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 jmiller@quantumdev.com

Finding the Index on an Incremental Encoder with a DMM

Sometimes you don’t have the right tools to do the job.

Lets say you needed to identify where the index pulse was firing on your incremental encoder, but you left your oscilloscope in your other jacket pocket, and now all you have on hand is a DMM.

Well fear not, finding the index with a multimeter is possible although a bit tedious.

The index fires once per revolution and at higher line counts this makes it VERY easy to miss.  Since there is some delay in a multimeter’s display time, you will need to rotate the encoder very slowly to catch a change in voltage level.

The Blue box has a nine-volt battery inside that I regulated down to 5Vdc for the encoder power.  I have pulled out connections to ground (Black wire) and the index channel (Orange wire). When the index fires, the voltage will go from zero to five volts.

Jim is an Applications Engineer with Quantum Devices Inc. A leading manufacturer of Optical Encoders.

What is meant by Rotary Incremental Encoder Index Pulse “gating”?


qd145_sealed_open

Gating refers to the width of the Z (index) pulse relative to the A and B channels. With ungated devices the edges of the Z pulse have no hard relation to A & B.

Gating to channel A, requires the Z pulse to be high once per revolution and only when A is high.

Gating to channels A&B high requires the Z pulse to be high once per revolution and only when A AND B are high.

Gating to A creates a Z pulse roughly 180 Electrical Degrees in duration while gating to A&B creates a Z pulse roughly 90 Electrical Degrees in duration.

Gating to a smaller duration increases the precision of the Z pulse, but also requires the motion system to be able to read the Z channel at a faster rate.

Gating is sometimes required by certain drive/amplifier/controller manufacturers.

QD145 Z (Index Pulse) Specifications:

incremental-encoder-gating1
Ungated Z
Z-A/A-Z min = 0 Electrical Degrees
Z-A/A-Z max= 225 Electrical Degrees
Z width min = 180 Electrical Degrees
Z width Max = 540 Electrical Degrees

Z Gated to A
Z width min = 135 Electrical Degrees
Z with max = A true

Z Gated to A&B
Z width min = 45 Electrical Degrees
Z width Max = A&B true

incremental-encoder-gated1

Ungated Z
Z true over A&B> 45 Electrical Degrees
Z-A&B/A&B-Z min = 0 Electrical Degrees
Z-A&B/A&B-Z max= 315 Electrical Degrees
Z width min = 180 Electrical Degrees
Z width Max = 540 Electrical Degrees

Z Gated to A
Z width min = 135 Electrical Degrees
Z with max = A true

Z Gated to A&B
Z width min = 45 Electrical Degrees
Z width Max = A&B true

Any optical encoder related questions can be posted here or sent to info@quantumdev.com .

For more information on optical encoders visit the main Quantum Devices Inc. Web site at http://www.quantumdev.com.

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