Choosing the resolution of a rotary incremental encoder
May 1, 2013
“What resolution of rotary encoder do I need?” is a question designers often ask themselves. A common mistake is that a higher resolution is always better. This may or may not be the case. Ultimately it is the nature of the application that decides.
Here are some things to take into consideration.
Higher line count encoders typically cost more. Higher line counts require tighter disk to sensor alignment and air gapping, as well as, improvements in the light source and the associated electronics. All of these factor in to an increase in cost as resolution increases.
Frequency Limit & RPM
It is important to note that interface electronics have a maximum input frequency that will cap the speed of the encoder. You would like to use a 5000 line count encoder in a 6000 RPM application, but if the encoder or the interface electronics is limited to 200 kHz, you can’t as this combination of RPM and resolution is putting out 500kHz.
Quantum Devices has a standard frequency response of 500kHz over most of their rotary encoder line.
How does the math work out?
Sometimes it is more important for a programmer to be able to make their math work cleanly than possibly introducing rounding errors that could compound over time. A 360 line count works nicely as one pulse equals one degree. A 3600 line count rotary encoder is popular for this same reason.
Does the performance of my application actually benefit from a higher line count?
Higher line count rotary encoders allow a drive or controller to make faster and more accurate decisions with regard to speed regulation, but much of that is application dependent. For applications that are running with a large inertial load, the added information from a higher line count may not improve the systems performance as there is only so much any motor can do to overcome the mass of the load. Higher resolution may add information that can aid in the electronic decision making, but the system may not be able to respond any better than with a lower line count device.
A crude analogy and the reason for the photo at the top:
A race car driver can certainly out-drive my elderly grandmother when it comes to auto racing, but I bet they are both on even ground with a lawn tractor. The race car driver has a much faster reaction time than my grandmother, but the lawn tractor cannot benefit from this, so the race car driver would be overkill for this application.
You can also see how my grandmother behind the wheel of a race car would limit the performance of the vehicle, much in the way that too low of resolution on a rotary encoder can limit speed regulation, acceleration/deceleration profiles and the precision to which a system can position.
Choosing the resolution that best suits your application is a careful balance between Cost, Frequency limitations, Math, and application limitations.