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.
Over the last few years there has been a lot of discussion on how we will meet the global energy demand in the future. And what will be the technologies to generate it? In the end it all comes down to the levelized cost of electricity (LCOE), which is the sum of all costs of a power plant divided by the total electricity that is generated over the plant’s lifetime. All companies in the renewable energy industry focus on reaching lower LCOE compared to conventional power generation (especially gas). Their biggest advantage is that there are no costs for fuel (sun light, wind, water).
Let’s take solar power as an example. Principally there are two ways to use the sun light: First it can be converted directly to electricity (photovoltaics). Second, it can be used indirectly by generating thermal energy (concentrated solar power). In order to reach higher efficiency solar trackers are used to orient photovoltaic panels, reflectors, or mirror towards the sun. On the other hand they add costs to the system. Therefore it must be carefully calculated whether a tracker (single or dual axis) is required or not (fixed installation).
Single axis trackers are used to position photovoltaic panels, parabolic troughs or linear Fresnel collectors from east to west on a north to south orientation. Depending on the required tracking accuracy different sensors are used for this task. As most of the photovoltaic trackers use electric linear actuators, very often inductive sensors are installed on the actuator for position feedback. They are cost optimized and are a standard feature in the actuators. Another option is to use inclination sensors that are directly mounted on the rotating shaft to provide angle feedback (e.g. in linear Fresnel plants). As inclinometers are mounted on the moving part, there is cable wear that could lead to failure over time. For high end tracking, as is required in parabolic trough plants, magnetic tape systems are used as rotary encoders. A magnetic tape is mounted around the shaft and a sensor head is installed on the frame of the tracker. The sensor counts the pulses accurately and provides continuous position feedback without any wear.
Dual axis trackers are used to position concentrated photovoltaic (CPV) panels, parabolic reflectors (dish) or mirrors (heliostats). Especially in central receiver plants high accuracy is required. They need high temperatures and therefore have to focus lots of light on a central receiver on top of a tower in the middle of the heliostat field. As there is an azimuth and an elevation axis, two position feedback systems are required. The elevation angle could be solved with an inclinometer, but this does not work for the azimuth position. Again, the position could be measured with embedded rotary encoders directly on the drive. But there is again backlash, and accuracy is of highest importance as heliostats could be one mile away from the central receiver. Magnetically coded position and angle measurement systems can be mounted on both axis (azimuth and elevation) and provide direct position feedback with highest accuracy.
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
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.