Something about Encoder resolution and accuracy

Stepper motor Encoders are at the heart of any closed loop servo system, providing feedback to the controller, which uses this information to determine if the motor reached the commanded position or velocity. Thus, encoder resolution and accuracy are essential to the proper operation of a closed-loop system.

Resolution is the distance over which a single encoder count takes place – it’s the smallest distance the encoder can measure. For rotary encoders, resolution is typically specified in terms of measuring units, or pulses, per revolution (PPR). Linear encoder resolution is most commonly specified as the distance over which the count takes place and is given in terms of microns (μm) or nanometers (nm). The resolution of an absolute encoder is specified in bits, since absolute encoders output binary “words” based on the encoder’s position.

Accuracy is the difference between the true position (or speed) of the device being measured and the position (or speed) reported by the encoder. For rotary encoders, it is specified in arcseconds or arcminutes, and for linear stepper motor encoders accuracy is typically given in microns.

Another term sometimes used in reference to encoder performance is “error.” Error is essentially the inverse of accuracy. In other words, accuracy specifies how close the encoder reading is to the true position, where error specifies how far the encoder is from the true position.


Factors that affect resolution
An encoder’s resolution is based on the number of lines (for an incremental encoder) or the pattern (for an absolute encoder) on the encoder disk or scale. Physically, resolution is fixed. Once an encoder is manufactured, there is no option to add more lines or patterns to the code disk.




Factors that affect accuracy
While the number of lines or measuring units determines resolution, accuracy is affected by the width and spacing of these lines or units. Inconsistent width and/or spacing will cause errors in the timing of the pulses. For absolute encoders, accuracy is influenced by the precision with which the pattern is placed on the code disk.



http://webautoshopper.com/cars/events/tops-on-choosing-a-stepper-motor-and-the-controller.html
http://bodybyhitch.com/shifouken43/blog/53/stepper-motor-controller-technology-overview

Most Effective Way to Commutate a BLDC DC Motor

Brushless DC motors
Brushless direct current electric motors, or BLDC motors for short, are electronically commutated motors powered by a DC electric source via an external motor controller. Unlike their brushed relatives, BLDC motors rely on external controllers to achieve commutation. Put simply, commutation is the process of switching the current in the motor phases to generate motion. Brushed motors have physical brushes to achieve this process twice per rotation, while BLDC motors do not, hence the name. Due to the nature of their design, they can have any number of pole pairs for commutation.

Motor commutation
Before delving too far into feedback options for BLDC motors, it is important to understand why they are necessary. BLDC motors come in single phase, 2-phase, and 3-phase configurations; the most common configuration being 3-phase. The number of phases matches the number of windings on the stator while the rotor poles can be any number of pairs depending on the application. Because the rotor of a BLDC motor is influenced by the revolving stator poles, the stator pole position must be tracked in order to effectively drive the 3 motor phases. Hence, a motor controller is used to generate a 6-step commutation pattern on the 3 motor phases. These 6-steps, or commutation phases, move an electromagnetic field which causes the permanent magnets of the rotor to move the motor shaft.

Position feedback
Since the inception of the brushless dc motor, Hall-effect sensors have been the workhorse for commutation feedback. For 3-phase control, only three sensors are required, and with a very low per-unit cost, they are easily the most economical option to achieve commutation from a pure BOM cost perspective. Hall sensors are embedded into the stator of the motor to detect rotor positon, which is used to switch the transistors in the 3-phase bridge to drive the motor. The three Hall-effect sensor outputs are commonly noted as the U, V, and W channels. While Hall sensors are an effective solution for commutating BLDC motors, they only address half the needs of a BLDC system.

How to Use BLDC Hall Sensors as Position Encoders
How to Use Hall Effect To Drive Brushless DC Motor