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INDUSTRIAL CONTROL HANDBOOK - 1.3 POSITION SENSORS

Many of the presence detectors already discussed are position sensors in that they measure the presence or non-presence of an object at a certain position. This section is devoted to sensors designed to measure where, within a range of possible positions, the target actually is.

Arrays of many switches can be used to find an object's actual position. Sensor array position sensors can be expensive because the cost of the sensor must include the cost of the switches, the cost of the connecting circuitry, and the cost of the controller required to evaluate the many signals from the switches. Arrays of switches can be economical if they are mass produced.

A single housing containing several contacts was once the preferred type of rotary shaft position sensor.

The rotary position of the shaft could be determined by counting the number of conductive tracks touching the rotating wiper (see Figure 1.15 (a)). The absolute encoder, a variation on this type of position sensor, is very popular and will be discussed later in this section.

Switches built into arrays in a single integrated circuit or printed onto a circuit board are other types of switch arrays that are economically viable.

 

Photodiodes, printed only millimeters apart in linear arrays on metal surfaces as shown in Figure 1.15 (b), are in use as position sensors. Columnated light is projected onto the photosensor array, and the location of the target can be determined by examining which of the photodiodes is not conducting because it isn't receiving light.

 

Pressure switch arrays (Figure 1.15 (c)) are available. They enable automated systems to find the location and orientation of parts resting on the switch array. (Pressure transducer arrays use the amount of pressure at each sensor in their evaluation programs.)

 

CCD cameras include light sensor arrays on a single 1C chip. Each sensor is usually used as a transducer, not as a switch. As the position-sensing task using switch arrays becomes more complex, the need for computerized analysis becomes greater, so most suppliers of switch arrays also supply controllers.

Fig. 1.14 Flow rate sensors

Fig. 1.14 Flow rate sensors

 

Fig. 1.15 Switch arrays as position sensors: (a) rotary position sensor; (b) photodiode array; (c) pressure switch array.

Fig. 1.15 Switch arrays as position sensors: (a) rotary position sensor; (b) photodiode array; (c) pressure switch array.

The majority of position sensors are not switch arrays, but transducers, or sometimes transducer arrays.

The inductive and optical transducers described in the section on non-contact presence sensors are available for use as position sensors. Instead of control circuitry containing switched output, when used as position sensors they must include control circuitry to output an analog value (e.g.. voltage or current) linearly proportional to the sensed distance from the transducer to the "target."

 

Potentiometers, also called "pots" or variable resistors, are making a comeback as position sensors. Pots, shown in Figure 1.16. can be used as linear or as rotary position sensors. Pots output a voltage proportional to the position of a wiper along a variable resistor.

A dependable position sensor with an intimidating name is the linear variable differential transformer, or LVDT. Shown in Figure 1.17. the LVDT is provided with AC at its central (input) coil. The transformer induces AC into the output coils above and below this input coil. Note that the two output coils are connected in series and are opposite wound. If the core is exactly centered, the AC induced into one output coil exactly cancels the AC induced in the other, so the LVDT output will be 0 VAC. The transformer core is movable in the LVDT housing. If the core lifts slightly, there is less voltage induced in the lower output coil than in the upper coil so a small AC output is observed, and this output voltage increases with increased upward displacement of the core. If the upper coil is wound in the same direction as the input coil this output voltage is in phase with the input AC. If the core displaces downward from center, output voltage will also increase proportionally with displacement, but the output AC waveform will be 180 degrees out of phase.

Fig. 1.16 Potentiometric position sensors

Fig. 1.16 Potentiometric position sensors

Magnetostrictive position sensors are recent arrivals as position sensors. They detect the location of a magnetic ring that slides along a conductive metal tube. A magnetostrictive position sensor is shown in Figure 1.18. To detect the position of the magnet, a pulse of DC current is introduced into the tube. Some time later, the current pulse reaches the magnet and passes through its magnetic field. When current moves across a field, the conductor experiences a force. The tube distorts, and a vibration travels back along the tube to a force sensor. The time elapsed between generation of the DC current and the time of the vibration is linearly related to the position of the magnet along the tube.

 

Capacitive position sensors have been used as dial position sensors in radios for years. (Their variable capacitance is used in the radio frequency selection circuitry, so perhaps calling them "sensors" in this application isn't quite right.) A capacitor increases in capacitance as the surface area of the plates facing each other increases. If one 180 degree set of plates is attached to a rotating shaft, and another 180 degree set of plates is held stationary as demonstrated in Figure 1.19, then capacitance increases linearly with shaft rotation through 180 degrees.

A wide assortment of position sensors work on the principle of reflected waveorms. Several reflected waveform principles are shown in Figure 1.20. The simplest of this category is the retroreflective light beam sensor (previously discussed, but this time using the transducer's analog output). The sensor's output is proportional to the amount of light reflected back into the light detector, and therefore proportional to the nearness of the reflective surface.

Fig. 1.17 The linear variable differential transformer (LVDT)

Fig. 1.17 The linear variable differential transformer (LVDT)


Fig. 1.18 The magnetostrictive position sensor. (By permission, Deem Controls Inc., London, Ontario, Canada.)

Fig. 1.18 The magnetostrictive position sensor. (By permission, Deem Controls Inc., London, Ontario, Canada.)


Just slightly more complex are the sensors, such as ultrasound scanners, in which a short pulse of energy (in this case, high frequency sound) is generated. The distance to the target is proportional to the time it takes for the energy to travel to the target and to be reflected back. As used in medical ultrasound scanners, a portion of the energy is reflected by each change in density of the transmission media, and multiple "target" locations can be found. Ultrasound is now available in inexpensive sensors to detect distances to solid objects.

Fig. 1.19 Capacitive position sensors: (a) variable plate engagement; (b) variable electrolyte presence

Fig. 1.19 Capacitive position sensors: (a) variable plate engagement; (b) variable electrolyte presence


More sophisticated and much more precise location measurements can be done with interferometer type sensors, which use energy in the form of (typically) light or sound. The transmitted wave interacts with the reflected wave. If the peaks of the two waveforms coincide, the resultant waveform amplitude is twice the original. If the reflected wave is 180 degrees out of phase with the transmitted wave, the resultant combined waveform has zero amplitude. Between these extremes, the combined waveforms result in a waveform that is still sinusoidal, but has an amplitude somewhere between zero and twice that outputted, and will be phase shifted by between 0 and 180 degrees. This type of sensor can determine distance to a reflective surface to within a fraction of a wavelength. Since some light has wavelengths in the region of 0.0005 mm, this leads to a very fine precision indeed. If laser light is used, the waveforms can travel longer distances without being reduced in energy by scattering.

Fig. 1.20 Reflected waveform sensors: (a) amount of reflected waveform; (b) time of travel; (c) interferometry. (Photograph of ultrasonic sensor by permission, OMRON Canada Inc./ Scarborough, Ontario, Canada.)

Fig. 1.20 Reflected waveform sensors: (a) amount of reflected waveform; (b) time of travel; (c) interferometry. (Photograph of ultrasonic sensor by permission, OMRON Canada Inc./ Scarborough, Ontario, Canada.)


Some position sensors are designed to measure the rotational position of shafts. Two similar types of shaft rotational position sensors are the rotary resolver and the rotary synchro.

The resolver is slightly less complex, so we will discuss it first. Figure 1.21 shows that in construction the resolver looks very similar to a DC motor. It has field windings (two of them, at 90 degrees to each other) and a winding in the rotor. The rotor winding is electrically connected to the outside world through slip rings (not a commutator). In operation, the resolver has more in common with a transformer than it does with a motor. The following description will examine only one possible connection method.

Synchros are different from resolvers in that they include a third "field" winding. The three "field" windings are at 120 degrees from each other. This extra winding allows synchros to be used where added precision is required.

Optical encoders are perhaps the most common shaft position sensor used today. As we will see, they are ideally suited for digital controller use. They come as either absolute or as incremental encoders. Of the two, the incremental encoder is most widely used, so we will discuss it first.

The incremental optical encoder, shown in Figure 1.22, consists of a light source, one or two disks with opaque and transparent sections, three light sensors, and a controller. In the single disk system (shown in the diagram), the disk is attached to the rotating shaft. The stationary light sensors detect light when the transparent section of the disk comes around. The encoder's controller keeps track of the shaft position by counting the number of times the sensors detect changes in light.

Resolution of the sensor increases with the number of transparent sections on the disks, so a two-disk system is more common than the single-disk system. In this type, both disks have finely etched radial transparent lines, as demonstrated with the upper incremental disk in Figure 1.22. One disk is held stationary while another is attached to the rotating shaft. The sensor will thus see light only when the transparent sections on both disks line up.

The controller must also detect the direction of rotation of the shaft. It keeps track of the rotary position by adding or subtracting from the last position every time light is received. Figure 1.22 shows a second track of transparent sections, at 90 degrees from the first. If the shaft is rotating in a clockwise direction, the outer light sensor sees light first. If rotating in a counterclockwise direction, the inner light sensor sees light first.

The third, inner sensor is used to initialize the count. When the light source is unpowered, motion of the shaft goes undetected, so position control systems using incremental encoders must always be "homed" after power has been off. The user first rotates the encoder shaft until it is within one revolution of its "home" position. The controller is then sent a signal that tells it to watch for output from the inner sensor and initialize its counter value when the sensor detects light. The shaft is then slowly rotated. When the one transparent section of the disk at the third sensor comes around, the count is set to zero.

Fig. 1.21 The rotary resolver

Fig. 1.21 The rotary resolver

Fig. 1.22 The incremental optical encoder, (a) Cutaway view of encoder; (b) the etched disk.

Fig. 1.22 The incremental optical encoder, (a) Cutaway view of encoder; (b) the etched disk.

The initiation circuit is then disabled and the counter proceeds to increment and decrement as the other two light sensors detect shaft rotation.

The incremental encoder must at least include circuitry to cause the light transducers to act as switches.

Nothing more is needed if a digital controller is programmed to initialize and keep track of the count. Optionally, the encoder supplier may include sufficient built-in features to initialize and maintain the count, so that a digital controller need only read the position when required. Some controllers are capable of providing an output signal at a preset position.

Unlike incremental optical encoders, absolute optical encoders do not require homing. These encoders consist of a light source, a rotating disk with more than three circumferential sets of transparent sections, a light sensor for each ring of slots, and a circuit card.

Fig. 1.23 Absolute optical encoder.

Fig. 1.23 Absolute optical encoder.

Fig. 1.23 Absolute optical encoder.


The Gray numbering system is used to prevent this type of large error in encoder output. In the Gray numbering system, only one sensor changes its value between one range and the next, so the potential error due to imprecision in transparent section placement or sensor switching times is minimized. Optional circuit boards are available to translate Gray binary numbers to natural binary numbers.

Although optical encoders have been discussed because they are so common, incremental and absolute encoders can be and are manufactured using non-optical switches.

GO TO NEXT PAGE: INDUSTRIAL CONTROL HANDBOOK - 1.4 VELOCITY AND ACCELERATION SENSORS

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