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14. Mechanical friction – especially stiction. 15. Effect of foreign matter on gear teeth. 16. Twist due to torque in shafts. 17. Shaft bending.
Each of these effects alone is probably not a major influence on accuracy. Te problem arises because all these effects stack together. A common misconception in No 1 is that an encoder with 1,000 counts per revolution is accurate to 1/1000th of a rev. Unfortunately, resolution is not the same as accuracy. For reliability, most engineers know that the
reliability of any mechanical system is proportional to the number of parts in it – especially moving parts. Gear, pulley or chain systems are susceptible to foreign matter. Tis can often be overlooked by engineers who expect their equipment will be operated within the specified envelope and that all servicing will be carried out by skilled personnel who always replace baffles and seals. Experience shows this is wishful thinking. Foreign matter often arises from unexpected, sometimes bizarre, conditions. Examples of foreign matter to consider are dust, sand, mud, rain, snow, ice, hail, condensation, insects, rodents, rodent waste, mould, fungus, rogue mechanical tools, rogue mechanical fasteners, swarf, particles, coffee, cola (which is corrosive), pollen, air-borne seeds, vegetation, water residue, smoke/cordite residue, insect faeces/secretions, snails, worms, brake/clutch dust, hair and textile fibres. Far fetched? No – experience shows that unexpected foreign matter is to be expected. As a general rule, if the position of an object is to be
measured accurately then the measurement should be made at, or close to, the object. Measuring shaft angle
directly simplifies the system and reduces tolerance stack up. Te result: improved accuracy and reliability. So why doesn’t everyone use direct measurement?
Te reason is that, until recently, large bore rotary encoders were disproportionately expensive, delicate and difficult to fit.
Ring style optical encoders have been around for
years but are expensive, bulky, need careful installation and are prone to failure with foreign matter. Similarly, large bore or ‘pancake’ resolvers have been around for years but their price, complex electrical supply/signal processing and bulk make them unsuitable for most mainstream applications. However, a new generation of inductive encoders
now enables simple, effective and accurate angle measurement for large diameter shafts. Tese devices work on similar principles to contactless resolvers and are just as robust and reliable.
Annular encoder Rather than wire spools or windings they use printed, laminar windings. Tese enable a low profile, annular encoder ideally suited to large through-shafts. Unlike their optical counterparts, these devices do not require precision installation and the electrical interface is straightforward – DC voltage in; absolute digital data out.
Te mechanical arrangement of these new generation devices is simple and eradicates all gearing. Te result: a simple, easy-to-install, compact, lightweight, low inertia, accurate and reliable solution. ●
Mark Howard is General Manager, Zettlex UK Ltd, Newton, Cambridge, UK.
www.zettlex.com
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