The concepts of accuracy and resolution are very specific but are quite frequently used too casually. To construct a precision positioning system with the expected results, it is necessary to be careful to keep the two concepts separated.
Resolution refers to the size of steps that can be made. Accuracy describes the repeatability of the step. It would be possible to construct a system with a very low resolution (large steps) and verry high accuracy. Such a system might step in units of 1 mm, but to an accuracy of a micrometer. Other systems might have an incredibly high resolution (very small steps), but an accuracy so low that that the repeatability of the system might be hundreds or thousands of steps.
The latter systems are much more common than the former. Motor-driven micrometers frequently use a small motor a with large gear ratio in the range of 100:1. If a high resolution encoder is attached to the motor, the resulting theoretical resolution might be half a million steps per mm. It is also common for such devices to have a certain degree of backlash in the gearhead. The backlash might be equal to 100 steps or more. This means that the high resolution is useless in terms of absolute accuracy, but can still be extremely useful in terms of relative motion.
In a scanning application, it would be expected that the theoretical resolution would be useful to a degree, but would have a relatively small cyclic error. However, to move away from a point and attempt to return to the same point would bring into play all the mechanical inaccuracies of the device, resulting in an error in absolute position of many times the resolution.
The message to be learned is that it is vital to the satisfactory choice of a precision positioning system to be constantly aware of the difference between these two concepts. If it is important to an application to be able to specify the actual position to a very high accuracy, then it is necessary to avoid being misled by the fact of a high resolution. Lower cost positioning stages use precision lead screws and a motor with a rotary encoder mounted on the motor shaft. The accuracy of positioning is then decreased by any backlash in the gearhead, if one is used, and the inaccuracy of the leadscrew. For extreme accuracy, the temperature must be taken into account because of the change in length of the stage and the leadscrew when the temperature chhanges.
In micro positioning , when the highest achievable accuracy of such systems is inadequate, it is necessary to replace the rotary encoderr on the motor shaft with a linear encoder mounted to measure the position of the stage, itself. This method bypasses all the factors that contribute to the positional errors. These encoders are much more expensive, but are inidspensable when the highest levels of accuracy are required.
When choosing a stage, the key specifications to look for are bi-directional repeatability and uni-directional repeatability. These specifications describe the usable accuracy of the system. The resolution of such systems is normally consistent with the achievable accuracy and need not be specified separately under most conditions.
Updated on ... October 05, 2007