion path planning,servo-loop closure, and sequence execution.
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