[Introduction]
As we all know, servo control systems require encoders with both speed and position feedback. When selecting an encoder, we must consider not only its type but also its interface, resolution, accuracy, and protection level to meet the user 's control requirements. In particular, encoder resolution and accuracy are closely related to motion control. Today, we'll discuss the resolution and accuracy of servo encoders.
1. Resolution
Resolution refers to the distance produced between each counting unit of the encoder; it is the smallest distance that the encoder can measure.
For rotary encoders, resolution is generally defined as the number of units or pulses (e.g. , PPR) measured per revolution of the encoder. For linear encoders, resolution is often defined as the distance between two quantization units, typically given in micrometers (μm) or nanometers (nm).
Absolute encoder resolution is generally defined in bits because the output of an absolute encoder is a binary "word" based on the encoder's actual position. One bit is a binary unit; for example, 16 bits equals 2^16, or 65536. Therefore, a 16-bit encoder provides 65536 quantization units per revolution.
2. Accuracy
Accuracy is a measure of the repeatable average deviation between the actual value and the set value under normal conditions. For rotary encoders, it is generally defined as arcseconds or arcminutes, while for linear encoders, accuracy is generally measured in micrometers.
An important point to note is that high resolution does not necessarily mean high accuracy. For example, consider two rotary encoders with the same accuracy : one with a resolution of 3600 PPR and the other with 10000 PPR. The lower-resolution encoder (3600 PPR) can provide a measurement distance of 0.1 °, while the higher-resolution encoder can provide a smaller measurement distance. However, their accuracy is the same; the higher-resolution encoder simply has the ability to reduce 0.1 ° to smaller incremental distances.
Encoder resolution and accuracy are two independent concepts. As shown in the figure above, two encoders have the same resolution (24 PPR) but different accuracies.
When discussing accuracy, we usually also consider another encoder performance metric—"repeatability." Accuracy refers to the degree of closeness between a measured value and the true value; without comparison to a standard, accuracy is meaningless. "Repeatability" refers to the ability to reproduce the same result under unchanged external conditions.
In some cases , repeatability may be more important than accuracy. This is because if a system is repeatable, errors can be eliminated through compensation. Generally, encoder repeatability is defined as a multiple of encoder accuracy, often 5 to 10 times the encoder accuracy value.
Let's look at a diagram to understand the relationship between the three :
When discussing accuracy, we often combine "accuracy" and "repeatability" into one concept, and we tend to think of accuracy as something that can be expressed as "realism." When we talk about accuracy, we usually mean "high accuracy with high repeatability."
3. Factors affecting encoder resolution
An encoder's resolution depends on its number of scribe lines (incremental encoder) or encoder code disk mode (absolute encoder). Generally, resolution is a fixed value; once an encoder is manufactured, it is impossible to increase the number of scribe lines or the encoding.
However, incremental encoders can increase resolution through signal subdivision. For example, a square wave incremental encoder (HTL/TTL) outputs an incremental square wave signal. By recording the rising and falling edges of each incremental channel (signal A) each time, the encoder resolution can be increased by two times. Thus, 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 frequency), as shown in the figure below.
For encoders that use sin/cos signals, we can subdivide the electrical signal with θ to provide higher resolution compared to square wave signals, as shown in the figure below.
4. Factors affecting encoder accuracy
Once the number of lines and measurement units of the encoder are determined, its accuracy is affected by the width and spacing of these lines or measurement units. Inconsistent width or spacing can lead to pulse errors. At the same time, some external factors can also affect the accuracy of the encoder. The accuracy of a rotary encoder mainly depends on the following aspects:
1) Directional deviation of the radial grating
2) Eccentricity of the engraved code disk relative to the bearing
3) Bearing radial deviation
4) Errors caused by the connection with the coupling
For linear encoders, the expansion of the scribe lines and mounting surface caused by temperature can also affect the encoder's accuracy. Consistent width and measurement gap are key factors affecting the accuracy of incremental encoders.
The relationship between resolution and accuracy in servo motor encoders is easily confused. Accuracy primarily depends on the encoder's manufacturing process, while resolution can be improved through subdivision. However, high resolution does not necessarily equate to high accuracy. For example , by using sin/cos incremental signals, Siemens servo motor encoders can achieve a resolution of up to 24 bits (16,777,216), resulting in a minimum unit that the encoder can describe as 0.07 arcseconds . However , its physical accuracy is only ±40 arcseconds. The accuracy provided by resolution is far greater than the encoder's actual physical accuracy.
However, for Siemens servo motor encoders using HTL or TTL types, the resolution can only be increased by 4 times. For example, encoders of the 1024SR or 2048SR type can provide a maximum resolution of 4096 or 8192, and the smallest unit that the encoder can describe after conversion is 5.27 arcminutes or 2.63 arcminutes, but its physical accuracy can be provided up to ±1 arcminute . The accuracy provided by the resolution is less than the actual physical accuracy of the encoder.
