Encoder Measuring Wheels

Throughout my career, people have asked which type of measuring wheel is the best for use with a rotary encoder. The short answer is, whichever one gives you the best result on your specific application. Obviously, the real answer is a bit more complicated.

There are four main types of encoder wheel:

  • Rubber or Rubber-coated
  • Magnetic
  • Phenolic
  • Knurled Steel or Aluminum
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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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I’m Back!

After a long absence, the website is back.  I’ve changed jobs a couple of times in the last few years, and my hosting company sold out to another company.  It took months for them to move everything over to the new company, and in the end they lost my database.  Shame on me for not keeping an up-to-date backup.  Yet, here we are.

Most of what was on the site was a version of a whitepage I’d written for some function or another, so it won’t take long to recover most of the material.  In reality, most of my traffic is from people looking for the correct methods to calculate things like encoder resolution, but I’m glad to provide a useful service, no matter how small.

If you’re in the industry, have an interest in things “roll forming”, and are looking to learn a few things about the process then bookmark the page and check back from time-to-time.