Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 7

Prototype early, prototype often, and test everything. Mobile robots are inherently complex devices with many interactions within themselves and with their environment. The result of theeffort, though, is exciting, fun, and rewarding. There is nothing like seeing an autonomous robot happily driving around, doing some useful taskcompletely on its own.This page intentionally left blank.AcknowledgmentsThis book would not even have been considered and would never havebeen completed without the encouragement and support of my loving wife, Victoria. Thank you so much.In addition to the support of my wife, I would like to thank Joe Jonesfor his input, criticism, and support. Thank you for putting up with mymany questions. Thanks also goes to Lee Sword, Chi Won, Tim Ohm,and Scott Miller for input on many of the ideas and layouts.

The processof writing this book was made much easier by iRobot allowing me to usetheir office machines. And, lastly, thanks to my extended family, especially my Dad and Jenny for their encouragement and patience.xxxvCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.Chapter 1Motor and MotionControl SystemsCopyright © 2003 by The McGraw-Hill Companies, Inc.

Click here for Terms of Use.This page intentionally left blank.INTRODUCTIONA modern motion control system typically consists of a motion controller, a motor drive or amplifier, an electric motor, and feedback sensors. The system might also contain other components such as one ormore belt-, ballscrew-, or leadscrew-driven linear guides or axis stages.A motion controller today can be a standalone programmable controller,a personal computer containing a motion control card, or a programmable logic controller (PLC).All of the components of a motion control system must work togetherseamlessly to perform their assigned functions.

Their selection must bebased on both engineering and economic considerations. Figure 1-1illustrates a typical multiaxis X-Y-Z motion platform that includes thethree linear axes required to move a load, tool, or end effector preciselythrough three degrees of freedom. With additional mechanical or electro-Figure 1-1 This multiaxis X-Y-Zmotion platform is an example ofa motion control system.34Chapter 1Motor and Motion Control SystemsFigure 1-2 The right-handedcoordinate system showing sixdegrees of freedom.mechanical components on each axis, rotation about the three axes canprovide up to six degrees of freedom, as shown in Figure 1-2.Motion control systems today can be found in such diverse applications as materials handling equipment, machine tool centers, chemicaland pharmaceutical process lines, inspection stations, robots, and injection molding machines.Merits of Electric SystemsMost motion control systems today are powered by electric motorsrather than hydraulic or pneumatic motors or actuators because of themany benefits they offer:• More precise load or tool positioning, resulting in fewer product orprocess defects and lower material costs.• Quicker changeovers for higher flexibility and easier product customizing.• Increased throughput for higher efficiency and capacity.• Simpler system design for easier installation, programming, andtraining.• Lower downtime and maintenance costs.• Cleaner, quieter operation without oil or air leakage.Electric-powered motion control systems do not require pumps or aircompressors, and they do not have hoses or piping that can leakChapter 1Motor and Motion Control Systems5hydraulic fluids or air.

This discussion of motion control is limited toelectric-powered systems.Motion Control ClassificationMotion control systems can be classified as open-loop or closed-loop.An open-loop system does not require that measurements of any outputvariables be made to produce error-correcting signals; by contrast, aclosed-loop system requires one or more feedback sensors that measureand respond to errors in output variables.Closed-Loop SystemA closed-loop motion control system, as shown in block diagramFigure 1-3, has one or more feedback loops that continuously compare thesystem’s response with input commands or settings to correct errors inmotor and/or load speed, load position, or motor torque.

Feedback sensorsprovide the electronic signals for correcting deviations from the desiredinput commands. Closed-loop systems are also called servosystems.Each motor in a servosystem requires its own feedback sensors, typically encoders, resolvers, or tachometers that close loops around themotor and load. Variations in velocity, position, and torque are typicallycaused by variations in load conditions, but changes in ambient temperature and humidity can also affect load conditions.A velocity control loop, as shown in block diagram Figure 1-4, typicallycontains a tachometer that is able to detect changes in motor speed. Thissensor produces error signals that are proportional to the positive or negative deviations of motor speed from its preset value.

These signals are sentFigure 1-3 Block diagram of abasic closed-loop control system.6Chapter 1Motor and Motion Control SystemsFigure 1-4 Block diagram of avelocity-control system.to the motion controller so that it can compute a corrective signal for theamplifier to keep motor speed within those preset limits despite loadchanges.A position-control loop, as shown in block diagram Figure 1-5, typically contains either an encoder or resolver capable of direct or indirectmeasurements of load position. These sensors generate error signals thatare sent to the motion controller, which produces a corrective signal foramplifier. The output of the amplifier causes the motor to speed up orslow down to correct the position of the load.

Most position controlclosed-loop systems also include a velocity-control loop.The ballscrew slide mechanism, shown in Figure 1-6, is an example ofa mechanical system that carries a load whose position must be controlledin a closed-loop servosystem because it is not equipped with position sensors. Three examples of feedback sensors mounted on the ballscrewmechanism that can provide position feedback are shown in Figure 1-7:(a) is a rotary optical encoder mounted on the motor housing with its shaftcoupled to the motor shaft; (b) is an optical linear encoder with its gradu-Figure 1-5 Block diagram of aposition-control system.Chapter 1Motor and Motion Control Systems7Figure 1-6 Ballscrew-drivensingle-axis slide mechanism without position feedback sensors.ated scale mounted on the base of the mechanism; and (c) is the less commonly used but moreaccurate and expensive laser interferometer.A torque-control loop contains electronic circuitry that measures the input current applied tothe motor and compares it with a value proportional to the torque required to perform thedesired task.

An error signal from the circuit issent to the motion controller, which computes acorrective signal for the motor amplifier to keepmotor current, and hence torque, constant.Torque- control loops are widely used in machine tools where the load can change due tovariations in the density of the material beingmachined or the sharpness of the cutting tools.Trapezoidal Velocity ProfileIf a motion control system is to achieve smooth,high-speed motion without overstressing the ser-Figure 1-7 Examples of position feedback sensors installedon a ballscrew-driven slide mechanism: (a) rotary encoder,(b) linear encoder, and (c) laser interferometer.8Chapter 1Motor and Motion Control SystemsFigure 1-8 Servomotors areaccelerated to constant velocityand decelerated along a trapezoidal profile to assure efficientoperation.vomotor, the motion controller must command the motor amplifier toramp up motor velocity gradually until it reaches the desired speed andthen ramp it down gradually until it stops after the task is complete.

Thiskeeps motor acceleration and deceleration within limits.The trapezoidal profile, shown in Figure 1-8, is widely used because itaccelerates motor velocity along a positive linear “up-ramp” until thedesired constant velocity is reached. When the motor is shut down fromthe constant velocity setting, the profile decelerates velocity along a negative “down ramp” until the motor stops.

Amplifier current and outputvoltage reach maximum values during acceleration, then step down tolower values during constant velocity and switch to negative values during deceleration.Closed-Loop Control TechniquesThe simplest form of feedback is proportional control, but there are alsoderivative and integral control techniques, which compensate for certainsteady-state errors that cannot be eliminated from proportional control.All three of these techniques can be combined to form proportionalintegral-derivative (PID) control.• In proportional control the signal that drives the motor or actuator isdirectly proportional to the linear difference between the input command for the desired output and the measured actual output.• In integral control the signal driving the motor equals the time integral of the difference between the input command and the measuredactual output.Chapter 1Motor and Motion Control Systems• In derivative control the signal that drives the motor is proportionalto the time derivative of the difference between the input commandand the measured actual output.• In proportional-integral-derivative (PID) control the signal thatdrives the motor equals the weighted sum of the difference, the timeintegral of the difference, and the time derivative of the differencebetween the input command and the measured actual output.Open-Loop Motion Control SystemsA typical open-loop motion control system includes a stepper motor witha programmable indexer or pulse generator and motor driver, as shown inFigure 1-9.

This system does not need feedback sensors because loadFigure 1-9 Block diagram ofan open-loop motion controlsystem.position and velocity are controlled by the predetermined number anddirection of input digital pulses sent to the motor driver from the controller.