In many industries, especially in Packaging, the need to minimize capital equipment costs drives engineers to implement low-cost, manual methods of size change (also called format change) on their machinery. In most cases, this means hand-driven cranks with mechanical dial pointers and/or mechanical revolution counters.
While cost is saved on the procurement side, cost is also shifted over to the operational side. Plant management is left with the task of keeping accurate records of various machine set-ups needed to run different products, as well as the task of training machine operators to perform all machine set-ups correctly. It doesn’t always go as smoothly as expected, and machine reformatting can result in longer downtime than planned, machine stoppages, and possibly excessive scrap.
The key to size-change improvement is capturing the linear movements of the machine components and bringing them into the control system, and then providing “smart” visual feedback to the machine operator during setup. For capturing machine position, a robust and cost-effective magnetic linear encoder is ideal. However, traditional linear encoders deliver an A-B quadrature incremental signal, which requires re-homing upon start-up or after a power loss. What’s needed is an absolute encoder signal, but that brings other challenges such as the cost and complexity of implementing an absolute signal like SSI (Synchronous Serial Interface).
Fortunately, there’s a new encoder interface option that eliminates the problem of non-absolute feedback and the hassle of absolute position signal interface: IO-Link. IO-Link is a multi-vendor, non-proprietary, device-level serial digital interface that can be aggregated onto today’s Ethernet industrial networks. Magnetic linear encoders are now available that feature absolute position indication combined with the ease and convenience of the IO-Link communication protocol.
Now we just need to provide visual feedback to the machine operator regarding which direction and how far to turn the hand cranks. Once again, IO-Link provides the answer in the form of an IO-Link-enabled, fully programmable multi-segment LED stack light. When a new machine set up is required, the position parameters are stored in the controller. The controller communicates over IO-Link to the LED stack lights, indicating to the operator which dials need to be turned and in which direction. For example, a horizontally mounted stack light could be lit red on the right half, indicating that the dial needs to be turned to the right. As the position moves closer to the proper setting, the red segments count down until the entire stack light goes green, indicating that the correct position for that axis has been reached. No paper records to maintain and store, and very little training required with the intuitive operator visualization.
For more information about IO-Link linear encoders click here, and to learn more about IO-Link programmable LED stack lights visit www.balluff.com.
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
When thinking about position sensing in machine tool applications typically glass scale systems for the CNC axis control come to your mind. These sensor principles are the most used ones in modern machine tools and are applied to requirements with resolutions to even submicrometer resolutions. Yet there are many other applications in metalworking which do not need these high end but also high priced measuring systems.
Loading and Unloading of Workpieces
In highly automated processes of loading and unloading workpieces the required repeatability of the motion axis positions is in hundreds of millimeters. This is accurate enough to achieve a reliable and accurate handling of the workpieces. Here magneticlinear encoder systems provide an optimum performance-to-cost-ratio. With significantly lower price levels compared to glass scale systems and much easier installation the total cost of ownership is much better compared to glass scale systems. These magnetic linear encoder systems are offered with both incremental and absolute output signals. Signal types for incremental outputs are quadrature or sinusoidal. Absolute outputs e.g. are used with the industrially standardized SSI and BISS interfaces. Now more and more popularity the recently also industrially standardized serial IO-Link interface has gained.
The non contact, wear free system is designed for a long lifetime and allows tolerances in alignment to a certain extent, which is especially relevant in applications of axis lengths of several meters.
Position sensing at rotating applications
The usage of CNC controls started with typically 3 axis (X-, Y, Z-). In the last years more and more 5 axis solutions have entered the market as they offer more flexibility in manufacturing. Additionaly the efficiency of these machines is higher as in many cases workpieces may be produced without the need of manually changing their orientation in the machining process.
Modular systems like rotary tables and swivel tables significantly increase the performance of machine tools. The highly compact design of magnetic rotary encoder systems supports the design of these mechatronic modular Systems.
Another advantage of the magnetic rotary encoder principle is the generous leeway in the center of the axis which allows more room for media such as coolants as well as the power supply and signal lines.
Besides the usage of glass scale systems for the classical 3-axis control of CNC machines the automation of Metalworking processes in machine tools more and more uses magnetic encoder systems thanks to their features like compact design, cost efficiency and easy installation. Drivers for the design of new machine tool concepts will be efficiency and flexibility. Definitely magnetic encoders support these demands.
More information about magnetic encoders is available here.
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.
Internally, absolute optical rotary encoders are comprised of:
An LED light source
A rotating coded disk to modulate the light beam from the LED
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:
Glass + Metal Film
Very flat, allowing for tighter air gap and higher resolution
Fragile; can shatter when exposed to high shock or severe vibration
More tolerant of high shock and vibration
Higher resolutions not feasible due to weakening of the disk caused by necessarily large number of slots
More robust than glass + metal
Susceptible to sag and flutter, requiring a higher air gap that limits resolution
Absolute magnetic rotary encoders are comprised of only two components:
An ASIC (application-specific integrated circuit) with integrated precision magnetic sensors
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.
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 Principles
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
There’s a cool new serial data interface coming on the scene.
It’s called BiSS (Bi-Directional Synchronous Serial), and is an open-source, free-to-license digital interface for sensors and actuators. BiSS is hardware compatible to the industrial standard SSI (Serial Synchronous Interface) but offers additional features and options. Here are a few highlights:
Serial Synchronous Data Communication
2 unidirectional lines Clock and Data
o Cyclic at high-speed (up to 10 MHz with RS422 and 100 MHz with LVDS)
o Line delay compensation for high-speed data transfer
o Request processing times for data generation at slaves
o Safety capable: CRC, Errors, Warnings
o Bus capability for multiple slaves and devices in a chain
o BiSS C (unidirectional) protocol: Unidirectional use of BiSS C
o BiSS C protocol: Continuous mode
o Operate actuators via two additional lines
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.