Laminar encoders ideal for linear drives
Dean Palmer reports on a Cambridge-based company whose linear encoder technology is not only low cost, but also accurate and customisable
"Until recently, one of the barriers preventing more widespread use of linear drives, has been the high cost of high accuracy position encoders. These encoders measure the relative position of the moving and stationary parts of the drive and are necessary for true, closed loop control," explained Dr Darran Kreit, technical manager at Cambridge-based technology start-up firm Zettlex. He continued: "To minimise costs, incremental encoders can be used, but this means that changes in position are measured rather than absolute position. Unfortunately, most incremental encoders are not very 'smart' and need a calibration step after every power up. They've also been known to lose count at high speed - leading to measurement errors and possibly system damage." One alternative, he said, is to use a linear potentiometer. However, while potentiometers can get around the expense issue and provide absolute position measurement, Kreit said they can also lead to much bigger service and warranty issues if they suffer from reliability or wear problems in dirty, wet or high duty-cycle applications. But Zettlex is helping to break this barrier with its range of inexpensive, high accuracy linear encoders. This is not the first time Eureka has written about the technology. Readers might recall the cover story in April 2005 about Zettlex's "slim, lightweight encoder" technology and, since then, the company seems to have gone from strength to strength. The firm's inductive technology uses printed laminations of conductors - one on the moving part and one on the stationary part - only one of which needs an electrical connection. Electromagnetic interaction between the two parts produces an electronic signature, from which an absolute position can be calculated. There is no physical contact between the moving and stationary parts so the encoders can be completely sealed to IP68 and their life is limited only by the life of the components that they are attached to. The technique is well suited to wet, submerged or intrinsically safe (potentially explosive) environments and Zettlex systems can be ATEX-approved. Linear versions of the technology can resolve to 0.2 micron, although some ongoing development will refine the resolution still further for 2007. Nevertheless, Zettlex sees its main market opportunity in providing industrial OEMs with a lower cost alternative to more 'mainstream' linearly variable differential transformers, magnetostrictive transducers and linear potentiometers in the range 1mm to 2,250mm. According to Kreit, key to the company's success to date has been reliable sensor operation even in the very toughest environments - whether that is extreme temperatures, rapid thermal cycling, high duty cycles, aggressive chemical attack or spiky EMC environments. He continued: "Undoubtedly, the high reliability factors for our sensors have been helped by the technology's requirement for an absolute minimum of mechanical parts - no bearings, seals, guides, o-rings, gaskets, bushes, etc. Also, there is no need for tight mechanical tolerances, gaps or alignment of the moving and stationary parts in order to achieve accurate measurement." Indeed, any errors or variability in mechanical offsets between the two moving parts can be programmed out by 'smart' control electronics during an initial, one-off sensor calibration step. This means that any supporting or surrounding mechanical components can be generously toleranced, readily assembled and therefore inexpensively produced. Thermal expansion or contraction effects on the sensor are also eliminated using a rather neat signal processing and measurement technique. Another unusual feature of Zettlex's technology is that the sensor's electronic controls can be configured to suit specific applications. For example, output signals can be generated for out-of-bounds movement, speeds outside pre-set limits or even after a pre-set number of running hours for system maintenance. The controls, explained Kreit, can also be configured to control multiple targets per sensor or multiple sensors. "This means that in some higher volume OEM applications, the cost of a centralised set of electronics can be amortised over a number of rotary and linear encoders," he added. The encoders are not just intended for linear displacement measurement - the company offers sensors in a wide range of shapes and sizes, including rotary, curvilinear, cylindrical, 2D & 3D. The main printed parts can be produced on flexible or rigid board to a specification or dimensions requested by individual customers.