Renewable Energy
Direct versus indirect measurement of shaft angle
Mark Howard discusses the challenges associated with traditional ‘indirect’ methods of measuring the angular position or speed of rotating shafts and a new, direct approach enabled by inductive sensors.
Mark Howard expone los retos asociados a los métodos tradicionales “indirectos” de medición de la posición angular o la velocidad de los ejes rotatorios, así como una nueva estrategia directa habilitada por los sensores inductivos.
Mark Howard erläutert die Schwierigkeiten mit traditionellen „indirekten“ Methoden zur Messung der Winkelstellung und Geschwindigkeit von Drehachsen und beschreibt einen neuen, direkten Ansatz mit induktiven Sensoren.
A
s a rule, it’s preferable to measure directly the position or speed of the object that you are interested in. In many cases, practical problems of physical environment
or limited space mean that this can be a challenge, particularly when measuring the angle of shafts with a diameter greater than a couple of inches. Te traditional approach is to measure angular position or speed indirectly – typically inferring the shaft’s position from measurements made elsewhere.
Indirect measurement Potentiometers, resolvers and optical encoders are the most common devices for measuring shaft angle. Potentiometers offer a simple, low cost solution but are unsuitable for harsh environments or continuous rotation. Resolvers are reliable in tough conditions but their high cost and bulk mean that they are rare outside the defence, aerospace, and oil and gas sectors. Optical encoders are not as robust as resolvers but are widely available and keenly priced. Most optical encoders have a small (typically <1/2-in diameter) input shaft with incremental pulse output. Trough-shaft versions are available but the bore is usually limited to <2-in. Above this, encoder prices increase dramatically and availability dwindles. So how do
you measure the angle of a large diameter through-shaft of, say, 3-in or larger? Traditionally, a smaller, secondary shaft is driven from the larger primary shaft and the angle of the secondary
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shaft is measured. In other words, the angle of the primary is measured indirectly or inferred. For many years, this has been the approach in gun turrets, rotary tables, radar antennae, security cameras, large motors, medical scanners and telescopes. As the secondary shaft is smaller, there is a wide choice of rotary encoders. If absolute (rather than incremental) angle of the primary shaft is required, then additional gearing or a multi-turn encoder can be used.
Problems
Te angle of the primary shaft is calculated from the angle of the secondary – assuming that their relative rotation is proportional. Not unreasonable? As ever, the devil is in the detail and, in practice, there are problems with this assumption.
As a general rule, if the required measurement accuracy is <1 degree, indirect measurement is probably not going to work reliably – or at least not for long. Tere are two parts to the problem – accuracy and reliability. Inaccuracy comes from the number of factors in the system’s tolerance stack up. For a system coupled by gears, these factors include, but are not limited to:
1. Encoder accuracy. 2. Encoder thermal coefficients – ie drift in output due to temperature.
3. Differential thermal expansion in gears, shafts, bearings, mounts, etc.
4. Gear backlash. 5. Gear wear. 6. Concentricity of gears on shafts. 7. Gear train/tooth strain versus torque. 8. Shaft concentricity. 9. Variation of gear position with shock or vibration.
10. Tolerance on gear tooth position. 11. Tolerance on primary and secondary shaft centres.
12. Variation in shaft centre distance - due to load/bearing clearances.
13. Variations from lubrication – due to amount, type and viscosity.
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