Absolutely Incremental – Innovations in Magnetic Linear Encoder Technology

Linear encoders – absolute or incremental?  Incremental encoders are simple, inexpensive, and easy to implement, but they require that the machine be homed or moved to a reference position.  Absolute encoders don’t require homing, but they’re usually more expensive, and implementation is a bit more involved.  What if you could get an incremental encoder that also gave you absolute position?  Would that be great, or what?  Read on.

IncrementalEncodersIncremental encoders are pretty simple and straightforward.  They provide digital pulses, typically in A/B quadrature format, that represent relative position movement.  The number of pulses the encoder sends out correspond to the amount of position movement.  Count the pulses, do some simple math, you know how much movement has occurred from point A to point B.  But, here’s the thing, you don’t actually know where you are exactly.  You only know how far you’ve moved from where you started.  You’ve counted an increment of movement.  If you truly want to know where you are, you have to travel to a defined home or reference position and count continuously from that position.

AbsoluteEncodersAbsolute encoders, on the other hand, provide a unique output value everywhere along the linear travel, usually in the form of a serial data “word”.  Absolute encoders tell you exactly (absolutely) where they are at all times.  There’s no need to go establish a home or reference position.

So absolute is better, yes?  If that’s so, then why doesn’t everyone use them instead of incremental encoders?

It’s because incremental encoders typically cost a lot less, and are much easier to integrate.  In terms of controller hardware, all you need is a counter input to count the pulses.  That counter input could be integral to a PLC, or it could take the form of a dedicated high-speed counter module.  Either way, it’s a fairly inexpensive proposition.  And the programming to interpret the pulse count is pretty simple and straightforward as well.  An absolute encoder will usually require a dedicated motion module with a Synchronous Serial Interface (SSI, BiSS, etc.).  These interfaces are going to be both more expensive and more complex than a simple counter module.  Plus, the programming logic is going to be quite a bit more involved.

So, yes, being able to determine the absolute position of a moving axis is undoubtedly preferable.  But the barriers to entry are sometimes just too high.  An ideal solution would be one that combines the simplicity and lower cost of an incremental encoder with the ability to also provide absolute position.

Fortunately, such solutions do exist.  Magnetic linear encoders with a so-called Absolute Quadrature interface provide familiar A/B quadrature signals PLUS the ability to inform the controller of their exact, absolute position.  Absolute position can be provided either on-demand, or every time the sensor is powered up.

How is this possible?  It’s really quite ingenious. You could say that the Absolute Quadrature encoders are “absolute on the inside, and incremental on the outside”.  These encoders use absolute-coded magnetic tape, and the sensing head reads that position (with resolution as fine as 1 µmeter and at lengths up to 48-meters, by the way).  But, during normal operation, the sensor head outputs standard A/B quadrature signals.  Remember though, it actually knows exactly where it is (absolute inside…remember?), and can tell you if you ask.  When requested (or on power-up, if that’s how you have it configured), the sensor head sends out a string, or burst, of A/B pulses equal to the distance between the home position and the current position.  It’s as if you moved the axis back to home position, zeroed the counter, and then moved instantly back to current position.  But no actual machine movement is necessary.  The absolute burst happens in milliseconds.

So, to sum it up, Absolute Quadrature linear encoders provide a number of advantages:

  • Economical: Compatible with standard A/B incremental interfaces – no absolute controller needed
    • No need to upgrade hardware; can connect to existing control hardware
    • Get the advantages of absolute, but maintain the simplicity of incremental; eliminate the need for homing
  • Easy implementation: Simple setup, no (or very minimal) new programming required
  • Accurate: Resolution down to 1 µm, over lengths up to 48 meters

If you’d like to learn more about linear encoders with Absolute Quadrature, go to: http://www.balluff.com/local/us/news/product-news/bml-absolute-quadrature/

Absolute Rotary Encoder Sensing Technology: Optical and Magnetic

When it comes to selecting the most appropriate position detection technology for an absolute rotary encoder application, it’s helpful to consider the general advantages and potential disadvantages of the two most common approaches: optical and magnetic.

Optical Encoders

Example of an absolute-coded optical encoder disk
Example of an absolute-coded optical encoder disk

Internally, absolute optical rotary encoders are comprised of:

  1. An LED light source
  2. A rotating coded disk to modulate the light beam from the LED
  3. An array of photodetectors to convert the impulses of light into electrical signals.

The spinning code disk contains a series of concentric tracks that each represent one bit of resolution, and each track is associated with a separate photodetector.

Among optical encoders, there are two main variations: optical mask and optical phased-array. Optical mask encoders are the more straightforward implementation. A grated mask featuring slots of the same size as the slots in the optical disk is placed on top of the photodetectors to prevent the light spilling over from one channel to another. The chief advantage of the optical mask encoder rests in the ability of the encoder manufacturer to offer a variety of resolutions with the same photodetector array simply by changing the optical code disk and associated mask. On the downside, very high-resolution optical mask encoders require a very small air gap between the mask and the disk of about 0.001…0.003″ (25…75μm). Reliably maintaining such a tight gap requires tight manufacturing and assembly tolerances, and can lead to problems in severe shock and vibration environments.

As a result of the limitations of optical mask encoders, phased-array encoders were developed. Rather than relying on only a single detector for each channel, an ASIC (application-specific integrated circuit) provides an array of very small photodetectors for each channel. The responses of these multiple detectors are averaged, producing a more robust detection signal that is less susceptible to variation than a single detector. This additional signal robustness can be used to relax mechanical construction and assembly constraints such as disk flatness, eccentricity, and misalignment. The end result is a wider air gap tolerance for phased-array encoders compared to the optical mask types.

Both optical mask and phased-array detection schemes offer similar application advantages and disadvantages. They are immune to intense magnetic fields found around MRI machines or DC injection braking of AC induction motors. Due to the wider gap between disk and detectors, phased-array encoders are more tolerant of shock and vibration.

Regardless whether optical mask or phased-array detection is employed, both variations are rather susceptible to environmental contamination. Particulates such as dirt, dust, or powders and liquids like water or oil can block or attenuate the optical signals, leading to output errors. Another environmental consideration is that elevated temperatures and temperature variations can accelerate LED aging, leading to reduced light output and less reliable signal detection over time.

Absolute optical encoders are typically available with resolutions ranging from 10-bit (1024 pulses / 360°) to 22-bit (4,194,304 pulses / 360°).

Optical Encoder Disks

There are three popular construction methods for optical disks, each having certain advantages or disadvantages:

  1. Glass + Metal Film
    1. Very flat, allowing for tighter air gap and higher resolution
    2. Fragile; can shatter when exposed to high shock or severe vibration
  2. Metal
    1. More tolerant of high shock and vibration
    2. Higher resolutions not feasible due to weakening of the disk caused by necessarily large number of slots
  3. Mylar
    1. More robust than glass + metal
    2. Susceptible to sag and flutter, requiring a higher air gap that limits resolution

Magnetic Encoders

Example of an absolute-coded magnetic encoder disk
Example of an absolute-coded magnetic encoder disk

Absolute magnetic rotary encoders are comprised of only two components:

  1. An ASIC (application-specific integrated circuit) with integrated precision magnetic sensors
  2. A magnetically-coded rotating disk made of rubber ferrite on a metal carrier substrate

The magnetic disk employs a coding scheme called the Nonius principle, consisting of two concentric, adjacent tracks of alternating north and south magnetic poles. The number of poles on each track differs, typically by one pole. For example, the outer track may have 32 poles and the inner track 31. Going around the disk, there is a continuing shift of pole alignment between the inner and outer track. At any given position around the disk, the offset angle between inner and outer poles is unique.

Example of absolute magnetic encoder internal components
Example of absolute magnetic encoder internal components

Two magnetic field sensors inside the ASIC each produce a sinusoidal signal in response to the north and south poles as they traverse over them. The phase shift between these two signals is unique for every position around the disk. Digital electronics convert this analog phase shift into a serial digital data value corresponding to the absolute rotary position of the disk around 360° of rotation. A great advantage of magnetic encoders is that the maximum gap between the sensing ASIC and the magnetic disk surface is larger than for optical mask encoders. A typical specification for a magnetic encoder gap would be 0.012″ ±0.008″ (0.33mm ±0.2mm), compared to an optical mask encoder requiring a gap of about 0.002″ ±0.001″ (50 μm ±25μm).

Magnetic encoders are extremely robust. Virtually immune to shock and vibration, they are also impervious to many kinds of particulates and liquid contaminants, including non-magnetic (non-ferrous) metal shavings and powders. This ability to tolerate contamination largely reduces or eliminates the need for costly sealed enclosures. The primary caveats when applying a magnetic encoder are the presence of very strong magnetic fields that could disrupt the encoder’s operation and the presence of ferrous particles or dust that would be attracted to the magnetic surface, where they would potentially cause distortion of the magnetic poles.

Although magnetic encoders don’t currently offer the highest levels of resolution available with some optical encoders, they do offer more than enough resolution for a wide range of applications. Absolute magnetic encoders are available with resolutions ranging from 12-bit (4096 pulses / 360°) to 17-bit (131,072 pulses / 360°).

Comparison of Optical and Magnetic Absolute Encoder Operating PrinciplesEncoderTable

Linear Sensor Basics: Absolute vs. Incremental

When it comes time to choose a linear position sensor, there’s a dizzying array of options and terminology to wade through.  In this series of articles, we’re going “back to basics” to try to shed some light on the sometimes confusing world of linear sensing options, technologies, and terminology.

First up, we’re going to take a look the two basic linear sensor measurement types: absolute measurement and incremental measurement.

Continue reading “Linear Sensor Basics: Absolute vs. Incremental”