An important component of motion control systems is nothing but encoders. An encoder sends feedback signal to the controller to determine factors such as position, speed, count and direction. Resolution, accuracy and precision are the important terms used to describe the capabilities of an encoder. People often use them interchangeably. Obviously, these three terms are independent of each other and are not interchangeable. Understanding the difference might become critical to your application.
Resolution
Resolution describes how finely a disk is divided. Considering optical encoders, the resolution specifies the number of times the output signal goes high per revolution. This number can be matched with the number of lines on a disk. As we look into a disk, it has lines and higher resolution means a multiple of the number of lines. These lines are the physical reality as it denotes the disk’s resolution. The value ranges from low numbers like 32 or 64 to much higher resolutions of 5000 or 10000 and beyond.
In an incremental encoders via quadrature, encoders are added with additional LED and photo sensor. As compared to the resolution indicated by the physical lines on the encoder disk, this type enables the resolution of an encoder to be multiplied by a factor of two or four.
Absolute encoders operate differently. Here each code is absolute, which means they output a unique code for each position on the disk. Resolution for absolute encoders is the number of positions per revolution the disk rotates through 360 degrees. Absolute encoder with a bar code can offer programmable resolution. Since the absolute encoders don’t use quadrature, there is no equivalent to the resolution multiplication that’s available with incremental encoders.
Accuracy
An encoder’s accuracy is the difference between the target position and actual position. The actual position is where the encoder really is. It might be off from the target position by a small amount as indicated by the range in the encoder’s accuracy specification. Considering rotary encoders, accuracy is measured in degrees, arcminutes, or arcseconds. Degrees might be sufficient for a low accuracy encoder. Fractions of a degree or arcminutes are suitable for encoders with medium accuracy. For ultra-high accuracy encoders, arcseconds is suitable. Which units to be used purely depends upon the encoder being measured.
The non-encoder parts of that system can have a dramatic impact on overall system accuracy. Encoder manufactures can control some of the factors that affect accuracy. But it is duty of end users to control application-specific factors.
Considering an example of a manufacturer offering two versions of a magnetic absolute encoder, one with 12-bit resolution and the other with 10-bit resolution. Even if the accuracy is same for both the encoders, the 12-bit version provides higher resolution. Encoders are a part of a larger motion control system. This defines the saying that accuracy and resolution are not interrelated.
Consider a machine designer who needs an encoder that can report positions every tenth of a degree. Here the designer needs more resolution, not more accuracy. A 12-bit encoder would be a good choice here.
Precision
The concept of precision applies to every component of a motion control system, not just the encoder. Consider a cut-to-length application, here an encoder is attached to a motor that drives a ball screw and actuator that positions wire for cutting. The system is set to cut the wire at exactly 15.00 inches. After the first four wires are cut, the following measurement result gives: 14.81 inches, 14.82 inches, 14.80 inches, and 14.81 inches. Here, the difference between readings is small. That means, the system has good precision. The error could be coming from nearly any component in the system. It can be the encoder, the motor, play in the threads of the ball screw, or in the bearings of the linear slide. To correct for this error, the differences in wire length can be used to calibrate the application. You can add 0.20 inches to the final target position of 15.00 inches to end up at a compensated position of 15.20 inches. So when the wires are then cut, they will be very close to the desired length of 15.00 inches. Precision is the amount that the successive measurements differ from each other. An encoder that has position errors which are repeatable may have good precision. This may not be perfectly accurate. The precision can be used to compensate for the inaccuracy of the encoder.
When choosing an encoder, better understanding of resolution, accuracy, and precision will help you make better decisions. Resolution refers to the number of cycles per revolution or cycles per inch of an encoder. Accuracy is the difference between target position and actual reported position. Precision is the difference between repeated measurements. With all these explanations, now it is easier to see why the terms resolution, accuracy, and precision are independent of each other.