|
 |
|
Encoders
Various types of encoders can be used for position feedback to a CNC controller. Two basic categories of encoders are rotary disc encoders attached to the end of the motor, and linear scale encoders attached to the stage. With rotary encoders, the radial position of the motor shaft is detected and used to calculate the linear position of the stage. With linear encoders, the actual linear position of the stage is directly detected.
|
 |
|
The encoder signal is generated when increment-marking lines on the encoder move past lines on the detector, creating an interference pattern. The resolution of the system is a function of the size of the increment: the number of lines per revolution on a rotary encoder or the number of lines per inch on a linear encoder. However, the number of lines per revolution or inch does not completely define the resolution.
With a Differential Line Driver Square-Wave Encoder, the passing of each line generates 2 square wave signals, with a 90-degree phase shift. Each pair of square waves has 4 readable points that supply position data to the controller. So, an encoder with 1000 lines per revolution yields 4000 counts per revolution using this feature known as
quadrature. The direction of motion is detected by comparing the phase relationship of the two signal channels.
An Amplified Sine-Wave encoder, as its name implies, sends sine wave and cosine wave output signals rather than two square-wave signals. Since sine and cosine waves are continuous, they contain a virtually infinite number of data points. With the use a high-resolution encoder signal multiplier circuit, these signals can be divided into many digital points, effectively multiplying the resolution by a factor of 5X to 1000X. As with square-wave encoders, amplified sine-wave encoders use the phase relationship between signals to detect direction of motion.
For most precision applications, closed loop encoder feedback is preferable because it yields the best accuracy and repeatability. With closed loop feedback, the controller tells the stage where to move, and the encoder sends data back to the controller confirming actual position. This position feedback is used to minimize positional error.
|
|
