How Accurate Are Optical Position Encoder ?

How Accurate Are Optical Position Encoder

When it comes to optical position encoders, accuracy stands out as one of the most important things to take into account. Accuracy can be defined as being the veracity degree and precision is reproducibility degree.

Understanding the optical position encoder revolves around the difference between precision and accuracy. This can be understood by thinking about an arrow that you fire towards a target. When referring to accuracy, we talk about how close the arrow reaches the bullseye. Precision is practically hitting the target and determining how close the arrows are to each other. When you shoot many arrows, precision refers to the cluster size that you get. If all the arrows are grouped, we are faced with a precise cluster.

The Accuracy Of The Optical Encoder

An optical encoder works through shining a source of light through or on an optical element. This is normally done with the use of a glass disk. Light passes through gratings or is blocked by them. A signal ends up being generated and is analogous to position.

Glass disks are really important since they have tiny features that are allowing the manufacturers claiming a high precision. The fact that is not always explicit is what goes on in the event that the tiny features end up being obscured by grease, dirt and dust. Determining accuracy is all about ignoring glass fragility, obscuration issues, temperature limits and optoelectronics life because these are problems that are known. The thing that is not known is how accurate optical encoder kits are.

Let’s think about an optical device that uses a nominal disk of one inch and an 18 bits resolution. Normally, claimed accuracy in this case is +/- 10 arc seconds. The problem is that this stated accuracy always assumes the fact that the disk will perfectly rotate in relation to read head and temperature is always constant.

Eccentricity in accuracy comes from various sources, including:

  • Hub concentricity when relative to optical disk
  • Glass disk concentricity
  • Hub perpendicularity based on optical disk plane
  • Shaft concentricity when hubs are mounted
  • Optical disk face parallelism with read head plane
  • Imperfect bearings alignment
  • Locating method
  • Thermal effects

The perfectly mounted optical disk needs fine engineering, which can increase costs. A measurement error is needed to determine real accuracy. When the mounting error is of 0.001 inches, we have a measurement error that is equivalent to angle with a subtended amount of 0.001 inches at optical track radius.

If tracks have a radius of 0.5 inches, the error is 412 arc-seconds or 2 milliradians. Basically, this means that the device that has the accuracy specification of 10 arc-seconds is 40 times less-accurate than what you see in the data sheet.

In the event that you buy an optical disk, it needs to be accurately positioned within 0.001 inches to do really well. You do not want to end up with a low accuracy.

To sum up, the information that you see in the data sheet is normally not accurate. It is really important to properly install the encoders to be sure that errors are low.

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