For servo control systems, encoders with speed feedback and position feedback are required. When choosingencoderWhen doing so, it is not only necessary to consider the type of encoder, but also the interface, resolution, accuracy, protection level, and other aspects of the encoder to meet the user's control requirements. Especially the resolution and accuracy of encoders are closely related to motion control. Today, we will talk about the resolution and accuracy of servo encoders#langco Precision Machinery

Resolution

Resolution refers to the distance generated between each counting unit of the encoder, which is the minimum distance that the encoder can measure.

For rotary encoders, resolution is generally defined as the unit or pulse measured by one revolution of the encoder (e.g. PPR). For linear encoders, resolution is often defined as the distance between two quantization units, typically given in micrometers (μ m) or nanometers (nm).

The resolution of an absolute value encoder is generally defined in the form of bits, as the output of the absolute value encoder is a binary "word" based on the actual position of the encoder. One bit is a binary unit, such as 16 bits equals 216 or 65536. Therefore, a 16 bit encoder provides 65536 quantization units per revolution.

Accuracy

Accuracy is used to measure the amount of repeatable average deviation between the actual value and the set value under normal circumstances. For rotary encoders, it is generally defined as arcseconds or arcminutes, while for linear encoders, accuracy is generally in micrometers.

An important point to note is that high resolution does not necessarily mean high accuracy. For example, two equally accurate rotary encoders, one with a resolution of 3600 PPR and the other with 10000 PPR. A low resolution encoder (3600 PPR) can provide a measurement distance of 0.1 °, while a high-resolution encoder can provide a smaller measurement distance, but the accuracy of both is the same. A high-resolution encoder only has the ability to reduce 0.1 ° to a smaller incremental distance.

Encoder resolution and accuracy are two independent concepts, with two encoders having the same resolution (24PPR) but different accuracies.

When we discuss accuracy, we usually also involve another performance metric of the encoder - "repeatability". Accuracy refers to the degree of closeness between measured values and true values. Without comparison with standards, accuracy cannot be discussed. Repeatability "refers to the ability to reproduce the same results without changing the external state. In some cases, "repeatability" may be more important than accuracy. This is because if the system has repeatability, errors can be eliminated through compensation. Generally speaking, the repeatability of an encoder is defined as the multiplier of the encoder accuracy, often ranging from 5 to 10 times the encoder accuracy value.

When we usually discuss accuracy, we often combine "accuracy" and "repeatability" into one, and we tend to think that accuracy is more represented by "realism". When we discuss accuracy, we often refer to "high repeatability and high precision".

Factors affecting encoder resolution

The resolution of an encoder depends on the number of engraved lines in its encoderIncremental encoder or encoder disc mode (absolute value encoder). Generally speaking, resolution is a fixed value, and once the encoder is manufactured, there is no way to increase the number of lines or codes. However, incremental encoders can increase resolution through signal subdivision. For example, a square wave incremental encoder (HTL/TTL) outputs incremental square wave signals, and by recording the rising and falling edges of each incremental channel (signal A) each time, the encoder resolution can be doubled. In this way, when we record the rising and falling edges of two channels (signals A and B), we can increase the encoder resolution by four times (4x).

For encoders using sin/cos signals, compared to square wave signals, we can use θ to segment the electrical signal to provide higher resolution.

Factors affecting encoder accuracy

After the number of lines and measurement units of the encoder are determined, the accuracy is affected by the width and spacing of these engraved lines or measurement units. Inconsistent width or spacing can lead to pulse errors.Meanwhile, some external factors can also affect the accuracy of the encoder. The accuracy of a rotary encoder mainly depends on the following aspects:

1) Direction deviation of radial grating

2) Eccentricity of the engraved code wheel relative to the bearing

3) Radial deviation of bearings

4) Error caused by connection with coupling

For linear encodersThe expansion of the engraved lines and installation surfaces caused by temperature can also affect the accuracy of the encoder, and consistent width and measurement gap are key factors affecting the accuracy of incremental encoders.

For servo motor encodersThe relationship between resolution and accuracy is easily confused. The accuracy mainly depends on the manufacturing process of the encoder, and the resolution can be improved through subdivision, but it does not mean that high resolution means that the encoder can achieve high accuracy.

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