Digital Sensors

Digital transducers are ideal devices for motion measurement. They produce a digital output which can be interfaced to the computer. They have become increasingly attractive because of the following properties.

• Signal conditioning simplicity

• Minor susceptibility to electro-magnetic interference While they are used to measure linear or angular displacement, digital transducers also are used to measure force, pressure, and liquid level with the appropriate mechanical or electromechanical translators.

1 Digital Encoders

Encoders are widely used for applications involving measurement of linear or angular position, velocity, and direction of movement. They are used not only as a part of computerized machines but also in many precision-measurement devices, motion control applications, and quality assurance of equipment at various stages of production. Encoders are used in tensile-test instruments to precisely measure the ball screw position used to apply tension or compression to the test specimen. They are used in automated test stands used when angular positions of windshield wiper drives and switch positions are tested.

The most popular encoders are linear- or rotary-type optical encoders. Other configurations, such as contact-type encoders, have serious limitations due to contact wear and low resolution.

2 Encoder Principle

An encoder is a circular device in the form of a disk on which a digital pattern is etched.

The inscribed pattern is sensed by means of a sensing head. The rotary disk is normally coupled to a shaft. As the shaft rotates, a different pattern is generated for each resolvable position.

The sensing mechanism can be a photoelectric device with slots acting as transparent optical windows.

An optical encoder generally is used to precisely measure rotational movement. Its main advan tages are simplicity, accuracy, and suitability for sensitive applications. Optical encoders are con sidered one of the most reliable and least expensive motion-feedback devices available and are used widely in a broad range of modern applications. Information obtainable from an optical encoder includes direction, distance, velocity, and position.

There are two types of encoders; incremental and absolute. An incremental encoder provides a simple pulse each time the object to be measured has moved a given distance. An absolute encoder provides a unique binary word coded to represent a given position of the object.

3 Incremental Encoders

Incremental encoders for angular measurement consist of a sensing shaft attached to a disk which is divided into an equal number of sectors on the circumference. In the linear type of encoders, there are equal segments along the length of travel. The readings are sensed by direct electrical contact with a brush or wiper or optically using optical slits or gratings. Since it counts the lines on a disk, the more lines, the higher the resolution. This specification is expressed as pulses per revolution, which is an important factor in encoder selection.

Incremental rotary encoders are very useful for measuring shaft rotation and primarily consist of three components: a light source, a coded wheel, and a photoelectric sensor. Ill. 25 shows an encoder measuring system which uses transmission gratings. As the movable grating translates with respect to a fixed grating, the pulses are counted to provide position information.

ILL. 25 GRATING TRANSDUCER PRINCIPLE

4 Absolute Encoders

The absolute encoder normally has a light source which emits a beam of light onto a photoelec tric sensor called a photo detector. This converts the receiving light into an electrical signal, as shown in Ill. 2 An optical encoded wheel (circular absolute grating) is mounted between the light source and photo detector. The encoded wheel has several concentric circular tracks that are divided into sectors. Manufactured into the surface of the coded wheel are alternating opaque and transparent sections. When the opaque section of the wheel passes in front of the light, the detector is turned off, and no signal is generated. When the transparent section of the wheel passes in front, the detector is turned on, and a signal is generated. The result is a series of sig nals corresponding to the rotation of the coded wheel. By using a counter to count these signals, it's possible to find out how far the wheel has rotated. Velocity information also can be obtained by differencing the pulses.

ILL. 26 OPTICAL ENCODING

Incremental encoders are more commonly used than absolute encoders because of their sim plicity and lower cost. Incremental encoders are used for both velocity and position measurement and are one of the most reliable and inexpensive devices available for this task.

5 Linear Encoder (Reflection Type)

Optical gratings are used both in linear and radial forms, with the latter being rotated directly by the lead screw or a rack-and-pinion arrangement. Recent years have seen increasing use of steel or steel-backed reflection scale grating, which for many engineering purposes is preferred to trans mission gratings because of the increased durability and rigidity of steel gratings in comparison to optical gratings. In linear reflection-type encoders, the light must pass to the scale grating through the index grating and be reflected back through the index grating to the photoelectric sensor.

Ill. 27 shows a linear measuring system using reflection gratings. The fixed portion of the transducer box consists of a source of light, associated optics, and the detection system. The out put of the detector is shown in the form of a digital read out. These types of transducers are popular in the machine-tool industry.

ILL. 27 LINEAR ENCODER (REFLECTION TYPE)

6 Moiré Fringe Transducers

The moiré fringe principle is used in some types of digital transducers. These transducers also are used to measure length, angle, straightness, and circularity of motion. The transducer can supply information about the variable required and is relatively unaffected by external effects. An essential element of a transducer is an optical grating. An optical grating consists of regular succession of opaque lines separated by clear spaces of equal width. The lines are at right angles to the length of the grating. When two sections of such a grating are superimposed with the lines at slight angle to each other, a moiré fringe pattern with approximately a sinusoidal distribution of intensity results from the integrated interference effects of the interaction of the lines on each grating.

When one grating is moved with respect to the other at right angles to its lines, the moiré fringe pattern travels at right angles to the direction of movement. The sense of movement depends on the sense of relative travel of the gratings. This principle is shown in Ill. 28.

ILL. 28 (A) MOIRE FRINGES (B) FRINGE SEPARATION

Analysis of geometric relationships between the moiré fringes and the grating pair producing them leads to a finer comprehension of the potentialities of the moiré fringe measuring techniques.

where

_A, _B = pitches of the gratings A and B, respectively

_ _ fringe separation

_ _ acute angle formed by the intersecting gratings

_ _ acute or obtuse angle between the lines of the first gratings and the fringe

7 Applications

ILL. 29 digital transducers for machine tool measurement

Whenever encoders are used, they have to be calibrated for that specific situation. This is important because of the differing sizes, resolution requirements, and the specific nature of the movement. e.g., Ill. 29 shows encoders that are mounted to measure the displacements in two axial directions of a high-precision machine tool.

The distance to be traveled and the direction of travel are transmitted to the processor as reference values. This data gives reference values to the controller and the drive motor. If these values don't agree, the motor continues the rotation. Once they agree, the processor sends a stop signal to the controller, indicating the final slide position. If a new reference value is provided, the process is continued.

Absolute encoders are used in applications where the location of an object or identifying its position is of special interest. Unlike the incremental encoder, which determines position by counting pulses from the datum, the absolute encoder reads the system of coded tracks to establish the position. These encoders don't lose position when power is off. Each position is uniquely identified by a nonvolatile position verification device. Absolute encoders are chosen for situations, where establishing position status is desired as well as the possibility of avoiding equipment dam age. This feature is useful in satellite tracking antennas, where occasional position verification is necessary, or in situations where an object is inactive for long periods of time or moves at very slow rates. Whenever the power is turned on, true position can be verified. Absolute encoders are not affected by stray signals from electrical noise and also can be used for serial data output for long distance transmission. The absolute encoder is either a linear or an angular type. They may be single or multi-turn devices-the latter having higher accuracy and resolution.

Application in the Manufacturing Industry

• Machine slide position in numerically controlled machine tools
• Vertical and horizontal boring machines and precision lathes
• Gauging applications, such as in measuring calipers or digital height gauges
• As extensometers and measuring scales in structural research

The savings in indirect operator time using a digital measurement system often justifies the capital cost of transducer and display devices. Other advantages include further savings resulting from reduced scrap, operator fatigue, improved floor-to-floor time, and easier fitting.

Encoders in various configurations are possible with scaling in units of millimeters or inches, while the use of dual inch-metric capability is popular in the machine-tool industry. Angular encoders are calibrated to read degrees, minutes, seconds of arc, or (alternatively) decimal fractions of the degree. it's common to attach optical shaft encoders to the lead screw of the machine tool to digitize the screw position. The use of linear encoders eliminates the error caused by backlash in the lead screw and other mechanical transmission systems.

For applications requiring high resolution, the size of the transparent and opaque sections must be made very small, and the light source must be properly aligned in order for the photo detector to sense a change in light. Multiplication techniques can be used to increase the resolution. Four times magnification is commonly achieved by externally counting the rising and falling edges of each channel. e.g., a 5,000 ppr quadrature encoder can generate 20,000 ppr using this technique.

SUMMARY Rotary Encoder

Encoders have both linear and rotary configurations. Rotary encoders are available in two forms.

1. Incremental encoders produce digital pulses as shaft rotates, allowing relative displacement of shaft to be measured.

2. Absolute encoders have a unique digital word corresponding to each rotational position of the shaft.

Incremental encoders (Ill. 30) are useful for measuring shaft rotation and consist of primarily three components: a light source, a coded wheel, and a photoelectric sensor. An incremental encoder provides a simple pulse each time the object to be measured has moved a given distance.

ILL. 30 Photodetectors, Coded disk, Light source

Moiré Fringe Transducer

ILL. 31

When two sections of optical gratings are superimposed with the lines at slight angle to each other, a moiré fringe pattern is generated (Ill. 31). The interference effect of the lines provides a sinusoidal distribution of intensity. When one grating is moved with respect to the other at right angles to its lines, the moiré fringe pattern travels at right angles to the direction of movement; the sense of movement depends on the sense of relative travel of the gratings.

Applications

• Encoders are used for measurement of linear or angular position, velocity, and direction of movement.

• Used in computerized manufacturing machines, motion-control applications, and quality assurance of equipment.

• Used in tensile-test instruments to precisely measure the ball screw position.

• Used in automated test stands used when angular positions of windshield wiper drives and switch positions are tested.

• Incremental encoders commonly are used for counting applications.

• The moiré fringe transducers also are used to measure length, angle, straightness, and circularity of motion.

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