Encoders as angular position indicators

How Does It Work:?

Shown in the diagrams are the important parts of a typical optical encoder. A transparent disk with equally spaced dark radial marks is attached to the rotation axis. A fixed light (LED) sent through the series of dark stripes is sensed with a photodetector. The electrical output is high with light and low with no light. As the axis rotates the electrical signal will change in step movement of the movement of the stripes beneath the light sensor. Two light sensor set near each other provide information to determine the difference in rotation direction. These also create a method of quadrupling the resolution of the encoder.

Incremental Encoder Electrical Output: - two channels A and B

Count at 1 per step (one cycle per optical mark) Count at 4 per step (four cycles per optical mark) Determine direction of moment

360 deg/revolution

Resolutions: Incremental Encoders

OpticalTicks

Cycles/Revolution ResolutionMax. Rotation8000 Hz read

MGIII

Max. Rotation50,000 Hz read

Ouranos

1000 4000 5.4' arc 2 rev/s 12 rev/s

2000 8000 2.7' arc 1 rev/s 6 rev/s

Encoder Interface

A special interface is needed between the encoder and the computer. This has a microprocessor (computer) that

(1) keeps track of the encoder position and direction of rotation by polling the optical sensors at a high rate,

(2) converting the information to numbers and (3) communication with the outside world (or other computer).

Operations of the Encoder Interface and Computer: (Interface has clock and microprocessor)

Clock and Microprocessor Counts and Interprets movement of the Encoder. Microprocessor receives commands from RS232 port and sends back status

and/or positions of encoder. Computer with database of astronomical objects and a control program

calculates geometry of the telescope and converts encoder positions to telescope direction angles.

Sky Programs show where the telescope in pointed in the Sky (When properly aligned!!!).

Commercial Computer Interfaces and/or Encoder Controllers

(only representative and not a complete list)

Orion Telescopes SkyView Pro IntelliScope

Mead - Magellan "Telescope Computer Systems" Celestron Astromaster Microguider (Nova Astronomics) Dave Ek's Digital Setting Circles Project Home Page TL SYSTEMS EzDSC "DSC object finder in your pocket" Jim's Mobile, Inc . (SGT-Max) Others.....

The Brooklyn Street Observatory Installed Ditial Setting Circles

An working system showing attachment of the encoders and wiring to the computer-encoder interface. A 33 cm Newtonian on Dobsonian mount using the Outranous interface (no longer available). Page of Details

Acadia University's Celestron 8 with Encoders and Computer Interface

An example of a telescope retrofit with encoders is the one shown below. It allows students not completely familiar with the sky to find celestial object to image with the CCD camera. The environment in which the telescope is used is light polluted which makes it all the more difficult to find celestial objects using the traditional star hopping using star patterns. The computer aided telescope

is a big help in this environment.

Earth Centred Universe 3.0A - Sky Program Orion Sky Wizard CTI (9 V battery required) 4000 cycles/rev encoders Santa Barbara Group ST-7 CCD Camera CCDOps DOS software Laptop with Windows 98

A couple of Sky Programs that can talk to an encoder interface

Earth Centred Universe Guide (Project Pluto)

L.Bogan Nov 2000

My Robot

From AlanMacek.com

There project is very much in progress. Unfortunately I don't work on it often enough so when I do work on it, it takes a while to get back to where I left off.

The latest version of the robot is powered by two modified servo motors. Shaft encoders on the wheels provide feedback to the control system but not much is done with that yet.

The control system is a PIC Microchip microprocessor. For testing, I'm using the 12F675.

Wheel Encoders - Motor Control - Object Detectors

Wheel Encoders

My plan is to use the wheel encoders to implement a dead reckoning navigation system. Also, the encoders might will be helpful to get the robot to drive in a straight line since the servos are not exactly matched. Seattle Robotics has a good article on dead reckoning and includes a good template for making your own encoders.

My sensor is the QRD1114 which is an analog 4 pin IR emitter/detector module. I feed the output from the sensor to an op-amp configured as a voltage comparator with the threshold voltage set by a voltage divider (the 10k/15k resistors). The output from the op-amp is digital.

The digital signal is connected to an IO pin of the 12F675 configured to interrupt on change. So every time a black/white boundary passes the sensor, an interrupt is triggered. In the interrupt handler, I determine which encoder has fired and increment a counter. The plan is that periodically, the processor will integrate these counters to determine its new location.

I placed the sensor and a couple of resistors next to the wheel (the dashed box) with the op-amp and remaining circuitry on the main board. A 3-wire lead connects between the

two sections. The circuit diagram above only shows the circuit for a single wheel. The other wheel is exactly the same, except it uses the second op-amp in the LM385 package.

Update (Oct 2005): I noticed that 'svo' has used and improved my circuit for the wheel encoders on his robot. Svo uses a similar circuit with general purpose IR transmitter/receivers and places the entire circuit inside a modified servo motor case. He has step-by-step instructions with photo illustrations.

The detector is mounted next to the servo motor with the sensor aligned near the rim of the wheel. The wheel has been removed but

the backside of the opposite wheel is visible in the background. A - the power shaft of the

servo motor. B - the sensor. C - leads to additional circuitry.

The back side of the wheel and encoder showing the encoder disk. There is about 2mm

between the encoder disk and the sensor.

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Motor Control

I recently moved from using modified servo motors (see in the above photos) to using GM2 geared motors. They have the advantage of better speed control using PWM control than the pulse control of the servos. I used the L293D dual 'push-pull' motor driver which allows directional control of two motors - it handles all the high current stuff that would overwhelm a microprocessor.

The challenge was to develop a PWM controller for two motors using a single 16F819 PIC microprocessor. Some of the bigger microprocessors (such as the 16F877A) have dual PWM controls built-in but the 819 only has one.

I wrote an assemble program which using a single 8bit timer (TMR0) to drive both channels of PWM as well as control direction.

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Object Detector

This is an older module. My current version does not have any object detection yet. The next version of the object detection will likely move more to the microprocessor by using the analog->digital converter.

Here is a circuit diagram of the IR detector unit I am using for object detection.

Here are the values I used:

R1 = 150 ohms - used for current limiting through the LED. R1 > Vcc/(Rated Current Max)

R2 = 220 Kohms - used in voltage division with detector. R3 = 4.7 Kohms R4 = 10 Kohms L1 is a cheap LED IR emitter T1 is a IR photo transistor - basically a variable resister depending on intensity of IR

light. OP1 is a Operational Amplifier. I used a LM358 package which includes two op amps. Vcc = +5 Volts

Notes:

R2 should be larger then the maximum resistance of the detector. Measure the resistance of the detector when it is pointing into a dark area and then choose the next larger resister. This means the voltage on the op amp is close to zero when there is no signal.

R3 and R4 determine the amplification of the op amp, gain = 1 + R4/R3. An appropriate ratio can be determined by connecting up the circuit and measuring the voltage entering the op amp and knowing the threshold value needed at Vout. Vout = (1 + R4/R3)Vin so just solve for the ratio using the values for Vout and Vin.

As stated above, I used a LM358 Dual op amp package. The specs and pin outs are available on the net at any of the semiconductor manufacturers such as National Semiconductor.

I used an approximately Vcc = 5 Volt power source on my robot but almost any power source greater then that can easily be used with appropriate values of R1, R2 and within the specs for the OP Amp chip.

The Vout signal is connected directly to an I/O pin of the Basic Stamp. The ground used for this circuit is required to be the same as the ground used by the processor.

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2 Unidades de tracción gemelas.

Vamos a construir dos unidades de tracción: motor+reductor+rueda idénticas, para poderlas utilizar en un robot de tracción diferencial. Todo ello, como no, con materiales de reciclaje. Utilizaremos piezas de CD-ROM, impresora y disquetera. 

Es un ejemplo de lo que puede hacerse con la chatarra informática y algo de imaginación.

Comenzamos con una cabeza lectora de una unidad grabadora de CDROM. Serán necesarias dos iguales. Lo que vamos a aprovechar es el motor y el reductor, que en este caso es angular. Casualmente en este modelo el motor lleva un encoder de una fase en el eje del motor, también podremos usarlo.

Se eliminan las guías y la cabeza y se corta el el chasis para dejar únicamente el motor y el reductor. 

Este es el despiece de lo que queda: el motor es un FF-050SK-13130 que funciona entre 2V y 4V y da 0,59 mNm a 7600 RPM. No es mucho pero con una reducción adecuada puede mover con soltura un microrobot. Puede verse el detector de herradura del encoder montado en un circuito impreso y el disco ranurado montado en el eje del motor. El encoder es de 30 pulsos por vuelta y la reducción es de 16 dientes a 53, es decir  3,3125:1, bastante poco. 

Como la reducción que proporciona los piñones en ángulo que ya tenemos es poca añadiremos otro piñón más. Compruebo entre la chatarra y tengo este reductor de una unidad de CDROM creative X24 (que malas salieron). Ya comenté en alguna ocasión que prefiero guardar los motores junto con sus reductores, suelen resultar más útiles. El piñón de la izquierda tiene el mismo módulo que el del reductor anterior y podría servir. La reducción resultante es de 16 dientes a 89, es decir 5.5625:1 que junto con el conjunto anterior se consigue una reducción total de 18,426 a 1.El resto del reductor y el motor puede utilizarse para otra unidad de tracción en el futuro. 

Como ruedas utilizaremos los rodillos de tracción de papel de una impresora HP690C. Estas máquinas llevan 3 rodillos en un eje de 8mm. Con una sola impresora destripada ya tenemos las dos ruedas, y la otra de repuesto. Como eje servirán unos tornillos de M4 que coinciden con el diámetro interior de los rodamientos de disquetera. El diámetro exterior de estos rodamientos es 8mm, perfecto para ajustarlos al taladro del eje de las ruedas. Las ruedas tienen un diámetro de 51,5 mm. En la foto pueden verse las ruedas con los rodamientos y los tornillos que hacen de eje. En una esta ya pegado el piñón de la transmisión. 

En esta foto puede verse los dos conjuntos ya montados. 

Conecto el encoder al frecuencímetro (usando el montaje que describí en "midiendo las revoluciones de un motor") y alimento el motor con distintas tensiones. Utilizo otro cuentarrevoluciones para medir la velocidad de la rueda.

VoltiosHz encoder

RPM motor

RPM rueda

M/S V lineal rueda

2V 1800 3600 195 0,5258

3V 285,6 0,7701

4V 3800 7600 402 1,084

5V 4600 9200 514,2 1,380

6V 612 1,6502

Como el encoder es de 30 pulsos por revolución  y 1 minuto son 60 segundos RPM = (HZ/30) * 60seg. Puede verse que a 4V las 7600 RPM coinciden con lo dicho por el fabricante del motor. A esta tensión se consigue una velocidad

lineal, con estas ruedas de 51,5 mm de diametro, de 1,084 m/seg que son 3,9 Km/h.

Para rizar el rizo acoplo otro detector de herradura (obtenido del detector de pista 0 de una disquetera de 3 y 1/2 pulgadas) al disco ranurado del encoder para obtener la segunda fase del encoder y poder determinar el sentido de giro. Hay que ajustar la posición para que  la salida esté desplazada 90 grados respecto del que había de origen. De esta forma, con un encoder de 2 fases de 30 pulsos por vuelta, se puede hacer un control muy preciso de la posición y de la velocidad. También se puede acoplar un mecanismo de ratón de bola para hacer el encoder completo, pero no con este reductor, no hay sitio.

Como no puedo resistirme a la tentación de ver las ruedas corriendo ya mismo, monto un triciclo con ¡un cartón!, pegamento térmico, una rueda de un mueble, las unidades de tracción y una pila de 4,5 V. ¡A correr!!!

Compruebo que puede salvar obstáculos como lapiceros, alicates etc. El peso del conjunto es de 290 gramos, lo lastro hasta 600 gramos y compruebo que sube pendientes de 30 grados con mucha soltura. Hay que lijar la goma de las ruedas porque se endurece superficialmente y no agarra bien (fallo común en todas las impresoras hp series 500 y 600 y reconocido por HP). El siguiente paso es construir un chasis en condiciones e ir pensando en la electrónica de control.

Conclusión: en un par de horas se han conseguido dos unidades de tracción con unas prestaciones bastante buenas ¡gratis! y hemos aprovechado unos cuantos cacharros viejos. 

Inicio.

mailto:helitp@arra

Rotary Encoder

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A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary. Rotary encoders are manufactured in two basic forms: 1) the absolute encoder where a unique digital word corresponds to each rotational position of the shaft, and 2) the incremental encoder, which produces digital pulses as the shaft rotates, allowing measurement of relative position of shaft. Most rotary encoders are composed of a glass or plastic slotted disk. As radial lines in each track interrupt the beam between a photoemitter-detector pair (or Optointerrupter), digital pulses are produced. Below is a figure of an encoder with a spinning codewheel and a stationary mask. This stationary mask is usually not used.

Absolute encoder

The optical disk of the absolute encoder is designed to produce a digital word that distinguishes N distinct positions of the shaft. For example, if there are 8 tracks, the encoder is capable of producing 256 distinct positions or an angular resolution of 1.406 (360/256) degrees. The most common types of numerical encoding used in the absolute encoder are gray and binary codes. To illustrate the acion of an absolute encoder, the

gray code and natural binary code dsisk track patterns for a simple 4-track (4-bit) encoder are illustrated in Fig 2 and 3. The linear patterns and associated timing diagrams are what the photodetectors sense as the code disk circular tracks rotate with the shaft. The output bit codes for both coding schemes are listed in Table 1.

Decimal code

Rotation range (deg.)

Binary code

Gray code

0 0-22.5 0000 0000

1 22.5-45 0001 0001

2 45-67.5 0010 0011

3 67.5-90 0011 0010

4 90-112.5 0100 0110

5 112.5-135 0101 0111

6 135-157.5 0110 0101

7 15.75-180 0111 0100

8 180-202.5 1000 1100

9 202.5-225 1001 1101

10 225-247.5 1010 1111

11 247.5-270 1011 1110

12 270-292.5 1100 1010

13 292.5-315 1101 1011

14 315-337.5 1110 1001

15 337.5-360 1111 1000

Table 1. 4-Bit gray and natural binary codes

Image:Conv.jpg

Fig 4. Gray code to binary code conversion

The gray code is designed so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions. This effect can be seen clearly in Table 1. For the gray code, the uncertainty during a transition is only one count, unlike with the binary code, where the uncertainty could be multiple counts.

Since the gray code provides data with the least uncertainty but the natural binary code is the preferred choice for direct interface to computers and other digital devices, a circuit to convert from gray to binary code is desirable. Figure 4 shows a simple circuit that utilizes exclusive OR gates (XOR) to perform this function.For a gray code to binary code conversion of any number of bits N, the most signficant bits (MSB) of the binary and gray code are always identical, and for each other bit, the binary bit is the exlcusive OR (XOR) combination of adjacent gray code bits.

Incremental encoder

The incremental encoder, sometimes called a relative encoder, is simpler in design than the absolute encoder. It consists of two tracks and two sensors whose outputs are called channels A and B. As the shaft rotates, pulse trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation. The code disk pattern and output signals A and B are illustrated in Figure 5. By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured. The A and B channels are used to determine the direction of rotation by assessing which channels "leads" the other. The signals from the two channels are a 1/4 cycle out of phase with each other and are known as quadrature signals. Often a third output channel, called INDEX, yields one pulse per revolution, which is useful in counting full revolutions. It is also useful as a reference to define a home base or zero position.

Figure 5 illustrates two separate tracks for the A and B channels, but a more common configuration uses a single track with the A and B sensors offset a 1/4 cycle on the track to yield the same signal pattern. A single-track code disk is simpler and cheaper to manufacture.

The quadrature signals A and B can be decoded to yield the direction of rotation as hown in Figure 6. Decoding transitions of A and B by using sequential logic circuits in different ways can provide three different resolutions of the output pulses: 1X, 2X, 4X. 1X resolution only provides a single pulse for each cycle in one of the signals A or B, 4X resolution provides a pulse at every edge transition in the two signals A and B providing four times the 1X resolution. The direction of rotation(clockwise or counter-clockwise) is determined by the level of one signal during an edge transition of the

second signal. For example, in the 1X mode, A= with B =1 implies a clockwise

pulse, and B= with A=1 implies a counter-clockwise pulse. If we only had a single output channel A or B, it would be impossible to determine the direction of rotation. Furthermore, shaft jitter around an edge transition in the single signal would result in erroneous pulses.

Connecting an Encoder to the PC/104 Stack

To connect an encoder to the PC/104 stack, you will have to create a suitable ribbon cable connector. The connector that came with the encoder will most likely not match the correct pinout to attach. Consult the PC104 I/O page to see how to connect the encoder.

Datasheets

Datasheet for the Maxon Tacho 103935 encoder: Media:maxon_tacho_103935_encoder.pdf

References

Introduction to Mechatronics and Measurement Systems, Histand & Alciatore, 1999 McGraw Hill

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Digital Encoders

A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or by decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary. Rotary encoders are manufactured in two basic forms: the absolute encoder where a unique digital word corresponds to each rotational position of the shaft, and the incremental encoder, which produces digital pulses as the shaft

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00311162255977 Search FORID:0 hades.mech.north

rotates, allowing measurement of relative position of shaft. Most rotary encoders are composed of a glass or plastic code disk with a photographically deposited radial pattern organized in tracks. As radial lines in each track interrupt the beam between a photoemitter-detector pair, digital pulses are produced.

Absolute encoder

The optical disk of the absolute encoder is designed to produce a digital word that distinguishes N distinct positions of the shaft. For example, if there are 8 tracks, the encoder is capable of producing 256 distinct positions or an angular resolution of 1.406 (360/256) degrees. The most common types of numerical encoding used in the absolute encoder are gray and binary codes. To illustrate the acion of an absolute encoder, the gray code and natural binary code dsisk track patterns for a simple 4-track (4-bit) encoder are illustrated in Fig 2 and 3. The linear patterns and associated timing diagrams are what the photodetectors sense as the code disk circular tracks rotate with the shaft. The output bit codes for both coding schemes are listed in Table 1.

Decimal code

Rotation range (deg.)

Binary code

Gray code

0 0-22.5 0000 0000

1 22.5-45 0001 0001

2 45-67.5 0010 0011

3 67.5-90 0011 0010

4 90-112.5 0100 0110

5 112.5-135 0101 0111

6 135-157.5 0110 0101

7 15.75-180 0111 0100

8 180-202.5 1000 1100

9 202.5-225 1001 1101

10 225-247.5 1010 1111

11 247.5-270 1011 1110

12 270-292.5 1100 1010

13 292.5-315 1101 1011

14 315-337.5 1110 1001

15 337.5-360 1111 1000

Table 1. 4-Bit gray and natural binary codes

The gray code is designed so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions. This effect can be seen clearly in Table 1. For the gray code, the uncertainty during a transition is only one count, unlike with the binary code, where the uncertainty could be multiple counts.

Since the gray code provides data with the least uncertainty but the natural binary code is the preferred choice for direct interface to computers and other digital devices, a circuit to convert from gray to binary code is desirable. Figure 4 shows a simple circuit that utilizes exclusive OR gates (XOR) to perform this function.For a gray code to binary code conversion of any number of bits N, the most signficant bits (MSB) of the binary and gray code are always identical, and for each other bit, the binary bit is the exlcusive OR (XOR) combination of adjacent gray code bits.

 

 

Fig 4. Gray code to binary code conversion

Incremental encoder

The incremental encoder, sometimes called a relative

encoder, is simpler in design than the absolute encoder. It consists of two tracks and two sensors whose outputs are called channels A and B. As the shaft rotates, pulse trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation. The code disk pattern and output signals A and B are illustrated in Figure 5. By counting the number of pulses and knowing the resolution of the disk, the angular motion can be measured. The A and B channels are used to determine the direction of rotation by assessing which channels "leads" the other. The signals from the two channels are a 1/4 cycle out of phase with each other and are known as quadrature signals. Often a third output channel, called INDEX, yields one pulse per revolution, which is useful in counting full revolutions. It is also useful as a reference to define a home base or zero position.

Figure 5 illustrates two separate tracks for the A and B channels, but a more common configuration uses a single track with the A and B sensors offset a 1/4 cycle on the track to yield the same signal pattern. A single-track code disk is simpler and cheaper to manufacture.

The quadrature signals A and B can be decoded to yield the direction of rotation as hown in Figure 6. Decoding transitions of A and B by using sequential logic circuits in different ways can provide three different resolutions of the output pulses: 1X, 2X, 4X. 1X resolution only provides a single pulse for each cycle in one of the signals A or B, 4X resolution provides a pulse at every edge transition in the two signals A and B providing four times the 1X resolution. The direction of rotation(clockwise or counter-clockwise) is determined by the level of one signal during an edge transition of the

second signal. For example, in the 1X mode, A= with B =1 implies a clockwise

pulse, and B= with A=1 implies a counter-clockwise pulse. If we only had a single output channel A or B, it would be impossible to determine the direction of rotation. Furthermore, shaft jitter around an edge transition in the single signal woudl result in erroneous pulses..

(Materials taken from Introduction to Mechatronics and Measurement Systems, Histand & Alciatore, 1999 McGraw Hill)

Bueno aqui hay una confusion mia, porque yo sabia que los encoder (en un topico mecanico) eran los que son discos perforados

el detalle es que la mayoria de los aparatos nuevos de audio, que trabajan digitalmente utilizan una especie de potenciometro "digital" y al abrirlos puedes verlos, investigue un poco y al buscar al fabricante (ALPS http://www.alps.com/ ) este los llama encoders ò switch/encoder y trabajan internamente de forma mecanica, bueno este es el tipo de encoder que quiero utilizar, espero no haber expresado mal mi primer post y haberte confundido.

SENSORES DE POSICIÓN

 

Potenciómetro angular

Es un transductor de posición angular, de tipo absoluto y con salida de tipo analógico. Básicamente es una resistencia de hilo bobinado en una pista de material conductor, distribuida a lo largo de un soporte en forma de arco y un cursor solidario a un eje de salida que pueda deslizar sobre dicho conductor . El movimiento del eje arrastra el cursor provocando cambios de resistencia entre éste y los extremos. De esta forma si se alimentan los extremos con una tensión constante Vo aparece en la toma de medida una tensión proporcional al ángulo girado a partir del origen. Interesa que esta variación sea lineal como se representa en la figura. En cuanto a la respuesta dinámica el

potenciómetro es un elemento proporcional sin retardo, pero la frecuencia de funcionamiento suele quedar limitada a 5 Hz por motivos mecánicos.

Potenciómetro angular

Encoders

Los encoders son dispositivos formados por un rotor con uno o varios grupos de bandas opacas y translúcidas alternadas y un estator con una serie de captadores ópticos que detectan la presencia o no de banda opaca. Existen dos tipos de encoders, incrementales y absolutos. Los primeros dan un determinado número de impulsos por vuelta y requieren un contador para determinar la posición a partir de un origen de referencia, los absolutos disponen de varias bandas en el rotor ordenadas según un código binario, y los captadores detectan un código digital completo que es único para cada posición del rotor.

Los encoders incrementales suelen tener una sola banda de marcas repartidas en el disco del rotor y separadas por un paso p. En el estator disponen de dos pares de emisor-receptor ópticos, decalados un número entero de pasos más ¼ de paso. Al girar el rotor genera una señal cuadrada, el decalaje hace que las señales tengan un desfase de ¼ de periodo si el rotor gira en un sentido y de ¾ si gira en el sentido contrario, lo que se utiliza para discriminar el sentido de giro.

Un simple sistema lógico permite determinar desplazamientos a partir de un origen, a base de contar impulsos de un canal y determinar el sentido de giro a partir del desfase entre los dos canales. Algunos encoders pueden disponer de un canal adicional que genere un pulso por vuelta y la lógica puede dar número de vueltas más fracción de vuelta.

Encoder Incremental

La resolución del encoder depende del número N de divisiones del rotor, es decir del número de impulsos por revolución. No debe confundirse lo que es resolución angular del encoder con la posible resolución de un sistema de medida de coordenadas lineales que dependerá de la desmultiplicación mecánica.

Los encoders absolutos disponen de varias bandas dispuestas en forma de coronas circulares concéntricas, dispuestas de tal forma que en sentido radial el rotor queda dividido en sectores, con combinaciones de opacos y transparentes que siguen un código Gray o binario .

Encoder absoluto

El estator dispone de un conjunto emisor-receptor ópticos para cada corona del rotor. El conjunto de información binaria obtenida de los captadores es único para cada posición del rotor y representa su posición absoluta. Se utiliza el código Gray en lugar de un binario clásico porque en cada cambio de sector sólo cambia el estado de una de las bandas, evitando errores por falta de alineación de los captadores. Para un encoder con N bandas, el rotor permite 2N combinaciones, la resolución será 360° entre los 2N sectores, por ejemplo para encoders de 12 y 16 bits se obtiene una resolución angular de 0.0879° y 0.00054° respectivamente.

Sincros y Resolvers

Un sincro es un transductor de posición angular de tipo electromagnético. Su principio de funcionamiento puede resumirse diciendo que se trata de un transformador, en el que uno de los devanados es rotativo.La configuración más habitual es :

Primario en el rotor y monofásico

Secundario en el estator y trifásico

En la Figura se representa el esquema de un Sincro con la configuración indicada. Cuando se aplica una tensión senoidal al devando primario, se recogen en los devanados secundarios de cada una de las fases tres tensiones, cuya amplitud y fase con respecto a la tensión del primario dependen de la posición angular del rotor.

Funcionamiento del Sincro

En caso de existir una sola fase en el estator existiría una indeterminación en el signo del ángulo, que desaparece para un estator trifásico.

Una configuración particular del Sincro es la del Resolver, cuyo principio de funcionamiento es análogo, con las siguientes particularidades constructivas :

Primario en el estator y bifásico

Secundario en el rotor, monofásico o bifásico.

En la Figura se representa de forma esquemática una configuración típica. Los devanados del estator se alimentan en serie, dando un campo estacionario sobre el eje y los devanados del rotor recogen distintas tensiones en función de 1 .

Esquema de resolver

Sensores Inductosyn

Es un transductor electromagnético utilizado para la medida de desplazamientos lineales, con precisión del orden de micras. Se emplea en máquinas medidoras de coordenada y máquinas herramientas de control numérico. El transductor consta de dos partes acopladas magnéticamente, una denominada escala fija y situada paralela al eje de desplazamiento y otra solapada a la anterior deslizante y solidaria a la parte móvil.

Inductosyn

La parte fija lleva grabado un circuito impreso con pistas en forma de onda rectangular con un paso p. La parte móvil tiene dos más pequeños, encarados con los de la escala, y

desfasados entre si un número entero de pasos mas ¼ de paso (análogamente a lo visto para encoders incrementales). Si se excita la parte fija con una señal alterna en cada uno de los circuitos de la parte deslizante se recoge una tensión que es función del desplazamiento lineal y el paso de onda de la escala. La amplitud varia entre un máximo y un mínimo según las que los circuitos fijo y móvil estén enfrentados o decalados ½ de paso. La medida se realiza sumando el número de ciclos de señal de salida completos, más la variación dentro de un ciclo. La indeterminación del sentido se resuelve comparando la fase de los dos captadores.

Sensores LASER

Los sensores LASER pueden utilizarse como detectores de distancias por análisis de interferencias (interferometría LASER). El principio de funcionamiento se basa en la superposición de dos ondas de igual frecuencia, una directa y la otra reflejada. La onda resultante pasa por valores máximos y mínimos al variar la fase de la señal reflejada. Los sensores industriales generan un haz de luz que se divide en dos parte ortogonales mediante un separador . Un haz se aplica sobre un espejo plano fijo, mientras el otro refleja sobre el objeto cuya distancia se quiere determinar, los dos haces se superponen de nuevo en el separador, de forma que al separarse el objeto se generan máximos y mínimos a cada múltiplo de la longitud de onda del haz. La distancia se mide contando dichas oscilaciones o franjas, obteniéndose una salida digital de elevada precisión.

Sensores ultrasónicos

Los sensores ultrasónicos emiten una señal de presión hacia el objeto cuya distancia se pretende medir, y miden el tiempo que transcurre hasta la recepción del eco reflejado. El más conocido es el SONAR o en la actualidad los sistema de ecografía, en el campo industrial se suelen emplear para controlar niveles de sólidos en depósitos, presencia de

obstáculos en celdas robotizadas, detección de grietas en la inspección de materiales o soldaduras.

Sensores magnetoestrictivos

Están también basados en la detección de un impulso ultrasónico generado por la deformación elástica que se produce en algunos materiales bajo el efecto de un campo magnético.

Básicamente se trata de una varilla de material magnético en la que se genera una perturbación ultrasónica mediante una bobina inductora, sobre la varilla se coloca un imán móvil que puede deslizarse. El imán provoca un cambio de permeabilidad en el medio y esto provoca una reflexión de la onda ultrasónica, pudiéndose detectar la distancia al imán por el tiempo en recibir el eco.

 

 

What is an incremental encoder ?

Incremental encoders are sensors capable of generating digital signals in response to rotary movement. They are employed to convert the rotary movement into electrical signals and to obtain position and speed measures . The encoder generates a signal for each incremental change in position . In onjunction with mechanical conversion devices, such as rack-and-pinions, measuring wheels or spindles, incremental encoders can also be used to measure linear movement.

Our incremental encoders are designed with an optical electronic circuit. With optical encoders , a grating disc made of metal or glass associated with a mask interrupts an infrared beam emitted by a transmitting gallium arsenide diode . The number of gratings (increments ) determines the system ‘s resolution , ie . the number of increments per rotation. Every time the infrared beam is interrupted, this is registered by a receiver and then processed electronically. To make the detection less sensitive to the light level, the receiver uses a differential measure between two photodiodes : one lighted and the other masked. The result is a square wave output signal.

Two shifted photosensitive diode arrays deliver squared signals ( A and B) in quadrature. The Phase shift ( 90° electric ) of signals A and B makes possible to determine the direction of rotation. In one direction, during the going upof the singal A the signal B is equal to 1. In the other direction, Deuring the going up the signal A, the signal B is equal to 0.

The Z or singnal zero comprises only one transparent window delivering one signal by turn. This signal is gated in synchronism with signals A and B. This zero signal determines a position of reference and allows reinitializing the system at each turn.

The electronic treatment delivers signals complementary to A, B and Z and makes possible to remove electric noises by using a differential transmission of the signals.

Output waveform: A leads B for clockwise rotion from front size

Product Articles

Products   >   Articles   >   Is a Modular Encoder Right for Your Application?

IntroductionModular, or "Kit", encoders are actually a specialized category of rotary encoders. The difference between a modular encoder and a standard encoder is that the modular encoder does not incorporate internal bearings into its design. Instead, it relies on the host, typically a motor, to supply the structural integrity necessary for proper operation. At first glance, this approach seems to be a good idea, since eliminating the bearings can lower the price of the encoder and give the overall encoder a lower height profile. For these reasons, Modular Encoders are often favored by motor manufacturers.

However, modular encoders are definitely not a panacea, and can be good or bad, depending upon personal experience. The root problem is not modular encoders themselves, but the fact that they are often misapplied, and a proper understanding of the tradeoffs involved will go a long ways towards preventing a bad experience. The primary purpose of this article is to help you determine whether or not a modular encoder is the right choice for your application. In addition, you will better understand the tradeoffs involved, and proper implementation, so that you will see only benefits. When applying modular encoders, there are several important considerations that often get overlooked, but, before we discuss them in detail, let's review what is critical for an encoder's reliable operation.

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Critical Design ElementsTo produce reliable output signals, proper axial and radial alignment between the rotating disk and the sensor must be established and maintained. It is here that the primary area for potential problems occurs. In standard encoders, this relationship can be properly calibrated at the factory, and can be more easily maintained in the field. However, since modular encoders do not have bearings, and their most common use involves mounting them on a motor, the motor's bearings and shaft assembly must serve as the encoder's optical platform. Understandably, Lenze AC Motors are designed to be motors and their design engineers rarely take into consideration the requirements represented by encoders. Even when the newer, more tolerant sensor technology is used (such as that in EPC's Model 121 Modular Encoders), the motor must still meet certain criteria. The two most common motor specifications that affect encoder operation are the shaft's axial movement (end float or end play) and the shaft's total indicated runout (TIR).

Typical Encoder Construction

Axial Motor Shaft Movement (End Float or End Play)End float refers to the amount of axial movement in the motor shaft. There are a number of factors that can contribute to axial motor shaft movement including part tolerances, bearing pre-load method, thermal expansion, and bearing wear over time. When an encoder is mounted, the amount of end float directly affects the encoder's air-gap (the distance between the sensor and the disk). It can be difficult to obtain end float specifications from the motor manufacture, and even when you do, the information may not be correct. Some motor designs mechanically lock the shaft's axial movement on the feedback end so that end float is minimal. Other motor designs often use a wave spring washer to take up any excess play and provide a pre-load force to the bearings. In this case, you cannot assume that the end float has been removed by the wave washer, since this is only true until an opposing axial force on the end of the shaft overcomes the spring force of the wave washer allowing the shaft to move. The results on a modular encoder mounted to this shaft could be disastrous, potentially causing the encoder disc to hit the sensor. Examples where this situation can occur are:

When a motor shaft is connected to a ball screw, as the motor changes direction, the force from the ball screw will also change directions. This alternating force may cause the motor shaft to move axially.If a sprocket, pulley, or gear with some side wobble is mounted on the motor shaft, the wobble may cause an alternating axial load to the motor shaft.

Total Indicated Runout (TIR)Total Indicated Run-out (TIR) measures the radial range of shaft movement about its centerline. If an encoder is to be mounted on the motor shaft, TIR should be measured at that point that represents the furthest extent of the encoder case. For example, if the encoder is one inch thick, TIR should be measured about 1" from the motor face. Although many encoders with new technology Sensors will continue to operate as TIR increases beyond the specified tolerance, accuracy will be sacrificed.

Encoders with BearingsOptical encoders (such as Encoder Products Company's Models 755A and 260) usually include internal bearings. With bearings, the amount of axial play is typically controlled to less than 0.0005". In addition, the disk is carefully aligned to the optics as part of the calibration procedure to keep radial run out less than 0.0002" typical. In this manner, the critical factors of end float and TIR are controlled and will not be affected by the motor shaft in normal operation. A stainless steel flex mount allows the encoder to tolerate increased TIR and end float from the motor without sacrificing encoder performance or damaging it.

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Selection CriteriaConsider these factors when choosing between a modular encoder and an encoder with bearings:

First and foremost, motor shaft end float and TIR must be within the encoder's specifications. This is so important that if you don't have (or can't get) this information on the motor, or don't trust what you have, then an encoder with bearings is a much safer choice.Modular encoders can be a good choice for high-speed applications, those above 10,000 RPM, because there are no speed limitations dictated by encoder bearings. For example, EPC's Model 121 Modular Encoder has been successfully operated at speeds in excess of 40,000 RPM.If the motor is to be used under considerable load, where the motor bearings could experience extra wear, then an encoder with bearings would be the better choice.Modular encoders are difficult to seal. If your application needs wash down, your environment is dirty, etc., then an encoder with bearings and seals should be your first consideration, effectively ruling out modular encoders, unless external protection is used.If your application involves a combination of high frequency response (> 200kHz), high temperatures (100C or higher), and higher resolutions (>2048 CPR), then an encoder with bearings is recommended. This combination requires that the air-gap be smaller and better controlled for long term reliability. An encoder with bearings simply provides a better optical platform.Lower resolutions in general (up to 1024 CPR) are more forgiving to end float and TIR, and are well suited for modular applications.If you plan to use a lot of encoders, then the lower price of a modular encoder could save you some money. On the other hand, the additional durability and ease of installation of an encoder with bearings might easily be worth the slightly higher price. In any case, you should carefully weigh the factors of long term support costs versus slightly lower acquisition costs before making your final decision.

Quick Selection Chart (listed in order of importance):

Parameter Attribute Use ModularUse Encoder with

bearings

Motor shaft end float and TIRWithin the encoder mfg.'s specifications

Yes Yes

Motor shaft end float and TIROutside the encoder mfg.'s specifications

No Yes

Motor shaft end float and TIRDon't have the information or don't trust

Not suggested Suggested

High-speed applications Above 10,000 RPM Good possibility Not suggested

Severe duty applicationMotor bearings have extra load and wear

Not suggested Suggested

Dirty environment May need seals Not suggested Suggested

Comb. of high freq. response, temp., CPR

>200kHz, >100C, >2048 CPR Not suggested Suggested

Lower resolution requirement <1024 cycles per revolution Good possibility Good

Number of units neededAcquisition cost vs. Life cycle cost

Consider if large volume

Good

If you have decided that a modular encoder is the right choice, Model 121 Self-Aligning Encoder is the best modular encoder anywhere. It's innovative, patent pending design eliminates the installation and mounting hassles typical of other modulars. As a result, the Model 121 does not need calibration gapping or special tools to install. In fact, it's three step installation is the simplest and quickest in the industry. What's more, the Model 121's all-metal construction will not warp or deflect (like non-metal designs), making it the most durable and reliable modular encoder available.