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Selecting the proper position sensor There are almost as many types of position sensors to choose from as there are motor types. However, the choice of sensor may be relatively simple if there are no extremely high precision requirements. The combination of a good quality gearhead and leadscrew will yield adequate repeatability for all but the most demanding applications. Given the ability to predict the position of the load from the rotor angle of the motor shaft, a low cost rotary incremental encoder is all that is needed. Such encoders with 2000 or more steps per revolution are readily available in the $25-50 range. The resolution of a system with a rotary encoder on the motor shaft is determined by the gearhead ratio (if any) and the pitch of the leadscrew (if any). A typical motorized micrometer application might use a micrometer head with a pitch of 0.5 mm, meaning that two revolutions of the micrometer shaft will cause an excursion of 1 mm. A typical gearhead ratio used in this application might be 20:1 or perhaps 256:1, depending on the need for greater resolution or greater velocity of motion. With a 2000-count encoder (see note 1), a 0.5 mm leadscrew pitch and a 20:1 gearhead, we will see 80,000 encoder counts for each mm of motion. Or, a resolution of 0.0000125 mm (0.0125 microns or 12.5 nanometers). A 6,000 RPM motor would be able to move the tip at 2.5 mm/sec. A 256:1 gearhead would be capable of a velocity of less than 20 microns/sec, but would have an astonishing (and unrealistic) resolution of 0.0000001 mm. The lesson here is that the gear ratio should be chosen to yield a resolution in keeping with the system accuracy. The relationship between accuracy and resolution is very complex and is discussed in a separate topic. For the moment, just be aware that there can be a tremendous difference between the two and don't expect to be able to position to an accuracy of 12.5 nanometers with any arrangement similar to the one described. An accuracy of 100 nanometers would be excellent. Ten micrometers (microns) would be a more realistic expectation for a motor driven micrometer. The choice between optical and magnetic encoders may resolve to simply a matter of cost, available resolution in the desired package or environmental factors. Unless there is some justification for a particular type, such as power consumption, magnetic field susceptibility or such, then it makes little difference which type is chosen. A more difficult choice lies between the use of a rotary or linear encoder and between incremental and absolute encoders. For the highest possible accuracy, it is necessary to mechanically connect the position sensor directly to the moving part. This arrangement bypasses the errors introduced by the gearhead, coupling and leadscrew. It also bypasses the effective resolution magnification of the gearhead and leadscrew. In the example given, a linear encoder would have to have 20 times as much resolution as the rotary encoder for equal increments of resolution. The cost impact of changing from rotary encoder to linear is non-trivial, but may be justified by the need. A top quality linear scale with 1 micron sensitivity will cost roughly 10 times the price of a rotary encoder. However, there is no alternative when the highest accuracy is required. The selection of an incremental or absolute encoder is determined by whether it is necessary to know the position after a power-off/power-on cycle without the necessity of moving to a reference position. Incremental encoders do not "remember" their position when power is off. Although it is a trivial matter to save the position at which power was removed, it is not possible to guarantee that there was no motion during the period that power was removed. For this reason, incremental encoder systems can measure only relative motion until some reference position is attained. Absolute encoders do not require a reference position to achieve their ultimate accuracy. However, their cost is many times higher than incremental encoders of any type. For a general rule of thumb, rotary incremental encoders are available for $10s of dollars, linear incremental encoders for $100s of dollars and absolute (digital) encoders for $1000s of dollars. In addition, most available servo controls provide interfaces for only incremental encoders. A special interface is required for the absolute encoder. Other forms of absolute position references are widely used in servo control and cost much less than the digital form. For many years, the resolver was the overwhelming choice of rotary position sensing. Resolvers are a form of rotary transformer that are used for resolutions up to approximately 4096 positions per revolution. A linear form of resolver, called an LVDT for "Linear Variable Displacement Transformer" performs a similar function. Both require special interfaces to convert the ratio of two AC signals into position data. Resolvers and LVDTs are priced somewhat less than high resolution linear encoders, not counting the electronics required to provide the signal conditioning function, which can be significant.
Summary of position sensors A quick glance at this summary reveals that nearly every type of position sensor is available in both digital and analog and usually with either optical or magnetic sensing.
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