What are the factors that affect the accuracy and resolution of the encoder

What are the factors that affect the accuracy and resolution of the encoder?

Motor servo control systems need to be equipped with encoders with speed feedback and position feedback. When selecting an encoder, we must not only consider the type of encoder, but also consider the interface, resolution, accuracy, and protection level of the encoder. Meet the user's control requirements. In particular, the resolution and accuracy of the encoder are closely related to motion control. Today we will talk to you about the factors that affect the accuracy and resolution of the encoder.

Encoder resolution (resolution) The resolution is the distance generated between each counting unit of the encoder, and is the smallest distance that the encoder can measure.

For a rotary encoder, the resolution is usually defined as the unit or pulse measured by one revolution of the encoder (for example, PPR). For linear encoders, the resolution is usually defined as the distance produced between two quantization units, and the predetermined unit is usually micrometer (μm) or nanometer (nm).

The resolution of an absolute encoder is generally defined in the form of bits. Because the output of an absolute encoder is a binary "word" based on the actual position of the encoder. 1 bit is a binary unit of 16 bits equal to 216 or 65536. Therefore, the 16-bit encoder provides 65536 quantization units per revolution.

What are the main factors that affect the accuracy and resolution of the encoder?

Accuracy is used to measure the size of the repeatable average deviation between the usual actual value and the set value. For rotary encoders, it is usually defined as arc seconds or arc minutes, and for linear encoders, the accuracy is usually micrometers.

The important point is that high resolution does not mean high accuracy. For example, for two rotary encoders with the same accuracy, one has a resolution of 3600 PPR and the other has a resolution of 10000 PPR. The low-resolution encoder (3600 PPR) can provide a measurement distance of 0.1°, while the high-resolution encoder can provide a smaller measurement distance. The accuracy of the two is the same, and the high-resolution encoder can only reduce 0.1° to a greater Small incremental distance

The resolution and accuracy of the encoder are two independent concepts. As shown in the figure, the two encoders have the same resolution (24PPR), but have different accuracy.

What are the main factors that affect the accuracy and resolution of the encoder?

When studying accuracy, the "reproducibility" of other encoder performance indicators is generally mentioned. Accuracy is how close the measured value is to the true value. If it is not compared with the reference, the accuracy cannot be said. "Reproducibility" refers to the ability to reproduce the same result without changing the external state. In some cases, "repeatability" is more important than accuracy. This is because if the system is reproducible, errors can be eliminated by compensation. Generally speaking, the reproducibility of the encoder is defined as the multiple of the encoder accuracy, usually 5 to 10 times the encoder accuracy value.

Next is a picture to feel the relationship between the three.

We usually think that when studying accuracy, we often put "precision" and "reproducibility" together, and accuracy tends to be expressed in terms of "fidelity". When discussing precision, it often refers to "high precision with high reproducibility".

What are the main factors that affect the accuracy and resolution of the encoder?

The resolution of an encoder depends on the number of reticles of the encoder (incremental encoder) or the encoder disc mode (absolute encoder). Generally speaking, the resolution is a fixed value. If the encoder is manufactured, the number of scribing lines and encoding cannot be increased. However, incremental encoders can improve resolution through signal subdivision. For example, a square wave incremental encoder (HTL/TTL) outputs an incremental square wave signal, and every time the rising and falling edges of each incremental channel (signal a) are recorded, it outputs 2 channels ( Signal a and

What are the main factors that affect the accuracy and resolution of the encoder?

For encoders using sin/cos signals, for square wave signals, as shown in the figure below, the electrical signal can be subdivided by θ to provide higher resolution.

What are the main factors that affect the accuracy and resolution of the encoder?

After the encoder line number and measurement unit are determined, the accuracy will be affected by the width and spacing of these engraved lines or measurement units. Inconsistent width or spacing will cause pulse errors. In addition, some external factors will also affect the accuracy of the encoder. The accuracy of the rotary encoder mainly depends on the following points.

1) Direction deviation of radial grating

2) The eccentricity of the engraved code disc relative to the bearing

3) The radial deviation of the bearing

4) Error caused by connection with the coupling

For linear encoders, the temperature-induced marking will affect the accuracy of the encoder as well as the expansion of the mounting surface. The consistent width and measurement gap are important factors that affect the accuracy of the incremental encoder.

For the motor encoder when servoing, the relationship between resolution and accuracy is very easy to confuse. The accuracy mainly depends on the manufacturing process of the encoder. The resolution is improved by subdivision, but high resolution does not mean that the encoder can achieve high accuracy. For example, by using sin/cos incremental signals, Siemens servo motor encoders can increase the resolution to up to 24 bits (resolution 16777216). After conversion, the smallest unit that the encoder can describe is 0.07 arc seconds, but the physical accuracy resolution The accuracy that can be provided is far greater than the actual physical accuracy of the encoder.

What are the factors that affect the accuracy and resolution of the encoder?

But for the Siemens servo motor encoder using HTL or TTL type, the resolution can only be increased by 4 times. Such as 1024 SR or 2048 SR type encoder, the highest resolution that can be provided is 4096 or 8192, the smallest unit that the encoder can describe after conversion is 5.27 arc minutes or 2.63 arc minutes, but its physical accuracy can provide up to ±1 arc minutes , The accuracy provided by the resolution is less than the actual physical accuracy of the encoder.