Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated, страница 14

The bidirectional digital linear actuator shown in Figure 1-33 can provide linear res-Figure 1-33 This light-duty linear actuator based on a permanent-magnet stepping motor hasa shaft that advances or retracts.Chapter 1Motor and Motion Control Systemsolution as fine as 0.001 in. per pulse. Travel per step is determined by thepitch of the leadscrew and step angle of the motor.

The maximum linearforce for the model shown is 75 oz.SERVOSYSTEM FEEDBACK SENSORSA servosystem feedback sensor in a motion control system transforms aphysical variable into an electrical signal for use by the motion controller. Common feedback sensors are encoders, resolvers, and linearvariable differential transformers (LVDTs) for motion and position feedback, and tachometers for velocity feedback. Less common but also inuse as feedback devices are potentiometers, linear velocity transducers(LVTs), angular displacement transducers (ADTs), laser interferometers,and potentiometers.

Generally speaking, the closer the feedback sensoris to the variable being controlled, the more accurate it will be in assisting the system to correct velocity and position errors.For example, direct measurement of the linear position of the carriagecarrying the load or tool on a single-axis linear guide will provide moreaccurate feedback than an indirect measurement determined from theangular position of the guide’s leadscrew and knowledge of the drivetrain geometry between the sensor and the carriage. Thus, direct positionmeasurement avoids drivetrain errors caused by backlash, hysteresis, andleadscrew wear that can adversely affect indirect measurement.Rotary EncodersRotary encoders, also called rotary shaft encoders or rotary shaft-angleencoders, are electromechanical transducers that convert shaft rotationinto output pulses, which can be counted to measure shaft revolutions orshaft angle.

They provide rate and positioning information in servo feedback loops. A rotary encoder can sense a number of discrete positionsper revolution. The number is called points per revolution and is analogous to the steps per revolution of a stepper motor. The speed of anencoder is in units of counts per second. Rotary encoders can measurethe motor-shaft or leadscrew angle to report position indirectly, but theycan also measure the response of rotating machines directly.The most popular rotary encoders are incremental optical shaft-angleencoders and the absolute optical shaft-angle encoders.

There are alsodirect contact or brush-type and magnetic rotary encoders, but they arenot as widely used in motion control systems.4344Chapter 1Motor and Motion Control SystemsCommercial rotary encoders are available as standard or catalog units,or they can be custom made for unusual applications or survival inextreme environments. Standard rotary encoders are packaged in cylindrical cases with diameters from 1.5 to 3.5 in. Resolutions range from 50cycles per shaft revolution to 2,304,000 counts per revolution.

A variation of the conventional configuration, the hollow-shaft encoder, eliminates problems associated with the installation and shaft runout of conventional models. Models with hollow shafts are available for mountingon shafts with diameters of 0.04 to 1.6 in. (1 to 40 mm).Incremental EncodersThe basic parts of an incremental optical shaft-angle encoder are shownin Figure 1-34. A glass or plastic code disk mounted on the encoder shaftrotates between an internal light source, typically a light-emitting diode(LED), on one side and a mask and matching photodetector assembly onthe other side. The incremental code disk contains a pattern of equallyspaced opaque and transparent segments or spokes that radiate out fromits center as shown.

The electronic signals that are generated by theencoder’s electronics board are fed into a motion controller that calculates position and velocity information for feedback purposes. Anexploded view of an industrial-grade incremental encoder is shown inFigure 1-35.Glass code disks containing finer graduations capable of 11- to morethan 16-bit resolution are used in high-resolution encoders, and plastic(Mylar) disks capable of 8- to 10-bit resolution are used in the morerugged encoders that are subject to shock and vibration.Figure 1-34 Basic elements ofan incremental optical rotaryencoder.Chapter 1The quadrature encoder is the most commontype of incremental encoder.

Light from theLED passing through the rotating code diskand mask is “chopped” before it strikes thephotodetector assembly. The output signalsfrom the assembly are converted into two channels of square pulses (A and B) as shown inFigure 1-36. The number of square pulses ineach channel is equal to the number of codedisk segments that pass the photodetectors asthe disk rotates, but the waveforms are 90º outof phase. If, for example, the pulses in channelA lead those in channel B, the disk is rotatingin a clockwise direction, but if the pulses inchannel A lag those in channel B lead, the diskis rotating counterclockwise.

By monitoringboth the number of pulses and the relativephases of signals A and B, both position anddirection of rotation can be determined.Many incremental quadrature encoders alsoinclude a third output Z channel to obtain azero reference or index signal that occurs onceper revolution. This channel can be gated to theA and B quadrature channels and used to trigger certain events accurately within the system.The signal can also be used to align the encodershaft to a mechanical reference.Motor and Motion Control Systems45Figure 1-35 Exploded view of an incremental optical rotaryencoder showing the stationary mask between the code wheeland the photodetector assembly.Figure 1-36 Channels A and Bprovide bidirectional positionsensing. If channel A leads channel B, the direction is clockwise; ifchannel B leads channel A, thedirection is counterclockwise.Channel Z provides a zero reference for determining the numberof disk rotations.46Chapter 1Motor and Motion Control SystemsAbsolute EncodersAn absolute shaft-angle optical encoder contains multiple light sourcesand photodetectors, and a code disk with up to 20 tracks of segmentedpatterns arranged as annular rings, as shown in Figure 1-37.

The codedisk provides a binary output that uniquely defines each shaft angle, thusproviding an absolute measurement. This type of encoder is organized inessentially the same way as the incremental encoder shown in Figure 135, but the code disk rotates between linear arrays of LEDs and photodetectors arranged radially, and a LED opposes a photodetector for eachtrack or annular ring.The arc lengths of the opaque and transparent sectors decrease withrespect to the radial distance from the shaft. These disks, also made ofglass or plastic, produce either the natural binary or Gray code. Shaftposition accuracy is proportional to the number of annular rings or trackson the disk.

When the code disk rotates, light passing through each trackor annular ring generates a continuous stream of signals from the detector array. The electronics board converts that output into a binary word.The value of the output code word is read radially from the most significant bit (MSB) on the inner ring of the disk to the least significant bit(LSB) on the outer ring of the disk.The principal reason for selecting an absolute encoder over an incremental encoder is that its code disk retains the last angular position of theencoder shaft whenever it stops moving, whether the system is shutdown deliberately or as a result of power failure. This means that the lastreadout is preserved, an important feature for many applications.Figure 1-37 Binary-code disk foran absolute optical rotaryencoder.

Opaque sectors represent a binary value of 1, and thetransparent sectors representbinary 0. This four-bit binary-codedisk can count from 1 to 15.Chapter 1Motor and Motion Control Systems47Linear EncodersLinear encoders can make direct accurate measurements of unidirectional and reciprocating motions of mechanisms with high resolution andrepeatability. Figure 1-38 illustrates the basic parts of an optical linearencoder. A movable scanning unit contains the light source, lens, graduated glass scanning reticule, and an array of photocells. The scale, typically made as a strip of glass with opaque graduations, is bonded to asupporting structure on the host machine.A beam of light from the light source passes through the lens, fourwindows of the scanning reticule, and the glass scale to the array of photocells.

When the scanning unit moves, the scale modulates the lightbeam so that the photocells generate sinusoidal signals.The four windows in the scanning reticule are each 90º apart in phase.The encoder combines the phase-shifted signal to produce two symmetrical sinusoidal outputs that are phase shifted by 90º. A fifth pattern onthe scanning reticule has a random graduation that, when aligned with anidentical reference mark on the scale, generates a reference signal.A fine-scale pitch provides high resolution. The spacing between thescanning reticule and the fixed scale must be narrow and constant toeliminate undesirable diffraction effects of the scale grating. The complete scanning unit is mounted on a carriage that moves on ball bearingsalong the glass scale.

The scanning unit is connected to the host machineFigure 1-38 Optical linearencoders direct light through amoving glass scale with accurately etched graduations to photocells on the opposite side forconversion to a distance value.48Chapter 1Motor and Motion Control Systemsslide by a coupling that compensates for any alignment errors betweenthe scale and the machine guideways.External electronic circuitry interpolates the sinusoidal signals fromthe encoder head to subdivide the line spacing on the scale so that it canmeasure even smaller motion increments.