## Absolute Position for Incremental Systems

In linear motion applications, it is often desirable to eliminate the need to make a homing run to re-acquire the reference position for an incremental linear encoder. The homing routine may need to be eliminated to save processing time, or it may not be practical…for example, if the machine can’t be moved following a loss of power due to some mechanical consideration. Additionally, to reduce costs and simplify system design, it would also be helpful to eliminate the need for home and limit switches.

Absolute linear encoders offer an upgrade path, however they also require changes on the controller side to more costly and difficult-to-implement serial interfaces like Biss C, EnDat®, SSI, and others.  These obstacles have limited the use of absolute encoders in the majority of linear motion applications.

Recently, an innovative encoder interface called Absolute Quadrature brings absolute encoder functionality to systems with controllers designed to accept a simple and commonly used A-B quadrature incremental interface.

This demonstration video from In-Position Technologies highlights the functionality and advantages of upgrading incremental positioning systems with an Absolute Quadrature encoder.

## 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.

Incremental 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.

Absolute 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

## Putting Linear Encoders Out of Sight and Out of Mind

Linear encoders can do a lot to improve factory automation. When used as secondary feedback they can greatly enhance the precision of motion control systems. They can act as a feedback device for automatic size change, and they can be used in gauging applications.

However, they can be troublesome to maintain. Most linear encoders are made from a glass strip or rod that is etched with index marks and read optically. These kinds of encoders can achieve very high accuracy…with high price points to match. However, a consistent problem in many factory automation environments is the mechanical fragility of the glass scale encoder. They can be easily broken by shock, vibration, or impact. The presence of dirt and liquids can also interfere with proper operation. Repair costs can become a problem, not to mention the cost of carrying the spare parts needed to cope with long lead times for replacements.

Depending on the resolution and accuracy class required, one alternative to these issues is the magnetic linear encoder. Today’s magnetic encoders can achieve resolution to 1 μm and accuracy to ±5 μm. Rather than index marks on glass, the scale consists of magnetic poles precisely located on a ferromagnetic strip of tape. A magnetic read head glides over the tape and outputs digital position signals. The magnetic system is much more tolerant of shock and vibration, and can tolerate most kinds of liquids and dirt. The main caveat is ferrous particles or chips; these can accumulate on the magnetic strip and cause position deviations.

Most magnetic linear encoders offer incremental signals, but a new option is absolute position over an SSI or BiSS-C serial interface. This allows the encoder to report position upon power-up, without the need for a time-wasting homing or reference run. This can be helpful in situations like a power outage, where it may not be possible to re-home the machine without damaging work in process and/or breaking tooling.

## Enhancing Stepper Motor Systems with Linear Encoders

Tabletop automation is a trend that is gaining momentum, especially in the fields of medical laboratory automation and 3D printing. Both of these applications demand a level of linear positioning accuracy and speed that might suggest a servomotor as a solution, but market-driven cost constraints put most servos out of financial consideration. New advances in stepper motor design, including higher torque, higher power ratings, and the availability of closed-loop operation via integrated motor encoder feedback are enabling steppers to expand their application envelope to include many tasks that formerly demanded a servo system.

Meeting the Demand for Even More Accurate, More Reliable Positioning

As tabletop automation development progresses, performance demands are increasing to the point that stepper systems may struggle to meet requirements. Fortunately, the addition of an external linear encoder for direct position feedback can enhance a stepper system to enable the expected level of reliable accuracy. An external linear encoder puts drive-mechanism non-linearity inside the control loop, meaning any deviations caused by drive component inaccuracy are automatically corrected and compensated by the overall closed-loop positioning system. In addition, the external linear encoder provides another level of assurance that the driven element has actually moved to the position indicated by the number of stepper pulses and/or the movement reported by the motor encoder. This prevents position errors due to stepper motor stalling, lost counts on the motor encoder, someone manually moving the mechanism against motor torque, or drive mechanism malfunction, i.e. broken drive belt or sheared/skipped gearing.

Incremental, Absolute, or Hybrid Encoder Signals

The position signals from the external encoder are typically incremental, meaning a digital quadrature square wave train of pulses that are counted by the controller. To find a position, the system must be “homed” to a reference position and then moved the required number of counts to reach the command position. The next move requires starting with the position at the last move and computing the differential move to the next command position. Absolute position signals, typically SSI (synchronous serial interface) provide a unique data value for each position. This position is available upon power-up…no homing movement is required and there is no need for a pulse counter. A recent innovation is the hybrid encoder, where the encoder reads absolute position from the scale, but outputs a quadrature incremental pulse train in response to position moves. The hybrid encoder (sometimes referred to as “absolute quadrature”) can be programmed to deliver a continuous burst of pulses corresponding to absolute position at power up, upon request from the controller, or both.

## Magnetic Linear Encoders – Tape Magnetization Technology

Precision Tape Magnetization Leads to Precision Position Measurement

The key to ultimate accuracy for any magnetic linear encoder system is the precision of the magnetic encoding on the tape (sometimes called a scale). Sensors inside the encoder read head respond to the strength and position of the magnetic flux coming from the magnetic poles encoded onto the tape. Precise placement of these poles – and just as importantly, the precise shape of these poles – is critical to the ultimate level of accuracy that can be delivered by the encoder system. Any inaccuracy in the position, strength, or shape of these fields will directly influence the accuracy of the encoder’s indicated position. This effect is amplified with increasing gap distance between the tape and the encoder read head. The further away from the tape, the weaker and more indistinct the shape and position of the magnetic poles becomes.

Not All Magnetic Tapes Are Created Equal

Many magnetic encoder tapes on the market are surface magnetized utilizing the conventional parallel magnetization process. This is a straightforward technique that results in an encoder tape that meets performance specifications at close gap settings between the read head and the tape.

A more recent tape magnetization process called Permagnet® produces magnetic poles with improved control over their strength, shape, and location on the tape. The best way to appreciate the advantages of this technology is to compare magnetic scans of some conventionally magnetized tapes to some examples of tapes encoded with the Permagnet® process. Note the visible difference in sharp definition of the magnetic poles that is produced by the newer technology.

Conventionally Magnetized Tape – Sample #1

Tape Magnetization with Permagnet® Technology – Sample #2

The stronger, more sharply-defined magnetic poles produced by Permagnet® technology enables encoders to be more tolerant of variation in the working distance between the encoder read head and the tape. Reduced dispersion and distortion of the magnetic fields at any distance within the specified working range reduces the influence of distance variation on the accuracy of the position measurement in real-world applications.

Summary of Application Benefits

• Improved linearity at close working distances for ultimate system accuracy
• Improved linearity at longer working distances
• Higher tolerance to deviations in the working distance, with reduced non-linearity
• Less need to closely control the working distance in the application, saving cost by reducing painstaking setup and alignment effort
• Full system accuracy, even if gap distance varies during operation
• Better linearity for any given pole spacing on the tape

## Direct load position sensing with secondary feedback encoders

Motion control system designers have found a way to eliminate or reduce common sources of position error, such as mechanical backlash, non-linearity, and hysteresis.  The method is called direct load position sensing and it employs linear encoders as a source of secondary position feedback.  Secondary feedback encoders supplement the indirect position measurement taken by a rotary shaft encoder by measuring the position of the moving load directly.

This method can save money by delivering the specified motion system performance at lower initial cost, and helps maintain system performance over time by getting around the problem of mechanical wear and tear degrading the accuracy of position measurements taken at the motor.

If you’d like to know more, there’s a White Paper available called “Motion Control Primer: Direct load position sensing with secondary feedback encoders”.

## What is “Minimum Edge Separation” and Why Should I Care?

When it comes to magnetic incremental linear encoders, sometimes the configurable performance parameters can be a tad obscure.  One of the most puzzling is the feature called “Minimum Edge Separation”.

Just for review, a magnetic incremental linear encoder system consists of a precision-encoded magnetic tape and a precision magnetic reader head.  It is a non-contact device and is an incremental measurement system.  Incremental means the position is given as a series of pulses which must be counted by the controller.  If the count is lost or corrupted for any reason (such as a power-down), the system must be re-zero’d to a known home position before controlled positioning/measuring can resume.

## Automatic Size Change on a Budget – Part 2

In last month’s discussion “Automatic Size Change on a Budget – Part I”, we talked about the designer’s dilemma: accomplish size change at low-cost with an unsophisticated manual size change mechanism, or achieve high-performance automatic size change using costly servo drives.  This month we will look at some alternatives that nicely bridge the gap between affordability and automatic (or, semi-automatic) operation.

## Resolution, Accuracy, and Repeatability

Written by: Bjoern Schaefer

Scott’s article on analog resolution provides a great understanding of analog signal evaluation, which I’d like to expand on with a brief summary about typical performance criteria of linear measurement systems.