Category: Length Control


Quadrature allows a length control system to count four counts for one pulse from a bi-directional (two-channel) encoder.

A two-channel encoder is capable of reporting direction as well as movement.  Typically, there is an A channel and a B channel.  As the shaft of the encoder turns, pulses from both channels are being sent to the length control system.  The two channels are always offset by 90 degrees, so one channel will always lead the other channel depending on direction.

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Encoder Basics

Since the majority of roll forming lines use a computer to control punch and shear operations, the majority are also using rotary encoders to measure length.  If the computer control system is the brain of the machine, then the encoder is the eyes and ears of the brain.  Everything the length control system knows about the material – distance, speed, direction of movement – comes from the encoder and the measuring wheel affixed to its shaft.  It is for this reason that nearly all length variance problems are due to the encoder, the measuring wheel, and how they are tracking the material.  The computer that controls the machine and its parameters are almost never the cause of length variance.  Variance usually comes from the real world.

To understand how the encoder plays such a vital role in controlling length, it’s critical to understand how an encoder works.  You must also understand some fundamentals about the geometry of the measuring wheel and how alignment and mechanical backlash (slop) affect consistency.  This post will cover the fundamentals of encoder function.

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Length Control

There are three primary methods of controlling length on a roll former – gauge bars, positive stop, and encoders.  The first two methods are completely mechanical, with the last being electro-mechanical.

Length Control Methods

  • Mechanical
    • Length Gauge Bar
    • Positive Stop
  • Electro-mechanical
    • Encoder

Length Gauge Bars and Positive Stops

Gauge bars and positive stops have been around since the start of flying die applications.  They are probably the oldest method of length control for roll forming applications, but they’re still in use today.  That’s because they typically offer the best accuracy and consistency for the lowest cost.

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