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Machines that move need a means of measuring movement. Since the machines from transport materials to the most complex machine tools have internal measuring mechanisms. The most common type of measurement component today is the encoder. There are three main groups of Encoders these are: optic (photoelectric), magnetic and mechanical contact. The photoelectric encoders in particular plays a significant role in machines tool and automation in general due to their reliability, accuracy and low cost. Encoders can be manufactured in either rotary or linear types. Most encoders today, operate detecting the photoelectric impulses caused by the movement of two very small graticule's. The detection unit of most incremental encoders consists of two light sources, on one side of the moving graticule. On the other side are two photo diodes. The light passing from one side to the other passes through the moving graticule and a phase plate that consists of two graticule's that are out of phase by 90' hence when the graticule lines line up on one detector the other is completely obscured. Using this technique two channels are produced the 'A' and the 'B' channel. By detecting the relationship between the phase of the resultant signals one can not only detect movement but also the direction of movement. The encoder also produces two signal per channel logic high or logic low. Since there are two channels and two states the encoder produces 4 pulses for every one line on the graticule. In other words a 1000 pulse per revolution encoder actually produces 4000 pulses per revolution.

 

Rotary Encoders In many machines, the movements and angular positions need to be transformed into electric signals either to display on a screen, automation or numeric control. They are also used to measure linear motion as for example the displacement that the rotation of a lead screw produces.

 

Incremental Encoder The incremental rotary encoders output signals are evaluated by an electronic counter in which the measured value is determined counting up each increment (when the photovoltaic cells receive the beam of light). Therefore the number of square waves output cycles is similar to the number of windows in the rotary disc mentioned above. When they are used for length measurement, the motors generally control a lead screw, producing a linear movement. The encoders are generally coupled to the rear of the servomotor. The quadrature encoders are the most common type of incremental encoders and widely used in all the automation forms. This encoder type has 2 channels, and it is denominated of quadrature because the 2 channels are out of phase 1/4 of cycle (90 degrees). When the encoder rotates clock wise the signal A is the first one in ascending to '1', if encoder rotates counter clock wise the signal B will be the first one in ascending, this way an electronic circuit detect the direction in which the motor is running. A third channel with a pulse for a turn is often in this encoders type and it is commonly called reference pulse.

 

Absolute Encoder Absolute rotary encoders provide a true absolute position immediately upon power-up within one revolution. The windows of the disc are made in such way that they generate a code. The most used code is the Gray code. The disc gives several signals (the number of the signals depends on the encoder) and each one represents a bit. The Gray code consists in that only one bit at the same time changes in the transition to the following number, and in the same way when occur an overflow. For example in the system BCD the transition of the digital values 0111 (7 decimal) to 1000 (8 decimal) all the bits changes and this is a problem for the electronic system detection because it can read a completely different number. Therefore for more reliable measuring it is commonly used the Gray code. It should be noted that rotary absolute encodes give a position relative to a zero position within 1 revolution. Applications that require many revolutions of the encoder will only gain the position within 1 revloution from the encoder, the exact position must be computed by a controller and this information may be lost on power down. The great advantage of the absolute encoder is that it detects the actual position without need of making the initial home routine, because each bit combinations of the Gray code fetched by the encoder represents a turn angle. Then when happening power interruptions or when the machine turn on the absolute encoder is able to detect the actual position. The inconvenience is the price, since it needs more sophisticated electronic and optic system that the incremental encoders. There are many absolute encoders versions available today and each one should be evaluated depending on the wanted application. But it is necessary to keep in mind that the encoder gives the absolute position of a single turn, so if the machine has motors that rotate more than a turn, it is not good practice to use this type of encoder.

 

Linear Encoders The linear encoders consists on a linear tape with windows (similar to that of the rotary encoders), a source of light and a photovoltaic cells, these 3 elements are housed together. The light is projected through the windows of the tape and detected by the photovoltaic cells. The tape is fixed but the source of light and the photovoltaic cells move at the same time, therefore the tape modulates the signal of light every time that passes through the windows. The photovoltaic cells receives the signal and transforms it into an electric signal, generally in a square wave. The encoder uses two channels, A and B and they are out of phase 90 grades, what allows to know the direction of the movement. This encoder is also incremental what allows to know the position by counting pulses. Linear encoders are preferred to the rotary encoders in the applications in that the final output of the system is a linear movement. As the reading head is directly in the load, a motion control can benefit of this. For example, when there is thermal expansion in the lead screw the linear encoder compensates the error, what doesn't happen to the rotary encoders. However this encoder is more expensive and difficult when they are over 1 metre in length.

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