jim_miller

Jim Miller

Quantum Device - Application/Design Engineer

Summary

Jim is employed with Quantum Devices Inc, a leading manufacturer of optical rotary incremental Encoders. He plays a vital role in optical encoder design and...

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Understanding Incremental Encoder Signals

Engineers using an incremental encoder for the first time often have a misunderstanding about what types of signals are provided by an encoder and how to interface an encoder to their application. They tend to know that an incremental encoder will be able to provide highly accurate positional information, but how to get this data from an incremental encoder is often a mystery.

While there are many variations of incremental encoders, the most common provide three main channels of information: A, B and Z.

Channels A and B provide identical square wave signals that correlate to the “line count” of the encoder. If the encoder has a resolution of 5,000 lines, there will be 5,000 pulses per revolution (PPR) on channel A and 5,000 PPR on channel B.

Channels A and B offset from each other by 90 electrical degrees. This offset is known as quadrature and is used to determine direction of rotation. The term quadrature comes from the 90 electrical degree offset, as there are four 90-degree segments in a 360 electrical degree cycle.

In one direction the leading edge of channel A will be before the leading edge of channel B. And in the opposite direction channel B will lead channel A.

Channels A & B (Incremental Channels)

Use only A (or only B) for an RPM or counting applications where the rotation is either unidirectional or where you don’t need to know direction.

Use A and B together to know direction. After two low pulses, the next high pulse indicates direction. This is due to the phasing offset between A and B of 90 electrical degrees, placing the signals in what is known as “quadrature”: http://quantumdevices.wordpress.com/2010/02/22/why-use-an-optical-quadrature-encoder-for-a-motor-encoder/.

These signals can also be used to set up an up/down counter.

Index pulse, also known as Z, marker, or I

The Index pulse is a pulse that occurs once per rotation. It’s duration is nominally one A (or B) electrical cycle, but can be “gated”: http://quantumdevices.wordpress.com/2009/03/27/what-is-meant-by-rotray-incremental-encoder-gating/ to reduce the pulse width.

The Index (Z) pulse can be used to verify correct pulse count.

The Incremental Encoder Index pulse is commonly used for precision homing. As an example, a lead screw may bring a carriage back to a limit switch. It is the nature of limit switches to close at relatively imprecise points. This gives a coarse homing point. The machine can then rotate the lead screw until the Z pulse goes high, identifying a homing point with much higher precision.

For a 5,000 line count encoder, this would mean locating a homing position to within 1/5,000 of a rotation or a precision of .072 Mechanical Degrees.

Commutation (UVW) signals are used to commutate or electrically time a brushless DC motor. I like to compare these signals to that of a distributor in a car. The commutation (sometimes called “Hall”) signals tell the motor windings when to fire.

You would need to have encoder commutation signals if the motor you are mounting the encoder to has a pole count and there is no other device doing the work of commutation. It is important to note that commutation signals need to be aligned or “timed”: http://quantumdevices.wordpress.com/2010/03/17/timing-a-bldc-motor-to-an-optical-encoder/ to the motor.

Single-ended vs. Differential

These terms refer not to the waveforms of signals, but instead to the way the signals are wired.

Single-ended wiring uses one signal wire per channel and all signals are referenced to a common ground.

TTL”: http://en.wikipedia.org/wiki/Transistor-transistor_logic and Open Collector are types of single-ended wiring.

Differential wiring uses two wires per channel that are referenced to each other. The signals on these wires are always 180 electrical degrees out of phase, or exact opposites. This wiring is useful for higher noise immunity, at the cost of having more electrical connections.

Differential wiring is often employed in longer wire runs as any noise picked up on the wiring is common mode rejected.

The image below shows an example of how differential wiring can common-mode-error out a noise component that is picked up along the wiring.

RS-422 is an example of differential wiring.

Tags: encoder, signals. incremental,

Comments on this post:

Muhammad Tayyab

2 years ago:  grt

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