Advertisement Feature Cover Story How to measure when you go large
Traditional rotary encoders can be readily fitted to small shafts, but what happens if your design needs a larger diameter through shaft or bore? Mark Howard of Zettlex describes the traditional approach and a new, robust, more accurate approach using inductive sensors
Each of these effects alone is proba- bly not a major influence on accuracy. The problem arises because all these effects stack together.
A new approach
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 the tolerance stack up. The result - improved accuracy and reliability.
So why doesn’t everyone measure directly? The reason is that until recently, large bore rotary encoders were disproportionately expensive, delicate and difficult to fit.
Ring style optical encoders are expensive, bulky, need careful installation and are prone to failure with foreign matter. Large bore or ‘pancake’ resolvers are reliable but their price, complex electronics and bulk make them unsuitable for many applications.
here are many devices for mea- suring shaft angle and the most popular usually have a small input shaft with incremental pulse or absolute digital output. Through shaft versions are available but the bore is usually limited to <2”. Above this, encoder prices increase dramatically and availabil- ity dwindles.
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So how do you measure the angle of a large diameter through shaft of, for example, 3” or larger? Traditionally, a smaller ‘secondary’ shaft is used and its motion geared to the larger ‘primary’ shaft. In other words, the angle of the primary is measured indirectly from the angle of the secondary. For many years, this has been the approach in gun turrets, rotary tables, radar anten- nae, security cameras, large motors, medical scanners and telescopes. Large bores are often needed for slip rings, cables, hydraulic pipes or fibre optics.
The motion of the secondary shaft is usually geared from the primary shaft but it could also be driven from a toothed belt or chain. Since the sec- ondary shaft is usually small, there is a choice of encoders and installation is easy. If absolute angle of the pri- mary shaft is required, then reduc- tion gearing or a multi-turn encoder can be used.
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‘Indirect’ approach - the problems The angle of the primary shaft is calcu- lated from the angle of the secondary - assuming that the rotation of the sec- ondary varies proportionately with the primary - not unreasonable? As ever, the devil is in the detail and in prac- tice, there are problems with this assumption. The problem comes in two parts - reliability and inaccuracy. Reduced reliability comes from the increased complexity and number of components. Inaccuracy comes from the number of factors in the system’s tolerance stack up. For a system cou- pled by gears, these factors include (but are not limited to): 1. Encoder accuracy.
2. Encoder thermal coefficients. 3. Differential thermal expansion. 4. Gear backlash and wear. 5. Concentricity of gears.
6. Gear train/tooth strain versus torque. 7. Shaft concentricity. 8. Variation of gear position with shock or vibration. 9. Tolerance on gear tooth position around the gear. 10. Tolerance and variation on primary and secondary shaft centres. 11. Variations in lubrication. 12. Mechanical friction - especially stiction at low speeds/torque. 13. Foreign matter on gear surfaces. 14. Twist due to torque. 15. Shaft bending.
APRIL 2013 Electrical Engineering
Above: Zettlex IncOders enable a simple, effective and accurate way to measure the angle of large diameter shafts, like those found in gun turrets, rotary tables, radar antennae (below), security cameras, large motors, medical scanners and telescopes
Zettlex IncOders enable a simple, effective and accurate way to mea- sure the angle of large diameter shafts. These devices work on similar principles to contactless resolvers and are just as reliable in harsh con- ditions. Rather than wire spools or windings they use printed, laminar windings. This enables a low profile, annular encoder ideally suited to large diameter shafts. The electrical interface is simple due to onboard electronics - DC voltage in, absolute digital data out. The mechanical arrangement of these new generation devices is simple and eradicates all gearing. The result - a simple, com- pact, lightweight, low inertia, accu- rate and reliable solution.
Zettlex
www.zettlex.com T: 01223 874 444
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