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Encoder resolution, accuracy and repeatability: What's the difference?

2020年8月20日

Resolution

The resolution is the smallest movement detected by the encoder. It is measured differently depending on the type of encoder:

In the case of a linear encoder, the resolution is the length of one measuring step. It is normally expressed in µm and calculated as the pole length (in µm) divided by the interpolation factor. In printing applications it is usually expressed as DPI (dots per inch).

The resolution of a rotary encoder can be expressed in arcseconds, arcminutes, degrees, grads or radians. For an absolute rotary encoder, the resolution is a number of measuring segments or units in one revolution, e.g. a 13-bit resolution of an absolute rotary encoder means that there are 213 = 8192 steps within a full 360° rotation. Incremental rotary encoder defines the resolution in PPR (pulses per revolution) and CPR (counts per revolution). PPR denotes a signal pulse from one rising edge to the next, while CPR marks rising and falling edges on both channels. CPR is achieved by electronically multiplying PPR by four.


Illustration of one CPR and PPR.


Resolution can fall within various categories, these can roughly be represented as:

  • For linear encoders:
    • High resolution: below 100 nm
    • Medium resolution: 200 nm – 10 µm
    • Low resolution: above 50 µm
  • For rotary encoders:
    • High resolution: above 18 bit
    • Medium resolution: 13 bit – 17 bit
    • Low resolution: bellow 12 bit

The selected encoder should have a resolution equal to or better than required by the application.



Accuracy

The accuracy is a measure of how close the output is to where it should be. It is the deviation between the actual position and the position reported by the encoder, or, in short, the maximum error of our measurements.

The accuracy of an encoder is a combination of the scale accuracy and the errors caused by the readhead. For rotary encoders it is usually expressed in arcseconds or degrees. The most common unit for indicating the accuracy of linear encoders is µm per unit of length of the scale (µm/m). Note that higher resolution does not automatically lead to higher accuracy.

With mounting error

Mounting error removed





Repeatability

Repeatability is the maximum difference between different measurements taken at the same actual position.

The unidirectional repeatability is the difference in reported position of the same physical point from multiple measurements taken while travelling in the same direction and under the same conditions. Electronic noise, temperature drift and ageing of components cause the encoder to report different values of the same position over time. The repeatability is often not explicitly stated but is included in the accuracy specification. As a general rule of thumb, the repeatability is usually about 5-10 times better (smaller) than the accuracy of a specific encoder.

Bidirectional repeatability is the difference in the reported position of the same physical point from several measurements when travelling in the opposite direction and under the same conditions.

A system can be very repeatable, but still not accurate enough. The difference between accuracy and repeatability is shown in Figure 6. Although good repeatability does not necessarily mean good accuracy, repeatability is the most important encoder parameter for many applications, e.g. for controlling a repetitive motion of a robot hand.

Difference between accuracy and repeatability.

Difference between accuracy and repeatability.

Posted in: Encoder knowledge