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The Basics of Optical Encoders

basics of optical encoders

Closed loop servo applications for brush commutated and brushless DC motors and gearmotors require positioning feedback from which crucial velocity and acceleration data are derived. With accurate feedback comes an opportunity for enhanced motor control and an even wider range of applications. One method to generate reliable position feedback is with two- or three-channel optical incremental encoders. In addition, DC motor and encoder combinations can be customized with differential line drivers to counter the effects of electrically noisy environments and to ensure uncorrupted positioning feedback from the encoder to the control circuit. This is especially important, because even one false signal adding to or subtracting from the position count has the potential to degrade the accuracy of the DC servo system.

Each TTL compatible optical incremental encoder typically contains a lensed Light Emitting Diode (LED) source, an integrated circuit (IC) with detectors and output circuitry, and a codewheel that rotates between the emitter and detector IC. In two-channel encoders, the outputs are two square waves in quadrature; three-channel encoders offer a third index channel output in addition to the two-channel quadrature. This third index channel is generated once for each full rotation of the codewheel and thus offers an ideal point of reference. For codewheels 2 in. or less in diameter, resolution generally can be specified up to 2048 counts per revolution (CPR).

Optical incremental encoders essentially translate the rotary motion of a shaft into either a two- or a three-channel digital output. The light from the LED source is collimated into a parallel beam by means of a single polycarbonate lens located directly over the LED. Opposite the emitter is an integrated detector circuit. This IC consists of multiple sets of photodetectors and the signal processing circuitry necessary to produce the digital waveforms.

Either a metal or film codewheel is employed to rotate between the emitter and detector, causing the light beam to be interrupted by the pattern of spaces and bars on the codewheel. The photodiodes that detect these interruptions are arranged in a pattern that corresponds to the radius and design of the codewheel. These detectors are also spaced such that a light period on one pair of detectors corresponds to a dark period on the adjacent pair of detectors. The photodiode outputs are then fed through the signal processing circuitry. Comparators receive these signals and produce the final outputs for the channels. Due to the integrated phasing technique, the digital output of one channel is in quadrature with that of the other (90 degrees out of phase).

In a three-channel encoder, the output of the comparator for the third channel is sent to the index processing circuitry along with the outputs of the other two channels. The final output of the third channel (generated once for each full rotation of the codewheel) can be gated to be coincident with the low states of the first two channels. The result is highly accurate signal feedback and reliable point of reference.

Encoders are designed to be mounted quickly and easily to a motor and will provide reliable motion detection in high-volume applications, including printers, plotters, tape drives, positioning tables, and industrial and factory automation equipment. As an alternative low cost solution for applications that need velocity feedback only, an option is a Rotary Pulse Indicator (RPI). This is a single-channel encoder with open collector or TTL compatible outputs and, because this is a single-channel device, direction information is not generated.

In encoder applications where a reduction in the effects of conducted and radiated noise is desired, the assembly can be customized with a differential line driver, which will enable improved signal integrity.

Differential circuits improve noise immunity by processing a signal that is the algebraic difference of two complementary signals at the input. The differential line driver receives the signal from the encoder and inverts polarity on one output to form complementary signals. A 5V input signal would transmit as 5V on one output and 0V on the other. Because the transmission lines are balanced and positioned closely, any noise induced in the circuit equally affects the signal amplitude, polarity, and phase in both wires.

The lines feed to a differential receiver, which re-inverts one input and adds the voltage in the lines, effectively canceling electromagnetic interference (EMI). Therefore, if a +1V noise spike enters the 5V system, the lines would carry 6V and 1V for the duration of the spike, then the receiver would invert the 1V input and detect the original 5V.

To ensure that noise equally affects both transmission lines, differential circuits commonly employ twisted-pair wiring, especially as transmission distances become longer. For shorter transmissions, ribbon cable suffices. With twisted-pair wiring, designers can achieve higher noise immunity, because the inductively coupled noise currents are out-of-phase and effectively cancel one another in each loop. Wires should be terminated at the receiver end only with a resistor equal to the differential line impedance.

In addition to reducing common-mode noise, the differential circuit also supports longer transmission distances by providing better noise margins and boosting signal output. Typically, optical incremental encoders only provide source current in the microamp range. Standard differential line drivers, however, provide up to 20 milliamps of drive current, which is five times more sinking current than average encoders. Designers can determine acceptable line length by examining the signal strength at the end of the line and the amplification possible with the receiver.

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