Does a higher resolution servo encoder always mean faster speed?
Servo encoder resolution refers to the precision or accuracy of the position or motion information that the encoder can provide. Specifically, it reflects the fineness with which the encoder converts motion into pulses or position signals. Encoder resolution is typically expressed in lines or pulses. Higher resolution means the encoder can provide more accurate position or motion feedback information.
However, there is no direct equivalence between servo encoder resolution and speed. High resolution primarily represents the encoder's accuracy and the level of detail in its position information, while speed is more related to the encoder's response speed. Response speed refers to how quickly the encoder reacts after receiving a signal. If the response is sluggish, it's like having a low-pass filter connected in series after an ideal sensor, which will cause phase lag in the entire closed-loop control system, affecting the high-frequency stability of the entire system.
In most applications, servo motor encoders have very fast feedback speeds and can be considered ideal sensors. However, some serial communication encoders with lower communication speeds (such as SSI) may experience delays due to their communication methods, thus limiting the bandwidth of the speed loop.
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 a rotary encoder Q, 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 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.
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 precision.
For example, two rotary encoders of the same precision, one with a resolution of 3600 PPR and the other with a resolution of 10000 PPR.
A low-resolution encoder (360° PPR) can provide a measurement distance of 0.1°, while a high-resolution encoder can provide a smaller measurement distance, but the accuracy of the two is the same. The high-resolution encoder simply has the ability to reduce 0.1° to a smaller incremental distance.
Encoder resolution and accuracy are two independent concepts. As shown in the diagram above, two encoders have the same resolution (24 PPR) but different accuracies. When discussing accuracy, we also usually involve another encoder performance metric—"repeatability".
Accuracy refers to the degree of closeness between a measured value and the true value. Without comparison with a standard, accuracy is meaningless.
"Repeatability" refers to the ability to reproduce the same result while keeping the external conditions unchanged. 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.
Difference between 18-bit and 24-bit resolution of servo encoders
The resolution of a servo encoder refers to the precision with which it measures and outputs position information. 18-bit and 24-bit resolutions represent the number of bits in the position value the encoder can output; there is a significant difference between the two.
1. **18-bit resolution**:
- An 18-bit resolution encoder can output up to 2^18 (2^62,144) different position values.
This resolution is typically used in general motion control applications, such as industrial automation and CNC machine tools, where high precision is not required.
- For applications with smaller motion ranges or slower speeds, 18-bit resolution is usually sufficient and relatively inexpensive.
2. **24-bit resolution**:
- A 24-bit resolution encoder can output up to 2^24 (16,777,216) different position values, which is higher than 18-bit resolution.
This high resolution is typically used in applications requiring high positional accuracy, such as precision machining, medical devices, and semiconductor manufacturing, where more precise positional control is needed.
- A 24-bit resolution encoder can provide finer position control and is suitable for applications requiring high-speed motion, high-precision positioning, or even higher precision.
- Due to their high precision and performance, 24-bit encoders are typically more expensive.
In general, 18-bit and 24-bit resolution servo encoders are suitable for different applications, and the choice depends on specific needs and budget. 18-bit resolution is typically suitable for general industrial control applications, while 24-bit resolution is more suitable for applications requiring higher positional accuracy.
