Finding the Index Pulse of an Incremental Encoder

The index pulse, often also called “Z” or “Marker pulse”, of an optical incremental encoder is a once per revolution digital pulse that is used for homing or count verification of incremental signals.

In the QD145 and QD200 series of encoders the index pulse fires when the mark on the top cover of the optical incremental encoder and the mark on the encoders shaft are aligned.

This mark also indicates the rising edge of the U channel for commutated optical incremental encoders.  Knowing the location of this edge is useful for the initial rough timing of Brushless DC motors.

Quantum Devices is a leading manufacturer of Optical Incremental Encoders.

Jim can be reached at 608.924.3000 or by e-mail at


Brushless Motors vs Brush Motors, what’s the difference?

What’s the difference between a Brushless Motor and a Brush Motor?

Well, the brushes of course.

Yeah, but what does that mean?

The principle behind the internal working of both a brushless DC motor and a brushed DC motor are essentially the same.  When the motor windings become energized, a temporary magnetic field is created that repels(and/or attracts) against permanent magnets.  This force is converted into shaft rotation, which allows the motor to do work.  As the shaft rotates, electric current is routed to different sets of windings, maintaining electromotive repulsion/attraction,  forcing the rotor to continually turn.

Construction differences

Brushes inside  electric motors are used to deliver current to the motor windings through commutator contacts.  Brushless motors have none of these current carrying commutators.  The field inside a brushless  motor is switched via an amplifier triggered by a commutating device, such as an optical encoder.

Windings are on the rotor (Rotating part of motor) for brush motors and on the stator (stationary part of motor) for brushless motors.

Brush Motor: windings on rotor, magnets on stator

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The Versatile QD787 Optical Encoder


This is a picture of the QDI Model 787 Hollowshaft encoder and a loose flex-mount showing the tabs bent 90 degrees for mounting to a surface larger than the diameter of the encoder.  The motor shaft would be inserted into the brass hub and tightened by two setscrews located 120 degrees apart.

Notice the groove in the housing of the encoder.  It is designed to allow infinite adjustability; allowing the technician to align the index pulse to a mechanical location on the machine used for homing or reference.  This is called an encoder “Servo Mount” and is intended to have “D” shaped washers used where after the alignment is complete, the larger diameter of the washer is inserted into the groove and screws tighten the encoder into place.  When mounting the encoder like this, the flex-mount is completely removed.

The housing of the encoder is made from Nickel coated, conductive carbon fiber that eliminates magnetic interference (EMI) generated by electric motors.  It is available in resolutions up to 2048 PPR or 8,192 quadrature counts per revolution (direct read) with index pulse being a standard feature.

Electrical output can be a Differential Line Driver for noise immunity via a ribbon cable with connector.

The QDI Model 787 encoder is about the diameter of a penny.

There are extended options that are not listed in the QDI online documentation for custom shaft lengths and diameters, making this encoder perfect for potentiometer replacement or other human interface applications.










For more information on the QD787, check out the Spec sheets for both incremental encoder and absolute encoder varieties.

Download the printable versions of the spec sheets for the Absolute Encoder (PDF),  and the Incremental Encoder (PDF)

The Quantum Devices INC. main home page can be reached here.